P/Ca in Carbonates as a Proxy for Alkalinity and Phosphate Levels

Understanding mechanisms, rates, and drivers of past carbonate formation provides insight into the chemical evolution of Earth's oceans and atmosphere. We paired geological observations with elemental and isotope geochemistry to test potential proxies for calcium-to-alkalinity ratios (Ca:ALK). Across diverse carbonate facies from Pleistocene closed-basin lakes in Owens Valley, CA, we observed less delta Ca-44/40 variation than theoretically predicted (>0.75 parts per thousand) for the very low Ca:ALK in these systems. Carbonate clumped isotope disequilibria implied rapid carbonate growth-kinetic isotope effects, combined with the diverse carbonate minerals present, complicated the interpretation of delta Ca-44/40 as a paleoalkalinity proxy. In contrast, we observed that the high phosphate concentrations are recorded by shoreline and lake bottom carbonates formed in 11 Pleistocene lakes at orders of magnitude greater concentrations than in marine carbonates. Because the maximum phosphate content of water depends on Ca:ALK, we propose that carbonate P/Ca can inform phosphate levels and thereby Ca:ALK of aqueous environments in the carbonate record.