Calcium isotopes in scleractinian fossil corals since the Mesozoic: Implications for vital effects and biomineralization through time

- δ44/40Ca was measured in well preserved fossil corals and bulk pelagic carbonates.
- δ44/40Ca compositions of pelagic carbonates remain relatively constant since ∼80 Ma.
- δ44/40Ca compositions of fossil corals increase by ∼1‰ between ∼160 Ma and today.
- The fossil coral record may reflect a vital effect of calcification.
- Possible dependence of coral Ca isotope discrimination on seawater [Ca].

We present a Cenozoic record of δ44/40Ca from well preserved scleractinian fossil corals, as well as fossil coral δ44/40Ca data from two time periods during the Mesozoic (84 and 160 Ma). To complement the coral data, we also extend existing bulk pelagic carbonate records back to ∼80 Ma. The same fossil corals used for this study were previously shown to be excellently preserved, and to be faithful archives of past seawater Mg/Ca and Sr/Ca since ∼200 Ma (Gothmann et al., 2015). We find that the δ44/40Ca compositions of bulk pelagic carbonates from ODP Site 807 (Ontong Java Plateau) and DSDP Site 516 (Rio Grande Rise) have not varied by more than ∼±0.20‰ over the last ∼80 Myr. In contrast, the δ44/40Ca compositions of Mesozoic and Early Cenozoic fossil corals are ∼1‰ lighter than those of modern corals.The observed change in coral δ44/40Ca does not likely reflect secular variations in seawater δ44/40Ca. Instead, we propose that it reflects a vital effect of calcification - specifically, a sensitivity of coral Ca isotope discrimination to changing seawater [Ca] and/or pH. Support for this hypothesis comes from the presence of an empirical correlation between our coral δ44/40Ca record and records of seawater [Ca] and pH since the Mesozoic (Lowenstein et al., 2003; Hönisch et al., 2012). We explore various mechanisms that could give rise to such a vital effect, including: (1) changes in calcification rate, (2) changes in proton pumping in exchange for Ca2+, (3) variable Rayleigh distillation from an isolated calcifying fluid, and (4) changes in the calcium mass balance of the extracellular calcifying fluid (termed here the “leaky Ca model”). We test for the dependence of seawater δ44/40Ca on external seawater [Ca] by measuring the δ44/40Ca of cultured corals grown in seawater solutions with [Ca] ranging from 10 to 15 mmol/kg. Corals grown under elevated [Ca] conditions show a slight, ∼0.15‰ depletion of δ44/40Ca at higher seawater [Ca] - a supportive but not definitive result.