PATTERNS IN LAMINATION OF EOCENE LACUSTRINE STROMATOLITES: LAMINATION AS INDICATORS OF LOCAL CLIMATIC CONDITIONS

Stromatolites are a result of microbe-sediment interactions that are responsive to environmental conditions. As they grow, stromatolites may act as “tape-recorders” of climatic patterns manifested as differences within their laminae. Laminae differ from one another in grain size, composition, fabric, color, and thickness. Stromatolites from the Fontenelle Tongue of the Eocene Green River Formation contain prominent, well-preserved lamination, and were examined for patterns and properties that may relate to the environmental conditions of ancient Lake Gosiute. High-resolution images of laminae were produced from cut slabs and thin sections. Luminance values of laminae were obtained using ImageJ by measuring luminance along lines drawn normal to growth. Each line was divided into line segments measuring 1 by 100 pixels. Gray-scale measurements for the segments were averaged and plotted along the line drawn normal to the growth of the stromatolite. Thus, each data point represents one line segment and a vertical increment of 84.7 μm. These data points were used to produce line luminance curves for each of 4 lines drawn. The curves were then compared to visual representations of stromatolite lamination (slabs, high resolution scans of slabs, and thin sections), correlated, and scrutinized for patterns and trends. The data points were also analyzed for periodicity using the Lomb-Scargle method. Preliminary results indicate a two-fold periodicity and a lesser three-fold pattern, which are likely climate signals. These are in keeping with paleobotanical evidence that indicates a dry-wet subtropical climate with three possible components: (1) a cool, dry, winter to early spring, (2) a warm, wet, late spring to early fall, and (3) a cool, wet to dry, late fall to winter.