Mg/Ca-temperature and seawater-test chemistry relationships in the shallow-dwelling large benthic foraminifera Operculina ammonoides
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 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
References (53)
- et al.
The effects of pCO2 and temperature on magnesium incorporation in calcite in seawater and MgCl2–CaCl2 solutions
Geochim. Cosmochim. Acta
(1991) - et al.
Li, Sr, Mg, and Na in foraminiferal calcite shells from laboratory culture, sediment traps, and sediment cores
Geochim. Cosmochim. Acta
(1985) - et al.
Eocene seasonality and seawater alkaline earth reconstruction using shallow-dwelling large benthic foraminifera
Earth Planet. Sci. Lett.
(2013) - et al.
Sr isotopes and pore fluid chemistry in carbonate sediment of the Ontong Java Plateau: calcite recrystallization rates and evidence for a rapid rise in seawater Mg over the last 10 million years
Geochim. Cosmochim. Acta
(2006) - et al.
Application of calcite Mg partitioning functions to the reconstruction of paleocean Mg/Ca
Geochim. Cosmochim. Acta
(2010) - et al.
Planktic foraminifers as recorders of seawater Ba/Ca
Marine Micropaleontol.
(2011) - et al.
Chemical evolution of seawater during the Phanerozoic: implications from the record of marine evaporites
Geochim. Cosmochim. Acta
(2002) - et al.
Uptake of sodium and potassium by calcite
Chem. Geol.
(1984) - et al.
Controls on shell Mg/Ca and Sr/Ca in cultured planktonic foraminiferan, Globigerinoides ruber (white)
Earth Planet. Sci. Lett.
(2008) - et al.
Experimental determination of barium uptake in shells of the planktonic foraminifera Orbulina universa at 22 °C
Geochim. Cosmochim. Acta
(1992)