Abstract
Abstract. Mg / Ca ratios in foraminiferal tests are routinely used as paleotemperature proxies, but on long timescales, they also hold the potential to reconstruct past seawater Mg / Ca. The impact of both temperature and seawater Mg / Ca on Mg incorporation in Foraminifera has been quantified by a number of studies. The underlying mechanism responsible for Mg incorporation in foraminiferal calcite and its sensitivity to environmental conditions, however, has not been fully identified. A recently published biomineralization model (Nehrke et al., 2013) proposes a combination of transmembrane transport and seawater leakage or vacuolization to link calcite Mg / Ca to seawater Mg / Ca and explains inter-species variability in Mg / Ca ratios. To test the assumptions of this model, we conducted a culture study in which seawater Mg / Ca was manipulated by varying [Ca2+] and keeping [Mg2+] constant. Foraminiferal growth rates, test thickness and calcite Mg / Ca of newly formed chambers were analyzed. Results showed optimum growth rates and test thickness at Mg / Ca closest to that of ambient seawater. Calcite Mg / Ca is positively correlated to seawater Mg / Ca, indicating that it is not absolute seawater [Ca2+] and [Mg2+] but their ratio that controls Mg / Ca in tests. These results demonstrate that the calcification process cannot be based only on seawater vacuolization, supporting the mixing model proposed by Nehrke et al. (2013). Here, however, we suggest transmembrane transport fractionation that is not as strong as suggested by Nehrke et al. (2013).
Highlights
Foraminiferal test Mg / CaCC is a proxy used in paleoceanography to reconstruct past seawater temperatures (e.g., Nürnberg et al, 1996; Lear et al, 2000)
Due to its relatively large size of > 1 mm, it is relatively easy to observe and handle. Because it was cost efficient and accessible, reef rubble with attached benthic Foraminifera was sampled in April 2012 from a coral reef aquarium at Burger’s Zoo, Arnhem, the Netherlands (Ernst et al, 2011)
The largest test size of 503 μm suggests that optimal growth conditions were attained at [Ca] = 17.9 mM and Mg / CaSW = 2.9, directly followed by the near-ambient control treatment at [Ca] = 9 mM and Mg / Ca = 5.7, with a final test size of 428 μm (Fig. 1b)
Summary
Foraminiferal test Mg / CaCC is a proxy used in paleoceanography to reconstruct past seawater temperatures (e.g., Nürnberg et al, 1996; Lear et al, 2000). Calcite Mg / CaCC is controlled by seawater Mg / CaSW (Segev and Erez, 2006; Evans and Müller, 2012). Biological processes involved in calcification complicate the relationships between Mg / CaCC, temperature and Mg / CaSW, which is apparent from large inter-species differences in Mg / Ca (Bentov and Erez, 2006). To improve the reliability of proxy relationships, it is necessary to understand the impact of cellular processes involved in calcification. E.g., Erez (2003), and Bentov et al (2009) suggested that foraminifers vacuolize seawater to acquire the ions needed for calcification. Seawater vacuolization would require the extraction of Ca2+ and CO23−
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