Laptev Sea Continental Margin Research Articles

FIRST DISCOVERY OF AUTHIGENIC CARBONATES ON THE LAPTEV SEA FLANK OF THE GAKKEL RIDGE (ARCTIC OCEAN)

The article describes the first find of authigenic carbonates on the southern flank of the Gakkel Ridge in the zone of its junction with the Laptev Sea continental margin of the Russian Federation. The samples are represented by dense magnesian calcites and aragonites, including rounded and angular fragments of terrigenous material, as well as microphytoplankton of different ages, spores and pollen of terrestrial and aquatic plants. Elemental and organochemical characteristics indicate the predominance of oxidizing or intermediate between oxidizing and reducing conditions of carbonate crystallization, which may be a consequence of their formation near the bottom surface. The isotopic composition of O, C, and Sr allows us to conclude that the diagenetic carbonates of the Gakkel Ridge were deposited mainly in isotopic equilibrium with bottom water at a temperature of about 0°C, which corresponds to measurements from the ship. A wide range of δ13С (–23.5 до –37.3) indicates that methane was an important, but not the only source of carbon in carbonates. The wide variations in the 87Sr/86Sr (0.70906–0.70933), which correlate with the δ13С values, show that the carbonate-forming fluid was not only modern sea water, but also diagenetic solutions coming from the sedimentary cover together with methane and the products of methane and organic matter oxidation. Intense discharge of heterogeneous methane-bearing fluids may be related to the high modern tectonic activity of the studied region.

STRUCTURE OF THE EARTH’S CRUST OF THE LAPTEV SEA CONTINENTAL MARGIN AND THE ADJACENT PART OF THE EURASIAN BASIN

A 3D model of the Earth’s crust for the Laptev Sea continental margin and the adjacent part of the Eurasian Basin was built using the latest seismic and gravity data. The thickness of the Earth’s crust in the research area equals 7–11 km, which corresponds to a highly extended continental or oceanic crust. Basement formation and sedimentation in this area most likely began in the Late Jurassic. The south-eastern part of the Eurasian Basin is separated from the rest of the basin by a dextral shear zone, the displacement along which during the Paleogene was more than 100 km.

Molecular Geochemistry of the Dispersed Organic Matter in the Late Cenozoic Sediments of the Laptev Sea Continental Margin and Adjacent Part of the Arctic Ocean

Abstract ––The main factors controlling the bulk sedimentation in the Siberian segment of the Lomonosov Ridge (axial part and western slope) and the Laptev Sea continental margin during the late Cenozoic were studied using a complex of geomorphological, lithological, and organic geochemical data. Samples for the study were collected during the cruises of R/V Akademik Fedorov in 2005 and 2007 and nuclear icebreaker Rossiya in 2007. Analysis of the group and molecular composition of the dispersed organic matter (DOM) in bottom sediments has shown that the input of terrigenous sediments enriched in the products of abrasion of lithified rocks determines sedimentation process on the continental slope of the Laptev Sea and in the Amundsen Basin. The individual characteristics of the DOM of the late Cenozoic sediments from the Lomonosov Ridge reflect the wide diversity of sedimentary sources and depositional environments. Subaqueous erosion of edaphogenic products and pre-Holocene sediments plays an important part in sedimentation together with terrigenous flow and ice transport.

Structure of the Khatanga-Lomonosov fracture zone according to seismic data

According to the results of geophysical data interpretation considered the structural features of the Khatanga-Lomonosov fracture zones (HLZ), adapted to the strip joint part on the Laptev Sea continental margin with the structures of the spreading of the Eurasian basin and the Lomonosov ridge. Its reflection in anomalous geophysical fields, in a wave seismic picture on sections is shown on the example of separate links of HLZ and the character of its kinematic interrelations with heterogeneous tectonic elements is illustrated. The tectonic nature, role and place of this fault zone in the geodynamic evolution of the region are substantiated.

Changes in the paleohydrological conditions in the Laptev Sea during the late Pleistocene and Holocene based on a study of aquatic palynomorphs

On the basis of aquatic palynomorph assemblages in sediment cores obtained from the eastern Laptev Sea shelf, major phases of environmental change associated with the last postglacial global sea-level rise can be recognized for the time since 17.5 calendar years BP (cal. ka). It is shown that in the time interval of 17.5–13.0 cal. ka in the western part of the sea there was a very cold-water sea basin with permanent sea-ice cover, 12.3–11.2 cal. ka the outer shelf were characterized by increased precipitation of river-loaded matter in a river-proximal environment of Anabara and Khatanga rivers, and the period of 11.2–7.0 cal ka was marked by enhanced influence of Atlantic water at the Laptev Sea continental margin. Modern-like environments were established in this part of the sea approximately 7.0 cal. ka.

Halocline water modification and along-slope advection at the Laptev Sea continental margin

Abstract. A general pattern in water mass distribution and potential shelf–basin exchange is revealed at the Laptev Sea continental slope based on hydrochemical and stable oxygen isotope data from the summers 2005–2009. Despite considerable interannual variations, a frontal system can be inferred between shelf, continental slope and central Eurasian Basin waters in the upper 100 m of the water column along the continental slope. Net sea-ice melt is consistently found at the continental slope. However, the sea-ice meltwater signal is independent from the local retreat of the ice cover and appears to be advected from upwind locations. In addition to the along-slope frontal system at the continental shelf break, a strong gradient is identified on the Laptev Sea shelf between 122° E and 126° E with an eastward increase of riverine and sea-ice related brine water contents. These waters cross the shelf break at ~ 140° E and feed the low-salinity halocline water (LSHW, salinity S < 33) in the upper 50 m of the water column. High silicate concentrations in Laptev Sea bottom waters may lead to speculation about a link to the local silicate maximum found within the salinity range of ~ 33 to 34.5, typical for the Lower Halocline Water (LHW) at the continental slope. However brine signatures and nutrient ratios from the central Laptev Sea differ from those observed at the continental slope. Thus a significant contribution of Laptev Sea bottom waters to the LHW at the continental slope can be excluded. The silicate maximum within the LHW at the continental slope may be formed locally or at the outer Laptev Sea shelf. Similar to the advection of the sea-ice melt signal along the Laptev Sea continental slope, the nutrient signal at 50–70 m water depth within the LHW might also be fed by advection parallel to the slope. Thus, our analyses suggest that advective processes from upstream locations play a significant role in the halocline formation in the northern Laptev Sea.

Reprint of: Benthic and planktic community changes at the North Siberian margin in response to Atlantic water mass variability since last deglacial times

Abstract The eastward penetration of Atlantic-derived water (ADW) into the Eurasian Basin of the Arctic Ocean was investigated at the western Laptev Sea continental margin for the time since c. 17.6 ka. Using a high-resolution investigation of the lithology, geochemistry, planktic and benthic foraminifers, and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized. In general, ADW was continuously present in the study area. Between 17.6 and 15.4 ka ADW manifested itself through open-water polynyas and associated upwelling events. Comparison between the Laptev Sea and northern Svalbard shelf using Cassidulina neoteretis allows assuming an unmodified subsurface inflow of ADW within its northern branch between 15.4 and 13.2, which was strongest after 14.7-ka and in line with the overall climate amelioration. A local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off-shelf displacement of ADW from the studied site as suggested by the disappearance of C. neoteretis between 12 and 7 ka. As evidenced by an abundance peak in Nonion labradoricum , the sea-ice marginal zone was located at the site around 12–11 ka but then shifted northward during the early Holocene warming. Enhanced ADW inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift-ice margin. The inflow of ADW during mid–late Holocene differed from deglacial times because of the combined influence of northern and eastern ADW branches.

Postglacial and Holocene environmental evolution of the Laptev Sea continental margin inferred from high-resolution records of benthic and planktic foraminifers

Postglacial and Holocene environmental evolution of the Laptev Sea continental margin inferred from high-resolution records of benthic and planktic foraminifers

Seismic geologic structure model for the sedimentary cover of the Laptev Sea part of the Lomonosov Ridge and adjacent parts of the Amundsen Plain and Podvodnikov Basin

Seismic data on the southern (Laptev Sea) extremity of the Lomonosov Ridge were used to develop a new structural model for the sedimentary cover. It permitted a correlation between the seismic cross-sections of the ridge crest and two deep-sea basins: the Podvodnikov Basin and the Amundsen Plain. It is the first time that a seismic model has taken into account both regional seismic-reflection profiles obtained from NP drifting ice stations and recent high-resolution CDP data. Our seismic model agrees both with geological data on the Laptev Sea continental margin and the data obtained from deep-sea drilling into the Lomonosov Ridge under the IODP-302 project. The sedimentary cover of the southern Lomonosov Ridge and adjacent parts of the Amundsen Plain and Podvodnikov Basin was dated at the Aptian–Cenozoic. The sedimentary section is divided by two main unconformities, of Campanian–Paleocene and Oligocene–Early Miocene ages. The cover contains a structurally complicated graben system, which is an extension of the New Siberian system of horsts and grabens, recognized in the shelf. Sedimentation began in the grabens in the Aptian–Albian and ended with their complete compensation in the Paleocene.

Modified Halocline Water over the Laptev Sea Continental Margin: Historical Data Analysis

Abstract Historical hydrographic data (1940s–2010) show a distinct cross-slope difference of the lower halocline water (LHW) over the Laptev Sea continental margins. Over the slope, the LHW is on average warmer and saltier by 0.2°C and 0.5 psu, respectively, relative to the off-slope LHW. The LHW temperature time series constructed from the on-slope historical records are related to the temperature of the Atlantic Water (AW) boundary current transporting warm water from the North Atlantic Ocean. In contrast, the on-slope LHW salinity is linked to the sea ice and wind forcing over the potential upstream source region in the Barents and northern Kara Seas, as also indicated by hydrodynamic model results. Over the Laptev Sea continental margin, saltier LHW favors weaker salinity stratification that, in turn, contributes to enhanced vertical mixing with underlying AW.

Benthic and planktic community changes at the North Siberian margin in response to Atlantic water mass variability since last deglacial times

The eastward penetration of Atlantic-derived water (ADW) into the Eurasian Basin of the Arctic Ocean was investigated at the western Laptev Sea continental margin for the time since c. 17.6 ka. Using a high-resolution investigation of the lithology, geochemistry, planktic and benthic foraminifers, and ostracods on a sediment core from 270 m water depth major steps in the environmental evolution of the region are recognized. In general, ADW was continuously present in the study area. Between 17.6 and 15.4 ka ADW manifested itself through open-water polynyas and associated upwelling events. Comparison between the Laptev Sea and northern Svalbard shelf using Cassidulina neoteretis allows assuming an unmodified subsurface inflow of ADW within its northern branch between 15.4 and 13.2, which was strongest after 14.7-ka and in line with the overall climate amelioration. A local freshwater event at 13 ka followed by shelf flooding and the establishment of a freshened shelf water mass resulted in an off-shelf displacement of ADW from the studied site as suggested by the disappearance of C. neoteretis between 12 and 7 ka. As evidenced by an abundance peak in Nonion labradoricum, the sea-ice marginal zone was located at the site around 12–11 ka but then shifted northward during the early Holocene warming. Enhanced ADW inflow since 7 ka correlated with climate cooling and southward retreat of the seasonal drift-ice margin. The inflow of ADW during mid–late Holocene differed from deglacial times because of the combined influence of northern and eastern ADW branches.

Properties of the Atlantic derived halocline waters over the Laptev Sea continental margin: Evidence from 2002 to 2009

[1] A series of transects carried out in 2002–2009 across the Laptev Sea continental margin show consistent cross‐slope differences of the lower halocline water (LHW). Over the slope the LHW core is on average warmer and saltier by 0.39°C and 0.26 practical salinity unit, respectively, relative to the off‐slope LHW. Underlying Atlantic water (AW) thermohaline properties exhibit an opposite pattern; it is colder and fresher over the slope and warmer and saltier off the slope. Although on‐slope and off‐slope LHWs have different formation histories, our results suggest that an important part of the heat and salt lost from the AW is gained by the overlying LHW over the continental slope area. This implies the role of enhanced vertical mixing over the sloping topography, which contributes to the difference between the on‐ and off‐slope LHW properties. The distribution of chemical tracers (dissolved oxygen and nutrients) provides further evidence supporting this interpretation and additionally suggests that the LHW may also be influenced by water from the outer shelf.

Origin of freshwater and polynya water in the Arctic Ocean halocline in summer 2007

Extremely low summer sea-ice coverage in the Arctic Ocean in 2007 allowed extensive sampling and a wide quasi-synoptic hydrographic and δ 18O dataset could be collected in the Eurasian Basin and the Makarov Basin up to the Alpha Ridge and the East Siberian continental margin. With the aim of determining the origin of freshwater in the halocline, fractions of river water and sea-ice meltwater in the upper 150 m were quantified by a combination of salinity and δ 18O in the Eurasian Basin. Two methods, applying the preformed phosphate concentration (PO *) and the nitrate-to-phosphate ratio (N/P), were compared to further differentiate the marine fraction into Atlantic and Pacific-derived contributions. While PO *-based assessments systematically underestimate the contribution of Pacific-derived waters, N/P-based calculations overestimate Pacific-derived waters within the Transpolar Drift due to denitrification in bottom sediments at the Laptev Sea continental margin. Within the Eurasian Basin a west to east oriented front between net melting and production of sea-ice is observed. Outside the Atlantic regime dominated by net sea-ice melting, a pronounced layer influenced by brines released during sea-ice formation is present at about 30–50 m water depth with a maximum over the Lomonosov Ridge. The geographically distinct definition of this maximum demonstrates the rapid release and transport of signals from the shelf regions in discrete pulses within the Transpolar Drift. The ratio of sea-ice derived brine influence and river water is roughly constant within each layer of the Arctic Ocean halocline. The correlation between brine influence and river water reveals two clusters that can be assigned to the two main mechanisms of sea-ice formation within the Arctic Ocean. Over the open ocean or in polynyas at the continental slope where relatively small amounts of river water are found, sea-ice formation results in a linear correlation between brine influence and river water at salinities of about 32–34. In coastal polynyas in the shallow regions of the Laptev Sea and southern Kara Sea, sea-ice formation transports river water into the shelf’s bottom layer due to the close proximity to the river mouths. This process therefore results in waters that form a second linear correlation between brine influence and river water at salinities of about 30–32. Our study indicates which layers of the Arctic Ocean halocline are primarily influenced by sea-ice formation in coastal polynyas and which layers are primarily influenced by sea-ice formation over the open ocean. Accordingly we use the ratio of sea-ice derived brine influence and river water to link the maximum in brine influence within the Transpolar Drift with a pulse of shelf waters from the Laptev Sea that was likely released in summer 2005.

Atmospheric controlled freshwater release at the Laptev Sea continental margin

Considerable interannual differences were observed in river water and sea-ice meltwater inventory values derived from d18O and salinity data in the Eurasian Basin along the continental margin of the Laptev Sea in the summers of 1993 and 1995, and in the summers of 2005 and 2006 during Nansen and Amundsen Basins Observational system (NABOS) expeditions. The annually different pattern in river and sea-ice meltwater inventories remain closely linked for all of the years studied, which indicates that source regions and transport mechanisms for both river water and sea-ice formation are largely similar over the relatively shallow Laptev Sea Shelf. A simple Ekman trajectory model for surface Lagrangian particles based solely on wind forcing can explain the main features observed between years with significantly different wind patterns and vorticities, and can also explain differences in river water distributions observed for years with a generally similar offshore wind setting. An index based on this simplified trajectory model is rather similar to the vorticity index, but reflects the hydrology on the shelf better for distinctive years. This index is not correlated with the Arctic Oscillation, but rather with a local mode of oscillation, which controls the outflow and distribution of the Eurasian Basin major freshwater source on an annual timescale. Keywords: Arctic Ocean halocline; atmospheric forcing; Laptev Sea; stable isotopes; water masses. (Published: 1 January 2011) Citation: Polar Research 2011, 30: 5858 - DOI: 10.3402/polar.v30i0.5858

Lithogenic and biogenic particle fluxes on the Lomonosov Ridge (central Arctic Ocean) and their relevance for sediment accumulation: Vertical vs. lateral transport

Investigations of lithogenic and biogenic particle fluxes using long-term sediment traps are still very rare in the northern high latitudes and are restricted to the arctic marginal seas and sub-arctic regions. Here data on the variability of fluxes of lithogenic matter, CaCO 3, opal, and organic carbon and biomarker composition from the central Arctic Ocean are presented for a 1-year period. The study was carried out on material obtained from a long-term mooring system equipped with two multi-sampling traps, at 150 and 1550 m depth, and deployed on the southern Lomonosov Ridge close to the Laptev Sea continental margin from September 1995 to August 1996. In addition, data from surface sediments were included in the study. Annual fluxes of lithogenic matter, CaCO 3, opal, and particulate organic carbon were 3.9, 0.8, 2.6, and 1.5 g m −2 y −1, respectively, in the shallow trap and 11.3, 0.5, 2.9, and 1.05 g m −2 y −1, respectively, in the deep trap. Both the shallow and the deep trap showed significant variations in vertical flux over the year. Higher values were found from mid-July to the end of October (total mass flux of 75–130 mg m −2 d −1 in the shallow trap and 40–225 mg m −2 d −1 in the deep trap). During all other months, fluxes were fairly low in both traps (most total mass flux values <10 mg m −2 d −1). The interval of increased fluxes can be separated into (1) a mid-July/August maximum caused by increased primary production as documented in high abundances of marine biomarkers and diatoms and (2) a September/October maximum caused by increased influence of Lena River discharge indicated by maximum lithogenic flux and large amounts of terrigenous/fluvial biomarkers in both traps. During September/October, total mass fluxes in the deep trap were significantly higher than in the shallow trap, suggesting a lateral sediment flux at greater depth. The lithogenic flux data also support the importance of sediment input from the Laptev Sea for the sediment accumulation on the Lomonosov Ridge on geological time scales, as indicated in sedimentary records from this region.

History of freshwater runoff across the Laptev Sea (Arctic) during the last deglaciation

Sediment core PS2458 from the Laptev Sea continental margin (983-m water depth) stems from a position close to the paleoriver mouth of Lena and Yana rivers. It was dated by AMS- 14C and analyzed in high resolution for oxygen isotopes of planktic foraminifers. Except the uppermost 100 cm and possibly the lowermost meter of the 8-m-long core, the sediments were deposited during the last deglaciation (14.5–8.0 cal-ka). According to 210Pb data, the uppermost 100 cm represents only the last 200 years. Planktic foraminifers are present throughout the dated deglacial interval, with the exception of a short time after ca. 13 cal-ka. Taking into account the global “ice volume effect” on the oxygen isotopic composition of the foraminifers, the isotopic record is considered to reflect salinity changes which were influenced by variable freshwater runoff and a growing marine influence during the postglacial transgression of the Laptev Sea shelf. The most conspicuous feature in the isotopic record is an outstanding peak dated to ca. 13 cal-ka. It is proposed that it represents a rapid outburst of large amounts of freshwater, possibly from an ice-dammed lake in the hinterland. Possible correlations to the onset of the cool Younger Dryas event in the northern hemisphere are discussed.

The Arctic plate boundary

Earthquakes provide information on the regional segmentation and seismotectonics of the poorly known boundary between the Eurasian and the North American plates from the Knipovich Ridge to the Laptev Sea continental margin. To this end, we have sorted earthquake epicenter locations and focal mechanism solutions from global and regional catalogs, assessed location errors and network detectabilities, and compiled a well‐constrained Arctic Catalog of events north of 72°N for the period 1955–1999. The seismological data are integrated with bathymetry and potential field data to locate ridge and transform segments along the Arctic mid‐oceanic ridge (MOR), and we suggest that the plate boundary is divided into four regional provinces. The Spitsbergen Transform System comprises a series of short ridge and transform segments. The seismicity is constrained to the plate boundary in the south but is diffuse in the north. Gakkel Ridge has clustered and focused seismicity and relatively elongated ridge segments cut by small‐offset transforms. A first‐order change in axis trend at 60°E divides the ridge into two areas of different morphology and geophysical character. The West Gakkel Ridge has an accentuated relief and high‐amplitude magnetic anomalies, whereas the East Gakkel Ridge has a smoother relief and lower magnetic amplitudes. At the Laptev Sea continental slope, the seismicity indicates that the change from ultraslow seafloor spreading to active continental rifting occurs over a less than 60 km wide continent–ocean transition (COT) with a 150–200 km long sheared margin segment.

Accumulation of particulate organic carbon at the Eurasian continental margin during late Quaternary times: controlling mechanisms and paleoenvironmental significance

Data on the amount and composition of organic carbon were determined in sediment cores from the Kara and Laptev Sea continental margin, representing oxygen isotope stages 1–6. The characterization of organic matter is based on hydrogen index (HI) values, n-alkanes and maceral composition, indicating the predominance of terrigenous organic matter through space and time. The variations in the amount and composition of organic carbon are mainly influenced by changes in fluvial sediment supply, Atlantic water inflow, and continental ice sheets. During oxygen isotope stage (OIS) 6, high organic carbon contents in sediments from the Laptev Sea and western East Siberian Sea continental margin were probably caused by the increased glacial erosion and further transport in the eastward-flowing boundary current along the continental margin. During OIS 5 and early OIS 3, some increased amounts of marine organic matter were preserved in sediments east of the Lomonosov Ridge, suggesting an influence of nutrient-rich Pacific waters. During OIS 2, terrigenous organic carbon supply was increased along the Barents and western Kara Sea continental margin caused by extended continental ice sheets in the Barents Sea (Svalbard to Franz Josef Land) area and increased glacial erosion. Along the Laptev Sea continental margin, on the other hand, the supply of terrigenous (organic) matter was significantly reduced due to the lack of major ice sheets and reduced river discharge. Towards the Holocene, the amount of total organic carbon (TOC) increased along the Kara and Laptev Sea continental margin, reaching average values of up to 0.5 g C cm −2 ky −1. Between about 8 and 10 ka (9 and 11 Cal ka), i.e., during times when the inner shallow Kara and Laptev seas became largely flooded for the first time after the Last Glacial Maximum, maximum supply of terrigenous organic carbon occurred, which is related to an increase in coastal erosion and Siberian river discharge. During the last 8000 years, the increased amount of marine organic carbon preserved in the sediments from the Kara and Laptev Sea continental margin is interpreted as a result of the intensification of Atlantic water inflow along the Eurasian continental margin.

Late Quaternary dinoflagellate cyst stratigraphy at the Eurasian continental margin, Arctic Ocean: indications for Atlantic water inflow in the past 150,000 years

Four sediment cores located at the Eurasian continental margin underlying the Atlantic layer have been studied for their dinoflagellate cyst content. Concentrations of distinct dinoflagellate cyst taxa display fluctuations in the late Quaternary, which are linked to changes in the inflow of relatively warm Atlantic surface and near-surface waters, resulting in increased local production of cysts in certain time intervals. Based on the assumption that marked changes in strength of inflow occurred synchronously at the Eurasian continental margin, concentration maxima can be used to correlate sediment cores. A dinoflagellate cyst record from the northern Barents Sea continental margin has been related to the stable oxygen isotope and paleomagnetic records to provide direct chronological information. The combination of these methods permits definition of stratigraphic sections equivalent to oxygen isotope stages in carbonate-poor sequences from the Eurasian continental margin. Previous age models of sediment cores are revised, based on dinoflagellate cyst abundance peaks and species distribution, but a firm chronostratigraphy of sedimentary sequences at the eastern Laptev Sea continental margin cannot be established because of the weak signal at the sites furthest from Fram Strait. In the past 150,000 years, the influence of Atlantic (sub-) surface waters generally decreased from west to east along the Eurasian continental margin, in particular during the glacials. Pronounced concentration maxima of cosmopolitan and temperate–subpolar dinoflagellate cysts indicate the inflow of Atlantic waters and seasonally increased production of cysts in the Holocene and Eemian. The Holocene is well-marked at the entire Eurasian continental margin but it is more difficult to assess the extent of (sub-) surface water inflow during the Eemian, which may have only reached the western Laptev Sea continental margin.

The Late Quaternary evolution of the western Laptev Sea continental margin, Arctic Siberia—implications from sub-bottom profiling

High-resolution seismic profiles (PARASOUND, 4 kHz) of the western Laptev Sea continental margin and the adjacent Vilkitsky Strait were studied in order to gain new evidence about the Weichselian glaciations in Central Siberia and to test reconstructions of maximum ice sheet extents. Four regionally correlatable seismic units, named I (youngest) to IV, were identified in the upper Quaternary sedimentary succession: (I) a thin drape; (II) prograding wedge-shaped deposits along the shelf edge; (III) layered sediments of draping and infilling character with increasing thickness towards the western Laptev Sea shelf edge and the Vilkitsky Strait; (IV) stacked debris-flow deposits. The thin drape of unit I is radiocarbon-dated to Holocene and mainly deposited during the transgression of the Laptev Sea. The wedge-shaped deposits of unit II are interpreted as river deltas, referring to point sources along the shelf edge during the Late Weichselian sea-level lowstand. This indicates that the river input across the Laptev Sea shelf was continuous during marine isotope stage (MIS) 2. The layered sediments of unit III suggest hemipelagic conditions indicative of a sea-level highstand. The pronounced thickening of unit III towards the shelf edge of the western Laptev Sea reflects the lowering of the global sea level during MIS 3. This is associated with increased riverine input due to the northward shift of the Siberian coastline. The stacked debris-flow deposits of unit IV extend continuously from the shelf edge in the Vilkitsky Strait to the continental rise of the western Laptev Sea continental margin. They indicate that large quantities of sediments were deposited directly on the upper continental slope during advances of the Kara Sea ice sheet to the shelf break. These ice-proximal conditions are presumably linked to the Middle Weichselian glaciation (MIS 4). Our evidence confirm earlier reconstructions, suggesting that in central Siberia, the Middle Weichselian glaciation (MIS 4) was of larger extent than the Late Weichselian glaciation (MIS 2).