Evaluating the Ca isotope proxy

The use of Ca isotopes as a proxy for mass flux imbalances in the Ca cycle is evaluated critically. A compiled Ca isotope record for the last 45 Ma, derived from bulk nannofossil ooze and with a temporal resolution of ∼0.5 Ma, and an interpretation of the record are presented in the context of the global Ca cycle. This analysis, which assumes that nannofossil ooze records isotopic variations in seawater, indicates a dynamic Ca cycle in the Cenozoic. Such dynamic behavior has serious implications for the C cycle and suggests feedbacks between the Ca and C cycles to stabilize, or buffer, the oceanic carbon reservoir. Alternative applications of the Ca isotope proxy are investigated, using numerical models to determine the extent to which Ca isotopes are sensitive to other aspects of the Ca cycle; the results of the simulations are applied to specific cases in the Cenozoic. The simulations illustrate how variations in the global fractionation factor between calcium carbonate and seawater can produce trends similar to those observed when comparing previously published Ca isotopic compositions of marine barite to the nannofossil ooze record. The large drop in the δ⁴⁴Ca value of bulk nannofossil ooze near the Eocene-Oligocene boundary can be reconciled in two ways, either as a substantial increase in weathering relative to sedimentation or as an indicator of changing depositional mode within the ocean. Though the preferred interpretation is not clear at present, it is evident that Ca isotopes stand to be a unique proxy for Ca cycling once the isotope systematics are elucidated.