Abstract

Element incorporation in shell calcite precipitated by foraminifera reflects the chemical and physical properties of the seawater the foraminifera lived in and can therefore be used to reconstruct paleo environmental conditions. One of the most prominent proxies for past seawater temperature is Mg/Ca of foraminiferal calcite. Still, in addition to seawater temperature, also biomineralization processes impact foraminiferal Mg/Ca values. As the impact of biomineralization plays a major role and is not necessarily constant, it is imperative to identify the mechanism by which Mg is incorporated and thereby understand how temperature influences Mg incorporation. Biomineralization is discriminating against Mg to different degrees and hence investigating the fractionation of Mg isotopes at different temperatures and for species with contrasting calcification pathways can be used to better understand the pathway of Mg during biomineralization. Overall, we observe that foraminifera with higher Mg content have δ26Mg values closer to those of seawater. Moreover, controlled temperature culture experiments show that parallel to an increase in Mg/Ca, δ26Mg in the tests of large benthic foraminifer Amphistegina lessonii decreases when sea water temperatures increase. This negative correlation between shell Mg/Ca and δ26Mg suggests a two-step control on the incorporation of Mg during biomineralization. Using a simple model, we can explain both trends as a result of a stable Mg pool, which is only little fractionated with respect to sea water and a temperature dependent Mg pool which shows a higher fractionation with respect to sea water during biomineralization. The stable, not much fractionated pool is relatively large in high Mg foraminifera, whereas for the low Mg foraminifera the transport of Mg over a cell membrane probably results in the observed inverse correlation. Here we present a model using the Mg isotope fractionation we established for A. lessonii to explain the general trends for both high- and low-Mg/Ca foraminifera. A process-based understanding remains crucial a robust interpretation of foraminiferal Mg-isotopes.

Highlights

  • Past climates and environments can be reconstructed using foraminiferal shell chemistry

  • Magnesium Isotope Analyses at NIOZ Samples of Amphistegina lessonii from the temperaturecontrolled culture experiment were measured at the Royal NIOZ, using triplicates, each consisting of 60 μg foraminiferal carbonate

  • We show a distinct negative relation between foraminiferal δ26Mg and Mg/Ca for the hyaline species A. lessonii (Figure 2)

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Summary

INTRODUCTION

Past climates and environments can be reconstructed using foraminiferal shell chemistry. The largest fractionation has been found for species with a low Mg content (Wombacher et al, 2011), which may reflect the cellular mechanism that they employ to create a supersaturated fluid with considerably less Mg than that of seawater This observation supports the hypothesis that occasional transport of Mg ions by transmembrane calcium transport (Nehrke et al, 2013; de Nooijer et al, 2017) is an important source for the Mg that is incorporated into the foraminiferal shell wall of hyaline foraminifera, since crossmembrane ion transport favours lighter isotopes over heavy ones. To investigate the link between temperature and foraminiferal Mg/Ca and δ26Mg, we cultured the large benthic species Amphistegina lessonii at a range of temperatures (18–26°C) and investigated the relationship between δ26Mg and Mg/Ca in a range of benthic foraminiferal species

MATERIALS AND METHODS
24 M gsample 24 M gstandard
RESULTS
DISCUSSION
CONCLUSION
Findings
DATA AVAILABILITY STATEMENT
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