Description: Highlights • Neogloboquadrina pachyderma morphotypes can have different isotope compositions. • Isotopic offsets between morphotypes depend on environmental background conditions. • Thin-shelled Neogloboquadrina pachyderma are good recorders of near-surface salinity. • A strong freshwater event off NE Greenland at 12.7 ka may be related to deglaciation. Abstract We report on stable oxygen and carbon isotope data obtained on two different morphotypes of polar planktic foraminifers, i.e., fully encrusted and minor encrusted Neogloboquadrina pachyderma, from a sediment core taken on the NE Greenland continental margin. These morphotypes are supposed to live at different water depths of the upper water column in the area which today is strongly stratified, with a low-saline, cold-water layer at the surface. The paired isotopic data sets inform on temporal variations of past water salinity and temperature in the preferred water depth ranges of the investigated morphotypes and allow conclusions on the stratification of the upper water column. The radiocarbon-dated sediment core covers the time interval from 21 to 4 cal-ka, but the early part of the deglacial interval (18.5–12.7 cal-ka) is not represented, probably due to intense erosion by bottom currents. In sediments from the late last glacial maximum, oxygen isotope differences between thin-shelled and thick-shelled N. pachyderma are low and point at a weaker stratification with less freshwater than today near the surface. The carbon isotopes indicate a dense, perennial sea ice cover, very limited bioproduction, and the presence of a subsurface Atlantic Water layer. In the late deglaciation until ∼10.3 cal-ka, the stable isotope values of both analyzed morphotypes are considerably lower, with significantly stronger amplitudes in the record of thin-shelled specimens than later on. The high-amplitude record stems from a laminated sediment sequence whose older part was deposited within just a few decades. The data are evidence of a strong freshwater event in the research area that probably started before 12.7 ka and may have reduced sea surface salinities by 4–5 practical salinity units. As freshwater sources we discuss both the disintegration of NE Greenland shelf ice and export from the Arctic Ocean interior. The event may have contributed to the weakening of the Atlantic meridional overturning circulation during the Younger Dryas cold event. For the early and mid-Holocene (10-4 cal-ka), the isotope data suggest a structure of the upper water column similar to today, with a well-developed halocline separating low-saline near-surface waters from the underlying Atlantic Water layer. A seasonally disintegrated sea ice cover allowed for a considerable planktic bioproduction.

Description: The final phase of the closure of the Panamanian Gateway and the intensification of Northern Hemisphere Glaciation (NHG) both occurred during the Late Pliocene. Glacial–interglacial (G–IG) variations in sea level might, therefore, have had a significant impact on the remaining connections between the East Pacific and the Caribbean. Here, we present combined foraminiferal Mg/Ca and δ 18O measurements from Ocean Drilling Program (ODP) Site 1241 from the East Pacific and ODP Site 999 from the Caribbean. The studied time interval covers the first three major G–IG Marine Isotope Stages (MIS 95–100, ∼2.5 Ma∼2.5 Ma) after the intensification of NHG. Analyses were performed on the planktonic foraminifera Neogloboquadrina dutertrei and Globigerinoides sacculifer, representing water mass properties in the thermocline and the mixed-layer, respectively. Changes in sea water temperature, relative salinity, and water column stratification strongly suggest that the Panamanian Gateway temporarily closed during glacial MIS 98 and 100, as a result of changes in ice volume equivalent to a drop in sea level of 60–90 m. Reconstructed sea surface temperatures (SST) from G. sacculifer show a glacial decrease of 2.5 °C at Site 1241, but increases of up to 3 °C at Site 999 during glacial MIS 98 and 100 suggesting that the Panamanian Gateway closed during these glacial periods. The Mg/Ca-temperatures of N. dutertrei remain relatively stable in the East Pacific, but do show a 3 °C warming in the Caribbean at the onset of these glacial periods suggesting that the closing of the gateway also changed the water column stratification. We infer that the glacial closure of the gateway allowed the Western Atlantic Warm Pool to extend into the southern Caribbean, increasing SST (G. sacculifer) and deepening the thermocline (N. dutertrei). Additionally, ice volume appears to have become large enough during MIS 100 to survive the relatively short lasting interglacial MIS 99 so that the gateway remained closed. Towards the end of MIS 98, during MIS 97 and into MIS 96 temperatures on both sides are mostly similar suggesting water masses exchanged again. Additionally, Caribbean variations in SST and δ18Owater follow a precession-like cyclicity rather than the obliquity-controlled variations characteristic of the East Pacific and many other tropical areas, suggesting that regional atmospheric processes related to the trade winds and the InterTropical Convergence Zone (ITCZ) had a dominant impact in the Caribbean.

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.