Carbonate-bound phosphate and Ca isotope range as measures of cation availability and relative alkalinity

Calcium to alkalinity ratios (Ca:ALK) of ancient systems have implications for the evolution of oceanic and atmospheric chemistry, understanding why carbonates form when and where they do, and ultimately, the interplay between carbonate chemistry and microbial life in deep time . In this study, we first test the expression of Ca isotope range (Δ^44/40Ca) in carbonates as a proxy for Ca:ALK, as it was first proposed in a study of hyperalkaline, cation-limited, and microbially rich Mono Lake waters (Ca:ALK = 10^-5 [ seawater Ca:ALK = 10^-2] , and 100× greater cell counts than s eawater ). Carbonates precipitating under low Ca :ALK conditions are expected to record Δ^44/40Ca range > 0.75‰ due to Rayleigh distillation from extremely Ca deplete water , whereas carbonates precipitating from waters with moderate Ca:ALK ( i.e. modern marine) yield very limited Δ^44/40Ca. However, we do not recover the Δ^44/40Ca expected of Ca-limited Rayleigh distillation in lake bottom sediments, shoreline crusts, and tufa carbonate from Mono Lake nor Pleistocene Lakes Russell and Searles. Further, we hypothesize that much of the measured range can be attributed to (1) a kinetic isotope effect due to fast rates of tufa carbonate precipitation , supported by disequilibrium carbonate clumped isotope Δ₄₇ values, and (2) aragonite vs calcite carbonate-water fractionation . The use of Δ^44/40Ca as a Ca:ALK proxy in c arbonate environments requires precisely locating drawdown/distillation events not always accessible in the rock record.

However, we present carbonate-bound phosphate concentration ([P]_carb) as a new proxy for Ca:ALK in modern and ancient carbonate systems. Mono Lake's d issolved PO₄^3- concentration ([P]_water ~1000× seawater) is high due t o N limitation, evaporative enrichment , and inhibited mineralization due to cation scarcity. The latter two mechanisms also concentrate ALK, making [P] a possible metric for relative alkalinity in some systems . Further, [P]_carb from Mono Lake, Searles, and Eocene Green River Fm are 1- 3 orders of magnitude higher than initial marine [P]_carb measurements from the Archean to modern. [P]_carb may be a valuable tool for characterizing [Ca]:ALK in ancient environments, for deciphering lacustrine vs. marine settings , and understanding the interplay of microbial life and aqueous chemistry (i.e. ALK, [P]).