Reassessing Mg/Ca temperature calibrations of Neogloboquadrina pachyderma (sinistral) using paired δ44/40Ca and Mg/Ca measurements

The Mg/Ca temperature calibration of the polar to subpolar planktonic foraminifera Neogloboquadrina pachyderma (sinistral) (sinistral indicates left coiling) was refined by a multiproxy approach combining hydrographic temperature and salinity data with Mg/Ca, delta Ca-44/40, and delta O-18 values from Holocene Nordic seas core top samples. Reliable Mg/Ca-based temperature estimates are limited to foraminiferal tests that calcified in water masses with temperatures above similar to 3 degrees C at habitat depth. In these samples, Mg/Ca and delta Ca-44/40 values are positively correlated (Mg/Ca (mmol/mol) = 0.77 (+/- 0.22) x delta Ca-44/40 (parts per thousand SRM 915a) + 0.52 (+/- 0.12); n = 20, R-2 = 0.76). Both Mg/Ca- and delta Ca-44/40-derived temperatures projected onto their corresponding depth intervals reveal that the apparent'' calcification depth of N. pachyderma (sinistral) averaging the specimens' whole life cycle is bound to an isopycnal layer defined by water densities (sigma(t)) between 27.7 and 27.8. This implies that N. pachyderma (sinistral) prefers gradually deeper habitats with increasing sea surface temperatures, thus counterbalancing absolute temperature variations. Consequently, the total temperature range recorded in this foraminiferal species is restricted and only partly reflects environmental changes. On the basis of the new Mg/Ca, delta Ca-44/40, and delta O-18 multiproxy data set, we propose a linear Mg/Ca temperature relation for high-latitude N. pachyderma (sinistral): Mg/Ca (mmol/mol) = 0.13 (+/- 0.037) T (degrees C) + 0.35 (+/- 0.17); T > 3 degrees C. In core top samples from polar waters with peak summer temperatures below similar to 3 degrees C, the temperature response in the Mg/Ca and delta Ca-44/40 proxy signal is inversed and poorly correlated. Both Mg/Ca- and delta Ca-44/40-derived temperature estimates pretend significantly higher calcification temperatures than maximum summer sea surface temperatures of these water masses.