Dramatic local environmental change during the Early Eocene Climatic Optimum detected using high resolution chemical analyses of Green River Formation stromatolites

The Eocene Green River Formation represents a system of lakes that covered parts of what is now Wyoming, Colorado, and Utah, and captures the Early Eocene Climatic Optimum (EECO, 52–50million years ago or Ma), the warmest period of the Cenozoic Era, and a period associated with very high levels of atmospheric CO2. Lakes, especially closed basin lakes, can respond dramatically to environmental change because of their sensitivity to precipitation and evaporation. In this study, stromatolites from the Rife Bed of the Green River Formation are used as fine-scale records of terrestrial paleoenvironmental change during a global hothouse climate, and to investigate how the environmental dynamics within the lake system affected the growth of stromatolites. The stromatolites are composed of branching microdigitate columns laminated on the 10–100μm scale. Laminae are grouped in cm-scale layers that alternate between calcite fan, micritic, and mixed microstructures. The fan layers are depleted in 18O, Na, and Mg/Ca. The micrite layers, in contrast, are comparatively enriched in 18O, Na, and Mg/Ca. The δ13C and δ18O are strongly positively correlated, suggesting the stromatolites formed in a closed basin lake, consistent with the regional stratigraphy. Additionally, clumped isotope analyses provide the first quantitative values for water temperatures in lake water from the Green River Formation (~35°C for micrite layers and ~28°C for fan layers). Changes in δ18O and sodium ion concentration are likely related to periods of evaporation and recharge, and thus can be used to estimate lake volume change during stromatolite growth. Two models, one using sodium ion concentrations in a conserved system, the other using Rayleigh fractionation and mixing equations to explain changes in oxygen isotopes, converge upon similar results for lake volume changes, revealing multiple episodes of meter-scale lake level rise and fall during the accretion of the ~30cm thick stromatolite. Because of the broad, flat bathymetry of the lake, such lake volume and depth changes would have been accompanied by shoreline shifts on the order of tens of kilometers while the stromatolites were actively growing, challenging the view of a single stromatolite paleoenvironment in the lake. Therefore, the fan microfabric, interpreted here as abiogenic in nature, formed in cooler waters when the lake was deeper, possibly below a thermocline. In contrast, the micrite microfabric, for which there is evidence of biogenicity, formed when the lake was shallow and warm. The alternation between biogenic and abiogenic microfabrics present in the Rife Bed stromatolites is hypothesized to result from dramatic changes in lake level influencing the microbiology and chemistry of the waters in which the stromatolites formed, indicating that stromatolite growth can occur under disparate conditions and therefore does not necessarily represent a single facies.