Description: Reliable estimates of bottom-water oxygen contents are crucial to understanding the formation of past oxygen-depleted environments. Here, we investigate the relationship between pore density in calcareous benthic foraminiferal tests and environmental factors like bottom-water oxygen and nitrate concentration, water depth, and temperature in living (Rose Bengal stained) specimens of the shallow-infaunal species Bolivina pacifica, and the two deep-infaunal species Fursenkoina mexicana, and Chilostomella oolina. Used samples span an oxygen-gradient (0.10 to 4.62 ml L−1) across oxygen minimum zones (OMZ) off Namibia and Pakistan. Bolivina pacifica and F. mexicana display an inverse correlation between pore density and in-situ bottom-water oxygen content (BW-O2), indicating a morphological response of the foraminifers to decreasing oxygenation. Supporting previous results, we suggest that both species may increase their pore numbers to improve the ability of oxygen uptake in low-oxygen environments. Comparison of the calculated pore densities for B. pacifica and F. mexicana with bottom-water nitrate concentration (BW-NO3−) and temperatures, however, illustrates that these factors might also influence the pore density. Our results for the deep-infaunal species C. oolina show no significant relationship between pore density and BW-O2. This suggests that C. oolina, rather than increasing its pore density, has another life-strategy to survive sustained low-oxic conditions, possibly nitrate respiration. The non-correlation between pore densities and BW-NO3−, however, suggests that pores are not involved in the denitrification process. According to our data we suggest that the pore density of some benthic foraminiferal species is controlled by BW-O2. This relation is, however, species-specific. Overall, our data suggest that this morphological response could provide the basis for an independent proxy for BW-O2 in future studies.

Description: We determined δ18OCib values of live (Rose Bengal stained) and dead epibenthic foraminifera Cibicidoides wuellerstorfi, Cibicides lobatulus, and Cibicides refulgens 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 δ18OCib data set is generated and compiled from the Arctic Ocean. For comparison, we defined Atlantic Water (AW), upper Arctic Bottom Water (uABW), and Arctic Bottom water (ABW) by their temperature/salinity characteristics and calculated mean equilibrium calcite δ18Oequ from summer sea-water δ18Ow and in situ temperatures. As a result, in the Arctic environment we compensate for Cibicidoides- and Cibicides-specific offsets from equilibrium calcite of − 0.35 and − 0.55 ‰, respectively. After this taxon-specific adjustment, mean δ18OCib values plausibly reflect the density stratification of principle water masses in the Nordic Sea and Arctic Ocean. In addition, mean δ18OCib from AW not only significantly differs from mean δ18OCib from ABW, but also δ18OCib from within AW differentiates in function of provenience and water mass age. Furthermore, in shallow waters brine-derived low δ18Ow can significantly lower the δ18OCib of Cibicides spp. and thus δ18OCib may serve as a paleobrine indicator. There is no statistically significant difference, however, between deeper water masses mean δ18OCib of the Nordic Sea, and of the Eurasian and Amerasian basins, and no influence of low-δ18Ow brines is recorded in Recent uABW and ABW δ18OCib of C. wuellerstorfi. This may be due to dilution of a low-δ18Ow brine signal in the deep sea, and/or to preferential incorporation of relatively high-δ18Ow 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, δ18OCib 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.