INTRA-LAMINATION ISOTOPIC VARIABILITY IN GREEN RIVER FORMATION STROMATOLITES: SIGNIFICANCE FOR STROMATOLITE-BASED PALEOCLIMATE MODELING OF THE EARLY EOCENE CLIMATIC OPTIMUM

The Early Eocene Climatic Optimum (EECO, 52-50 Ma) was a period of global hothouse climate associated with elevated CO₂ levels, providing a useful analogue for future climate regimes influenced by increasing greenhouse gases. The EECO is recorded in the Green River Formation, an extensive paleolake system within present-day Wyoming, Utah, and Colorado. Closed-basin lacustrine deposits can provide detailed records of climate variation, as lake volume and shoreline position can fluctuate dramatically with changes in evaporation and precipitation. Variation in lake hydrology in turn produce distinct petrographic and isotopic patterns in the rock record.

Stromatolites from the Green River Formation have been used to construct models of lake volume change during the EECO, where changes in d¹³C and d¹⁸O values between layers were related to periods of precipitation and evaporation on the time scale of stromatolite formation (e.g., Frantz et al., 2014). This study examines d¹³C and d¹⁸O values sequentially along several laminae from a Green River stromatolite to assess the significance of intra-lamination variation for paleohydrologic modeling.

The stromatolite originates from the LaClede Bed of the Laney Member, Green River Formation, and has been used to reconstruct paleo-lake levels (Frantz, 2013). Multiple samples were drilled along the same lamination (~2mm) within the LaClede Bed stromatolites and analyzed for d¹³C and d¹⁸O; care was taken to avoid drilling into the underlying layers. In this case, d¹³C and d¹⁸O levels remain similar throughout individual stromatolite layers and within error of the isotopic measurements. Thus, the intra-lamination variability throughout the LaClede Bed stromatolite is statistically insignificant and would have little effect on EECO paleoclimate or paleohydrologic modeling.