The calcium isotopic composition of carbonate hardground cements: A new record of changes in ocean chemistry?

Reconstructing changes in the calcium isotopic composition (δ⁴⁴Ca) of the ocean over Earth history has been challenging. This difficulty is due to the large range of calcium isotope fractionation factors during mineral precipitation and the potential for overwriting the initial δ⁴⁴Ca of minerals during shallow marine diagenesis. We present a new δ⁴⁴Ca record measured in carbonate hardground cements, an inorganic carbonate-mineral precipitate that rapidly forms at or near the sediment-water interface. The range in the δ⁴⁴Ca for any particular carbonate hardground cements is between 0.05 and 0.56‰. In some cases, the progressive increase in the δ⁴⁴Ca during precipitation can be observed, consistent with precipitation in a ‘closed-system’. Our data show an average calcium isotope fractionation during carbonate hardground cement precipitation that is −0.57 ± 0.27‰, similar to the calcium isotope fractionation factor for inorganic calcite precipitates in previous laboratory and modelling studies, and closer to what is considered a kinetic end member calcium isotope fractionation than growth at equilibrium. This is consistent with the rapid carbonate mineral precipitation expected for carbonate hardground cements. Our δ⁴⁴Ca record over the Phanerozoic is similar to other calcium-bearing mineral records over the same time interval, with average δ⁴⁴Ca becoming lower going back in time by about 0.5 to 0.7‰. Our results add further support for the evolution of seawater δ⁴⁴Ca over time, and we discuss the possible causes of these changes with suggestions for future studies.