Arctic Ocean benthic foraminifera Mg/Ca ratios and global Mg/Ca-temperature calibrations: New constraints at low temperatures

We explore the use of Mg/Ca ratios in six Arctic Ocean benthic foraminifera species as bottom water palaeothermometers and expand published Mg/Ca-temperature calibrations to the coldest bottom temperatures (< 1 degrees C). Foraminifera were analyzed in surface sediments at 27 sites in the Chukchi Sea, East Siberian Sea, Laptev Sea, Lomonosov Ridge and Petermann Fjord. The sites span water depths of 52-1157 m and bottom water temperatures (BWT) of -1.8 to +0.9 degrees C. Benthic foraminifera were alive at time of collection, determined from Rose Bengal (RB) staining. Three infaunal and three epifaunal species were abundant enough for Mg/Ca analysis. As predicted by theory and empirical evidence, cold water Arctic Ocean benthic species produce low Mg/Ca ratios, the exception being the porcelaneous species Quinqueloculina arctica. Our new data provide important constraints at the cold end (< 1 degrees C) when added to existing global datasets. The refined calibrations based on the new and published global data appear best supported for the infaunal species Nonionella labradorica (Mg/Ca = 1.325 +/- 0.01 x e(boolean AND)(0.065 +/- 0.01 x BWT), r(2) = 0.9), Cassidulina neoteretis (Mg/Ca = 1.009 +/- 0.02 x e(boolean AND)(0.042 +/- 0.01 x BWT), r(2) = 0.6) and Elphidium clavatum (Mg/Ca = 0.816 +/- 0.06 + 0.125 +/- 0.05 x BWT, r(2) = 0.4). The latter is based on the new Arctic data only. This suggests that Arctic Ocean infaunal taxa are suitable for capturing at least relative and probably semi-quantitative past changes in BWT. Arctic Oridorsalis tener Mg/Ca data are combined with existing O. umbonatus Mg/Ca data from well saturated core-tops from other regions to produce a temperature calibration with minimal influence of bottom water carbonate saturation state (Mg/Ca = 1.317 +/- 0.03 x e(boolean AND)(0.102 +/- 0.01 BWT), r(2) = 0.7). The same approach for Cibicidoides wuellerstorfi yields Mg/Ca = 1.043 +/- 0.03 x e(boolean AND)(0.118 +/- 0.1 BWT), r(2) = 0.4. Mg/Ca ratios of the porcelaneous epifaunal species Q. arctica show a clear positive relationship between Mg/Ca and Delta[CO32-] indicating that this species is not suitable for Mg/Ca-palaeothermometry at low temperatures, but may be useful in reconstructing carbonate system parameters through time. (C) 2018 The Authors. Published by Elsevier Ltd.