The calcium isotopic composition of carbonate hardground cements: A new record of changes in ocean chemistry?

Erhardt, Andrea M. and Turchyn, Alexandra V. and Bradbury, Harold J. and Dickson, J. A. D. (2020) The calcium isotopic composition of carbonate hardground cements: A new record of changes in ocean chemistry? Chemical Geology, 540. p. 119490. ISSN 0009-2541 DOI https://doi.org/10.1016/j.chemgeo.2020.119490

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Abstract

Reconstructing changes in the calcium isotopic composition (δ44Ca) of the ocean over Earth history has been challenging. This difficulty is due to the large range of calcium isotope fractionation factors during mineral precipitation and the potential for overwriting the initial δ44Ca of minerals during shallow marine diagenesis. We present a new δ44Ca record measured in carbonate hardground cements, an inorganic carbonate-mineral precipitate that rapidly forms at or near the sediment-water interface. The range in the δ44Ca for any particular carbonate hardground cements is between 0.05 and 0.56‰. In some cases, the progressive increase in the δ44Ca during precipitation can be observed, consistent with precipitation in a ‘closed-system’. Our data show an average calcium isotope fractionation during carbonate hardground cement precipitation that is −0.57 ± 0.27‰, similar to the calcium isotope fractionation factor for inorganic calcite precipitates in previous laboratory and modelling studies, and closer to what is considered a kinetic end member calcium isotope fractionation than growth at equilibrium. This is consistent with the rapid carbonate mineral precipitation expected for carbonate hardground cements. Our δ44Ca record over the Phanerozoic is similar to other calcium-bearing mineral records over the same time interval, with average δ44Ca becoming lower going back in time by about 0.5 to 0.7‰. Our results add further support for the evolution of seawater δ44Ca over time, and we discuss the possible causes of these changes with suggestions for future studies.

Item Type:	Article
Uncontrolled Keywords:	2019AREP; IA76
Subjects:	01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions:	01 - Climate Change and Earth-Ocean Atmosphere Systems 08 - Green Open Access
Journal or Publication Title:	Chemical Geology
Volume:	540
Page Range:	p. 119490
Identification Number:	https://doi.org/10.1016/j.chemgeo.2020.119490
Depositing User:	Sarah Humbert
Date Deposited:	09 Apr 2020 16:57
Last Modified:	01 Mar 2021 18:38
URI:	http://eprints.esc.cam.ac.uk/id/eprint/4670

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