To see the other types of publications on this topic, follow the link: Climatic changes Paleoclimatology Palynology.
Dissertations / Theses on the topic 'Climatic changes Paleoclimatology Palynology'
Create a spot-on reference in APA, MLA, Chicago, Harvard, and other styles
Consult the top 43 dissertations / theses for your research on the topic 'Climatic changes Paleoclimatology Palynology.'
Next to every source in the list of references, there is an 'Add to bibliography' button. Press on it, and we will generate automatically the bibliographic reference to the chosen work in the citation style you need: APA, MLA, Harvard, Chicago, Vancouver, etc.
You can also download the full text of the academic publication as pdf and read online its abstract whenever available in the metadata.
Browse dissertations / theses on a wide variety of disciplines and organise your bibliography correctly.
1
Norman, Kelly. "A high resolution re-examination of vegetation and climate change in the Jarbidge Mountains of northeastern Nevada from 4, 000 to 2000 cal yr BP." abstract and full text PDF (free order & download UNR users only), 2007. http://0-gateway.proquest.com.innopac.library.unr.edu/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1442872.
Full text
2
Heimhofer, Ulrich. "Response of terrestrial palaeoenvironments to past changes in climate and carbon-cycling : insights from palynology and stable isotope geochemistry /." Zurich : [Swiss Federal Institute of Technlogy], 2004. http://e-collection.ethbib.ethz.ch/show?type=diss&nr=15463.
Full text
3
Serefiddin, Feride Schwarcz H. P. "Paleoclimate models for western North America as inferred from speleothem isotope records /." *McMaster only, 2003.
Find full text
4
Osterberg, Erich Christian. "North Pacific Late Holocene Climate Variability and Atmospheric Composition." Fogler Library, University of Maine, 2007. http://www.library.umaine.edu/theses/pdf/OsterbergEC2007.pdf.
Full text
5
Lisiecki, Lorraine E. "Paleoclimate time series : new alignment and compositing techniques, a 5.3-MYR benthic [exponents] d18O stack, and analysis of Pliocene-Pleistocene climate transitions /." View online version; access limited to Brown University users, 2005. http://wwwlib.umi.com/dissertations/fullcit/3174639.
Full text
6
Weirauch, Daniel R. "A high-resolution record of climate instability spanning ~1.0 million years across the mid-Pleistocene transition." Access to citation, abstract and download form provided by ProQuest Information and Learning Company; downloadable PDF file, 131 p, 2007. http://proquest.umi.com/pqdweb?did=1472642111&sid=21&Fmt=2&clientId=8331&RQT=309&VName=PQD.
Full text
7
Sun, Yuanyuan, and 孙嫒嫒. "Cenozoic climatic and environmental changes in the Qaidam Basin." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2014. http://hdl.handle.net/10722/210238.
Full textAbstract:
Large discrepancies remain regarding the timing of Cenozoic paleoclimatic and paleoenvironmental transitions in the central Asia. The first order driving force behind these changes has been intensively debated. Global climate change, the uplift of Tibetan Plateau, and the evolution of Paratethys sea have been proposed as three major candidates. To understand the evolutionary history of climate and environment of the region and controlling factors responsible for these paleoclimatic and paleoenvironmental changes, a combined study utilizing multiple proxies, including microfossils, bulk carbonate carbon and oxygen isotopes, long chain alkenones, plant n-alkane-based indices (carbon preference index, average chain length and Paq) and compound-specific carbon and hydrogen isotopes of higher plant n-alkanes, was carried out on a long, continuous and well-dated section in Dahonggou, Qaidam Basin, northern Tibetan Plateau. A parallel study was also carried out in another relatively shorter section in the Xunhua Basin, northern Tibetan Plateau. Six intervals of paleoclimatic and paleoenvironmental transitions over an interval of ~35 Myr can be recognized in the studied sections, including Late Eocene-Early Oligocene gradual drying (prior to ~30 Ma), Middle Oligocene aridification (~30-26 Ma), Late Oligocene-Early Miocene wetting (~26-21 Ma), Early Miocene drying (~21-17 Ma), Middle Miocene climatic optimum (~17-13 Ma), and deteriorated climate since the late Middle Miocene (~13 Ma onwards). The reconstructed onsite C4 plant abundance including occurrence of C4 plants and their thriving and the followed decreasing, a sensitive indicator of available moisture level in the environment, agrees well with these intervals.
Microfossils and long-chain alkenones suggest that a relic sea existed in the Qaidam Basin during the Middle Miocene, thus falsifying any hypothesis of significant variations in elevations of northern Tibetan Plateau prior to the Middle Miocene. The relatively stable elevations since the Eocene and before the Middle Miocene of, respectively, the central-southern part and northern part of the Plateau reveals an insignificant role of Tibetan Plateau uplift in controlling the evolution of central Asian climate and environment during the early Cenozoic. However, the Middle Miocene marine transgression and the rapid plateau-scale uplift since the late Middle Miocene probably contributed to the Middle Miocene climatic optimum and the initiated aridification afterwards in the central Asia. A comparison of proxy records in the northern Tibetan Plateau with the global benthic oxygen isotope record suggests a tight relation between the climatic/environmental transitions in the central Asia and global climatic changes. This lends support to the hypothesis that global climate, by controlling the moisture supply to the continental interior, played the dominant role in the evolution of climate and environment of central Asia during the Cenozoic time.<br>published_or_final_version<br>Earth Sciences<br>Doctoral<br>Doctor of Philosophy
8
Holt, Sabine. "Palaeoenvironments of the Gulf of Carpentaria from the last glacial maximum to the present, as determined by foraminiferal assemblages." School of Earth and Environmental Sciences - Faculty of Science, 2005. http://ro.uow.edu.au/theses/409.
Full textAbstract:
This thesis presents a palaeoenvironmental study of the Gulf of Carpentaria, northern Australia, from around the Last Glacial Maximum (LGM) to the present. Foraminifers, microscopic unicellular aquatic organisms, occur throughout the sediment in the time frame studied. Data on the species composition and preservation of the microfossils found in the Gulf of Carpentaria cores are utilised to reconstruct past environments by comparison to the known assemblages of living foraminifers in various modern environments. The Gulf of Carpentaria is a shallow epicontinental sea, situated between Australia and Papua New Guinea, and is a maximum of 70m deep. It is separated from the Pacific Ocean to the east by Torres Strait, which is 12m deep at its shallowest, and from the Indian Ocean and Arafura Sea to the west by the Arafura Sill, which is 53m below sea-level (bpsl) at its shallowest. For at least ten thousand years in the lead up to the LGM (which reached its peak about twenty thousand years ago), and for about ten thousand years after, sea levels were lower than the 53m-deep Arafura Sill. The continental shelf in the Gulf of Carpentaria area between Australia and Papua New Guinea was exposed, creating a land bridge between the two islands, and a lake developed in the Carpentaria Basin. This palaeolake is termed Lake Carpentaria (named by Torgersen et al., 1983). Documentation of the timing in fluctuations in the extent and salinity of Lake Carpentaria provides information on local and regional climatic systems, such as the Australian summer monsoon. Constraining the nature and timing of the postglacial rise in sea-level which flooded the lake provides evidence for global eustatic sea-level reconstructions. Analysis of sediment cores from the Gulf of Carpentaria, beginning around 40ka cal BP (forty thousand calendar years before present), shows the existence of Lake Carpentaria (a large, non-marine water body of fluctuating extent) until sealevel rose over the Arafura Sill and inundated the palaeolake around 10.5ka cal BP. The earliest studied phase dates to around 40ka cal BP which is a marineinfluenced brackish water lacustrine facies where Lake Carpentaria is briefly at its maximum extent: 12m deep in its deepest section. The existence of such a large body of water (around 150,000km2) supports the existence of a strong Walker Circulation in the region enhancing precipitation. Between 40ka and 18.8ka cal BP the non-marine, increasingly saline, Lake Carpentaria decreased to 7m maximum water depth, adding to the evidence of aridity around the LGM in northern Australia. At 18.8ka cal BP the lake freshened and monospecific bivalve, foraminiferal and ostracod populations dominated the still shallow (around 8m deep) lake. The lake was expanding, and from around 15±2ka cal BP, fluctuations are noted in the general trend of increasing precipitation. The recorded variations in precipitation intensity may result from stronger seasonality (i.e. monsoons) and/or interdecadal variability (e.g. El Niño Southern Oscillation). At 12.7ka cal BP Lake Carpentaria was at around 12m maximum water depth – the maximum documented extent in the studied period. At this stage there was some exchange of waters with the Arafura Sea via tidal outlet channels in the Arafura Sill (indicating sea-level around 60m below present), seen as a marine influence beginning in the western margins at 12.7ka cal BP. At 12.4ka cal BP the sealevel had risen to the same height as water levels within the lake (58m bpsl). By 12.2.ka cal BP sea-level was up to 2m higher than the previous lake level, and flowed into the lagoonal Lake Carpentaria via channels in the Arafura Sill. By 10.5ka cal BP the sea-level had overtopped the highest surface of the 53m-deep Arafura Sill and the transition to marine conditions began in the Gulf of Carpentaria, confirming the accepted models of sea-level rise.
9
Grigg, Laurie Davis. "Millennial-scale vegetation and climate variations in the Pacific Northwest during the last glacial period (60,000-16,000 cal yr B.P.) /." view abstract or download file of text, 2000. http://wwwlib.umi.com/cr/uoregon/fullcit?p9998032.
Full textAbstract:
Thesis (Ph. D.)--University of Oregon, 2000.<br>Typescript. Includes vita and abstract. Includes bibliographical references (leaves 237-250). Also available for download via the World Wide Web; free to University of Oregon users.
10
Shunk, Aaron Driese Steven G. "Late Tertiary paleoclimate and stratigraphy of the Gray Fossil Site (eastern TN) and Pipe Creek Sinkhole (northcentral IN)." Waco, Tex. : Baylor University, 2009. http://hdl.handle.net/2104/5303.
Full text
11
Smith, Alexander Ryan. "Sedimentology and Stratigraphy of Miocene-Age Glacial Deposits, Friis Hills, Antarctica." Thesis, North Dakota State University, 2011. https://hdl.handle.net/10365/29316.
Full textAbstract:
The Friis Hills is an isolated plateau standing as much as 600 m above surrounding topography in the McMurdo Dry Valleys region or Antarctica.Preserved on the plateau surface is a sequence of early to middle Miocene-aged dritis. At the eastern edge of the plateau, these drifts fill a shallow paleovalley to a depth of at least 35 m. The drills are exposed in a natural cross-section where modern topography crosscuts the paleovalley. Establishing an age and an environmental interpretation for these deposits is important because Antarctic paleoclimate records are lacking from the Mid-Miocene Climate Optimum. Two drifts fill the ancient paleovalley in the eastern Friis Hills. The upper drift is here named Cavendish drift: the lower is here named Friis drift. Cavendish can be subdivided into three units, whereas Friis drift can be subdivided into two units. Each of these units is a horizontal bed that laps on paleovalley sidewalls. The lowest, Friis II, is a compact diamicton that is overlain by a nearly in-situ bedded volvanic ash. Based on [20]Ar/[39]Ar dating, the ash is 19.76 [plus/minus] 0.07 Ma old. A second diamicton, Friis I, conformably blankets Friis II and was discovered to hold fossileferous interbeds. Both Friis I and II contain erratic clasts and both are lodgemont tills deposited from small, locally derived, alpine glaciers. Bedrock striations show ice flow to the northeast at azimuths between 025? to 032?, parallel to the trend of the paleovalley axis. Above these, Cavendish I. II. and III were deposited when thick ice covered the Friis Hills. Where the Cavendish drift laps onto paleovalley sidewalls, bedrock striations show ice flow from 077? to 150?. Cavendish drift was deposited sometime alter 19.8 Ma but before 14 Ma. when the Dry Valleys glacial records show that regional glaciers became cold-based. Downcutting eventually isolated the Friis Hills plateau, resulting in the preservation of the drift sequence. This event was most likely associated with growth or the East Antarctic Ice Sheet 14 Ma ago. This age constraint means that the tills preserved in the Friis Hills date from a time just before the East Antarctic Ice Sheet expanded and became a permanent feature. Based on the age-dated stratigraphy presented in this thesis, future work focusing on fossiliferious interbeds could provide unique and important constraints on Miocene climate change.<br>North Dakota State University. Department of Geosciences
12
Buffen, Aron Maurice. "Abrupt Holocene climate change: Evidence from a new suite of ice cores from Nevado Coropuna, southwestern Peru and recently exposed vegetation from the Quelccaya Ice Cap, southeastern Peru." Columbus, Ohio : Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1218568566.
Full text
13
Young, Seth Allen. "A chemostratigraphic investigation of the late Ordovician greenhouse to icehouse transition oceanographic, climatic, and tectonic implications /." Columbus, Ohio : Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1201628490.
Full text
14
Araki, Ricardo 1966. "A história do clima de São Paulo." [s.n.], 2012. http://repositorio.unicamp.br/jspui/handle/REPOSIP/286914.
Full textAbstract:
Orientadores: Luci Hidalgo Nunes, Christian Pfister<br>Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Geociências<br>Made available in DSpace on 2018-08-21T15:40:51Z (GMT). No. of bitstreams: 1
Araki_Ricardo_D.pdf: 3861461 bytes, checksum: 37262bd50f9c4575fdfea9afa2e03e46 (MD5)
Previous issue date: 2012<br>Resumo: Esta pesquisa teve por objetivo reconstruir a historia climática do estado de São Paulo, desde sua colonização européia até o inicio do século 20. Fontes históricas de diferentes naturezas como jornais, diarios, almanaques, cartas, relatos de viagens, entre outros foram consultadas no intuito de comprovar hipótese que manifestações da Pequena Idade do Gelo, que entre os séculos 14 e 19 afetaram sensivelmente as sociedades do Hemisfério Norte, teriam também sido sentidas no território paulista. Mais de 7 mil consultas foram feitas, resultando em 1355 registros sobre tempo e clima entre 1550 e 1927, que após serem revisadas possibilitaram sistematizar 685 eventos que contêm claramente a data, local e descrição do fenômeno climático. A quantidade de registros mencionando frio, temperaturas baixas e geadas praticamente se equipara aos registros sobre chuvas, sendo também mais que o dobro do número de informações sobre calor, corroborando a hipótese de que também no estado de São Paulo o período atual tem sido menos frio do que o passado<br>Abstract: This research aimed to reconstruct the climatic history of São Paulo State, from the colonial period to the beginning of the 20 century. Distinct historical sources like journals, diaries, almanacs, letters, expedition records, among others, were consulted to verify the hypothesis, i.e., that manifestations of the Little Ice Age, which significantly affected the Northern Hemisphere societies between 14 and 19 century, were also experienced in the territory of São Paulo. More than 7,000 queries were made, resulting in 1,355 weather and climate records between the years of 1550 and 1927, which were revised and then systematized into 685 events that contain clearly date, localization and the phenomenon description. The quantity of data mentioning cold, low temperatures and frosts is practically the same of the number of records of rain, and more than twice of warm events, reinforcing the hypothesis that also in São Paulo the current period has been less cold than the past<br>Doutorado<br>Análise Ambiental e Dinâmica Territorial<br>Doutor em Ciências
15
Hekkers, Michael Leslie. "Climatic and Spatial Variations of Mount Rainier's Glaciers for the Last 12,000 Years." PDXScholar, 2010. https://pdxscholar.library.pdx.edu/open_access_etds/4951.
Full textAbstract:
Regional paleoclimatic proxies and current local climate variables and were analyzed to reconstruct paleoglaciers in an effort to assess glacier change On Mount Rainier. Despite the dry and generally warm conditions (sea surface temperatures (SST) -0.15°C to +1.8°C relative to current temperatures), the previously documented McNeeley II advance (10,900 - 9,950 cal yr B.P.) was likely produced by air temperature fluctuations. The average SST record and the terrestrial climate proxies show cooling temperatures with continued dryness between McNeeley II and the Burroughs Mountain advance (3,442 - 2,153 cal yr B.P.). The paleoclimate during the Burroughs Mountain advance was both cool and warm (SST temperatures -0.55°C to +0.5°C) and was the wettest of the Holocene.
A combination of statistical and deterministic equilibrium line altitude (ELA) models was used to produce Holocene ELAs between 1,735 -2,980 m. Glacial advances were predicted 10,990, 10,170, 9,260, 8,200, 6,490, 3,450 and 550 - 160 cal yr. B.P. Two glacier flow models were produced simultaneously to constrain glacial extent through the Holocene. Model I is based on current mass balance parameters and produced lengths for the Nisqually and Emmons glaciers 3.7 - 14.2 km and 4.2 - 17.1 km respectively. Glaciated area ranged from 26 to 327 km2. Model 2 is tuned to the Garda advance and produced lengths 2.6-10.6 km and 2.3-13.9 km. Glaciated area ranged from 11 to 303 km2. The first two advances were similar in elevation and GIS-modeled extent to McNeely II moraines. The following three advances were not detected in the geologic record. The 3,450 cal yr. B.P. advance was the largest of the late-Holocene (ELA 1,800 - 1,817 m) and was ~200 m lower than the geologic record. The ELAs of the Garda advance were modeled (1,944 - 1,983 m) and are similar to previous reconstructions. North-south spatial variations in glacial extent increase during periods of recession as the southern glaciers receive more ablation than northern glaciers.
Early humans could have accessed the alpine environments as high as 1,730-2,980 m. The early Holocene glacial extent allowed the highest (2,980 m) 11,150 cal yr. B.P. and lowest (1,730 m) 10,990 cal yr. B.P. alpine access. Glacial retreat (2,727 m 10,400 cal yr. B.P.) was followed by an advance (1,929 m 10,170 cal yr. B.P.) and another retreat (2,951 m 10,050 cal yr. B.P.). Ice gradually descended and limited access to 1,820 m 6,490 cal yr. B.P. Glacial extents remained largely unchanged until the historic era when paleohumans would have had access to alpine environments at 2,000 m.
16
Robbins, John A. "Stable isotopes, marine paleoclimates, and human subsistence on California's Channel Islands." Ann Arbor, Mich. : ProQuest, 2007. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:1442835.
Full textAbstract:
Thesis (M.S. in Geology)--S.M.U., 2007.<br>Title from PDF title page (viewed Mar. 18, 2008). Source: Masters Abstracts International, Volume: 45-05, page: 2387. Adviser: Robert Gregory. Includes bibliographical references.
17
Helama, Samuli. "Millennia-long tree-ring chronologies as records of climate variability in Finland /." Helsinki : S. Helama, 2004. https://oa.doria.fi/bitstream/handle/10024/2685/millenni.pdf.
Full text
18
Reade, Hazel. "Climate variability and early human occupation in the Gebel Akhdar, Libya : evidence from inter- and intra-tooth isotope analysis." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.648105.
Full text
19
Doughty, Alice Marie. "10Be Cosmogenic Exposure Ages of Late Pleistocene Moraines Near the Maryburn Gap of the Pukani Basin, New Zealand." Fogler Library, University of Maine, 2008. http://www.library.umaine.edu/theses/pdf/DoughtyAM2008.pdf.
Full text
20
Sánchez, López Giomar. "North Atlantic Oscillation imprints in the Central Iberian Peninsula for the last two millennia: from ordination analyses to the Bayesian approach." Doctoral thesis, Universitat de Barcelona, 2016. http://hdl.handle.net/10803/400758.
Full textAbstract:
The climate variability of the Iberian Peninsula (IP) can be explained in terms of relatively few large-scale atmospheric modes, such as the North Atlantic Oscillation (NAO), the East Atlantic (EA) and the Scandinavian (SCAND) patterns. The present-day IP climatology clearly show that the NAO is the most prominent mode, especially in winter. However, the recent investigations have highlighted that other climate modes play a key role in both modulating the NAO-climate relationship and controlling certain meteorological parameters, although little is known about the past evolution of these climate interactions. Furthermore, there is a reasonable understanding of the past NAO evolution in the northern and the southern IP, but almost no information is available in the Central IP. Within this framework, the main aim of this PhD thesis is to characterize the impacts of the NAO on the Central IP over the last 2,000 years.
The conceptual lake model formulated to understand the present-day influence of the NAO on the limnological evolution of Peñalara (2016 m asl) and Cimera (2140 m asl) alpine lakes (Iberian Central Range, ICR) was established using Pearson's correlation coefficients between seasonal data of the NAO index, climatic data (i.e., precipitation and air temperature data) and ice phenology records from both lakes. The results suggest that the effects of the NAO are only reflected in the thawing process via the air temperature and the insulating effect of snow accumulation on the ice cover. An altitude component is evident in our survey because the effects of the NAO on Peñalara Lake are restricted to winter, whereas for higher Iberian alpine lakes (i.e., Redon Lake, Pyrenees), the effects extend into spring. A latitudinal component is also clear: in northern Europe, the NAO signal is primarily reflected in lake ice phenology via the air temperature, whereas our results confirm that in southern Europe, the strong dependence of both precipitation and temperature on the NAO determines the importance of these climatic variables for lake ice cover.
The past NAO impacts on the Central IP were determined by the multi-proxy characterization of the sediments of Peñalara and Cimera lakes using ordination statistical analyses. This approach was used to reconstruct the intense runoff events, the lake productivity and the soil erosion in the Cimera Lake catchment and to interpret these factors in terms of temperature and precipitation variability in the ICR for the last two millennia. The spatio-temporal integration of this reconstruction with other Iberian reconstructions was employed to identify the main climate drivers over this region. During the Roman Period (RP; 200 BC – 500 AD), the generally warm conditions and the E–W humidity gradient in the IP indicate a dominant interaction between a negative NAO phase and a positive EA phase (NAO-–EA+), whereas the opposite conditions during the Early Middle Ages (EMA; 500 – 900 AD) indicate a NAO+–EA- interaction. The dominantly warm and arid conditions during the Medieval Climate Anomaly (MCA; 900 – 1300 AD) and the opposite conditions during the Little Ice Age (LIA; 1300 – 1850 AD) in the IP indicate the interaction of the NAO+–EA+ and NAO-–EA-, respectively. Furthermore, the higher solar irradiance and fewer tropical volcanic eruptions during the RP and MCA may support the predominance of the EA+ phase, whereas the opposite conditions during the EMA and LIA may support the predominance of the EA- phase. In addition, evidence of African dust inputs in these lakes could denote a coupled displacement between the Intertropical Convergence Zone and the NAO during the study period.
Finally, a Bayesian random walk-modularised model was formulated to quantitatively reconstruct the evolution of the NAO impacts in the ICR (NAOICR) for the last two millennia using the raw chemical element profiles obtained from the Cimera Lake sediments using an X-Ray-Fluorescence Avaatech® core scanner. The obtained quantitative values of the NAOICR were in accordance with previously reconstructed precipitation and temperature conditions. In addition, the comparison of the NAOICR with other NAO approaches show that the local impact of the NAO can also display global aspects of this climate mode and that this impact reconstruction could<br>La variabilidad climática en la Península Ibérica (PI) puede ser explicada por un reducido número de modos atmosféricos como la Oscilación del Atlántico Norte (siglas en inglés NAO), el Atlántico Este (siglas en inglés EA) o el Escandinavo, aunque la NAO es el predominante, sobre todo en invierno.
La construcción de un modelo conceptual basado en los coeficientes de correlación de Pearson entre el índice de la NAO, datos climáticos (temperatura, precipitación y nieve) y registros de la cubierta de hielo de lagos alpinos (Peñalara, 2016 m.; Cimera, 2140) situados en el Sistema Central Ibérico (SCI) ha permitido entender los efectos actuales de la NAO en estos lagos. Los resultados sugieren que dichos efectos sólo se reflejan en el proceso de deshielo de la cubierta a través de la temperatura del aire y del efecto aislante de la nieve acumulada en ella durante el invierno. A mayor altitud, este efecto se extiende hasta primavera y a mayor latitud este efecto depende principalmente sólo de la temperatura.
El análisis multiproxy de los sedimentos lacustres de estos lagos ha permitido realizar una reconstrucción climática en el SCI para los últimos dos mil años. La integración espacio-temporal de dicha reconstrucción con otras reconstrucciones en la PI ha permitido determinar los principales factores climáticos que actuaron durante ese periodo. Las interacciones entre NAO y EA, la variabilidad solar y la actividad volcánica y quizás el movimiento acoplado entre la Zona de Convergencia Intertropical y al NAO fueron probablemente las principales causas de dicha variabilidad climática.
Finalmente, el desarrollo de un modelo matemático basado en estadística bayesiana ha permitido reconstruir cuantitativamente el impacto de la NAO en el SCI (NAOSCI) para este periodo usando datos geoquímicos obtenidos mediante el análisis de fluorescencia de rayos X de los sedimentos de Cimera. A pesar de que el modelo no pudo ser validado por correlación cruzada, los valores de NAOSCI están en concordancia con la reconstrucción climática previamente obtenida. La comparación de NAOSCI con otras reconstrucciones de NAO sugiere que este impacto regional podría mostrar aspectos globales de este modo climático y por tanto, con algunas mejoras podría considerarse un índice regional para toda la PI.
21
Wayolle, Audrey A. J. "Multiscale soil carbon distribution in two Sub-Arctic landscapes." Thesis, University of Stirling, 2011. http://hdl.handle.net/1893/6502.
Full textAbstract:
In recent years, concern has grown over the consequences of global warming. The arctic region is thought to be particularly vulnerable to increasing temperatures, and warming is occurring here substantially more rapidly than at lower latitudes. Consequently, assessments of the state of the Arctic are a focus of international efforts. For the terrestrial Arctic, large datasets are generated by remote sensing of above-ground variables, with an emphasis on vegetation properties, and, by association, carbon fluxes. However, the terrestrial component of the carbon (C) cycle remains poorly quantified and the below-ground distribution and stocks of soil C can not be quantified directly by remote sensing. Large areas of the Arctic are also difficult to access, limiting field surveys. The scientific community does know, however, that this region stores a massive proportion (although poorly quantified, soil C stocks for tundra soils vary from 96 to 192 Gt C) of the global reservoir of soil carbon, much of it in permafrost (900 Gt C), and these stocks may be very vulnerable to increased rates of decomposition due to rising temperatures. The consequences of this could be increasing source strength of the radiatively forcing gases carbon dioxide (CO2) and methane (CH4). The principal objective of this project is to provide a critical evaluation of methods used to link soil C stocks and fluxes at the usual scales spanned by the field surveys (centimetre to kilometre) and remote sensing surveys (kilometre to hundreds of kilometres). The soil C distribution of two sub-arctic sites in contrasting climatic, landscape/geomorphologic and vegetation settings has been described and analysed. The transition between birch forest and tundra heath in the Abisko (Swedish Lapland) field site, and the transition between mire and birch forest in the Kevo (Finnish Lapland) field site span several vegetation categories and landscape contexts. The natural variability of below-ground C stocks (excluding coarse roots > 2 mm diameter), at scales from the centimetre to the kilometre scale, is high: 0.01 to 18.8 kg C m-2 for the 0 - 4 cm depth in a 2.5 km2 area of Abisko. The depths of the soil profiles and the soil C stocks are not directly linked to either vegetation categories or Leaf Area Index (LAI), thus vegetation properties are not a straightforward proxy for soil C distribution. When mapping soil or vegetation categories over large areas, it is usually necessary to aggregate several vegetation or soil categories to simplify the output (both for mapping and for modelling). Using this approach, an average value of 2.3 kg C m-2 was derived both for soils beneath treeless areas and forest understorey. This aggregated value is potentially misleading, however, because there is significant skew resulting from the inclusion of exposed ridges (with very low soil C stocks) in the ‘treeless’ category. Furthermore, if birch trees colonise tundra heath and other ‘open’ plant communities in the coming decades, there will likely be substantial shifts in soil C stocks. This will be both due to direct climate effects on decomposition, but also due to changes in above- and below-ground C inputs (both in quantity and quality) and possibly changes in so-called root ‘priming’ effects on the decomposition of existing organic matter. A model of soil respiration using parameters from field surveys shows that soils of the birch forest are more sensitive to increases in mean annual temperature than soils under tundra heath. The heterogeneity of soil properties, moisture and temperature regimes and vegetation cover in ecotone areas means that responses to climate change will differ across these landscapes. Any exercise in upscaling results from field surveys has to indicate the heterogeneity of vegetation and soil categories to guide soil sampling and modelling of C cycle processes in the Arctic.
22
Hansen, Christel Dorothee. "The characterisation of an openwork block deposit, northern buttress, Vesleskarvet, Dronning Maud Land, Antarctica." Thesis, Rhodes University, 2014. http://hdl.handle.net/10962/d1013138.
Full textAbstract:
Investigating openwork block accumulation has the potential to further our understanding of rock weathering, the control of geological structure on landforms, the production of substrates for biological colonisation and the impacts of climate change on landform development and dynamics. Various models for the development of these landforms have been proposed. This includes in situ weathering, frost heave and wedging. Furthermore, it has been suggested that cold-based ice has the potential to preserve these features rather than to obliterate them. Blocky deposits are also frequently used as proxy evidence for interpreting palaeoclimates. The morphology and processes acting on a blockfield located on the Northern Buttress of the Vesleskarvet Nunataks, Dronning Maud Land, Antarctica (2°W, 71°S) were investigated and characterised. Given block dimensions and orientations that closely resembled the parent material and only small differences in aspect related characteristics observed, the blockfield was found to be autochthonous with in situ block production and of a young (Holocene) age. Small differences in rock hardness measurements suggest some form of aspect control on rock weathering. South-facing sides of clasts were found to be the least weathered. In comparison, consistently low rock hardness rebound values for the north-facing aspects suggest that these are the most weathered sides. Additional indicators of weathering, such as flaking and pitting, support analyses conducted for rock hardness rebound values. Solar radiation received, slope gradients and snow cover were found to influence weathering of clasts across the study site. Furthermore, ambient temperatures and wind speed significantly influenced near-surface ground temperatures dynamics. However, the lack of a matrix and paucity of fine material in textural analyses suggest a limited weathering environment. It is suggested that the retreat of the Antarctic ice sheet during the last LGM led to unloading of the surface, causing dilatation and subsequent fracturing of the bedrock along pre-existing joints, leading to in situ clast supply. Subsequent weathering and erosion along other points or lines of weakness then yielded fines and slight edge rounding of clasts.
23
Guillon, Sebastien. "Dynamique de la végétation alluviale côtière dans le Sud-Est de la France (bassins versants du Loup et de la Cagne, Alpes-Maritimes) au cours de la première moitié de l' Holocène." Phd thesis, Université Nice Sophia Antipolis, 2014. http://tel.archives-ouvertes.fr/tel-01064064.
Full textAbstract:
Au cours de la première moitié de l'Holocène les conditions climatiques, eustatiques et anthropiques connaissent de nombreuses et importantes modifications. Parmi celles-ci, les données. Parmi celles-ci, les données régionales relatives au quart nord-ouest du bassin méditerranéen révèlent plusieurs variations pluriscalaires et significatives des conditions d'humidité ainsi que des températures. Parallèlement, la hausse du niveau marin enregistre des vitesses très élevées et la région de l'arc Liguro-Provençal voit l'installation des premières communautés agro-pastorales de la culture " Impressa ", en l'occurrence dès le début du 6ème millénaire cal. BCE. Afin de qualifier la réponse de la végétation alluviale côtière, entre la fin du 8ème et le 5ème millénaire, face à l'évolution de ces conditions, l'analyse pollinique à haute résolution de deux séquences sédimentaires alluviales (bassin du Loup et de la Cagne) à été réalisée. Grâce à une approche pluridisciplinaire (carpologie, ostracologie, sédimentologie...) et méthodologique inédite (analyse du transport pollinique fluviatile) les résultats montrent une évolution précise des écosystèmes végétaux côtiers et alluviaux. Au sein de cette évolution, la remontée du niveau marin joue un rôle fondamental comme en témoigne l'expansion littorale des aulnaies marécageuses. Le forçage climatique joue également un rôle important. Les étés plus humides du 6ème millénaire participe à la diffusion du sapin à basse altitude, alors que l'augmentation de la fréquence des sécheresses estivales enregistrées à partir du 5ème millénaire favorise le développement d'une végétation sclérophylle à bruyère arborescente. La néolithisation de la région participe également au façonnage des paysages littoraux. La récurrence du type pollinique Cerealia (gr. Hordeum) dès les premières décennies du 6ème millénaire atteste de l'importance des plaines alluviales côtières dans l'économie de production des premiers groupes néolithiques.
24
Partin, Judson Wiley. "Stalagmite reconstructions of western tropical pacific climate from the last glacial maximum to present." Diss., Georgia Institute of Technology, 2008. http://hdl.handle.net/1853/22556.
Full textAbstract:
The West Pacific Warm Pool (WPWP) plays an important role in the global heat budget and global hydrologic cycle, so knowledge about its past variability would improve our understanding of global climate. Variations in WPWP precipitation are most notable during El Niño-Southern Oscillation events, when climate changes in the tropical Pacific impact rainfall not only in the WPWP, but around the globe. The stalagmite records presented in this dissertation provide centennial-to-millennial-scale constraints of WPWP precipitation during three distinct climatic periods: the Last Glacial Maximum (LGM), the last deglaciation, and the Holocene. In Chapter 2, the methodologies associated with the generation of U/Th-based absolute ages for the stalagmites are presented. In the final age models for the stalagmites, dates younger than 11,000 years have absolute errors of ±400 years or less, and dates older than 11,000 years have a relative error of ±2%. Stalagmite-specific 230Th/232Th ratios, calculated using isochrons, are used to correct for the presence of unsupported 230Th in a stalagmite at the time of formation. Hiatuses in the record are identified using a combination of optical properties, high 232Th concentrations, and extrapolation from adjacent U/Th dates. In Chapter 3, stalagmite oxygen isotopic composition (d18O) records from N. Borneo are presented which reveal millennial-scale rainfall changes that occurred in response to changes in global climate boundary conditions, radiative forcing, and abrupt climate changes. The stalagmite d18O records detect little change in inferred precipitation between the LGM and the present, although significant uncertainties are associated with the impact of the Sunda Shelf on rainfall d18O during the LGM. A millennial-scale drying in N. Borneo, inferred from an increase in stalagmite d18O, peaks at ~16.5ka coeval with timing of Heinrich event 1, possibly related to a southward movement of the Intertropical Convergence Zone (ITCZ). An inferred precipitation maximum (stalagmite d18O minimum) during the mid-Holocene in N. Borneo supports La Niña-like conditions and/or a southward migration of the ITCZ over the course of the Holocene as likely mechanisms for the observed millennial-scale trends. In Chapter 4, stalagmite Mg/Ca, Sr/Ca, and d13C records reflect hydrologic changes in the overlying karst system that are linked to a combination of rainfall variability and cave micro-environmental effects. Dripwater and stalagmite geochemistry suggest that prior calcite precipitation is a mechanism which alters dripwater geochemistry in slow, stalagmite-forming drips in N. Borneo. Stalagmite Mg/Ca ratios and d13C records suggest that the LGM climate in N. Borneo was drier and that ecosystem carbon cycling may have responded to the drier conditions. Large amplitude decadal- to centennial-scale variability in stalagmite Mg/Ca, Sr/Ca and d13C during the deglaciation may be linked to deglacial abrupt climate change events.
25
Fawcett, Peter J. "Simulation of climate-sedimentary evolution a comparison of climate model results to the geologic record for India and Australia /." 1994. http://catalog.hathitrust.org/api/volumes/oclc/33920205.html.
Full text
26
Sosdian, Sindia Maria. "Climate transitions across the cenozoic insight from elemental ratios in benthic foraminifera and marine gastropods." 2008. http://hdl.rutgers.edu/1782.2/rucore10001600001.ETD.17576.
Full text
27
Oosting, Antje Margriet. "Palaeoenvironmental and climatic changes in Australia during the early cretaceous = Palaeomilieu en- klimaatsveranderingen in Australië gedurende het vroeg krijt /." 2004. http://eprints.jcu.edu.au/1803.
Full textAbstract:
Thesis (Ph.D.) - James Cook University, 2004.<br>"Met een samenvatting in het Nederlands" -- T.p. "Ter verkrijging van der graad van doctor aan de Universiteit Utrecht, op gezang van de Rector Magnificus, Prof. Dr. W. H. Gispen, ingevolge het besluit van het College voor Promoties in het openbaar te verdedigen op donderdag 30 septembber 2004 des morgens om 10:30 uur" -- T.p. Typescript (photocopy) Bibliography: leaves 175-181.
28
Brenner, Logan. "Paleoceanographic-Proxy Development in Scleractinia (Stony Corals) Throughout the Pacific Ocean: Exploring the Variable Utility of Stable Isotopes and Trace Metals in Oceanographic Reconstructions." Thesis, 2017. https://doi.org/10.7916/D8P279DP.
Full textAbstract:
Scleractinia (stony corals) are powerful tools in the field of paleoceanography, allowing researchers to reconstruct past ocean conditions based on variations in coral geochemistry. As corals regularly accrete their aragonite skeletons they preserve a history of climate on regional to global scales. Often able to provide centuries long continuous records of climate, an individual coral colony can provide insight into significant environmental perturbations. If preservation permits, fossil corals can be used to evaluate climate thousands of years in the past. Researchers use paleoclimate proxies, which are indirect geochemical fingerprints of environmental conditions, to create paleoclimate time series. Paleoclimate proxies are prevalent throught the literature and while many are well constrained by years to decades of use, individual conditions unique to study sites and timescale prevent the use of blanket assumptions regarding their interpretation. In this dissertation I illustrate the varied ways that the same or similar coral-based climate proxies can be used to reconstruct past ocean conditions.
Part I (Ch. 2, 3) presents two studies based along of the Pacific Coast of Panamá to examine the influence of the seasonal migration of the Intertropical Convergence Zone (ITCZ). I analyzed a long-term coral δ18O time series from a living massive Porites colony to address low-frequency variation overprinted by the wet-dry seasonality. The coral record uncovered a clear decadal (~11 year) cycle in coral δ18O-inferred precipitation. I propose this mode is related to basin-wide processes, specifically a component of the Pacific Decadal Oscillation, which describes large-scale patterns in sea surface temperature (SST) and precipitation influencing marine ecosystems. In Chapter 2, I supplement the coral δ18O record with a coral Ba/Ca time series from a different coral colony. Coral Ba/Ca can be used as a proxy for river discharge (Q), although this practice is relatively new. Our coral record outlined seasonal variation in river Q and can also be used to identify past El Niño events and prolonged periods of drought. Uncovering a geochemical indicator of El Niño in this region is particularly powerful since conditions become warm and dry, which negate each other in coral δ18O rendering the proxy unable to consistently identify these climate events. This chapter furthers the community’s understanding of the many ways that trace metals can be used in paleoceanographic research, specifically to constrain local hydroclimate.
In Part II (Ch. 4, 5) I present two studies in the Great Barrier Reef (GBR) based on coral δ18O and Sr/Ca records from modern and fossil Isopora, a coral species that is nearly completely absent from the paleoceanographic literature. Although this suite of climate proxies is similar to those used in Part I, in Part II the GBR corals provide a history of sea surface temperature rather than hydroclimate, which is due to prevailing local environmental conditions over a given timescale. In Chapter 4 I developed the first modern Sr/Ca- and δ18O-Sea Surface Temprature (SST) calibration using Isopora, which approaches those calculated for the commonly used Porites corals. Using Isopora in Pacific-based paleoceanographic research allows us to analyze coral records from reefs that might not be dominated by Porites. In Chapter 5 I applied the new Isopora Sr/Ca- and δ18O-SST calibrations to fossil corals recovered during Integrated Ocean Discovery Program (IODP) 25. The fossil corals date beyond the Last Glacial Maximum (LGM, ~20 kyr BP) to 25 kyr BP. In the Pacific SST change since the LGM is better constrained for more equatorial locations so our fossil samples from the GBR extend the geographic network of LGM-aged coral-based climate proxies. I measured ~5-7°C of cooling in the GBR at the LGM compared to today. The SST change through the LGM deglaciation provides valuable understanding of reef resilience and future risk of or adaptability to climate change.
Each chapter in this dissertation uses similar strategies but provides a unique perspective on past climate change in the tropical Pacific. This dissertation identifies the many ways that coral proxies can be utilized with specific examples of the ways in which interpretation can vary. It is necessary to consider the environmental specifics of a given region before blindly interpreting paleo-proxy data. Furthermore, coral-based proxy records are supremely powerful tools in exploring and uncovering past climate histories of a given region. Coral-records can supplement and extend the limited instrumental record with centuries to millenia long information on SST and hydroclimate. These data can improve climate models, further our knowledge of coral reef growth, and deepen our understanding of regional hydroclimate, which are all vital to our understanding of global climate.
29
Fitchett, Jennifer Myfanwy. "Towards a multi-proxy holocene palaeoenvironmental and palaeoclimatic reconstruction for Eastern Lesotho." Thesis, 2016. http://hdl.handle.net/10539/19361.
Full textAbstract:
Thesis submitted in fulfilment of the academic requirements for the degree of Doctor of Philosophy
School of Geography, Archaeology and Environmental Studies
University of the Witwatersrand, Johannesburg
October 2015<br>The eastern Lesotho highlands observe climate patterns distinct from adjacent lower altitude
regions, representing a niche environment with unique biodiversity, comprising well-adapted but
restricted biomes. With a heavy reliance on subsistence agriculture, Lesotho faces risks to both the
economy and individual livelihoods, should current rates of climate change persist or intensify.
Furthermore, eastern Lesotho serves as southern Africa’s primary water catchment, with
precipitation exceeding evaporation. Any changes in the climate and hydrological systems, as are
likely under climate change scenarios, would compromise biomes, livelihoods, and water security
both locally and regionally. Climate change research in eastern Lesotho, is thus of particular value,
yet meteorological data are sparse and the palaeoenvironmental history remains poorly resolved.
This research presents the first multi-proxy Holocene palaeoenvironmental and palaeoclimatic
reconstruction for eastern Lesotho. This reconstruction is developed from the results from pollen,
diatom and sediment analyses, extracted from sediment cores obtained from two peat bogs at Sani
Valley (~2,800 m.asl) and Mafadi Wetland (~3,390 m.asl), and from an exposed gully-sidewall profile
at Sekhokong (~2,950 m.asl), approximately 1km south of the Sani Valley site. The reconstructions
are temporally constrained by AMS radiocarbon dates obtained for all three sites.
Mafadi Wetland demonstrates marked differences to the lower altitude sites, including slower
sedimentation rates, a decrease in pollen and diatom taxa diversity, and an increase in the relative
abundance of ice-tolerant diatom taxa. The microtopography of the three sites influences the rates
of sedimentation, sediment properties, pollen composition, and distinct palaeoenvironmental and
palaeoclimatic reconstructions for each site. The Sekhokong record commences in the late
Pleistocene, with a wet period from ~13,180-10,850 cal. yr BP, interrupted by a dry period from
~13,080-12,830 cal. yr BP. From ~10,550-6,420 cal. yr BP, the Sekhokong record indicates a drier
climate with a slow transition to warmer, wetter conditions. The Mafadi Wetland record commences
with cold, wet conditions from ~8,140-7,580 cal. yr BP, followed by a warmer, drier period from
~7,520-6,680 cal. yr BP. Thereafter, greater microclimatic differences are apparent. For Sekhokong,
warmer, dry conditions are inferred for ~6,420-6,000 cal. yr BP, followed by cold, wet conditions
from ~6,000-5,450 cal. yr BP. Warmer, dry conditions commence earlier at Mafadi Wetland, from
~6,160-5,700 cal. yr BP, coinciding with the initiation of a longer wet period at Sani Valley, from
~6,200-4,900 cal. yr BP. At Sekhokong, a dry, warmer period follows from ~5,450-3,700 cal. yr BP. At
Sani Valley, drier conditions are evident from ~4,770-4,470 cal. yr BP, followed by a cold, wet period
from ~4,460-2,260 cal. yr BP. For Mafadi Wetland, these cold, wet conditions endure longer, from
~5,600-1,100 cal. yr BP. This overlaps with similarly cool, wet conditions at Sekhokong, from ~3,650-
1,200 cal. yr BP. By contrast, dry conditions are evident at Sani Valley, from ~2,260-1,350 cal. yr BP.
For all three sites, ~1,000 cal. yr BP to present is characterised by progressive drying, with discrete
wet events. Pronounced cold events are detected at ~12,660 cal. yr BP, ~8,400-8,000 cal. yr BP and
~150 cal. yr BP.
The results of this study indicate similarities with records from adjacent studies in western Lesotho
and South Africa, although with notable variability in the timing of events. The palaeoenvironmental
reconstructions for eastern Lesotho, and their comparison with existing studies, provide valuable
information to improve the understanding of southern African Holocene climates, and to facilitate
the development of high resolution, accurate climate models for the eastern Lesotho region.
30
Leland, Caroline Wogan. "Impacts of Partial Cambial Dieback on Tree-Ring Records from Ancient Conifers." Thesis, 2019. https://doi.org/10.7916/d8-4tdx-2898.
Full textAbstract:
Tree-ring records from long-lived trees are instrumental for understanding climate variability during the Common Era. Some of the oldest and most valuable conifers used to reconstruct past climate exhibit strip-bark morphology, in which vertical segments of the tree have died in response to environmental stress. This form of localized stem mortality, also referred to as partial cambial dieback, is particularly common on conifers growing in xeric, cold, or exposed environments. Some studies note that strip-bark trees have increasing ring-width trends relative to trees with a fully living stem circumference, but there is substantial uncertainty as to what extent partial cambial dieback can influence tree-ring records and subsequent climate reconstructions. This dissertation explores the environmental drivers of partial cambial dieback on Siberian pine (Pinus sibirica Du Tour) from Mongolia, the effect of cambial dieback on the radial growth and physiology of affected trees, and methods for reducing strip-bark biases in tree-ring records.
Chapter 1 assesses the causes and radial growth impacts of partial cambial dieback on Siberian pine trees growing on an ancient lava flow in central Mongolia. Using a combination of field observations and dendrochronological methods, this chapter demonstrates that strip-bark trees from this site exhibit dieback primarily on the southern side of stems, and that dieback was most common during a cold and dry period in the mid-19th century. Given the directionality and timing of dieback on these strip-bark trees, it is hypothesized that localized mortality events are linked to physiological injuries spurred from solar heating combined with unfavorable climatic conditions. This chapter also reveals that strip-bark trees from this site have increasing radial growth trends relative to trees with a full circular morphology (“whole-bark” trees). Strip-bark trees showed an especially rapid increase in ring widths following the cambial dieback period in the mid-19th century, providing initial evidence that dieback events can lead to increasing ring widths in strip-bark Siberian pine.
Chapter 2 seeks to discern the physiological mechanisms of increasing radial growth trends in the Siberian pine strip-bark trees using stable carbon and oxygen isotopes from tree rings. One simple hypothesis is that strip-bark trees show increasing ring-width trends because radial growth is restricted to a smaller stem area after cambial dieback events. Conversely, some studies have hypothesized that increasing ring widths in strip-bark trees reflect a CO2 fertilization effect on growth that is not readily apparent in whole-bark trees. This chapter finds that strip-bark and whole-bark trees responded similarly to increasing atmospheric CO2 and climate variability in their radial growth and leaf-level gas exchange inferred from tree-ring stable isotopes. However, strip-bark and whole-bark trees showed notably different behavior following documented cambial dieback events. After dieback events, strip-bark trees exhibited an increase in ring widths and an enrichment in stable carbon and oxygen isotopes that was not apparent in whole-bark trees. These results further support the notion that partial cambial dieback leads directly to increasing ring widths in strip-bark trees, and that this response could reflect an increase in the ratio of leaf to live stem area after dieback occurs.
Chapters 1 and 2 demonstrate that partial cambial dieback events and morphological changes impact the radial growth and physiology of strip-bark trees. Therefore, prior to developing climate reconstructions, it is necessary to remove variance associated with these non-climatic, morphological changes in tree-ring series. Chapter 3 outlines two chronology development methods for reducing strip-bark biases in tree-ring records. These methods, applied to Siberian pine and Great Basin bristlecone pine (Pinus longaeva Bailey), successfully reduce a strip-bark bias without removing low-to-medium frequency climate variance inferred from whole-bark trees, which were not impacted by dieback activity. While one approach directly corrects the bias in strip-bark series using a whole-bark chronology as a target, another method is based on the development of a low-percentile chronology, which can be applied to a site collection where the stem morphology of individual trees is unknown. Some limitations and caveats of these methods are discussed in context of the analyzed tree species.
The findings from this dissertation have significantly contributed to our understanding of the radial growth and physiological responses of Siberian pine to partial cambial dieback and environmental changes. This dissertation also provides new methods for removing strip-bark biases in tree-ring chronologies. The conclusions presented here have important implications regarding the potential effects of partial cambial dieback on tree-ring records from other tree species and climate reconstructions derived from them. Continued and detailed study of the causes and impacts of partial cambial dieback on other tree species will be critical for understanding the interactions between ancient trees and their environment, and for improving the reliability of climate reconstructions based fully or partly on strip-bark trees.
31
Rao, Mukund Palat. "Hydroclimate variability and environmental change in Eurasia over the past millennium and its impacts." Thesis, 2020. https://doi.org/10.7916/d8-890y-wb66.
Full textAbstract:
Streamflow records in many regions of Eurasia including South Asia are short and fragmentary. This makes it challenging to contextualise natural climate variability relative to anthropogenic climate change and evaluate the severity of recent extreme events. In the first section of the dissertation (Chapter 1 and 2) we use tree rings to reconstruct centennial-scale streamflow of the Indus and Brahmaputra Rivers in South Asia for the past six centuries. These two rivers and their tributaries are fed by Himalayan glaciers and the South Asian Monsoon and are the basis of economic, social, and cultural life of over 700 million people in the region. These centennial reconstructions of past discharge provide valuable information about long-term hydroclimate variability, drought and flood hazard. They also help us to interpret recent climate extremes relative to those in the past and benchmark projections of climate change for the region.
Large tropical (and extratropical) volcanic eruptions can release large quantities of reflective sulphate aerosols into the stratosphere where they may persist for up to 3 years. In Chapter 3 we discuss how these aerosols can impact European and Mediterranean hydroclimate by causing dry conditions over northwestern Europe and the British Isles and wet conditions over the western Mediterranean. We examine this hydroclimate response using Superposed Epoch Analysis (SEA), a statistical method used to identify consistent responses to events by testing for the possibility of random occurrence. Finally, in Chapter 4 we describe in further detail (cf. Chapter 3) the modified double-bootstrap SEA that we developed in Chapter 3 to examine the uncertainty inherent in SEA within a probabilistic framework. We describe our modified SEA by applying it to two datasets, a reconstruction of northern hemisphere summer temperature for the past millennium, and a compilation of tree ring fire scar records for the western U.S. Using these two datasets we examine post-eruption northern hemisphere cooling following volcanism and the synchrony between drought conditions and fire events in the western U.S. respectively.
32
Friel, Charlotte. "Diatom Records of Holocene Climatic and Hydrological Changes in the Western Hudson Bay Region, Canada." Thesis, 2011. http://hdl.handle.net/1807/30595.
Full textAbstract:
Rapidly changing climates in northern Canada make the western Hudson Bay region an area of high importance for paleoenvironmental studies. Long-term changes in assemblages of diatoms (microscopic algae) were analyzed from lake sediment cores from Baker Lake, Nunavut, and Lake AT01, northern Ontario, to track responses to past environmental changes. Diatom assemblages dating to 6700 years ago in AT01 were initially characterized by cold- tolerant Fragilarioid assemblages, but shifted to an assemblage dominated by large benthic species and Cymbella diluviana consistent with the timing of the Holocene Thermal Maximum after 6300 years BP. A possible drainage event in Lake AT01 may have added significant hydrologic control on the diatom assemblages. The post-industrial period is marked by the largest compositional shifts in both records. Assemblages during the 20th century are indicative of reduced ice cover and enhanced thermal stratification linked to a climate regime shift noted in Hudson Bay since the mid-1990’s.
33
Ali, Guleed. "Late Glacial and Deglacial Fluctuations of Mono Lake, California." Thesis, 2018. https://doi.org/10.7916/D8CC2BNN.
Full textAbstract:
Anthropogenic climate change risks significant changes in the global distribution of precipitation. Across the western United States, modelling studies show significant reductions in wetness that imply weighty societal and ecological impacts. But the validity of the model projections need to be ground-truthed. Paleo-hydroclimate data are useful reference points to assess a model’s ability to hindcast past hydroclimate. If the hindcast matches the paleodata, it brings confidence to a model’s ability to predict future hydroclimatic change.
The foremost metric of hydroclimate in the geologic record is the surface area of lakes in hydrologically closed basins. In such basins, a lake’s surface area is determined by the balance between precipitation and evaporation. The lake will expand when the balance is positive, and it will contract when the balance is negative.
In this dissertation, I develop a 25-9 ka record of lake fluctuation from the Mono Basin, a hydrologically closed basin in east-central California. I deduced the fluctuations using three pieces of evidence: stratigraphy; geomorphology; and geochronology. These pieces of evidence were determined from a study of the Mono Basin’s Late Pleistocene lithostratigraphic unit: the Wilson Creek Formation.
There are 19 tephra intercalated in the Wilson Creek tephra. They are named by their reverse depositional order (Ash 19 is the oldest and Ash 1 is the youngest). Uncertainty on their ages cause confusion as to the paleo-hydroclimate record of the Mono Basin. The age of Ash 19, for example, is important because its deposition marks the onset of relatively high lake levels that occurred during the last glaciation. There are two principal interpretations of Ash 19’s age: 40 ka, which is based on lacustrine macrofossil 14C data; and 66 ka, which is underpinned by paleomagnetic intensity data. In chapter 2, I tested these end-member interpretations. I used the U/Th method to date carbonate deposits that underlie and cut across Ash 19. The U/Th data show that Ash 19 must have been deposited between these two dates: 66.8 ± 2.8 ka; and 65.4 ± 0.3 ka. These dates are, therefore, more consistent with the 66 ka interpretation of Ash 19’s age. Thus the onset of relatively high lake levels in the Mono Basin corresponds with the rapid drawdown of atmospheric CO2 during Marine Isotope Stage 4. The coincidence between the drop in atmospheric CO2 and lake level rise is suggestive of a causal link.
In chapter 3, I determined Mono Lake's fluctuations 25-9 ka. This time encompasses three climatic intervals: the coolest time of the last glaciation, termed the Last Glacial Maximum (LGM); the period corresponding to the rapid termination of the last glaciation, termed the deglaciation; and the early Holocene, a period of inordinate warmth that immediately followed the last glaciation’s termination. In this study, I used stratigraphic and geomorphic evidence in conjunction with 14C and U/Th dates. I measured the 14C dates on bird bones and charcoal. And I measured the U/Th dates on carbonates. Together the data showed that the lake's rises and falls concurred with North Atlantic climate. Periods of aberrant warmth in the North Atlantic concurred with low stands of Mono Lake. On the other hand, extreme cooling in the North Atlantic correlated with Mono Lake high stands. The timing of these lake fluctuations also corresponds with variations in other tropical and mid-latitude hydroclimatic records. The global harmony in the hydroclimatic records suggests a unifying conductor. I hypothesize that the conductor is tropical atmospheric circulation.
In chapter 4, I present evidence on the peculiar case of an extreme low stand of Mono Lake. The low stand is dubbed the “Big Low”. The principal evidence underpinning the Big Low derives from a sedimentary sequence exposed along the canyon walls of Mill Creek. The strata show that the lake fell below 1,982 m between the deposition of Ashes 5 and 4—making this low stand the lowest recognized level of Mono Lake during the Wilson Creek Formation. Observations from dispersed sequences corroborate this interpretation. And three data constrain the age of the Big Low to be between ~24.4-20.5 ka: a carbonate U/Th date on a littoral conglomerate associated with the Big Low; a carbonate U/Th date that underlies Ash 4; and a carbonate U/Th date that cuts across Ash 5. Thus the interval that the Big Low must occur within encompasses the LGM. The timing of this low stand, therefore, corresponds with summer temperature minima, suggesting that the fall was due not to an increase in evaporation but due to a decrease in precipitation. This finding is counter to conventional wisdom: that the LGM was a relatively wet interval. In addition, both the documentation of a low stand during glacial maximum conditions and the inference that precipitation must have been reduced are contrary to previous published interpretations from model and paleoclimatic data. These discrepancies raise significant questions about our understanding of the regional expression and forcing of hydroclimate across the western United States during the LGM. Because of this period’s importance to ground-truthing climatic models, additional evidence on the geographic extent of this unexpected result is essential.
34
Boswell, Steven M. "Enhanced Surface Melting of the Fennoscandian Ice Sheet during Stadials." Thesis, 2018. https://doi.org/10.7916/D8572VRZ.
Full textAbstract:
Unexpected melting of Northern Hemisphere ice sheets during periods of regional cooling characterizes the climate of the last glacial period. While the Heinrich Events are the most well-studied example of this phenomenon, Samuel Toucanne and colleagues recently documented evidence of Fennoscandian Ice Sheet melting during Heinrich Stadials, the cold periods during which Heinrich Events occur. In this dissertation, I use the geographic provenance of sediments in the Bay of Biscay, a proxy for Fennoscandian Ice Sheet melting, along with other paleoclimate records to: (1) demonstrate the persistence of abrupt Fennoscandian Ice Sheet melting as a feature of the Pleistocene climate system, (2) develop a self-consistent explanation for the synchronous melting of ice sheets in the North Atlantic region, and (3) elucidate the timing of abrupt climate change in the Northern Hemisphere.
I begin by introducing a framework for inferring the subglacial transport distance of fine sediments from coupled provenance and grain size analyses. This chapter untangles the relationships between the source, size, transport history, and geochemical signature of glacigenic sediments in northern Europe, clarifying the geographical significance of sediment provenance in the Bay of Biscay. I then develop a new method for the spectral analysis of unevenly sampled time series. In the following chapter, I apply the new spectral method to time series of Fennoscandian Ice Sheet melting, Laurentide Ice Sheet melting, and solar activity changes during the last glacial period. Doing so reveals a coherence between ice sheet melting and solar activity and helps explain the quasi-periodic melting of ice sheets on millennial timescales. I then extend the neodymium isotope provenance record of Fennoscandian Ice Sheet melting through Marine Isotope Stage 6, demonstrating that enhanced summertime melting of the FIS during Heinrich Stadials is a recurring feature of glacial periods. In the final chapter, I document a relationship between the occurrence of abrupt ice sheet melting in the Northern Hemisphere and the precession of Earth’s spin axis to reveal an astronomical forcing of millennial-scale climate change.
35
Bishop, Daniel Alexander. "Attributing the Causes of a Century of Hydroclimatic Change in the United States." Thesis, 2021. https://doi.org/10.7916/d8-cwj0-2058.
Full textAbstract:
Hydroclimate in the United States (US) is climatologically divided by the 100th meridian. The semi-arid western US has experienced high-amplitude multidecadal swings in drought and soil moisture variability over the last millennium, culminating in anthropogenic warming-driven drying into the early part of the 21st century. In sharp contrast, the climatologically humid eastern US has experienced century-long increases in precipitation and soil moisture, and generally less warming than in the west, creating a fascinating wetting east – drying west contrast over North America. In eastern North America, a large proportion of the annual precipitation trend was driven by fall-season increases in the southeastern US (SE-US). A rigorous examination of this region would lead to greater insight into the broader causes of hydroclimatic change across North America.
The objectives of this dissertation are to (1) identify the large-scale drivers of increased fall precipitation in the SE-US and (2) contextualize and evaluate the causes of regional-to-continental scale changes in soil moisture availability across North America.The first three research chapters of my dissertation focus on my first objective to address the causes of the 20th-century fall precipitation trend. In my first research chapter, I identify and describe fall-season precipitation increases in the SE-US. I show that fall precipitation in the SE-US has increased by nearly 40% during 1895-2016 due to increased circulation-driven moisture transport from the Gulf of Mexico into the SE-US, likely associated with a strengthening or relocation of the North Atlantic Subtropical High (NASH). The NASH is a semipermanent high pressure system located over the North Atlantic that directs moisture transport into the SE-US. Using atmospheric general circulation models forced by sea-surface temperatures (SSTs) and anthropogenic emissions, I demonstrate that models have the capability to simulate a precipitation response to the NASH, but the observed precipitation trend was extremely unlikely in both forced and unforced scenarios. This indicates that the fall precipitation trend was likely caused by processes not well represented in these models, suggesting more work is needed to address why models are unable to simulate observed circulation and SE-US precipitation trends. SST-forced simulations do simulate an enhanced, although displaced to the northwest, NASH and greenhouse gases appear to weakly increase the likelihood of fall wetting.
In the first research chapter, I evaluated the proximate drivers of the SE-US fall precipitation variability and trends, working towards the goal of identifying the ultimate driver of observed NASH intensification and SE-US wetting. As a next step, it is important to understand how the increases in precipitation have been delivered, particularly given that fall overlaps with the peak of Atlantic hurricane season. In the second research chapter, I complete a daily-scale decomposition of storm types and precipitation intensity in the SE-US to understand how different precipitation events influenced the fall precipitation increase. I show that increases in SE-US fall precipitation occurred largely as a result of highest-intensity non-tropical (mostly frontal) precipitation days (72% contribution to the fall precipitation trend). In contrast, precipitation from tropical cyclones, a major contributor to extreme fall precipitation, demonstrated a nominal but positive contribution to the trend (13%). Nearly all of the precipitation was delivered on the most extreme (top 5%) intensity precipitation days. These results suggest the observed increase in SE-US fall precipitation has critical implications for flash flood risk from high-intensity rainfall events should the trend continue through the 21st century.
Once I identified the types and intensity of storms that influence the fall precipitation trend, I sought to diagnose the physical causes of increased circulation into the SE-US and resultant increases in fall precipitation in the third research chapter. I find that fall precipitation was facilitated by an increase in zonal sea-level pressure (SLP) gradient over the Gulf of Mexico, almost entirely driven by increased SLP along the western edge of the NASH. The zonal SLP gradient was linked to an upper-tropospheric wave train over the North Pacific and North America, leading to increased circulation into the SE-US from the Gulf of Mexico. SST-forced simulations are capable of simulating the spatial features of the NASH and wave train but lack the circulation trends that lead to increased zonal SLP gradient and fall precipitation. The models simulated an enhanced tropical-to-subtropical wave train which increased subsidence and SLP over the subtropical Atlantic Ocean and North America and led to a stronger, more expansive modeled NASH intensification relative to reanalyses, suggesting there exists a stronger atmospheric response to tropical SSTs in models. Due to these discrepancies between models and reanalyses, we can anticipate limitations when using atmospheric models forced by observed SSTs to assess regional climate change in the North Atlantic basin. More research will be needed to understand the physical processes that influence this divergence. The ultimate cause of increased fall moisture transports into the SE-US and resulting precipitation increases remains elusive, but this work improves our understanding of the succession of climatic events that contribute to increased fall precipitation and identify key areas of research needed to reduce uncertainty in SST-forced models.
In the final research chapter, I address my second dissertation objective and broaden the focus to all of North America to investigate and contextualize the recent increase in the contrast between soil moisture anomalies in eastern and western North America, termed the east-west North American aridity gradient. Positive aridity gradient values refer to periods during which soil moisture anomalies are more positive in eastern relative to western North America. Using observed and tree-ring reconstructed summer soil moisture anomalies, I show that the 2001-2020 aridity gradient was more positive than any 20-year period since 1400 CE, which followed the most negative aridity gradient during 1976-1995. Using hydrologic models, I find the 2001-2020 aridity gradient anomaly was predominantly driven by century-long summer precipitation increases in the East and to a lesser degree by annual temperature and humidity trends and spring precipitation decreases in the West. Model-simulated anthropogenic trends have minimal effects on the aridity gradient trend due to high inter-model spread in modeled precipitation trends and larger warming effects in the East relative to the West. My findings reveal significant uncertainty in how human and natural systems will be impacted by changes in future water resource availability and provide a benchmark for evaluating North American hydroclimatic change in a warming world through the end of the 21st century.
36
Lund, David Charles. "Millennial-scale surface and deep water oscillations in the N.E. Pacific : implications for late pleistocene climate change." Thesis, 1997. http://hdl.handle.net/1957/28117.
Full text
37
Doughty, Alice Marie. "¹⁰Be cosmogenic exposure ages of late pleistocene moraines near the Maryburn Gap of the Pukani Basin, New Zealand /." 2008. http://www.library.umaine.edu/theses/theses.asp?highlight=1&Cmd=abstract&ID=GEO2008-003.
Full text
38
Sandstrom, Robert Michael. "Geochronology and reconstruction of Quaternary and Neogene sea-level highstands." Thesis, 2021. https://doi.org/10.7916/d8-1xn4-vb62.
Full textAbstract:
Understanding the past sensitivity of ice sheets and sea level rise in a warmer climate is essential to future coastal planning under the threat of climate change, as accurately modeling impending scenarios depends primarily on data from the past. Extreme warm events during the Quaternary and Neogene periods hold much of the information needed to predict future global climate conditions due to anthropogenic and natural forcings, and may provide unique glimpses of how much future sea level rise can be expected on both short- and long-term timescales. Constraining global mean sea level (GMSL) during past warm periods becomes increasingly difficult the further back in time one goes, especially as precise dating of globally distributed paleoshorelines, along with long-term vertical displacement rates, is essential for establishing GMSL and ice volume history. However, placing chronological constraints on shorelines beyond the limit of U-series radiometric dating (~600 kyr), or at high latitude sites lacking coral, has remained elusive. Even relatively recent warm periods, such as the Last Interglacial (~117-129 ka) has proved challenging for reconstructing GMSL, primarily due to uncertainties in long-term vertical deformation rates and timing of when the highstand occurred. The first two chapters of this thesis address the dating of carbonate shorelines older than ~500 kyr through refinement of the strontium isotope stratigraphy dating methodology. I apply these techniques to a well-known location with numerous uplifted fossil shorelines (Cape Range, Western Australia) to provide the first geochemically derived ages on three fossil shorelines spanning the Pleistocene to the Miocene. Accurate dating and mapping at this location allows correction of long-term vertical displacement. In the last chapter, I use these rates of uplift, in conjunction with twenty new 230Th/U-ages on corals from Western Australia, to refine the timing and peak elevation of the Last Interglacial sea level highstand.
Chapter 1 re-evaluates strontium isotope stratigraphy dating techniques for chronologically constraining fossil shorelines from ~0.5 to >30 Ma. Using marine terraces from South Africa, Western Australia, and the Eastern United States as examples, this chapter presents a refined sampling and dating methodology to overcome limitations on diagenetically altered samples, which are ubiquitous in older carbonate shorelines. Discussion on best practices for constraining maximum or minimum ages includes a novel scoring methodology for alteration and a sequential leaching procedure that is specifically suited for shallow-water biogenic carbonate fauna.
In Chapter 2, I apply the revised strontium isotope stratigraphy dating methodology to three previously unknown aged terraces in Cape Range, Western Australia. The results obtained show Late-Miocene, Late-Pliocene and Mid-Pleistocene shorelines, which I then use to reconstruct the vertical uplift history of the anticlinal structure and relative rates of deformation. This study is the first to directly date the three terraces, and provides the deformation history necessary for constraining Last Interglacial sea level at Cape Range. In addition, we are able to place maximum relative sea level constraints on all three of these older shorelines.
Chapter 3 builds upon the previous chapter by focusing on the Last Interglacial sea level history along ~300 km of coastline in Western Australia (Cape Range and Quobba). This chapter presents new U-series ages on multiple coral heads that are among the highest in-situ corals ever dated in Western Australia, with ages spanning from ~125.3 – 122.6 ka. Detailed geomorphic analysis, particularly at Cape Range, constrains the relative sea level highstand to 6.9 ± 0.4 m. When glacial isostatic adjustment models are applied to the age and elevation data, the resulting Eemian GMSL highstand occurred between 125.5-123.0 ka and reached an elevation between 4.9 and 6.7 m. This is later in the Interglacial and lower in elevation than many recent studies suggest.
This dissertation focuses on refining sea level highstands from the Last Interglacial to the Late Miocene in a relatively small (but historically important) region of Western Australia. However, the methodologies presented here provide a powerful multi-proxy dating and mapping approach, which, when applied to regions with multiple marine terraces, can greatly improve the reliability of younger shoreline elevations by reducing neotectonic and dynamic topography uncertainties. The carbonate screening techniques and 87Sr/86Sr stratigraphy dating described here are applicable to a wide range of marine carbonates, with the ability to place accurate chronologic constraints on shorelines from 0.5 to >30 Ma. As I show in chapter 3, when combined with 230Th/U-dating on Late Pleistocene coral in places where multiple marine terraces exist, valuable long-term vertical deformation constraints can allow for far more accurate analysis of sea level in younger paleo shorelines (i.e. Last Interglacial).
39
Yatsko, Andrew. "Late Holocene paleoclimatic stress and prehistoric human occupation on San Clemente Island." 2000. http://catalog.hathitrust.org/api/volumes/oclc/48393634.html.
Full text
40
Barbafiera, Mario. "An examination of palaeo-sea surface temperatures and thier influence on Southern African palaeoclimates over the last 20000 years." Thesis, 2016. http://hdl.handle.net/10539/20579.
Full textAbstract:
A Dissertation Submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, for the Degree of Master of Science
Johannesburg, 1997<br>The palaeoclimate of southern Africa for the last twenty thousand
years is investigated through the production of palaeoclimate maps
based on an analysis of literature and data from the ongoing PASH
project. The trends from the maps are compared with published
reviews and modelling studies of the region' s palaeoclimates, and
are contextualised in terms of global palaeoceanographic and
palaeoclimatic dynamics. The changes evident in the past climate
of southern Africa tend to mirror global climate changes, however,
evidence exists that the timing of climate changes between the two
hemispheres are not quite synchronous, and may 110tfit present
theories of global palaeoclimate dynamics,
41
Rose, Cassaundra Ashley. "Late Cenozoic Evolution of Aridity and C4 Vegetation in North Africa." Thesis, 2015. https://doi.org/10.7916/D8NK3DS6.
Full textAbstract:
Northern Africa has experienced major shifts towards aridity and extensive C4 vegetation over the late Cenozoic, but due to a scarcity of spatially and temporally extensive paleoenvironmental records, the timing, patterns, and causes of these shifts are still under debate. Both long-term aridification and large amplitude orbital-scale climate variability have been recognized, with little understanding of how these two patterns relate to each other over time. African’s climate and environmental history of the last 7 Myr is of particular interest because hydrological and vegetation variability is considered the driving selection mechanism for human evolution. In addition, the age of the initiation of desert conditions in the modern Sahara desert, Earth’s largest warm desert and the largest source of dust to the modern atmosphere, is unknown.
The stable isotope ratios of carbon and hydrogen in sedimentary plant leaf wax biomarker compounds have recently been shown to quantitatively track source vegetation photosynthetic pathways and the hydrogen isotope composition of plant source water, which is dominantly controlled by the amount of precipitation in Africa. These proxies have been applied to reconstruct long-term vegetation changes in East Africa and SW Africa over the last 14 Ma, as well as orbital-scale variability from various locations around the African continent, but they have not been extended further back in time or combined in tandem to robustly assess both long-term and orbital-scale climate and vegetation variability and how they relate to each other.
In this thesis, I have utilized quantitative plant leaf wax stable isotope proxies to examine both orbital-scale and long-term changes in North African aridity and vegetation from a variety of regions over the last 25 Ma, with particular emphasis on the last 4.5 Ma. In Chapter 2, I investigated the evolution of hydrological and vegetation gradients from the equator to the sub-Sahara in NW Africa over the last 25 Myr using leaf wax stable isotopes at two marine sediment core locations, producing the longest existing leaf wax stable isotope record in Africa to my knowledge, and one of the longest such records globally. In this study I found that NW African environments were remarkably similar at both latitudes from 25 – 10 Ma, but at 10 Ma C4 vegetation abruptly expanded in the north, indicating sudden aridification in the Sahara region at that time. The hydrogen isotope record was stable long-term, with variability similar to that of known orbital-scale cyclicity in the Pliocene and Pleistocene, possibly suggesting that orbital-scale cyclicity or other factors obscured or were larger than any long-term changes in the hydrogen isotope ratio of precipitation. Saharan aridification at 10 Ma is consistent with climate model predictions of aridity due to the closure of the Tethys Seaway connection between the Indian Ocean and Mediterranean Sea near that time. The 10 Ma expansion in C4 vegetation is earlier than most other regions globally.
To examine long-term changes in orbital-scale variability in the Eastern Sahara and Mediterranean Sea, I constructed a record of eastern Mediterranean sedimentary leaf wax carbon and hydrogen isotopes, leaf wax abundance, lignin biomarkers, and oxygen isotope ratios of planktonic foraminifera G. ruber during two 100-kyr periods of equal eccentricity near 3.0 and 1.7 Ma (Chapter 3). I found that precession-scale variability dominates the record during both periods, and Eastern Saharan precipitation and the vegetation assemblage, which was C4-dominated, do not change on average between the two periods.
Chapter 4 extended the eastern Mediterranean record of Chapter 3 by sampling leaf wax stable isotopes in sapropel sediments (deposited during North African humid periods) at ~0.25 Myr resolution back to 4.5 Ma, placing the orbital-scale Chapter 3 results in long term context. I found that Eastern Saharan environments were persistently C4-dominated (>68%) throughout the entire interval, and that long-term hydrogen and carbon variability were similar in magnitude to orbital-scale cycles back to 4.5 Ma, strongly indicating that orbital-scale variability has been the dominant environmental control in NE Africa since the early Pliocene. This record contrasts sharply with observations of a transition from C3-C4 mixed vegetation to abundant C4 grasslands in East Africa over the same period of time. The results may suggest that long-term precipitation shifts did not occur in NE Africa since the Pliocene, or that the resolution of this approach is not sufficient to detect long-term shifts. It is likely that NW Africa also experienced similarly large hydrological variability over the same period of time, which may explain the unclear long-term hydrological signal in Chapter 2. The results emphasize that East Africa has not been representative of northern Africa as a whole since the Pliocene.
42
Mabry, James Brice. "The impact of glaciation and climate change on biogeochemical cycling and landscape development." Thesis, 2012. http://hdl.handle.net/1805/2761.
Full textAbstract:
Indiana University-Purdue University Indianapolis (IUPUI)<br>Lake cores from Dry Lake, California and Crystal Lake, Illinois were analyzed to identify climate variability and characterize landscape response to glacial/deglacial climate transitions.
Geochemical analysis of the Dry Lake sediment prior to the 8.2 kyr event revealed average values for percent total organic carbon to be 4% with a range of 0.2% to 15.2%. The average decreased to approximately 2.1% with a range of 0.4% to 5.3% during and after the event. Occluded phosphorus averaged 488 µg/g before the 8.2 kyr event and 547 µg/g after but was much lower during the event at 287 µg/g. These results were interpreted as an environment which began as warm, wet, and productive then quickly turned colder and drier during the 8.2 kyr event which resulted in a resetting of soil development. The higher temperatures returned after the 8.2 kyr event which allowed for continued soil development despite its drier climate. Previous research corroborated these conclusions.
The Crystal Lake geochemical record was very different from Dry Lake. Percent total organic carbon averaged 6.7% with a range of 3.9% to 8.5% during the Younger Dryas but recorded a lower average before and after at 4.9% and 4.6% respectively. Occluded phosphorus acted similarly with a higher average during the cooling event, 2626 µg/g, and lower averages before and after, 1404 µg/g and 1461 µg/g, respectively. This was interpreted as continued productivity and soil development through the cold period which was attributed to a change in biomass.
43
Stamps, Lucas G. "A Laminated Carbonate Record of Late Holocene Precipitation from Martin Lake, LaGrange County, Indiana." Thesis, 2016. http://hdl.handle.net/1805/10030.
Full textAbstract:
Indiana University-Purdue University Indianapolis (IUPUI)<br>Precipitation trends and their driving mechanisms are examined over a variety of spatial and temporal scales using a multi-proxy, decadally-resolved sediment record from Martin Lake that spans the last 2300 years. This unique archive from a northern Indiana kettle lake documents significant climate variability during the last 2 millennia and shows that the Midwest has experienced a wide range of precipitation regimes in the late Holocene. Three independent proxies (i.e., oxygen and carbon isotopes of authigenic carbonate and %lithics) record variations in synoptic, in-lake and watershed processes related to hydroclimate forcing, respectively. Together, these proxies reveal enhanced summer conditions, with a long period of water column stratification and enhanced summer rainfall from 450 to 1200 CE, a period of time that includes the so-called Medieval Climate Anomaly (950-1300 CE). During the Little Ice Age, from 1260 to 1800 CE, the three proxy records all indicate drought, with decreased summer rainfall and storm events along with decreased lake stratification. The Martin Lake multi-proxy record tracks other Midwest climate records that record water table levels and is out-of-phase with hydroclimate records of warm season precipitation from the High Plains and western United States. This reveals a potential warm season precipitation dipole between the Midwest and western United States that accounts for the spatial pattern of late Holocene drought variability (i.e., when the Midwest is dry, the High Plains and the western United States are wet, and vice versa). The spatiotemporal patterns of late Holocene North American droughts are consistent with hydroclimate anomalies associated with mean state changes in the Pacific North American teleconnection (PNA). Close associations between late Holocene North American hydroclimate and records of Northern Hemisphere temperatures and the Pacific Ocean-atmosphere system suggests a mechanistic linkage between these components of the global climate system that is in line with observational data and climate models. Based on our results, predominantly –PNA conditions and enhanced Midwestern summer precipitation events are likely to result from continued warming of the climate system. In the western United States, current drought conditions could represent the new mean hydroclimate state.