Mercury Burial in Modern Sedimentary Systems of the East China Marginal Seas: The Role of Coastal Oceans in Global Mercury Cycling

Coastal oceans, the transition zones between terrestrial and oceanic systems, are susceptible to anthropogenic mercury (Hg) inputs and are regarded as critical dynamic interfaces of the global Hg cycle. However, the extent to which coastal oceans are accountable for sequestering Hg remains largely unknown owing to the lack of data on high-resolution Hg accumulation in marine sediments. Synthesizing the results of this study (eight cores and 212 surface sediments) and the literature (three cores and 149 surface sediments), we provide a quantitative evaluation of the biogeochemical cycle of sedimentary Hg in the East China Marginal Seas (ECMS), including the response of the coastal marine sediments to anthropogenic disturbance as well as both human-derived and natural Hg burial fluxes. We find a linear increase in Hg accumulation since the 1950s (2.0 +/- 2.5% yr(-1)) and a decline in Hg accumulation between 2010 and 2016. Modern burial fluxes of total and anthropogenic Hg in the ECMS (covering similar to 4.8 x 10(5) km(2) of sea surface) were estimated to be 89.1 +/- 48.3 and 35.9 +/- 33.1 Mg yr(-1), respectively. Using a compilation of 688 surface sediments and 131 sediment cores (819 samples in total) distributed globally in coastal oceans, we estimate that approximately 1,590 (range: 1,190-2,760) Mg yr(-1) (Method 1) and 540 (range: 310-960) Mg yr(-1) (Method 2) Hg are accumulated in coastal ocean regions. Our findings suggest that coastal oceans are likely the largest global marine sinks for Hg and play a dominant role in regulating the oceanic Hg cycle and budgets. Plain Language Summary The mercury (Hg) burial in the Eastern China Marginal Seas (ECMS) is a dynamic component of the global Hg cycle because it receives high delivery rates of Hg fluxes from the Chinese rivers, atmospheric deposition, and via other pathways, yet our understanding of accumulation and quantity of anthropogenic Hg stored in the ECMS remains poorly constrained. Modern Hg accumulation rate in the ECMS based on surface sediments and high-resolution cores from muddy zones reveals that similar to 40% of the contemporary Hg buried and stored in the ECMS was of anthropogenic origin. The temporal variation in sedimentary Hg accumulation over the past 120 years could be explained by changes in Chinese energy consumption, atmospheric Hg emission, and sediment fluxes associated with river damming. We directly quantified the contribution of coastal oceans to the burial of marine Hg in the ocean using two distinct approaches, including assessing Hg accumulation rates from sediment core data, as well as determining Hg content in marine sediments and sediment delivery rates. We conclude that due to high rates of terrestrial sediment delivery and increased levels of anthropogenic Hg, the coastal oceans exhibit high Hg accumulation rates on average and provide a large sink for global anthropogenic Hg burial.