RATE OF LAMINATION FORMATION OF A MICRODIGITATE STROMATOLITE FROM WALKER LAKE, WESTERN NEVADA

In general, the processes that control stromatolite laminae formation are poorly understood, partially because of the paucity of modern examples to study. Typically, laminations are alternately interpreted to be the record of the daily, yearly, or perhaps seasonal response of a microbial community to some environmental forcing, but the inability to reliably date ancient stromatolites on such a fine scale precludes a better understanding of the lamination forming process. Modern marine stromatolites are known, but their coarse texture and millimeter-scale laminations make them less than ideal analogues for Precambrian stromatolites, which are typically composed of finer grained material and display sub-millimeter scale lamination.

Walker Lake, an alkaline lake in western Nevada, contains microdigitate stromatolites that texturally resemble finely laminated Precambrian forms. The young age of the Walker Lake stromatolites provides a unique opportunity to study the rate of laminae formation in a way not possible with Precambrian forms. Five ¹⁴C dates spanning 32mm were taken from a Walker Lake stromatolite. These dates indicate that stromatolite growth initiated ~3300ybp (assuming a constant CO₂ residence time of 300yrs) and grew as Walker Lake underwent major lake level fluctuations. In the basal 7mm, counts of laminations reveal an average rate of formation of one lamination every 5.6 ± 1.5yrs. As the stromatolite grew, overall carbonate accretion rate dropped, but the frequency of lamina formation increased (laminations themselves became thinner), averaging one lamination about every1.5 ± 1yrs. Towards the top, the stromatolite texture changes from finely to weakly laminated, and lamination frequency decreases back to a rate of one lamination every 4.5±1.2yrs. These results show that stromatolite laminations cannot be taken as daily, yearly or seasonal deposits without additional information, and that one stromatolite may record significant changes in lamination frequency, likely closely tied to the environment in which they formed. Interestingly, the lower and upper part of the stromatolite appear to respond to some forcing averaging approximately 4-6 years, which is similar to certain wet/dry climate cycles that affect the region, whereas the central portion records nearly yearly lamination.