Taxon-specific epibenthic foraminiferal δ18O in the Arctic Ocean: realtionship to water mass, deep circulation, and brine formation

Abstract

We determined δ¹⁸O_Cib values of live (Rose Bengal stained) and dead epibenthic foraminifera<em> Cibicidoides wuellerstorfi, Cibicides lobatulus</em>, and <em>Cibicides refulgens </em>in surface sediment samples from the Arctic Ocean and the Greenland, Iceland, and Norwegian seas (Nordic Sea). This is the first time that a comprehensive δ¹⁸O_Cib data set is generated and compiled from the Arctic Ocean. For comparison, we defined Atlantic Water (AW), upper Arctic Bottom Water (uABW), and ABW by their temperature/salinity characteristics and calculated mean equilibrium calcite δ¹⁸O_equ from summer sea-water δ¹⁸O_w and <em>in situ</em> temperatures accordingly. As a result, in the Arctic environment we compensate for <em>Cibicidoides</em>- and <em>Cibicides</em>-specific offsets from equilibrium calcite of -0.35 and -0.55 ‰, respectively. After this taxon-specific adjustment, mean δ¹⁸O_Cib values plausibly reflect the density stratification of principle water masses in the Nordic Sea and Arctic Ocean. In addition, mean δ¹⁸O_Cib from AW not only significantly differs from mean δ¹⁸O_Cib from ABW, but also δ¹⁸O_Cib from within AW differentiates between provenience and water mass age. Furthermore, in shallow waters brine-derived low δ¹⁸O_w can significantly lower the δ¹⁸O_Cib of <em>Cibicides</em> spp. and thus δ¹⁸O_Cib may serve as a paleobrine indicator. There is no statistically significant difference, however, between deeper water masses mean δ¹⁸O_Cib of the Nordic Sea, and the Eurasian and Amerasian basins, and no influence of low-δ¹⁸O_w brines is recorded in Recent uABW and ABW δ¹⁸O_Cib of <em>C. wuellerstorfi.</em> This may be due to dilution of a low-δ¹⁸O_w brine signal in the deep sea, and/or to preferential incorporation of high-δ¹⁸O_w brines from high-salinity shelves. Although our data encompass environments with seasonal sea-ice and brine formation supposed to ultimately ventilate the deep Arctic Ocean, δ¹⁸O_Cib from uABW and ABW do not indicate negative excursions. This may challenge hypotheses that call for enhanced Arctic brine release to explain negative benthic δ¹⁸O spikes in deep-sea sediments from the late Pleistocene North Atlantic Ocean.