Geochemistry of sediments of stage 5 of the Nordic seas

Stable isotope, foraminifera and ice rafted detritus (IRD) records covering the last interglacial (the Eemian) from 7 sediment cores in a transect from the Norwegian to the Greenland Sea are presented. The percentages of Neogloboquadrina pachyderma (s.) and Globigerina quinqueloba, foraminiferal content, and to some extent planktonic stable isotope records, demonstrate marked, regional changes in surface water conditions. Importantly, the variability in the abundances of subpolar foraminifera and foraminiferal content are not coherent, implying that these two types of proxies fluctuated independently of each other and most likely reflect changes in sea surface temperature and surface water carbonate productivity, respectively. Paleoceanographic reconstructions demonstrate significant movements of the oceanographic fronts. At the warmest periods, the Arctic front was located far west of the present-day location, at least within the Iceland Sea region. At 126-125 ka, this was most probably due to a stronger or more westerly located Norwegian current. Within the later warm intervals, higher heat flux to the western part of the basin reflects a combination of a stronger Irminger current and/or a weaker east Greenland current. During the main cold spell at ~124 ka, a diffuse Arctic front had a more southeasterly location than today, and intrusion of Atlantic surface waters was probably limited to a narrow corridor in the Eastern Norwegian Sea. A general correspondence between minima in sea surface temperatures and light benthic delta18O may indicate enhanced influx of freshwater to the basin within the cold events. At least in the Norwegian Sea, we find some evidence that the changes in surface water conditions are associated with changes in deep water ventilation. The majority of the fluctuations may be related to occasional breakdown or reduction of the thermohaline circulation within the Nordic seas. In the earliest Eemian, this could result from meltwater forcing. During the remaining part of the last interglacial the fine balance between temperature and salinity, which the deep water formation is depending on, may have been disturbed by periodic increases in fresh water supply or variable influx of warm Atlantic surface waters.

Age determination and stable isotope record of sediment core MR06-04_PC23A

Millennial-scale variability in the behavior of North Pacific Intermediate Water during the last glacial and deglacial period, and its association with Dansgaard-Oeschger (D-O) cycles and Heinrich events, are examined based on benthic foraminiferal oxygen and carbon isotopes (d18Obf and d13Cbf) and %CaCO3 using a sediment core recovered from the northeastern slope of the Bering Sea. A suite of positive d18Obf excursions at intermediate depths of the Bering Sea, which seem at least in part associated with increases in the d18Obf gradients between the Bering and Okhotsk Seas, suggest the Bering Sea as a proximate source of intermediate water during several severe stadial episodes in the last glacial and deglacial period. Absence of such d18Obf gradients during periods of high surface productivity in the Bering and Okhotsk Seas, which we correlate to D-O interstadials, suggests a reduction in intermediate water production in the Bering Sea and subsequent introduction of nutrient-rich deep waters from the North Pacific into intermediate depths of the Bering Sea. We argue that a reorganization of atmospheric circulation in the high-latitude North Pacific during severe cold episodes in the last glacial and deglacial period created favorable conditions for brine rejection in the northeastern Bering Sea. The resulting salinity increase in the cold surface waters could have initiated intermediate (and deep) water formation that spread out to the North Pacific.

(Table 2) Trace metal concentrations in ODP Hole 169-1033B

We measured the concentrations of redox-sensitive trace metals (Mn, V, Mo, U, Cd and Re) in sediments from ODP Leg 169S Hole 1033B in Saanich Inlet, British Columbia, to determine changes in redox conditions associated with the onset of laminated sediments at ~12.5 kyr. The most striking result is a large peak in authigenic Re along with detrital levels of Mo at the glacial terrigenous clay-diatomaceous sediment transition. In contrast, the underlying glacial terrigenous clay, which extends throughout the bottom section of the core, is chemically similar to detrital concentrations, either Cowichan River particulates or average shale values. These data suggest a period of oxic bottom waters but reducing pore-waters. This could be due to the dramatic transformation of Saanich Inlet during the late deglaciation from an open bay to an inlet, which restricted circulation and slowed bottom water oxygen renewal. A peak and gradual increase in authigenic Mn in younger sediments subsequent to the Re peak suggests that increasingly oxic conditions followed the authigenic enrichment in Re. These conditions could be connected to the Younger Dryas cooling period, which was coincident with an increase in well oxygenated upwelled waters on the west coast of North America that form the bottom waters of Saanich Inlet. Metal concentrations in a gray clay bed (~11 kyr) are similar to their concentrations in the glacial terrigenous clay, implying that they have a common source. Authigenic enrichments of Re with little authigenic Mo and Cd suggest that before the deposition of this bed, bottom waters were oxic and pore-water oxygen was consumed in the top centimeter or less. Laminations above the clay layer suggest anoxic conditions, which are also indicated by higher authigenic Mo and Cd and slightly lower Re/Mo ratios in these sediments. The basin remained mostly anoxic after the gray clay was emplaced, as seen by continuous authigenic enrichment of the redox-sensitive trace metals. These results are consistent with increased stratification of the water column, brought about by an influx of fresh water to the basin by a large flood.

Sedimentological, mineralogical, biogeochemical and micropaleontological parameters on sediment core PS69/849-2, Burton Basin, MacRobertson Shelf, East Antarctica

A multi-proxy study including sedimentological, mineralogical, biogeochemical and micropaleontological methods was conducted on sediment core PS69/849-2 retrieved from Burton Basin, MacRobertson Shelf, East Antarctica. The goal of this study was to depict the deglacial and Holocene environmental history of the MacRobertson Land-Prydz Bay region. A special focus was put on the timing of ice-sheet retreat and the variability of bottom-water formation due to sea ice formation through the Holocene. Results from site PS69/849-2 provide the first paleo-environmental record of Holocene variations in bottom-water production probably associated to the Cape Darnley polynya, which is the second largest polynya in the Antarctic. Methods included end-member modeling of laser-derived high-resolution grain size data to reconstruct the depositional regimes and bottom-water activity. The provenance of current-derived and ice-transported material was reconstructed using clay-mineral and heavy-mineral analysis. Conclusions on biogenic production were drawn by determination of biogenic opal and total organic carbon. It was found that the ice shelf front started to retreat from the site around 12.8 ka BP. This coincides with results from other records in Prydz Bay and suggests warming during the early Holocene optimum next to global sea level rise as the main trigger. Ice-rafted debris was then supplied to the site until 5.5 cal. ka BP, when Holocene global sea level rise stabilized and glacial isostatic rebound on MacRobertson Land commenced. Throughout the Holocene, three episodes of enhanced bottom-water activity probably due to elevated brine rejection in Cape Darnley polynya occured between 11.5 and 9 cal. ka BP, 5.6 and 4.5 cal. ka BP and since 1.5 cal. ka BP. These periods are related to shifts from warmer to cooler conditions at the end of Holocene warm periods, in particular the early Holocene optimum, the mid-Holocene warm period and at the beginning of the neoglacial. In contrast, between 7.7 and 6.7 cal. ka BP, brine rejection shut down, maybe owed to warm conditions and pronounced open-water intervals.

Seasonal changes in sediment core PS1795-2

The Weddell Sea and the associated Filchner-Rønne Ice Shelf constitute key regions for global bottomwater production today. However, little is known about bottom-water production under different climate and icesheet conditions. Therefore, we studied core PS1795, which consists primarily of fine-grained siliciclastic varves that were deposited on contourite ridges in the southeastern Weddell Sea during the Last Glacial Maximum (LGM). We conducted high-resolution X-ray fluorescence (XRF) analysis and grain-size measurements with the RADIUS tool (Seelos and Sirocko, 2005, doi:10.1111/j.1365-3091.2005.00715.x) using thin sections to characterize the two seasonal components of the varves at sub-mm resolution to distinguish the seasonal components of the varves. Bright layers contain coarser grains that can mainly be identified as quartz in the medium-to-coarse silt grain size. They also contain higher amounts of Si, Zr, Ca, and Sr, as well as more ice-rafted debris (IRD). Dark layers, on the other hand, contain finer particles such as mica and clay minerals from the chlorite and illite groups. In addition, Fe, Ti, Rb, and K are elevated. Based on these findings as well as on previous analyses on neighbouring cores, we propose a model of enhanced thermohaline convection in front of a grounded ice sheet that is supported by seasonally variable coastal polynya activity during the LGM. Accordingly, katabatic (i.e. offshore blowing) winds removed sea ice from the ice edge, leading to coastal polynya formation. We suggest that glacial processes were similar to today with stronger katabatic winds and enhanced coastal polynya activity during the winter season. Under these conditions, lighter coarser-grained layers are likely glacial winter deposits, when brine rejection was increased, leading to enhanced bottom-water formation and increased sediment transport. Vice versa, darker finer-grained layers were then deposited during less windier season, mainly during summer, when coastal polynya activity was likely reduced.

Zooplankton abundance and vertical fluxes of sedimentary matter and chemical elements in the White Sea from 15 to 24 June, 2000

A multidisciplinary oceanographic survey of the White Sea was carried out in the Gorlo Straight, Basin, and Kandalaksha Bay regions including estuaries of Niva, Kolvitza and Knyazhaya rivers. Hydrophysical study in the northern part of the Basin revealed long-lived step-like structures and inversions in vertical profiles of temperature and salinity, which formed due to tidal mixing of saline and cold Barents Sea waters and warmer White Sea waters in the Gorlo Straight. Biological studies revealed the main features of spatial distribution, as well as qualitative and quantitative composition of phyto- and zooplankton in all studied areas; tolerance of main zooplankton species to fresh water influence in estuaries was shown. Study of suspended matter in estuaries clearly demonstrated physicochemical transformations of material supplied by the rivers. Data on vertical particle flux in the deep part of the Kandalaksha Bay showed difference between the upper and near-bottom layers, which could result from sinking of spring phytoplankton bloom products and supply of terrigenic suspended matter from the nepheloid layer formed by tidal currents.

Stable oxygen and carbon isotope data for planktic and benthic foraminifera from southern Atlantic of DSDP Hole 74-525A and ODP Hole 113-690C

Published stable isotope records in marine carbonate are characterized by a positive d18O excursion associated with a negative d13C shift during the early Maastrichtian. However, the cause and even the precise timing of these excursions remain uncertain. We have generated high-resolution foraminiferal stable isotope and gray-scale records for the latest Campanian to early Maastrichtian (73-68 Ma) at two Ocean Drilling Program sites, 525 (Walvis Ridge) and 690 (Weddell Sea). We demonstrate that the negative d13C excursion is decoupled from the d18O increase with a lag of about 600 ka. Our d13C records (both planktic and benthic) show an amplitude for the negative excursion of 0.7 per mill that falls between about 72.1 and 70.7 Ma. Our planktic d18O records indicate an overall increase of 1.2 per mill from 73 to 68 Ma at Site 690, whereas at Site 525 they record a slightly smaller increase (1 per mill) that peaks around 70.1 Ma with decreasing values thereafter. Our benthic d18O data indicate an increase of 1.5 per mill at Site 525 and 0.7 per mill at Site 690 between about 71.4 and 69.9 Ma. Benthic d18O values show different baseline values at the two sites before and after the excursion, but the larger increase at Site 525 means that the values attained at the peak of the excursion are similar at the two sites. We interpret this observation in terms of water mass changes. The excursion is interpreted to reflect a cooling of bottom waters in response to the strengthening contribution of intermediate- to deep-water production in the high southern latitudes rather than increased ice volume. The associated carbon cycle perturbations that we observe are interpreted to reflect a weakening of surface water stratification and increased productivity, as supported by our gray value data.