11:30 AM - 11:45 AM
[MIS16-20] Terrestrial environmental response to the global climate change during the early to middle Eocene (52.7-43.8 Ma): evidence from lacustrine deposits of the Green River Formation, Utah, USA
Keywords:lake sediment, Eocene, greenhouse, paleoclimate, carbon cycle, Milankovitch cycle
The evolution of early Eocene climate and oceans has been the focus of significant study due to the elevated atmospheric CO2 levels (>1,000 ppm) of this warmest interval of the Cenozoic. In contrast, the terrestrial response during this period is less understood. This study focused on the Early Eocene Climatic Optimum (EECO) recorded in the Uinta Basin’s lower to middle Green River Formation, western United States, ca. 52.7-43.8Ma This section (ca. 1000 m in thickness) affords a high temporal resolution of 1-2 m (10-20 kyrs resolution).
In this study, detailed lithofacies descriptions of lake level change, as well as elemental and mineralogical analyses, were used to quantitatively reconstruct the terrestrial environment. Alterations in lithofacies allow us to subdivide the succession into the following nine paleoenvironmental stages: the fluvio-lacustrine stage; the deep-lake stage; the fluvio-lacustrine stage; the deep-lake stage; the shallow lake stage; the evaporative lake stage; the shallow lake stage; fluctuating lake stage; and the fluvio-lacustrine stage, in ascending order. By comparing the lithofacies and the composition of major elements, Ca/Al can be utilised as a proxy of lake level, Mn/Fe as an indicator of lake bottom redox conditions, and K/Al as a proxy for chemical weathering rates. Each paleoclimatic proxy suggests that the Green River Formation had high lake levels, an anoxic lake bottom environment, and high chemical weathering during the EECO. Mineral composition changes reflect variation in the degree of chemical weathering during and after the EECO. In concert, these results suggest abundant precipitation during the EECO in the mid-latitudes of North America. In addition, our results correspond to the positive δ13C trend in deep-sea benthic foraminifera and insolation phase change during the early to middle Eocene, indicating that the mid-latitudes of the Northern Hemisphere may have become wetter during the "hothouse" period of EECO which probably correspond to the northward expansion of the intertropical convergence zone due to global warming.
In this study, detailed lithofacies descriptions of lake level change, as well as elemental and mineralogical analyses, were used to quantitatively reconstruct the terrestrial environment. Alterations in lithofacies allow us to subdivide the succession into the following nine paleoenvironmental stages: the fluvio-lacustrine stage; the deep-lake stage; the fluvio-lacustrine stage; the deep-lake stage; the shallow lake stage; the evaporative lake stage; the shallow lake stage; fluctuating lake stage; and the fluvio-lacustrine stage, in ascending order. By comparing the lithofacies and the composition of major elements, Ca/Al can be utilised as a proxy of lake level, Mn/Fe as an indicator of lake bottom redox conditions, and K/Al as a proxy for chemical weathering rates. Each paleoclimatic proxy suggests that the Green River Formation had high lake levels, an anoxic lake bottom environment, and high chemical weathering during the EECO. Mineral composition changes reflect variation in the degree of chemical weathering during and after the EECO. In concert, these results suggest abundant precipitation during the EECO in the mid-latitudes of North America. In addition, our results correspond to the positive δ13C trend in deep-sea benthic foraminifera and insolation phase change during the early to middle Eocene, indicating that the mid-latitudes of the Northern Hemisphere may have become wetter during the "hothouse" period of EECO which probably correspond to the northward expansion of the intertropical convergence zone due to global warming.