Mercury in dated sediment cores from coastal ponds of St Thomas, USVI

We collected seven cores from five coastal ponds in St Thomas, USVI, dated them via ²¹⁰Pb, and measured their Hg profiles. Levels ranged from 20 to 100ngg^-1 and always increased upward to the surface or a shallow mixed zone. Taking into account differences in sediment accumulation rates and the presence or absence of a mixed zone, all sites revealed similar deposition histories. Mercury levels were at low, possibly pristine, values until the early to mid-20^th century. Then they then doubled or quadrupled steadily until about 1990, after which Hg became relatively constant. We conclude that Hg in the sediments of these ponds came from long range deposition directly to the ponds and to their watersheds. Constant levels in recent years may be caused by transfer from watersheds, a pathway that is delayed compared to direct deposition from the atmosphere.