Sedimentary lycopane/n‐ C 31 ratio as a proxy for bottom water hypoxia in coastal ocean

Abstract Bottom‐water hypoxia on continental shelves is intensifying under climate change and eutrophication, yet long‐term records remain scarce. Here we show that the sedimentary lycopane/n‐C 31 ratio reliably records bottom‐water oxygenation in the seasonally hypoxic Yangtze River Estuary. In surface sediments, lycopane/n‐C 31 correlates negatively with dissolved oxygen, reflecting preferential preservation of lycopane under low‐oxygen conditions. Despite multiple potential sources, sedimentary lycopane is dominated by bottom particulate deposition, making the ratio a robust redox proxy in dynamic coastal margins. Sediment core records reveal a progressive intensification of hypoxia since 2005, consistent with trace‐metal enrichment factors (Mo EF /U EF ) and complementary biomarkers. This deoxygenation coincides with warming, reduced sediment supply, and enhanced export production. Our results suggest that lycopane/n‐C 31 is a sensitive indicator of bottom‐water oxygen variability in the Yangtze River Estuary, with potential for reconstructing modern decadal‐scale hypoxia trends in seasonally hypoxic coastal systems.