Calcium isotopes in the global biogeochemical Ca cycle: Implications for development of a Ca isotope proxy

In the marine realm, we evaluate the effect of a variable fractionation factor accompanying global removal of Ca from the ocean on seawater δ44Ca and suggest methods by which such a mechanism can be recognized in the rock record. Experimental data suggest that there is considerable leverage (< 1‰) in the fractionation factor to change seawater δ44Ca; the simulations presented demonstrate that when changes in the global fractionation factor drive seawater δ44Ca variability, the isotopic composition of the output flux is not representative of seawater δ44Ca evolution. This behavior is distinct from seawater δ44Ca variability driven by the δ44Ca of the weathering flux and by Ca mass flux imbalances into and out of the ocean. Thus, the successful application of a Ca isotope proxy for reconstructing seawater δ44Ca requires the measurement of at least two distinct phases, a “passive” tracer to constrain seawater δ44Ca and a tracer that characterizes the δ44Ca of the output flux. This requires robust and well understood mineral proxy archives, the study of which should be a high priority focus of future research.