NEW TERMINOLOGY FOR CALCITE STEM CASTS FORMED IN STANDING WATER: A CASE STUDY FROM THE SEMIARID WESTERN USA

In semiarid regions such as the playas of the Great Salt Lake Desert, calcite forms around local vegetation and creates stem casts. The casts are preserved at the surface as short (0.5-6cm) upright and/or overturned tubes that exhibit an apron at the sediment surface. The casts do not form under typical pedogenic or phreatic conditions and are not “rhizoliths” proper, since they do not form beneath the sediment surface or around a plant root. At this time, no terminology exists to describe the formation and origin of these carbonates, and their presence in the geologic record (and geochemical signatures) may previously have been interpreted as pedogenic. This research proposes new terminology for calcite stem casts formed in standing water above the sediment/surface water interface, and was inspired by Dr. Richard L. Hay and his geochemical research on rhizolith carbonates found at Olduvai Gorge, Tanzania.

Carbonate and water samples were collected from a playa in eastern Nevada and geochemically analyzed. Stem casts are only found near saline springs on the playa (E.C. >10mS/cm). The initial geochemical conditions for the springs are dictated by local hydrology: freshwater springs appear in the north to the east of a broad alluvial fan, and more saline springs emerge to the south where the alluvial fan influence diminishes. The temperatures and δ¹⁸O_water values of the springs, along with the δ¹⁸O_calcite values of the stem casts, confirm that the calcite forms in equilibrium with standing water. The δ¹³C_calcite values coupled with the stem cast morphology indicate that the calcite forms exclusively around saltgrass (Distichlis sp.). Thus, local hydrology dictates the presence of the casts (i.e., saltgrass distribution) as well as the initial geochemical conditions under which the calcite forms. As the residence time of water on the playa increases, evaporation and biological processes alter the aquatic chemistry and initiate calcite precipitation around saltgrass stems growing in standing water. The origin of the stem casts makes them faithful recorders of ambient water conditions (e.g., geochemistry) as well as local hydrologic parameters (e.g., water depth) when the playa surface is flooded, but their geological application is restricted to a partial annual signal (e.g., saltgrass growing season).