Exploring the implications of a dynamic Ca cycle in the Cenozoic

Recent work measuring the Ca isotopic composition of Cenozoic marine sediments has resulted in a variety of Ca isotope curves and related interpretations. Such isotope records may be interpreted as representing changes in the Ca isotopic composition of either seawater or the output flux (e.g., nannofossil ooze, foraminiferal tests, or barite). Variability in surface ocean temperature and/or the mode of carbonate deposition in the ocean, as they affect the fractionation factor between seawater and the primary sedimentation flux, have been suggested as causes of change. Within the context of the global Ca cycle, the levers that drive change are the isotopic composition of the weathering flux and the balance between the Ca input and output fluxes to and from the ocean. In many cases, it is quite reasonable to explain the observed variations in the Ca isotope record as being due to imbalances in the weathering (FW) and sedimentation (FSed) fluxes. The current work offers a critical analysis of the hypothesis that imbalances between the weathering and sedimentation fluxes affects the Ca isotopic of the ocean over million-year time scales. Employing numerical models of various complexities, this study addresses the major questions surrounding the interpretation of Ca isotopes as a relative weathering proxy: (1) can the Ca flux into the ocean due to weathering be significantly greater than or less than the sedimentation flux on long time scales, (2) do imbalances in the relative weathering flux (FW/FSed) result in reasonable changes in ocean alkalinity, and (3) over what time scales are changes in the fractionation factor visible in either a tracer proxy (e.g., barite) or a primary component of the output flux (e.g., nannofossil ooze)? These questions must be resolved if Ca isotopes are to be used as a robust proxy for past temperature and/or weathering regime. Simple box models coupling the Ca and C cycles show that FW/FSed decreases of 5 to 10% over tens of thousands of years can drive ocean alkalinity to zero. This is not a realistic scenario, requiring a more in-depth look at Ca cycling and the chemistry of the ocean. This study, therefore, evaluates existing Ca isotope records in the context of temperature change, hydrothermal exchange, and varying contributions to the terrestrial weathering flux.