Magnesian calcite solid solution thermodynamics inferred from authigenic deep-sea carbonate

Journal Article · 18 January 2019 · Geochimica et Cosmochimica Acta

DOI: https://doi.org/10.1016/j.gca.2019.01.006 · OSTI ID:1580380

Lammers, Laura Nielsen ^[1]; Mitnick, Elizabeth H. ^[2]

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1. Univ. of California, Berkeley, CA (United States); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
2. Univ. of California, Berkeley, CA (United States)

Magnesian calcite is perhaps the most well studied solid solution in the geosciences due to the widespread use of marine carbonates to reconstruct paleoenvironment. Despite decades of research, the low temperature thermodynamic properties of magnesian calcite in seawater are poorly constrained, largely because very slow reaction kinetics prevent the direct measurement of equilibrium distribution coefficients ($$K_d^{Mg}$$) for anhydrous Mg-bearing minerals. In this study, we use the Mg content of authigenic calcite formed in deep-sea marine carbonate sediments to determine the dependence of $$K_d^{Mg}$$ on temperature and aqueous Mg/Ca between ~2 and 25 °C. We find that the solid activity coefficient of magnesite in Mg-calcite is strongly temperature dependent in this range, leading to predicted exsolution of Mg at low temperatures. At the temperatures typical of ocean bottom water, equilibrium Mg distribution coefficients are at least an order of magnitude lower than values inferred from inorganic calcite growth experiments. Moreover, the equilibrium temperature dependence of $$K_d^{Mg}$$ agrees well with field-based paleotemperature calibrations determined for low-Mg benthic and planktonic foraminifera at temperatures <20 °C. Partitioning of Mg in foraminiferal tests is expected to be highly dependent on the Mg/Ca ratio in the calcifying fluid, so ignoring secular variations in seawater Mg/Ca can lead to significant underestimation of paleotemperatures.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1580380
• Journal Information: Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: C Vol. 248; ISSN 0016-7037
• Publisher: Elsevier; The Geochemical Society; The Meteoritical SocietyCopyright Statement
• Country of Publication: United States
• Language: English