Paleoclimate reconstruction using stable isotopes of freshwater phreatic carbonates

The research presented here explores the formation of phreatic carbonates in both temperate and semi-arid regions and investigates the possibility that phreatic carbonates record ambient water conditions during formation. The dissertation also determines the controls of oxygen and carbon isotope values in phreatic carbonates and provides a foundation for the interpretation of phreatic rhizolith stable isotopic signatures. A variety of methods were used in order to address these objectives, including thin section microscopy, XRD, CL, EDX, SEM, and stable isotope analysis. Green Lake, NY was chosen as a temperate end-member because precipitation is high and evaporation is low to moderate. Calcite is currently forming in the Green Lake waters. It is coincident with summer algae and bacteria blooms and therefore provides a geologically instantaneous record of current lake water geochemistry. Pilot Valley, UT/NV was chosen as a semi-arid end-member because evaporation is high and rainfall is low. Its hydrology is vastly different from Green Lake and rhizoliths are abundant on the playa. In addition, data from Pleistocene carbonates at Olduvai Gorge are presented as a case study for interpreting stable isotope values from ancient samples. Conclusions from the temperate site are: (1) carbonate production is confined to the summer months and is biologically mediated, (2) calcite precipitates in oxygen isotopic equilibrium, but is in disequilibrium with respect to DIC delta13 C, (3) a large range in carbon isotope values (due to biological effects) accompanies a small range in oxygen isotope values, (4) formation rates of rhizoliths are 0.5 mm/6 months, and (5) preservation potential of rhizoliths is good. Conclusions from the semi-arid site are: (1) carbonate precipitation is constrained to spring and early summer, (2) rhizolith stable isotopic composition is influenced by the geochemistry of the springs near their formation sites, (3) rhizolith formation occurs initially as a carbonate sheath that is then infilled with secondary micritic material, (4) a large range in oxygen isotope values results from extensive evaporation at this site, but the carbon isotope range is not yet fully understood, and (5) preservation potential of rhizoliths at this semi-arid site is poor. These conclusions shed light on the modern precipitation of calcite in hydrologic end-members.