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Characterization of Natural and Affected Environments
Climate, fire, and vegetation mediate mercury delivery to mid-latitude lakes over the Holocene David P. Pompeani, Colin A. Cooke, Mark B. Abbott, and Paul E. Drevnick Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01523 • Publication Date (Web): 27 Jun 2018 Downloaded from http://pubs.acs.org on June 28, 2018
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Environmental Science & Technology
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Climate, fire, and vegetation mediate mercury delivery to mid-latitude lakes over the Holocene David P. Pompeani*1,2, Colin A. Cooke3,4, Mark B. Abbott2, Paul E. Drevnick5 1
Department of Geography, Kansas State University, Manhattan, Kansas 66506, United States 2 Department of Geology & Environmental Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States 3 Environmental Monitoring and Science Division, Alberta Environment and Parks, Government of Alberta, Edmonton, Alberta T5J 5C6, Canada 4 Department of Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada 5 Environmental Monitoring and Science Division, Alberta Environment and Parks, Government of Alberta, Calgary, Alberta T2E 7J2, Canada
*Corresponding author:
[email protected] TOC Art
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Environmental Science & Technology
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Abstract 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 timescales. Here we present a 9,000-year mercury record from sediment in Copper Falls; a small closed-basin lake on the Keweenaw Peninsula. Prior to abrupt increases in the 19th and 20th centuries, mercury remains at relatively low concentrations for the last 9,000 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 yr1 ) 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.
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Environmental Science & Technology
Introduction Mercury (Hg) contamination and its subsequent methylation and bioaccumulation in aquatic ecosystems have resulted in elevated levels found in fish. Lake sediments contaminated with mercury represent a continuing source of toxicity in aquatic environments that impact wildlife and humans through food or water consumption. The rise in mercury in lake sediments deposited during the 20th century is widely recognized to have resulted from anthropogenic emissions1, 2. Research aimed at understanding the consequences of aquatic mercury contamination in lakes has focused mainly on recent mercury contamination patterns and bioaccumulation in food webs3, 4. However, few studies in North America have investigated factors that have influenced atmospheric mercury deposition to lakes prior to Anthropogenic increases. By analyzing mercury concentrations in a sediment core over the Holocene, fluxes rates can be inferred and compared with other proxy records to provide insight into mechanisms that modulate mercury delivery to lakes. Cannon, et al. 5 documented 11,000 years of mercury deposition in sediments at Elk Lake in Minnesota (47.190°, -95.218°). The highest concentration of mercury occurred briefly circa 7,900 years before present (yr BP). Paleoclimate proxy evidence suggests a period of warming and drying leading to the replacement of the forests around the lake with the northward expansion of prairie vegetation during this time6. Cannon et al 5 speculate that mercury sequestered in soil organic matter on the forest floor was released due to soil alteration by climatic and vegetational changes occurring across the region. The mercury appears to have been transported and deposited from the soil into Elk Lake as wind-blown dust 5. The current geographic distribution of the mercury-laden dust and its impact on lakes in the region is poorly constrained because of the lack of proxy evidence. Forest fires can increase mercury concentration in lakes7 and their sediments8. Therefore, changes in the occurrence of forest fires over time could impact long-term trends in sediment mercury concentrations. Forest fires liberate mercury sequestered in soils and vegetation9, which can then be transported into lake sediment via groundwater or as airborne fallout. Analyzing >125 µm-sized charcoal particulate concentrations in lake sediment provides useful means to reconstruct local fire events, because charcoal particulates are transported a short distance (
