Victoria A. Petryshyn

Stromatolite lamination frequency, Walker Lake, Nevada: Implications for stromatolites as biosignatures

Lamination in stromatolites (considered some of the oldest fossils on Earth) is commonly interpreted to record the periodic response of a microbial community to daily, seasonal, or perhaps yearly environmental forcing, but the inability to date ancient stromatolites precludes an understanding of the lamination formation processes. We use high-resolution 14C dating of Holocene stromatolites from Walker Lake, Nevada (United States), to construct a record of lamination rate over the course of accretion. Laminae formed with a period of 5.6 ± 1.6 yr/lamination at the base of the structure, 1.6–2.8 ± 1.9 yr/lamination in the middle, and 4.5 ± 0.8 yr/lamination at the top of the laminated portion. The predominant 4−6 yr periodicity indicates that lamination formation is likely more closely related to regional climate forcing (e.g., El Nino–Southern Oscillation) versus the typical diurnal or seasonal changes in microbial mats traditionally assumed for most ancient stromatolites. Thus, generalizations regarding the influence of microbial mats on stromatolite lamination and the use of stromatolites as biosignatures need careful consideration.

Grain trapping by filamentous cyanobacterial and algal mats: implications for stromatolite microfabrics through time.

Archean and Proterozoic stromatolites are sparry or fine-grained and finely laminated; coarse-grained stromatolites, such as many found in modern marine systems, do not appear until quite late in the fossil record. The cause of this textural change and its relevance to understanding the evolutionary history of stromatolites is unclear. Cyanobacteria are typically considered the dominant stromatolite builders through time, but studies demonstrating the trapping and binding abilities of cyanobacterial mats are limited. With this in mind, we conducted experiments to test the grain trapping and binding capabilities of filamentous cyanobacterial mats and trapping in larger filamentous algal mats in order to better understand grain size trends in stromatolites. Mats were cut into squares, inclined in saltwater tanks at angles from 0 to 75° (approximating the angle of lamina in typical stromatolites), and grains of various sizes (fine sand, coarse sand, and fine pebbles) were delivered to their surface. Trapping of grains by the cyanobacterial mats depended strongly on (i) how far filaments protruded from the sediment surface, (ii) grain size, and (iii) the mats incline angle. The cyanobacterial mats were much more effective at trapping fine grains beyond the abiotic slide angle than larger grains. In addition, the cyanobacterial mats actively bound grains of all sizes over time. In contrast, the much larger algal mats trapped medium and coarse grains at all angles. Our experiments suggest that (i) the presence of detrital grains beyond the abiotic slide angle can be considered a biosignature in ancient stromatolites where biogenicity is in question, and, (ii) where coarse grains are present within stromatolite laminae at angles beyond the abiotic angle of slide (e.g., most modern marine stromatolites), typical cyanobacterial-type mats are probably not solely responsible for the construction, giving insight into the evolution of stromatolite microfabrics through time.

Analysis of growth directions of columnar stromatolites from Walker Lake, western Nevada

Samples of digitate, branching, columnar stromatolites were collected from the steep sides and near horizontal top of four in situ boulders located on the southwestern side of Walker Lake, Nevada, to test the widely held assumption that stromatolite column formation represents a phototropic response. We would predict that the columns on the steeply dipping sides of the boulder would bend upwards toward the light during growth if phototropism was significant during stromatolite morphogenesis. Angle of growth measurements on >300 stromatolites demonstrate that the stromatolites grew nearly normal to their growth surface, regardless of the inclination of their growth surface. No significant differences in the distribution of growth angles between north-, south-, east-, or west-facing samples were observed, and stromatolite lamina thickness did not systematically vary with position on the boulder. The lack of a strong phototropic response does not rule out a biological origin for the Walker Lake structures, but it does suggest that phototropic growth was not a dominant factor controlling stromatolite morphogenesis in these stromatolites and that column formation cannot be uniquely attributed as a phototropic response in stromatolites. It is interesting to note that the morphology of the stromatolites on the top of the boulder is identical to stromatolites on the steep sides. Stromatolite morphogenetic models that predict branching typically require a vertically directed sedimentary component, a feature that would have likely affected the stromatolites on the tops of the boulders, but not the sides, suggesting that other factors may be important in stromatolite morphogenesis.