MODERN 'RHIZOLITH' FORMATION ON A CLOSED-BASIN PLAYA, PILOT VALLEY, NV

On a semi-arid playa in Nevada, calcium carbonate infills stem and root holes and creates rhizoliths that can be used as tools to reconstruct the landscape. Pilot Valley, NV is a closed-basin system situated on the northeastern border of the state. During the spring snowmelt, water drains to the east off the Pilot Mountains into the basin as surface runoff and as groundwater beneath the alluvial fans. Springs emerge at the toe of the alluvial fans and tap both near-surface fresh water and recirculating saline groundwater. During the summer, evaporation on the playa is intense and by late summer (July-August) the playa is dry during most years. The evaporative concentration of the playa waters triggers the precipitation of calcite casts around plant stems, thereby suggesting that the rate of rhizolith formation is seasonal. Saltgrass is prolific and can tolerate the extreme evaporative conditions on the playa. The size of the rhizoliths is consistent with their formation around these saltgrass stems. Rhizoliths do not extend more than 1cm below the sediment interface and do not exceed 4 centimeters in total length. Most of the casts no longer surround the plant stem or root and have been infilled with sediment and calcite. Stable isotope analyses of the rhizoliths suggest that they form in equilibrium with ambient water conditions in the summer, and are therefore phreatically-formed rather than pedogenic. The phreatic origin implies that the rhizoliths must be underwater during calcite precipitation and therefore, each rhizolith is a proxy for water depth during its formation. More research needs to be done on the Pilot Valley rhizoliths to confirm their rate of formation and preservation potential before these carbonates can be used to accurately interpret fossil (phreatic) rhizoliths.