Littérature scientifique sur le sujet « Terrigenous dissolved carbon »

Shelf seas provide valuable ecosystem services, but their productivity and ecological functioning depend critically on sunlight transmitted through the water column. Anthropogenic reductions in underwater light availability are thus a serious threat to coastal habitats. The flux of light-absorbing coloured dissolved organic matter (CDOM) from land to sea may have increased world-wide, but how this has altered the availability and spectral quality of light in shelf seas remains poorly known. We present time-series data from the Sunda Shelf in Southeast Asia, where the monsoon-driven reversal in ocean currents supplies water enriched in CDOM from tropical peatlands for part of the year, resulting in 5- to 10-fold seasonal variation in light absorption by CDOM. We show that this terrigenous CDOM can dominate underwater light absorption at wavelengths up to 500 nm, and shift the underwater irradiance spectrum towards longer wavelengths. The seasonal presence of terrigenous CDOM also reduces the 10% light penetration depth by 1-5 m, or 10-45%. We estimate that on average 0.6 m, or 25%, of this terrigenous CDOM-mediated shoaling might be attributable to the enhanced input of dissolved organic matter following peatland disturbance. The seasonal change in the light environment is correlated with changes in phytoplankton absorption spectra that suggest a photo-acclimation response, and we infer that terrigenous CDOM likely contributes to limiting the depth distribution of photosynthetic corals. The results reveal an ecologically important but largely overlooked impact of human modifications to carbon fluxes that is likely increasingly important in coastal seas.

The concentrations of dissolved organic carbon (DOC) and its light-absorbing fraction (chromophoric dissolved organic matter; CDOM) in surface waters, particularly those draining organic-rich peatlands, have dramatically increased over the past decade due to climate change and human disturbance. To explore the spatiotemporal dynamics of DOC and CDOM in surface waters of the northeastern Qinghai-Tibetan Plateau, we collected water samples from two rivers in the Zoige alpine wetland and from two rivers in its adjacent alpine-gorge region, during wet and dry seasons. DOC concentration ranged from 4.82 mg·L−1 to 47.83 mg·L−1, with a mean value of 15.04 mg·L−1, 2.84 times higher than the global average. The Zoige rivers had higher DOC concentration and highly terrigenous CDOM. Significantly higher DOC concentration was observed for the Zoige rivers in the wet season compared to the dry season. In contrast, the alpine-gorge rivers had higher DOC levels in the dry season. No significant correlations were observed between DOC and CDOM at all rivers due to the influence of autochthonous sources on the alpine-gorge rivers and intensive photochemical degradation of terrigenous DOM in the Zoige rivers. Significant relationships between CDOM and specific ultraviolet absorbance at 254 nm (SUVA254) and between CDOM/DOC and SUVA254 were observed, indicating that the aromaticity of DOM in the rivers was mainly determined by CDOM. Moreover, the DOC/CDOM properties of the Hei River indicate critical human-induced water quality degradation. High DOC level and high browning degree were found in rivers in the Zoige alpine wetland, indicating that large amounts of terrigenous DOC were released to the aquatic systems of the region.

Abstract. South-East Asia is home to one of the world's largest stores of tropical peatland and accounts for roughly 10 % of the global land-to-sea dissolved organic carbon (DOC) flux. We present the first ever seasonally resolved measurements of DOC concentration and chromophoric dissolved organic matter (CDOM) spectra for six peatland-draining rivers and coastal waters in Sarawak, north-western Borneo. The rivers differed substantially in DOC concentration, ranging from 120–250 µmol L−1 (Rajang River) to 3100–4400 µmol L−1 (Maludam River). All rivers carried high CDOM concentrations, with a350 in the four blackwater rivers between 70 and 210 m−1 and 4 and 12 m−1 in the other two rivers. DOC and CDOM showed conservative mixing with seawater except in the largest river (the Rajang), where DOC concentrations in the estuary were elevated, most likely due to inputs from the extensive peatlands within the Rajang Delta. Seasonal variation was moderate and inconsistent between rivers. However, during the rainier north-east monsoon, all marine stations in the western part of our study area had higher DOC concentrations and lower CDOM spectral slopes, indicating a greater proportion of terrigenous DOM in coastal waters. Photodegradation experiments revealed that riverine DOC and CDOM in Sarawak are photolabile: up to 25 % of riverine DOC was lost within 5 days of exposure to natural sunlight, and the spectral slopes of photo-bleached CDOM resembled those of our marine samples. We conclude that coastal waters of Sarawak receive large inputs of terrigenous DOC that is only minimally altered during estuarine transport and that any biogeochemical processing must therefore occur mostly at sea. It is likely that photodegradation plays an important role in the degradation of terrigenous DOC in these waters.

The present work is devoted to geochemical studies of the Bazhenov Formation in the north of the West Siberian Petroleum Basin. The object is the Upper Jurassic–Lower Cretaceous section, characterized by significant variations in total organic carbon content and petroleum generation potential of organic matter at the beginning of the oil window. The manuscript presents the integration of isotopic and geochemical analyses aimed at the evaluation of the genesis of the rocks in the peripheral part of the Bazhenov Sea and reconstruction of paleoenvironments that controlled the accumulation of organic matter in sediments, its composition and diagenetic alterations. According to the obtained data, the sediments were accumulated under marine conditions with a generally moderate and periodically increasing terrigenous influx. The variations in organic matter composition are determined by redox conditions and terrigenous input which correlate with the eustatic sea level changes during transgressive/regressive cycles and activation of currents. Transgression is associated with an intensive accumulation of organic matter under anoxic to euxinic conditions and insignificant influence of terrigenous sources, resulting in the formation of rocks with oil-generating properties. During the regression periods, the terrigenous sedimentation increased along with the dissolved oxygen concentration, and deposits with low organic matter content and gas-generating properties were formed.

Cinq des plus grands fleuves mondiaux sont en Arctique et transportent des quantités importantes de carbone dissous organique (COD) et inorganique (CID) dans l’Océan Arctique (OA). La réponse de l’océan côtier à ces apports est encore incertaine, ce qui est un frein à l’estimation des flux air/mer de CO2 dans cette région. Dans un contexte de réchauffement climatique et de changement rapide de l’environnement arctique, il est donc important de mieux comprendre l’effet de ces apports de carbone terrigène sur les flux de CO2 dans les panaches fluviaux. Le modèle couplé océan/glace/biogéochimie ECCO-Darwin est utilisé afin d’étudier la réponse du sud-est de la mer de Beaufort aux apports de carbone dissout du fleuve Mackenzie des échelles synoptiques à interannuelles. Ce modèle régional intègre le tout premier forçage interannuel journalier de COD terrigène provenant du Mackenzie estimé grâce à la fusion de données in situ et de données satellites acquis aux trois embouchures principales du delta. Nous observons que la variabilité interannuelle du débit du Mackenzie module localement les flux air/mer de CO2 dans le panache fluvial côtier. Le CID terrigène contribue deux fois plus que le COD terrigène au dégazage du panache. Avec le dégel du pergélisol, les incertitudes sur la dégradation du COD terrigène dans les panaches fluviaux sont nombreuses. La variabilité des flux air/mer de CO2 liée à la dégradation bactérienne est estimée à ±0.39 TgC yr−1 en 2009. D’autres processus biophysiques contribuent également à cette variabilité comme la floculation du COD terrigène (+0.14 TgC yr−1 absorbé par l’océan) et la stratification verticale induite par le panache (+0.35 TgC yr−1 rejeté par l’océan). Ce travail de thèse met en lumière l’importance d’inclure une représentation réaliste du continuum terre/mer dans les modèles régionaux arctiques afin d’améliorer les estimés de flux de carbone dans cet océan changeant et fortement altéré par les modifications de ses bassins versants
About 10 % of atmospheric carbon dioxide is sequestered in the ocean above 60°N, half of which is in coastal seas where 10 % of the global riverine freshwater volume flows in. Five of the world’s largest rivers convey in the Arctic Ocean (AO) huge quantities of dissolved carbon in the organic (DOC) and inorganic (DIC) form. The response of the coastal ocean to this supply is still highly uncertain, which makes the assessment of air-sea CO2fluxes challenging in this remote region. It is thus timely to gain a better understanding of the impact of terrestrial carbon released by watersheds on air-sea CO2 fluxes in Arctic rivers plumes, especially in a context of global warming. In the present PhD thesis, the ECCO-Darwin ocean-sea ice-biogeochemical model is used to investigate the synoptic to interannual response of the South eastern Beaufort Sea (Western AO) to the Mackenzie River’s carbon exports. The model includes the very first daily terrestrial DOC (tDOC) runoff forcing estimated through merging riverine in situ measurements and coastal remotely sensed data at three major delta outlets, over the last two decades (2000-2019). We find that interannual variability in river discharge modulates localized air-sea CO2flux in the coastal plume with riverine DIC contributing twice as much as riverine DOC to CO2 outgassing. As current knowledge on tDOC remineralization in Arctic plume regions is still uncertain, the range of air-sea CO2 flux variability due to microbial remineralization is estimated to ±0.39 TgC yr−1 in 2009. Other biophysical processes also contribute to the high CO2 flux variability, such as tDOC flocculation (+0.14 TgC yr−1 in gassing) and enhanced plume stratification (+0.35 TgC yr−1 outgassing). To conclude, the work presented here intends to pave the way toward a better representation of the land-to-ocean continuum (LOAC) in regional Arctic models with the aim to improve the simulated carbon cycle in rapidly changing Arctic watersheds and coastal seas