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

ELSEVIER

Marine Geology 143 (1997) 1699188

Late Weichselian deglacial history of the Svyataya (Saint) Anna Trough, northern Kara Sea, Arctic Russia Leonid Polyak a,cv*,Steven L. Forman b, Frances A. Herlihy a,1, Gennady Ivanov ‘, Pyotr Krinitsky d a Byrd Polar Research Center, The Ohio State University, Columbus,, OH 43210, USA b Department of Geological Sciences, University of Illinois at Chicago, Chicago,, IL 60607, USA ’ VNII Okeangeologia (Research Institute for Geology and Mineral Resources of the Ocean), St. Petersburg 190121, Russia ’ Polar Marine Geological Expedition, Lomonosov-St. Petersburg, Russia Received

15 February

1997; accepted

6 May 1997

Abstract Marine sediment core and seismic records from the Svyataya (Saint) Anna Trough provide new insight into the distribution of Late Weichselian glacial coverage, ice retreat pattern, and post-glacial environments in the northern Barents and Kara seas. These records indicate that the Saint Anna Trough was filled with grounded glacier ice, which likely reached the shelf edge during the Late Weichselian maximum. Several radiocarbon dates suggest early deglaciation of the deep axial part of the trough prior to 13.3 ka. Two sandy beds in the deglacial section of the cores

imply distinct pulses of iceberg calving and/or melting, which were probably associated with stepwise retreat of the ice margin. Morainic ridges and glacial-sole markings in the western part of the Saint Anna Trough indicate that the northern-central Barents Sea was a site of a large ice mass during deglaciation; a smaller ice cap is inferred for the Northern Kara Plateau. At later stages, ice retreat on the western flank of the trough was directed towards FranzJosef Land, and was presumably facilitated by a separation of the Barents Sea and Novaya Zemlya ice domes. Deglaciation of the Saint Anna Trough was completed by ca. 10 ka. High post-glacial sediment fluxes between 10 and 8 ka were probably related to sea-floor/coastal erosion and/or Siberian river discharge during the rising sea level. 0 1997 Elsevier Science B.V. Keywords: Kara Sea; Russian Arctic; bottom features; seismic stratigraphy;

1. Introduction Paleoglaciological models of the Late Weichselian glacier ice over the Barents and Kara seas have portrayed variably the extent and volume of ice, * Corresponding author. Tel.: + 1 (614) 292-2602; fax: + 1 (614) 292-4697; e-mail: [email protected] ‘Present address: Battelle Memorial Institute, Environmental Restoration Department, Columbus, OH 43201, USA. 0025-3227/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZI SOO25-3227(97)00096-O

glacial geology; postglacial environment

placement of ice divides, and deglacial history (Grosswald, 1980; Velichko et al., 1984; Forman et al., 1995; Lambeck, 1996). These discrepancies are mainly associated with terrestrial glacial records (e.g. Grosswald, 1994; Tveranger et al., 1995), while marine geologic studies during the past two decades concur that grounded ice occupied most of the Barents Sea during the Late Weichselian (Elverhrai and Solheim, 1983; Vorren et al., 1988; Elverhari et al., 1990; Polyak et al.,

1995). The marine evidence is largely based on the ubiquitous occurrence of stiff glacigenic diamictons overlying pre-Quaternary strata, separated by a prominent erosional unconformity (Upper Regional Unconformity: Solheim and Kristoffersen, 1984). Marine fauna in glacimarine sediments overlying these diamictons yields 14C ages starting at 13- 14 ka, which implies that at least the upper part of the diamictons reflects the Late Weichselian glaciation. A critical constraint on ice-sheet geometry and deglaciation pattern is the complex subglacial topography of the Barents and Kara seas. which is characterized by deep transverse troughs opening to the Arctic Ocean and the NorwegianGreenland Sea ( Fig. 1; Grosswald, 1980: Siegert and Dowdeswell. 1996). Marine geological studies of the western Barents Sea show that the Bear Island Trough and other marginal troughs served as major conduits for grounded-ice flow from the central areas of the shelf, associated with extensive erosion of pre-glacial deposits and accumulation of sediment in trough mouth fans (e.g. Vorren et al., 1988; S&tern et al., 1992; Faleide et al., 1996). Estimates based on bathymetry and sediment thickness suggest that the large transverse troughs at the northern margin of the Barents and Kara seas (Franz-Victoria, Svyataya Anna, and Voronin troughs) could have had a comparable constraint on glacier movement and sediment erosion and redeposition (Rasmussen and Fjeldskaar, 1996; Vagnes, 1996). During deglaciations. the marginal troughs were sites of initial ice retreat. presumably facilitated by sea-level rise (Vorren et al.. 1990. 1995). et al.. 1988; Elverhoi Reconstruction of the little known deglaciation history of the northern troughs is particularly important for understanding sediment and meltwater inputs from marginal seas into the Arctic Ocean during the last glacial cycle. Recent marine geological results from the FranzVictoria Trough show that during the Late Weichselian it was filled by grounded ice flowing from the central-northern Barents Sea, and that the deglaciation of the trough occurred as early as 13-l 5 ka (Polyak and Solheim, 1994; Lubinski et al., 1996: Herlihy, 1996; Solheim et al., 1996). These conclusions are consistent with sea-level

data from the adjacent Franz-Josef Land .4rchipelago (Forman et al.. 1996). Still enigmatic is the role of the largest trough in the Barents and Kara seas. the Svyataya (Saint) Anna Trough, in ice-sheet movement and disintegration processes. Bathymetry of the adjacent Nansen Basin resembles the large depositional fans fed by glaciated troughs in the western Barents Sea, implying that the Saint Anna Trough was also affected by glacier erosion at times during the Quaternary ( Vagnes, 1996 ). However, existing paleoglaciological models are not constrained by direct geological data from this area, which results in large discrepancies between the minimum and maximum reconstructions of the Barents Sea Ice Sheet (e.g. Lambeck, 1996). In this paper, we present new data on the glacial extent and post-glacial environments in the Saint Anna Trough, and discuss its significance for the demise of the Late Weichselian Barents Sea Ice Sheet.

2. Study area, materials, and methods The Saint Anna Trough is a major pathway for exchange of water, sediment, and ice between the Arctic Ocean and the Barents and Kara seas (Figs. 1 and 2). The trough contains two deep basins with > 500 m water depth separated by a sill which is almost 200 m shallower. The southern basin is immediately adjacent to Novaya Zemlya, whereas the larger northern basin extends to the continental slope of the Arctic Ocean and attains depths of > 600 m. Shelf-originated surface and bottom waters flow northward along the trough into the Arctic Ocean. In contrast, intermediate Atlantic-derived water can move southward into the Saint Anna Trough at depths of 100 to 400 m (e.g. Timofeev, 1961; Hanzlick and Aagaard, 1980; Pfirman et al., 1994). Early Russian expeditions and USCG cruises have provided information on lithology, geochemistry and microfauna of recent sediments from the Saint Anna Trough (Yermolaev, 1948; Klenova, 1960; Kulikov, 1961; Turner and Harris, 1970; Andrew and Kravitz, 1974). However, these studies did not establish a definitive stratigraphic

L. Polyak et al. / Marine Geology 143 (1997) 169-188

84”

80’

76”

72”

64” , Fig. 1. Overview

map with simplified

bathymetry

(300 and 1000 m) and location

framework because of the absence of reliable age control and seismic stratigraphy. In August and September 1994, a joint Russian-American-Norwegian expedition on board R/V Professor Logachev implemented a multidisciplinary investigation of the northern areas of the Barents and Kara seas. Major goals of this cruise included characterizing present oceanographic conditions and retrieving deglacial

of the study area (shaded ).

and Holocene sedimentary records from the Saint Anna Trough (Fig. 2). Acoustic records with subbottom penetration were retrieved by a hull-mounted ORE echosounder tuned at 8.8 kHz. Non-penetration 12 kHz records were used for increasing the bathymetry sounding coverage. Two side-scan sonar profiles were run across the western flank of the Saint Anna Trough with a 30 kHz system. Additionally,

172

81

78

77

76 *

sediment core stations penetration 8.8 kHz echosounder

--C--,

12 kHz echo-sounder

profiles

profiles

30 kHz side-scan profiles airgun profile (WV Geolog Fersman, 1992)

Fig. 2. Study area map showing location of sediment cores and sounding tracks (R.!V Prof>sso~ Loguclze~. 1994). Water depth (m) is shown in solid and dotted contour lines (Cherkis et al., 1991 ); area with water depth >400 m is shaded. Sediment core stations mentioned in this paper are numbered.

173

L. Polyak et al. f Marine Geology 143 (1997) 169-188 Table 1 Core locations Core No.

Core length (cm)

Lat. ‘N

Lot&E

Water depth (m)

1 11 29 67

161 151 168 235

81’59.69 81’29.48 79O59.60 7819.54

67 32.77 70’07.21 69O56.96 70”02.07

633 632 605 443

Table 2 Physical standard

properties deviations

of major

lithostratigraphic

units in the Saint Anna

Unit/environment:

Upper/marine

Intermediate/glacimarine

Seismic signature: Color:

Stratified/transparent Brown to olive gray

Background Stratified/transparent Gray with dark-gray

No. of samples Bulk density (g/cm3) Water content (%) Sand (%a) Silt (%) Clay (%) Shear strength (kPa)

51 1.6kO.1 43+6 6*2 40+7 52+9 7+3 (16)

Number

measurements

of shear strength

Trough

Sandy layers or brown

Partially

a seismic airgun line across the southern basin of the trough was obtained in 1992 with two 40 cubic inch sleeve guns (R/V Geolog Fersman; Solheim, 1993). Sediments were sampled by gravity and box corers. All cores were measured for volume magnetic susceptibility, and selected cores were later

in mean

values

and

indurated

Turbidites Opaque/chaotic Gray to dark gray

layers

I 1.9kO.l 25k3 31+5 30*4 35*4 15k6 (4)

(9)

is shown in parentheses.

7, 11, 29, 67) shown

Lower/glacial

22 1.8kO.l 33*4 6*4 38+9 52k7 6*2

(cores

oxidation

8 1.8+0.0 24kl 45k6 28+6 26k5 43k26 (4)

35 1.9+0.1 24k2 23*2 32k3 43+3 26+27

(12)

bed OX-II is omitted.

analyzed for specific gravity, water content, shear strength, grain-size, organic matter, carbonate content, and paleontological composition at the Byrd Polar Research Center. Duplicate cores were investigated at the University of Tromso with the emphasis on the Holocene sedimentary environ-

Table 3 AMS t“C dates Lab No. a GX-21798 AA-16849 AA-16848 GX-21067 GX-20859 AA-19029 AA-20482 GX-20860 AA-19030 GX-20861 AA-19031

Core No. 6 7 29 67

Core depth 120-125 5 73375 95 17 50 92 115 171 190 214

(cm)

Lithologic intermediate upper intermediate intermediate upper upper upper upper upper upper upper

unit

Material BF, PF, BF, BF, BF, BF, BF, M, M, M, M,

’

mixed N. pachyderma M. barleeanus M. barleeanus, C. lobatulus E. excavatum, M. barleeanus M. barleeanus, C. lobatulus mixed Yoldiella sp. Yoldiella sp. Yoldiella sp. Yoldiella sp.

a AA = NSF Arizona Facility; GX = Geochron Laboratories. b BF = benthic foraminifers; PF = planktonic foraminifers: M = bivalve mollusks.

Uncorrected 8,680 * 195 1,870k 50 13,710+ 130 13,730+ 110 3.420+ 70 6.285+ 75 7,175+ 160 8,610,115 9,225 i 120 9,390 f 120 10,010+ 90

age (?I

yr B.P.)

NE

SW Novaya Zemlya T

1 km H

-

400 mwd -

Fig. 3. Airgun profile across the southern basin of the Saint .Anna (Fig. 2 for location) (courtesy of A. Solheim. Institute). LIand h represent surface and bottom reflectors. respectively. of the lower seismo-stratigraphic unit.

ments (Kolstad, 1996). Cores 7. 11, 29 and 67 collected from a longitudinal transect along the trough axis were studied in detail to characterize the deglacial record (Fig. 2: Tables 1 and 2; Herlihy, 1996). Age control is provided by eleven 14C ages on foraminiferal tests and AMS small mollusks (Table 3). Laboratory-reported. S13C-normalized ages were reservoir-corrected by subtraction of 400 year (Stuiver and Braziunas. 1993). Mineralogical analysis of the fine sand faction (0.06330.1 mm) was performed on cores 7 and 67 at the P.P. Shirshov Institute 01 Oceanology.

3. Seismic stratigraphy and landforms Echo-sounder that sediments

and airgun seismic records show above bedrock in the Saint Anna

Fig. 4. X.8 kHz echo-sounder profile across the southern surface reflector of the lower seismo-stratigraphic unit.

Norwegian

Polar

Trough are easily subdivided into two major seismo-acoustic units: the opaque or chaotic-signature lower unit and the acoustically laminated upper unit, separated by a marked reflector (I (Figs. 3~ 6). A similar stratigraphic sequence is identified by seismic surveys in other areas of the Barents Sea (e.g. Elverhoi and Solheim, 1983; Gataullin et al., 1993; Polyak et al., 1995). The airgun profile across the southern basin of the Saint Anna Trough shows that the lower unit reaches a thickness of 70 m in the center of the basin and thins to less than the airgun resolution ( < 5-~IO m) at shallower depths while acquiring a rough surface (Fig. 3). The lower unit truncates dipping bedrock strata with a sharp boundary h. The bedrock in the northern and eastern Barents Sea is largely represented by dipping strata of weakly lithified deformable Mesozoic sediments ( Yashin et al.. 1985; Johansen et al., 1993; Epshtein

basin of the Saint Anna

Trough

at - 77 15’N (Fig. 2 for location).

(I=

L. Polyak et al. / Marine Geology 143 (1997) 169-188

S -

600 mwd -

175

I10 msm

1 km

-7.5

-

600 mwd -

Fig. 5. Longitudinal (67”30’E) 8.8 kHz echo-sounder profile at the northern end of the Saint Anna Trough (Fig. 2 for location). a and b represent surface and bottom reflectors, respectively, of the lower seismo-stratigraphic unit.

-

600 mwd -

, Fig. 6. 8.8 kHz echo-sounder profile across the eastern flank of the Saint Anna Trough at 81”lO’N (Fig. 2 for location). a and b represent surface and bottom reflectors, respectively, of the lower seismo-stratigraphic unit.

and Gataullin, 1993). Outcrops of Lower Cretaceous sands on the Northern Kara Plateau islands (Dibner and Zakharov, 1970), together with seismic data (Vinogradov et al., 1987) indi-

cate that bedrock in the Saint Anna Trough has similar Mesozoic strata. The upper seismo-stratigraphic unit might be largely masked by a bubble pulse in the airgun

trough axi transverse

ridges

flutes

1 km

hummock field Fig. 7. 30 k&

side-scan

I

I profile across the axial part of the Saint Anna Trough

at 80 N

( Fig. 2 for locatIon). Background

water deplh

590 m.

record, but adjacent echo-sounder profiles clear11 show that the unit has a laminated acoustic signature grading to transparent with decreasing thickness (Fig. 4). These sediments conformably

drumlin-like features

transverse ridges

W

blanket a rough surface (u) of opaque, highimpedance deposits of the lower unit, mostly unpenetratable by the echo-sounder. The upper unit is traced on echo-sounder records throughout

hummock field

\

f

520 m

I 380 m

f

1 km

E

i ,

slope

4

iceberg scours Fig. 8. 30 kH2 side-scan shown by the image.

profile across

the western

flank of the Saint Anna Trough

at 80 N (Fig. 2 for location).

Water depths

are

L. Polyak et al. / Marine Geology 143 (1997) 169-188

transverse ridges /

anastomosing ridges I

sinuous transverse ridges /

I

1 km

I

Fig. 9. 30 kHz side-scan profiles across the shallow area at the western side of the Saint Anna Trough at 80”N (A) and 79”35’N (B) (Fig. 2 for location). Background water depths: 340 m (A) and 300 m (B).

the Saint Anna Trough and typically has 1 to 3 m thickness, increasing to > 10 m thickness in local depocenters along the trough periphery. In the northern part of the trough, another marked reflector (6) is identified at 3 to 10 m below the sea floor, presumably indicating a thin lower unit (Figs. 5 and 6). The origin of underlying deposits cannot be derived from an echo-sounder record, but considering the stratigraphic position and the sharp, smooth character of the reflector, we suggest that it is the reflector b seen in the airgun profile. The echo-sounder and side-scan sonar records reveal a complicated system of erosional and depositional morphological features, including numerous furrows and ridges of variable size, associated almost exclusively with the upper surface of the lower seismo-acoustic unit (Figs. 4-9). Large ridges, up to 30 m high and > 5 km wide, are

recorded mostly at the western flank and adjacent axial part of the trough. Only one conspicuous ridge of under 20 m high is detected on the eastern flank (Fig. 6). Mapping of major ridges on multiple intersecting profiles suggests arcuate shapes which form an echelon-like set of subparallel belts (Fig. 10). The ridge belts are most apparent at depths between 350 and 550 m; at shallower depths, a high density of ridge-and-furrow features complicates the correlation of profiles. Side-scan sonograms in the central part of the Saint Anna Trough reveal numerous smaller ridges and hummocks with prevailing widths of 10 to 30 m and heights of ~5 to 10 m. These topographic features are most clearly visible in the axial part of the trough at water depths exceeding 400 m due to the sparseness of superimposed furrows and a thin upper seismo-acoustic unit at

60

,65

/

,70

.

I

1

l

80,E

I

i

,300

78

l

$“.

sediment

cores

side-scan

profiles

-

inferred general

I

echosounder

--

\I\\ sets of minor transverse ~3

to the larger ridge belts, and may grade into fields of small hummocks. Several sites also have a perpendicular generation of linear ridges with even denser spacing (Fig. 7). In some areas, they grade into chains of elongated mounds with somewhat larger dimensions of 50 to 100 m in width and > 10 m in height (Fig. 8). Multiple furrows, up to 200 m wide and 20 m deep, are identified at the sea floor on acoustic and side-scan sonar records, commonly truncating both large and small ridges. These incisions are typically ‘v-shaped’ and may have berms. The furrows are most abundant at the trough periphery in water depths of ~400 m, but are also common at depths reaching 550 m, especially on the crests of large ridges in the central part of the trough. Side-scan sonograms reveal diverse orientations for furrows, with linear, winding or parabolic pathways that often intersect (Figs. 8 and 9).

400

BARENTS SEA

-

,75

T

major morainic ridges

glacier-margin

profiles ridges

P flutes retreat

Fig. IO. Distribution of identified bedforms based on echosounder and side-scan data. Orientation of major morainic ridges is inferred from side-scan sonograms and from correlation between echo-sounder profiles; where correlation was not available, ridges are shown perpendicular to echo-sounder lines. Minor morphological features (glacial-sole markings) are interpreted from side-scan sonograms. Water depth (m) is shown in 100 m contour lines (Cherkis et al.. 1991 ).

these levels. The most common directional features are sets of slightly curved, sinuous, or bifurcating ridges spaced at 50 to 200 m intervals (Figs. 7 and 8). In shallow areas, they may be more winding and irregular, forming an anastomosing pattern (Fig. 9). These features appear to be subparallel

4. Sediment stratigraphy The lithology of sediment cores recovered from the Saint Anna Trough is divided into three major lithostratigraphic units, based on sediment color, structure, physical properties, magnetic susceptibility. grain size, organic matter and carbonate contents, as well as fauna1 and mineral composition (Table 2; Figs. 11 and 12; Herlihy, 1996: Kolstad. 1996). The basal unit is a gray to dark-gray, stiff, massive, almost unfossiliferous diamicton with clayey matrix, fairly high sand content of 20-30%. and common subangular coarse clasts. The small volume of grain-size samples precludes an accurate quantification of the coarse component, but the X-ray images show abundant clasts up to 10 cm in size. The diamicton is distinguished by high bulk-density ( 1.8-2.0 g/cm3), modest water content of around 25%, and variable shear strength averaging at 25 kPa and in some places reaching > 70 kPa. The sediment appears homogeneous with occasional sandy lenses (core 29) or clay interclasts (core 7). The heavy mineral composition is dominated by minerals of authigenic origin, mainly pyrite and siderite (Fig. 11). Together with

179

L. Polyak et al. / Marine Geology 143 (1997) 169-188

MS, 1O-5SI

Water&tent,%

c calcareous ct arenaceous Foraminifers/g

e TOC, wt%

SaKl, wt%

Heavy minerals,%

a

13.3 ka

1

2

Bulk density, g/cm +-

3o

2

4

Gravel, wt%

0

0:1

0.2

PyroxenelPyritelSyderite

TCaCO,, wt% &

Fig. 11. Sediment characteristics of the outer Saint Anna Trough, exemplified by core 7 (Fig. 2 for location). Lithologic symbols as in Fig. 12. Solid lines separate lithostratigraphic units; dashed lines show positions of dark-gray sandy layers. Corrected 14C ages are shown on the right. MS= magnetic susceptibility; TOC= total organic carbon; TCaCO, = total calcium carbonate.

relatively high organic matter and carbonate contents ( l-2% and 0.1-O-2%, respectively), this composition is common for basal diamictons in the Barents Sea and is considered to be inherited from redeposited Mesozoic bedrock (Yashin et al., 1985; Elverhoi et al., 1989; Polyak and Solheim, 1994). The overlying intermediate unit occurs in cores collected from depths of > 550 m, and does not exceed 1 m in thickness. The unit comprises several lithologic facies, but is characteristically composed of gray, relatively soft clayey mud, often with black iron-sulfide speckles and traces of bioturbation (Table 2; Figs. 11 and 12). The bottom 10 to 20 cm of sediment has a slightly brownish color and displays sandy bedding which varies from faint banding visible on X-ray images to well-

developed gradational sequences of Bouma A and B types (core 29; Bouma, 1969). The overlying gray mud contains two dark-gray diamictic intercalations with sharp boundaries, high sand contents (up to 40%) and a composition similar to the basal diamicton. In particular, the heavy minerals in sandy layers largely contain ‘authigenits’ in contrast to the enclosing sediment characterized by the dominance of clinopyroxenes (Fig. 11). Grain-size samples from the sandy layers lack coarse detritus, but the additional examination of cores revealed clasts of > 5 mm in size. The intermediate unit appears to be mostly unfossiliferous, but contains a pronounced abundance spike of calcareous foraminifers centered at 5-10 cm below the lower sandy bed. Foraminiferal assemblage in this spike is dominated by benthic

Water depth (ml

N

turbldltes

mollusc shells dark-gray sandy layers

600-

11

7

.__-.--___---_ Wwr Unit

lnterfnedia te

‘.

I i I I 1I Sand Sand Sand

Fig. 12. (‘orrelation magnetic

susceptibility

MS 60 +t++-tl Sand

50%

50 wt%

of sediment

co,-es along a north

south transect

25%

50%

in the Saint Anna Trough

(Fig. :! for locations).

Solid curves=

( MS); dotted curves = sand content

Cussidulirzu testis (up to 60%) and contains planktonic forms. The top of the intermediate unit is marked by a bright reddish- to yellowish-brown band that is partially indurated. The brownish coloration of the unit’s bottom and top is accompanied by lows in magnetic susceptibility values and carbonate and organic carbon contents. Previous sediment core studies from the Saint Anna Trough indicate that similar layers are enriched in iron and manganese oxides, which is reflected in the brownish-red color of the sediment (Yermolaev. 1948; Turner and Harris. 1970). These oxidized beds (designated as OX-11 and OX-III) as well as the two dark-gray sandy beds are used as lithostratigraphic markers due to their distinctive appearance and consistent occurrence throughout the trough (Fig. 12 ). The upper unit is a homogeneous, brown to olive-gray. soft clayey mud with low sand content

10%) and bulk density ( 1.5-I .7 gjcm”). high water content (over 40%). signs of bioturbation, and variable amounts of calcareous and arenaceous microfossils (Table 2; Figs. I 1 and 12). In cores from the inner trough, the lower part of this unit commonly contains abundant iron-sulfide speckles, calcareous foraminifers and small bivalve mollusks. This part of the section is topped by an oxidized bed (OX-I ). The dominant heavy mineral for the upper unit is clinopyroxene, which attains highest concentrations in the lower part of the unit. The overall thickness of the upper unit increases about tenfold from 30 cm to > 3 m southwards along the trough axis, towards Novaya Zemlya; elevated thicknesses also occur in local depressions along the eastern trough periphery. The increase in thickness is accompanied by a more grayish coloration of sediment due to abundant iron sulfides. (under

L. Polyak et ul. / Marine Geology 143 (1997) 169-188

5. Age control

The chronostratigraphic framework is provided by eleven 14C AMS ages on small paired bivalve mollusks or foraminifers picked from abundance spikes (Table 3); 18 additional ages were obtained from the duplicate cores (Kolstad, 1996). Due to the limited availability of datable material, most of the ages are from the upper unit and range from 1.4 to 9.6 ka (Fig. 12); a maximum age of 9.9 ka is reported for this unit based on duplicate core dating. Three ages obtained from the foraminiferal abundance level in the intermediate unit of cores 7, 29, and duplicate core 7 yield almost identical ages of ca. 13.3 ka, while a correlative date from core 6 gave a significantly younger age of ca. 8.3 ka. The latter sample had a small weight (0.25 mg of recovered carbon), and should be considered less reliable (H. Krueger, pets. commun., 1996). Assigning ages to major lithostratigraphic units identified in the Saint Anna Trough cores is aided by the occurrence of prominent lithologic features, notably the unit boundaries and positions of sandy and oxidized beds (Fig. 12). Correlation is further enhanced by similar magnetic susceptibility (MS) records between cores, with the lowest MS values recorded for the oxidized beds. We infer an age of > 13.3 ka for the boundary between the basal and intermediate unit and an extrapolated age of approximately 10 ka for the transition from the intermediate to upper unit, based on available i4C ages and lithologic correlation.

6. Depositional environments The correlation of echo-sounder records with sediment cores in the Saint Anna Trough indicates that the basal lithological unit (diamicton) corresponds to the top part of the high-impedance, chaotic-signature, lower seismo-stratigraphic unit. Its sharp bottom reflector (b) appears to be erosional and is correlated to the Upper Regional Unconformity, which separates the glacigenic sediments from bedrock in the Barents Sea (Solheim and Kristoffersen, 1984). In contrast to the mostly smooth bottom reflector, the unit’s upper surface

181

(a) is disturbed by multiple ridges, hummocks, and furrows (Figs. 49) which indicate a variety of complex sub- and pro-glacial processes. We interpret the large ridge belts as marginal morainic complexes, likewise the prominent ridge structures recorded in the Barents Sea along the northern and southern margins of the Bear Island Trough, around Svalbard and northern Novaya Zemlya (e.g. Elverhoi and Solheim, 1983; Vorren and Kristoffersen, 1986; Epshtein and Gataullin, 1993). The furrows are easily interpreted as iceberg plowmarks, which are common features on glaciated shelves and adjacent slopes (e.g. Solheim et al., 1988; Dowdeswell et al., 1992; Vogt et al., 1994). Small ridge-and-hummock features at the Saint Anna Trough bottom are similar to glacialsole markings inferred from side-scan sonograms of glaciated shelves, including areas recently vacated by grounded ice (Solheim et al., 1990; Josenhans and Zevenhuizen, 1990; Solheim, 1991; Pudsey et al., 1994). Small ridges subparallel to major morainic belts appear to be minor transverse morainic ridges (e.g. Clayton and Moran, 1974; Sugden and John, 1976). We interpret sets of transverse ridges in the axial part of the trough as De Geer moraines (Figs. 7 and 8; cf. Solheim et al., 1990; Josenhans and Zevenhuizen, 1990), believed to be formed by molding or pushing of subglacial sediment near the grounding line of a readvancing glacier (Zilliacus, 1989; Lundquist, 1989). Anastomosing ridges at shallower depths (Fig. 9) look more like cross-valley moraines of Baffin Island, which are also believed to be related to a readvancing submarine glacier margin ( Andrews and Smithson, 1966). In contrast to transverse ridges, intersecting linear ridges and elongated mounds are interpreted as flutes and drumlin-like features (Figs. 7 and 8; cf. Solheim et al., 1990; Josenhans and Zevenhuizen, 1990; Fader et al., 1997) which are created by deformation of substratum beneath a glacier in the direction of its movement, especially by streamlined readvances of a glacier margin (e.g. Boulton, 1976; Lundquist, 1990). The combination of seismo-acoustic characteristics and sedimentologic properties of the diamicton suggest that it is a glacigenic seismo- and lithostratigraphic unit identified elsewhere in the

Barents Sea (e.g. Elverhoi and Solheim, 1983; Gataullin et al.. 1993; Polyak et al., 1995). Accordingly, it is inferred that the diamicton was formed subglacially or in association with an icesheet grounding line. Moderate density and shear strength values of the Saint Anna Trough diamicton (Table 2) are similar to subglacial facies from other Barents Sea troughs (Elverhoi et al., 1990; %&tern et al.. 1992: Lubinski et al., 1996), which were supposedly the outlets for grounded-ice flow from the shelf interior. The diamicton is recognized in sediment cores and on echo-sounder records throughout the Saint Anna Trough area to at least 82”N and a water depth of 630 m, which suggests that the grounded ice filled the entire trough and extended to the shelf edge. The apparently conformable contact between the diamicton and overlying sediments, and radiocarbon ages above the diamicton indicate that its deposition likely terminated in the Late Weichselian. The two upper lithological units (intermediate stratified sediment and upper homogeneous mud), equivalent to the laminated upper seismo-stratigraphic unit, were deposited during and after the deglaciation of the Saint Anna Trough and adjacent shelf. Their lithostratigraphy and seismic geometry is similar to other postglacial sedimentary sequences from the Barents Sea (cf. Elverhoi and Solheim, 1983; Gataullin et al., 1993; Polyak et al., 1995: Lubinski et al., 1996). Early deglaciation of the Saint Anna Trough was characterized by variable sediment input, reflected in the deposition of sandy turbidites or laminated and slightly oxidized finer-grained sediments at the bottom of the intermediate unit. The lamination implies periodic input of sediment with little bioturbation, which is common in proximal glacimarine environments (Powell, 1983; Eyles et al.. 1985). Oxidation of sediment suggests relatively low deposition rates, perhaps associated with a reduction of calving/melting due to a stabilization of a glacier front at shallower water depths. or with a permanent sea-ice cover (cf. Hebbeln and Wefer, 1991). Variable patterns of glacimarine sedimentation continued during the later phases of deglaciation. Two distinct sandy diamictic beds within the inter-

mediate unit (Figs. I 1 and 12) have sharp contacts and massive internal structure, which indicates rapid deposition with little sorting. A sediment composition of these beds is similar to the basal diamicton and includes abundant reworked pyrite and siderite grains which can not survive a long transportation in water or sediment flows. We conclude that the sandy beds were deposited by iceberg rafting (cf. Eyles et al., 1985; Dowdeswell et al.. 1994), and reflect two major pulses of iceberg calving and/or melting in the Saint Anna Trough. The impact of icebergs on deglacial environments in the trough is underscored by abundant plowmarks of up to 200 m wide, which are common at water depths down to 550 m. This depth and the dimensions of plowmarks are significantly greater than those expected from modern icebergs in the Saint Anna Trough area (Voevodin. 1972: Abramov, 1992), but are similar to iceberg gouges off some Greenland outlet glaciers (Dowdeswell et al., 1992). This comparison suggests that the deglacial iceberg calving in the Saint Anna Trough originated from fast-flowing glaciers with relatively short floating termini (tens of kilometers). Sediment cores from depths less than 550 m lack the intermediate lithologic unit, which was probably disturbed by iceberg plowing ( Fig. 12). The relatively soft basal lithological unit of these cores exemplified by core 67 may be an iceberg-turbation diamicton, which is not easily distinguishable from a basal till of a marine-based ice sheet (Hald et al., 1990; Dowdeswell et al.. 1994). Stratigraphic levels with patches of reduced organic matter (iron sulfides), traces of bioturbation, and foraminiferal fauna in the intermediate unit imply the presence of biota in the Saint Anna Trough at least episodically during deglaciation. A foraminiferal spike dated to 13.3 ka contains planktonic forms and relatively abundant benthic Cussidulina teretis, which are commonly connected with advected subsurface Atlantic waters and/or increased surface productivity (e.g. Tamanova, 1970; Wollenburg, 1995; Jennings and Weiner. 1996). Similar spikes in a comparable stratigraphic position are recorded for the Franz-Victoria Trough ( Polyak and Solheim, 1994; Lubinski et al.,

L. Polyak et al. / Marine Geology 143 (1997) 169-188

1996). These events may reflect the advection of Atlantic waters to the northern margin of the Barents and Kara seas with ice-sheet retreat, as early as 13.3 ka. Data from the continental slope and adjacent Nansen Basin indicate that Atlanticwater inflow north of the Barents Sea occurred even during the Late Weichselian glacial maximum (Solheim et al., 1996; Knies and Stein, 1996). The combination of extensive iceberg discharge with relatively warm intermediate water may have resulted in episodes of significant ice melting indicated by the spikes of low 6180 in records from the Franz-Victoria and Saint Anna troughs and the adjacent Nansen Basin (Stein et al., 1994; Lubinski et al., 1996; Norgaard-Pedersen, 1996; Polyak et al., 1996). The transition from the intermediate glacimarine unit to the upper marine unit in the Saint Anna Trough is marked by a pronounced oxidation bed (OX-II; Figs. 11 and 12). Buried oxidation fronts imply dramatic decreases in sedimentation rates leading to the formation of iron and manganese oxides and dissolution of biogenic carbonate in the subsurface sediment (cf. Wilson et al., 1986; Buckley and Cranston, 1988). Extrapolation of available radiocarbon ages dates the oxidation bed OX-II to approximately 10-l 1 ka. The drop in sedimentation at the end of deglaciation implied by the oxidized bed could result from the withdrawal of glaciers from the shelf and/or may reflect perennial sea-ice cover. The upper unit was deposited after ca. 10 ka under full marine conditions. During the early Holocene, the interior of the Saint Anna Trough, especially its southeastern part was characterized by high sedimentation rates reaching > 1 m/1000 year around 9 ka (Fig. 12; Kolstad, 1996), which favored the formation of iron sulfides and preservation of carbonates. This interval of high sediment input cannot reflect glacier activity from Franz-Josef Land because its coasts were largely deglaciated by 10 ka, and some outlet glaciers were even behind present limits in the early Holocene (Forman et al., 1996). The deglaciation pattern of Novaya Zemlya is less understood, but available sea-level observations from the northern part of the archipelago present no evidence for significant ice lingering into the Holocene (Forman et al.,

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1995; Zeeberg, 1997). The rapid sedimentation more likely reflects winnowing of glacigenic sediments from submerged banks (cf. Vorren et al., 1984; Hald et al., 1996), and increased sediment input from Siberian rivers and/or coastal erosion during a rising eustatic sea level. High contents of clinopyroxene in heavy minerals at this stratigraphic level, reaching 50% in core 67, may indicate a strong Siberian-river component (cf. Levitan et al., 1996), although the discrimination of this source from another pyroxene province, western Franz-Josef Land (Klenova, 1960) needs further investigation. Sedimentation rates dropped to low near-modern values after 8 ka, reflecting a larger distance from sediment sources with the stabilization of sea level, and/or establishment of lag deposits on banks, and possibly heavier sea-ice cover. The decrease in sediment delivery is marked with an oxidation bed (OX-I) in the deep part of the Saint Anna Trough (Fig. 12).

7. Deglaciation pattern and chronology The identification of a glacigenic diamicton along the axis of the Saint Anna Trough provides the first evidence of the extension of grounded ice to the shelf edge in this area. These results are consistent with an undated, bathymetrically inferred accretionary fan in front of the Saint Anna and Voronin troughs (Vagnes, 1996). According to recent sedimentological and seismic data from the continental slope, the last ice-sheet advance to the northern margin of the Barents Sea occurred at approximately 20 ka (Knies and Stein, 1996; Solheim et al., 1996). The three radiocarbon ages of 13.3 ka from deglacial sediments provide a minimum age for retreat of a grounded-ice margin from the deep, axial part of the Saint Anna Trough (over 550 m water depth). The dated level overlies > 30 cm of post-glacial sediment, including the slightly oxidized bed (OX-III; Fig. 12). Assuming relatively slow sedimentation for this interval inferred from sediment oxidation, we suggest that the deep Saint Anna Trough could have become ice-free as early as 14+ ka, which is close to the oldest post-glacial dates of 14-15 ka from the outer Franz-Victoria Trough and the western

Barents Sea margin (Elverhoi et al.. 1995; Solheim et al., 1996). The inferred early deglaciation of the deep Saint Anna Trough underscores the importance of bathymetry control on marine-based ice sheets (cf. Hughes, 1977). We believe that the retreat of a glacier grounding line progressed up-trough with the rising sea level. The glacial-sole markings, thin glacimarine cover over a glacigenic diamicton. and abundant iceberg plowmarks indicate that icemargin retreat in the axial part of the trough was rapid, and was accompanied by an extensive iceberg discharge (cf. Vorren et al., 1988: Elverhoi et al., 1990; Polyak et al., 1995). Possibly. the initial massive iceberg-calving event is reflected by a prominent light 6180 spike in planktonic foraminiferal records from the adjacent Eurasian Basin dated to approximately 14--16 ka (Stein et al.. 1994; Nsrgaard-Pedersen, 1996 ). Further deglaciation of the Saint Anna Trough progressed with unknown rates. and was completed by ca. 10 ka. Multiple morainic ridge belts and glacial-sole marking zones reflect a potentially stepwise ice-margin retreat ( Figs. 10 and 13 ). similar to deglacial patterns from the western and southern Barents Sea areas (Vorren et al., 1988; Elverhoi et al., 1990; Lehman and Forman. 1992: Polyak et al., 1995). Two sandy layers within the intermediate unit deposited between ca. 13 and 10 ka in the deep axial part of the Saint Anna Trough appear to represent two major episodes of iceberg calving and/or melting (Fig. 12). A sediment record from the Franz-Victoria Trough also contains two sandy spikes within a similar stratigraphic interval (Lubinski et al.. 1996). Additional chronologic control and provenance studies are needed to test whether these spikes in the FranzVictoria and Saint Anna troughs mark regional iceberg-calving events along the northern margin of the Barents and Kara seas. The orientation of the small morphological features and the larger morainic ridges recorded in the western part of the Saint Anna Trough suggest a general south- to southwestwards glacier-margin retreat at an early stage, and a potential subsequent shift to a northwestern direction towards FranzJosef Land at depths of < 400 m (Figs. 10 and 13). This change in the retreat pattern may be

associated with a separation of the Northern Barents Sea and Novaya Zemlya ice domes. A relatively small but distinct ridge at the eastern flank of the Saint Anna Trough may indicate the presence of an ice cap on the Northern Kara Plateau (Figs. 4 and 10). Deposition of marine sediments since ca. 10 ka implies that the trough was completely vacated of glacier ice by this time. which is consistent with terrestrial data from western Franz-Josef Land indicating that glaciers retreated behind the coastline by 10.4 ka (Forman et al., 1996). The overall reconstruction of glaciermargin positions during deglaciation of the Saint Anna Trough is in agreement with the pattern of glacioisostatic rebound on Franz-Josef Land. pointing at the northern Barents Sea as a major ice center of the Late Weichselian Barents Sea Ice Sheet (Fig. 13; Forman et al., 1995). A thick accumulation of glacigenic sediment in the southern basin of the Saint Anna Trough (Fig. 3) is presumably caused by glacier ice from Novaya Zemlya and agrees with extensive morainic ridges reported west of the archipelago (Merklin et al., 1992; Epshtein and Gataullin, 1993; Gataullin and Polyak. 1997). However, the presence of the Novaya Zemlya ice in the Saint Anna Trough is not clearly reflected in the distribution of morainic ridges (Fig. lo), which may result from insufficient echo-sounder coverage of the southern part of the trough.

8. Conclusions ( 1 ) During the last glaciation, the Saint Anna Trough was filled with a grounded ice sheet extending to the shelf edge to at least 82”N and a water depth of 630 m. (3) We infer that deglaciation of the Saint Anna Trough started prior to 13.3 ka and advanced up-trough as calving of icebergs progressed. An undated initial pulse of iceberg discharge may have caused the early light #*O spike in planktonic foraminiferal records from the Eurasian Basin ‘“C-dated to 14-16 ka. (3) The subsequent deglaciation of the Saint Anna Trough appears stepwise and probably included two major pulses of iceberg calving

L. Polyak et al. / Marine Geology 143 (1997) 169-188

Greenland

185

Sea

Fig. 13. Deglaciation patterns in the Saint Anna Trough and adjacent areas of the Barents Sea. Solid lines are positions of retreating glacier margins inferred from the distribution of glacier bedforms, seismo- and lithostratigraphy in the Saint Anna Trough (this study), and morainic ridge belts west of Novaya Zemlya (Epshtein and Gataullin, 1993; Gataullin and Polyak, 1997). Dashed lines show hypothetical continuation of inferred glacier margins. Dotted lines are isobases (m) of post-glacial emergence since 5 ka (Forman et al., 1995). Area with modern water depths >200 m is shaded.

and/or melting between ca. 13 and 10 ka. Intervals with benthic and planktonic biota started at ca. 13.3 ka, and were possibly associated with the advection of intermediate Atlantic waters. (4) The major ice masses during deglaciation were located west and south of the Saint Anna Trough; however, a smaller ice cap is also inferred for the Northern Kara Plateau. As deglaciation progressed, ice retreat at the western side of the trough was directed towards eastern Franz-Josef Land. (5) The Saint Anna Trough was completely vacated of grounded ice by ca. 10 ka. The trans-

ition from glacimarine to marine environments was presumably accompanied by a drop in sedimentation rates. A temporary increase in sediment flux occurred between 10 and 8 ka, possibly caused by greater sediment input from Siberian rivers and sea-floor/coastal erosion during lower sea level.

Acknowledgements

This research was supported by ONR contract NOOO14-93-1-0995 and NSF grant OPP-9223493. We thank the crew and shipboard party of R/V

Projixwr

Loguchrv

headed by V.N. lvanov (PMGE, Sevmorgeologia, Russia). A. Solheim (Norwegian Polar Research Institute) kindly contributed the airgun seismic record. Yu.P. Goremykin (PMGE) performed digital processing of side-scan sonograms. M.A. Levitan (P.P. Shirshov Institute of Oceanology, Russia) supervised mineralogical analysis. Discussions with M. Hald and V. Kolstad (University of Tromso. Norway) were most helpful for core data interpretation. Critical reviews by D.J. Lubinski and W.F. Manley helped to improve the manuscript. This is Byrd Polar Research Center contribution No. 1065.

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