In the meso-oligotrophic Bay of Biscay, a diminishing downward organic matter flux with depth is accompanied by an important decrease of the live foraminiferal density. Although bottom water oxygenation is not directly influenced by organic matter input, the oxygenation of interstitial waters and the primary redox fronts do change in response to variations of the organic matter flux. The occurrence of deep and intermediate infaunal taxa can be linked to fundamental redox fronts and putative associated bacterial consortia. Our data are in agreement with the TROXmodel, which explains the benthic foraminiferal microhabitat as a function of organic flux and benthic ecosystem oxygenation. Both the depth of the principle redox fronts and the microhabitat of deep infaunal species show important increases with depth. At the deepest oligotrophic stations, deep infaunal faunas become relatively poor. Therefore, the exported flux of organic matter appears to be the main parameter controlling the composition and the vertical distribution of benthic foraminiferal faunas below the sediment-water interface. The oxygenation of pore waters plays only a minor role. A species-level adaptation of the TROX-model is presented for the Bay of Biscay. r

No abstract

[1] 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, d13 C values of cytoplasmcontaining epibenthic F. wuellerstorfi are by up to 4.4% lower than at control site, thus representing the lowest values hitherto reported for this species. Live C. neoteretis and C. reniforme reach d13 C values of À7.5 and À5.5% Vienna Pee Dee Belemnite (VPDB), respectively, whereas d13 C values of their empty tests are higher by 4% and 3%. However, d13 C 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 d 13C 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 d13 C values of bottom-water-dissolved inorganic carbon (DIC) are not significantly depleted, we conclude that low test d 13C values of F. wuellerstorfi are the result of incorporation of heavily 13 C-depleted methanotrophic biomass that these specimens feed on rather than because of low bottom water d 13 C DIC . Alternatively, the pogonophores, which are rooted at depth in the upper sediment column, may serve as a conduit for depleted d 13 C DIC that ultimately influences the calcification process of F. wuellerstorfi attached to the pogonophoran tube well above the sediment/water interface. The lowest d 13 C of live specimens of the endobenthic C. neoteretis and C. reniforme are within the range of pore water d 13 C DIC values, which exceed those that could be due to organic matter decomposition, and thus, in fact, document active methane release in the sediment.

The intensity of the Oxygen minimum zone (OMZ) of the eastern North Pacific (ENP) experienced strong variations during the last glacial, mirroring changes in the balance between export production (O2 consumption) and water mass ventilation (O2 renewal). In this paper we present a new benthic foraminiferal assemblages record from Core MD02‐2508, recovered from the Pacific slope off Baja California, Mexico. The record reflects oxygen conditions at the northern limit of the OMZ during the last 80 kyr. We statistically identified three assemblages, characteristic of dysoxic, suboxic, and oxic conditions, which we used to produce the first semiquantitative reconstruction of [O2] for the northeastern Pacific Ocean. Our results show that the estimated [O2] covaries with δ18O records from the North Greenland Ice Core Project. Oxygen concentrations overall exhibit moderate values (~1 mL.L−1) during stadials, reaching ~ 2 mL.L−1 during stadials corresponding to Heinrich events in the Atlantic Ocean. Conversely, bottom waters at the core location were strongly depleted in oxygen (<0.5 mL.L−1) during interstadials. Benthic foraminiferal abundance increased with higher export production as recorded by geochemical tracers (Cd/Al ratio). This export production signal increases (decreases) with a fall (rise) in [O2] during interstadials (stadials), suggesting a relationship between both parameters during these intervals. The influence of ventilation on oxygenation is also a key player. O2 pulses suggested by the downcore records of serial/spiral test ratio and abundance of oxic species may be explained by enhanced ventilation during Heinrich stadials, in agreement with latest modeling‐based oceanic circulation reconstructions.

No abstract

Benthic foraminiferal assemblages and geochemical tracers (δ18O, δ13C and 14C) have been analyzed on benthic and planktonic foraminifera from core MD77‐176, located in the northern Bay of Bengal, in order to reconstruct the evolution of intermediate circulation in the northern Indian Ocean since the last glaciation. Results indicate that during the Last Glacial Maximum (LGM), Southern Sourced Water masses were dominant at the core site. A high relative abundance of intermediate and deep infaunal species during the LGM reflects low oxygen concentration and/or mesotropic to eutrophic deep water conditions, associated with depleted benthic δ13C values. During the Holocene, benthic foraminiferal assemblages indicate an oligotropic to mesotrophic environment with well‐ventilated bottom water conditions compared with LGM. Higher values for benthic foraminifera δ13C and B‐P 14C age offsets suggest an increased contribution of North Atlantic Deep Water to the northern Bay of Bengal during the Late Holocene compared to the LGM. Millennial‐scale events punctuated the last deglaciation, with a shift in the δ13C and the ɛNd values coincident with low B‐P 14C age offsets, providing strong evidence for an increased contribution of Antarctic Intermediate Water at the studied site. This was associated with enhanced upwelling in the Southern Ocean, reflecting a strong sea‐atmospheric CO2 exchange through Southern Ocean ventilation during the last deglaciation.

No abstract

No abstract