Leonid Polyak

Ice shelves in the Pleistocene Arctic Ocean inferred from glaciogenic deep-sea bedforms

It has been proposed that during Pleistocene glaciations, an ice cap of 1 kilometre or greater thickness covered the Arctic Ocean. This notion contrasts with the prevailing view that the Arctic Ocean was covered only by perennial sea ice with scattered icebergs. Detailed mapping of the ocean floor is the best means to resolve this issue. Although sea-floor imagery has been used to reconstruct the glacial history of the Antarctic shelf , little data have been collected in the Arctic Ocean because of operational constraints. The use of a geophysical mapping system during the submarine SCICEX expedition in 1999 provided the opportunity to perform such an investigation over a large portion of the Arctic Ocean. Here we analyse backscatter images and sub-bottom profiler records obtained during this expedition from depths as great as 1 kilometre. These records show multiple bedforms indicative of glacial scouring and moulding of sea floor, combined with large-scale erosion of submarine ridge crests. These distinct glaciogenic features demonstrate that immense, Antarctic-type ice shelves up to 1 kilometre thick and hundreds of kilometres long existed in the Arctic Ocean during Pleistocene glaciations.

Pleistocene stratigraphy and paleoenvironmental variation from Lomonosov Ridge sediments, central Arctic Ocean

Abstract High resolution seismoacoustic chirp sonar data and piston cores were collected from the Lomonosov Ridge in the central Arctic Ocean (85°–90°N; 130°–155°E). The chirp sonar data indicate substantial erosion on the ridge crest above 1000 mbsl while data from deeper sites show apparently undisturbed sedimentation. Piston cores from both the eroded ridge crest and the slopes have been analyzed for a variety of properties, permitting inter-core correlation and description of paleoenvironmental change over time. Based on the evidence of extensive sediment erosion at depths above 1000 mbsl, we infer that the top of the Lomonosov Ridge has been eroded by grounded ice during a prominent glacial event that took place during MIS 6 according to a newly published age model. This event is coeval with a dramatic shift from low amplitude glacial–interglacial variability to high amplitude variability recorded in the sedimentary record. The new age model used in our study is based on nannofossil biostratigraphy and correlation between sedimentary cycles and a low-latitude oxygen isotope record and confirmed by paleomagnetic polarity studies where negative paleomagnetic inclinations are assigned to excursions. Due to the controversy between this age model and age models that assign the negative paleomagnetic inclinations to polarity reversals, we provide a correlation to Lomonosov Ridge core PS2185-6 [Spielhagen et al., Geology, 25 (1997) 783]. According to the latter age models, the Lomonosov Ridge was eroded by ice grounding much earlier, at MIS 16.

Freshwater and Atlantic water inflows to the deep northern Barents and Kara seas since ca 13 14C ka:: foraminifera and stable isotopes

Foraminiferal stable isotopes and assemblages from Franz Victoria and St. Anna troughs provide a valuable record of freshwater and Atlantic Water flows to the northern Barents and Kara seas from deglaciation to present. The d 18 O and d 13 C of planktonic Neogloboquadrina pachyderma (s) and benthic Elphidium excavatum were up to1.4 % lower than present at ca 13, 11.5, and 10 14 C ka (global sea-level corrected), mostly reflecting substantial freshwater inputs coincident with glacial–marine sediment deposition. Cassidulina teretis exceeded 40% of benthic foraminifera ca 13 and 10 14 C ka, indicating subsurface penetrations of Atlantic Water. The transition to postglacial marine conditions is marked by a � 1% rise in foraminiferal d 18 O and a sharp fall in % C. teretis soon after 10 14 C ka. These changes imply reduced inputs of freshwater and Atlantic Water. Subsequent isotopic and foraminiferal assemblage variations reflect changing Atlantic Water conditions ‘‘upstream’’ in the Nordic Seas and shifts between the warm Fram Strait and cold Barents Sea branches of Atlantic Water. We hypothesize that glacial-isostatically induced deepening by up to � 150 m influenced Atlantic Water inflows to the northern Barents Sea during deglaciation and the Holocene. Thus, effects of isostatic recovery have to be factored into paleoceanographic reconstructions. # 2001 Elsevier Science Ltd. All rights reserved.

Contrasting glacial/interglacial regimes in the western Arctic Ocean as exemplified by a sedimentary record from the Mendeleev Ridge

Distinct cyclicity in lithology and microfaunal distribution in sediment cores from the Mendeleev Ridge in the western Arctic Ocean (water depths ca. 1.5 km) reflects contrasting glacial/interglacial sedimentary patterns. We conclude that during major glaciations extremely thick pack ice or ice shelves covered the western Arctic Ocean and its circulation was restricted in comparison with interglacial,modern-type conditions. Glacier collapse events are marked in sediment cores by increased contents of ice-rafted debris,notably by spikes of detrital carbonates and iron oxide grains from the Canadian Arctic Archipelago. Composition of foraminiferal calcite N 18 O and N 13 C also shows strong cyclicity indicating changes in freshwater balance and/or ventilation rates of the Arctic Ocean. Light stable isotopic spikes characterize deglacial events such as the last deglaciation at ca. 12 14 C kyr BP. The prolonged period with low N 18 O and N 13 C values and elevated contents of iron oxide grains from the Canadian Archipelago in the lower part of the Mendeleev Ridge record is interpreted to signify the pooling of freshwater in the Amerasia Basin,possibly in relation to an extended glaciation in arctic North America. Unique benthic foraminiferal events provide a means for an independent stratigraphic correlation of sedimentary records from the Mendeleev Ridge and other mid-depth locations throughout the Arctic Ocean such as the Northwind and Lomonosov Ridges. This correlation demonstrates the disparity of existing age models and underscores the need to establish a definitive chronostratigraphy for Arctic Ocean sediments. ; 2003 Elsevier B.V. All rights reserved.

BENTHIC FORAMINIFERAL ASSEMBLAGES FROM THE SOUTHERN KARA SEA, A RIVER-INFLUENCED ARCTIC MARINE ENVIRONMENT

Calcareous foraminifers and hydrographic parameters in 113 bottom samples from the southern Kara Sea were examined to improve the usage of foraminifers as paleoenvironmental proxies for river-dominated high-latitude continental shelves. Foraminiferal taxa form a succession from near-estuarine to distal open-sea locations, characterized by a gradual increase in salinities. Foraminiferal assemblages are discriminated into three groups: river-proximal, -intermediate, and -distal. This succession appears to be controlled by a combination of feeding conditions and bottom salinities, and are related to riverine fluxes of freshwater, organic matter, and sediments. Morphological and behavioral adaptations of foraminifers to specific environments are discussed.

Late Weichselian deglacial history of the Svyataya (Saint) Anna Trough, northern Kara Sea, Arctic Russia

Abstract Marine sediment core and seismic records from the Svyataya (Saint) Anna Trough provide new insight into the distribution of Late Weichselian glacial coverage, ice retreat pattern, and post-glacial environments in the northern Barents and Kara seas. These records indicate that the Saint Anna Trough was filled with grounded glacier ice, which likely reached the shelf edge during the Late Weichselian maximum. Several radiocarbon dates suggest early deglaciation of the deep axial part of the trough prior to 13.3 ka. Two sandy beds in the deglacial section of the cores imply distinct pulses of iceberg calving and/or melting, which were probably associated with stepwise retreat of the ice margin. Morainic ridges and glacial-sole markings in the western part of the Saint Anna Trough indicate that the northern-central Barents Sea was a site of a large ice mass during deglaciation; a smaller ice cap is inferred for the Northern Kara Plateau. At later stages, ice retreat on the western flank of the trough was directed towards Franz-Josef Land, and was presumably facilitated by a separation of the Barents Sea and Novaya Zemlya ice domes. Deglaciation of the Saint Anna Trough was completed by ca. 10 ka. High post-glacial sediment fluxes between 10 and 8 ka were probably related to sea-floor/coastal erosion and/or Siberian river discharge during the rising sea level.

Radiocarbon content of pre‐bomb marine mollusks and variations in the 14C Reservoir age for coastal areas of the Barents and Kara Seas, Russia

Fourteen mollusks, collected alive between 1900 and 1945 from the Russian Barents and Kara seas, were analyzed by AMS 14C dating to evaluate variations in the 14C marine reservoir for arctic coastal sites, which is important for correcting ages in paleoenvironmental time-series and advancing understanding of the exchange of carbon. The 14C ages on the mollusks reveal a range of marine reservoir values (R(t)) from 159 14C yr to 764 14C yr. The oldest R(t) values of 764 to 620 14C yr are for the bivalve Portlandia arctica, which often inhabit cold and low salinity waters and muddy substrates. The depleted 14C content for this bivalve reflects possibly the incorporation of old carbon from freshwater inputs and/or the consumption of old organic matter from the underlying sediments and pore waters. Other mollusks with sessile habitats and pelagic food sources gave significantly lower R(t) values between 159 and 344 14C yr. The youngest R(t) values indicate enrichment in 14C and may partially reflect enhanced transfer of 14C-enriched CO2 from the atmosphere to the ocean surface with wind-generated wave agitation. This study underscores that a variety of processes can lead to variable 14C depletion and enrichment of surface waters yielding a ca. 600 year age span for contemporaneous arctic mollusks. There may be added uncertainty in the 14C reservoir correction for deposit-feeder species such as Portlandia sp. and perhaps for certain benthic foraminifera (e.g. Nonion labradoricum) because these taxa often incorporate old organic matter from the substrate. A reservoir correction of ≥700 years may be more appropriate for infaunal, deposit-eater species, particularly in glacier-dominated environments. Mollusks and foraminifera with sessile habits and pelagic food sources should be selected preferentially for 14C dating, because their shells may more closely reflect the 14C content of the global-ocean mixed layer.

Late-glacial and Holocene paleoceanography and sedimentary environments in the St. Anna Trough, Eurasian Arctic Ocean margin

Based on stratigraphical analysis of twelve sediment cores from the Saint Anna Trough, we reconstruct changes in paleoceanography and sedimentary environment during the last deglaciation and the Holocene. Detailed analysis of benthic and planktic foraminiferal fauna, stable oxygen and carbon isotope analysis, lithostratigraphy and radiocarbon dates, are used to reconstruct the following evolution: After the deglaciation of the Saint Anna Trough >13,300 yr B.P. until 9500 yr B.P., the environment was mainly characterised by low biogenic production, carbonate dissolution and deposition of proximal to distal glaciomarine sediments. Intervals with high abundance of the benthic foraminifer Cassidulina teretis, may indicate influx of Atlantic Water at bottom. The transition into the present interglacial started at 9500 yr B.P. reflected by increased production of foraminifera and bivalves. After 8000 yr B.P. there was a marked drop in planktic δ18O followed by a rise in planktic foraminifera and subsequently an increase of C. teretis. These paleoceanographic changes reflect increased heat transport into the area and are coupled to changes in Nordic seas. The early Holocene warming of the Saint Anna Trough were delayed by ca. 2000 years relative to the northeast Nordic seas. Early Holocene sedimentation rates were relatively high (>100 cm/1000 yr), declining drastically after 8000 yr B.P. (<50 cm/1000 yr). This was presumably caused by reduction in winnowing and/or riverine input.

Icebreaker expedition collects key Arctic seafloor and ice data

The recently completed Healy-Oden Trans-Arctic Expedition 2005 (HOTRAX′05) retrieved 29 piston cores averaging nearly 12 meters in length from a complete transect across the central Arctic Ocean (Figure 1). These cores provide a critically-needed sample cache for both a pan-Arctic stratigraphy and a long-awaited paleoclimate record that it is hoped will greatly improve the understanding of how deepwater is exchanged between Arctic basins, how the climate system in the Arctic works over longer time intervals, and how the Arctic system interacts with global systems. The coring was done from the U.S. Coast Guard Cutter (USCGC) Healy, while oceanographic measurements were made from the Swedish icebreaker Oden. In addition to coring and oceanography, HOTRAX mapped the seafloor with multibeam bathymetry and collected chirp sonar profiles that not only mapped the strata to a sub-bottom depth of 50–100 meters, but also provided detailed information on the geologic context of the core sites.

New constraints on the limits of the Barents-Kara ice sheet during the Last Glacial Maximum based on borehole stratigraphy from the Pechora Sea

A new, 14 C-verified borehole stratigraphy provides the first age-controlled reconstruction of the late Quaternary glacial history of the Pechora Sea (southeasternmost Barents Sea). A complete glaciation of the Pechora Sea is confirmed for middle Weichselian time, prior to ca. 35‐ 40 ka. Composition of glacial erratics indicates that ice was moving from or across southernmost Novaya Zemlya and Vaygach Island. After a brief interstadial period with normal marine conditions, the Pechora Sea was affected by a drop in sea level and a drier climate. Subsequently, the late Weichselian Barents-Kara ice sheet occupied the northwestern part of the Pechora Sea, but did not reach the coast of the Pechora lowland, as previously believed. These data provide a new constraint on the Last Glacial Maximum (LGM) ice-sheet limits in the Eurasian Arctic. The inferred direction of the Last Glacial Maximum ice movement in the Pechora Sea was from the northeast, but with a stronger northern component than the penultimate glaciation. The ice sheet retreated early, ca. 13 ka, after which the shallow Pechora Sea was subjected to strong erosion during the postglacial sea-level rise.