Sediments of Coeur d'Alene Lake, ID, are heavily contaminated with mine tailings that contain high levels of iron, lead, zinc, arsenic, and other trace elements. These tailings originate from the Silver Valley mining district drained by the South Fork of the Coeur d'Alene River. The possibilities that either lake eutrophication or the develop ment of a seasonally anoxic hypolimnion could mobilize trace elements from sediments into overlying waters led us to evaluate their phase associations. Analysis of ∼0.5 m gravity cores reveals these sediments to be highly reduced, and the trace elements therein predominantly associated with an operationally defined sulfidic phase. Vertical patterns of metal distribution suggest that Fe, Mn, and As have mobilized toward the sediment−water interface; these patterns are consistent with diagenetic solubilization. This is not the case for Zn, Pb, and other trace elements whose maximum abundance is generally found in deeper sediments. We postulate that metal sulfide formation and metal binding with organic material restricts mobilization of most trace elements. The abundance of redox-active elements such as As, Fe, and Mn is highly correlated by depth. The abundance of less redox-sensitive elements such as Pb and Zn is also highly correlated; however, the two groups correlate poorly with one other.

No abstract

No abstract

No abstract

To improve future predictions of anthropogenic climate change, a better understanding of the relationship between global temperature and atmospheric concentrations of CO2 (pCO2), or climate sensitivity, is urgently required. Analyzing proxy data from climate change episodes in the past is necessary to achieve this goal, with certain geologic periods, such as the Miocene climatic optimum (MCO), a transient period of global warming with global temperatures up to ~7°C higher than today, increasingly viewed as good analogues to future climate under present emission scenarios. However, a problem remains that climate models cannot reproduce MCO temperatures with less than ~800 ppm pCO2, while most previously published proxies record pCO2 < 450 ppm. Here, we reconstructed MCO pCO2 with a multitaxon fossil leaf database from the well‐dated MCO Lagerstätte deposits of Clarkia, Idaho, USA, using four current methods of pCO2 reconstructions. The methods are principally based on either stomatal densities, carbon isotopes, or a combination of both—thus offering independent results. The total of six reconstructions mostly record pCO2 of ~450–550 ppm. Although slightly higher than previously reconstructed pCO2, the discrepancy with the ~800 ppm required by climate models remains. We conclude that climate sensitivity was heightened during MCO, indicating that highly elevated temperatures can occur at relatively moderate pCO2. Ever higher climate sensitivity with rising temperatures should be very seriously considered in future predictions of climate change.