Stromatolites As Fine Records of Terrestrial Environmental Conditions: Examples from the Eocene Green River Formation (Wyoming)

Abstract:

Stromatolites are layered structures that form subaqueously, thereby recording chemical information about their formation environment. As such, these accretionary structures are useful tools for fine-timescale environmental reconstructions. High-resolution geochemical analyses of stromatolites that formed in paleolake Gosiute (Eocene Green River Formation) provided novel information about terrestrial environmental variability during the Early Eocene Climatic Optimum (EECO), the period with the highest temperatures and atmospheric CO₂ levels in the Cenozoic. Stromatolites from the ~51 Ma Rife Bed of the Tipton Shale Member of the Green River Formation record dramatic changes in lake volume (and correspondingly, water depth and shoreline) indicating the environment during the peak of the EECO was more variable than previously appreciated. A second set of stromatolites from the ~49 Ma Lower Laclede Bed of the Laney Member of the Green River Formation record transient periods of basin closure during a time when the basin is generally considered to have been balanced-filled. In addition, the results reveal that basin filling after desiccation was not continuous, but fluctuated before becoming an open system, further indicating local climate variability during the EECO. In both cases, major environmental changes are reflected not only in the recorded chemistry, but also in changes in stromatolite microfabric. In addition, clumped isotope paleothermometry provided estimates of water temperature from the evolving lake, which for most of its existence was so massive that it would have influenced regional climate. These and other studies demonstrate that stromatolite laminae can be used to understand fine-scale environmental variability in ancient lacustrine systems.