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, δ44/40Ca, and δ18O 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 ∼3°C at habitat depth. In these samples, Mg/Ca and δ44/40Ca values are positively correlated (Mg/Ca (mmol/mol) = 0.77 (±0.22) × δ44/40Ca (‰ SRM 915a) + 0.52 (±0.12); n = 20, R2 = 0.76). Both Mg/Ca‐ and δ44/40Ca‐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 (σt) 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, δ44/40Ca, and δ18O 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 (°C) + 0.35 (±0.17); T > 3°C. In core top samples from polar waters with peak summer temperatures below ∼3°C, the temperature response in the Mg/Ca and δ44/40Ca proxy signal is inversed and poorly correlated. Both Mg/Ca‐ and δ44/40Ca‐derived temperature estimates pretend significantly higher calcification temperatures than maximum summer sea surface temperatures of these water masses.