A 600-kyr reconstruction of deep Arctic seawater δ18O from benthic foraminiferal δ18O and ostracode Mg/Ca paleothermometry.

The oxygen isotopic composition of benthic foraminiferal tests (δ ¹⁸O_b) is one of the preeminent tools for correlating marine sediments and interpreting past terrestrial ice volume and deep-ocean temperatures. Despite the prevalence of δ ¹⁸O_b applications to marine sediment cores over the Quaternary, its use is limited in the Arctic Ocean because of low benthic foraminiferal abundances, challenges with constructing independent sediment core age models, and an apparent muted amplitude of Arctic δ ¹⁸O_b variability compared to open ocean records. Here we evaluate the controls on Arctic δ ¹⁸O_b by using ostracode Mg/Ca paleothermometry to generate a composite record of the δ ¹⁸O of seawater (δ ¹⁸O_sw) from fourteen sediment cores in the intermediate to deep Arctic Ocean (700–2700 m) covering the last 600 kyr. Results show that Arctic δ ¹⁸O_b was generally higher than open ocean δ ¹⁸O_b during interglacials but was generally equivalent to global reference records during glacial periods. The reduced glacial-interglacial Arctic δ ¹⁸O_b range resulted in part from the opposing effect of temperature, with intermediate-to-deep Arctic warming during glacials counteracting the whole-ocean δ ¹⁸O_sw increase from expanded terrestrial ice sheets. After removing the temperature effect from δ ¹⁸O_b, we find that the intermediate-to-deep Arctic experienced large (≥ 1 ‰) variations in local δ ¹⁸O_sw, with generally higher local δ ¹⁸O_sw during interglacials and lower δ ¹⁸O_sw during glacials. Both the magnitude and timing of low local δ ¹⁸O_sw intervals are inconsistent with the recent proposal of freshwater intervals in the Arctic Ocean during past glaciations. Instead, we suggest that lower local δ ¹⁸O_sw in the intermediate-to-deep Arctic Ocean during glaciations reflected weaker upper ocean stratification and more efficient transport of low- δ ¹⁸O_sw Arctic surface waters to depth by mixing and/or brine rejection. [ABSTRACT FROM AUTHOR]