Abstract

Abstract. The geochemical composition of deep-sea benthic foraminiferal calcite is widely used to reconstruct sea floor paleoenvironments. The calibration of the applied proxy methods has until now been based on field observations in complex natural ecosystems where multiple factors are interfering. However, laboratory experiments with stable physico-chemical conditions appear to be the ideal way to evaluate the influence of a single parameter. In this paper, we present the oxygen isotopic composition of deep-sea benthic foraminiferal shells entirely calcified under controlled experimental conditions over a large temperature range (4 to 19 °C). The new laboratory protocols developed for this study allowed us to produce large quantities of shells in stable conditions, so that also the shell size effect could be investigated. It appears that when considering a narrow test size range, the curve describing the temperature dependency of δ18O in Bulimina marginata is parallel to the thermodynamically determined curve observed in inorganically precipitated calcite (−0.22‰ °C−1). This observation validates the use of δ18O of this benthic species in paleoceanographical studies. Over the studied size range (50 to 300 μm), the effect of test size was 0.0014‰ μm−1, confirming previous suggestions of a substantial test size effect on δ18O of benthic foraminifera. This study opens new perspectives for future proxy calibrations in laboratory set-ups with deep-sea benthic foraminifera (e.g. quantification of the influence of the carbonate chemistry).

Highlights

  • Stable oxygen isotopes of carbonate microfossils are one of the most widely used tools in paleoceanography

  • In order to obtain the results presented in this paper, we developed new laboratory protocols to produce large quantities of Bulimina marginata shells under controlled and stable conditions and over a large range of temperatures (4–19 ◦C), making it possible to investigate the influence of temperature on the δ18O of deep-sea benthic foraminiferal calcite

  • Knowing that shell size may have an effect on isotope ratio in foraminifera (Spero and Lea, 1996; Bemis et al, 1998; Elderfield et al, 2002; Schmiedl et al, 2004), our data were treated according to four different size fractions to consider this possible effect on B. marginata: ≤150 μm, 150–200 μm, 200–250 μm and >250 μm

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Summary

Introduction

Stable oxygen isotopes of carbonate microfossils are one of the most widely used tools in paleoceanography. Temperature and the isotopic composition of the seawater influence the 18O/16O composition of foraminiferal calcite, and other factors, such as the carbonate ion effect (Spero et al, 1997; Zeebe, 1999; Rathmann and Kunhert, 2008), vital effects (Duplessy et al, 1970) and diagenetic processes may strongly influence the δ18O of carbonate microfossils. Since many of these factors co-vary in the natural environment, only culture experiments can precisely reveal the influence of a single parameter, such as temperature. The growth of deep-sea benthic foraminifera takes much longer

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