Chronology of the atmospheric mercury in Lagoa da Pata basin, upper Rio Negro region of Brazilian Amazon.

Abstract

We present prehistoric mercury accumulation rates in a dated sediment core from Lagoa da Pata, a remote lake in Sao Gabriel da Cachoeira, northern Amazon. The sediment samples were subdivided for mercury and radiocarbon analyses. A group of 18 samples have been prepared at ANU for (super 14) C dating by accelerator mass spectrometry (AMS). The dating results show a good correlation with depth in the core, down to 41,500 BP. Three distinct sections are clearly identified in the core. They consist of upper and lower organic-rich layer, separated by an inorganic layer which represents a short period of rapid accumulation around 18 ka BP. The mercury accumulation rate is found to be larger in the upper layer (18 ka to present) than in the lower one (41 ka to 25 ka), by a factor of three. The larger accumulation rate of mercury is probably associated with warmer temperatures and a higher frequency of forest fires during the Holocene.