Climate, Fire, and Vegetation Mediate Mercury Delivery to Midlatitude Lakes over the Holocene

The rise in mercury concentrations in lake sediment deposited over the last ∼150 years is widely recognized to have resulted from human activity. However, few studies in the Great Lakes region have used lake sediment to reconstruct atmospheric mercury deposition on millennial time scales. Here we present a 9000-year mercury record from sediment in Copper Falls; a small closed-basin lake on the Keweenaw Peninsula. Prior to abrupt increases in the 19 th and 20 th centuries, mercury remains at relatively low concentrations for the last 9000 years. Higher mercury fluxes in the early Holocene (3.4 ± 1.1 μg m –2 yr –1 ) are attributed to drier conditions and greater forest fire occurrence. The gradual decline in mercury flux over the middle to late Holocene (1.9 ± 0.2 μg m –2 yr –1 ) is interpreted to reflect a transition to wetter conditions, which reduced forest fires, and promoted the development of soil organic matter and deciduous forests that sequestered natural sources of mercury. The Copper Falls Lake record highlights the sensitivity of watersheds to changes in mercury inputs from both human and natural forcings, and provides millennial-scale context for recent mercury contamination that will aid in establishing baseline values for restoration efforts.