Mg/Ca-temperature and seawater-test chemistry relationships in the shallow-dwelling large benthic foraminifera Operculina ammonoides

Evans, David, Erez, Jonathan, Oron, Shai and Müller, Wolfgang (2015) Mg/Ca-temperature and seawater-test chemistry relationships in the shallow-dwelling large benthic foraminifera Operculina ammonoides. Geochimica et Cosmochimica Acta, 148, 325-342. (doi:10.1016/j.gca.2014.09.039).

Abstract

The foraminifera Mg/Ca palaeothermometer contributes significantly to our understanding of palaeoceanic temperature
variation. However, since seawater Mg/Ca has undergone large secular variation and the relationship between seawater
and test Mg/Ca has not been calibrated in detail for any species with a substantial fossil record, it is only possible to assess
relative temperature changes in pre-Pleistocene fossil samples. In order to establish the basis of accurate quantitative
Mg/Ca-derived deep-time temperature reconstructions, we have calibrated the relationship between test Mg/Ca, seawater
chemistry and temperature in laboratory cultures of the shallow-dwelling large benthic species Operculina ammonoides.
Operculina has a fossil range extending back to the early Paleogene and is the nearest living relative of the abundant genus
Nummulites. We find a temperature sensitivity of 1.7% C^-1 and a linear relationship between the Mg distribution coefficient
and seawater Mg/Ca (Mg=Ca_sw) with m = -1.9 x 10^-3, within error of the equivalent slope for inorganic calcite. The higher
test Mg/Ca of O. ammonoides compared to inorganic calcite may be explained by an elevated pH of the calcifying fluid, implying that these foraminifera do not modify the Mg/Ca ratio of the seawater from which they calcify, differentiating them in this
respect from most other perforate foraminifera. Applying these calibrations to previously published fossil data results in
palaeo-Mg=Ca_sw reconstruction consistent with independent proxy evidence. Furthermore, our data enable accurate absolute
palaeotemperature reconstructions if Mg=Ca_sw is constrained by another technique (e.g. ridge flank vein carbonate; fluid
inclusions). Finally, we examine Li, Na, Sr and Ba incorporation into the test of O. ammonoides and discuss the control
exerted by temperature, seawater chemistry, saturation state and growth rate on these emerging proxies.

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Contributors

Author: David Evans

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