ALBERT

Description: Live (Rose Bengal stained) benthic foraminifera were investigated in surface sediment samples from the Okhotsk Sea to reveal the relationship between faunal characteristics and environmental parameters. Live benthic foraminifera were quantified in the size fraction 〉 125 µm in the upper 8 cm of replicate sediment cores, recovered with a multicorer at five stations along the Sakhalin margin, and at three stations on the southwestern Kamchatka slope. The stations are from water depths between 625 to 1752 m, located close or within the present Okhotsk Sea oxygen minimum zone, with oxygen levels between 0.3 and 1.5 ml l- 1. At the high-productivity and ice-free Kamchatka stations, live benthic foraminifera are characterized by maximal standing stocks (about 1700-3700 individuals per 50 cm2), strong dominance of calcareous species (up to 87-91% of total live faunas), and maximal habitat depths (down to 5.2-6.7 cm depth). Vertical distributions of total faunal abundances exhibit a clear subsurface maximum in sediments. At the Sakhalin stations, which are seasonally ice-covered and less productive, live benthic foraminifera show lower standing stocks (about 200-1100 individuals per 50 cm2), lower abundance of calcareous species (10-64% of total live faunas), and shallower habitat depths (down to 2.5-5.4 cm depth). Faunal vertical distributions are characterized by maximum in the uppermost surface sediments. It is suggested that 1) lower and strongly seasonal organic matter flux, caused by the seasonal sea ice cover and seasonal upwelling, 2) lower bottom water oxygenation (0.3-1.1 ml l- 1), and 3) more pronounced influence of carbonate undersaturated bottom water along the Sakhalin margin are the main factors responsible for the observed faunal differences. According to species downcore distributions and average living depths, common calcareous species were identified as preferentially shallow, intermediate and deep infaunal. Foraminiferal microhabitat occupation correlates with the organic matter flux and the depth of the oxygenated layer in sediments.

Description: Biogeochemical, benthic and planktonic foraminiferal data of core MD01-2415 were investigated to compare variations of paleoproductivity and oxygen minimum zone (OMZ) intensity on the northern slope of the Okhotsk Sea during the Termination (T) I – marine iotope stage (MIS) 1 and TII – MIS 5e. We also estimated changes of the sea surface temperature (SST) based on the planktonic foraminiferal percentages. The IMAGES core MD01-2415 (53 57.09N; 149 57.52E) was collected on the northern slope at water depth 822 m during the WEPAMA 2001 cruise of the R/V Marion Dufresne. The 46.23 m-long core represents the last 1.1 million years (Nürnberg and Tiedemann, 2004). Our study focuses on two intervals 0-3.1 and 7.6-8.5 m, which correspond to the TI – MIS 1 (the last 16 ka) and TII – MIS 5e (116-134 ka), respectively. The previous core age model (Nürnberg and Tiedemann, 2004) was improved basing on four AMS14C dates on foraminifers. Foraminifers were quantified in the size fraction 〉μm, For the SST estimation, planktonic foraminifers re-counted in the size fraction 〉150 μm. Faunal analysis were performed in 67 and 63 samples collected each 5 and 1-2 cm throughout the TI – MIS 1 and TII – MIS 5e sediments, respectively. It gives a temporal resolution about 300 years for the both studied intervals. The C/N Ratio, total organic carbon (TOC) and CaCO3 contents were obtained at 5 cm sample spacing that corresponds to a temporal resolution 300-600 years. Benthic foraminiferal assemblages indicate that the Okhotsk Sea OMZ was intensified during the TI and TII – early MIS 5e (127-134 ka), whereas it was weakened during the MIS 1 and late MIS 5e. The increased marine (mainly coccoliths) productivity and terrestrial organic matter (OM) input from the submerged shelves occurred during the TI and TII in the Okhotsk Sea (Seki et al., 2009; Nürnberg and Tiedemann, 2004) that likely controlled the deglacial intensifications of the OMZ (Bubenshchikova et al., 2010). The stronger OMZ is reconstructed during the TI (i.e. Preboreal) based on maximum of the Dysoxic bolivinids: mainly Bolivina spissa. The less pronounced OMZ is recovered during the TII – early MIS 5e by the dominance of the Suboxic cassidulinids: Cassidulina laevigata/ carinata. The TOC and C/N Ratio values are found to be higher in the TI than in the TII – early MIS 5e. It suggests higher input of terrestrial OM and supports stronger OMZ during the TI. Planktonic foraminiferal assemblages indicate that the summer SST increased during the TI and TII – early MIS 5e and decreased during the MIS 1 and late MIS 5e. It is evidenced from the higher contribution of temperate-to-subpolar Globigerina bulloides up to 28% during the TI and up to 40% during the TII – early MIS 5e in the almost monospecies assemblages dominated by polar Neogloboquadrina pachyderma sinistral. The deglacial warmings of surface water could have caused stratification, inhibited diatom growth and stimulated coccolithophorid productivity in the Okhotsk Sea (Seki et al., 2009). It agrees with the reconstructed deglacial intensifications of the OMZ by benthic foraminiferal data. References Bubenshchikova, N.V., Nurnberg D., Gorbarenko, S.A., Lembke-Jene, L., 2010. Variations of the oxygen minimum zone of the Okhotsk Sea during the last 50 Kyr as indicated by benthic foraminiferal and biogeochemical data. Okeanologiya 50 (1), 93-106. Nürnberg, D., Tiedemann, R., 2004. Environmental change in the Sea of Okhotsk during the last 1.1 million years. Paleoceanography 19, PA4011, doi:10.1029/2004Pa001023. Seki, O., Sakamoto, T., Sakai, S., Schouten, St., Hopmans, E., Sinninghe Damste J.S., Pancost, R. D., 2009. Large changes in seasonal sea ice distribution and productivity in the Sea of Okhotsk during the deglaciations. Geochemistry. Geophysics. Geosystems. 10 (10), Q 10007, doi:10.1029/2009GC002613

Description: ABSTRACT FINAL ID: PP41C-1794 Today’s knowledge about the past variability in North Pacific Intermediate Water (NPIW) formation and ventilation is still limited and rests largely on paleoceanographic time series from the eastern Pacific and regions distal to principal formation regions. Okhotsk Sea Intermediate Water (OSIW) is presumed to be the most important precursor for the formation of NPIW and is today also the main contributor to mid-depth ventilation in the North Pacific. Formation of OSIW at the northern Sakhalin margin is closely coupled to the basin's boundary conditions such as freshwater river discharge from the Amur and the dynamics of sea-ice. Earlier works implied significant ventilation changes in the NW-Pacific during the Last Glacial Maximum (LGM) with a better ventilated North Pacific Intermediate Water (NPIW) compared to today, with a clear boundary to less-ventilated Pacific Deep Water masses around 2000 m. In our study we used a comprehensive suite of more than 20 sediment cores along lateral and vertical transects to gain a good coverage of the basin and complement existing older data. We focused on intermediate water depths between 600 and 2000 m to better reconstruct the glacial ventilation history of OSIW. Our results show - contrary to earlier results - that no clear boundary can be defined between better and less well ventilated water masses at 2000 m water depth based on stable carbon isotope data of benthic foraminifera. Overall, the water column structure more resembles a vertically pattern similar to modern oceanographic conditions during the LGM, albeit overall slightly better ventilated than today. An upper part of better ventilated OSIW (700-1000 m) might stem from the inflow area near the Kamchatka continental margin. For the Holocene, a smaller sub-set of gravity cores off the Sakhalin continental margin was chosen to investigate the history of rapid changes in the formation of OSIW. A high-resolution sediment core from the Kamchatka continental margin proximal to the inflow of NW-Pacific water masses completes our set. According to our AMS 14C-based age models, maximum sedimentation rates range from 30 to 160 cm/kyr, while average sample spacing varies between 5 and 150 years. Planktic and benthic foraminiferal δ18O data from the Sakhalin margin suggest a high variability in the formation of OSIW determined by large fluctuations in the discharged Amur’s freshwater volume, influencing the stratification of local surface water masses and formation of sea ice. The δ13C data of benthic foraminifera reveal that a principal mid-Holocene shift in surface and intermediate water hydrography occurred in the Okhotsk Sea around 4.5 kyr BP, with better OSIW ventilation during the late Holocene and intermittent cessation of ventilated OSIW during the early part. The shift of baseline conditions into a different state at 4.5 kyr BP was accompanied by significant changes in the frequency and amplitude of shorter-term centennial-scale variability in both upper mixed layer and intermediate waters in the Okhotsk Sea.

Description: ABSTRACT FINAL ID: PP41C-1793 At present, the Okhotsk Sea contributes to ventilation of intermediate NW-Pacific via production of the oxygenated Okhotsk Sea Intermediate Water (OSIW: ~200-800 m). In the Okhotsk Sea, oxygen minimum zone appears as water layer between ~800 and 1500 m water depths with oxygen contents 0.3-1.5 ml/l. The paleoreconstructions of the OSIW production and the OMZ intensity in the Okhotsk Sea are important to understand the glacial-interglacial variations of oxygenation / ventilation of intermediate NW-Pacific. This study presents high-resolution benthic foraminiferal and sedimentological data for the last 115 ka of the IMAGES core MD01-2415 (Figure 1). The resulting reconstructions of the OMZ intensity for core MD01-2415 are compared with those for the KOMEX cores: LV28-2-4, LV28-40-5 and LV28-43-5 covering the last 52, 80 and 46 ka, respectively. To reconstruct intensity of the OMZ, we applied downcore distributions of the relative abundance (%) and accumulation rate (AR, sp. cm/kyr) of the dominant benthic foraminifera and assemblages indicative of the different bottom water oxygenation (Figure 1). The Dysoxic (0.1-0.3 ml/l), Suboxic (0.3-1.5 ml/l) and Oxic (1.5-6 ml/l) assemblages were obtained by grouping of all species of cores. Additional proxies included the sediment color* b, magnetic susceptibility, the percentage and AR of total organic carbon, calcium carbonate and biogenic opal for cores under study. We discuss probable causes of the reconstructed intensification of the Okhotsk Sea OMZ during the Termination I – Holocene (with maximum in the Preboreal) and the OMZ weakening during the Marine Isotope Stages 2-5d.