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

At the Earth's surface, calcium (Ca) is a critical element at a variety of scales: it is soluble in natural waters, a biological nutrient, and a major constituent of the dominant mineral sink for carbon in the ocean (CaCO3). There is a 4‰ variation in the Ca isotopic composition (44Ca/40Ca expressed as δ44Ca) of various Ca reservoirs on Earth, suggesting Ca isotopes as a promising tracer of Ca cycling in both the present and the past. Fifteen years of high precision Ca isotope measurements has revealed much about the behavior of Ca isotopes in the Earth surface environment, but there remain fundamental questions concerning how Ca isotopes are used to elucidate the marine and terrestrial Ca cycles. The current work presents a data compilation of over 70 published Ca isotope studies, totaling over 2600 measurements presented on a common delta scale, that includes data on rivers and groundwater, dust, soils and soil pore fluids, vegetation, rainwater, silicate minerals/rocks, and authigenic marine minerals (carbonates, sulfates, and phosphates, both modern and ancient). The data compilation suggests that: (1) there is a significant difference between carbonate (0.60‰) and silicate δ44Ca (0.94‰); (2) riverine δ44Ca (0.88‰) does not simply reflect the compiled carbonate δ44Ca; and (3) terrestrial vegetation exhibits the largest range of Ca isotopic compositions ~ 3.5‰ in the terrestrial setting. We discuss these observations in the context of the global Ca cycle, exploring the extent to which seawater δ44Ca variability is feasible and how we can achieve accurate reconstructions of seawater δ44Ca 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 δ44Ca 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 δ44Ca 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, δ44Ca. A steady state biosphere has little leverage to alter riverine δ44Ca, except in cases where the δ44Ca of the recycling flux is isotopically distinct from the δ44Ca of the uptake flux. A non-steady state biosphere can substantially impact both soil and riverine δ44Ca, 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 · 1015 mol Ca), we suggest a decaying biosphere has the potential to impact riverine δ44Ca 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‰) 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 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