COUPLED STRATIGRAPHY, PETROGRAPHY, AND Δ47 OF ANCIENT WALKER LAKE, NEVADA REVEALS UNIQUE ANALOG FOR STUDYING PROTEROZOIC STROMATOLITE FORMATION AND CLIMATIC FORCINGS

Throughout Earth history, stromatolites have ranged broadly in their complexity and the degree to which textures contain evidence for biogenicity. Archean and Proterozoic stromatolites are typically composed of sparry crust microfabrics and/or are fine-grained/finely laminated, whereas coarse-grained stromatolites appear much later in the Phanerozoic. Investigations of modern textural analogs of these stromatolites can be useful to understand the changes in stromatolite microstructure throughout Earth history.

Stromatolites with a sparry crust microstructure reminiscent of Proterozoic microstructures are found 60 meters above current lake level near Walker Lake, Nevada. During the Late Pleistocene, Walker Lake was part of Lake Lahontan, a large glacial lake covering a significant portion of northwestern Nevada. The stromatolites formed between ~41,460 to ~35,680 (radiocarbon ages on carbonate, calibrated YBP, IntCal13) over 14 cm, placing the age of formation within Marine Isotope Stage (MIS) 3, a time noted for significant climatic shifts. Clumped Isotope (Δ₄₇) analysis revealed large temperature fluctuations during formation of the stromatolites. Using changes in conservative trace metals within the stromatolite, we modeled the fluctuation of volume of Walker Lake to be almost 50% over the course of of stromatolite accretion.

Several unexpected results emerge from the environmental scenario for Walker Lake during stromatolite accretion. With respect to climate studies, the stromatolites appear to represent an archive of climate that records dramatic lake level and temperature fluctuations during MIS 3. As an analogue to help understand ancient stromatolite formation, a rate of accretion can be calculated ~25 microns per year. Although we do not know how rapidly Proterozoic stromatolites grew, an understanding of more modern examples provides bounds to understand ancient examples. Furthermore, environmental conditions changed significantly during stromatolite accretion, but in general the microstructure remained similar. Based on our analyses, we propose Walker Lake stromatolites as a useful analog for understanding Proterozoic stromatolite formation and an archive for more recent climate change.