Mercury levels in the South China Sea

Mercury is a potent neurotoxin. It enters the atmosphere when coal is burned and precious metals are mined. Carried by winds, Hg dissolves into seawater, where it emerges, much concentrated and to much alarm, in the flesh of large edible fish. But Hg also evaporates from seawater. To determine the net flux of marine Hg, Chun-Mao Tseng of National Taiwan University in Taipei and Carl Lamborg of the Woods Hole Oceanographic Institution in Massachusetts carried out an ambitious experiment. From 2003 to 2007, they and their colleagues made seasonal cruises across the South China Sea (SCS), taking measurements as they went. For such a study, the SCS is ideal. Not only do coal-fired plants abound in southern China, but the prevailing weather patterns are strongly seasonal. Indeed, when Tseng and Lamborg analyzed their data, they found pronounced seasonal behavior. In winter, when sea surface temperatures are low and monsoon winds are high, the SCS acts as a net sink of atmospheric Hg. In summer, when the meteorological conditions reverse, the SCS acts as a net source. Annually, the source dominates the sink: Even though the SCS makes up 1% of the world’s sea surface, it accounts for 2.6% of global Hg emission. Regardless of net flux, Tseng and Lamborg found that the concentration of dissolved Hg in the SCS was at least nine times higher than in the open ocean. (C.-M. Tseng, C. H. Lamborg, S. C. Hsu, Geophys. Res. Lett. 40, 167, 2013.)