Elevated weathering rates in the rocky mountains during the early eocene climatic optimum

During the chemical weathering of silicate minerals, atmospheric carbon dioxide is incorporated into carbonate minerals and buried. As the rate of silicate weathering is thought to increase in response to increasing atmospheric CO"2 concentrations, this represents an important negative feedback mechanism. Quaternary records of weathering reflect a narrow range of pCO"2 (180-300p.p.m.v.); therefore, the extent of this feedback has been difficult to predict for increasing concentrations of atmospheric CO"2. However, high CO"2 levels of up to 1,125p.p.m.v. have been suggested for the Early Eocene Climatic Optimum (52 to 50 million years ago). Here, we combine ⁴⁰Ar/³⁹Ar ages and the measured volumes of river-derived sediments and sodium-bearing evaporites to determine rates of physical erosion and chemical weathering in the Green River Basin, western United State of America, during the Early Eocene Climatic Optimum. We find physical erosion rates of 420±79tkm²yr^-1 and chemical weathering rates of 62.5±21.9tkm²yr¹. The calculated denudation rates of 175±30mMyr¹ rival the highest documented non-glacial Quaternary rates for crystalline bedrock. We suggest that elevated atmospheric CO"2 levels during the Early Eocene epoch led to enhanced silicate dissolution rates, and thus to increased production of loose rock material and higher rates of physical weathering and denudation.