Description: We provide high-resolution foraminiferal stable carbon isotope (d13C) records from the subarctic Pacific and Eastern Equatorial Pacific (EEP) to investigate circulation dynamics between the extra-tropical and tropical North Pacific during the past 60 kyr. We measured the d13C composition of the epibenthic foraminiferal species Cibicides lobatulus from a shallow sediment core recovered from the western Bering Sea (SO201-2-101KL; 58°52.52' N, 170°41.45' E; 630 m water depth) to reconstruct past ventilation changes close to the source region of Glacial North Pacific Intermediate Water (GNPIW). Information regarding glacial changes in the d13C of sub-thermocline water masses in the EEP is derived from the deep-dwelling planktonic foraminifera Globorotaloides hexagonus at ODP Site 1240 (00°01.31' N, 82°27.76' W; 2921 m water depth). Apparent similarities in the long-term evolution of d13C between GNPIW, intermediate waters in the eastern tropical North Pacific and sub-thermocline water masses in the EEP suggest the expansion of relatively 13C-depleted, nutrient-enriched, and northern-sourced intermediate waters to the equatorial Pacific under glacial conditions. Further, it appears that additional influence of GNPIW to the tropical Pacific is consistent with changes in nutrient distribution and biological productivity in surface-waters of the glacial EEP. Our findings highlight potential links between North Pacific mid-depth circulation changes, nutrient cycling, and biological productivity in the equatorial Pacific under glacial boundary conditions.

Description: Changes in heat transport associated with fluctuations in the strength of the Atlantic meridional overturning circulation (AMOC) are widely considered to affect the position of the Intertropical Convergence Zone (ITCZ), but the temporal immediacy of this teleconnection has to date not been resolved. Based on a high-resolution marine sediment sequence over the last deglaciation, we provide evidence for a synchronous and near-linear link between changes in the Atlantic interhemispheric sea surface temperature difference and continental precipitation over northeast Brazil. The tight coupling between AMOC strength, sea surface temperature difference, and precipitation changes over northeast Brazil unambiguously points to a rapid and proportional adjustment of the ITCZ location to past changes in the Atlantic meridional heat transport.

Description: The Eastern Equatorial Pacific (EEP) is a key area to understand past oceanic processes that control atmospheric CO2 concentrations. Many studies argue for higher nutrient concentrations by enhanced nutrient transfer via Southern Ocean Intermediate Water (SOIW) to the low-latitude Pacific during glacials. Recent studies, however, argue against SOIW as the primary nutrient source, at least during early Marine Isotope Stage 2 (MIS 2), as proxy-data indicate that nutrients are better utilized in the Southern Ocean under glacial conditions. New results from the subarctic Pacific suggest that enhanced convection of nutrient-rich Glacial North Pacific Intermediate Water (GNPIW) contributes to changes in nutrient concentrations in equatorial sub-thermocline water masses during MIS 2. However, the interplay between SOIW versus GNPIW and its influence on the nutrient distribution in the EEP spanning more than one glacial cycle are still not understood. We present a carbon isotope (d13C) record of sub-thermocline waters derived from deep-dwelling planktonic foraminifera Globorotaloides hexagonus in the EEP, which is compared with published d13C records around the Pacific. Results indicate enhanced influence of GNPIW during MIS 6 and MIS 2 compared to today with largest contributions of northern-sourced intermediate waters during glacial maxima. These observations suggest a mechanistic link between relative contributions of northern and southern intermediate waters and past EEP nutrient concentrations. A switch from increased GNPIW (decreased SOIW) to diminished GNPIW (enhanced SOIW) influence on equatorial sub-thermocline waters is recognized during glacial terminations and marks changes to modern-like conditions in nutrient concentrations and biological productivity in the EEP.

Description: Surface sediment samples taken with a multiple corer from the Polar Front region along two profiles in the eastern South Atlantic sector between 35° and 57°S have been investigated for their benthic foraminiferal content. Live fauna was differentiated from the death assemblage. Based on the dead assemblages, potential fossil assemblages (dead fauna reduced by non-resistant agglutinants) were calculated for comparison with upper Neogene core material. The distribution of the potential fossil assemblages defined by Q-mode principal component analysis mirrors the Recent hydrographic, as well as the trophic situation in the eastern South Atlantic (ocean circulation and productivity). Two Southern Component Bottom Water Faunas reflect the northward flow of Antarctic Bottom Water (AABW) and Lower Circum-Polar Deep Water (LCDW): an assemblage dominated by the arenaceous Multifidella nodulosa below the Calcite Compensation Depth (CCD), and a Nuttallides umbonifer dominated assemblage above the CCD. In the region of highest bio-silicious primary productivity, south of the Polar Front and north of the winter sea-ice limit, these faunas are overprinted by a Southern High-productivity Fauna, dominated by infaunal species such as Pullenia bulloides, Melonis pompilioides and M. zaandamae. North Atlantic Deep Water (NADW) meets the Circum-Polar Deep Water (CDW) at intermediate depth, as reflected in the distribution of the Northern Component Deep Water Fauna, dominated by Epistominella exigua. The high-productivity assemblage overprinting this fauna, the Northern High-productivity Fauna, is dominated by the shallow endobenthic Bulimina aculeata. A Strong Bottom-current Fauna, dominated by Angulogerina angulosa, characterizes areas between 400 and 900 m water depth with sandy sediment, independent from water mass characteristics and food supply.

Description: Negative stable carbon isotopic excursions have been observed throughout most of the mid-depth (~1000-3000m) Atlantic Ocean during Heinrich Stadial 1 (HS1) and the Younger Dryas (YD). Although there is agreement that these mid-depth excursions were in some way associated with a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), there is still no consensus on the precise mechanism(s). Here, we present benthic stable carbon and oxygen isotopic (d13C and d18O) records from five cores from the western equatorial Atlantic (WEA). Together with published benthic isotopic records from nearby cores, we produced a WEA depth transect (~800-2500m). We compare HS1 and YD data from this transect with data from previously published North- and South Atlantic cores and demonstrate that the largest negative d13C excursions occurred in the WEA during these times. Moreover, our benthic d18O records require the presence of two water masses flowing from the Southern Ocean, bisected by a Northern Component Water (NCW). Given that d18O is a conservative water mass tracer, we suggest that d13C was decoupled from water mass composition and do not correspond to simple alternations between northern and southern sourced waters. Instead, d13C behaved non-conservatively during HS1 and the YD. Consistently with our new 231Pa/230Th record from the WEA transect, that allowed the reconstruction of AMOC strength, we hypothesize that the negative d13C excursions reflect an increase in the residence time of NCW in response to a weakened AMOC, allowing for a marked accumulation of 13C-depleted respired carbon at the mid-depth WEA.

Description: Under modern conditions only North Pacific Intermediate Water is formed in the northwest Pacific Ocean. This situation might have changed in the past. Recent studies with general circulation models indicate a switch to deep-water formation in the northwest Pacific during Heinrich Stadial 1 (17.5-15.0 ka) of the last glacial termination. Reconstructions of past ventilation changes based on paleoceanographic proxy records are still insufficient to test whether a deglacial mode of deep-water formation in the North Pacific Ocean existed. Here we present deglacial ventilation records based on radiocarbon-derived ventilation ages in combination with epibenthic stable carbon isotopes from the northwest Pacific including the Okhotsk Sea and Bering Sea, the two potential source regions for past North Pacific ventilation changes. Evidence for most rigorous ventilation of the intermediate-depth North Pacific occurred during Heinrich Stadial 1 and the Younger Dryas, simultaneous to significant reductions in Atlantic Meridional Overturning Circulation. Concurrent changes in d13C and ventilation ages point to the Okhotsk Sea as driver of millennial-scale changes in North Pacific Intermediate Water ventilation during the last deglaciation. Our records additionally indicate that changes in the d13C intermediate-water (700-1750 m water depth) signature and radiocarbon-derived ventilation ages are in antiphase to those of the deep North Pacific Ocean (〉2100 m water depth) during the last glacial termination. Thus, intermediate- and deep-water masses of the northwest Pacific have a differing ventilation history during the last deglaciation.

Description: To investigate the use of benthic foraminifera as a means to document ancient methane release, we determined the stable isotopic composition of tests of live (Rose Bengal stained) and dead specimens of epibenthic Fontbotia wuellerstorfi, preferentially used in paleoceanographic reconstructions, and of endobenthic high-latitude Cassidulina neoteretis and Cassidulina reniforme from a cold methane-venting seep off northern Norway. We collected foraminiferal tests from three push cores and nine multiple cores obtained with a remotely operated vehicle and a video-guided multiple corer, respectively. All sampled sites except one control site are situated at the Håkon Mosby mud volcano (HMMV) on the Barents Sea continental slope in 1250 m water depth. At the HMMV in areas densely populated by pogonophoran tube worms, d13C values of cytoplasm-containing epibenthic F. wuellerstorfi are by up to 4.4 per mil lower than at control site, thus representing the lowest values hitherto reported for this species. Live C. neoteretis and C. reniforme reach d13C values of -7.5 and -5.5 per mil Vienna Pee Dee Belemnite (VPDB), respectively, whereas d13C values of their empty tests are higher by 4 per mil and 3 per mil. However, d13C values of empty tests are never lower than those of stained specimens, although they are still lower than empty tests from the control site. This indicates that authigenic calcite precipitates at or below the sediment surface are not significantly influencing the stable isotopic composition of foraminiferal shells. The comparatively high d13C results rather from upward convection of pore water and fluid mud during active methane venting phases at these sites. These processes mingle tests just recently calcified with older ones secreted at intermittent times of less or no methane discharge. Since cytoplasm-containing specimens of suspension feeder F. wuellerstorfi are almost exclusively found attached to pogonophores, which protrude up to 3 cm above the sediment, and d13C values of bottom-water-dissolved inorganic carbon (DIC) are not significantly depleted, we conclude that low test d13C values of F. wuellerstorfi are the result of incorporation of heavily 13C-depleted methanotrophic biomass that these specimens feed on rather than because of low bottom water d13CDIC. Alternatively, the pogonophores, which are rooted at depth in the upper sediment column, may serve as a conduit for depleted d13CDIC that ultimately influences the calcification process of F. wuellerstorfi attached to the pogonophoran tube well above the sediment/water interface. The lowest d13C of live specimens of the endobenthic C. neoteretis and C. reniforme are within the range of pore water d13CDIC values, which exceed those that could be due to organic matter decomposition, and thus, in fact, document active methane release in the sediment.