Two Centuries of Coral Skeletons from the Northern South China Sea Record Mercury Emissions from Modern Chinese Wars

The contemporary mercury (Hg) cycle in the world's oceans has been greatly affected by human activities. However, we are still lacking reliable, long-term, and continuous records of Hg in seawater. Here, we report for the first time on using annually banded Porites coral skeletons from the northern South China Sea (SCS) as an archive for recording changes of seawater dissolved Hg spanning the past two centuries. We developed a combustion-trapping method to preconcentrate ultratrace Hg concentrations from coral aragonitic skeletons for highly accurate total Hg measurements. Results show that Hg in the coral skeletons ranges from 0.3 to 5.1 pmol/g and is discriminated against Ca during coral skeletal calcification. Preindustrial (1798-1832) Hg levels in coral skeletons were found to be approximately 0.5 pmol/g. The highest Hg concentrations (3-5 pmol/g) were observed during the WWII period (1933-1942). Other distinct Hg maxima (similar to 3 pmol/g) are observed for the periods 1833-1847, 1858-1862, 1918-1927, 1978-1982, and 1988-1992, with the first four coinciding with contemporary Chinese wars. Our study suggests that the production and use of ammunitions in those wars likely account for the primary Hg emission sources in the northern SCS before 1950, and coral is potentially a robust indicator of historical, regional Hg contamination events.