Basin-wide investigation of terrestrial biomarker source to sink transport along a major modern fluvial system

Abstract:

Organic carbon discharged by rivers and buried in continental margin sediments represents an important carbon sink and a valuable record of information on past environmental variations on the continents. In this context, it is crucial to understand the sources of carbon in river basins and the factors that influence biomarker signals during transport from the continental source to the oceans. In this study, we adopt a source-to-sink approach where concentration and compositional variations in branched glycerol dialkyl glycerol tetraethers (brGDGTs), plant wax fatty acids and lignin phenols in fine-grained (<63 µm) riverbank sediment deposits from the Danube River are investigated. Using this multi-proxy approach, we seek to establish geochemical “fingerprints” of the largest tributaries and follow the evolution of these signatures along the course of the river, from headwater tributaries to its delta. Spatial trends in biomarker concentrations along the river transect are assessed by normalizing them on the mineral-specific surface area of the sediment. Surface area normalized brGDGT concentrations show a clear trend to decreasing values (ng brGDGT m^-2) from the upper to the lower catchment. The distributions of brGDGTs, a group of soil bacterial membrane lipids that has been shown to record local environmental parameters, reflect the trend of increasing air temperature along the course of the Danube. This trend suggests an increasing contribution of soil organic carbon from tributaries with a more continental and warmer climate in the lower Danube basin to the OC that is finally delivered to the delta. More detailed insights into the evolution of the organic carbon composition within the Danube river basin will stem from compound-specific δD, δ¹³C and ultimately ¹⁴C measurements on terrestrial biomarkers in riverbank and suspended sediments.