Multiple sources and extensive degradation of terrestrial sedimentary organic matter across an energetic, wide continental shelf

Abstract

The fate of organic carbon in marine sediments is closely linked to atmospheric CO2 concentrations and thus to climate variability over geological time scales. The East China Sea (ECS) is characterized not only by massive terrestrial organic matter (TOM) inputs from the Yangtze (Changjiang) River but also by a shallow ( 500 km) continental shelf with widespread relict sands, such that it is distinct from other continental margins. We examine the diagenesis of sedimentary TOM and the impacts of relict sands on TOM in the ECS. The results reveal that bulk δ13Corg values correlate with terrestrial biomarker concentrations in muddy and accumulative areas; in contrast, depleted δ13Corg values around −24.5‰ co-occur with almost absent terrestrial biomarkers in sandy/erosional areas. We suggest that mixing of contemporary TOM and marine OM dominates in the muddy/accumulative shelf areas, whereas a putative relict OM, associated with relict sands, appears to be significant in many sandy/erosional shelf areas. Given the global occurrence of relict sands, a persistent amount of relict OC (e.g., 0.1%) may complicate TOM budget calculations. In addition, our observations reveal that TOM is extensively partitioned and degraded in the estuary and continues to be partitioned and degraded during the along-shore and across-shelf transport, which is reflected by decreases in terrestrial biomarker concentrations and increases in degradation indices. This study highlights the unique and dynamic role of shallow and wide continental shelves with massive relict sands on TOM cycling.