David P. Pompeani

Lake Sediments Record Prehistoric Lead Pollution Related to Early Copper Production in North America

The mining and use of copper by prehistoric people on Michigans Keweenaw Peninsula is one of the oldest examples of metalworking. We analyzed the concentration of lead, titanium, magnesium, iron, and organic matter in sediment cores recovered from three lakes located near mine pits to investigate the timing, location, and magnitude of ancient copper mining pollution. Lead concentrations were normalized to lithogenic metals and organic matter to account for processes that can influence natural (or background) lead delivery. Nearly simultaneous lead enrichments occurred at Lake Manganese and Copper Falls Lake ∼8000 and 7000 years before present (yr BP), indicating that copper extraction occurred concurrently in at least two locations on the peninsula. The poor temporal coherence among the lead enrichments from ∼6300 to 5000 yr BP at each lake suggests that the focus of copper mining and annealing shifted through time. In sediment younger than ∼5000 yr BP, lead concentrations remain at background levels at all three lakes, excluding historic lead increases starting ∼150 yr BP. Our work demonstrates that lead emissions associated with both the historic and Old Copper Complex tradition are detectable and can be used to determine the temporal and geographic pattern of metal pollution.

Ocean‐atmosphere forcing of centennial hydroclimate variability in the Pacific Northwest

Reconstructing centennial timescale hydroclimate variability during the late Holocene is critically important for understanding large-scale patterns of drought and their relationship with climate dynamics. We present sediment oxygen isotope records spanning the last two millennia from 10 lakes, as well as climate model simulations, indicating that the Little Ice Age was dry relative to the Medieval Climate Anomaly in much of the Pacific Northwest of North America. This pattern is consistent with observed associations between the El Nino–Southern Oscillation (ENSO), the Northern Annular Mode, and drought as well as with proxy-based reconstructions of Pacific and Atlantic ocean-atmosphere variations over the past 1000 years. The large amplitude of centennial variability indicated by the lake data suggests that regional hydroclimate is characterized by longer-term shifts in ENSO-like dynamics and that an improved understanding of the centennial timescale relationship between external forcing and drought is necessary for projecting future hydroclimatic conditions in western North America.

Copper mining on Isle Royale 6500–5400 years ago identified using sediment geochemistry from McCargoe Cove, Lake Superior

Isle Royale, in Lake Superior, contains evidence of indigenous copper mining; however, the timing and geographical extent of mining activity is poorly known. We analyzed metal, carbon, nitrogen, and organic matter concentrations to document past mining pollution in sediment cores recovered from McCargoe Cove; a long, narrow inlet of Lake Superior on Isle Royale that receives drainage from a watershed that contains numerous ancient copper mines. At McCargoe Cove, concentrations of lead, copper, and potassium increase in the sediments after ad 1860 and between 6500 and 5400 years before ad 1950 (yr BP). Metal pollution increases at McCargoe Cove exceed natural (or background) levels and coincide with radiocarbon dates associated with copper artifacts and existing lead pollution reconstructions from lakes on the Keweenaw Peninsula. Interestingly, a coherent cessation of lead emissions at multiple study sites after ~5400 yr BP coincides with the onset of dry conditions found in regional paleoclimate proxy records. After ~5000 yr BP, lead concentrations on both Isle Royale and the Keweenaw Peninsula remain at background levels until the onset of modern lead pollution ~ad 1860.

Late-Holocene Indian summer monsoon variability revealed from a 3300-year-long lake sediment record from Nir’pa Co, southeastern Tibet

Sedimentological and geochemical results from Nir’pa Co, an alpine lake on the southeastern Tibetan Plateau, detail late-Holocene Indian summer monsoon (ISM) hydroclimate during the last 3300 years. Constrained by modern calibration, elevated silt and lithics and low sand and clay between 3.3 and 2.4 ka and 1.3 ka and the present indicate two pluvial phases with lake levels near their current overflow elevation. Between 2.4 and 1.3 ka, a sharp increase in sand and corresponding decrease in lithics and silt suggest drier conditions and lower lake levels at Nir’pa Co. Hydroclimate expressions in the sedimentological proxies during the Medieval Climate Anomaly (MCA) and ‘Little Ice Age’ (LIA) are not statistically significant, suggesting that these events were minor compared to the millennial scale variability on which they were superimposed. However, decreasing sand and increasing lithics and silt during the MCA between 950 and 800 cal. yr BP may suggest briefly wetter conditions, while increasing sand and reduced lithics and silt from 500 to 200 cal. yr BP suggest potentially drier conditions during the LIA. Similarities with regional records from lake sediment and ice cores and speleothem records from the central and eastern Tibetan Plateau, India, and the Arabian Sea, suggest generally coherent late-Holocene ISM variability in these regions. Increased late-Holocene ISM intensity occurred during times when Tibetan Plateau surface air temperatures were warmer, Indo-Pacific sea surface temperatures were elevated, and the tropical Pacific was in a La Niña–like mean state. Conversely, aridity between 2.4 and 1.3 ka occurred in concert with cooling on the Tibetan Plateau and in the Indo-Pacific with more El Niño–like conditions in the tropical Pacific. Differences with western Tibetan records may reflect a weakened ISM and stronger westerlies in this region during the late-Holocene.

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 19th and 20th 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.

Reconstructing Early Industrial Contributions to Legacy Trace Metal Contamination in Southwestern Pennsylvania

Early industrial trace metal loadings are poorly characterized but potentially substantial sources of trace metals to the landscape. The magnitude of legacy contamination in southwestern Pennsylvania, the cradle of North American fossil fuel industrialization, is reconstructed from trace metal concentrations in a sediment core with proxies including major and trace metal chemistry, bulk density, and magnetic susceptibility. Trace metal chemistry in this sediment record reflects 19th and 20th century land use and industry. In particular, early 19th century arsenic loadings to the lake are elevated from pesticides used by early European settlers at a lakeside tannery. Later, sediment barium concentrations rise, likely reflecting the onset of acidic mine drainage from coal operations. Twentieth century zinc, cadmium, and lead concentrations are dominated by emissions from the nearby, infamous Donora Zinc Works yet record both the opening of a nearby coal-fired power plant and amendments to the Clean Air Act. The impact of early industry is substantial and rivals more recent metal fluxes, resulting in a significant potential source of contaminated sediments. Thus, modern assessments of trace metal contamination cannot ignore early industrial inputs, as the potential remobilization of legacy contamination would impact ecosystem and human health.