Leonid Polyak | Ohio State University (original) (raw)
Papers by Leonid Polyak
Multibeam bathymetry and sub-bottom profiler data acquired in 2011 from R/V Marcus Langseth in a b... more Multibeam bathymetry and sub-bottom profiler data acquired in 2011 from R/V Marcus Langseth in a broad grid over the Chukchi Sea margin reveal multiple glacigenic features on the top and slopes of the outer Chukchi Shelf/Rise and adjacent Borderland. Glacial lineations record a complex pattern of erosion likely formed by both local glaciation and far-traveled ice shelves/streams sourced from the Laurentide, and possibly East Siberian ice sheets. Multiple till units and stacked debris flows indicate recurrent glacial grounding events. Composite till wedges of several hundred meters thick extend the shelf edge by 10-20 km in places. Distribution of ice-marginal features on the Chukchi Rise suggests stepwise deglacial retreat towards the shelf, backing up the broad bathymetric trough at the eastern side of the Rise. Glacigenic features other than extensive iceberg scouring cannot be identified above 350-m depth, and no glacigenic bedforms are present on the current-swept shallow shelf. Despite the resulting uncertainty with the southern extent of the glaciation, the data suggest a widespread grounded-ice presence on the northern Chukchi Shelf, which makes it an important, previously underestimated component of the Arctic paleo-glacial system.
Sediment cores from the Northwind Ridge, western Arctic Ocean, including uniquely preserved calca... more Sediment cores from the Northwind Ridge, western Arctic Ocean, including uniquely preserved calca-
reous microfossils, provide the first continuous proxy record of sea ice in the Arctic Ocean encompassing
more than half of the Quaternary. The cores were investigated for foraminiferal assemblages along with
coarse grain size and bulk chemical composition. By combination of glacial cycles and unique events
reflected in the stratigraphy, the age of the foraminiferal record was estimated as ca 1.5 Ma. Foraminiferal
abundances, diversity, and composition of benthic assemblages, especially phytodetritus and polar
species, were used as proxies for sea-ice conditions. Foraminiferal Assemblage Zone 2 in the Lower
Pleistocene indicates diminished, mostly seasonal sea ice, probably facilitated by enhanced inflow of
Pacific waters. A gradual decrease in ice-free season with episodes of abrupt ice expansion is interpreted
for the Mid-Pleistocene Transition, consistent with climatic cooling and ice-sheet growth in the Northern
Hemisphere. A principal faunal and sedimentary turnover occurred near the EarlyeMiddle Pleistocene
boundary ca 0.75 Ma, with mostly perennial sea ice indicated by the overlying Assemblage Zone 1. Two
steps of further increase in sea-ice coverage are inferred from foraminiferal assemblage changes in the
“Glacial” Pleistocene by ca 0.4 and 0.24 Ma, possibly related to hemispheric (Mid-Brunhes Event) and
Laurentide ice sheet growth, respectively. These results suggest that year-round ice in the western Arctic
was a norm for the last several 100 ka, in contrast to rapidly disappearing summer ice today.
Journal of Quaternary Science, 2001
Palynomorphs were analysed in two sediment cores from the southeastern Barents Sea representing t... more Palynomorphs were analysed in two sediment cores from the southeastern Barents Sea representing the past 8.3 and 4.4 kyr. High dinocyst contents and species diversity enabled the application of the best analogue method to quantitatively reconstruct sea-surface salinities, temperatures and ice cover using 677 modern reference sites from the North Atlantic and Arctic seas, including new data from the Barents Sea reported here. At the southern core site, where waters are affected by the Atlantic inflow, sea-surface conditions were relatively warm and stable between ca. 8000 and 5000 calendar yr BP. In contrast, the past 5 kyr had periods with cooler temperatures and extended ice cover, fluctuating mostly at 1–1.5 kyr frequencies at both sites. Most pronounced coolings occurred around 8.1, 5, 3.5–3.2 and 2.5 ka. The northern site additionally shows younger cooling events, tentatively dated to 1.4, 0.3 and 0.1 ka. Identified variations in sea-surface conditions indicate changes in Atlantic water inputs to the Barents Sea. Our results generally correlate to palaeoclimatic reconstructions from northwestern Eurasia, exemplified by palynological records from Karelia. This correlation suggests that sea-surface variations in the Barents Sea reflect large-scale changes in atmospheric and oceanic interactions between the North Atlantic and the Arctic. Copyright © 2001 John Wiley & Sons, Ltd.
Eos, Transactions American Geophysical Union, 2002
Boreas, 1993
Sparker and shallow drilling data indicate that the Quaternary deposits in the Central Deep of th... more Sparker and shallow drilling data indicate that the Quaternary deposits in the Central Deep of the Barents Sea are mainly composed of glacigenic sediments. They comprise basal till and proximal and distal glaciomarine sediments deposited during the last glacial cycle. Apparent glaciotectonic features imply strong glacial erosion of Mesozoic bedrock. The general ice movement is assumed to have been from off Novaya Zemlya and it is concluded that the whole eastern Barents Sea was covered by the Late Weichselian ice-sheet.
Global and Planetary Change, 2001
High resolution seismoacoustic chirp sonar data and piston cores were collected from the Lomonoso... more High resolution seismoacoustic chirp sonar data and piston cores were collected from the Lomonosov Ridge in the central Ž . Arctic Ocean 858-908N; 1308-1558E . 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 w Ž .
Progress in Oceanography, 2006
Past changes in the Arctic Ocean and its marginal seas have been profound, even during the last 1... more Past changes in the Arctic Ocean and its marginal seas have been profound, even during the last 10,000 years. Understanding these changes, such as those occurring during the transition from glacial to interglacial climates, are important for research on modern processes, because this knowledge provides a framework and unique perspective in which to view the modern physical and biological processes. This paper discusses our current understanding of past environmental change and processes relative to those currently in progress. Special emphasis is placed on the most recent transition from a glacial state to the modern interglacial conditions.
Deep-sea Research Part I-oceanographic Research Papers, 2007
Seafloor mapping of the central Lomonosov Ridge using a multibeam echo-sounder during the Beringi... more Seafloor mapping of the central Lomonosov Ridge using a multibeam echo-sounder during the Beringia/Healy-Oden Trans-Arctic Expedition (HOTRAX) 2005 shows that a channel across the ridge has a substantially shallower sill depth than the $2500 m indicated in present bathymetric maps. The multibeam survey along the ridge crest shows a maximum sill depth of about 1870 m. A previously hypothesized exchange of deep water from the Amundsen Basin to the Makarov Basin in this area is not confirmed. On the contrary, evidence of a deep-water flow from the Makarov to the Amundsen Basin was observed, indicating the existence of a new pathway for Canadian Basin Deep Water toward the Atlantic Ocean. Sediment data show extensive current activity along the ridge crest and along the rim of a local Intra Basin within the ridge structure. r
Quaternary Research, 2007
At least two episodes of glacial erosion of the Chukchi margin at water depths to ∼ 450 m and 750... more At least two episodes of glacial erosion of the Chukchi margin at water depths to ∼ 450 m and 750 m have been indicated by geophysical seafloor data. We examine sediment stratigraphy in these areas to verify the inferred erosion and to understand its nature and timing. Our data within the eroded areas show the presence of glaciogenic diamictons composed mostly of reworked local bedrock. The diamictons are estimated to form during the last glacial maximum (LGM) and an earlier glacial event, possibly between OIS 4 to 5d. Both erosional events were presumably caused by the grounding of ice shelves originating from the Laurentide ice sheet. Broader glaciological settings differed between these events as indicated by different orientations of flutes on eroded seafloor. Postglacial sedimentation evolved from iceberg-dominated environments to those controlled by sea-ice rafting and marine processes in the Holocene. A prominent minimum in planktonic foraminiferal δ 18 O is identified in deglacial sediments at an estimated age near 13,000 cal yr BP. This δ 18 O minimum, also reported elsewhere in the Amerasia Basin, is probably related to a major Laurentide meltwater pulse at the Younger Dryas onset. The Bering Strait opening is also marked in the composition of late deglacial Chukchi sediments.
Eos, Transactions American Geophysical Union, 2005
Earth Surface Processes and Landforms, 2008
The last decade of geophysical seafloor mapping in the Arctic Ocean from nuclear submarines and i... more The last decade of geophysical seafloor mapping in the Arctic Ocean from nuclear submarines and icebreakers reveals a wide variety of glaciogenic geomorphic features at water depths reaching 1000 m. These findings provide new and intriguing insights into the Quaternary glacial history of the Northern Hemisphere. Here we integrate multi-and single beam bathymetric data, chirp sonar profiles and sidescan images from the Chukchi Borderland and Lomonosov Ridge to perform a comparative morphological seafloor study. This investigation aims to elucidate the nature and provenance of ice masses that impacted the Arctic Ocean sea floor during the Quaternary. Mapped glaciogenic bedforms include iceberg keel scours, most abundant at water depths shallower than ~350-400 m, flutes and megascale glacial lineations extending as deep as ~1000 m below the present sea level, small drumlinlike features and morainic ridges and grounding-zone wedges. The combination of these features indicates that very large glacial ice masses extended into the central Arctic Ocean from surrounding North American and Eurasian ice sheets several times during the Quaternary. Ice shelves occupied large parts of the Arctic Ocean during glacial maxima and ice rises were formed over the Chukchi Borderland and portions of the Lomonosov Ridge. More geophysical and sediment core data combined with modeling experiments are needed to reconstruct the timing and patterns of these events.
Quaternary Science Reviews, 2004
Numerous short sediment cores have been retrieved from the central Arctic Ocean, many of which ha... more Numerous short sediment cores have been retrieved from the central Arctic Ocean, many of which have been assigned sedimentation rates on the order of mm/ka implying that the Arctic Basin was starved of sediments during Plio-Pleistocene times. A review of both shorter-term sedimentation rates, through analysis of available sediment core data, and longer-term sedimentation rates, through estimates of total sediment thickness and bedrock age, suggests that cm/ka-scale rates are pervasive in the central Arctic Ocean. This is not surprising considering the physiographic setting of the Arctic Ocean, being a small land-locked basin since its initial opening during Early Cretaceous times. We thus conclude that the central Arctic Ocean has not been a sediment starved basin, either during Plio-Pleistocene times or during pre-Pliocene times. Rigorous chronstratigraphic analysis permits correlation of sediment cores over a distance of B2600 km, from the northwestern Amerasia Basin to the northwestern Eurasia Basin via the Lomonosov Ridge, using paleomagnetic, biostratigraphic, and cyclostratigraphic data. r
Eos, Transactions American Geophysical Union, 2001
Geophysical Research Letters, 1997
Fourteen mollusks, collected alive between 1900 and 1945 from the Russian Barents and Kara seas, ... more 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 palcoenvironmental time-series and advancing understanding of the exchange of carbon. The •4C ages on the mollusks reveal a range of marine reservoir values (R(0) from 159 •4C yr to 764 •4C yr. The oldest R(t) values of 764 to 620 •4C 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 •4C yr. The youngest R(0 values indicate enrichment in •4C and may partially reflect enhanced transfer of •4C-enriched CO2 from the atmosphere to the ocean surface with wind-generated wave agitation. This
Quaternary Science Reviews, 2004
In Arctic and sub Arctic seas, shell growth and/or secondary calcite overgrowth of Neogloboquadri... more In Arctic and sub Arctic seas, shell growth and/or secondary calcite overgrowth of Neogloboquadrina pachyderma (left coiled)-Npl-occur along the pycnocline, and their d 13 C and d 18 O-values are size and weight dependent. However, whereas the Npl 18 O data from the NW Atlantic indicate near-equilibrium conditions with ambient waters and a positive relationship between shell weight and 18 O-content, assemblages from box-cored sediments of the Chukchi Sea (western Arctic) are depleted by B2% with respect to equilibrium values with modern conditions, and depict a negative relationship between shell weight and its d 18 O-value (À0.1570.03%/mg on VPDB scale). A similar feature is also depicted by the dextral form of N. pachyderma (Npd). We associate the reverse shell-size or weight vs. d 18 O relationship to the reverse temperature gradient observed along the thermocline between the surface cold and dilute water layer, and the underlying near 3 C-warmer saline North Atlantic water mass. The analysis of two late to post-glacial sedimentary sequences from the Chukchi Sea indicates that such a water mass stratification with a reverse thermocline persisted throughout the Holocene, thus reflecting an early onset of the modern-like linkage between the Arctic Ocean and the North Atlantic. Moreover, lower d 18 O-values in both Npl and Npd together with larger d 18 O-gradients between the different shell sizes at ca 9-7 ka BP suggest B3 C higher temperatures in the upper North Atlantic water mass, in comparison with the present (approximately +1 C, at the study site), thus likely a higher inflow rate of this water mass during the early Holocene. r
Palaeogeography Palaeoclimatology Palaeoecology, 2004
Distinct cyclicity in lithology and microfaunal distribution in sediment cores from the Mendeleev... more 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. ß
Global and Planetary Change, 2002
Several sediment cores from the Kara Sea have been analyzed with a special emphasis on heavy-mine... more Several sediment cores from the Kara Sea have been analyzed with a special emphasis on heavy-mineral, foraminiferal, and stable-isotopic composition of sediments. The temporal variations of these characteristics, controlled by 14 C ages in some cores, allow us to reconstruct the history of latest Weichselian and Holocene sedimentation in the Kara Sea in relation to the discharge of the large Siberian Ob' and Yenisey rivers. We conclude that during the Last Glacial Maximum (LGM) these rivers were blocked by the Barents -Kara ice sheet that occupied the northern part of the Kara Sea. This blockage likely created a proglacial basin on the Kara Sea shelf; its effect on the Siberian drainage needs further investigation. With deglaciation and sea-level rise, river mouths migrated across the Kara Sea shelf to their modern estuaries. This retreat, combined with high early-Holocene insolation levels, had a profound influence on biota and sedimentation throughout the Kara Sea. Subsequent changes in riverine discharge did not affect the Kara Sea as strongly as during deglaciation and the early Holocene. A significant increase in discharge is inferred to have started after ca. 1.5 ka. D
Quaternary Science Reviews, 2010
Arctic sea-ice extent and volume are declining rapidly. Several studies project that the Arctic O... more Arctic sea-ice extent and volume are declining rapidly. Several studies project that the Arctic Ocean may become seasonally ice-free by the year 2040 or even earlier. Putting this into perspective requires information on the history of Arctic sea-ice conditions through the geologic past. This information can be provided by proxy records from the Arctic Ocean floor and from the surrounding coasts. Although existing records are far from complete, they indicate that sea ice became a feature of the Arctic by 47 Ma, following a pronounced decline in atmospheric pCO2 after the Paleocene–Eocene Thermal Optimum, and consistently covered at least part of the Arctic Ocean for no less than the last 13–14 million years. Ice was apparently most widespread during the last 2–3 million years, in accordance with Earth's overall cooler climate. Nevertheless, episodes of considerably reduced sea ice or even seasonally ice-free conditions occurred during warmer periods linked to orbital variations. The last low-ice event related to orbital forcing (high insolation) was in the early Holocene, after which the northern high latitudes cooled overall, with some superimposed shorter-term (multidecadal to millennial-scale) and lower-magnitude variability. The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.
Geological Society, London, Special Publications, 1996
Quaternary sediment records from the southeastern Barents Sea were used to reconstruct palaeocean... more Quaternary sediment records from the southeastern Barents Sea were used to reconstruct palaeoceanographic environments since the last deglaciation based on lithology, foraminiferal assemblages and AMS lac dates. After the ice sheet retreat at 13 ka BP two pulses of glacimarine sedimentation occurred, separated by non-deposition between approximately 12 and 10.5 kaBP. Proximal sedimentation from meltwater overflows prevailed during the first pulse, whereas the second pulse reflects distal glacimarine environments with some inflow of Atlantic water. Full marine environments were established by 9.3kaBl', associated with increased Atlantic water advection, intensified currents and reduced sedimentation rates. The maximum Atlantic influence lasted until approximately 5 ka BP, followed by an advance of Arctic surface water with winter sea ice cover.
Multibeam bathymetry and sub-bottom profiler data acquired in 2011 from R/V Marcus Langseth in a b... more Multibeam bathymetry and sub-bottom profiler data acquired in 2011 from R/V Marcus Langseth in a broad grid over the Chukchi Sea margin reveal multiple glacigenic features on the top and slopes of the outer Chukchi Shelf/Rise and adjacent Borderland. Glacial lineations record a complex pattern of erosion likely formed by both local glaciation and far-traveled ice shelves/streams sourced from the Laurentide, and possibly East Siberian ice sheets. Multiple till units and stacked debris flows indicate recurrent glacial grounding events. Composite till wedges of several hundred meters thick extend the shelf edge by 10-20 km in places. Distribution of ice-marginal features on the Chukchi Rise suggests stepwise deglacial retreat towards the shelf, backing up the broad bathymetric trough at the eastern side of the Rise. Glacigenic features other than extensive iceberg scouring cannot be identified above 350-m depth, and no glacigenic bedforms are present on the current-swept shallow shelf. Despite the resulting uncertainty with the southern extent of the glaciation, the data suggest a widespread grounded-ice presence on the northern Chukchi Shelf, which makes it an important, previously underestimated component of the Arctic paleo-glacial system.
Sediment cores from the Northwind Ridge, western Arctic Ocean, including uniquely preserved calca... more Sediment cores from the Northwind Ridge, western Arctic Ocean, including uniquely preserved calca-
reous microfossils, provide the first continuous proxy record of sea ice in the Arctic Ocean encompassing
more than half of the Quaternary. The cores were investigated for foraminiferal assemblages along with
coarse grain size and bulk chemical composition. By combination of glacial cycles and unique events
reflected in the stratigraphy, the age of the foraminiferal record was estimated as ca 1.5 Ma. Foraminiferal
abundances, diversity, and composition of benthic assemblages, especially phytodetritus and polar
species, were used as proxies for sea-ice conditions. Foraminiferal Assemblage Zone 2 in the Lower
Pleistocene indicates diminished, mostly seasonal sea ice, probably facilitated by enhanced inflow of
Pacific waters. A gradual decrease in ice-free season with episodes of abrupt ice expansion is interpreted
for the Mid-Pleistocene Transition, consistent with climatic cooling and ice-sheet growth in the Northern
Hemisphere. A principal faunal and sedimentary turnover occurred near the EarlyeMiddle Pleistocene
boundary ca 0.75 Ma, with mostly perennial sea ice indicated by the overlying Assemblage Zone 1. Two
steps of further increase in sea-ice coverage are inferred from foraminiferal assemblage changes in the
“Glacial” Pleistocene by ca 0.4 and 0.24 Ma, possibly related to hemispheric (Mid-Brunhes Event) and
Laurentide ice sheet growth, respectively. These results suggest that year-round ice in the western Arctic
was a norm for the last several 100 ka, in contrast to rapidly disappearing summer ice today.
Journal of Quaternary Science, 2001
Palynomorphs were analysed in two sediment cores from the southeastern Barents Sea representing t... more Palynomorphs were analysed in two sediment cores from the southeastern Barents Sea representing the past 8.3 and 4.4 kyr. High dinocyst contents and species diversity enabled the application of the best analogue method to quantitatively reconstruct sea-surface salinities, temperatures and ice cover using 677 modern reference sites from the North Atlantic and Arctic seas, including new data from the Barents Sea reported here. At the southern core site, where waters are affected by the Atlantic inflow, sea-surface conditions were relatively warm and stable between ca. 8000 and 5000 calendar yr BP. In contrast, the past 5 kyr had periods with cooler temperatures and extended ice cover, fluctuating mostly at 1–1.5 kyr frequencies at both sites. Most pronounced coolings occurred around 8.1, 5, 3.5–3.2 and 2.5 ka. The northern site additionally shows younger cooling events, tentatively dated to 1.4, 0.3 and 0.1 ka. Identified variations in sea-surface conditions indicate changes in Atlantic water inputs to the Barents Sea. Our results generally correlate to palaeoclimatic reconstructions from northwestern Eurasia, exemplified by palynological records from Karelia. This correlation suggests that sea-surface variations in the Barents Sea reflect large-scale changes in atmospheric and oceanic interactions between the North Atlantic and the Arctic. Copyright © 2001 John Wiley & Sons, Ltd.
Eos, Transactions American Geophysical Union, 2002
Boreas, 1993
Sparker and shallow drilling data indicate that the Quaternary deposits in the Central Deep of th... more Sparker and shallow drilling data indicate that the Quaternary deposits in the Central Deep of the Barents Sea are mainly composed of glacigenic sediments. They comprise basal till and proximal and distal glaciomarine sediments deposited during the last glacial cycle. Apparent glaciotectonic features imply strong glacial erosion of Mesozoic bedrock. The general ice movement is assumed to have been from off Novaya Zemlya and it is concluded that the whole eastern Barents Sea was covered by the Late Weichselian ice-sheet.
Global and Planetary Change, 2001
High resolution seismoacoustic chirp sonar data and piston cores were collected from the Lomonoso... more High resolution seismoacoustic chirp sonar data and piston cores were collected from the Lomonosov Ridge in the central Ž . Arctic Ocean 858-908N; 1308-1558E . 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 w Ž .
Progress in Oceanography, 2006
Past changes in the Arctic Ocean and its marginal seas have been profound, even during the last 1... more Past changes in the Arctic Ocean and its marginal seas have been profound, even during the last 10,000 years. Understanding these changes, such as those occurring during the transition from glacial to interglacial climates, are important for research on modern processes, because this knowledge provides a framework and unique perspective in which to view the modern physical and biological processes. This paper discusses our current understanding of past environmental change and processes relative to those currently in progress. Special emphasis is placed on the most recent transition from a glacial state to the modern interglacial conditions.
Deep-sea Research Part I-oceanographic Research Papers, 2007
Seafloor mapping of the central Lomonosov Ridge using a multibeam echo-sounder during the Beringi... more Seafloor mapping of the central Lomonosov Ridge using a multibeam echo-sounder during the Beringia/Healy-Oden Trans-Arctic Expedition (HOTRAX) 2005 shows that a channel across the ridge has a substantially shallower sill depth than the $2500 m indicated in present bathymetric maps. The multibeam survey along the ridge crest shows a maximum sill depth of about 1870 m. A previously hypothesized exchange of deep water from the Amundsen Basin to the Makarov Basin in this area is not confirmed. On the contrary, evidence of a deep-water flow from the Makarov to the Amundsen Basin was observed, indicating the existence of a new pathway for Canadian Basin Deep Water toward the Atlantic Ocean. Sediment data show extensive current activity along the ridge crest and along the rim of a local Intra Basin within the ridge structure. r
Quaternary Research, 2007
At least two episodes of glacial erosion of the Chukchi margin at water depths to ∼ 450 m and 750... more At least two episodes of glacial erosion of the Chukchi margin at water depths to ∼ 450 m and 750 m have been indicated by geophysical seafloor data. We examine sediment stratigraphy in these areas to verify the inferred erosion and to understand its nature and timing. Our data within the eroded areas show the presence of glaciogenic diamictons composed mostly of reworked local bedrock. The diamictons are estimated to form during the last glacial maximum (LGM) and an earlier glacial event, possibly between OIS 4 to 5d. Both erosional events were presumably caused by the grounding of ice shelves originating from the Laurentide ice sheet. Broader glaciological settings differed between these events as indicated by different orientations of flutes on eroded seafloor. Postglacial sedimentation evolved from iceberg-dominated environments to those controlled by sea-ice rafting and marine processes in the Holocene. A prominent minimum in planktonic foraminiferal δ 18 O is identified in deglacial sediments at an estimated age near 13,000 cal yr BP. This δ 18 O minimum, also reported elsewhere in the Amerasia Basin, is probably related to a major Laurentide meltwater pulse at the Younger Dryas onset. The Bering Strait opening is also marked in the composition of late deglacial Chukchi sediments.
Eos, Transactions American Geophysical Union, 2005
Earth Surface Processes and Landforms, 2008
The last decade of geophysical seafloor mapping in the Arctic Ocean from nuclear submarines and i... more The last decade of geophysical seafloor mapping in the Arctic Ocean from nuclear submarines and icebreakers reveals a wide variety of glaciogenic geomorphic features at water depths reaching 1000 m. These findings provide new and intriguing insights into the Quaternary glacial history of the Northern Hemisphere. Here we integrate multi-and single beam bathymetric data, chirp sonar profiles and sidescan images from the Chukchi Borderland and Lomonosov Ridge to perform a comparative morphological seafloor study. This investigation aims to elucidate the nature and provenance of ice masses that impacted the Arctic Ocean sea floor during the Quaternary. Mapped glaciogenic bedforms include iceberg keel scours, most abundant at water depths shallower than ~350-400 m, flutes and megascale glacial lineations extending as deep as ~1000 m below the present sea level, small drumlinlike features and morainic ridges and grounding-zone wedges. The combination of these features indicates that very large glacial ice masses extended into the central Arctic Ocean from surrounding North American and Eurasian ice sheets several times during the Quaternary. Ice shelves occupied large parts of the Arctic Ocean during glacial maxima and ice rises were formed over the Chukchi Borderland and portions of the Lomonosov Ridge. More geophysical and sediment core data combined with modeling experiments are needed to reconstruct the timing and patterns of these events.
Quaternary Science Reviews, 2004
Numerous short sediment cores have been retrieved from the central Arctic Ocean, many of which ha... more Numerous short sediment cores have been retrieved from the central Arctic Ocean, many of which have been assigned sedimentation rates on the order of mm/ka implying that the Arctic Basin was starved of sediments during Plio-Pleistocene times. A review of both shorter-term sedimentation rates, through analysis of available sediment core data, and longer-term sedimentation rates, through estimates of total sediment thickness and bedrock age, suggests that cm/ka-scale rates are pervasive in the central Arctic Ocean. This is not surprising considering the physiographic setting of the Arctic Ocean, being a small land-locked basin since its initial opening during Early Cretaceous times. We thus conclude that the central Arctic Ocean has not been a sediment starved basin, either during Plio-Pleistocene times or during pre-Pliocene times. Rigorous chronstratigraphic analysis permits correlation of sediment cores over a distance of B2600 km, from the northwestern Amerasia Basin to the northwestern Eurasia Basin via the Lomonosov Ridge, using paleomagnetic, biostratigraphic, and cyclostratigraphic data. r
Eos, Transactions American Geophysical Union, 2001
Geophysical Research Letters, 1997
Fourteen mollusks, collected alive between 1900 and 1945 from the Russian Barents and Kara seas, ... more 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 palcoenvironmental time-series and advancing understanding of the exchange of carbon. The •4C ages on the mollusks reveal a range of marine reservoir values (R(0) from 159 •4C yr to 764 •4C yr. The oldest R(t) values of 764 to 620 •4C 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 •4C yr. The youngest R(0 values indicate enrichment in •4C and may partially reflect enhanced transfer of •4C-enriched CO2 from the atmosphere to the ocean surface with wind-generated wave agitation. This
Quaternary Science Reviews, 2004
In Arctic and sub Arctic seas, shell growth and/or secondary calcite overgrowth of Neogloboquadri... more In Arctic and sub Arctic seas, shell growth and/or secondary calcite overgrowth of Neogloboquadrina pachyderma (left coiled)-Npl-occur along the pycnocline, and their d 13 C and d 18 O-values are size and weight dependent. However, whereas the Npl 18 O data from the NW Atlantic indicate near-equilibrium conditions with ambient waters and a positive relationship between shell weight and 18 O-content, assemblages from box-cored sediments of the Chukchi Sea (western Arctic) are depleted by B2% with respect to equilibrium values with modern conditions, and depict a negative relationship between shell weight and its d 18 O-value (À0.1570.03%/mg on VPDB scale). A similar feature is also depicted by the dextral form of N. pachyderma (Npd). We associate the reverse shell-size or weight vs. d 18 O relationship to the reverse temperature gradient observed along the thermocline between the surface cold and dilute water layer, and the underlying near 3 C-warmer saline North Atlantic water mass. The analysis of two late to post-glacial sedimentary sequences from the Chukchi Sea indicates that such a water mass stratification with a reverse thermocline persisted throughout the Holocene, thus reflecting an early onset of the modern-like linkage between the Arctic Ocean and the North Atlantic. Moreover, lower d 18 O-values in both Npl and Npd together with larger d 18 O-gradients between the different shell sizes at ca 9-7 ka BP suggest B3 C higher temperatures in the upper North Atlantic water mass, in comparison with the present (approximately +1 C, at the study site), thus likely a higher inflow rate of this water mass during the early Holocene. r
Palaeogeography Palaeoclimatology Palaeoecology, 2004
Distinct cyclicity in lithology and microfaunal distribution in sediment cores from the Mendeleev... more 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. ß
Global and Planetary Change, 2002
Several sediment cores from the Kara Sea have been analyzed with a special emphasis on heavy-mine... more Several sediment cores from the Kara Sea have been analyzed with a special emphasis on heavy-mineral, foraminiferal, and stable-isotopic composition of sediments. The temporal variations of these characteristics, controlled by 14 C ages in some cores, allow us to reconstruct the history of latest Weichselian and Holocene sedimentation in the Kara Sea in relation to the discharge of the large Siberian Ob' and Yenisey rivers. We conclude that during the Last Glacial Maximum (LGM) these rivers were blocked by the Barents -Kara ice sheet that occupied the northern part of the Kara Sea. This blockage likely created a proglacial basin on the Kara Sea shelf; its effect on the Siberian drainage needs further investigation. With deglaciation and sea-level rise, river mouths migrated across the Kara Sea shelf to their modern estuaries. This retreat, combined with high early-Holocene insolation levels, had a profound influence on biota and sedimentation throughout the Kara Sea. Subsequent changes in riverine discharge did not affect the Kara Sea as strongly as during deglaciation and the early Holocene. A significant increase in discharge is inferred to have started after ca. 1.5 ka. D
Quaternary Science Reviews, 2010
Arctic sea-ice extent and volume are declining rapidly. Several studies project that the Arctic O... more Arctic sea-ice extent and volume are declining rapidly. Several studies project that the Arctic Ocean may become seasonally ice-free by the year 2040 or even earlier. Putting this into perspective requires information on the history of Arctic sea-ice conditions through the geologic past. This information can be provided by proxy records from the Arctic Ocean floor and from the surrounding coasts. Although existing records are far from complete, they indicate that sea ice became a feature of the Arctic by 47 Ma, following a pronounced decline in atmospheric pCO2 after the Paleocene–Eocene Thermal Optimum, and consistently covered at least part of the Arctic Ocean for no less than the last 13–14 million years. Ice was apparently most widespread during the last 2–3 million years, in accordance with Earth's overall cooler climate. Nevertheless, episodes of considerably reduced sea ice or even seasonally ice-free conditions occurred during warmer periods linked to orbital variations. The last low-ice event related to orbital forcing (high insolation) was in the early Holocene, after which the northern high latitudes cooled overall, with some superimposed shorter-term (multidecadal to millennial-scale) and lower-magnitude variability. The current reduction in Arctic ice cover started in the late 19th century, consistent with the rapidly warming climate, and became very pronounced over the last three decades. This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.
Geological Society, London, Special Publications, 1996
Quaternary sediment records from the southeastern Barents Sea were used to reconstruct palaeocean... more Quaternary sediment records from the southeastern Barents Sea were used to reconstruct palaeoceanographic environments since the last deglaciation based on lithology, foraminiferal assemblages and AMS lac dates. After the ice sheet retreat at 13 ka BP two pulses of glacimarine sedimentation occurred, separated by non-deposition between approximately 12 and 10.5 kaBP. Proximal sedimentation from meltwater overflows prevailed during the first pulse, whereas the second pulse reflects distal glacimarine environments with some inflow of Atlantic water. Full marine environments were established by 9.3kaBl', associated with increased Atlantic water advection, intensified currents and reduced sedimentation rates. The maximum Atlantic influence lasted until approximately 5 ka BP, followed by an advance of Arctic surface water with winter sea ice cover.