The distribution and coordination of trace elements in Krithe ostracods and their implications for paleothermometry

Branson, O. and Redfern, S. A. T. and Elmore, A. C. and Read, E. and Valencia, S. and Elderfield, H. (2018) The distribution and coordination of trace elements in Krithe ostracods and their implications for paleothermometry. Geochimica Et Cosmochimica Acta, 236. pp. 230-239. ISSN 0016-7037 DOI https://doi.org/10.1016/j.gca.2017.12.005

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Official URL: https://doi.org/10.1016/j.gca.2017.12.005

Abstract

The Mg and Sr content of ostracod valves have been used to reconstruct past temperature and salinity, and their stable isotopes have been used to reveal aspects of marine, lake and estuary hydrology. However, significant uncertainties surround ostracod calcification processes, the incorporation mechanisms of trace elements, and the sensitivity of proxy tracers to complex confounding factors. The valves of most ostracods are composed of micron-scale crystalline grains embedded in an organic matrix. The fine-scale geochemistry of these structures, and the nature of the influence of biological mineralisation processes on valve chemistry, remain poorly constrained. We have performed sub-micron resolution X-ray microscopy of a marine Krithe ostracod valve, and determined the chemical coordination of Mg, and the distribution of Mg, Na and S throughout the crystal-organic valve structure. These trace elements display systematic sub-micron-scale compositional variations within the mineral grains and inter-granular matrix of the valve ultrastructure. These patterns imply that Krithe biomineralisation processes significantly modulate trace element incorporation at the sub-micron scale. Thus Krithe chemical composition is likely to be decoupled to some extent from the water in which they calcified. Most importantly, Mg K-edge Near-Edge X-Ray Absorption Fine Structure (NEXAFS) spectra, and the coincidence of high-Mg regions with S-rich organic layers reveal that Mg is not primarily hosted in the calcite structure in the valve. Our results highlight the need to understand the processes that drive this fine-scale chemical heterogeneity and their influence on connections between the external environment and valve geochemistry, if ostracods are to be used as sources of paleoenvironmental proxies.

Item Type: Article
Uncontrolled Keywords: 2017AREP;
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
08 - Green Open Access
12 - PhD
Journal or Publication Title: Geochimica Et Cosmochimica Acta
Volume: 236
Page Range: pp. 230-239
Identification Number: https://doi.org/10.1016/j.gca.2017.12.005
Depositing User: Sarah Humbert
Date Deposited: 13 Dec 2019 17:49
Last Modified: 13 Dec 2019 17:49
URI: http://eprints.esc.cam.ac.uk/id/eprint/4594

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