Determining anthropogenic and natural controls on the global mercury cycle using lake sediment and ice cores

While anthropogenic emissions of the toxic element mercury (Hg) to the atmosphere are currently rising, major uncertainties remain in determining the relative impacts of anthropogenic activity and natural processes on the global Hg cycle. In this dissertation, I develop and interpret records of atmospheric Hg deposition from lake sediments and glacial ice in order to determine the anthropogenic and natural controls on atmospheric Hg deposition during the Holocene. These records consist of four ∼400-year lake sediment records from southeastern Peru, a ∼12,300-year lake sediment record also from southeastern Peru, and a 600-year glacial ice record from Mt. Logan, Yukon Territory, Canada. Moving forward in time within these records, atmospheric Hg deposition was remarkably constant during the pre-anthropogenic period (12.3 to 3.5 ka) and did not respond to local changes in temperature and precipitation. Volcanic signals are absent from all of the records in this dissertation despite large volcanic events that occurred regionally proximal to some of the study sites. The influence of anthropogenic activity on atmospheric Hg deposition in the lake Yanacocha in southeastern Peru began in ∼1200 B.C., likely due to the local transport of pre-Incan mining dust that continued for nearly a millennium. Hg emissions derived from Incan and colonial mining from A.D. 1400 to 1820 were likely not transported globally, evidenced by background-level Hg fluxes registered during this time in the lake sediment cores from southeastern Peru and in the Mt. Logan ice core. All of the records register a secular rise in atmospheric Hg deposition beginning in ∼A.D. 1850 and lasting until the most recent archive material (A.D. 1998 to 2011). The only record to exhibit a peak in Hg fluxes during the "Gold Rush" between A.D. 1860 and 1920 is the Mt. Logan ice core. More recent emissions from artisanal and small-scale gold mining are not apparent in the Hg records from lakes in southeastern Peru, located ∼100 km down-wind of the largest such mining center in Peru. An overall three-fold increase of Hg fluxes since pre-anthropogenic time supports previous Hg records and counters emissions estimates that are currently used in global Hg models.