Green River formation stromatolites as a paleoclimate indicator: an investigation of the early Eocene climatic optimum through mass spectrometry, micro-X-ray fluorescence spectroscopy, and petrography

2019-05-06The Early Eocene Climatic Optimum (EECO, 52-50 Ma), a global hothouse climate associated with high temperatures and high CO₂ levels, marked the longest sustained warming period of the Cenozoic Era. The EECO is well recorded in the Eocene Green River Formation, a sedimentary formation deposited in a closed-basin lake system. The Green River closed-basin lake system, especially Lake Gosiute, had a highly dynamic lake volume and dramatically variable shoreline levels, because of its sensitivity to evaporation and precipitation. This study uses stromatolites from the LaClede Bed of the Green River Formation to create a fine-scale record of terrestrial paleoenvironmental and paleoclimate changes during the EECO. The stromatolites of the LaClede Bed were deposited in Lake Gosiute, and track the fluctuations of lake volume and shoreline levels through its depositional history. ❧ In a closed-basin lake where water enters solely through ground water, rivers, and precipitation, periods of lake filling and evaporation are recorded in the chemistry of lake water, which in turn can be recorded within the carbonate of stromatolites. Changes in δ¹³C, δ¹⁸O, and certain elemental abundances recorded in stromatolites are likely related to periods of precipitation and evaporation, which can be used to estimate shifting lake shore levels. First, light microscopy and petrographic methods were used to analyze the carbonate fabrics of the distinct stromatolite laminae to better characterize the environmental conditions in which they were deposited. Then, isotopic perturbations and elemental abundances were measured to assess climate change during the deposition of the stromatolites. In particular, this study introduces a novel application of micro-XRF spectroscopy to stromatolite-based paleoclimate reconstruction, where elemental abundances were analyzed via micro-XRF spectroscopy on the micrometer scale, which allows for stromatolite laminae analyses on a scale previously not presented. Thirteen distinct elemental abundances (Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, and Sr) were measured as vertical transects through the stromatolite laminae, and magnesium (used as a conservative lake level indicator), iron (used as a redox-sensitive biologic indicator), and strontium (a redox-insensitive elemental marker) are highlighted. Using these three lines of evidence, the LaClede stromatolites record a complex history of lake filling and evaporation during the EECO, rather than a simple transgressive system as previously hypothesized