Elsevier

Geochimica et Cosmochimica Acta

Volume 148, 1 January 2015, Pages 325-342
Geochimica et Cosmochimica Acta

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

https://doi.org/10.1016/j.gca.2014.09.039Get rights and content

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/Casw) with m = −1.9 × 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/Casw reconstruction consistent with independent proxy evidence. Furthermore, our data enable accurate absolute palaeotemperature reconstructions if Mg/Casw 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.

Introduction

The Mg/Ca thermometer is an established palaeoclimatic tool and provides one of the most accurate quantitative techniques in Pleistocene-Holocene ocean temperature reconstruction. Notwithstanding the wealth of information on the climate system gained from the such studies, many of the most interesting intervals with respect to understanding the controls on Earth system sensitivity lie further back in time (Haywood et al., 2011). Since the initial development of the foraminifera Mg/Ca temperature proxy (Nürnberg et al., 1996, Rosenthal et al., 1997), many more species have been investigated and it is now well known that modern foraminifera exhibit a wide range of Mg/Ca ratios that are controlled by calcification physiology as well as temperature (summarised in Bentov and Erez, 2006).

The Mg/Ca palaeothermometer has been applied throughout the Cenozoic (e.g. Lear et al., 2000), although it is now clear that there are fundamental complications with the use of this proxy deeper in geological time, on top of the so-called ‘vital effects’ which introduce unknown error when applying calibrations to extinct foraminifera. This is principally because the dependence of test Mg/Ca (Mg/Catest) on seawater Mg/Ca (Mg/Casw) is both non-linear and poorly known for all species abundant in the fossil record (see Evans and Müller, 2012, for an overview). Furthermore, the highest resolution Mg/Casw data available (Fantle and DePaolo, 2006) suggest a significant rise (∼2×) over the last 4 Ma, implying that even poorly-corrected or uncorrected Mg/Ca data from the Pliocene may result in inaccurate palaeotemperature estimates. In order for fossil foraminifera Mg/Ca data to yield accurate absolute temperature reconstructions, both a Mg/Ca-temperature and a Mg/Catest–Mg/Casw calibration is required, along with knowledge of Mg/Casw for the time of interest. As far as we are aware, this has not yet been achieved for any species. Here, we focus on Operculina ammonoides (Family: Nummulitidae), a species closely related to both Heterostegina depressa, for which a trace element study has been performed (Raitzsch et al., 2010), and the genus Nummulites (within the same sub-family) which were widespread throughout the Paleogene (sub)tropics to the extent that they are the principal component of some shallow water carbonates (e.g. Guido et al., 2011). Because of the abundance of the nummulitids in the fossil record they represent an under-utilised early-mid Cenozoic palaeoclimate archive.

Operculina are symbiont-bearing, shallow-dwelling benthic foraminifera with a peak abundance-depth range comparable to surface-dwelling planktic foraminifera (Evans et al., 2013, and references therein). The hyaline (glassy) appearance of the test is the result of the non-random orientation of the calcite crystals. Chambers are perforate and lamellar; calcite is mineralised each side of an organic matrix with the addition of a new layer to the entire outer test every time a new chamber is deposited (Reiss, 1958). Previous analyses of a number of fossil and recent non-cultured nummulitids have shown that the alkali earth metal distribution coefficients and their response to temperature and seawater chemistry variation are within error, therefore calibrations based on extant species can be applied to other species within this family in the fossil record (Evans et al., 2013). In order to facilitate comparison to previous work (Raitzsch et al., 2010, Evans and Müller, 2013, Evans et al., 2013) and because different parts of the test have subtly different X/Ca ratios, we focus our geochemical measurements on the marginal cord, the thickened test margin which plays an important reproductive and inter-chamber cytoplasm transport role.

In order to (1) investigate the controls on trace element incorporation in these LBF, (2) provide the basis of more accurate Mg/Ca-based deep-time (pre-Pleistocene) temperature reconstruction and (3) place constraints on the nummulitid biomineralisation mechanism, we present the first coupled temperature-seawater chemistry-test chemistry calibration for a foraminifera. Whilst we present spatially-resolved data for a suite of commonly analysed elements measured by laser-ablation ICPMS, we focus on the Mg/Ca ratio because of its potential for palaeoclimate reconstruction and the importance of understanding Mg incorporation for the assessment of biomineralisation models.

Section snippets

Culture

All culturing work was carried out at the Institute of Earth Sciences, The Hebrew University of Jerusalem. O. ammonoides were collected from the sediment surface from the northernmost Gulf of Eilat (north beach, Eilat) in May 2012 at a depth of 10–15 m. Water temperature at the time of collection was 22 °C. O. ammonoides were by far the most abundant organism in the sediment and were sampled from the 1.0–1.3 mm size fraction. Live foraminifera were identified as being those which climbed container

Results

Compositional data along with calculated physiological and carbonate chemistry parameters are shown in Table 2, Table 3.

Mg/Ca-derived palaeoreconstruction

Accurate pre-Pleistocene Mg/Ca palaeothermometry requires a good understanding of the relationship between Mg/Casw,Mg/Catest and temperature, as well as an independent estimate of seawater Mg/Ca for the time interval of interest (Evans and Müller, 2012). Given that the majority of proxy and model data show that seawater was characterised by lower Mg/Ca throughout almost all of the Cenozoic compared to the present day (e.g. Stanley and Hardie, 1998, Coggon et al., 2010), Mg/Ca data from fossil

Conclusion

We have performed laboratory calibrations on the shallow-dwelling large benthic foraminifera species O. ammonoides, principally in order to investigate the control exerted by temperature and seawater Mg/Ca on Mg incorporation in the calcite test. Based on laser-ablation ICPMS measurements at sub-chamber resolution facilitating unequivocal discrimination of calcite precipitated during culture, we find a Mg/Ca-temperature sensitivity of ∼1.7% °C−1, in good agreement with the field calibration of

Acknowledgements

DE acknowledges a NERC postgraduate studentship at Royal Holloway University of London. The authors acknowledge the Israel Science Foundation for funding the experimental part of this research (ISF grant 551/10 to J.E.). We are grateful to Simon Chenery and Tom Barlow (NERC Isotope Geosciences Laboratory) for performing the seawater analyses and for subsequent discussions of the data. We are indebted to the associate editor Yair Rosenthal and three anonymous reviewers for providing detailed and

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