ANALYSIS OF GROWTH DIRECTIONS OF DOMED MICRODIGITATE STROMATOLITES FROM WALKER LAKE, WESTERN NEVADA

Stromatolites are commonly defined as laminated organo-sedimentary structures built by the trapping and binding and/or precipitation of minerals via microbial processes. Typically, stromatolites have a domed morphology that is commonly taken as an indication of microbial phototaxis and growth towards incident light. Despite the fact that stromatolites may represent evidence for some of the oldest life on Earth, much about the processes that control stromatolite form remains enigmatic. Complicating this is the fact that several studies have demonstrated that the presence of microbial activity is not necessarily a prerequisite for this growth pattern.

Walker Lake, an alkaline lake in western Nevada, contains recent, undeformed, finley laminated, domed microdigitate stromatolites that texturally resemble Precambrian forms. The stromatolites are found encrusting boulders near the southwestern shore of the lake. If the domed morphology of the stromatolites is a record of the response of phototaxis by phototrophic organisms, it should be represented in the angle of growth, where stromatolites growing off the sides of the boulders should show an inclination to grow at angles up towards the light.

Samples of stromatolites were taken from the north, south, east, and west sides of four in situ boulders located on the southwestern side of Walker Lake, both from the steep sides and shallowly angled top. Angle of growth measurements were made on more than 300 stromatolites, with 90 degrees representing surface normal growth, <90 degrees representing growth “up” towards incident light, and >90 degrees representing growth “down” away from incident light. 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, or 90 degree growth, though some grow both “up” toward light as well as “down” away from it. There are no significant differences in the distribution of growth angles between north, south, east or west facing samples. While this evidence does not rule out a biological origin for the structures, it does indicate that phototaxis and growth towards incident light was not a dominant factor in the morphogenesis of the stromatolite’s domed morphology.