The role of climate: 71 ka of atmospheric mercury deposition in the southern hemisphere recorded by Rano Aroi Mire, Easter Island (Chile)

Abstract

The study of mercury accumulation in peat cores provides an excellent opportunity to improve the knowledge on mercury cycling and depositional processes at remote locations far from pollution sources. We analyzed mercury concentrations in 150 peat samples from two cores from Rano Aroi (Easter Island, 27° S) and in selected vegetation samples of present-day flora of the island, in order to characterize the mercury cycling for the last ~71 ka BP. The mercury concentrations showed values ranging between 35 and 200 ng g−1, except for a large maxima (~1000 ng g−1) which occurred at the end of the Last Glacial Maximum (LGM, ~20 ka cal BP) in both peat cores. Low temperatures during the LGM would accelerate the atmospheric oxidation of Hg(0) to divalent mercury that, coupled with higher rainfall during this period, most likely resulted in a very efficient surface deposition of atmospheric mercury. Two exceptional short-lived Hg peaks occurred during the Holocene at 8.5 (350 ng g−1) and 4.7 (1000 ng g−1) ka cal BP. These values are higher than those recorded in most peat records belonging to the industrial period, highlighting that natural factors played a significant role in Hg accumulation—sometimes even more so than anthropogenic sources. Our results suggest that wet deposition, linked to atmospheric oxidation, was the main process controlling the short-lived Hg events, both in the mire and in the catchment soils