Revealing Their True Stripes: Mg/Ca Banding in the Paleogene Planktonic Foraminifera Genus Morozovella and Implications for Paleothermometry.

The Mg/Ca ratio of foraminiferal calcite is a widely used empirical proxy for ocean temperature. Foraminiferal Mg/Ca‐temperature relationships are based on extant species and are species‐specific, introducing uncertainty when applying them to the fossil tests of extinct groups. Many modern species show remarkable heterogeneity in their intra‐test Mg distributions, typically due to the presence of high Mg bands, which have a biological origin. Importantly, banding patterns differ between species, which could affect Mg/Ca‐temperature relationships. Few studies have looked at intra‐test variability in Mg/Ca ratios in extinct species of foraminifera, despite the obvious implications for paleothermometry. We used electron probe microanalysis (EPMA) to investigate intra‐test Mg distributions in the fossil tests of two species of planktonic foraminifera from the extinct muricate mixed‐layer‐dwelling genus Morozovella, commonly used in Paleogene sea surface temperature reconstructions. Both M. aragonensis and M. crater show striking Mg banding patterns with multiple high and low Mg/Ca band pairs throughout the test wall in all chambers. The intra‐test Mg variability in M. aragonensis and M. crater is similar to that in modern species widely used in paleoclimate reconstructions and banding patterns are consistent with published growth models for modern forms, albeit with subtle differences. The presence of Mg bands supports the application of Mg/Ca‐palaeothermometry in extinct Morozovella species as well as the utility of EPMA for examining preservation of foraminifera tests in paleoclimatological studies. However, we emphasize the importance of rigorous assessments of inter‐ and intra‐test Mg variability when using microanalytical techniques for foraminiferal Mg/Ca paleothermometry. Plain Language Summary: It is widely accepted that the incorporation of Mg into the calcium carbonate shells ("tests") of foraminifera (single‐celled marine organisms) is dependent on temperature. As such, measurement of Mg/Ca ratios in fossil tests from deep‐sea sediments has become an important working tool for reconstructing past seawater temperatures. However, Mg/Ca‐temperature relationships are based on modern foraminifera and are species‐specific, which introduces uncertainty when we apply them to extinct groups. Furthermore, modern foraminifera tend to have distinct high Mg bands in their test walls and these bands are biologically controlled, with differences in banding patterns between species. Few studies have compared banding patterns in extinct foraminifera species with those in modern forms, despite the potential implications for the accuracy and precision of Mg/Ca paleotemperature reconstructions. We used electron probe microanalysis to investigate the distribution of Mg within the well‐preserved fossil tests of two species belonging to the extinct mixed layer dwelling genus, Morozovella, which is commonly used in Paleogene (∼66–23 million years ago) ocean temperature reconstructions. Our results show striking Mg banding patterns in M. aragonensis and M. crater similar to those described in modern species. Overall, our data support the use of these extinct species in Mg/Ca paleothermometry. Key Points: Alternating bands of high and low Mg/Ca calcite are found throughout the tests of Morozovella aragonensis and M. craterIntra‐test Mg distributions in these species are similar to those in modern species upon which Mg/Ca‐temperature calibrations are basedOur results support the use of EPMA in evaluating the preservation of fossil foraminifera [ABSTRACT FROM AUTHOR]