Environmental versus biological controls on Mg/Ca variability in Globigerinoides ruber (white) from core top and plankton tow samples in the southwest Pacific Ocean

Abstract

[1] Laser ablation inductively coupled plasma mass spectrometry was used to analyze the individual chambers from tests of foraminiferal fossil and plankton tow Globigerinoides ruber from the southwest Pacific Ocean, from latitudes 3°S to 42°S. The variability of Mg/Ca between chambers of an individual (intraindividual) and individuals of the same population (interindividual), is such that when converted to temperature, the extent of intra-individual and interindividual variability appears to exceed that attributable to either calcification or seasonal temperature variability. The pooled mean chamber Mg/Ca from each core top and plankton tow site demonstrates a significant (p < 0.05) positive correlation with temperature. We derive chamber-specific calibrations where Mg/Ca Ch_F-2 = 0.798 exp 0.070 T , Mg/Ca Ch_F-1 = 0.891 exp 0.067 T and Mg/Ca Ch_F = 0.590 exp 0.072 T . We do not observe any bias between the two morphotypes Gs. ruber ruber and Gs. ruber pyramidalis. The chamber-specific calibrations potentially offset Mg/Ca-based temperature reconstructions if used on bulk (whole) test Mg/Ca or applied to misidentified chambers. Nevertheless, these calibrations can be used to reliably estimate sea surface temperature. Although there is a general overriding temperature control on Mg/Ca, we show that removal of the effect of temperature at each site reveals a lognormal Mg/Ca distribution. This suggests that Mg/Ca variability at each site is also affected by biological mechanism(s) that may control the distribution of interindividual Mg/Ca. In addition, other TE/Ca data (Al/Ca and Mn/Ca) from laser ablation trace element depth profiles can be used to identify detrital or diagenetic phases that may bias the trace element/Ca signal.

Evaluating controls on planktonic foraminiferal geochemistry in the Eastern Tropical North Pacific

“…5). Within each shell, Mg/Ca ratios generally decrease in subsequent chambers, consistent with previous studies (Anand and Elderfield, 2005;Bolton et al, 2011). There is an average 31% reduction in Mg/Ca observed between the penultimate and final chamber, nearly identical to what is seen in G. bulloides tests from the southwest Pacific Ocean (Marr et al, 2011).…”

“…the development and calibration of this proxy have come from laboratory culture, sediment traps, plankton tows and core-tops, and nearly all regions of the world's oceans. Consistent throughout all of these calibration studies is an exponential Mg/Ca ratio:temperature sensitivity of 8-10% change in ratio per • C (Anand et al, 2003;Bolton et al, 2011;Dekens et al, 2002;Lea et al, 1999;Martínez-Botí et al, 2011;Mashiotta et al, 1999;McConnell and Thunell, 2005;Regenberg et al, 2009). The influences of secondary factors such as salinity and carbonate chemistry are still being evaluated, and in many cases appear to be species -and/or regionspecific (Arbuszewski et al, 2010;Dekens et al, 2002;Evans et al, 2016;Hertzberg and Schmidt, 2013;Hönisch et al, 2013;Kısakürek et al, 2008;Russell et al, 2004).…”

“…4b, c). Based on the crystal structure, the calcite overgrowth appears to be inorganic (Bolton et al, 2011;Hathorne et al, 2009) and is thicker than the chamber calcite. This secondary layer is not present in the G. bulloides specimen with low Mg/Ca (Fig.…”

Calibration of the boron isotope proxy in the planktonic foraminifera Globigerinoides ruber for use in palaeo-CO2 reconstruction

“…In addition, material from sediment traps in the Cariaco Basin was used to further test for biases introduced during sedimentation. This array of sites covered as broad a range of in situ d 11 B borate as possible for regions inhabited by G. ruber, and serves to test for regional variations (as seen in Mg/Ca, Bolton et al, 2011) that might stem from genotype differences (Darling and Wade, 2008). The locations of these core-top and sediment trap sites are shown in Fig.…”

Encrustation and trace element composition of &lt;i&gt;Neogloboquadrina dutertrei&lt;/i&gt; assessed from single chamber analyses – implications for paleotemperature estimates

“…In both cases a different calibration is needed to convert individual layer Mg/Ca to temperature. Additionally, Bolton et al (2011) have shown that for G. ruber Mg/Ca temperature calibrations differ for individual chambers. The choice of a particular calibration can thus not be justified in the absence of independent determination of the depth of formation of both layers.…”