Radiogenic ⁴⁰Ca in Seawater: Implications for Modern and Ancient Ca Cycles

Radiogenic ⁴⁰Ca is preferentially concentrated in the continental crust through the decay of radioactive ⁴⁰K and may have the potential to be used as a tracer for Ca fluxes to the ocean through time. Numerous published flux estimates suggest that rivers are the dominant source of Ca to the oceans. This conflicts, however, with conclusions drawn from previous radiogenic Ca data suggesting that seawater Ca has been dominated by weathering/hydrothermal alteration of oceanic crust throughout Earth history. We attempt to address this discrepancy by carrying out a larger number of radiogenic Ca measurements on materials that represent modern seawater, marine carbonates, and ocean floor basalt. We find that the ⁴⁰Ca/⁴⁴Ca composition of the oceanic crust and mantle appear to be different from modern seawater and marine carbonates, such that the latter are measurably enriched in radiogenic ⁴⁰Ca (ε_Ca = +1.1 ± 0.3, 2SE) relative to basalts and deep-sea hydrothermal fluid. This observation is consistent with most other data available in the literature. The results are also consistent with Sr isotope data and confirm that continental sources of Ca (mainly from rivers and groundwater) dominate the modern seawater budget. We find that off-axis Ca fluxes from the low temperature alteration of the oceanic crust are not large enough to change this balance. The Ca isotope data measured and compiled here also suggest that bulk-silicate earth ⁴⁰Ca/⁴⁴Ca is 1.2 ± 0.3 ε-units lower than reference material SRM915a and that variations in seawater ε_Ca in the geologic past are likely too small to be resolved with current analytical techniques.