Ostracode Mg/Ca paleothermometry in the North Atlantic and Arctic oceans: Evaluation of a carbonate ion effect

The reconstruction of deep‐sea bottom water temperature (BWT) is important to assess the ocean's response to and role in orbital‐ and millennial‐scale climate change. Deep‐sea paleothermometry employs magnesium to calcium (Mg/Ca) ratios in calcitic benthic microfaunas (foraminifera, ostracodes) as a primary proxy method. Mg/Ca paleothermometry may, however, be complicated by bottom water carbonate ion chemistry, which might affect Mg/Ca ratios in shells. To address temperature and carbonate ion influence on Mg/Ca ratios, we studied Mg/Ca ratios in the benthic ostracode genus Krithe in the North Atlantic and Arctic oceans using a 686‐specimen core top collection, including 412 previously unpublished analyses. Mg/Ca ratios are positively correlated to temperature in multiple species from the North Atlantic [BWT = (0.885 × Mg/Ca) − 5.69, r2 = 0.73] and for K. glacialis in the Arctic Ocean and Nordic Seas [BWT = (0.439 × Mg/Ca) − 5.14, r2 = 0.50], consistent with previously published calibrations. We found no evidence for a relationship between Krithe Mg/Ca and carbonate ion saturation in the North Atlantic Ocean, Nordic Seas, and Arctic Ocean, supporting the use of Krithe Mg/Ca for reconstructing past BWT. Key Points Krithe Mg/Ca positively relates to BWT in the North Atlantic and Arctic oceans Carbonate ion saturation state cannot be shown to relate to Krithe Mg/Ca Changes in carbonate ion saturation should not affect Krithe Mg/Ca BWT records