Constraints of organic carbon burial on sedimentary mercury in the Bohai Sea over the past century

In the global warming scheme, organic carbon burial and its environmental effects on coastal shelves are important in the marine carbon cycle, and a window for understanding the impact of modern human activities on coastal ecosystems. A 53-cm long core was acquired by box-coring in 2013 from the central mud deposition area of the Bohai Sea, North China. The high-resolution sedimentary records of total organic carbon (TOC), stable carbon isotopes (δ¹³C), biomarkers, and sedimentary mercury over the past century were comprehensively analyzed, and the evolutionary characteristics of organic carbon from various sources and constraining effects on mercury burial were clarified. By correcting the δ¹³C of organic carbon for the Suess effect and applying a two-endmember mixing model, we estimated that marine organic carbon contributes approximately 59% to 90%, with a marked increase since the 1970. The trends of marine organic carbon align closely with those of brassicasterol and dinosterol, indicating a significant increase in marine primary productivity since the 1970, due likely to increased nutrient input and climate warming. The burial records of sedimentary mercury are generally consistent with the trends of the aforementioned productivity parameters and exhibit a significant positive correlation with marine organic carbon and other indicators since the 1970. This suggests that the increase in phytoplankton productivity in recent decades may have played a crucial role in the accumulation of sedimentary mercury. Therefore, biological processes may have a significant impact on the sequestration and fate of mercury in coastal areas. A further comparison of the relative changes in recent anthropogenic atmospheric mercury emissions and sedimentary mercury burial rates revealed a significant decline in the burial efficiency relative to emissions after the 2000s. This decline may be related to concurrent changes in nutrient levels and phytoplankton community structure, such as a notable increase in dinoflagellates, which potentially reduced the efficiency of biological mercury scavenging.