Ca Isotopes in Evaporite Minerals from Death Valley, California: No Evidence for Non-Biological Fractionation during Precipitation

It has been proposed that Ca isotope fractionation is produced mainly by biological processes, and hence that Ca isotope effects may be a tracer of biological activity in the geologic records of both Earth and Mars. However, Ca isotope fractionation has been produced for rapid precipitation of calcite and aragonite in laboratory experiments, and this fractionation is interpreted to be kinetic% by some investigators and equilibrium by others. Whether the experimental results apply to natural conditions is unknown. To investigate whether there is Ca isotope fractionation in natural inorganic processes, we have measured evaporite minerals precipitated in the Badwater salt pan, Death Valley, California. Death Valley is the hottest and driest desert in North America. Average summer temperatures are 37.8° C with a maximum record of 56.7° C and an annual average of 26° C. Potential evaporation is about 3.8~m/yr, which is 100 times average precipitation. A continuous 200,000-year record of closed-basin calcite and sulfate precipitation is available from a 186-meter sediment core into Badwater Basin. The δ18O and δD variations of fluid inclusion waters in halite are consistent with flooding-dissolution-evaporation cycles (Yang et al., 1995; 1997). The δ18O records of calcite and sulfate reflect the timing and driving forces of late Quaternary paleoclimatic changes (Yang et al., 1998; 2005). The longer-term (96,000, 39,000 and 21,000 years) fluctuations match Milankovitch orbital forcing, and are likely to be global in origin; the shorter-term (14,000 and 8,000 years) fluctuations probably reflect regional climatic and/or hydrologic forcing. Excursions in calcite δ18O are similar to those of δ18O in sulfate in the Death Valley core, and mimic those in marine carbonate (SPECMAP) and polar ice in the Summit ice core (GRIP), Greenland. Preliminary study of Ca isotopes in the calcite and sulfate minerals from the Badwater saline sediment core shows no significant variations through the core, regardless of the evaporation-dissolution-flooding cycles indicated by O and D isotopes, evaporative chemical evolution of the saline lake waters, mineralogy, depositional environment and glacial-interglacial climatic periods. The δ44Ca value of both carbonates and sulfates (-0.3‰~ relative to bulk Earth Ca or -1.2‰~ relative to seawater) is also nearly identical to that expected for dissolved Ca in local runoff. These results indicate that the Ca isotope fractionation factor associated with precipitation of both carbonate and sulfate under natural conditions is very close to unity, that is, α = 1.0000~ ± ~0.0001. This suggests that the fractionation factor of approximately 0.9985 observed for the formation of bone and shell in both vertebrates and invertebrates is not an equilibrium thermodynamic effect.