PH-CONTROLLED OXYGEN ISOTOPE VARIATIONS IN MIOCENE LACUSTRINE CALCITE

Spring-deposited calcite in the middle Miocene Barstow Formation consists of alternating bands of fibrous calcite and micrite. Fibrous bands are 300 to 400 microns thick, and micrite bands are 10 microns thick. These banded deposits formed around vents, constructing pipes up to 20 cm in diameter. Carbonate microsamples (11 to 30 µg) were drilled from a thin section using a fully automated Merchantek microdrill. Three samples (base, middle, and top) were taken from single bands of fibrous calcite, and three bands were sampled at several centimeter spacings. The d18O in the pipe ranged from –6.87 to –6.37 permil. Within each band, d18O increased 0.2 to 0.5 permil. within individual bands. External precision (1s) is better than + 0.08 permil. The variations within single growth bands of fibrous calcite indicate a cyclic mechanism that resulted in a ~0.5 permil decrease in d18O from the base to the tips of the fibers. The small overall variation in the pipe, the cyclicity, and the increase in d18O argue against temperature and nonequilibrium effects. Rather, the cyclic increase in bands is probably the result of change in pH. Because the bicarbonate ion is isotopically heavier than the carbonate ion, calcite shows oxygen isotopic depletion with increasing pH. A pH change from 7.8 to 9 can cause a decrease of 1.5 permil in calcite. Because there were no carbonate aquifers in the recharge area of the Barstow basin, spring water that entered the lake was likely more acidic than the lake water. As seasonal recharge of spring water began, the precipitation of calcite was dominated by the low-pH spring water and has the heaviest d18O value. As spring flow waned, the proportion of high-pH lake water in the mixture increased, causing the d18O of the calcite to decrease.