Transport and evolution of terrestrial organic carbon signals along the Danube River basin
Embargoed until 2019-12-15
Freymond, Chantal V.
- Doctoral Thesis
Rivers are the main land-to-ocean transport mechanism for transfer of particulate organic carbon (OC) mobilized on the continents to the ocean. Steady deposition and burial of terrigenous OC on continental margins act as a net sink for carbon, including that additionally mobilized by human intervention in the global carbon cycle. At the same time, valuable and continuous archives of environmental conditions on the continents are built up through the accumulation of sediment in front of river systems. However, the evolution of OC as it moves through river basins from biological source to the depositional sink, and the corresponding timescales of this carbon transfer are not well constrained. In this thesis, these questions are addressed with a detailed basin-wide investigation of the Danube River system, the second largest river basin in Europe. River sediment was collected along the Danube mainstem from headwaters to the Black Sea and at 12 tributaries close to their confluences with the Danube. The fine fraction (<63 μm) of this sediment was analyzed for sedimentological parameters (mineralogy, grain size, mineral-specific surface area), as well as the abundances, distributions and isotopic compositions (13C, 14C) of bulk OC and specific biomarker compounds. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) were evaluated as indicators of soil- derived OC, whereas long-chain leaf wax fatty acids (FAs) and lignin phenols served as markers for higher plant-derived OC. Total OC, FA and brGDGT concentrations normalized to mineral-specific surface area show a systematic decrease of 70–80% along the course of the Danube, suggesting significant reduction in OC loading due to either degradation or dilution with a high-surface-area but low-OC source such as loess, which occurs extensively in the middle and lower Danube basin. The evolving characteristics of brGDGT distributions as well as plant wax FA stable isotopic signatures suggest significant inputs of locally-derived OC to the sediment load along the course of the Danube river mainstem. A mixing model including bulk OC δ13C and 14C further reveals that soil is the main contributor of OC in fluvial sediments throughout the basin, however, the contribution from loess increases from the middle to the lower basin, accounting for up to 37% of OC close to the river terminus at the Black Sea. The integrated transport or residence time of total OC and higher plant derived OC in the Danube is determined to be 2,130 and 2,790 years, respectively. Assuming these ages correspond to average residence times, model calculations that take into account accompanying changes in OC loading imply that complete degradation would occur with transport time >2,700 years. These calculations suggest that the majority of the upstream OC signal would be degraded by the time it reaches the terminus of the river. It is concluded that the combined effect of loss due to degradation and loess OC addition leads to the observed decrease in OC and biomarker loadings as well as to the gradual aging trend along the river. Additionally to the longitudinal investigation of river sediment along the Danube, an in-depth study of the bulk and molecular composition of suspended particulate matter in depth profiles across a river cross- section close to the outflow of the Danube was performed. Instantaneous OC and biomarker fluxes to the Black Sea were determined with this ‘picket-fence’ approach. Comparing brGDGT and FA-derived proxies and bulk and compound-specific stable isotope values from suspended matter in the cross-section to sediment deposited on riverbanks reveals that the fine fraction of freshly deposited sediment on riverbanks may serve as a proxy for the suspended particulate matter in a well mixed river such as the Danube. The finding that about 8 kg particulate OC per second are discharged by the Danube underlines the importance of global riverine OC export as a transport mechanism for terrestrial OC to the ocean. Further, this thesis shows that the OC load in a river evolves during transport, and highlights the importance of provenance studies and consideration of continental residence times for robust interpretation of paleo- environmental signals preserved in sedimentary records. Show more
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ContributorsExaminer: Eglinton, Timothy I.
Examiner: Peterse, Francien
Examiner: Lupker, Maarten
Examiner: Bianchi, Thomas S.
SubjectOrganic carbon; Biomarkers; Danube River Basin; CARBON CYCLE (GEOCHEMISTRY); Radiocarbon C-14
Organisational unit03868 - Eglinton, Timothy I. / Eglinton, Timothy I.
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Is identical to: http://hdl.handle.net/20.500.11850/129791
Is identical to: http://hdl.handle.net/20.500.11850/228339
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