Benthic foraminiferal Mg/Ca-paleothermometry: a revised core-top calibration

Abstract

Core-top samples from different ocean basins have been analyzed to refine our current understanding of the sensitivity of benthic foraminiferal calcite magnesium/calcium (Mg/Ca) to bottom water temperatures (BWT). Benthic foraminifera collected from Hawaii, Little Bahama Bank, Sea of Okhotsk, Gulf of California, NE Atlantic, Ceara Rise, Sierra Leone Rise, the Ontong Java Plateau, and the Southern Ocean covering a temperature range of 0.8 to 18°C were used to revise the Cibicidoides Mg/Ca-temperature calibration. The Mg/Ca–BWT relationship of three common Cibicidoides species is described by an exponential equation: Mg/Ca = 0.867 ± 0.049 exp (0.109 ± 0.007 × BWT) (stated errors are 95% CI). The temperature sensitivity is very similar to a previously published calibration. However, the revised calibration has a significantly different preexponential constant, resulting in different predicted absolute temperatures. We attribute this difference in the preexponential constant to an analytical issue of accuracy. Some genera, notably Uvigerina, show apparently lower temperature sensitivity than others, suggesting that the use of constant offsets to account for vital effects in Mg/Ca may not be appropriate. Downcore Mg/Ca reproducibility, as determined on replicate foraminiferal samples, is typically better than 0.1 mmol mol⁻¹ (2 S.E.). Thus, considering the errors associated with the Cibicidoides calibration and the downcore reproducibility, BWT may be estimated to within ±1°C. Application of the revised core-top Mg/Ca–BWT data to Cenozoic foraminiferal Mg/Ca suggests that seawater Mg/Ca was not more than 35% lower than today in the ice-free ocean at 50 Ma.

Introduction

Evaluation of past climate change is dependent on proxy records for physical and chemical conditions of Earth’s past. Many different proxies for marine and continental temperatures exist, including approaches as diverse as coral strontium/calcium thermometry, groundwater noble gas measurements, and leaf margin analysis Wolfe 1979, Beck et al 1992, Stute et al 1995. No single proxy used in isolation is ideal; this observation is exemplified by the uncertainties associated with perhaps the most reliable and established quantitative proxy for past global temperature change, the oxygen isotopic composition of benthic foraminifera Shackleton and Kennett 1975, Miller et al 1987. The main uncertainty associated with interpreting such records stems from the fact that the oxygen isotopic composition of foraminiferal calcite is dependent on both the temperature and also the oxygen isotopic composition of the seawater in which the foraminifera lived. The oxygen isotopic composition of seawater is largely determined by global ice volume, because the lighter isotope of oxygen is preferentially stored in ice sheets, although additional geographical variations in the oxygen isotopic composition of seawater associated with salinity variations also exist. A salinity-independent quantitative paleothermometer would therefore not only have the advantage of determining past ocean temperatures, but also global and local variations in seawater salinity.

The magnesium/calcium (Mg/Ca) of benthic foraminiferal calcite has been proposed as such a proxy for bottom water temperatures (BWT), and an empirical temperature calibration based on a study of surface sediments intersecting the subtropical thermocline has been constructed (Rosenthal et al., 1997). The Rosenthal et al. (1997) calibration was based on a 300 to 1600 m depth transect from the Little Bahama Bank, covering a temperature range of 4 to 18°C. Support for calcite Mg/Ca as a paleotemperature proxy is found in laboratory experiments Katz 1973, Burton and Walter 1991, Hartley and Mucci 1996 and empirical and culture studies of marine biogenic calcites such as ostracodes (Dwyer et al., 1995), corals (Mitsuguchi et al., 1996), and foraminifera Nürnberg et al 1996, Lea et al 1999, Toyofuku et al 2000. The temperature dependence of foraminiferal Mg/Ca probably involves a temperature effect on both the inorganic distribution coefficient, and also the physiologic processes that affect the uptake of Mg into the calcite test (Rosenthal et al., 1997). The published calibration has been used in both Quaternary and Cenozoic studies of climate change Lear et al 2000, Billups and Schrag, Martin et al, but is in need of refinement for several reasons. (1) The current calibration contains no Mg/Ca data for BWT less than 4°C, which reflects most of the modern deep ocean. (2) The calibration is based on only one benthic foraminiferal species from one locality. (3) There is significant scatter at the high temperature end of the published calibration, and it is not clear whether this is expected in all foraminifera living at these temperatures, or if it is caused by a process specific to the Little Bahama Bank. Reducing this scatter would improve the accuracy of temperature estimates made using the calibration. (4) The foraminifera analyzed for the published calibration were not prepared using the rigorous cleaning technique now commonly used by most laboratories (Boyle and Keigwin, 1985/86). (5) Subsequent analyses of core-top benthic foraminiferal Mg/Ca are not in agreement with the original calibration (Martin et al in press). For these reasons we have refined the existing calibration by studying numerous species from different ocean basins, covering a wider temperature range, and have prepared all samples using a rigorous cleaning technique, which limits contamination from clays, organic matter, and metal oxides.