LOCAL ENVIRONMENTAL CHANGE DURING THE EARLY EOCENE CLIMATIC OPTIMUM DETECTED USING CHEMICAL ANALYSES OF A GREEN RIVER FORMATION (LACUSTRINE) STROMATOLITE

The Eocene Green River Formation is one of the largest and best-studied paleolake systems in the world. Green River Formation sediments record the Early Eocene Climatic Optimum (EECO), with high global temperatures correlated to high levels of greenhouse gases in the atmosphere. Stromatolites from the upper Rife Bed at the Boar’s Tusk outcrop (near Rock Springs, Wyoming) formed at the height of the EECO—a time when the lake in which they formed (Lake Gosiute) was transitioning from a balanced-filled to underfilled, net-evaporative system. As finely-laminated structures that form over geologically short periods, stromatolites have the potential to record local environmental changes at high temporal resolution during this notable period in the Earth’s history.

The stromatolites in question are composed of branching microdigitate columns that are laminated on the 10-100µm scale. Laminae are grouped in cm-scale bands that alternate between micritic, calcite fan, and mixed microstructures. The micrite bands, on average, are enriched in ¹⁸O, carbonate associated sulfate (CAS), and Mg/Ca compared to the fan layers, indicating formation of micritic bands during net-evaporation in closed systems versus the fans. Furthermore, the magnetic susceptibility, commonly used to assess detrital input to lacustrine systems, is higher in the micrite versus the fan microstructures suggesting formation of micritic layers closer to clastic sources versus fan layers. The geochemical evidence is consistent with stromatolite growth during both expansion and contraction (evaporation) of the ancient lake, where the micritic layers formed during evaporation and the fans formed during lake expansion/introduction of additional fresh water. Thus, the alternating phases of stromatolite growth, on the mm scale, tell a story of dramatic lake volume fluctuation. Preliminary clumped isotope measurements, from which source water temperatures can be derived, are available for the micrite and fan layers of the stromatolite and will be discussed pending replication.