Walker Lake, Nevada Stromatolites: An in-Depth Study of Texture, Growth Rate, and Morphology

A variety of carbonate deposits (“tufas”) are associated with Walker Lake, an alkaline lake in western Nevada, including microdigitate stromatolites that texturally resemble finely laminated Precambrian forms. The stromatolites, found encrusting boulders near current lake level, form cm-scale microdigitate columns with finely laminated inner portions. Stromatolites are commonly thought to form by the trapping and binding and/or precipitation of minerals through microbial processes. Precambrian stromatolites constitute some of the oldest evidence for life on Earth, but their growth rate and the significance of their texture and morphology are poorly understood. The young age of the Walker Lake stromatolites and the temporally well-constrained history of the lake provide an opportunity to study morphogenesis in a way not possible with Precambrian forms.

A comparison of preliminary ¹⁴C dates and available lake level reconstructions reveals that the stromatolites started forming at water depths of 30 meters or more and continued to grow as lake level fell between 2800 and 2000 B.P. Accretion rate slowed as lake depth shallowed. A detailed study of growth direction was undertaken by sampling stromatolites that were encrusting all exposed sides of an in situ boulder located on the northwest side of Walker Lake. Samples were taken from the north, south, east, and west sides of the boulder, both from the steep (>75 degree) sides and from the top. Though incident light would have been most intense at the top of the boulder, the stromatolites show no evidence of growth towards a light source; rather, most of the stromatolites display surface normal growth. While this evidence does not rule out a biological origin for the structures, it does indicate that these particular stromatolites were not formed in response to incident light, and it is not clear how trapping and binding would produce laminae with the orientations noted here.