OS53B-1097
Salinity of the Early and Middle Eocene Arctic Ocean From Oxygen Isotope Analysis of Fish Bone Carbonate
Plate tectonic reconstructions indicate that the Arctic was largely isolated from the world ocean during the early and middle Eocene, with exchange limited to shallow, and possibly intermittent, connections to the North Atlantic and Tethys (via the Turgay Strait). Relative isolation, combined with an intensification of the hydrologic cycle under an Eocene greenhouse climate, is suspected to have led to the development of a low-salinity surface water layer in the Arctic that could have affected deep and intermediate convection in the North Atlantic. Sediment cores recently recovered from the Lomonosov Ridge by the IODP 302 Arctic Coring Expedition (ACEX) allow for the first assessment of the salinity of the Arctic Ocean during the early and middle Eocene. Stable isotope analysis performed on the structural carbonate of fish bone apatite from ~30 samples between the ages of ~55 and ~44 myr yielded $\delta$$^{18}$O values between -6.84\permil and -2.96\permil VPDB, with a mean value of -4.89\permil. From the $\delta$$^{18}$O values we calculate that the Arctic Ocean was probably brackish during most of the early and middle Eocene, with an average salinity of 19 to 24\permil. Negative excursions in the $\delta$$^{18}$O record ( < -6\permil) indicate three events during which the salinity of the Arctic surface waters was severely lowered: the Paleocene Eocene Thermal Maximum (PETM), the {\it Azolla} event at ~49 Ma, and a third previously unidentified event at ~46 Ma. During the PETM, low salinities developed under conditions of increased regional precipitation and runoff associated with extreme high latitude warmth and possible tectonic uplift in the North Atlantic. During the other two low-salinity events, sea level was lowered by ~20-30 m, implying a possible severing of Arctic connections to the world ocean. The most positive $\delta$$^{18}$O value (-2.96\permil) occurs at ~45 Ma, the age of the youngest dropstone discovered in the ACEX sediments, and may therefore correspond to a climatic cooling rather than a high salinity event.
OS53B-1098
Quaternary Inferences on Central Arctic Ocean Circulation and Sediment Provenance Based on Diffuse Spectral Reflectance Analysis
Reconstruction of Arctic Ocean surface circulation during the quaternary provides an important constraint on the potential magnitude of future Arctic change based on the observed range of natural variation in this climatically sensitive region. The second leg of the 2005 Healy-Oden Trans-Arctic Expedition (HOTRAX) recovered 21 jumbo piston cores across the Arctic Ocean, spanning the Alpha-Mendeleev and Lomonosov ridges, providing unprecedented coverage of the major Arctic basins. Inter-comparison of shipboard lithostratigraphy with Multi- Sensor core logs enables preliminary correlation of material from the Alpha-Mendeleev Ridges, and with less certainty, the Lomonosov Ridge. Some unique stratigraphic features such as a distinctive change in lithology and prominent IRD layers provide the basis for correlation with previously developed stratigraphies. This overall correlation is confirmed by 1 cm post-cruise color reflectance measurements and indicates an overall increase in sedimentation rates from the Alpha Ridge to the southern Mendeleev Ridge, and even higher rates for the Lomonosov Ridge. Principle Component Analysis of the reflectance data using representative cores raised from the major Arctic Ocean ridge systems extract components interpreted to be related to smectite-chlorite (53%), illite (39%), and goethite (4%). Together these three components account for 95% of the variance in the data set. In addition, the first two components can be used to reconstruct downcore sediment Mn oxide fluctuations, a useful proxy for glacial-interglacial intervals. Downcore analysis of the resulting components indicates an inverse correlation between smectite-chlorite which reaches maxima during interglacial and interstadial intervals when sediment Mn is also high, and illite and goethite which reach maxima during glacial and stadial intervals when sediment Mn is low. These variations are likely driven by contrasts in glacial vs. interglacial sedimentation regimes, bottom water residence times, and changes in the interplay of the Beaufort Gyre and the Transpolar Drift.
OS53B-1099
New Insights in Sea Ice Drift Based on Dirty Ice Samples Collected in 2005 by the HOTRAX Expedition
Several dirty sea ice samples collected over the rarely visited Alpha Ridge and from other central Arctic Ocean areas are compared to ice-drift back trajectories from International Arctic Buoy Programme. Both the back trajectories and Fe grain fingerprint sources suggest Russian sources near the Laptev Sea (New Siberian Islands), but the Fe grain sources also indicate other Russian sources such as the Kara Sea and even some sources were traced to the northern Arctic Canadian Archipelago. Drift patterns based on back trajectories show that a mix of Russian and N. American sources is possible because the location where the ice was sampled is close to the area where the Trans-Polar Drift carrying Russian ice joins that portion of the Beaufort Gyre moving toward Fram Strait and carrying N. American ice. The Fe grain sources also show that both Russian and N. American ice can occur juxtaposed within very short distances of tens of meters due to mixing of ice floes. This supports earlier findings in the Beaufort Gyre area where Russia floes co-mingled with N. American floes due to repeated separation and rejoining of ice floes. Two dirty ice samples north of Barrow, Alaska indicate a Bering Strait/Chukchi Sea source. The back trajectories in this area do not show any ice originating from any nearshore sources off Alaska nor northern Canada, but only a tight clockwise rotation over the past several years prior to 2005. Thus the Chukchi Sea source is entirely feasible in light of this back-trajectory data. If so this is the first direct evidence of ice in the Beaufort Gyre originating from the Chukchi Sea area.
OS53B-1100
Quaternary Foraminiferal Assemblages From IODP-ACEX Cores, Central Arctic Ocean
Foraminferal assemblages from the Integrated Ocean Drilling Program's (IODP) Arctic Coring Expedition (ACEX) 302 Hole 4-C (87$^{\circ}$52' N, 136$^{\circ}$11' E; 1288 m water depth; 0-25 meters below sea floor (mbsf)) show fluctuations in species dominance between interglacial (mixed agglutinated and calcareous benthic, plus planktonic) and glacial (mainly agglutinated benthic) periods of the last 1.6 Ma. Interglacial assemblages are most common above 2 mbsf and in thin carbonate-rich zones down to 17 mbsf and include the polar planktonic species {\it Neogloboquadrina pachyderma} (mostly left-coiling) and benthic taxa {\it Cassidulina teretis}, {\it Oridorsalis tener}, {\it Cibicides wuellerstorfi}, and {\it Quinqueloculina spp}. Downcore, the interglacial changes to almost entirely benthic species ({\it Cassidulina reniforme}, {\it C. teretis}, {\it Dentalina spp.}, {\it Elphidium spp.}). From 5 to 20 mbsf agglutinated foraminifera alternate cyclically among {\it Cyclammina pusilla}, {\it Psammosphaera fusca}, {\it Trochammina lomonosovensis}, and {\it Rhabdammina spp.}, most likely reflecting ecological response of benthic organisms to orbital-scale variability in surface productivity, food availability, sea- ice cover, and other factors.
OS53B-1101
Holocene Paleoceanography of the Chukchi Sea / Alaskan Margin, Western Arctic Ocean
A multi-proxy approach to the analysis of deep-sea sediment cores has been used to investigate paleoceanographical changes in the western Arctic. This approach includes stable isotopic analysis of{ \it Neogloboquadrina pachyderma} (Np), the dominant planktonic foraminifera in the Arctic, which lives along the pycnocline that separates the cold, low salinity surface water from the underlying warmer water mass of North Atlantic origin. Palynological analyses focus on dinocyst assemblages that are usually well preserved in Arctic seas and include several species, thus allowing treatment of data (modern analogue technique) for quantitative reconstruction of sea-surface parameters, including maximum summer temperature and seasonal extent of sea- ice cover (cf. de Vernal et al., Paleoceanography, 2005).\\Work in the Chukchi Sea has shown that larger (i.e., heavier, possibly mature) specimens of Np are characterized by a lower oxygen isotopic composition relative to smaller (i.e., possibly juvenile) specimens that live at shallower depths, reflecting calcification of larger Np shells on top of the warmer North Atlantic Water mass (cf. Hillaire-Marcel et al., Quat. Sci. Rev., 2004). These results suggest that a reverse thermocline (warmer water underlying colder water) persisted throughout the Holocene, and that the gradients between surface and subsurface waters were stronger during the early Holocene, which may indicate greater influx of North Atlantic Water at this time. It is also possible that the isotopic composition of the planktonic foraminifera was influenced by enhanced sea-ice formation and sinking of isotopically-light brines during the early Holocene. This second hypothesis is compatible with reconstructions from dinocysts that suggest maximum sea-ice extent during the early Holocene.\\Preliminary results from core HLY0501-05 collected in the Beaufort Sea off the Alaskan Margin during Leg 1 of the HOTRAX 2005 Expedition show results consistent with those of the Chukchi Sea. Dinocyst assemblages (concentration ranging 1000-5000 cysts/g) are characterized by a slight increase in diversity and the occurrence of subpolar taxa in the upper part of the sequence. The results suggest a trend of decreasing sea-ice cover extent during the late Holocene.
OS53B-1102
Stratigraphy and Sedimentary Processes of Drift Deposits in the Yermak-Plateau Area (Arctic Ocean)
During the Polarstern Expedition ARK-XX/2 (2004) major sediment drifts were documented in Parasound and multichannel seismic profiles from the Fram Strait/Yermak Plateau area. Bottom and sub-bottom reflection patterns obtained by Parasound characterize the uppermost sediments in terms of their acoustic behavior. This can be used to interpret the sedimentary environments and their changes in space and time. The topography of the Yermak Plateau is marked by a number of small subplateaus which appear to be uplifted areas or relicts of basement reaching into relatively shallow water. Along the steep flanks of these sub-plateaus, Parasound indicates well-stratified sediments, which decrease in thickness significantly over short lateral distances down- slope directly adjacent to steep slopes. In some places, the deeper areas are also marked by erosional discontinuities. The overall character of the pattern is typical for marine drift deposits, which are controlled by the influence of bottom currents. Studies of these sediment drift deposits including investigations of sediment cores may give important information about changes in paleoclimate and paleoceanographic circulation patterns through time. These sediment cores are analysed for reconstruction of the history and formation processes of sediment drifts on the Yermak Plateau. Major objectives are (1) establishing of a stratigraphic framework including acoustic profiles and well-constrained chronology (AMS$^{14}$C and stable isotopes), (2) a high resolution reconstruction of the paleoclimatic and paleoceanographic circulation patterns in the Arctic gateway area, using sedimentological and geochemical methods (grain size analysis, TOC, physical properties) and (3) the study of past (bottom) water mass exchange between the Arctic and North Atlantic oceans. The main focus of this work is the stratigraphic framework, using Parasound profiling data, Multi-Sensor-Core-Logging data and data from two selected sediment cores representing a low bottom-current speed (high accumulation rates) and a high bottom-current speed environment (low accumulation rates).
OS53B-1103
The Hinlopen/Yermak Megaslide (north of Svalbard, Arctic Ocean): Size, Timing and Triggering
With increasing interest in slope stability issues on continental shelves the causes and trigger mechanism of submarine slides get more and more into the scientific focus. The extent of the Hinlopen/Yermak Megaslide north of Spitsbergen has been revised based on new acoustic and detailed bathymetric data. Its true geometry, with an affected area of at least 10,000 km$^{2}$ and more than 2400 km$^{3}$ involved sedimentary material, puts the megaslide among the largest exposed submarine slides worldwide. Details from the its internal structure give evidence for one main failure event during MIS 3 followed by repeated minor events. The megaside's geometry and internal physical appearance point to a tectonically induced partial shelf collapse around 30 kyr. BP. The timing coincides with the transition of the Kapp Ekholm Interstadial into Glaciation G of Svalbard (Mangerud et al., 1998) and the build-up phase of the Svalbard-Barents Sea Ice Sheet. Thus, the megaslide occurred during a period of falling sea level, increasing ice volume and, presumably, increasing glacio-tectonic activity. We conclude that the Hinlopen/Yermak Megaslide has been the consequence of the rapid onset of LGM-glaciation resulting in a drastic sea level drop, asymmetrical ice loading and a fore bulge development. For the final trigger we assume a magnitude-amplified earthquake positioned below or close to the SE-Sophia Basin. Thus the slide is climatically controlled. References: Mangerud, J., Dokken, T., Hebbeln, D., Heggen, B., Ingolfsson, O., Landvik, J. Y., Mejdahl, V., Svendsen, J. I., Vorren, T. O. (1998). �Fluctuations of the Svalbard-Barents Sea ice sheet during the last 150 000 years.� Quaternary Science Reviews 17: 11-42. Winkelmann, D., W. Jokat, F. Niessen, R. Stein, and A. Winkler (2006 a), Age and extent of the Yermak Slide north of Spitsbergen, Arctic Ocean, Geochem. Geophys. Geosyst., 7, Q06007, doi:10.1029/2005GC001130.
OS53B-1104
The Innuitian Ice Sheet Collapse History and its Relationship to the Laurentide Ice Sheet Collapse Events in Arctic Ocean Cores
Ice rafted debris (IRD) events in Arctic deep-sea cores traced to the Innuitian ice sheet (IIS) and Laurentide ice sheet (LIS) occur together but with the IIS usually preceding the LIS events. Detrital Fe grain fingerprinting (chemical composition of individual grains measured by electron microprobe) are used to match significant numbers of these Fe grains to grains with the same composition found in glacial deposits near the Arctic outlets of these large ice sheets. Fe grain abundances in several cores can be matched to precise glacial sources and these grains increase and decrease rapidly for what appears to be short durations suggesting ice sheet collapse or rapid iceberg calving. In some cores these two ice sheets either appear to collapse simultaneously or the separate ice sheet collapse events cannot be resolved. The lead-time between the Innuitian and Laurentide collapse events is usually less than 500-1000 years in cores with the moderate resolution. If this can be confirmed, then it is similar to lead times suspected for the Scandinavian ice sheet collapse prior to Heinrich Events, the Laurentide ice sheet collapse events that shed thousands of icebergs into the Atlantic through Hudson Straits. Finally, this study demonstrates that the Innuitian Ice Sheet produced significant IRD deposits in the Arctic prior to the last glaciation. In several instances the IIS signature is stronger than the Fe grain count from the larger LIS in Arctic cores. Thus the Innuitian ice sheet is not just a Late Pleistocene phenomenon and may even have influenced the multiple collapses of the Laurentide ice sheet over the last 80,000 years.
OS53B-1105
Marine Sediments Indicate Trends in Eurasian Ice Sheet Configurations During the Last 500 ky
Sediment core data of the last 500 ky from the Norwegian-Greenland Sea show a stepwise trend of decreasing fluxes of ice-rafted debris (IRD) during "full" glaciations, i.e., marine isotope stages (MIS) 12, 6, and 2. Strongest IRD deposition occurred in MIS 12, while it was lower in MIS 6 and 2. We interpret this observation as evidence for a decreasing discharge of sediment-laden icebergs from northwestern European ice sheets during peak glacials. Results from the ESF-programme QUEEN suggest that developments in the Norwegian-Greenland Sea were accompanied by synchronous trends in the Arctic. Field observations from Russia and central Siberia provide evidence for at least four pre-Weichselian glaciations. Maximum southern extents were diachronous in the various areas before MIS 6, but they reached much further to the south than ever after. However, it was only the vast MIS 6 glaciation which, for the first time, also reached the N. Eurasian shelf break, forming an almost 2000 km long calving line. The discharge of icebergs from this ice margin is reflected in Arctic Ocean deep-sea sediments by a thick IRD-rich layer, deposited on top of older, usually more fine-grained sediments. During cold times ensuing MIS 6 (MIS 5b, 4, and 2) ice sheet sizes progressively decreased and shifted towards Scandinavia, NW Europe, and the Barents Sea. The observed developments in the Norwegian-Greenland Sea and Eurasian Arctic during the last 500 ky indicate a progressive migration of glaciation limits towards the north, accompanied by a westward shift of glaciation centers across northwest Eurasia. These regional shifts were forced by major changes in oceanic heat transport which essentially influenced the pathways of atmospheric moisture transfer across northern Eurasia. It can be suspected that, within this process, the Arctic climate system became more and more sensitive to external (e.g., anthropogenic) forcings.
OS53B-1106
Thorium-230 Stratigraphy of Alpha Ridge Sediment (Arctic Ocean)
The Alpha Ridge (central Arctic Ocean) is characterized by very uniform sedimentary deposition essentially linked to vertical particulate rain. This property led us to investigate the behavior of U-series isotopes (Th-230 and Pb- 210) in such a setting, i.e., one without significant sedimentary advection. Two sites cored with a 70 cm-long multicorer during the 2005 Hotrax Expedition and located about 20 nautical miles apart and at different water depths (core 11: 2644 m and core 12: 1585 m) were selected for the purpose of this study. Lead-210 profiles are practically identical in both cores, with high activities at the surface (>30 dpm/g), followed by a first minimum at 1 cm ( < 10 dpm/g), and then increasing values down to approximately 7 cm (about 15 dpm/g), and a gradual decrease below this core depth. This pattern suggests significant bioturbation, at least down to 8-10 cm, and some Pb-210 diffusion below. At three distinct depths the Th-230 activities are above supported Th-230 values (approx. 1.2 dpm/g): from 0 to 8 cm (with a maximum ranging 25-30 dpm/g), 15 to 20 cm (up to 7 dpm/g) and 26 cm to core bottom (34 and 38 cm, respectively in cores 12 and 11). Here again, despite their large bathymetric difference, the two sites yielded almost identical 230Th-profiles. 230Th-activities are highly correlated with the CaCO3 content, allowing for the decay of the excess-thorium 230 (230Thxs) downcore. Assuming a linear initial relationship between CaCO3 and Th-230xs, the assignment of the lowermost part (below 37 cm) of core 11 to the oxygen isotope 5e seems probable. Maximums in organic carbon and carbonate contents at the base and top of the cores would support this interpretation. In such settings, Th-230 reveals useful data to constrain the stratigraphy of the late Pleistocene sediments, and may compensate for the absence of a viable oxygen isotope stratigraphy.
OS53B-1107
Downcore 231Pa and 231Pa/230Th Records From the Central Arctic Ocean
The particle-reactive radionuclides 231Pa and 230Th, produced by natural U decay in seawater, are removed from the ocean by adsorption to particles and subsequent burial in sediments. 230Th production and removal are thought to largely balance each other locally in most of the world ocean, while 231Pa has a longer residence time in the ocean and is thus susceptible to transport by advection or eddy diffusion. Our previous work indicates that the sedimentary 230Th budget is approximately balanced in the central western Arctic, allowing the use of sedimentary 230Th as a normalizing tool, and hence allowing us to investigate sedimentary budgets of 231Pa. We have generated downcore records from the Arctic of sedimentary U, Pa, Th concentrations and 231Pa/230Th ratios by ICP-MS. These records provide new information on the budget, intra-basin transport, and potential export of 231Pa from the Central Arctic over the last 30 kyr, with implications for our understanding of particle fluxes and water column transport of 231Pa during the glacial, deglacial, and Holocene periods.
OS53B-1108
Paleoceanographic Investigations of the Alpha Ridge, Central Arctic Ocean: New Stratigraphic Data and Their Implications for Paleoclimatological Reconstructions
Marine-geological investigations were carried out on three sediment cores recovered on the Alpha Ridge (Amerasian Basin), during RV POLARSTERN Cruise ARK-XIV/1a (1998) in order (1) to establish a stratigraphic framework and (2) to reconstruct the Quaternary paleoenvironmental history of this poorly known area. These sediment cores with lengths of up to 7.2 m, were studied using a multi-proxy approach including geophysical (MSCL logger), paleomagnetic (inclination, declination, intensity), sedimentological (lithology, X-ray images, grain size, silt granulometry), mineralogical (bulk and clay mineralogy by XRD) and geochemical (organic carbon, sulfur, Th230 excess) data which provide the most detailed insight into the sedimentary record of the Alpha Ridge to date. Based on these new data, a lateral correlation to older cores from the Alpha Ridge (e.g., CESAR core 83- 14) as well as the intensively studied cores PS2185 and 96/12-1pc on the adjacent Lomonosov Ridge can be realised by comparison of lithology, sand content, paleomagnetic data and clay mineral distribution. The age model developed so far for the Lomonossov Ridge can be tied to the Alpha Ridge supporting the scenario of higher sedimentation rates (cm/ky) for this part of the Central Arctic Ocean rather than the model of reduced sedimentation (mm/ky) as documented in earlier publications. In one of our cores (PS51/038-4), new Th230 excess data suggest an age younger than 350 ka at a paleomagnetic excursion boundary which formerly was interpreted as Brunhes-Matuyama boundary according to the low sedimentation rate model. This polarity change can be well correlated throughout the entire Central Arctic Ocean. However, due to the uncertainties in the interpretation of the paleomagnetic record (chrons vs. excursions) and the scarcity of datable fossiliferous material, the lowermost unit of core PS51/038-4 still remains stratigraphically uncertain. Based on our still preliminary age model, the extrapolated age for the total depth of the core (7.2 m) suggests a maximum age of about 1.0 to 1.2 Ma.
OS53B-1109
Magnetic Correlation and Chronology of Sediment-cores (HOTRAX) From the Alpha Ridge and Lomonosov Ridge; Preliminary Results and Some Questions
The chronology of Plio-Pleistocene sediment-cores from Central Arctic Ocean retrieved during the Healy-Oden Transarctic expedition in 2005 is attempted constructed using paleomagnetic reversal/excursion stratigraphies and relative paleointensity records combined with stratigraphic variations of mineral magnetic properties. Shipboard whole core magnetic susceptibility measurements revealed surprisingly similar records in cores retrieved between the Mendeleev Ridge and Alpha Ridge; 7 cores can be tied together by more than 15 characteristic susceptibility features, enabling a very detailed and precise correlation. This result suggests lateral uniformity of sediment-composition and only minor variation in deposition rates across this part of the Arctic. Paleomagnetic directions and relative paleointensity-data have so far been obtained from two cores; one from the Alpha Ridge-area, and one collected within the conspicuous depression (gap) across the Lomonosov Ridge. The two cores reveal a large number of short-duration polarity reversals (i.e. excursions), as is a characteristic feature of Arctic Ocean sediment cores. The highly varying lengths of the reversed �polarity' intervals may reflect local variations in accumulation/erosion rates. Relative paleointensity (RPI) curves may be correlated with SINT- 800. However, most �excursions' are not associated with intensity-lows, questioning the reality of these excursions as well as the reliability of using SINT-800 as a dating tool. A previously investigated core from the Lomonosov Ridge (AO96-12pc1) also carries a large number of �excursions' with varying lengths. Although the RPI-curve resembles the Alpha Ridge RPI-records, any correlation with SINT-800 to obtain a chronology is presently questionable. We address the following questions: A) Are all - or some of - the inferred excursions artifacts due to undisclosed post-depositional processes? B) May RPI-variations be invariant to geomagnetic field-directions? C) May the geomagnetic field produce �excursions' confined to polar (i.e. high-latitudes) regions only?
OS53B-1110
Holocene Calcareous Nannofossils from a HOTRAX site on the Lomonosov Ridge
A fairly diversified Holocene calcareous nannofossil assemblage occurs in a trigger core recovered by the HOTRAX expedition (Darby et al., 2005) from an area on the central Lomonosov Ridge where the ridge morphology is characterized by a 1 km local depression. The investigated core was taken from this depression, and where subbottom profiles indicated expanded sediment sections and locally higher depositional rates. Fourteen samples in the upper 66 cm contain calcareous nannofossils. An area of 1.25 square mm on conventional smear-slides was investigated using a polarizing light microscope. The assemblage is dominated by Emiliania huxleyi and small- and medium-sized Gephyrocapsa. Gephyrocapsa caribbeanica is common in two samples. Other components are Coccolithus pelagicus, rather common at the 5-6 cm level but rare or absent below, Calcidiscus leptoporus, with a few specimens in seven samples, Syracosphaera spp., present in 10 of the 14 coccolith-bearing samples, Pontosphaera spp., a few rare occurrences, and Helicosphaera carteri, occurring with a single specimen in the sample at 60-61 cm. Moreover, a single sample (42-43 cm) holds abundant specimens of the calcareous dinocyst Thoracosphaera spp. Minor reworking of chiefly Mesozoic forms is observed in eight samples. The three most nannofossil rich samples between 42 and 56 cm yielded about 1500 to 3000 specimens per 1.25 square mm. These relatively high concentrations of Holocene calcareous nannofossils in a central Arctic Ocean setting only 180 km away from the North Pole probably reflects the influence of the warm Holocene climate optimum that lasted between about 9 ka and 5 ka. It follows that sedimentation rates in this Holocene sequence are on the order of about 6 to 8 cm/1000 years, which is the highest Holocene sedimentation rate ever recorded from the central Lomonosov Ridge. Darby, D., Jakobsson, M., and Polyak, L., 2005. EOS 86(52): 549, 552.
OS53B-1111
Linking Stratigraphy on the Lomonosov Ridge With Standard Lithological Units of the Amerasian Basin, Arctic Ocean
Short sediment cores taken from the ice-island T3 between 1963 and 1973 were used by Clark et al. (1980) to establish a lithological classification for the Amerasian Basin in the Arctic Ocean. Over a 310 cm long stratigraphic section, synthesized from 67 cores, 13 units were recognized and labeled A to M. This lithostratigraphic model has since been applied to correlate sediment cores over large distances on the Amerasian side, i.e., between cores on the Alpha Ridge, the Northwind Ridge and the Mendeleev Ridge. The chronostratigraphy for this lithostratigraphic model was developed by the identification of the Brunhes-Matuyama paleomagnetic reversal boundary (781 ka) and the assumption of linear sedimentation rates in the synthesized lithologic model. However, recent studies have shown that this measured magnetic polarity change rather represents a short-time excursion within the Brunhes chron, implying an order of magnitude higher sedimentation rates than originally assumed. We have investigated whether it is possible to link the lithostratigraphy from the Alpha Ridge to cores taken on the Lomonosov Ridge which, in turn, have been linked to cores in the Eurasian Basin. The cores selected for this comparison were cores B-8 and B-24 from the LOREX �79 ice island drift expedition, core PS2185-6 from the ARK-VIII/3 expedition of 1991 and core 96/12-1pc from the Arctic Ocean 96 expedition. Averaged sedimentary proxies (paleomagnetism, grain size, and microfossil abundance) from selected sets of T3 cores and published by Clark et al. (1980), have been used here to correlate the included Lomonosov Ridge cores. A lithostratigraphic connection between the Amerasian Basin and the Lomonosov Ridge stratigraphies was partly successfully established, although Clark's lithostratigraphic units A to M could not be recognized in the Lomonosov Ridge. Nevertheless, the established correlations clearly show that the Amerasian Basin is characterized by lower sedimentation rates when compared to the rates obtained from the Lomonosov Ridge and the Eurasian Basin , but that the Amerasian Basin rate are not as low as Clark and others previously suggested.
OS53B-1112
Bathymetry and deep-water exchange at the central Lomonosov ridge
Seafloor mapping using multibeam echo-sounder during the HOTRAX 2005 expedition at the central Lomonosov Ridge showed that a postulated (~2500m) deep channel in the IBCAO bathymetry does not exist. The multibeam survey along the ridge crest gave a maximum sill depth at of about 1870 m. A hypothesized exchange of deep water from the Amundsen Basin to the Makarov basin in this area could not be confirmed. Instead a flow of deep water from the Makarov to the Amundsen Basin was observed, indicating the existence of another pathway for Amerasian Basin Deep Water towards the Atlantic Ocean. Sediment data indicates extensive current activity along the ridge crest and at the rim of a local intra-basin within the ridge structure.
OS53B-1113
Evidence for the Mid-Cenozoic Uplift of the Lomonosov Ridge
Results from drilling on the Lomonosov Ridge during IODP Leg 302, the Arctic Coring Expedition (ACEX), have shown that one of the most profound changes in the character of sedimentation in the Central Arctic Ocean was the mid-Cenozoic shift from freshwater influenced biosilica rich deposits of the Eocene, to the fossil poor glaciomarine silty clays of the Miocene (Moran et al., 2006). In the ACEX record, this shift culminates in a ~ 5 meter interval where the two modes of sedimentation are captured in centimeter scale cross-banding and is preceded by a 25 million year hiatus. Micropaleontological, sedimentological and geochemical results from ACEX reveal a growing freshwater influence in sediments leading up to the hiatus. A mid-Eocene onset of tectonic uplift, resulting in the vertical migration of Lomonosov Ridge through a fresh to brackish surface water lense, can explain these observations. Uplift and subaerial exposure of the ridge accounts for the 25 Myr hiatus and is followed by rapid mid-Miocene subsidence where the cross-banded sediments describe the sinking of the ridge below high-energy surface waters. Further constraints on the timing of these events are provided by seismic observations on the depth to oceanic basement in the adjoining Amundsen basin, where a similar anomalous phase of subsidence has been reported (Weigelt and Jokat, 2003; Jokat and Micksch, 2004). The inferred uplift of the ridge coincides with the northward impingement of Greenland on the growing Eurasian basin (Brozena et al., 2003), with resumed subsidence closely following a change in the location of the Euler pole for the North American and Eurasian plates that ended a period of transpression along the Laptev Sea shelf (Drachev et al., 1998). These results suggest that the Cenozoic geodynamic evolution of the central Arctic Ocean is linked to changes in far field tectonic stresses. Unraveling the magnitude and extent of these influences is critical for interpreting sedimentological and geophysical records used to document the evolution of the modern cryosphere.
OS53B-1114
Evidence of an Asteroid Impact in the Central Arctic Ocean?
Revaluation of single channel seismic reflection data from ice station T-3 (1967-74) acquired over the submarine Alpha Ridge in the central Arctic Ocean, supplemented by new multi-channel data, show spatially restricted massive disturbance of sub-bottom sediments within a 200 x 600 km area. Deposits have been locally disrupted down to at least 500 meter below the bottom, and have suffered intensive local erosion. Mass wasting is abundant. At this point, we are not able to neither document a likely cause for each of these types of stratigraphic disturbance nor a direct relationship between them. However, we note that: 1) tectonic movements normally involve the whole stratigraphic column and are not depth limited as observed here; 2) ground motion may trigger mass wasting, but is less likely to generate intense bottom current erosion; and 3) enhanced bottom currents are basin-wide phenomena and only disrupt stratigraphic continuity down to the deepest erosion level. As a working hypothesis, we suggest the spectrum and scale of drastic, spatially restricted and apparently geologically short-lived environmental changes are best explained by the effect of a shock wave from impact of an extra-terrestrial body into the central Arctic Ocean, the T3-Healy asteroid. The timing of an impact is unknown, but may be ?Plio-Pleistocene. Healy-2005 seismic team: Tore Arthun, Hans Berge, Vibeke Bruvoll and Erik Grindvoll of University of Bergen, Norway, Hedda Breien, University of Oslo, Norway, Dayton Dove, University of Alaska, Fairbanks, Paul Henkart, Scripps Institution of Oceanography, USA, Nina Ivanova, University of Uppsala, Sweden, Fredrik Ludvigsen, Thor Heyerdahl High School, Larvik, Norway, and Karina Monsen, Alta High School, Alta, Norway.
OS53B-1115
Marvin Spur - Lomonosov Ridge Relationships Based on Reflection Seismic Profiling Near the North Pole
Reflection seismic profiles acquired from the drifting ice-station NP-28 in 1988-1989 and other neighboring profiles provide evidence of the character and origin of the Marvin Spur and Lomonosov Ridge. The NP-28 seismic images of the sedimentary successions capping the Ridge can be correlated with those of the AWI- 91090 profile, which was calibrated by the ACEX drilling at 88� N. Along the AWI line, the most prominent reflector package marks the base of the Cenozoic (Paleocene) section and its angular unconformity to underlying Mesozoic strata. The NP-28 ice-station crossed the Lomonosov Ridge three times, near the North Pole. In each profile, the Marvin Spur is also imaged, in one case below the floor of the Makarov Basin and in the two others as a narrow ridge parallel to the Lomonosov Ridge. A prominent composite reflector occurs at a few hundred meters depth in the sedimentary successions on both the Lomonosov Ridge and the Marvin Spur, underlain disconformably by less regular reflectors, dipping towards the Amerasian Basin. Correlation of both the seismic images and velocities (Vp) with the AWI-91090 profile suggests that this composite NP-28 reflector marks the base of the Cenozoic. The reflection profiles across the Lomonosov Ridge and Marvin Spur are similar; probably the Spur is a narrow sliver of thinned continental crust that was rifted off the Ridge. Towards the Greenland margin, the trough between the Lomonosov Ridge and the Marvin Spur narrows and the two appear to merge. Towards the Siberian margin, the trough widens and the crest of the Marvin Spur sinks beneath the Makarov Basin. It has been also imaged further along strike beneath this basin in the TRA(b)-90 profile (Langinen et al, ICAM-IV in press), where the composite reflector marks a clear unconformity capping the Spur and adjacent older successions. These lines of seismic evidence need to be tested by piston coring and drilling. They emphasize the importance of Cenozoic faulting for controlling the central Arctic bathymetry and the complexity of the boundary zone between the Lomonosov Ridge and the Amerasian Basin. Thinned continental crust is probably present even beneath parts of the Makarov Basin.
OS53B-1116
HLY0602: An integrated geophysical and geological study of the western Canada Basin
The {\it USCGC Healy} cruise, HLY0602, departed Barrow on 19 July 2006 and ended prematurely on the 22nd of August in Nome, Alaska. HLY0602 was an integrated geophysical and geological study of the western Canada Basin including Northwind Ridge, Chukchi Cap and the Mendeleev Ridge. The IBCAO chart of Arctic bathymetry (Jakobsson, et al., 2000) gives the impression that there is comprehensive bathymetric coverage of the western Canada Basin. While in general, the IBCAO coverage is accurate, there are a number of places where multibeam data indicate significant discrepancies. For instance, the large north-south trough on the eastern margin of Chukchi Cap at $163\deg$W appears on the IBCAO map to have a possible seamount on the eastern edge of the trough at $77.9\deg$N. We surveyed that region and found an extremely flat-floored trough with a depth of 2708 $\pm$ 5 m with no sign of a seamount within 10 km of where it is shown on the IBCAO map. On Chukchi Cap there is an apparent ~ 900 m deep trough in the center that is in fact no deeper than ~ 700 m. Multibeam bathymetric surveying of Mendeleev Ridge confirmed the numerous pockmarks found by HLY0504 with even greater concentrations of the pockmarks found to the south along Mendeleev Ridge. A number of major slump features were found on the northern margin of Arlis Plateau at the southern end of Mendeleev Ridge. If the pockmarks are associated with high gas content, then the level of organic rich sediments may be similar to those found on the Lomonosov Ridge by IODP drilling (Backman et al., 2006). {\it Backman, J., Moran, K., McInroy, D.B., Mayer, L.A., and the Expedition 302 Scientists, 2006. Proc. IODP, 302: Edinburgh (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.302.2006} {\it Jakobsson, M., Cherkis, N.Z., Woodward, J., Coakley, B., Macnab, R., 2000. A new grid of Arctic bathymetry: a significant resorce for scientists and mapmakers. EOS Transactions 81(9), 89, 93, 96.}
OS53B-1117
Stratigraphy, Structure, and Origin; A Geophysical Survey of the Mendeleev Ridge
The Mendeleev Ridge is a broad, aseismic ridge that extends from the Siberian Shelf into the central Arctic Ocean. While it is continuous with the Alpha Ridge and is inferred to be an oceanic plateau, it may have had a distinct and separate history. The origin of the Mendeleev ridge has only rarely been visited and, as a result, understanding the history of this region has largely been based on the presumption of a common origin for both features. In late summer 2005, a geophysical survey was conducted from USCGC Healy over the Mendeleev Ridge as part of a trans-arctic crossing. During this survey ~730 km of seismic reflection data was recovered over the ridge along with co-registered gravity and bathymetry data and seismic refraction profiles. The seismic source was two 250 cu in G-guns. The streamer length was limited by ice conditions to 300 meters. Wear and tear caused by towing the streamer through the ice pack eliminated hydrophones, so the number of active channels ranged from 24 to as few as 11. The seismic reflection data requires significant trace editing to eliminate random electrical noise and frequency- wave number filtering to eliminate low velocity noise caused by the streamer traveling through heavy ice. After trace editing the data are stacked and migrated with constant water velocity. Stacking velocities are used as input into initial ray tracing models. Derived boundary velocities from ray tracing models will be reapplied to the migration of reflection data and are converted through empirical relationships into densities, and used as input into gravity models. Brute stacked reflection images of the Mendeleev Ridge reveal pervasive extensional faulting of the basement and lower sediment layers, and a continuous, undeformed pelagic sediment layer mantling the ridge, indicative of recent tectonic inactivity. The age of the unconformity underlying this layer should date the end of significant deformation of the Alpha and Mendeleev Ridges. Consistency of modeled seismic velocities from the upper basement will provide some insight into the nature of the crustal material. Upper basement velocities estimated from the sonobuoy data range from 3.7-5.0 km/s, suggestive of a heterogeneous upper crust. Initial gravity models suggest that much of the amplitude variation over the Mendeleev Ridge is accommodated by invoking a single, continuous density layer for the crust. Future work will include: developing a structural map of the Mendeleev ridge as well as the refining of both the ray- tracing and gravity modeling in an attempt to better understand the crustal style of the ridge.
OS53B-1118
Acquisition of long-offset seismic refraction data in the Chukchi Borderlands and Mendeleev Ridge
The deep structure of the Chukchi Borderlands and Mendeleev Ridge is important for our understanding of the tectonic history of the western Arctic Ocean. Our constraints on the crustal structure of this region are sparse because the nearly continuous ice cover makes the acquisition of marine seismic refraction data difficult. In July and August of 2006 we gathered a unique seismic refraction data set on the Chukchi Borderlands and Mendeleev Ridge utilizing USCGC Healy and two helicopters. In order to obtain seismic refractions from an air-gun source over long offsets, we placed seismic instruments on the sea ice by helicopter. Each of the stations was equipped with a geophone, hydrophone, GPS unit and radio. The instruments were left on ice for several days, making occasional radio contact with either the ship or the helicopter to give us their latest location. We deployed an array of 12 instruments across the Northwind Escarpment into the Canada Basin, 13 instruments on an east-west transect across Chukchi Cap, and 14 seismometers on a refraction line parallel to the crest of Mendeleev Ridge. One instrument on the Chukchi Cap was lost at sea, but the other instruments were successfully retrieved with their refraction data. The instrument arrays recorded air-gun shots over distances up to 150 km. We will use the first-arrival time data to estimate the two-dimensional seismic velocity structure along the three profiles that were gathered on this cruise.
OS53B-1119
Structural Style of the Chukchi Borderlands From Marine Seismic Data Collected on the USCGC {\it Healy} in 2005
In August and September 2005, the U.S. Coast Guard Icebreaker {\it Healy} crossed the Arctic Ocean beginning in Alaska and ending in Norway. The cruise covered most of the major ridges and basins that make up the Arctic Ocean. New seismic reflection and refraction data was aquired over the Northwind Ridge and Chukchi Borderlands region and out onto the Mendeleev Ridge. A seismic source consisting of two 8-l (250 cu. in.) airguns was used and shots were recorded on a 300 m analogue streamer. The streamer consisted of 24 channels early on, but was reduced to 16 channels later in the cruise to preserve spare sections. As ice damage to the streamer accumulated, the number of active channels with good data decreased. The ice conditions were relatively light across the Chukchi borderlands and Mendeleev Ridge and high quality data was recorded on most of the channels throughout this region. In addition to the reflection data, we deployed sonobuoys to record wide- angle data. A large majority of these recorded excellent arrivals through the sediments and most have clear basement refractions, providing important velocity control on the area. In this contribution, we summarize the key seismic reflection and refraction data collected over the Chukchi Borderlands. The data provide constraints on the style and amount of extension the region experienced. The implications of these results for the tectonic evolution of the region and it's relation to surrounding areas will be discussed.
OS53B-1120
The Geological Setting of Hydrothermal Vent Sites on Gakkel Ridge
In 1998 and 1999, the Science Ice Exercises (SCICEX) mapped the fine-scale textures of the flanks and axial valley of Gakkel Ridge, the slowest-spreading mid-ocean ridge on Earth (full-spreading rates < 1.33 cm/yr). Sidescan data collected during the SCICEX expeditions showed the locations and distribution of lightly sedimented volcanic flows and faults including a small volcano near $85\deg$N, $85\deg$�E associated with >250 teleseismic events that occurred in 1999 [M�ller and Jokat, 1999; Edwards et al., 2001]. During the 2001 Arctic Mid-Ocean Ridge Expedition (AMORE), hydrothermal plume reconnaissance conducted during rock sampling operations revealed evidence of abundant hydrothermal venting on the Gakkel Ridge [Edmonds et al., 2003]. Comparison of the plume distributions with multibeam bathymetry data collected during AMORE showed that vent plumes were closely associated with topographic highs located inside the axial valley, with the largest and highest-temperature plume coinciding with the $85\deg$E volcano. We describe the geological setting of vent plumes discussed in Edmonds et al. [2003] by integrating water column information from the AMORE program with detailed textural data from the SCICEX surveys and develop predictions for locations where hydrothermal venting is likely to occur on ultra-slow spreading mid-ocean ridges. Our efforts focus on five hydrothermal sites identified by Edmonds et al. [2003] and Baker et al. [2004] ($7.5\deg$E, $37\deg$�E, $43\deg$E, $55\deg$E and $85\deg$E). We co-register the observed plume distributions with interpretative maps showing the locations of tectonic and volcanic features such as faults and reflective lava flows in order to characterize the hydrothermal sites. These results are compared with similar interpretative products for the $69\deg$E region and other sites where plume signals were observed but the hydrothermal activity could not be localized based on the strength of the hydrothermal signals or the occurrence of ``near-field`` signatures such as distinct layering or temperature anomalies. Finally, we examine reflective regions not associated with plumes, either because no hydrothermal anomalies were detected or the regions were not hydrographically surveyed during the AMORE program. Comparison of the different classes of vent sites are used to produce an improved model for predicting where hydrothermal venting might be observed during future Gakkel Ridge expeditions.
http://www.soest.hawaii.edu/HMRG/Aagruuk/index.p hp
OS53B-1121
Testing Plate Reconstructions For The High Arctic Using Crustal Thickness Mapping From Gravity Inversion
The plate tectonic history of the Amerasia Basin (High Arctic) and its distribution of oceanic and continental lithosphere is poorly known. A new method of gravity inversion with an embedded lithosphere thermal gravity anomaly correction has been applied to the NGA (U) Arctic Gravity Project data to predict crustal thickness and to test different plate reconstructions within the Arctic region. Two end member plate reconstruction models have been tested: in one model the Mendeleev Ridge is rifted from the Canadian margin while in the other it is rifted from the Lomonosov Ridge. The inversion of gravity data to map crustal thickness variation within oceanic and rifted continental margin lithosphere requires the incorporation of a lithosphere thermal gravity anomaly correction for both oceanic and continental lithosphere. Oceanic lithosphere and stretched continental margin lithosphere produce a large negative residual thermal gravity anomaly (up to -380 mGal), for which a correction must be made in order to determine realistic Moho depth by gravity anomaly inversion. The lithosphere thermal model used to predict the lithosphere thermal gravity anomaly correction may be conditioned using plate reconstruction models to provide the age and location of oceanic lithosphere. Two end-member plate reconstruction models have been constructed for the opening of the Amerasia Basin and used to determine lithosphere thermal gravity anomaly corrections: in one model the (presumably) continental Mendeleev Ridge is rifted from the Canadian margin in the Jurassic while in the other it is rifted off the Lomonosov Ridge (Eurasia Basin) in the Late- Cretaceous. Crustal thickness predicted by gravity anomaly inversion for the two plate reconstructions is significantly different in the Makarov Basin because of their different lithosphere thermal gravity corrections. The plate reconstruction with younger Makarov Basin ages gives a crustal thickness of the order 6-8 km thinner than the older Makarov Basin model. A crustal thickness of approximately 20 km has been obtained from seismic refraction data (Lebedeva-Ivanova et al., 2006) which would imply a Late Mid-Cretaceous age for the Makarov Basin. In this case plume-related forces may have contributed to the opening of this basin, as regional plate tectonics predict compression and not extension in the Makarov Basin area at this time.
OS53B-1122
The Paleoenvironmental Evolution of the Eastern Arctic Ocean in the past 3.6 Million Years
The Plio-/Pleistocene stratigraphy and paleoenvironmental history of the Yermak Plateau area has been studied by a multi-parameter approach on ODP Leg 151 Holes 910A and 911A using various sedimentological, mineralogical, micropaleontological and geochemical methods. Major steps in the long-term development of glaciations on the Northern Hemisphere such as the intensification of glaciations in the late Pliocene and the Mid-Pleistocene transition are reflected in the records from the Yermak Plateau. In particular, the northern Barents Sea Ice Sheet showed fluctuations obviously coeval with other segments of the Northern Hemisphere Ice Sheets. Mineralogical parameters indicate that the individual Eurasian ice sheets reacted differently to global cooling. Apparently, long-term centers of glaciations shifted from the Kara Sea to the Barents Sea since the late Pliocene. Since Northern Hemisphere Glaciations intensified at around 2.7 to 2.3 Ma, the Barents Sea became the major center of glaciations in the Eurasian Arctic. Fluctuations in ice sheets obviously occurred on various time scales, and were partly related to stronger inflow of Atlantic waters into the Arctic Ocean. Such an important phase was the late Pliocene warming event that preceeded the first advance of the Northern Barents Sea ice sheet to the shelf edge. Superimposed on the long-term paleoenvironmental evolution, distinct short-term variability is reflected in various proxies (eg. organic matter contents). Spectral analyses revealed frequencies close to those of orbital variations, suggesting that the Northern Barents Sea continental margin is a sensitive region to decipher climate change.