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

• Published in: Geophysical research letters Vol. 47; no. 21
• Main Authors: Ingalls, Miquela, Blättler, Clara L., Higgins, John A., Magyar, John S., Eiler, John M., Fischer, Woodward W.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16-11-2020
• Subjects: Alkalinity; Aqueous environments; Atmospheric chemistry; Ca isotopes; Calcium; Calcium carbonate; Calcium carbonates; Calcium ions; Calcium isotopes; Calcium phosphates; Carbonate minerals; Carbonate rocks; Carbonates; Chemical evolution; closed‐basin lakes; clumped isotopes; Geochemistry; Groundwater; Groundwater sources; Isotopes; Lake bottom; Lakes; Mineralization; Minerals; Ocean floor; Oceans; Phosphate; Phosphates; Pleistocene; Seawater; Sedimentary facies; Shorelines; Stratigraphy
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL088804

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 δ44/40Ca variation than theoretically predicted (>0.75‰) 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 δ44/40Ca 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.
Plain Language Summary
Carbonate minerals record information about the local environments in which they form, for example, along the margins of a lake or on the seabed, as well as the aqueous and atmospheric chemistry at the time of mineralization (e.g., pCO2 and pH). This information is recorded in both the chemical signatures and textures carbonate rocks acquire during precipitation. Formation of calcium carbonate requires both a source of calcium (Ca2+) and carbonate (CO32− or carbonate alkalinity [ALK]). The ratio of calcium to alkalinity (Ca:ALK) in lake or ocean water influences the rate of carbonate formation, the textures that carbonate will develop, and the chemical signatures recorded by the carbonate mineral. In this study, we tested several approaches to identify very low Ca:ALK chemistry in modern and ancient lakes. We found that water with extremely low levels of calcium and high levels of alkalinity forms carbonates with a characteristic “tufa” texture in stratigraphic positions tied to riverine and groundwater sources of Ca2+. Moreover, the phosphate concentrations in these rocks were orders of magnitude higher than carbonates that precipitate under the high Ca:ALK conditions of modern oceans. Together, the results illustrated that identification of carbonate tufa textures in the rock record and phosphate measurements of carbonate rocks can be used to study the ancient environments and mechanisms by which carbonate rocks formed.
Key Points
Calcium‐to‐alkalinity ratios (Ca:ALK) in water determine rates and mechanisms of carbonate salt formation, with shoreline tufa facies as a useful indicator of low Ca:ALK aqueous chemistry in the rock record
Calcium isotope proxy for Ca:ALK in these materials is complicated by rate and mineralogical impacts on carbonate‐water fractionation
Carbonate P/Ca provides another means to assess Ca:ALK via the phosphate content of ancient aqueous environments