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

• Published in: Earth-Science Reviews
• Main Authors: Fantle, Matthew S., Tipper, Edward T.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: Calcium; Stable calcium isotopes; Global Ca cycle; Chemical weathering; Geochemical proxy; MC-ICP-MS; IONIZATION MASS-SPECTROMETRY; ION-EXCHANGE CHROMATOGRAPHY; HIGH-PRECISION MEASUREMENT; ANCIENT MARINE CARBONATES; BASE-POOR FOREST; PLANKTONIC-FORAMINIFERA; PORE-FLUID; EXPERIMENTAL IDENTIFICATION; CA-44/CA-40 FRACTIONATION; Planktonic foraminifera
• Online Access: https://research-portal.st-andrews.ac.uk/en/publications/7724e751-ca11-44b7-94d7-1d4883741a2c; https://doi.org/10.1016/j.earscirev.2013.10.004

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%0 variation in the Ca isotopic composition (44CarmCa expressed as Oita) 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 modem and ancient). The data compilation suggests that: (1) there is a significant difference between carbonate (0.60%0) and silicate 644Ca (0.94%0); (2) riverine 644Ca (0.88%0) does not simply reflect the compiled carbonate 644Ca; and (3) terrestrial vegetation exhibits the largest range of Ca isotopic compositions -3.5%0 in the terrestrial setting. We discuss these observations in the context of the global Ca cycle, exploring the extent to which seawater 644Ca variability is feasible and how we can achieve accurate reconstructions of seawater 644Ca 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 modem system during continental cycling, the 644Ca range of riverine inputs to the ocean is considerably smaller than the variability ...