PHREATICALLY-FORMED RHIZOLITHS AS A TOOL FOR PALEOLANDSCAPE RECONSTRUCTION

Stable isotope records from terrestrial carbonates are providing important information in basin-scale and regional climate studies. However, the rate of formation and timing parameters of calcite precipitation in modern environments need to be fully understood in order to use these records. This study used stable isotope records generated from phreatically-formed calcite rhizoliths in modern lake and playa settings to evaluate their use in interpreting records from fossil rhizoliths. A temperate site in upstate New York (Green Lake) was selected to characterize the isotopic signature in an open system. The δ¹³C vs. δ¹⁸O plots from this site show a steep linear trend, indicating a large range in carbon and a small range in oxygen isotope data. We interpret this to reflect the seasonal phytoplankton bloom during the warm, summer months, producing a large range of δ¹³C values. The low variability in the δ¹⁸O values reflects the relatively narrow temperature range during which the photosynthesis occurs plus the open nature of Green Lake, which minimizes the evaporative concentration of ¹⁸O in the lake waters. Carbonate rhizoliths were also collected from a semi-arid playa in Nevada (Pilot Valley). The Pilot Valley data show a large spread in the δ¹⁸O values, which probably reflect the high evaporation that occurs on the closed-basin playa. However, the δ¹³C vs. δ¹⁸O plots show a negative linear trend, which is puzzling. It appears that the trend in the data is caused by overflowing of springs in the region (both freshwater and saline) altering the initial isotopic values of the rhizoliths that form on the spring's shores. The springs tap different source waters at depth and therefore the waters from which the rhizoliths form have various initial δ¹³C values, and possibly δ¹⁸O values. In both the temperate and semi-arid sites the δ¹⁸O values are consistent with predicted δ¹⁸O values based on local temperature and δ¹⁸O_water values. The δ¹³C values are less clear but seem to record the ambient δ¹³C value of the water. More work at the Pilot Valley site needs to be done to identify the initial δ¹³C values of various springs.