Timing and mechanism for intratest Mg/Ca variability in a living planktic foraminifer

Chamber formation leads to Mg/Ca banding in the planktonic foraminifer Neogloboquadrina pachyderma

“…We can therefore reasonably conclude that the Mg/Ca banding in N. pachyderma is also unlikely to be result of organic layering in the test wall. As such, consistent with most recent hypotheses on Mg/Ca banding (de Nooijer et al, 2014;Spero et al, 2015), we propose that such banding is intrinsic to the calcification mechanism of planktonic foraminifera. Below we discuss Mg/Ca heterogeneity in the lamellar and crust calcite, as well as the paleoceanographic implications of this intratest compositional variability.…”

“…universa the POM occurs in a broad band of low Mg/Ca (Eggins et al, 2004;Spero et al, 2015) and Sadekov et al (2005) showed that high Mg/Ca bands in Globorotalia truncatulinoides are not associated with embedded organic layers. We can therefore reasonably conclude that the Mg/Ca banding in N. pachyderma is also unlikely to be result of organic layering in the test wall.…”

“…Others have also shown that high Mg/Ca bands do not occur at the location of organic layers, rendering this explanation for intratest trace element banding unlikely (Eggins et al, 2004;Sadekov et al, 2005;Spero et al, 2015).…”

“…However, Mg/Catemperature relationships are species specific and remain (largely) empirical and many studies have shown a tight biological control on Mg incorporation in foraminiferal calcite (Anand et al, 2003;Bentov and Erez, 2006;de Nooijer et al, 2014;Jonkers et al, 2012;Jonkers et al, 2013a;Lea et al, 1999;Spero et al, 2015). In particular, the Mg distribution within single foraminifera tests shows large, non-temperature related variability (Eggins et al, 2003;Eggins et al, 2004;Kunioka et al, 2006;Sadekov et al, 2005).…”

“…Alternative hypotheses involve the participation of a tiny amount of seawater (which has high Mg concentrations) at the initiation of chamber formation, which would result in a high Mg band at the beginning (inner edge) of each lamina (de Nooijer et al, 2014), or an active role for mitochondria in Mg removal from the site of calcification, such that their reduced activity or density, possibly diurnally modulated, may cause high Mg bands (Spero et al, 2015). Nevertheless, the origin of Mg-banding is not yet fully understood and the suggested mechanisms are to a large extent based on observations from a single tropical and symbiont-bearing species O. universa, which forms a unique spherical outer chamber and hence differs from other species of planktonic foraminifera (Spero, 1988).…”