The data compilation suggests that: (1) there is a significant difference between carbonate (0.60鈥? and silicate 未44Ca (0.94鈥?; (2) riverine 未44Ca (0.88鈥? does not simply reflect the compiled carbonate 未44Ca; and (3) terrestrial vegetation exhibits the largest range of Ca isotopic compositions ~ 3.5鈥?in the terrestrial setting. We discuss these observations in the context of the global Ca cycle, exploring the extent to which seawater 未44Ca variability is feasible and how we can achieve accurate reconstructions of seawater 未44Ca over geologic time scales.
The current study presents simple mass balance models that quantify the leverage of inputs to change the Ca isotopic composition of the ocean, as this directly impacts the manner in which Ca isotopes are interpreted. Although Ca fractionates isotopically in the modern system during continental cycling, the 未44Ca range of riverine inputs to the ocean is considerably smaller than the variability observed in putative seawater proxies such as nannofossil ooze and marine barite. In the terrestrial realm, plants exhibit a wide 未44Ca range and there is evidence that Ca fluxes via biomass degradation are significant at the catchment scale. We therefore assess the ability of the continental biosphere to influence riverine, and consequently seawater, 未44Ca. A steady state biosphere has little leverage to alter riverine 未44Ca, except in cases where the 未44Ca of the recycling flux is isotopically distinct from the 未44Ca of the uptake flux. A non-steady state biosphere can substantially impact both soil and riverine 未44Ca, driving exchangeable Ca either heavier or lighter depending on the magnitude of the recycling flux relative to the uptake flux. Based on estimates of the size of the global biosphere (~ 1.5 路 1015 mol Ca), we suggest a decaying biosphere has the potential to impact riverine 未44Ca by tenths of a permil over time scales < 10 ka. At catchment scales, transient isotope effects related to biosphere cycling of Ca can be sizeable (order 1-2鈥? in soils, and variable over time, suggesting Ca as a useful tracer of biosphere dynamics.
In the marine realm, we evaluate the effect of a variable fractionation factor accompanying global removal of Ca from the ocean on seawater 未44Ca and suggest methods by which such a mechanism can be recognized in the rock record. Experimental data suggest that there is considerable leverage (< 1鈥? in the fractionation factor to change seawater 未44Ca; the simulations presented demonstrate that when changes in the global fractionation factor drive seawater 未44Ca variability, the isotopic composition of the output flux is not representative of seawater 未44Ca evolution. This behavior is distinct from seawater 未44Ca variability driven by the 未44Ca 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 未44Ca requires the measurement of at least two distinct phases, a 鈥減assive鈥?tracer to constrain seawater 未44Ca and a tracer that characterizes the 未44Ca 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.