Impacts of anthropogenic forcing on source variability of sedimentary organic matter in the Yellow River estuary over the past 60 years

To evaluate changes in the source of sedimentary organic matter (OM) in the Yellow River estuary, a sediment core collected in eastern Laizhou Bay was analyzed for total organic carbon (TOC), stable carbon isotopes of TOC, and biomarkers. The results showed a decreasing trend in terrestrial OM (TOM), but an increasing trend in marine OM (MOM) over the past 60 years. TOM was subdivided into soil OM and plant OM in a three end-member mixing model. The soil OM gradually decreased over the past 60 years, with a significant decline since the 1980s, while the plant OM gradually increased. This reveals that the reduction in TOM was caused mainly by the decreased input of soil OM. The reduced TOM contribution can be attributed primarily to dam construction and a decline in precipitation, whereas the elevated MOM contribution was caused by enhanced marine productivity driven by a rise in nutrient inputs.