Journal
GEOCHIMICA ET COSMOCHIMICA ACTA
Volume 222, Issue -, Pages 535-549Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.gca.2017.11.011
Keywords
Calcium; Sr/Ca; Mg/Ca; Element partitioning; Distribution coefficients; Coral; Aragonite
Categories
Funding
- Sigma Xi
- Geological Society of America
- Stable Isotope Laboratory at the University of Miami
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The skeletal composition of calcifying organisms, in particular Mg/Ca and Sr/Ca ratios, have been widely used to understand fluctuations in seawater chemistry throughout the Phanerozoic. While the success of applying these data to the geologic record depends on a knowledge of the distribution coefficients for these elements (D-Mg and D-Sr), there are scarcely any studies which have described how these values vary as a result of changing seawater Mg/Ca ratios. To address this, we have cultured the scleractinian coral, Pocillopora damicornis, in seawater with ranges of Mg and Ca concentrations. Here, we demonstrate that Mg/Ca and Sr/Ca ratios of coral skeletons correlate with total seawater Mg/Ca and Sr/Ca molar ratios, but that apparent D-Mg and D-Sr values do not remain constant across the range of experimental seawater treatments, with D-Mg values significantly increasing with seawater Mg/Ca ratios and D-Sr values significantly increasing with seawater Ca concentrations. These trends are not rate dependent and may be best explained by a Rayleigh distillation model, in which the calcifying space is semi-isolated from seawater during skeletogenesis (i.e. leaky). As there is a slight increase in D-Mg and decrease in D-Sr values between our Jurassic and Modern seawater treatments, the application of a constant distribution coefficient to estimate changes in ancient seawater chemistry may underestimate seawater Mg/Ca ratios and overestimate Sr/Ca throughout the Mesozoic and Cenozoic. We suggest that interpretations of seawater chemistry from fossil corals may be improved by using the relationships derived for skeletal and seawater Mg/Ca and Sr/Ca ratios established by our experiments, as they incorporate the effect of seawater Mg/Ca ratios on skeletal Mg/Ca and Sr/Ca ratios. (C) 2017 Elsevier Ltd. All rights reserved.
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