www.elsevier.comrlocatergloplacha
The extent of the Late Weichselian ice sheet in the southeastern Barents Sea
Valery Gataullin
a,1, Jan Mangerud
b,2, John Inge Svendsen
c,)aOil and Gas Research Institute, 5 Exporta Street, LV-1226 Riga, LatÕia
bDepartment of Geology, UniÕersity of Bergen, Allegt. 41, N-5007 Bergen, Norway
cCentre for Studies of the EnÕironment and Resources, UniÕersity of Bergen, Høyteknologisenteret, N-5020 Bergen, Norway Received 15 January 2000; accepted 23 May 2001
Abstract
We have compiled a large number of seismic records and descriptions of sediment cores obtained from the southeastern
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Barents Sea Pechora Sea by former Soviet Union institutions. Five major seismostratigraphic units SSU-I–V were
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recognized. The oldest till SSU-V is mainly confined to the southernmost area and is covered by a 100–150-m-thick wedge
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of Middle Weichselian marine sediments SSU-IV distal to the mouth of the Pechora River. Three Late Weichselian ice sheet margins are identified on the Pechora Sea shelf. The oldest is named the Kolguev Line and it marks the southern limit
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of the youngest till sheet SSU-III in the Barents Sea as well as the northern, erosional limit of SSU-IV and -V. The Kolguev Line marks the maximum extension of the Barents Ice Sheet during the Late Weichselian. The Kurentsovo Line, which is located 50–100 km further to the north, is much more expressed than the Kolguev Line and corresponds with long
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ice-pushed bedrock ridges and till ridges. Up to 100-m-thick accumulations of glaciomarine sediments SSU-II were mapped on the southern side of the Kolguev Line whereas less than 10–20 m where found on the northern side of the inferred ice sheet margin, indicating that the ice front remained at this position for considerable time. The youngest line is the southern continuation of the Admiralty Banks moraines, which have previously been mapped along the western margin of Novaya Zemlya. The Kolguev and Kurentsovo lines were both formed by an ice sheet centered in the Barents Sea, whereas the Admiralty Banks moraines were deposited from an ice sheet over Novaya Zemlya during the final stage of the Late Weichselian, possibly during the Younger Dryas. Submerged shorelines of Late WeichselianrEarly Holocene ages have been identified on the shelf down to a water depth of 50–70 m indicating a modest glacio-isostatic depression that partly compensated the sea-level fall during the last glacial maximum.q2001 Elsevier Science B.V. All rights reserved.
Keywords: Late Weichselian; Barents Sea; ice sheet; sea level; glaciation; seismostratigraphy
)Corresponding author. Fax:q47-55-58-9687.
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E-mail addresses: gtln@dtrans.lv V. Gataullin ,
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jan.mangerud@geol.uib.no J. Mangerud ,
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john.svendsen@smr.uib.no J.I. Svendsen .
1Currently at the Byrd Polar Research Center, Ohio State University, 1090 Carmack Rd., Columbus, OH 43210, USA. Fax:
q1-614-2924697.
2Fax:q47-55-58-9416.
1. Introduction
Comprehensive investigations during the last decade have provided unambiguous evidence for the existence of a major Late Weichselian ice sheet on the Barents Sea shelf that inundated Svalbard and
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Franz Josef Land e.g. Landvik et al., 1998 . Until
0921-8181r01r$ - see front matterq2001 Elsevier Science B.V. All rights reserved.
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PII: S 0 9 2 1 - 8 1 8 1 0 1 0 0 1 3 5 - 7
recently, it was a common view that the southeastern margin of this shelf-centered ice sheet expanded well onto the Russian mainland Peltier, 1994; Grosswald,Ž 1998 . New land-based data from Russia contradict. this view ŽAstakhov et al., 1999; Forman et al., 1999; Mangerud et al., 1999 . According to a recent.
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reconstruction by Svendsen et al. 1999 , subse-
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quently supported by Forman et al. 1999 , the Late Weichselian ice sheet terminated on the continental shelf in the southeastern Barents Sea and along the eastern margin of the Novaya Zemlya Trough in the
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Kara Sea Fig. 1 . Ongoing land-based investigations indicate that the Russian mainland was inundated by two preceding ice sheet advances after the last inter- glacial, one during the Early Weichselian 80–100Ž
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ka and another during the Middle Weichselian 50–
60 ka. ŽAlexanderson et al., 2001; Henriksen et al., 2001; Houmark-Nielsen et al., 2001; Mangerud et al., 2001 ..
In order to locate and document the Late Weich- selian ice sheet boundary, we have investigated a large volume of geological and geophysical data from the continental shelf in the southeastern corner
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of the Barents Sea, the Pechora Sea Figs. 1 and 2 . The Quaternary deposits in this region have previ- ously been investigated in connection with geotech- nical exploration of oil- and gas-prone fields ŽKrapivner et al., 1988; Onishenko and Bondarev, 1988; Okuneva and Gataullin, 1990; Melnikov and Spesivcev, 1995; Melnikov et al., 1997 . Here, we. present a reinterpretation of the seismic profiles em- phasizing the ice sheet history. The sedimentological and stratigraphic interpretations were verified by borehole data. The distribution of glacial and marine sediments has been used to ascertain the Late Weich- selian ice sheet extent. Age constrains based on a series of AMS dates from selected boreholes are
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presented in a separate paper by Polyak et al. 2000 .
Fig. 1. Map of the Barents and Kara Seas showing the maximum ice sheet limits during EarlyrMiddle Weichselian and Late Weichselian
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according to Svendsen et al. 1999 . The approximate location of the Admiralty Bank moraines to the west of Novaya Zemlya is from
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Gataullin and Polyak 1997b . The study area in the Pechora Sea is framed.
2. Material and methods
The investigated shallow seismic reflection pro- files comprise about 22,000 km of lines that were obtained in 1978–1997 by various institutions and
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expeditions of the former USSR Fig. 2 . The data include mainly single-channel sparker profiles Ž18,000 km. recorded by using a low frequency Ž150–600 Hz sound source towed at a water depth. of 1 m. These records have a penetration of 100–250 m and a resolution of 5–10 m. Some sparker profiles
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were run with higher frequencies up to 1–2.5 kHz and a tow depth of 50–60 m which gave better
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resolution up to 1 m , but less penetration. We have also investigated all available high frequency 5.6–Ž
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8.8 kHz echosounding profiles 2500 km and Para-
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sound profiles 1500 km , enabling a resolution of less than 1 m. These latter profiles illustrate the structure of the upper soft sediments, but do not penetrate compact tills.
The seismic lines were usually spaced at intervals of around 10–20 km. On some oil- and gas-prone areas, more detailed surveys have been provided, with a spacing from 3 to less than 1 km between the profiles. On the basis of all these seismic profiles, continuous geological cross-sections at a horizontal scale of 1:500,000 were constructed. A sound veloc- ity of 1460 mrs was used for calculating the water depths and 1500–1600 and 1600–1800 mrs were used for estimating the thickness of soft marine
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sediments and compacted glacial diamictons tills , respectively.
Fig. 2. Location map showing the analyzed seismic profiles and sediment cores.
()V.Gataullinetal.rGlobalandPlanetaryChange312001453–474
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Fig. 3. Sediment cores from the southern Pechora Sea. The cores are plotted approximately from WNW–ESE. The seismostratigraphic units SSU-I–V are indicated and
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correlated. The core locations are shown on Fig. 2. AMS radiocarbon dates are from Polyak et al. 2000 . Notice that core no. 111 is located to the north of the Kolguev Line whereas all other cores shown on this figure are located to the south of this line.
Interpretation of seismic profiles in the shallow Pechora Sea is complicated by numerous multiples.
Moreover, according to our interpretation the Quater- nary sediments are often gas-saturated andror frozen.
The mapping of areas shallower than 40–50 m is therefore primarily based on sediment core data ob- tained from approximately 50 boreholes drilled by
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Arctic Marine Geotechnical Expedition AMIGE ,
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Murmansk, Russia Fig. 2 . A selection of these cores is shown on Fig. 3 and some are indicated on
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the seismic profiles Figs. 9–12 . The average length of the boreholes is 50–70 m, maximum 130 m, but not all of them penetrated into the pre-Quaternary
bedrock. The core diameter is 76–89 mm, the core recovery varies from 40% to 100% depending on the lithology. Descriptions of sedimentary structures and measurements of physical properties, such as den- sity, water content and undrained shear strength, were performed on board the ship for all boreholes ŽAMIGE and Research Institute for Marine Geology and Geophysics, Riga, Latvia, unpublished reports ..
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Some bore-holes marked in Fig. 2 by a star were investigated in more detail using magnetic suscepti- bility, grain size, mineralogical composition, petrog- raphy and roundness of stones, geochemistry and
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biostratigraphy Gataullin, 1992 . Samples from key
Fig. 4. Bathymetric map constructed from the geophysical and navigation data. Inferred Late Weichselian ice sheet limits are indicated with solid lines. The locations of described submerged shorelines in the southern Pechora Sea are also marked.
boreholes were recently AMS radiocarbon dated ŽPolyak et al., 2000 ..
Based on the continuous tracing of seismic boundaries, we have constructed a bathymetric map ŽFig. 4 and four isopach maps showing the thickness. and distribution of the main seismostratigraphic units ŽFigs. 5–8 . All maps were compiled in a digitized. Mercator projection at a working scale of 1:500,000.
3. Bathymetry
Most of the investigated area is shallower than 100 m and is characterized by a flat and even sea floor bounded by the Geese Trough and the Southern Novaya Zemlya Trough where the water depths reach
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200–300 m Fig. 4 . The slopes of the Novaya Zemlya Trough are steep on the Novaya Zemlya side and gentle on the other. At the southern end of Novaya Zemlya, the coastline is strongly indented with numerous bays, ledges and small islands sub- parallel to the general geological structures. The Kara Gate Strait between Novaya Zemlya and Vaigach Island is an S-shaped, 5–10-km-wide chan- nel with a sill water depth of 105–110 m. The sea floor topography in this channel is very uneven with many sub-parallel troughs and rough ridges reflect- ing the underlying bedrock structures, indicating a thin and discontinuous cover of marine and glacial sediments.
4. Seismic stratigraphy and sediment distribution
4.1. Pre-Quaternary bedrock
Precambrian granites and gneisses outcrop near the Kola Peninsula west of the study area OkulitchŽ et al., 1989 . Paleozoic terrigenous and carbonaceous. rocks, Triassic red clay- and silt stone and Jurassic black clay-, silt- and sand stones occur in narrow zones along the Kola–Timan coast of mainland Rus- sia and along Novaya Zemlya. In the rest of the Pechora Sea, the Quaternary sequence is underlain by Cretaceous sediments comprising more than 2000 m in thickness and consisting of dark gray clay, silt
and sand Geologija SSSR, 1970; Gramberg, 1988;Ž Okulitch et al., 1989 ..
4.2. Glaciotectonic deformation of the bedrock
The borings show that the lower part of the sampled bedrock sediments have an undisturbed hor- izontal bedding. Above, there are small folds that gradually increase in size upwards, and mylonitized
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fractures dipping 25–408 become frequent. Within the uppermost 5–10 m, the bedrock is often strongly deformed and displays a banded structure distinctly different from the primary bedding. Small thrust faults, disharmonic drag and flow folds are common.
These features indicate that the bedrock has experi- enced a strong shear stress from glacial flow Gataul-Ž lin et al., 1993; Gataullin and Polyak, 1997a . On. several seismic profiles, parallel dipping reflectors within the Cretaceous sequence become abruptly fractured near the surface of the bedrock. We inter- pret all these features as the result of glacio-tectonic dislocation of the pre-Quaternary strata. In the area with Triassic rocks, the glacio-tectonic deformations are confined to a thin zone, usually not exceeding 1 m, whereas in the soft Cretaceous sediments the deformed zone may be as thick as 20–25 m.
4.3. Quaternary deposits
The base of the Quaternary deposits, which are resting on Mesozoic strata, is normally recognized as an erosional unconformity that corresponds with the
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so-called Upper Regional Unconformity URU in the western Barents Sea Solheim and Kristoffersen,Ž 1984 . The thickness of the Quaternary sequence. varies from a few meters in the northern part of the study area to more than 100 m further south. In the northwestern part, we have mapped three major seis- mostratigraphic units, SSU-I–III, which have been previously distinguished in the Barents Sea EpshteinŽ and Gataullin, 1993; Gataullin et al., 1993; Polyak et al., 1995 . These units were interpreted as Holocene.
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marine sediments SSU-I , Late Weichselian glacio-
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marine sediments SSU-II , and Late Weichselian till ŽSSU-III . In the southeastern Pechora Sea, we have. identified two additional units, SSU-IV and -V, which appear to be older. A description of all units is given below.
4.4. SSU-III and -V— Weichselian tills
SSU-V is the oldest Quaternary seismo-strati- graphic unit. However, it is absent in the northern area where SSU-III is normally the lowermost unit.
These two units have similar characteristics, and in several places, they are difficult or impossible to distinguish from each other. We have therefore con- structed an isopach map showing the composite dis-
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tribution of both units Fig. 5 .
On the seismic records, an opaque chaotic type of wave pattern with short, diversely oriented reflectors ŽFigs. 9–12 characterizes SSU-III and -V. The base. of these units nearly everywhere forms a strong,
relatively even or smoothly undulating reflector in- terpreted as the top of the bedrock, the Upper Re-
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gional Unconformity URU . This reflector is less distinct or sometimes disappears completely, when the bedrock is glacially disturbed. Dipping, sub- parallel reflectors in this case recognize the bedrock surface. At depths more than 100–120 m below sea level, the top surfaces of SSU-III and -V are irregu- lar and hummocky, whereas at shallower depths they are normally smoothed.
SSU-III can be traced continuously from the cen- tral Barents Sea in the northwest Gataullin et al.,Ž 1993; Polyak et al., 1995 and well into the Pechora. Sea. According to our mapping, the southern bound-
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Fig. 5. Isopach map showing the combined thickness of SSU-III and -V Late Weichselian and EarlyrMiddle Weichselian tills . Inferred
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Late Weichselian ice sheet limits are marked with solid lines. The data base seismic profiles and cores used for constructing this particular isopach map is shown on the inserted map.
ary of SSU-III corresponds with the Kolguev Line ŽFig. 5, see below . SSU-V is mostly confined to the. southeastern part of the Pechora Sea where it is underlying SSU-IV. To the northwest, this unit can be traced underneath SSU-III for some distance and is seen sporadically on the profiles from the areas farther north.
The sediment cores show that SSU-V and -III are composed of firm, dark-grey, muddy diamictons with
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a pebble )10 mm content of up to 5–6%. The diamictons are usually overconsolidated, with bulk densities of up to 2.3 grcm3and a low water content Ž10–20% of wet weight . The lowermost parts of the. units often contain detached and deformed blocks of
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soft local Mesozoic mainly Cretaceous deposits, reflecting subglacial entrainment of the substratum ŽSættem, 1990; Gataullin et al., 1993; Gataullin and Polyak, 1997a . Up-section, the amount of bedrock. inclusions gradually diminishes and the diamictons become more homogeneous with a higher content of long transported pebbles. The hard-rock erratics con-
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sist predominantly 30–40% of dark-grey and black microcrystalline limestone’s, characteristic for the Paleozoic strata of Novaya Zemlya, Vaigach Island and Pai-Khoi Ridge. In the eastern part of the Pe- chora Sea, where the diamictons are represented by SSU-V, the amount of limestone increases consider- ably and reaches up to 75% ŽGataullin, 1992;
Epshtein et al., 1999 . The pebbles are mostly angu-. lar or sub-angular with ice-grounded facets that oc- casionally are striated. The diamictons contain usu- ally broken mollusc shells as well as Quaternary and Cretaceous foraminifers that display features of rede- position from older deposits cf. Hald et al., 1990;Ž Polyak and Mikhailov, 1996 ..
Sediments with similar acoustic and lithological characteristics are found throughout the Barents Sea and are interpreted as glaciogenic sediments, mainly basal tills Solheim and Kristoffersen, 1984; VorrenŽ et al., 1989; Elverhøi et al., 1990; Sættem et al., 1992; Gataullin et al., 1993 . We interpret SSU-III as. the southern continuation of the Late Weichselian till sheet in the Barents Sea, whereas SSU-V is consid- ered to represent an older till bed. We will not discuss SSU-V further in this paper, and here just state that we correlate it with an extensive Middle Weichselian glaciation that ended at the Markhida Moraine in the Pechora River Valley Astakhov etŽ
al., 1999; Mangerud et al., 1999, 2001; Svendsen et al., 1999; Henriksen et al., 2001 . In some areas,. SSU-III and -V may include iceberg turbated sedi- ments and redeposited glacial sediments. For exam- ple, the diamictic wedges to the south of Kolguev Island may be a debris flow andror a solifluction
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tongue Figs. 11 and 12 . The absence of or patchy distribution of SSU-V to the north of the Kolguev Line is interpreted as a result of glacial erosion by the Late Weichselian ice sheet, subsequently deposit- ing SSU-III.
4.5. SSU-IV— MiddlerLate Weichselian prodeltaic marine sediments
SSU-IV is confined to the southern part of the study area where it occurs stratigraphically above
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SSU-V Figs. 3 and 6 . However, we will emphasize that the seismic signature of SSU-IV and -II are similar in some areas, so the apparent sharp geologi- cal limits of the two units may not always be real.
On the southern slope of the Novaya Zemlya Through, the seismic profiles reveal a large, up to
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100–150-m-thick sediment wedge Fig. 6 with pro- nounced progradational pattern of seismic signature ŽFig. 9 . A similar sequence, up to 60-m thick, is. found in the Pomorsky Strait, between Kolguev Is-
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land and the mainland Figs. 11 and 12 . In the shallow southern Pechora Sea, the distribution of SSU-IV is mainly based on core data Fig. 6 insetŽ and Figs. 2 and 3 . These cores penetrate into a dark. gray till correlated with SSU-V that is overlain by weakly laminated marine, prodeltaic mud of SSU-IV.
A series of AMS dates conducted on mollusc shells, benthic foraminifers and terrestrial plant detritus from these sediments yielded ages in the range 23–42 ka ŽPolyak et al., 2000. ŽFig. 3 . In the sediment cores,. this unit is truncated by an erosional boundary and covered by cross-stratified sand of early Holocene age. The SSU-IV fills a valley-shaped depression extending from the Pechora River delta towards the
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Kara Gate Strait Fig. 6 . Our interpretation is that the lower part of this marine sequence accumulated in relatively deep water during the Middle Weich- selian. The younger part of SSU-IV was deposited during a northward progradation of the Pechora River delta, probably reflecting a falling sea level. Due to subsequent erosion, the exact age of the youngest
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Fig. 6. Isopach map of SSU-IV MiddlerLate Weichselian marinerprodeltaic sediments . Inferred ice sheet limits are marked with solid
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lines. The data base seismic profiles and cores used for constructing this particular isopach map is shown on the inserted map.
part of this sequence is unknown and may extend well into the Late Weichselian.
4.6. SSU-II— Late Weichselian glaciomarine sedi- ments
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SSU-II has an uneven pounded distribution and is mainly confined to areas deeper than 40–60 m ŽFigs. 7, 9 and 10 . Sediments of this unit only. occasionally fill small local depressions on till sur- faces in the shallow part of the shelf. SSU-II can be traced more continuously in deeper water with an average thickness of about 10–20 m. Much thicker accumulations were recorded in paleo-depressions to
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the south of the Geese Bank 100 m , to the north-
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east of the Kolguev Island 50–60 m and in the
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Southern Novaya Zemlya Trough 75–80 m Fig.
7 . SSU-II is easily distinguished from the underly-. ing SSU-III by its clearly stratified acoustic signature with continuous sub-parallel, sometimes slightly wavy reflectors. In general, the sediments drape the underlying substratum and tend to smooth out the uneven topography of the till surface. Thick deposits on-lapping the limiting slopes characterize the cen- tral parts of the depressions. In areas shallower than 90–100 m, the boundary between SSU-II and -III is even and smooth, in contrast to areas at deeper water depths, which are characterized by hummocky till surfaces. We interpret the smooth boundary as a
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Fig. 7. Isopach map of SSU-II Late Weichselian glaciomarine sediments . Inferred ice sheet limits are marked with solid lines. The data
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base seismic profiles and cores used for constructing this particular isopach map is shown on the inserted map.
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result of marine mainly wave erosion caused by a low relative sea level during the Late Weichselian
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andror Early Holocene see below .
The cores show that sediments corresponding to SSU-II in deep water consist of laminated, brown and gray clays, similar to those described in detail west of the study area Polyak et al., 1995; PolyakŽ and Mikhailov, 1996 . The lamination and the fact. that this unit rests directly on a till surface suggests that it is a glacio-marine clay that accumulated on the sea floor in front of an ice sheet. However, a considerable component of these sediments may have been derived from erosion of the shallow areas of the Pechora Sea during a lower relative sea level. During the post-glacial sea level rise, the coastline and the
near-shore areas were exposed for wave erosion causing a high sedimentation rate in offshore depres- sions. In a core from the Kurentsovo depression in
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the southern part of the Geese Trough Fig. 10 , the sediments that correspond to the lower part of SSU-II is characterized by a varve-like lamination. This, together with the absence of foraminifers, may sug- gest a glacio-lacustrine environment. In shallower areas, the sediments are more coarse grained, and in the southern part of the Pechora Sea, the lower part of SSU-II consist of an up to15-m-thick sandy se-
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quence Fig. 10, profile 4B; profile 7, borehole 3 . SSU-II is considered to be a time transgressive unit that accumulated in front of the retreating ice sheet.
Radiocarbon dates from adjacent parts of the Barents
Sea shelf indicate that glacio-marine sedimentation ŽSSU-II. in areas to the north and west of the Pechora Sea started to accumulate on the till surface ŽSSU-III at around 13–14 ka Polyak et al., 1995;. Ž Vorren and Laberg, 1996 . This age is considered as. a minimum age for the base of SSU-II in the Pechora Sea. The upper boundary of SSU-II is dated to around 10–9 ka when the Holocene marine sedi-
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ments SSU-I started to accumulate Polyak et al., 1995 ..
4.7. SSU-I— Holocene marine sediments
SSU-I is the uppermost seismostratigraphic unit in the Barents Sea and it is in most areas less than
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5–10-m thick Fig. 8 . In cores, the sediments usu- ally contain well-preserved marine macro- and mi- crofossils. In water depths exceeding ca. 120 m, the SSU-I sediments typically consist of olive gray mud with scattered ice rafted pebbles. At these water depths, SSU-I is often conformably draping SSU-II and the boundary between these units can only be
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recognized from high-resolution records Fig. 9B . In areas shallower than 120 m, sandy sediments usually represent SSU-I with a pronounced erosional uncon- formity along the lower boundary.
The thickest part of SSU-I is found around the Kanin Peninsula and to the east of Kolguev Island ŽFig. 8 , where up to 50-m-thick sediment sequences,.
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Fig. 8. Isopach map of SSU-I Holocene marine sediments . Inferred ice sheet limits are marked with solid lines. The data base seismic profiles and cores used for constructing this particular isopach map is shown on the inserted map..
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Fig. 9. Interpreted seismic profiles 1 to 3 across the southern Novaya Zemlya Trough Fig. 2 , showing the distribution of the
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seismostratigraphic units SSU-I, -II, -III, -IV and -V . Pronounced reflectors and boundaries between seismostratigraphic units are marked with solid lines. Poorly seen reflectors are indicated by broken lines and assumed boundaries are indicated by dotted lines. Borehole no. 207,
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234 and 210–218 are marked on the profiles. Upper panel A and B shows seismic profiles marked on profile 3. Small numbers given in
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the boreholes are depths below the sea floor of lithological boundaries interpreted to correspond with the seismic units.
including ridges that we interpret as buried coastal
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bars, have been recorded Figs. 2, 11 and 12 . The long ridges along the southern side of the Pomorsky Strait, which are resting on a flat erosional surface, reveal large scale internal cross stratification Fig.Ž 11 . This erosional surface has been sampled in. many cores from adjacent parts of the shelf and is normally recognized as a sharp boundary between Middle Weichselian marine mud and overlying sandy sediments of Holocene age. In this area, we have not seen sediments that could be interpreted as a possible till and there are no indications of glaciotectonic
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disturbances of the underlying unit SSU-IV . An- other accumulation, up to 40-m thick, is located
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outside the Pechora River delta Fig. 8 . Probably due to permafrost andror gas saturation, the seismic data are difficult to interpret in this shallow area, but some profile fragments show a progradational se-
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quence. The borehole data Figs. 2 and 3 suggest that this sequence fills a broad, up to 50-km-wide valley, that is incised into the underlying sediments ŽFig. 8 . In all boreholes outside the Pechora River. delta, the lowermost sediments consist of cross-be- dded coarse-grained sand grading upwards into a dark gray, laminated clay enriched with organic ma- terial and plant macrofossils. We interpret this as a transgressive, pro-deltaic sequence of the Pechora River.
Many radiocarbon dates indicate that the accumu- lation of SSU-I in the southeastern Barents Sea
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started about 10–9.5 ka Polyak et al., 1995 . Holocene radiocarbon ages have also been obtained from the base of SSU-I in several sediment cores
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from the Pechora Sea Polyak et al., 2000, Fig. 3 . Therefore, in general, this unit is considered to be of Holocene age, even though the lower boundary is diachronous as a result of the Holocene transgres- sion.
4.8. Ice marginal features
Along the southwestern margin of the Novaya Zemlya Trough, there is a chain of pointed ridges, 20–30-km long, 2.5–10-km wide and 20–25-m high ŽFig. 4 . These features are characterized by a rough. surface with a series of small, 3–5-m high, irregular ridges, in contrast to the surrounding flat and smooth sea floor. The seismic reflection indicates that the
ridges are composed of Cretaceous bedrock, but unlike the adjacent strata, the internal structures along
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the ridge zone are indistinct Fig. 10, profile 5 . We postulate that this is due to intensive glaciotectonic deformation and conclude that the ridges are ice pushed and define a former ice margin that we name
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the Kurentsovo Line after the Kurentsovo oil field . A pronounced east–west trending thickening of the
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till cover SSU-III, Fig. 5 near the Geese Trough, interpreted as a buried end moraine, is considered to represent a western continuation of this margin Fig.Ž 10, profile 6 . The interpretation that this line marks. a former ice sheet margin is supported by the up to 100-m-thick accumulations of rhythmically bedded
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sediments SSU-II, Fig. 7 south of the line whereas the sediment cover to the north is much thinner. This implies that the sediments to the north of the Kurent- sevo Line is somewhat younger than the thick accu- mulation of sediments to the south of this line, i.e.
that SSU-II is a time-transgressive unit that accumu- lated in front of a retreating ice sheet. Similar ice marginal features have been identified in the western Barents Sea where they are related to the Late Weichselian ice sheet recession ŽElverhøi et al., 1990 ..
We cannot fully rule out the possibility that the Kurentsovo Line marks the ice front position during the Late Weichselian glacial maximum, but we find it more probable that it represents a retreat stage.
According to our interpretation, the maximum ice sheet extension is shown by the Koguev Line, some 50–100 km to the south of the Kurentsevo Line.
However, there are no ice marginal features or thick- ening of glaciomarine sediments associated with the Kolguev Line. There are a pronounced thickening of
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older marine sediments SSU-IV along the southern side of this line, but these accumulations prograded from the mainland and were not deposited from a former ice sheet margin to the north. According to our interpretation of the seismic profiles, SSU-IV Žand also SSU-V sediments are almost totally miss-. ing north of the Kolguev Line, due to the Late Weichselian glacial erosion. We therefore infer that the Kolguev Line represents the outermost ice sheet limit during the Late Weichselian. However, the apparent lack of marine glaciomarine sediments in front of the Kolguev Line is difficult to understand because we postulate considerable glacial erosion to
()V.Gataullinetal.rGlobalandPlanetaryChange312001453–474
Ž . Ž .
Fig. 10. Interpreted seismic profiles 4–7, showing the distribution of the seismostratigraphic units SSU-I, -II, -III and -V in the central part of the study area Fig. 2 . Pronounced reflectors and boundaries are marked with solid lines. Broken lines indicate poorly seen reflectors. Borehole no. 3, 32–36 and 37 are marked on the profiles.
Ž .
Enlarged seismic profiles for segments of the lines are shown as insets. Small numbers given in the boreholes are depths below the sea floor of lithological boundaries interpreted to correspond with the seismic units.
Ž .
Fig. 11. Seismic profile 8 across the Pomorsky Strait between the Kolguev Island and the Pechora Lowland Fig. 2 . Pronounced reflectors
Ž .
and boundaries between seismostratigraphic units SSU-I–V are marked with solid lines. Poorly seen broken lines indicate reflectors and assumed boundaries are indicated by dotted lines. Shallower than 50 m b.s.l. the sediments are gas-saturated andror frozen. Only a few, poorly visible reflectors can be recognized in these sediments. Borehole B-104 is marked on the profile. At the bottom are original seismograms, and above are interpreted versions of the same. On bore hole B-104, depths of lithological boundaries are given on the right
Ž .
side and AMS dates from Polyak et al. 2000 on the left.
Ž .
Fig. 12. Seismic profile 9 across the Pomorsky Strait between the Kolguev Island and the Pechora Lowland Fig. 2 . Lines and interpretation are similar as described for Fig. 11. Borehole B-141 on the Kolguev Island is marked on the northwestern end of the profile. At the bottom is an original seismogram, and above an interpreted version of the same.
remove units SSU-IV and -V inside the line. With this interpretation, we have to postulate that the ice proximal sediments that presumably accumulated in front of the ice sheet were eroded away during a period with low relative sea level around the Weich- selianrHolocene transition.
Outside the Gusinaya Zemlya Peninsula on No- vaya Zemlya, we have mapped a pronounced thick- ening of glaciogenic sediments that circumscribe the
Ž .
coastline Figs. 5 and 10 . This arched accumulation is considered to be the southern extension of a huge end-moraine complex that can be traced for hundreds of kilometers on the shelf to the west of Novaya
Ž .
Zemlya Fig. 1 , the so-called Admiralty Bank Moraines Epshtein and Gataullin, 1993; GataullinŽ and Polyak, 1997b . In contrast to the northern area,. where the end-moraines can be seen as pronounced ridges, it is recognized mainly from the increased
Ž .
thickness up to )50 m of the glaciogenic sedi-
Ž .
ments in the southern area Fig. 5 . There is also a
Ž .
thickening of the glaciomarine sediments SSU-II
Ž .
on the distal side of this line Fig. 7 . On some seismic profiles crossing the Admiralty Bank Line ŽFig. 10, profile 6 , there is a weak internal reflector. inside SSU-III, which may indicate that this till unit is composed of two depositional events.
4.9. Submerged shorelines and shallow marine fea- tures
On the seismic profiles, we have identified ter- races, ridges, erosional surfaces and possible deltaic sequences that are interpreted as submerged shore- lines andror shallow marine deposits Figs. 4 Figs.Ž 9–12 . These features occur at different water depths. down to at least 50–60 m below present sea level.
Often they are elongated deposits along the bathy- metric contour lines. Both the seismic profiles and the sediment cores reflect a continuous erosional unconformity down to this level. Evidently, the ridges are younger than this erosional surface. Here, we only describe some of these features.
A well-defined, ca. 20-km-wide terrace is located at a water depth of 50 m along the southern slope of
Ž .
the Novaya Zemlya Trough Fig. 9, profile 3 . The terrace is composed of an up to 20-m-thick sequence of large-scale cross-stratified sediments, probably
Ž .
sand SSU-I . It is overlying SSU-II and -III with a pronounced erosional unconformity at the base. We infer that the terrace is a prograding beach deposit or perhaps a delta that formed when the sea level was about 50 m lower than today. The erosional lower boundary may indicate a preceding sea level as low as 70 m below present, but this is more uncertain.
North and east of Kolguev Island there are some large, 12–20-km-long, 2–5-km-wide and 20–25-m- high plateaus at water depths of 40 to 50 m Fig. 10,Ž profile 7 . The seismic profiles reveal a large-scale. internal cross-stratification reflecting an offshore de- positional direction during deposition of these sedi- ments. The deposits, which probably consist of sand, occur stratigraphically above SSU-III and -II and are ascribed to SSU-I. The inferred shallow marine fea- tures are partly draped by marine mud of the same seismo-stratigraphic unit, probably reflecting a rising sea level.
Between Kolguev Island and the mainland, there are some smaller, 1.5–2-km-wide and 10-m-high ridges resting on an erosional unconformity at around
Ž .
60 m b.s.l. Fig. 11 . The ridges, which presumably consist of sand, are completely covered by younger marine mud. We interpret all these ridges as sub-
Ž .
merged sand bars barrier beaches that were formed when the sea level was some 40–50 m lower than today. The underlying erosional surface indicates that the relative sea level was somewhat lower prior to the formation of the ridges, probably at least 55 m lower than today. The lower boundary of these ridges is correlated with the erosional unconformity at the base of SSU-I in core 210–218 outside the Pechora
Ž .
River delta Fig. 3 . The sandy marine sediments
Ž .
above this boundary 44 m b.s.l. started to accumu- late during the Early Holocene before 9.2 ka. Thus, we assume that the sand ridges were formed close to the PleistocenerHolocene boundary.
5. Discussion
5.1. Sea leÕel history and glacio-isostatic moÕements of the crust
During the deposition of SSU-IV, the shallow shelf along the Pechora Lowland was submerged.
The occurrence of Middle Weichselian marine silt and clay up to a water depth of around 20 m coresŽ 112 and 114, Fig. 3 indicates that the contempora-. neous sea level was higher and that the seafloor at this time was below the wave base. This assumption is supported by the fact that SSU-IV is even more
Ž .
fine-grained than Holocene sediments SSU-I at
Ž .
comparable water depths Fig. 3 . However, observa- tions from coastal sections indicate that relative sea level has not been higher than today during the last
Ž .
30–40 ka Mangerud et al., 1999 . We therefore conclude that the relative sea level during the Mid- dlerLate Weichselian transition was between 0 and y20 m. The prograding SSU-IV indicates that the sea level was falling during deposition of this unit.
Probably, the highest relative sea level occurred shortly after the preceding deglaciation from theŽ Markhida Moraine. some 40–60,0000 years ago ŽMagerud et al., 2001 . The youngest date on marine. molluscs from SSU-IV is 29.3 ka from about 50 m
Ž .
b.s.l. Core 234, Fig. 3 . However, also the date 23.6
Ž .
ka on plant detritus core 234, Fig. 3 is probably from marine sediments.
We have not been able to identify shorelines that can be correlated with the Late Weichselian glacial maximum; all mapped beaches and shallow marine features seems to be younger. During the glacial maximum and the first retreat phases, fine-grained
Ž .
glaciomarine sediments SSU-II accumulated on the till surfaces of SSU-III up to at least 50 m b.s.l., which implies that the relative sea level was higher than this during the early stage of deglaciation.
SSU-IV, -III, and partly of -II are truncated by a pronounced erosional unconformity that can be traced down to 60–70 m below the present sea level Fig.Ž 11 . Off shore the Pechora River, this unconformity. forms a more than 40-m-deep valley, probably in- cised into SSU-V–IV by fluvial erosion, and subse-
Ž .
quently filled with Holocene sediments Fig. 8 . Deposits interpreted as beach sediments have been recognized on the seismic profiles down to water depths of 50–60 m Fig. 9, profile 3 and Fig. 10,Ž profile 7 . The fact that the till surface at these sites. is smoothed above and hummocky below 60–70 m, supports the conclusion that lowest relative sea level occurred after the late Weichselian glacial maximum.
We conclude that the late Weichselian minimum relative sea level in the southern Pechora Sea was
between 50 and 70 m below the present. Nearly all areas to the south of the Kolguev Line are shallower
Ž .
than 60 m Fig. 4 and were sub-aerially exposed during this low-stand. As long as the ice sheet blocked the drainage across the Barents Sea shelf in the north and west, the only connection to the ocean was through the Kara Gate Strait between Novaya Zemlya and Vaigach Island. The occurrence of lit- toral sediments at a water depth of 50 m cutting
Ž .
deeper water facies of SSU-II Fig. 9, profile 3 indicates a drop in relative sea level after the ice sheet recession had started. The regression minimum was presumably attained at the final stage of the Late Weichselian or even during the Early Holocene.
After the period with low relative sea level, the shelf was again transgressed when SSU-I accumu- lated. A series of radiocarbon dates from core 210–
Ž .
218 Fig. 3 shows that marine sediments started to accumulate on the erosional surface at 44 m b.s.l.
Ž .
shortly before 9.2 ka Polyak et al., 2000 . In several cores from offshore the Pechora Lowland, Veinbergs
Ž .
et al. 1995 report that the marine sediments are underlain by terrestrial peat of Holocene age. In one core from a water depth of 15–17 m, a 0.4-m-thick peat layer was radiocarbon dated to ca. 10 ka show- ing that much of the sea level rise occurred during the Holocene. Drowned Holocene peat below sea level has also been found along the coast, which is presently submerging at a rate of 2.4 mm per year ŽNikonov, 1980 ..
From the evidence discussed above, we have con- structed a relative sea level curve for the southern
Ž .
Pechora Sea Fig. 13 . We infer that there was a gradual lowering of the sea level from 30 ka until the final stage of the Late Weichselian when the regres- sion rate accelerated. Minimum sea level at around 60 m b.s.l. was probably attained 12–10 ka. A rapid sea level rise occurred during the Early Holocene with a slow down from about 7 to 8 ka. Realizing the uncertainties in both dating and depths, we still believe the curve outline the main pattern of the sea level changes.
The observed sea level variations in the Pechora
Ž .
Sea is significantly different from the eustatic global sea level changes during the last 35 ka, which imply some glacio-isostatic movements in the Pechora Sea.
The amplitude of the isostatic movements can be reconstructed by subtracting the eustatic from the
Ž .
Fig. 13. A Inferred relative sea level curve for the southern Pechora Sea for the last 35 ka. B Eustatic sea level curve,Ž .
Ž .
simplified from Chapell and Shacleton 1986 and Fairbanks Ž1989 . C The resultant isostatic curve when B is subtracted. Ž . Ž .
Ž . from A .
Ž .
local sea level Fig. 13 . The resultant curve reflects a glacio-isostatic depression of the crust in the order of 40–50 m during the period from 30 to 20 ka,
followed by a rapid rebound during the Late Weich- selian deglaciation. Most of the uplift was accom- plished by 10 ka and during the Early Holocene, the uplift was apparently much slower. The total iso- static uplift from ca. 20 to 5 ka was in the order of 100 m. During the last few thousand years, the crust has been subsiding, causing the ongoing transgres- sion along the coast.
5.2. The ice sheet extent during the Late Weichselian glacial maximum
The sediment stratigraphy in the Pechora Sea substantiate the conclusions from the land-based in- vestigations that the Late Weichselian ice sheet boundary was located well off the present coastline ŽAstakhov et al., 1999; Mangerud et al., 1999 .. Thick accumulations of marine sediments of Middle
Ž .
and Late Weichselian age SSU-IV and -II evidently postdate the last glaciation in the Pechora Sea. Ac- cordingly, the Late Weichselian ice sheet boundary must have been located to the northwest of this sediment sequence. To identify possible ice sheet margins in this area, we have used three independent criteria: 1 the southern extension of the youngestŽ . till sheet in the Barents Sea; 2 the occurrence ofŽ . ice-marginal ridges; and Ž .3 the distribution of glaciomarine sediments. We have located three pos- sible glacier margins: From south to north, these include the Kolguev Line, the Kurentsovo Line and the Admiralty Bank Line.
Ž .
The Late Weichselian till SSU-III and the over-
Ž .
lying glaciomarine sediments SSU-II are confi- dently traced from the central Barents Sea to the ice-pushed ridges along the Kurentsovo Line. The pronounced thickening of SSU-II on the southern side of the ridges indicate that the ice front halted for
Ž .
some time at this position Figs. 7 and 10 . SSU-II is here up to 100-m thick, and is interpreted as consist- ing of sediments that accumulated in front of the ice sheet. The Kurentsovo Line marks both morphologi- cally and sedimentologically the most distinct termi- nal position. The question is if it marks the very outermost position of the ice sheet margin or only a retreat stage.
According to our mapping, SSU-III continues well beyond the Kurentsovo Line and southward to the
Ž .
Kolguev Line Fig. 10 , indicating that the limit of
the Late Weichselian ice sheet was located 50–100 km to the south of the Kurentsovo Line. The appar-
Ž .
ent lack of Middle Weichselian sediments SSU-IV to the north of this boundary support this interpreta-
Ž .
tion Fig. 6 . On the other hand, in contrast to the Kurentsovo Line, ice marginal features and pro-gla- cial sediments are apparently lacking along the Kolguev Line. Whether the Late Weichselian glacial maximum is represented by the Kolguev or the Kurentsevo Line is a minor uncertainty compared to most cited alternatives the last decades.
The down warping of the crust that occurred after 30 ka reflects a growing ice load in the Barents Sea region. We infer that the maximum ice sheet position was attained more or less simultaneously with the western margin of the Barents Ice Sheet about 20–15
Ž .
ka Landvik et al., 1998 , but we are not yet able to determine the exact age of this event. Our conclusion that the ice sheet terminated off the coast explains the missing emerged shores along the Pechora coast, in contrast to other areas of the Barents Sea region that evidently were covered by the Late Weichselian
Ž .
ice sheet cf. Landvik et al., 1998 .
5.3. Deglaciation
The rapid uplift, which occurred after the ice sheet retreated from the maximum position, indicate that a glacial unloading occurred at around 15–12 ka ŽFig. 13 . This is consistent with data from the. central Barents Sea, which apparently was deglaciated shortly before 12.8 ka Polyak et al.,Ž 1995 ..
After the ice sheet receded from the Kurentsovo Line, the ice dispersal center shifted to Novaya Zemlya before the Admiralty Bank moraines were deposited. Probably they were deposited by a local ice cap centered over the Novaya Zemlya after deglaciation of the southeastern Barents Sea shelf ŽSvendsen et al., 1999 . According to our mapping,. these moraines seem to cross the southern part of the island. The seismic profiles that cross the offshore moraines may suggest that they reflect a glacier
Ž .
readvance Fig. 10, profile 6 . We hypothesize that these moraines were formed during the Younger Dryas cooling event, but this needs to be tested by future investigations.
6. Conclusions
Ž .1 The maximum extent of the Barents Ice Sheet during the Late Weichselian has been mapped on the shallow shelf from the southern tip of Novaja Zemlya towards Kolguev Island. This line, which has been termed the Kolguev Line, mark the southern limit of
Ž .
the youngest till sheet SSU-III in the Barents Sea as well as the northern erosional limit of Middle
Ž .
Weichselian marine sediments SSU-IV recorded in the Pechora Sea.
Ž .2 Another ice sheet boundary, named the Kurentsovo Line, is located 50–100 km further to the north. The Kurentsevo Line corresponds with end moraines, ice-pushed bedrock ridges and up to 100- m-thick accumulations of glaciomarine sediments ŽSSU-II ..
Ž .3 Submerged shorelines of Late Weichselianr Early Holocene ages have been identified on the shelf down to water depths of 50–70 m. The occur- rence of fine grained Middle Weichselian marine sediments up to ca. 20 m b.s.l. indicate a sea level drop in the order of 30–40 m during the Late Weichselian period.
Acknowledgements
This paper is a contribution to the European Commission project Ice Sheets and Climate in the Eurasian Arctic at the Last Glacial Maximum Eura-Ž sian Ice Sheets under the EnÕironment and Climate. Research Program ENV4-CT97-0563, Climate andŽ Natural Hazards and the Russian–Norwegian pro-. ject Paleo EnÕironment and Climate History of the
Ž .
Russian Arctic PECHORA . Both project forms part of the European Science Foundation Program Qua- ternary EnÕironment of the Eurasian North ŽQUEEN . Valery Gataullin, financially supported. by the Norwegian Research Council, compiled seis- mic profiles and core data obtained by various re- search institutes in the former Soviet Union. These include: Research Institute for Marine Geology and
Ž .
Geophysics NIIMorgeo, Riga, Latvia , Research In- stitute of Geology and Mineral Resources of the World Ocean VNIIOkeangeologia, St. Petersburg,Ž
. Ž
Russia , All-Union Geological Institute VSEGEI,
St. Petersburg, Russia , Shirshov’s Institute of. Oceanology, Russian Academy of Sciences IORAN,Ž Moscow, Russia , Arctic Marine Geotechnical Expe-.
Ž .
dition AMIGE, Murmansk, Russia , Baltic Marine Geotechnical Expedition BMGE, Kaliningrad, Rus-Ž
. Ž
sia , Marine Arctic Geological Expedition MAGE, Murmansk, Russia , Polar Marine Geological Expe-.
Ž .
dition PMGRE, Lomonosov, Russia . Jan Sverre Laberg and an anonymous person reviewed the pa- per. To all these institutions, we offer our sincere thanks.
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