OS52B-01
Quaternary Paleoceanography of the Arctic Ocean
Reconstructing Quaternary paleoclimate history of the central Arctic Ocean has been inhibited by the lack of long sediment cores, uncertain chronology, and the scarcity of calcareous microfossil proxy records. The 2004 IODP ACEX cruise to the central Arctic ($87.87\deg$N, $136.18\deg$E) recovered cores containing the first complete Quaternary record of foraminifera from the Lomonosov Ridge (1288 m water depth) allowing a us to examine the evolution of sea ice during orbital cycles of the past 1.6 million years. The ACEX record is characterized by a quasi-cyclic pattern of agglutinated benthic assemblages during 41-ka obliquity cycles of the Matuyama, followed significant faunal changes during the mid-Pleistocene shift to 100-ka cycles 0.9-1.0 Ma. Intermittent preservation of calcareous benthic and planktic (mainly N. pachyderma sin) taxa characterizes the last 500 ka, including a calcareous assemblage representing Marine Isotope Stage 11. Comparison to other Arctic records suggests orbital-scale changes in sea ice, surface ocean productivity, and food availability might be Arctic wide phenomena. The Arctic paleoclimate record will be discussed in terms of deep-water formation, inflowing warm Atlantic water and the progressive development of ice cover during glacial periods.
OS52B-02
Arctic Ocean Freshwater Perturbations of Thermohaline Overturning Circulation Over the Past 0.5 Million Years
Compared to the understanding gained of other ocean basins, knowledge of Arctic Ocean sedimentary history remains in a preliminary state. The reason is not the lack of any effort but with the many experimental difficulties involved in the analysis of Arctic sediments. Here, we present a continuous and exceptionally well-dated marine sediment record from the Arctic Ocean gateway documenting the strong imprint of Arctic Ocean freshwater pulses on the climate regime in the Nordic Sea region throughout the past 500 ka. To obtain the necessary information on freshwater influence, we produced a continuous stack of published and new stable oxygen and carbon isotope data of the planktic foraminifera Neogloboquadrina pachyderma, sin. in ODP Leg 151 Hole 910A with partly a centennial resolution. d18O values of N. pachyderma sin. mainly reflect the regional salinity variations in the upper water column of the Arctic Ocean. Downcore deviations from the global pattern were interpreted as 18O-depleted freshwater events during glacial terminations and/or collapses of ice- dammed lakes in the circum-Arctic throughout the past 180 ka. Accompanying low d13C values support changes in water mass properties of surface waters in the Arctic Ocean and are indicative for an inhibited vertical water mass exchange. Arctic Ocean freshwater pulses to the gateway region are reflected in the isotopic record as excursions of variable amplitude, duration and regional extent. Well-constrained are events during the Weichselian when outflow of freshwater from Barents-Kara Sea ice-sheets and/or collapsing ice-dammed lakes occurred at 130 ka, 80-75 ka and 52 ka disturbing the meridional overturning circulation in the Nordic Seas. Within the youngest portion of Hole 910A, meltwater injections are consistently in timing and duration with these regional changes in the hydrological cycle of the Arctic Ocean that punctuated glacial terminations during the Early Weichselian. Beyond the Weichselian, more than 12 comparable freshwater events occur throughout the entire record indicating that such episodic discharge to the gateway region caused fundamental perturbations of thermohaline circulation during the Pleistocene. Most spectacular are freshwater events during the penultimate deglaciation (MIS 5/6) and MIS 11/12. Freshwater released by collapsing Arctic ice sheets/ice dammed lakes during both periods disturbed the hydrological regime of the gateway region and destabilized the meridional overturning circulation during the interglacials.
OS52B-03 INVITED
Late Quaternary History of Surface and Intermediate Water Masses in the Eastern and Central Arctic Ocean
The Atlantic Water (AW) inflow through the Fram Strait and across the Barents Sea and the freshwater runoff and brine rejection from sea ice formation on and close to the surrounding shelves are important processes for the formation of modern water masses in the interior Arctic Ocean. We use microfossil data, isotopic data from planktic and benthic foraminifers, as well as 143Nd/144Nd ratios in leached sediment coatings from several well-dated Arctic sediment cores to reconstruct the variable influence of both processes on surface and intermediate waters in the Late Quaternary. High abundances of planktic foraminifers and mostly high planktic carbon isotope values are used as indicators of a relatively strong inflow of well-ventilated AW at the (sub)surface level. During the last ca. 250 ky such events usually were of a relatively short duration (10 ky or less). They occurred during full interglacials (i.e., marine isotope stages (MIS) 5e and 1) and during interstadials within the Saalian and Weichselian glaciations at times when the northern Eurasian shelves were ice-free. An exception is the last 50 ky interval, when a continuous inflow is recorded. Low Nd ratios indicate also a strong AW contribution to the intermediate waters since 50 ka and during MIS 5e. This conclusion is corroborated by high benthic carbon isotope values, indicative of well- ventilated waters at ca. 1500 m water depth. The good correlation between microfossil contents and insolation values suggests a dominant solar control of the AW inflow history. The time intervals of extended glaciations in northern Eurasia at ca. 190-130, 80-90, and 50-60 ka were characterized by a weak or lacking AW inflow and a recurring strong influx of freshwater from melting ice sheets and/or discharge from previously ice-dammed lakes. High Nd ratios indicate a significant contribution of brines from northern Siberian shelf/ice sheet margins to intermediate waters on the Lomonosov Ridge. The strongest freshwater events occurred at ca. 130 and 52 ka and resulted in an enhanced stratification of the upper water masses and poorly ventilated halocline waters.
OS52B-04
Severnaya Zemlya, Arctic Russia: a Nucleation Area for Kara Sea Ice Sheets During the Middle to Late Quaternary
Quaternary glacial stratigraphy and relative sea-level changes reveal at least four expansions of the Kara Sea ice sheet over the Severnaya Zemlya Archipelago, Russian Arctic. Major glaciations followed by deglaciation and marine inundation occurred during MIS 10-9, MIS 8-7, MIS 6-5e and MIS 5d-3. The MIS 6-5e event, associated with marine limit at 140 m a.s.l., implies ice-sheet thickness of more than 2000 m. This is consistent with evidence of ice grounding at 1000 m water depth in the central Arctic Ocean. Till fabrics and glacial tectonics record repeated expansions of local ice caps, suggesting wet-based ice cap advance followed by cold-based regional ice-sheet expansion. Local ice caps over highland sites along the perimeter of the shallow Kara Sea appear to have repeatedly fostered initiation of a large Kara Sea ice sheet, with exception of the Last Glacial Maximum (MIS 2), when Kara Sea ice did not impact Severnaya Zemlya.
OS52B-05
Shaping the seafloor by glacial ice and deep ocean currents; morphology along the HOTRAX 2005 transect
The Healy-Oden Trans-Arctic Expedition 2005 (HOTRAX'05) completed a more than 6500 km transect across the Arctic Ocean; from Bering Strait via the North Pole to the Fram Strait. Along this transect multibeam bathymetry and chirp sonar subbottom profiles were collected continuously providing new information on the seafloor morphology and the uppermost 50-100 m of the sediment stratigraphy. Here we integrate the HOTRAX'05 datasets with bathymetry and chirp sonar data previously collected in 1998 and 1999 from nuclear submarines during the SCICEX project and in 2003 from USCGC Healy. HOTRAX'05 mapped more extensive and deeper (to ~1000 m present water depth) glaciogenic features on the Chukchi Borderland than previously discovered and together with the SCICEX/Healy 2003 data a spatial view on the seafloor glacial morphology is provided. Depths as shallow as 800 m mapped on the Mendeleev Ridge show no signs of glacial erosion, instead prominent pockmarks are evident in the otherwise undisturbed seafloor consisting of conformably draped sediments. On the Alpha Ridge, the HOTRAX'05 multibeam bathymetry gives a first 3D-view on the extensive mudwave fields described by Hall (1979; Sedimentary Geology) from data collected during the ice island T3's drift 1967 through 1970. With 5-30 m amplitudes and wavelengths occasionally larger than 500 m, the most prominent mudwaves occurs in water depths between 2300 and 2200 m. Spatial patterns of separate mudwave sets appears to indicate different directions of current flows across the Alpha Ridge. The new bathymetry collected during HOTRAX'05 has been incorporated into the International Bathymetric Chart of the Arctic Ocean (IBCAO) database resolving the long-standing question regarding the deepest depth of a potential deepwater overflow passage in the central Lomonosov Ridge. Instead of a 2500 m deep channel, the updated bathymetry shows a depth of only 1870 m. The new HOTRAX'05 chirp data reveal extensive current activity in the area of the Lomonosov Ridge where the deepest threshold between the Eurasian and Amerasian Basins now is mapped to 1870 m.
OS52B-06
The Century-Scale History of the Arctic Oscillation Based on Detrital Fe Grain Provenance of Russian Sea Ice Origins in the Arctic Off Alaska
Detailed Fe grain provenance for a 19.9 meter long piston core (HLY02-JPC16) with about 17.5 m of Holocene sediment provides the highest resolution yet for the ice-rafted detritus history in the Beaufort Sea, Arctic Ocean. The presence of Fe grains matched to Russian shelves throughout this core, located 125 km north of Alaska in 1300 m water depth, indicates the variable but consistent influx of ice from these Russian shelves to this part of the Beaufort Sea throughout the Holocene. Russian sources include all of the Siberian seas (Chukchi Sea to Barents Sea). The dominant North American sources are the northern Queen Elizabeth Islands (Arctic Canadian Archipelago) followed by the Banks Island area. The variable influx of Russian grains into the Beaufort Gyre is consistent with data from another core (P1) from the Chukchi Shelf edge near the Chukchi Borderland. JPC16 provides much greater detail than this Chukchi core and shows that the influx is primarily on a centennial scale, much like P1. If the influx of Russian grains (sea ice) is due to the Trans Polar Drift moving closer to North America similar to what occurs during a positive phase of the Arctic Oscillation, then the Fe grains are a proxy for the AO. If so, the AO has been a consistent phenomenon throughout the Holocene and it has frequencies that are greater than can be seen in Modern observations extending back only about 60 years.
OS52B-07 INVITED
Low 18O-contents in Arctic Neogloboquadrina pachyderma shells: A proxy of brine formation rate?
In the Arctic Ocean, the cold water foraminifera {\it Neogloboquadrina pachyderma} (Np), left as right coiled, likely forms its shell along the pycnocline between the cold, dilute, surface water and the warmer, saline North Atlantic Water (NAW), due to salinity conditions in the surface water mass below optimum values for the species (~ 35 psu;[1]). However, $\delta$$^{18}$O-values in Np shell still present negative offsets with isotopic equilibrium conditions for a calcite precipitated at mid-pycnocline depth. This offset ranges from �1 (Arctic seas) to �3 per mil (Canada Basin, East Siberian Sea), although temperature gradients along the pycnocline still result in predictable isotopic shifts from small (juvenile?) to large (mature?) shells [2]. The precise mechanism responsible for the $^{18}$O offset is not known, but it seems linked to rate of sea-ice formation or to its seasonal duration (e.g., [3]). The freezing of low $^{18}$O-content sea-surface waters rejects isotopically-light brines that sink to the pycnocline. We hypothesize that Np-shell growth occurs in such high-salinity/low-$\delta$$^{18}$O water droplets or thin layers sinking to the pycnocline. In vitro experiments [4] have indeed shown that formation of new shell-chambers could still occurs in salinities of up to 58 psu, and that some specimens could survive 82 psu for at least a week. Thus, in this hypothesis, isotopic offsets in Np would relate to the rate of brine formation. In the modern Arctic Ocean, mixing of these brines into NAW and export of surface water and sea ice into the North Atlantic would contribute maintaining steady-state conditions, thus resulting in an asymptotic offset value near �2.5/-3 per mil in Np shells. From this viewpoint, the greater offsets in the western Arctic and East Siberian Sea areas (up to �3 per mil), compared with the eastern Arctic Ocean (appr. �1 per mil), would reflect a difference in sea-ice formation rates along the shelves. Such isotopic offsets maintained in the Chukchi Sea during most of the Holocene, with possibly larger offsets early on. We thus infer continuous sea-ice formation and greatest brine production in the early Holocene. Records from this area also illustrate some decoupling between surface-water conditions, as reconstructed from dinocyst assemblages, and conditions prevailing on top of the NAW, as indicated by the size dependent $^{18}$O-gradients in Np. In addition, the 9-8 ka interval depicts a large offset between small and large specimens, suggesting much warmer conditions in the NAW than in the surface water [2]. However, between 7 and 6 ka BP, these size-dependent gradients nearly vanished, suggesting a weakening of the pycnocline. This likely resulted from a higher surface salinity and less sea-ice formation, as also indicated by dinocyst assemblages. [1] Hilbrecht: http://www.ngdc.noaa.gov/mgg/geology/hh1996/pachy.html; [2] Hillaire-Marcel et al., 2004: Quaternary Sci. Rev. 23; [3] Bauch et al., 1997: Earth Planet. Sci. Lett. 146; [4] Spindler, 1996: Proc. NIPR Symp. Polar Biol. 9.
OS52B-08
Sediment Records From the Alaskan/Chukchi Slope Suggest High Meltwater Inputs From the Northern Laurentide Margin During the Last Deglaciation
Several sediment cores averaging ~15m in length were collected from the Chukchi Sea margin (down to 1200 mwd) northwest of Point Barrow, Alaska, by USCGC Healy on the 1st HOTRAX 2005 leg. These cores exhibit an up to 10m-thick, laminated brown-grey sedimentary sequence underneath a bioturbated, organic-rich unit of Holocene age. The brown-grey sequence is mostly fine grained, but contains intervals enriched in coarse grains (dropstones) and clay clumps. Diffuse Spectral Reflectance data indicate that the brown coloration results from the presence of iron hydroxides, primarily goethite. We infer that this iron-hydroxide deposition was controlled by the mixing of fresh and marine waters, as has been exemplified by some field and experimental data on estuarine environments. Considering the geographic location of the core sites and the stratigraphic position of this deposit, we suggest that it was formed by sediment discharged from the northwestern margin of the Laurentide ice sheet during the last deglaciation, with brown layers and dropstone-rich intervals indicating pulses of meltwater and iceberg discharge, respectively. Correlation with an age-constrained sediment core collected off the Chukchi margin further northwest indicates that the brown-grey sequence was deposited between ca. 13 to 10 kyr BP, with the largest iron-hydroxide and dropstone spikes occuring at the bottom of this interval. These results are consistent with modeling data indicative of a voluminous Laurentide meltwater discharge into the Arctic Ocean during the Younger Dryas, especially at the onset of this cooling.