THE MORPHOGENESIS OF WALKER LAKE, NEVADA STROMATOLITES

A variety of tufa deposits, including microdigitate stromatolites, are associated with Walker Lake, Nevada. The young age of these structures and the temporally constrained history of Walker Lake provide a unique opportunity to study stromatolite morphogenesis in a way not possible with Precambrian forms. Walker Lake is alkaline (pH 9.4), 28 m deep, and stratifies annually, becoming anoxic below 20 meters in the summer. The stromatolites are found encrusting boulders near current lake level. They form cm-scale digitate columns with three distinct portions: a finely laminated inner portion, a weakly laminated midsection, and a non-laminated crust. The laminated portion represents a closer textural analogue to Precambrian stromatolites than do their modern marine counterparts. 14C dates reveal that the laminated portion formed between 2885±20 and 2405±20 BP. When compared to available lake level history, the structures formed most rapidly while lake level was high; accretion rates slowed and lamination disappeared as lake level dropped. If correct, the stromatolites started to form at ~30 m water depth and growth slowed as lake depth shallowed. High-resolution ICP-MS laser ablation data demonstrates a notable cyclicity of redox-sensitive elements such as uranium and phosphorous; cyclicity is noted at several scales, including the lamina scale. Our data reveal that U and P are anti-correlated. In the absence of oxygen, soluble U(VI) is more easily reduced to insoluble U(IV); this reaction occurs in the sediments, thus, less U is available for incorporation into carbonates. Conversely, P becomes more mobile under anoxic conditions released during the reduction of iron oxyhydroxides. We interpret the U and P cyclicity to reflect oxygen concentration at the depth and time of formation and to indicate the strong control of lake chemistry on stromatolite morphogenesis. No microfossils or microporosity were noted in the finely laminated portion; in contrast, nearby non-laminated tufa mounds incorporate a significant microbiota. Furthermore, compound specific C-isotopes of n-alkanes reveal no difference between the stromatolites and the surrounding environment. While biological processes cannot be ruled out, the data do not necessarily point towards a primarily biological origin for Walker Lake stromatolites.