MARINE QEOLOQY - CORE · Sediment transport via sea ice is expected to contribute significantly to...

ELSEVIER Marine Geology 119 (1994) 185 214

MARINE QEOLOQY

tNTERNATIONAL JOURNAL OF MARINE G£OLOG~ GEOCHEMISTRY AND GEOPHYSICS

Sediments in Arctic sea ice: Implications for entrainment, transport and release

D. N/arnberg a, I. Wollenburg a, D. Dethleff b, H. Eicken a, H. Kassens b, T. Letzig b, E. Reimnitz c, j. Thiede b

" Alfred-Wegener-Institutefor Polar and Marine Research, Columbusstrafle, D-2 7515 Bremerhaven, Germany b GEOMAR, Researeh Center for Marine Geosciences, Wischhofstrafle 1 3, D-24148 Kiel, Germany

c U.S. Geological Survey, MS999, 345 Middlefield Road, Menlo Park, CA 94025, USA Energiesysteme Nord GmbH (ESN), Walkerdamm 17, D-24103 Kiel, Germany

(Received March 22, 1993; revision accepted November 2, 1993)

Abstract

Despite the Arctic sea ice cover's recognized sensitivity to environmental change, the role of sediment inclusions in lowering ice albedo and affecting ice ablation is poorly understood. Sea ice sediment inclusions were studied in the central Arctic Ocean during the Arctic 91 expedition and in the Laptev Sea (East Siberian Arctic Region Expedition 1992). Results from these investigations are here combined with previous studies performed in major areas of ice ablation and the southern central Arctic Ocean. This study documents the regional distribution and composition of particle-laden ice, investigates and evaluates processes by which sediment is incorporated into the ice cover, and identifies transport paths and probable depositional centers for the released sediment.

In April 1992, sea ice in the Laptev Sea was relatively clean. The sediment occasionally observed was distributed diffusely over the entire ice column, forming turbid ice. Observations indicate that frazil and anchor ice formation occurring in a large coastal polynya provide a main mechanism for sediment entrainment. In the central Arctic Ocean sediments are concentrated in layers within or at the surface of ice floes due to melting and refreezing processes. The surface sediment accumulation in central Arctic multi-year sea ice exceeds by far the amounts observed in first-year ice from the Laptev Sea in April 1992.

Sea ice sediments are generally fine grained, although coarse sediments and stones up to 5 cm in diameter are observed. Component analysis indicates that quartz and clay minerals are the main terrigenous sediment particles. The biogenous components, namely shells of pelecypods and benthic foraminiferal tests, point to a shallow, benthic, marine source area. Apparently, sediment inclusions were resuspended from shelf areas before and incorporated into the sea ice by suspension freezing.

Clay mineralogy of ice-rafted sediments provides information on potential source areas. A smectite maximum in sea ice sediment samples repeatedly occurred between 81°N and 83°N along the Arctic 91 transect, indicating a rather stable and narrow smectite rich ice drift stream of the Transpolar Drift. The smectite concentrations are comparable to those found in both Laptev Sea shelf sediments and anchor ice sediments, pointing to this sea as a potential source area for sea ice sediments.

In the central Arctic Ocean sea ice clay mineralogy is significantly different from deep-sea clay mineral distribution patterns. The contribution of sea ice sediments to the deep sea is apparently diluted by sedimentary material provided by other transport mechanisms.

0025-3227/94/$7.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0025-3227 (94)00036-K

186 D. Nfirnberg et al./Marine Geology 119 (1994) 185~14

1. Introduction

The Arctic sea ice cover is thought to strongly influence the global climatic evolution (Clark, 1990; Untersteiner, 1990). Extent, composition and thickness as well as drift patterns of sea ice influence the exchange of heat and gases between ocean and atmosphere, affecting the global energy balance and oceanic circulation (Aagaard et al., 1985; Clark, 1990).

Recent studies show that sea ice in the central Arctic Ocean transports significant loads of marine sediments originating most probably from surrounding shallow shelf regions (Pfirman et al. 1990; Wollenburg, 1993; Ftitterer et al., 1992). Early observations of sea ice sediments were made during Fridtjof Nansen's 1893-1896 Fram expedition to the eastern Eurasian Basin (Nansen, 1897, 1906; Gran, 1904) and by subsequent Arctic expeditions (Tart, 1897; Kindle, 1909; Poser, 1933; Sverdrup, 1931, 1938; Usachev, 1938). Eolian transport was suggested to be the dominant entrainment factor for the sea ice sediments in these early studies. Specific examinations of the sediment content (Mullen et al., 1972; Darby et al., 1974; Larssen et al., 1987; Pfirman et al., 1989a, 1990; Wollenburg, 1993) and of sediment incorporation processes (Barnes and Reimnitz, 1974; Reimnitz and Barnes, 1974; Barnes et al., 1982; Osterkamp and Gosink, 1984; Reimnitz et al., 1987, 1992; Reimnitz and Kempema, 1987, 1988; Thiede et al., 1988; Kempema et al., 1989; Dethleff et al., 1993), however, stress other processes as also being important for sediment entrainment. The relative importance of different processes will be discussed below.

To date, distinct source areas for sea ice sediments have not been located exactly. This is due, on the one hand, to the complexity of the drift regime in the Arctic Ocean and on the other hand due to the relatively small data base on sea ice sediments. Based on the recent large-scale ice drift pattern determined from buoy data (Colony et al., 1991), most investigations point to the broad (>500 km) and shallow (<50 m) Siberian continental shelf as the most probable site for sediment entrainment (Fig. 1).

Sediment transport via sea ice is expected to

contribute significantly to deep-sea sedimentation, at least in regions of ice ablation (Pfirman, 1990; Wollenburg, 1993). In order to understand the influence of recent sea ice transport on marine sediments, the relative importance of various processes responsible for the terrigenous sedimentation in the deep sea (i.e. oceanic currents, ice transport, gravity flows) have to be evaluated.

This paper includes the quantitative analysis of the lithogenous and biogenous components contained in sea ice and icebergs from the Eurasian part of the Arctic Ocean. The regional distribution is documented and annual variations in sediment content and distribution are assessed. The contribution of sea ice rafted sediments to deep-sea sedimentation in Arctic regions is discussed in combination with an attempt to identify transport pathways and depositional centers. Potential source areas for sedimentary material and processes by which sedimentary material is incorporated and redistributed during alternating melting/ freezing cycles are also reviewed. Identification of sea ice sediments in the deep-sea record and the subsequent differentiation from iceberg-material is generally problematic. For paleoceanographic and paleoclimatic reconstructions, a tracer indicating sea ice coverage in space and time would be of considerable importance. Up to now, dropstones defined as terrigenous components larger than 500/~m, are used to indicate ice transport or ice coverage (Molnia, 1972; Bischof et al., 1990). Differentiation between iceberg and sea ice sediments is of fundamental importance for under- standing the paleo-environment and the evolution of the Arctic polar cap during the geological past and will be discussed.

2. Methods and data base

2.1. Areas of investigation

Central parts of the Transpolar Drift have been systematically sampled in the framework of this study (Fig. 2; Table 1). Field work was performed during the International Arctic Ocean Expedition Arctic 91 aboard the RV Polarstern (Germany) and USCGC Polar Star (USA). Results from

6 0 °

D. Niirnberg et al./Marine Geology 119 (1994) 185 214

90 ° 120 °

187

1 5 0 °

1 8 0 °

1 5 0 °

6 0 ° 9 0 ° 1 2 0 °

Fig. 1. Ice drift pattern in the Arctic Ocean, after Gordienko and Laktionov (1969), with surrounding shallow (< 30 m), potential sediment source areas (stippled).

related studies in Fram Strait and the Greenland Sea carried out between 1987 and 1991 by some of the co-authors (e.g. RV Polarstern cruises ARK IV, V, VI and VII) will be incorporated in this analysis. During these expeditions, "dirty ice" was recovered from the European sector of the Arctic Ocean (Fig. 2), termed central Arctic Ocean in this context, and main ablation areas between approximately 72°N and 90°N.

This large data set on sediment-laden ice has been expanded during the East Siberian Arctic Region Expedition (E.S.A.R.E.'92) to the Lena

Delta and broad shelf areas in the Laptev Sea in 1992 (73°-79°N and 122°-141°E, see also Dethlett et al., 1993). Since this area is regarded as one ot the main Arctic "ice factories" with large sea ice export rates (Zakharov, 1976), sea ice sediments are presumed to be incorporated here and subsequently transported with the Transpolar Drift. During the summer, the Laptev Sea is ice-free over wide areas. Between October and December, fast ice builds up from shallow coastal areas to the 20-30 m isobath. The winter situation is characterized by the occurrence of an approximately

85 ° N 80 ° N

~maya ya

188 D. Nfirnberg et al./Marine Geology 119 (1994) 185-214

80 ° E

osef

0

Fig. 2. Chart of the Arctic Ocean showing sample locations of sea ice sediments during Arctic 91 from the RV Polarstern. The suffix H denotes helicopter sampling sites.

1800 km long, narrow zone of open water forming a polynya on the mid-shelf (Dethleff et al., 1993), which separates the fast ice from the drift-ice (Fig. 3).

Approximately 60 ice cores were collected at 91 sites during Arctic 91. In addition to the ice cores, approximately 130 "dir ty" surfaces on the ice floes, on icebergs and in cryoconites were sampled. In the Laptev Sea area, approximately 60 ice cores and 60 snow surface samples were obtained at 18 stations. Observations on surface characteristics, including thickness of snow cover, temperature and freeboard of floes were performed routinely (Eicken and Haas, 1992; Dethleff et al., 1993). Cores were obtained with a 10 cm ice corer, usually through the entire floe thickness within "dirty ice" patches.

Visual observations of the percentage of "dir ty" patches in the ice surface were made during Arctic 91 aboard RV Polarstern every 2 hours for about 15 minutes (depending on the ship's progress and the variability of ice conditions). These observa-

tions were made from the ship's bridge, roughly 17 m above sea level, covering a strip extending max. 5 km to either side of the ship's track (Eicken and Haas, 1992).

Investigations on sediment concentrations in ice cores and surface samples, grain size distribution, and lithic and biogenous sediment components have been carried out here and related to previous studies (Wollenburg, 1993; Reimnitz et al., 1992; Dethleff et al., 1993). Sediment concentrations were determined by vacuum filtering the melted ice samples and calculating the weight of particulate matter per liter of melt water. In order to derive an estimate of the ice floe sediment load and the downcore distributional pattern, selected ice cores were sampled at 2 cm intervals and analysed. Surface sediment concentration was calculated from the uppermost centimetres of the ice floes. In the course of Arctic 91, increasing snow thickness hindered recognition of "dir ty" ice. Therefore, snow was consequently removed before sampling the upper ice layers.

D. Niirnberg et al./Marine Geology 119 (1994) 185-214 189

Table 1 Sediment concen t ra t ion (mg/ l mel t water) in ice cores, floe surfaces and snow f rom the Arct ic ice cover. D a t a are der ived f rom

Arctic 91 and E .S .A.R.E '92 exped i t ions to the cent ra l Arct ic Ocean and the Lap tev Sea, respectively. Sample type, la t i tude and long i tude of sampl ing sites are indicated. For Arctic 91 data , sea ice coverage and percentage of dir ty ice are added

Sta t ion # Desc r ip t ion La t i tude Long i tude Sed./ice Sea ice cover Di r ty ice

Polarstern Arctic 91 ( °N) ( m g / l ) (%) (%)

218 surf. I 81°45.32 ' 029"56.17'E 160.74 > 9 5 1-5 218 surf. II 81°45.32 r 029°56.17'E 148.64

219 surf. I 82°46.50 ' 029°53.40'E 12.58 > 9 5 5 -20 220 surf. I 83°59.30 ' 030°28.60'E 1168.85 220 surf. II 83"59.30' 030~'28.60'E 3076.94 90 5 -20 220 surf. V 83°59.30 ' 030~'28.60'E 1581.29 222 surf. I 84~'52.70 ' 037°56.20'E 0.00 95 1 5 223 surf. 85°27.80 ' 044°29.50'E 3956.80 95 1-5

223 surf. I 85°27.80 ' 044°29.50'E 1335.84 223 surf. I I 85c'27.80 ' 044°29.50'E 666.00 223 surf. IV 85°27.80 ' 044°29.50'E 6701.45 223 surf. IV 85°27.80 ' 044°29.50'E 6153.05 224 surf. I 85°47.70 ' 050°49.30~E 5551.71 90-95 1 5 224 surf. II 85°47.70 ' 050°49.30'E 17,676.91 224 surf. IV 85047.70 ' 050°49.30'E 31,490.82

225 surf. I 86014.50 ' 059°12.70'E 1192.92 95 1 5

225 surf. I 86014.50 ' 059°12.70'E 2898.92

225-11 surf. I Heli 86°14.50 ' 059°12.70'E 2219.80

225-12 surf. II Heli 86°14.50 ' 059°12.70'E 56,944.45

226 surf. I 86°19.50 ' 059°24.10'E 160.62 75 95 1 5

226 surf. | 86°19.50 ' 059°24.10'E 1222.04

226 surf. I I 86~26.50 ' 060°02.00'E 957.35

227 surf. I 86"51.60' 059°44.70'E 2743.00 90-95 1 5

228-01 surf. I 86°53.46 ' 056°55.00'E 7022.28 80 1 5

228 surf. I Hel i 86°53.46 ' 058°51.33'E 6736.40 228 surf. I I Hel i 86°53.46 ' 058°51.33'E 4539.19

230-01 surf. I 87035.30 ' 060°44.10'E 3503.47 85 5--20

230-01 surf. I I 87o35.30 ' 060°44.10'E 5166.80

230 surf. I Heli 87°34.64 ' 064"33.15'E 0.00

230 surf. I I Hel i 87°40.95 ' 064°33.15'E 1956.55

231 surf. I I Hel i 87°35.70 ' 069 '10.10 'E 30,655.04 90 1 5

232 surf. I 87°15.30 ' 072°47.60'E 11.22 85 1 5

232 surf. I 87°15.30 ' 068°17.70'E 1228.93

233 surf. I 87°18.30 ' 069°13.60'E 274.63 90 98 1 5 233 surf. II 87°18.30 ' 069°13.60rE 327.71

234 surf. I 87°29.30 ' 090°56.90'E 1624.55 90 95 1 5 234 surf. I Heli 87°25.87 ' 089°43.59'E 9398.17 234 surf. I I Heli 87°23.42 ' 090°22.98'E 2319.94

234 surf. I I I Heli 87°22.76 ' 092°42.54'E 1664.56

234 surf. IV Hel i 87°24.12 ' 092°42.54'E 17,475.33 234 surf. V Hel i 87026.25 ' 093°42.42'E 1361.48

234 surf. VI Hel i 87°29.45 ' 094°07.55'E 339.57

235 surf. I 87°33.70 ' 103°29.90'E 644.86 95 99 1 5

239 surf. I 88°02.10' 134°46.00'E 56.38 > 95 1 5 240 surf. I 88°00.00 ' 158°51.80'E 679.24 95 5 20 240 surf. I I a 88°01.35 ' 160°36.60'E 1054.29 240 surf. P6 Hel i 87°56.00 ' 157°50.96'E 176.26 240 surf. P6-2 Heli 87056.00 ' 157°50.96'E 158.90 240 surf. P6-3 Heli 87056.00 ' 157°50.96'E 286.06 240 surf. P6-4 Heli 87°56.00 ' 157°50.96'E 0.00

190

Table 1 (conitinued)

D. Niirnberg et al./Marine Geology 119 (1994) 185-214

Station # Description Latitude Longitude Sed./ice Sea ice cover Dirty ice

Polarstern Arctic 91 (°N) (mg/1) (%) (%)

240 surf IIb, clean snow 88°00.00 ' 158°51.80'E 0.00 244 surf. Ib 87°36.20 , 148°54.00'E 54.29 95 5-20 244 surf. Ia 87°36.20 ' 148°54.00'E 67.54 244 surf. II 87°36.20 ' 148°54.00'E 196.44 250 surf. I North Pole 89°59.80 ' lI5°00.00'W 303.38 95-99 1-5 250 surf. I North Pole 89059.80 ' l15°00.00'W 498.97 250 surf. North Pole 89°59.80 ' 115°00.00'W 372.00 250 surf. Nor th Pole 89°59.80 ' 115°00.00'W 252.83 250 surf. North Pole 89°59.80 ' 115°00.00'W 365.59 252 surf. I 88o59.90 ' 009°24.90'E 0.00 >95 1-5 253 surf. I 87°30.60 ' 011°34.20'E 12.09 85 1-5 253 surf. II 87°30.60 ' 011 °34.20'E 28.04 254 surf. Ia 86o45.40 ' 009°56.90'E 0.00 90-95

254 surf. Ib 86°45.40 ' 009°56.90'E 0.00 254 surf. II 86°45.40 ' 009°56.90'E 371.77

254 surf. IIb 86°45.40 , 009~'56.90'E 0.00

254 surf. IIc 86°45.40 ' 009°56.90'E 552.61

256 surf. 86°07.80 ' 005°02.90'E 0.00 93-99

256-11 surf. 1 86°07.80 ' 005°02.90'E 0.00 257 surf. Ia 85045.70 ' 004°I0.00'W 20,182.97 >90

257 surf. Ia 85°45.70 ' 004c'10.00'W 19,778.43

257 surf. Id 85°45.70 ' 004°10.00'W 0.00

258 surf. Ia 85°33.80 ' 009°03.30'W 0.00 >90 1 5

258 surf. Ib 85°33.80 ' 009°03.30'W 0.00 259 surf. I Heli 85°16.41 ' 014"01.20'W 0.00 95 1 5

259 ice edge surf. 85°19.70 ' 014°13.80'W 0.00 261 surf. I 85°06.40 ' 014°22.90'W 0.00 >95 5-20

263 surf. I Heli 85°03.62 ' 011°16.27'W 0.00 95 1 5

263 surf. IIb Heli 85°03.31 ' 011 °16.27'W 0.00 264 surf. I 84o38.80 ' 006°47.10'W 252.42 >95 1 5

264 surf. I 84°38.80 ' 006°47.10'W 0.00 264 surf. II 84o38.80 ' 006°47.10'W 1007.24

264 surf. II 84°38.80 ' 006°47.10'W 0.00 266-01 surf. 1 gravel 83o59.90 ' 000"05.70'W 20,505.77 >95

267 surf. 1 heli 83°36.64 ' 004°40.10'E 4567.95 85 90 1 5

267 surf. 1II heli 83°33.55 ' 004c'32.33'E 37,595.40 267 surf. V hell 83o35.74 ' 005°27.28'E 0.00

267 surf. VIII heli 83°42.85 ' 005°07.07'E 0.00

268 surf. 1I 83008.25 ' 008°20.13'E 0.00 95

232 surf. I mud hole 87°15.30 ' 072°47.60'E 4892.63 85 237 surf. iceberg 87°46.10 ' 108°38.90'E 16.48 >98

237 surf. iceberg 87°46.10 ' 108°38.90'E 13.48

237-11 surf. 1 iceberg 87°46.10 ' 108°38.90'E 8.98 238 surf. I iceberg 88°01.70 ' I19°51.40'E 40.56 97 99

239 surf.Ila ridge 88°02.10 ' 134°46.00'E 1910.26 95

239 surf.IIb ridge 88°02.10' 134°46.00'E 348.85

245 surf. I iceberg 87026.55 ' 148°27.80'E 512.07 >98 245 surf. Ia iceberg 87°31.80 ' 144°15.80'E 196.47 245 surf. Ib iceberg 87°31.80 ' 144°15.80'E 434.12 245 surf. Ic iceberg 87°31.80 ' 144°15.80'E 230.81 248 surf. I iceberg 88°44.10 ' 126°59.70'E 0.00 >98 248 surf. II iceberg 88°44.10 ' 126°59.70'E 0.00 248 surf. l I I iceberg 88°44.10' 126"59.70'E 95.46



Stat ion # Descr ip t ion La t i tude Long i tude Sed./ice Sea ice cover Di r ty ice

Polarstern Arctic: 91 ( ° N ) ( m g / l ) (%) (%)

248 surf. IV iceberg 88°44.10 ' 126°59.70'E 135.35

258 3 - 4 m dep th 85033.80 ' 009°03.30 'W 0.00 90

258 1.5-2 m dep th 85°33.80 ' 009°03.30 'W 0.00

259 surf. VI heli, - 1 . 5 85°16.41 ' 014°01.20 'W 0.00 95

263 surf. I I a heli, 1.2 m 85°03.31 ' 011°16.27 'W 0.00 95

266 surf. 85°03.31 ' 000c~05.70'W 0.00 > 9 5

266-01 surf. I gravel 83°59.90 ' 000°05.70 'W 20,505.77

Polar Star

228-1-1

228-1-3

228-1-5

228-1-7

228-1-9

229-1-1

229-1-3

229-1-5

229-1-7

229-1-9

230-1-1

230-1-3

230-1-5

230-1-7

230-1-9

231-1-1

231-1-2

231-1-3

231-1-4

231-1-5

231-1-6

232-1-1

232-1-3

232-1-5

232-1-7

233-1-2

233-1-3

234-1-1

234-1-3

236-1-1

236-1-3

236-1-5

236-1-7

236-1-9

239-1-I

239-1-3

239-1-5

239-1-7

239-1-9

241-1-1

241-1-2

241-1-3

241-1-4

241-1-5

Arct ic '91 ice samples ice core 82°26.00 ' 30°32.00'E 2.50

ice core 82°26.00 ' 30°32.00'E 2.79

ice core 82°26.00 ' 30°32.00'E 3.69

ice core 82°26.00 ' 30°32.00'E 1.28

ice core 82°26.00 ' 30°32.00'E 4.95

ice core 83°21.00 ' 32°51.00'E 5.73

ice core 83°21.00 ' 32°51.00'E 5.68

ice core 83°21.00' 32°51.00'E 1.91

ice core 83°21.00 ' 32°51.00'E 2.61

ice core 83°21.00 ' 32°51.00'E 4.42

ice core 83°43.00 ' 33°22.00'E 2.19

ice core 83°43.00 ' 33°22.00% 2.09

ice core 83°43.00 ' 33°22.00'E 2.51

ice core 83°43.00 ' 33°22.00'E 7.33

ice core 83°43.00 ' 33°22.00'E 11.36

tu rb id ice chunks 84°08.00 , 33°45.00'E 31013.40

turb id ice chunks 84°08.00 ' 33°45.00'E 854.10




turbid ice chunks 84008.00 ' 33°45.00'E 1226.30

Ice core 84°11.50 33°58.50'E 4.00

ice core 84°11.50 33°58.50'E 6.00

ice core 84°11.50 33°58.50'E 3.40

ice core 84°11.50 33°58.50'E 5.55

ice core 84 ° 15.00' 34039.00'E 60.56

ice core 84°15.00' 34°39.00'E 111.79

ice core 84°21.00 ' 35°08.00'E 1.95

ice core 84021.00 ' 35°08.00'E 2.66

ice core 84°30.00 ' 34"32.00'E 8.94

ice core 84030.00 ' 34°32.00'E 178.78

ice core 84°30.00 , 34°32.00'E 6.01

Ice core 84030.00 ' 34°32.00'E 125.48

~ce core 84°30.00 ' 34°32.00'E 101.52

ice core 84o49.00 ' 39°57.00'E 6.72

ice core 84o49.00 ' 39°57.00'E 4.64

ice core 84049.00 ' 39°57.00'E 2.20

ice core 84049.00 ' 39°57.00'E 3.48

ice core 84049.00 ' 39°57.00'E 3.76

ice core 84o19.00 ' 45° 10.00'E 121.64

ice core 84°19.00 ' 45°10.00'E 163.84

ice core 84°19.00 ' 45°10.00'E 61.32

~ce core 84 ° 19.00' 45 ° 10.00'E 26.48

ice core 84°19.00 ' 45°10.00'E 34.68



Sta t i on # D e s c r i p t i o n L a t i t u d e L o n g i t u d e Sed. / ice Sea ice cove r D i r t y ice

Polarstern Arctic 91 ( ° N ) (mg/1) (%) (%)

241-2-1

242-1-1

242-1-3

242-1-5

242-1-7

242-1-9

243-1-1

243-1-3

243-1-5

243-1-7

243-1-9

244-1-2

244-1-7

244-1-3

244-1-1

244-1-4

244-1-5

244-1-6

244-1-8

244- I -9

244-1-10

245-1-1

245-1-3

245-1-5

245-1-7

245-1-9

P o l a r S t a r A r c t i c 91

228 - 1-2

228-1-4

228-1-6

228-1-8

228-1-10

229-1-2

229-1-4

229-1-6

229-1-8

229-1-10

230-1-2

230-1-4

230-1-6

230-1-8

230-1-10

232-1-2

232-1-4

232-1-6

232-1-8

234-1-2 234-1-4

236-1-2

236-1-4

236-1-6

236-1-8

t u r b i d ice c h u n k s 84°14.00 ' 44°48 .00 'E 1261.24

ice co re 84°03.00 ' 46°44 .00 'E 7.44

ice co re 84°03.00 ' 46°44 .00 'E 33.36

ice co re 84°03 .00 ' 46°44 .00 'E 306.56

ice co re 84°03.00 ' 46°44 .00 'E 10.68

ice co re 84°03.00 ' 46°44 .00 'E 2 .84

ice core 83039.00 ' 47°06 .00 'E 2.92

ice co re 83°39.00 ' 47°06 .00 'E 738.64

ice co re 83°39.00 ' 47°06 .00 'E 1200.64

ice co re 83°39 .00 ' 47°06 .00 'E 34.52

ice co re 83 c'39.00' 47 °06.00 'E 28.16

core , 1 0 - 2 2 c m 83°01.00 ' 48°07 .00 'E 1.57

core , 0 10 c m 83°01.00 ' 48°07 .00 'E 5.58

core , 0 10 cm 83°01.00 ' 48°07 ,00 'E 9.65

core , 0 - 1 0 c m 83°01.00 ' 48°07 .00 'E 85.84

core , 1 0 - 2 2 c m 83°01.00 ' 48°07 .00 'E 6.30

core , 0 - 1 0 c m 83°01.00 ' 48°07 .00 'E 22.00

core , 1 0 - 2 2 c m 83°01.00 ' 48°07 .00 'E 46.08

core , 10 22 c m 83°01.00 ' 48°07 .00 'E 3.80

core , 0 - 1 0 c m 83°01.00 ' 48°07 .00 'E 8.05

core , 1 0 - 2 2 c m 83°01.00 ' 48°07 .00 'E 2.78

ice co re 82°40.00 ' 49 ° 14.00 'E 47 .00

ice co re 82°40.00 ' 49 ° 14.00 'E 4 .92

ice co re 82°40.00 ' 49 ° 14.00 'E 6.16

ice core 82°40.00 ' 49°14 .00 'E 45.88

ice co re 82°40.00 ' 49 ° 14.00 'E 33.04

snow samples s n o w 82°26 .00 ' 30°32 .00 'E 1.16

s n o w 82°26.00 , 30°32 .00 'E 1.96

s n o w 82°26.00 ' 30°32.00 'E 3.66

s n o w 82°26.00 ' 30°32 .00 'E 3.41

s n o w 82°26.00 ' 30°32 .00 'E 16.65

s n o w 83 °21.00 ' 32 °51.00 'E 3.03

s n o w 83°21.00 ' 32°51.00 'E 0.97

s n o w 83 °21.00 ' 32 °51.00 'E 4.78

s n o w 83o21.00 ' 32°51.00 'E 1.96

s n o w 83°21.00 ' 32°51 .00 'E 4.08

s n o w 83°43.00 ' 33°22 .00 'E 1.03

s n o w 83 °43.00 ' 33 °22.00 'E 0.84

s n o w 83°43.00 ' 33°22.00 'E 0.99

s n o w 83°43.00 ' 33°22 .00 'E 3.78

s n o w 83°43.00 ' 33°22.00 'E 6.41

s n o w 84°11.50 ' 33'>58.50'E 2.42

s n o w 84"11 .50 ' 33°58 .50 'E 2.26

s n o w 84°11.50 ' 33°58 .50 'E 11.11

s n o w 84 ° 11.50' 33 °58.50 'E 1.73

s n o w 84°21.00 ' 35°08 .00 'E 2.68

s n o w 84°21.00 ' 35°08.00 'E 1.60 s n o w 84o30.00 ' 34°32 .00 'E 1.91

s n o w 84°30.00 ' 34°32 .00 'E 42.21

s n o w 84o30.00 ' 34°32 .00 'E 1.68

s n o w 84°30.00 ' 34°32 .00 'E 18.00

D. Nfirnberg et al./ Marine Geology 119 (1994) 185-214 193


Stat ion # Descr ip t ion La t i tude Long i tude Sed./ice Sea ice cover Di r ty ice


236-1-10 snow 84030.00 ' 34°32.00'E 135.30

239-1-2 snow 84o49.00 ' 39°57.00'E 2.62

239-1-4 snow 84°49.00 ' 39°57.00'E 2.34

239-1-6 snow 84°49.00 ' 39°57.00'E 1.52

239-1-10 snow 84049.00 ' 39°57.00'E 5.76

242-1-2 snow 84°03.00' 46°44.00'E 4.17

242-1-4 snow 84003.00 ' 46°44.00'E 29.94

242-1-6 snow 84o03.00 ' 46°44.00'E 280.77

242-1-8 snow 84°03.00 ' 46°44.00'E 8.04

242-1-10 snow 84°03.00' 46°44.00'E 6.39

243-1-2 snow 83039.00 ' 47°06.00'E 1.34

243-1-4 snow 83°39.00 ' 47°06.00'E 5668.28

243-1-6 snow 83°39.00 ' 47°06.00'E 9520.74

243-1-8 snow 83039.00 ' 47°06.00'E 3.79

243-1-10 snow 83039.00 ' 47°06.00'E 131.35

245-1-2 snow 82040.00 ' 49 ° 14.00'E 2.89

245-1-4 snow 82°40.00 ' 49°14.00% 75.69

245-1-6 snow 82040.00 ' 49 ° 14.00'E 4438.67

245-1-8 snow 82°40.00 ' 49 ° 14.00'E 19.59

245-1-10 snow 82°40.00 ' 49 ° 14.00'E 45.12

E.S.A.R.E. '92 snow samples 981-1 snow A 71o45.00 ' 127°09.00'E 197.17

981-1 snow B 71045.00 ' 127°09.00'E 107.24

981-1 snow C 71 °45.00' 127°09.00'E 202.92

100-1 snow A 72030.00 ' 130°00.00'E 49.76

100-1 snow B 72030.00 ' 130°00.00'E 14.25

100-1 snow C 72o30.00 ' 130°00.00'E 4.20

100-2 snow A 73029.00 ' 129°58.00'E 2.60

100-2 snow B 73029.00 ' 129°58.00'E 1.55

100-2 snow C 73o29.00 ' 129°58.00'E 12.86

101-1 snow A 73o45.00 ' 128°15.00'E 2.44

101-1 snow B 73°45.00 ' 128°15.00'E 7.02

101-1 snow C 73045.00 ' 128°15.00'E 23.21

102-1 snow A 74°06.00 ' 125°01.00'E 1.90

102-1 snow B 74o06.00 ' 125°01.00'E 19.14

102-1 snow C 74006.00 ' 125°01.00'E 2.59

102-2 snow A 74029.00 ' 129°50.00'E 13.73

102-2 snow B 74°29.00 ' 129°50.00'E 13.38

102-2 snow C 74°29.00 ' 129°50.00'E 12.12

103-11 snow A 71o48.00 ' 129°00.00'E 28.62

103-11 snow B 71°48.00' 129°00.00'E 37.41

103-11 snow C 71 °48.00' 129°00.00'E 40.74

107-1 snow 75°59.00 ' 137°53.00'E 33.73

107-1 snow 75°59.00 ' 137°53.00'E 29.86

108-1 snow A 76059.00 ' 134°58.00'E 10.27

108-1 snow B 76059.00 ' 134°58.00'E 5.90

108-1 snow C 76°59.00 ' 134°58.00'E 2.41

108-2 snow B 76°00.00 ' 133°05.00'E 6.39

108-2 snow C 76000.00 ' 133°05.00'E 27.85

108-22 snow A 76000.00 ' 133°05.00'E 41.71

111-11 snow A 79000.00 ' 140°01.00'E 3.04

111-11 snow B 79000.00 ' 140°01.00'E 3.68



Sta t i on # D e s c r i p t i o n L a t i t u d e L o n g i t u d e Sed. / ice Sea ice cove r D i r t y ice


111-11 s n o w C 79°00.00 ' 140°01.00 'E 7.40

112-2 s n o w A 77°48.00 ' 140°04 .00% 36.56

112-2 s n o w B 77°48 .00 ' 140°04 .00% 19.84

112-2 s n o w C 77°48.00 ' 140°04.00 'E 32.95

112-3 s n o w A 77°29.00 ' 133°11.00 'E 11.55

112-3 s n o w B 77°29.00 ' 133°11.00 'E 0.01

112-3 s n o w C 77°29.00 ' 133°11.00 'E 3.19

113-1 s n o w A 75°29.00 ' 132°12.00 'E 1.58

113-1 s n o w B 75o29.00 ' 132°12.00 'E 0.50

113-1 s n o w C 75°29 .00 ' 132°12.00 'E 0.39

113-2 s n o w A 74°11.00 ' 130°28.00 'E 67.64

113-2 s n o w B 74°11.00 ' 130°28.00 'E 32.68

113-2 s n o w C 74°11.00 ' 130°28.00 'E 27.61

E. S. A. R.E. "92 ice samples 100-11

100-12

100-21

101-11

102-11

102-12

102-21

103-11

103-11

108-12

108-13

108-21

111-10

111-11

l l l - l l b

l l l - 1 3 a

l l l - 1 3 b

111-14

111-15

111-16

111-17

111-18

111-19

l l l - l 1 2 a

l l l - l 1 2 b

111-2

112-2

112-31

113-12

113-13

113-13

113-15 113-16

113-21

114-1

114-12

114-13

114-14

ice co re

ice co re

ice co re

ice co re

sed. l ayer

ice co re

4 repr . c h u n k s

ice co re

6 repr . pieces

ice co re

t u r b i d ice

ice co re

6 repr . pieces

7 repr . pieces

7 repr . pieces

6 repr . pieces

6 repr . pieces

6 repr . pieces

6 repr . pieces

6 repr . pieces

7 repr . pieces

7 repr . pieces

7 repr . pieces

7 repr . pieces

7 repr . 31eces

ice co re

t u r b i d ice

ice co re

ice co re

a n c h o r ice

4 repr . c h u n k s

4 repr . c h u n k s t u r b i d ice

ice co re

repr . c h u n k s

t u r b i d ice

t u r b i d ice

t u r b i d ice

72°30 .00 ' 130°00.00 'E 30.19

72°30.00 ' 130°00.00 'E 63.89

73°29.00 ' 129°58 .00 'E 14.46

73 °45.00 ' 128 ° 15.00 'E 2.50

74°06.00 ' 125°01.00 'E 28.41

74°06.00 ̀ 125°01.00 'E 25.71

74029.00 ' 129°50.00 'E 5.68

71 °48.00 ' 129°50.00 'E 210.91

71 °48.00 ' 129 °50.00 'E 2.32

76°59.00 ' 134°58.00 'E 8.37

76°59.00 ' 134°58.00 'E 50.66

76°00.00 ' 133°05.00 'E 8.07

79°00.00 ' 140°01.00 'E 2.91

79°00 .00 ' 140°01.00 'E 1.60

79°00.00 ' 140°01.00 'E 102.64

79 °00.00 ' 140°01.00 'E 47 .26

79°00.00 ' 140°01.00 'E 156.03

79°00.00 ' 140°01.00 'E 0.92

79o00.00 ' 140°01.00 'E 1.48

79°00.00 ' 140°01.00 'E 1.14

79°00.00 ' 140°01.00 'E 57.33

79°00 .00 ' 140°01.00 'E 55.84

79°00.00 ' 140°01.00 'E 10.42

79000.00 ' 140°01.00 'E 52.96

79°00.00 ' 140°01.00 'E 8.22

79°00.00 ' 140°01.00 'E 4.21

77°48.00 ' 140°04.00 'E 132.41

77°29.00 ' 133 11.00'E 61.69

75°29.00 ' 132 ° 12.00 'E 98.02

75°29.00 ' 132 °12.00,E 783.98

75 o29.00' 132 ° 12.00 'E 140.41

75°29.00 ' 132 ° 12.00 'E 272.20 75°29.00 ' 132°12.00 'E 16.70

74 ° 11.00' 130°28.00 'E 6.82

77°44.00 ' 140°21.00 'E 160.75

77°44.00 ' 140°21.00 'E 255.52

77°44.00 , 140°21.00 'E 184.74

77°44 .00 ' 140°21.00 'E 963.67

D. Nfirnberg et al./Marine Geology 119 (1994) 185-214 195


Station # Polarstern

Description Arctic 91

Latitude Longitude Sed./ice (°N) (mg/1)

Sea ice cover (%)

Dirty ice (%)

114-14 114-15b 114-16

turbid ice turbid ice turbid ice

77+44.00 , 140°21.00'E 276.22 77°44.00 ' 140°21.00'E 492.53 77°44.00 , 140°21.00'E 328.11

E.S.A.R.E. "92

~ i 112"2 m 114-1

I1113-2 %

I-1

) 100-2 fastice

s m

Laptev Sea Polynya ~ Freshwater Ice generalized, mid winter (1992)

Fig. 3. Bathymetric chart o f the Laptev Sea indicating ice sampling sites. The extent of fast ice and fresh-water ice as observed during April 1992 field work is shown.

During the Laptev Sea expedition, 6-7 ice chunks of variable length judged representative of the entire ice column were selected from the core for calculation of the sediment load. This method was used because of logistical constraints in Siberia. The sediment load in snow was averaged from several 1-3 1 samples taken up-wind of contaminating helicopter activities on the ice. The

sand content was derived by wet-sieving of sea ice sediment samples.

Sea floor surface sediments and sediment cores have been recovered during Arctic 91 (i.e. Stein et al., 1994-this volume) in parallel to the ice- sampling program. These samples were analysed to identify the recent and fossil contribution of Arctic sea ice on deep-sea sedimentation. In the


Laptev Sea, a few bottom samples were taken to a maximum water depth of 64 m. The parallel sampling of sea floor and sea ice sediments was carried out to elucidate entrainment processes.

Grain size and component analyses have been performed on sediments from sea ice and the sea floor. Amounts of sediment components of Arctic 91 bulk samples are estimated from smear slides, whereas Laptev Sea data are derived by analysing the > 63 #m fraction. Percentages of terrigenous components (quartz, feldspar, heavy minerals, mica etc.) are calculated without considering the biogenous portion, since it is assumed that the majority of the biogenous components (diatom sub-ice flora) grew in the ice during the drift (cf. Abelmann, 1992). Clay mineralogy was determined by X-ray diffractometry. Texturally-oriented samples have been produced by vacuum filtration through a 0.15/~m filter (Lange, 1982) and were subsequently measured by a Philips X-ray dif- fraction system with cobalt Kalpha-radiation. Semiquantitative analyses are based on peak areas of the four clay minerals smectite, chlorite, illite and kaolinite. Peak areas are multiplied by "Biscaye factors" (Biscaye, 1965). Clay mineral percentages are not weighed according to bulk sediment, since additional minerals occurring in the clay fraction (e.g. feldspar) respectively in the silt and sand fraction (e.g. phyllosilicates) have not been considered. Organic carbon contents were determined on sediment bulk samples using a LECO-CS analyser. Carbonate contents were calculated as (total c a r b o n - total organic carbon) x 8.333.

3 . R e s u l t s

3.1. Small scale distribution of sea ice sediments

Numerous recent publications have shown that fine-grained, dispersed sediment inclusions in sea ice are much more important for ice rafting in the Arctic Basin than all other types of inclusions (i.e. Reimnitz et al., 1987, 1993; Kempema et al., 1989; Pfirman et al., 1989a,b, 1990; Wollenburg, 1993). These studies and our investigations indicate the

following characteristic types of surface and subsurface sediment accumulations:

Surface accumulations." - Sediment patches (10-20 m in diameter) with

diffusely distributed sediment particles restricted to the sea ice surface. Sediment covers as much as 50% of ice surfaces. Particles normally form small aggregates.

- Sediment surface layers (10-15 cm in thickness) at the interface between snow and ice. No sediment below this layer is observed.

- Sediment within cryoconites (10 30 cm depth, 3 5 cm in diameter) located in melt water ponds and at ice floe edges. Sediment accumulations easily disperse during sampling. The surroundings of melt water ponds often show high sediment concentrations and are slightly elevated relative to the ponds.

- Sparse sediment accumulations mostly restricted to one side of ice ridge surfaces, implying recent eolian transport.

Subsurface accumulations: - One or more sediment layers or lenses within

the interior of the floe. Horizontal layers can often be traced over hundreds of metres. Ridging of ice floes lead to inclined sediment layers.

- Sediment diffusely distributed over a broad range of the first-year ice column, such as in Laptev Sea. This turbid ice (Kempema et al., 1989) was especially found in a region of ice rubble 16 km west of the New Siberian Islands and over a distance of 70 km to the south thereof.

- Delicately structured sediment inclusions, 5-10 cm in diameter, with sediment and fibrous organic matter, which are strongly suggestive of entrainment by anchor ice. These so-called "puff balls" were exclusively found in Laptev Sea fast ice.

3.2. Regional distribution of sea ice sediments

Shipboard observations during Arctic 91 provide insights into the distributional pattern of sediments on and within sea ice. Similar to previous studies (Thiede et al., 1988; Pfirman et al., 1990;

D. Niirnberg et aL /Marine Geology 119 (1994) 185~14 197

Wollenburg, 1993; Ftitterer et al., 1992), our observations show that large areas are discolored by particulate matter (Fig. 4). Discolored patches of 10 20 m diameter with diffusely distributed sediment particles in the upper 3 cm of sea ice prevail. Such patches can cover as much as 50% of the total sea ice surface (Ftitterer et al., 1992). Coverage normally ranged between 0 and 20% (Fig. 4).

On the northbound leg of the RV Polarstern cruise (stations 217-250, Fig. 2) "dirty ice" was restricted mainly to the sea ice surface (Fig. 4).

The southwesternmost stations (stations 259-264) are characterized by thick and dark sediment layers in the interior of ice floes, which range up to 5 m thick.

In first year ice of the Laptev Sea, turbid ice prevails. Nearly total snow cover during April prevented the kind of visual assessment of regional sediment discoloration that is possible from an ice- breaking vessel. Sparse sediment discoloration of the young snow cover, nevertheless, was observed locally, where it could be explained by wind transport from nearby land.

85 ° N 80 ° N

80 ° E

o

( ~ Sed. accumulations on ice surface and layers in interior of floe 5-20% of sea ice cover

Q Sed. accumulalion reslricled to ice surface 5-20% of sea ice cover

0 Sed. accumulations reslricted !o ice surface 1-5% of sea ice cover

• Sed. accumulations restricted lo ice surface 0-1% of sea ice cover

Fig. 4. Chart showing areas where sediment inclusions in the Arctic sea ice and icebergs were observed during Arctic 91 by scientists aboard RV Polarstern and U S C G C Polar Star. Sediment abundance is indicated.

198 D. Niirnberg et aL/Marine Geology 119 (1994) 185 214

3.3. Sediment concentration

In the central Arctic Ocean, highest sediment concentrations were found on the surfaces of multi- year ice floes. Surface samples show concentrations ranging between ca. 10mg/1 and 56,000rag/1 (Table 1). Concentrations vary laterally even over distances of a few metres. A gradual increase in ice surface sediment concentrations between 85°N and 88°N corresponds to the center of the Transpolar Drift (Fig. 5).

The vertical distribution of sediment within the ice column was determined for 3 representative ice cores (Fig. 6). Concentrations at the surface range up to ~ 7000 rag/1. Below approximately 10 cm, sediment concentrations rapidly decrease. The high sediment accumulation near the floe surface is presumably due to redistribution processes during several freezing/melting cycles (Pfirman, 1990). Distinct sediment layers within floes may have concentrations as high as 2000 mg/1 (Wollenburg, 1993). These layers sometimes extend throughout many floes (Krause et al., 1991a).

The sediment concentration in snow samples from the Laptev Sea is generally low and ranges between approximately 0.01 and 100 rag/1 (Fig. 7; Table 1). Since these sediments occur in fresh snow, they are suggestive of eolian transport. A few exceptionally high values of ca. 100-200 rag/1 were measured in snow samples on Lena River

ice, where they were due to strong local orographic winds and erosion of nearby land surfaces.

Sediment concentrations in turbid ice cores from the Laptev Sea (Fig. 7; Table 1) are generally higher than in corresponding snow samples. Concentrations range between approximately 0.09 and 800 mg/1, which is comparable to central Arctic Ocean ice cores, but generally lower than in corresponding surface samples. Highest sediment concentrations occur at the northernmost sampling sites in the Laptev Sea, situated within a regime where ice is presumably supplied to the Transpolar Drift from east of the New Siberian Islands. Here, values range from ca. 29 to 800 mg/1. Exceptionally high sediment concentrations of as much as ca. 3800 mg/1 were measured near Bennett Island, northeast of the New Siberian Islands. In contrast, ice cores retrieved from the fast ice area to the south have sediment concentrations of only ca. 2.5 to 27 mg/1. High mean values of ca. 47 rag/1 and 262 rag/1 were also measured (1) in the ice cover of the winter Lena outflow (Station 100-1 in Fig. 3), and (2) in anchor ice southeast of the polynya (Station 113-1 in Fig. 3). Sediment concentrations on Greenland Sea floe surfaces are generally lower than central Arctic Ocean sediment concentrations, with values between 1 mg/1 and ca. 600 rag/1 (Wollenburg, 1993) (Fig. 7).

3.4. Grain size analyses

60000

50000,

E 40000" v

6 30000"

0

~ 20000 m

10000"

0 A A a

80 82 90

i " I J F

I • . o . . . .

i , T • • I 1 '

84 86 88 Lat i tude (N)

Fig. 5. Sediment concentrations of Arctic sea ice observed during Arctic 91. Between approximately 85°N and 88°N, corresponding to the central area of the Transpolar Drift, sediment concentrations are highest.

Grain sizes of sea ice sediments gathered from RV Polarstern during Arctic 91, were estimated from smear slides (Ft~tterer et al., 1992). These analyses compare with laboratory grain size investigations on sea ice samples gathered from USCGC Polar Star during Arctic 91 (Table2) and Wollenburg's (1993) grain size data from sea ice in the Fram Strait, Greenland Sea and the southern central Arctic Ocean.

Generally, sampled sea ice sediments are clayey silts or silty clays (up to ca. 75% silt and 90% clay) (Fig. 8). At a few sampling sites, sediments with up to 85% fine sand appear. These patches of coarse sediment accumulations appear randomly and occur directly beside patches of clayey to silty sediment composition. Iceberg sediments exhibit the same grain size spectrum as sea ice sediments

D. Niirnberg et aL /Marine Geology 119 (1994) 185-214 199

10

p 20

g t-

g ~° a

40

50

C e n t r a l Arc t ic O c e a n ARK 83 -221 -12

(mg/I) 300 600

" i . . . . . . . -'

, ~

ARK 83-221-22

(rag/I)

20 60 100 0 ' " ' " ' ' '

10 ~

20- I

:30-

40 .

50

ARK 83 -224-02

(mg/I)

0 3500 700C

20 :

30"

40

;0

L a p t e v S e a

(rag/I)

20 40 60

50- }

: 00-~

50- ----.0--.-- loo-12 / I • I~-21

• 10~-21

• 102 -12

t O O " + 101-11

100 -12

£ 112-31

150 ' ; : I

Fig. 6. Sediment distribution (mg/l) within ice floes of the central Arctic Ocean compared to those of the Laptev Sea. Note different scales.

Floe surface 10oo001-- , g -

: 1 i ° d o I i ° ~ a

~ , loooo-.,t . . . . . . . + ...... ÷ " ~ m -

: i i o , o o o ~ - - - ~ - - ~ - ~ _ -

¢ i i ~JB= ® 1oo',]--°-:;- .......... :,~--.-.~.-_1. O ~ ~o 17i

o ~o L...=~ L~........~=..].|... • o ~ o. q ~

Ice c o l u m n

!-I~

°'° g~ e

o ( °° 8

| i , .

70 74 78 82

Latitude (N)

Central Arctic Ocean [ ] Ice cores and surface samples

[] Surface samples (Wollenburg, 1991) • Iceberg samples

86 90 70 74 78 82 86 90

Latitude (N)

Laptev Sea O Ice cores and surface samples

Greenland Sea & Surface samples (Wollenburg, 1991)

Fig. 7. Sediment concentrations in sea ice from the central Arctic Ocean (this study), the Greenland Sea (Wollenburg, 1993), and the Laptev Sea (Dethleff et al., 1993). Data are differentiated according to sediment concentrations in floe surfaces and concentrations representing the total ice column beneath.


Table 2 Grain size analyses of sea ice and iceberg sediments f rom the central Arctic Ocean and for sea ice and sea floor sediments from the Laptev Sea. Grain size analyses performed during Arctic 91 are partly based on estimations from smear slides

Sample Gravel Sand Silt Clay Total (%) (%) (%) (%) (%)

Sea ice samples from Central Arctic Ocean (RV Polarstar) 228-1-11 1.5 30.6 67.9 100.0 229-1-11 0.3 19.9 79.8 100.0 229-1-12 0.2 27.6 72.2 100.0 231-1-7 10.9 41.4 47.7 99.9 233-1-11 0.7 44.2 55.1 100.0 236-1-11 0.1 83.8 5.2 10.9 100.0 236-1-11 a 83.9 5.2 10.9 99.9 241-1-6 0.6 4.5 94.8 100.0 241-1-7 2.3 11.9 85.8 100.0 241-2-2 0.1 11.8 88.1 99.9 242-1-11 0.0 5.9 94.1 100.0 245-1-11 0.0 19.6 80.4 100.0

Sea ice samples from Central Arctic Ocean (Arctic 91 R V Polarstern) (based on smear slide estimations) AR 83217-1 10.0 30.0 60.0 100.0 AR 83217-2 5.0 35.0 60.0 100.0 AR 83217-3 5.0 48.0 47.0 100.0 AR 83218-1 2.0 60.0 38.0 100.0 AR 83218-2 10.0 50.0 40.0 100.0 AR 83220-1 5.0 25.0 70.0 100.0 A R 83220-2 8.0 22.0 70.0 100.0 A R 83220-3 10.0 65.0 25.0 100.0 AR 83220-4 10.0 60.0 30.0 100.0 AR 83223-1 5.0 60.0 35.0 100.0 A R 83223-2 3.0 37.0 60.0 100.0 A R 83223-3 15.0 50.0 35.0 100.0 AR 83224-1 10.0 45.0 45.0 100.0 AR 83224-2 5.0 75.0 20.0 100.0 AR 83224-3 20.0 60.0 20.0 100.0 AR 83224-4 30.0 50.0 20.0 100.0 AR 83224-5 25.0 50.0 25.0 100.0 AR 83224-6 b 10.0 50.0 40.0 100.0 AR 83224-7 b 20.0 50.0 30.0 100.0 AR 83225-1 8.0 32.0 60.0 100.0 AR 83225-2 20.0 50.0 30.0 100.0 A R 83225-3 40.0 40.0 20.0 100.0 A R 83226-1 2.0 48.0 50.0 100.0 AR 83226-2 8.0 45.0 47.0 100.0 AR 83227-1 20.0 45.0 35.0 100.0 AR 83227-2 4.0 26.0 70.0 100.0 AR 83228-1 2.0 23.0 75.0 100.0 AR 83228-2 10.0 50.0 40.0 100.0 AR 83228-3 15.0 50.0 35.0 100.0 A R 83228-4 b 2.0 48.0 50.0 100.0 A R 83229-1 5.0 30.0 65.0 100.0 AR 83230-1 1.0 29.0 70.0 100.0 AR 83230-2 10.0 40.0 50.0 100.0

Table 2 (continued)

Sample Gravel Sand Silt Clay Total

(%) (%) (%) (%0 (%)

AR 83230-3 2.0 38.0 60.0 100.0 A R 83230-4 2.0 28.0 70.0 100.0 A R 83231-1 70.0 13.0 17.0 100.0 AR 83232-1 1.0 30.0 69.0 100.0 AR 83232-2 5.0 70.0 25.0 100.0 A R 83233-1 2.0 23.0 75.0 100.0 AR 83233-2 20.0 80.0 100.0 AR 83234-1 2.0 28.0 70.0 100.0 AR 83234-2 60.0 30.0 10.0 100.0 AR 83234-3 40.0 40.0 20.0 100.0 A R 83234-4 5.0 40.0 55.0 100.0 AR 83234-5 85.0 10.0 5.0 100.0 AR 83234-6 4.0 54.0 42.0 100.0 A R 83234-7 3.0 47.0 50.0 100.0 A R 83235-1 5.0 45.0 50.0 100.0 A R 83239-1 1.0 29.0 70.0 100.0 AR 83239-2 2.0 29.0 69.0 100.0 AR 83239-3 b 1.0 70.0 29.0 100.0 AR 83240-1 2.0 38.0 60.0 100.0 AR 83240-2 5.0 60.0 35.0 100.0 AR 83240-3 5.0 40.0 55.0 100.0 AR 83240-4 5.0 60.0 35.0 100.0 AR 83240-5 2.0 40.0 58.0 100.0 AR 83244-1 5.0 45.0 50.0 100.0 AR 83244-2 2.0 43.0 55.0 100.0 A R 83244-3 5.0 45.0 50.0 100.0 A R 83250-1 10.0 40.0 50.0 100.0 A R 83252-1 5.0 70.0 25.0 100.0 A R 83253-1 20.0 80.0 100.0 AR 83253-2 3.0 45.0 52.0 100.0 A R 83254-1 1.0 29.0 70.0 100.0 A R 83254-2 2.0 30.0 68.0 100.0 AR 83254-3 1.0 35.0 64.0 100.0 AR 83254-4 1.0 34.0 65.0 100.0 AR 83254-5 2.0 28.0 70.0 100.0 AR 83256-1 5.0 50.0 45.0 100.0 AR 83256-2 3.0 57.0 40.0 100.0 AR 83256-3 2.0 55.0 43.0 100.0 A R 83257-1 10.0 60.0 30.0 100.0 AR 83257-2 8.0 60.0 32.0 100.0 AR 83257-3 12.0 58.0 30.0 100.0 A R 83257-4 20.0 60.0 20.0 100.0 AR 83258-1 80.0 20.0 100.0 AR 83258-2 10.0 70.0 20.0 100.0 AR 83259-1 1.0 14.0 85.0 100.0 AR 83261-1 5.0 35.0 60.0 100.0 A R 83263-1 5.0 30.0 65.0 100.0 A R 83264-1 8.0 42.0 50.0 100.0 A R 83264-1 60.0 20.0 20.0 100.0 A R 83266-1 60.0 25.0 15.0 100.0 A R 83266-2 5.0 45.0 50.0 100.0 AR 83267-1 2.0 55.0 43.0 100.0 AR 83267-2 10.0 60.0 30.0 100.0

D. Nfirnberg et al./Marine Geology 119 (1994) 185 214 201

Table 2 (continued)

Sample Gravel Sand Silt Clay Total (%) (%) (%) (%) (%)

AR 83267-3 5.0 45.0 50.0 100.0 AR 83267-4 10.0 90.0 100.0 AR 83268-1 10.0 90.0 100.0

Iceberg sample AR 83237-1 5.0 50.0 45.0 100.0 AR 83238-1 10.0 60.0 30.0 100.0 AR 83245-1a 10.0 80.0 10.0 100.0 AR 83245-1b 5.0 50.0 45.0 100.0 AR 83245-1b 5.0 15.0 80.0 100.0 AR 83245-1c 2.0 20.0 78.0 100.0 AR 83248-1 4.0 47.0 49.0 100.0 AR 83248-3 5.0 45.0 50.0 100.0 AR 83248-4 10.0 50.0 40.0 100.0

Sample Description Sand Silt Clay Total (%) (%) (%) (%)

Sea ice samples from Laptev Sea (ESARE '92) 113-15 turbid ice 0.6 981 47 55 cm 78.5 114-16 ice 1.9 102-11 40-52 cm 4.7 111-19 ice core 0.4 113-12 0-34 cm 4.2 108-13 turb. ice 3.7 111-18 ice core 0.9 114-14 turb. ice 0.0 100-12 0-21 cm 0.0 l l l - 13b ice core 0.0 113-1 anchor ice 2.9 111-1 turbid ice 2.0

Seafloor sediments from Laptev sea 112-33 seafloor 1.1 108-2b seafloor 21.3 108-2 seafloor 21.1 108-23 seafloor 16.1 108-14 seafloor 4.8 112-11 seafloor 1.3 108-2 seafloor 6.3 112-2 snapper seafl. 33.1 112-2 seafloor 28.5

76.4 23.0 100.0 20.1 1.4 100.0 80.0 18.0 100.0 86.0 9.3 100.0 92.9 6.7 100.0 70.5 25.3 100.0 86.1 10.2 100.0 92.4 6.7 100.0 72.2 27.8 100.0 95.1 4.9 100.0 72.1 27.9 100.0 62.1 35.0 100.0 49.0 49.1 100.0

(ESARE "92) 51.3 47.6 100.0 41.2 37.5 100.0 40.1 38.8 100.0 41.6 42.3 100.0 56.5 38.8 100.0 52.8 45.9 100.0 57.3 36.4 100.0 34.3 32.6 100.0 9.8 61.8 100.0

aGravel was composed of 2 shell fragments, bSediment from cryoconites.

(Fig. 8; Table 2). In a few cases, coarse components (pebbles up to 5 cm in diameter) occur on multi-year floes.

In the Laptev Sea ice, clay and silt fractions also dominate (Fig. 8; Table2). In sea ice from the

drift and fast ice areas, the > 63 gm grain fraction ranges between 0 and 5% of the total sample. In contrast to sea ice, the sediments in Lena River ice are dominated by > 63/~m fraction ranging up to 78% of the total sediment. Since the sand fraction in shelf bottom sediments is significantly higher than in sea ice (Table 2), the prevailing low sand content observed in sediment in the ice of the Laptev Sea suggests preferential entrainment of the fine fraction, probably due to incorporation of resuspended sediments.

3.5. Sediment components and sediment variability

Central Arctic sea ice sediments consist mainly of terrigenous material (Fig. 9). Quartz, feldspar and clay minerals are dominant in the >63 #m fraction with a quartz/feldspar ratio around 1:1 (Wollenburg, 1993). Maximum concentrations of rounded quartz are observed between 85°N and 88°N. Mineral accessories are mica, pyrite, silt- stone fragments, glauconite and heavy minerals (hornblende, pyroxene, magnetite, t i tanite-- 0.47-8.38 wt.-%, Wollenburg, 1991). In general, muscovite is more abundant than biotite.

Clay mineralogy shows significant regional differences, with a smectite maximum in the central Arctic Ocean between 81°N and 83°N (Fig. 10). In 1987, the smectite percentage increased in this region to as high as 63 peak area-% compared to 22 peak area-% to the north and south of this region (Wollenburg, 1993). In the 1991 data, smectite values of ca. 42-56 peak area-% again appear in this area (Table 3), whereas to the north, concentrations were below 40 peak area-%. Due to the high smectite values in this region, sea ice sediments are depleted in illite (ca. 20 peak area-%) and chlorite (ca. 15-20 peak area-%). Pronounced maxima or minima similar to those for smectite are not obvious along the transect for illite, chlorite and kaolinite. Illite ranges between ca. 20 and 50 peak area-% and chlorite between ca. 10 and 18 peak area-%. Kaolinite generally has concentrations of ca. 15 30 peak area-%.

Mean smectite concentrations of 20-40 peak area-% observed in the central Arctic Ocean ice cover are comparable to values observed in the eastern Laptev Sea (Fig. 11; Table 3). Here, smec-

202 D. Ntirnberg et aL/Marine Geology 119 (1994) 185-214

Greenland Sea Fram Strait Southern Arctic Ocean

Sand pies

Silt Q a y

W o l l e n b u r g (1991)

Central Arctic Ocean Laptev Sea Sand Sand ~ e Sea ice samples

[] Iceberg samp es / / ~ . . . . [] / \ g M l v e r ICe

Cryoconile samples m / \ o Sea ce 0 Sea ice samples, / " \

Ni t Qay Silt a a y ARCTIC "91 E.S.A.R.E. "g2

Fig. 8. Sand, silt and clay fractions for sediments in sea ice presented in triangular diagrams, the peaks corresponding to 100% concentration. Data sets are separated according to areas of investigation. Left diagram shows data from Wollenburg (1993) gathered between 1987 and 1990, middle diagram shows data gathered during Arctic 91, and right diagram shows data from the Laptev Sea (E.S.A.R.E'92).

Other

I ~ Quartz r

Dis

0

Clay minerals

Based on smear slide investigations (ARCTIC "91)

Fig. 9. Mean abundances of most important sediment components in sea ice estimated from smear slide investigations. Terrigenous components dominate, however, a strong regional variability of sediment composition can be observed.

tite ranges between ca. 20-40 peak area-% in both shelf sediments and sea ice sediments from the fast ice area (anchor and turbid ice). Sediments sampled in drift ice further offshore, in contrast, are characterized by smectite concentrations below ca. 10 peak area-%.

Wollenburg (1993) and Reimnitz et al. (1993)

observed relatively low amounts of calcium carbonate in sea ice sediments (max. 0.6wt.-%). Maximum concentrations are due to detritus, shells and tests of calcitic organisms (pelecypods, foraminifers). Calcium carbonate in both Laptev Sea ice and bottom sediments is also generally below 1 wt.-% (Table 4). An exceptionally high value of 2.8 wt.-% in anchor ice is presumably due to the entrainment of calcarous macro-organisms, since the clay and silt fractions are barren of calcium carbonate.

Total organic carbon (TOC) is quite variable in central Arctic Ocean sea ice (ca. 0.6-6.4 wt.-%, Table4; Wollenburg, 1993). Maximum concentrations appear in sediments from the Transpolar Drift and from melt water ponds (4-6 wt.-%) (Wollenburg, 1993). Sea ice sediments in the Laptev Sea have TOC-concentrations of 1.5- 5.7 wt.-%, and they are mainly associated with the clay fraction. Shelf sediment TOC values are definitely lower (approximately 0.5-1.3 wt.-%).

The biogenous sediment components (Fig. 9) indicate a highly diverse ice flora (cf. Abelmann, 1992). Marine planktic and benthic diatoms are dominant. Especially at the bottom sides of the ice floes a rich diatom flora has been observed which colors the ice dark brown. Typical ice diatoms are Melosira arctica. In addition, centric and


Smectite (%) in sea ice

~g

20°W

Fig. 10. Smectite data (peak area-%) from central Arctic Ocean sea ice sediments show maximum concentrations between approximately 81 °N and 83°N decreasing northward. This maximum has been repeatedly observed during 1987 (Wollenburg, 1993) and 1991 (Arctic 91).

204 D. Niirnberg et al./Marine Geology 119 (1994) 185 214

Table 3 Clay minerals (peak area-%) in sea ice and sea floor sediments from the central Arctic Ocean and the Laptev Sea

Peak area-% Peak area-% Peak area-% Peak area-% Sum-% Sample Description Smectite Illite Chlorite Kaolinite

Sea ice samples from Central Arctic Ocean (Arctic 91 ) 217 sea ice 218 sea ice 220 sea ice 223 sea ice 224 sea ice 227 sea ~ce 231 sea ice 234 sea ice 240 sea ice 267 sea ice 83601 sea ice

56 21 9 14 100 42 24 13 21 100 33 36 13 18 100 39 29 10 22 100 21 35 15 30 101 37 28 12 23 100 31 30 10 29 100 31 34 12 23 100 12 51 18 19 100 28 39 12 21 100 21 38 17 24 100

Sea ice samples from Laptev Sea (ESARE'92) Fast ice area 113-1 anchor ice 45 26 113-15 turbid ice 42 30 113-12 ice core 23 39 Dri~ ice area 100-12 ice core 7 64 108-13 turbid ice 8 68 111-13 ice core 9 65 111-18 ice core 2 72 111-1 turbid ice 14 52 114-16 turbid ice 2 71

Seafloor samples j~om Laptev Sea (ESARE'92)

17 12 100 16 12 100 22 16 100

24 5 100 17 7 100 20 6 100 20 6 100 25 9 99 22 5 100

112-2 seafloor 13 52 26 9 100 108-14 seafloor 28 43 20 9 100 108-23 seafloor 39 38 16 8 100 108 -2 seafloor 28 45 19 8 100 108-2b seafloor 28 45 19 9 100 108-2 seafloor 28 45 18 8 100 112-1 seafloor 27 44 20 9 100 112-2 seafloor 18 52 22 8 100 112-33 seafloor 33 40 18 9 100

pennate diatoms like Nitzschia sp., Thalassiosira sp., and Chaetoceros sp. occur. Small gray aggregates ( < 2 ram) composed of centric and pennate diatoms and diatom spines are observed both in fresh snow and on ice surfaces. Of minor importance is the fauna including marine planktic tintinnids, silicoflagellates, copepods, radiolarians, dinocysts, pelecypods and amphipods. According to Gunilla Gard from University of Goteborg/ Sweden (pers. commun., 1992), no coccoliths have been observed in sea ice sediments gathered during

Arctic 91. Benthic foraminifers, which in general belong to the littoral-neritic zone, and planktic foraminifers are rare. Calcitic foraminifers are

Cibicides sp., Haynesina sp., Elphidium spp. and Neogloboquadrina pachyderma. Ffitterer et al.

(1992) describe concentrations of N. paehyderma in sea ice of up to several hundred empty tests per liter ice. Arenaceous foraminifers also occur in sea ice (Wollenburg, 1993).

During the Arctic 91 expedition, patches of "red snow" were observed at approximately 86°50'N,

D. Niirnberg et aL/Marine Geology 119 (1994) 185~14 205

L a p t e v S e a

F a s t ice area 113-1 Dri f t ice area ~o8-~

,,oof.r,-,oo i . . . . . . . . . . . . . . . . . . ,oo ! - - - - - - - - - - - • , ! i , o o

~=-° ! iSmeot"4 i iJ 7 0 0 ! - . . . . . . . . . =. . . . . . . . . . . . . . . . . . . . . ! . . . . . . . . . . . . . . . . . . . . .~ . . . . . . . . . . . . . . . . . . . + " l l ' " ' " i 5 0 0

! i A ! i Chl°riteNI I tO S00 | i J Ik_ i Illite i Kaolinite.'.[[ I 300

3oo ..... : i .................. i . . . . . . . . . . . . . . . . ~: . . . . . . .

1 0 0 ~ 100

2 4 6 8 10 12 14 16 18

1 12-1 108-2

[ S e a i c e s e d i m e n t i L ~ ~ ~ M o S 2

i Smectite i i | :, . . . . . . $ . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . i . . . . . . . . i ~ Chlonte ~- | i i Ilhte i Kaolinitei

4 6 8 10 12 14 16 18

7 0 0 . . . . . . . . . " . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . . . :" . . . . . . . . . . . . . . . . . . . ~" . . . . . . 9 0 0 i

! ........ ~ ~ i - ~Ch,o.te ~ ' l i I | i A i Illite iKao nte l I 700| Smectite: i Chorite.h-i i!

soo i i ~ ! : ~ Illite ~ . . : , o o l ~ : ~ ; ~ .................... ~...K~o,~,t~..j ~ .... 8 ' . . . . . . . . . . . . . . . . ' . . . . . . . . . . . . . . . . . . . . oo < o L ill

100

2 4 6 8 10 12 14 16 18 2 4 6 8 10 12 14 16 18

° 2 T h e t a ° 2 T h e t a

Fig. 11. Representative X-ray diffractograms from Laptev Sea differentiated into shelf bottom and sea ice samples from the drift- ice area and the fast ice area, respectively (see Fig. 3). High smectite concentrations from typical anchor ice sediments of 45 peak area-% (upper left diffractogram, see also Table 3) correspond to both Laptev Sea shelf bottom sediments (lower diagrams, see also Table 3) and central Arctic Ocean sea ice sediments (see Fig. 10 and Table 3).

59°45'E and 87°30'N, 103°30'E due to abundant growth of a coccal chlorophyte. This unicellular organism contains bright red to orange pigment bodies and is quite common in surface and subsurface sea ice sediments. However, it is not found in deep-sea sediments. Plant fragments and large logs (< 5 m long, < 30 cm in diameter) have been found in the central Arctic Ocean, either lying on the ice surface or protruding from the ice by 2 m, which might be an indication for the amount of ablation since entrainment.

The terrigenous components of Laptev Sea ice and shelf sediments are dominated by quartz, feldspar, rock fragments, mica, clay and heavy minerals. Biogenous particles > 63/tm are negligible in

sea ice sediments. Plant remains were observed only in ice samples from the Lena River and in anchor ice samples. In contrast to the central Arctic, where diatoms were often the dominant biogenous component in sea ice (Ft~tterer et al., 1992), the Laptev Sea ice samples are nearly devoid of diatoms except for a slight brown discoloration at the bottom of floes. In sea floor samples, biogenous particles >63/ t in are rare, mostly consisting of benthic foraminifers (Elphidium spp., Miliolinella spp., Labrospira crassimorgo), pelecypods, and ostra- cods (Paracyprideis pseudopunctillata, Sarsicy- theridea bradii, Heterocyprideis sorbyana according to W.M. Briggs from University of Colorado at Boulder/USA, pers. commun., 1993).

206 D. Niirnberg et al./Marine Geology 119 (1994) 185~14

Table 4 Calcium carbonate and organic carbon concentrations in shelf bottom and sea ice sediments from the Laptev Sea

Sample Description Co~g (wt.%) CaCO 3 (wt.%)

Seafloor samples from Laptev Sea (ESARE '92) 1081-4 0-1 cm 0.94 0.50 1081-4 0-5 cm 0.97 0.22 1081-4 0-5 cm 0.94 0.37 1081-4 0-5 cm 0.94 0.10 108-2 0-5 cm 1.15 0.25 108-2 0 5 cm 1.29 0.13 108-2-1 0 5 cm 1.19 0.50 108-2-3 0 1 cm 1.21 0.00 108-2-3 0-5 cm 1.16 0.25 112-1-1 0-1 cm 1.32 0.42 112-1-1 0-5 cm 1.23 0.62 112-2 0 1 cm 0.65 0.35 112-2 0-5 cm 0.58 0.06 112-2 0 5 cm 0.70 0.50 112-3 0-1 cm 1.19 0.84

Sea ice samples from Laptev Sea (ESARE '92) 111 - 1 turbid ice 1.56 0.96 113-1 anchor ice 5.74 2.79

4. Discussion

4. l. Sea ice sediment sources

General sea ice motion within the Arctic Ocean is well-known from drifting research stations and automatic buoy data (Colony et al., 1991). Arctic shelf areas are the most important sites for sea ice formation. Largest annual sea ice export rates, up to 500,000 km 2, are reported from the Laptev Sea (Zakharov, 1976). This sea belongs to the most extensive shallow shelf regions bordering the Arctic Ocean (Fig. 1). The shelf edge situated in ca. 50-60 m water depth is as far as 500 km from the continent. The Kara Sea exports approximately 230,000km z of ice annually (Zakharov, 1976). The Barents, East Siberian and Chukchi Seas are of minor importance for sea ice export, though they are partly much wider than the Laptev Sea (Gordienko and Laktionov, 1969; Zakharov, 1976; Vinje, 1985; 1987). Whether extent of source areas, transport paths and ablation areas of Arctic sea ice are reflected by sedimentary inclusions in the sea ice is of main interest of our study. Potential

sedimentary tracers identified in the deep-sea sedimentary record could then be used to reconstruct paleo-drift regimes and the evolution of the Arctic ice cover.

In general, it is still difficult to associate central Arctic Ocean sea ice sediments with distinct and regionally restricted source areas simply by their biogenic and/or mineralogic composition. The presence of benthic shallow-water foraminifers (Elphidium spp.) in sea ice sediments from the Laptev Sea as well as from the central Arctic Ocean definitely points to shelf areas as potential sources. According to Abelmann (1992), the diatom assemblages seen in multi-year sea ice of the Transpolar Drift between approximately 83°N and 86°N are dominated by marine-?brackish benthic species, which most probably originate in Siberian shelf waters and are incorporated as a bottom ice assemblage. During seasonal basal freezing and surface melting, these diatoms eventu- ally appear at the floe surface.

Quartz grains >63/am are one of the main components of the terrigenous coarse fraction found in sea ice. Although angular quartz dominates, sediments abundant in rounded quartz exist between ~85°N and 88°N. The presence of rounded grains, which are due to strong grain abrasion during long (fluvial) transport (Leeder, 1982), may indicate that fluvially transported material is incorporated into sea ice after being deposited on the shelf and subsequently resuspended. Our investigations in the Laptev Sea (see Dethleff et al., 1993) show that huge amounts of sediments are transported by the Lena River. Delicately structured sediment inclusions, which are strongly suggestive of entrainment by anchor ice, prevail in the river ice (Dethleff et al., 1993). From the air such sediment-laden fresh water ice could be traced far seaward off the Lena Delta, indicating that river discharge dominates the hydrography well into freeze-up (Dethleff et al., 1993). The importance of river-ice sediments for the Arctic sea ice, nevertheless, is assessed to be rather small since most of the river-ice melts in place. This process leads to deposition of fluvial sediments onto the shallow shelves.

Clay mineralogy aids in relating central Arctic Ocean sea ice sediments to restricted source areas


(Wollenburg, 1993). High smectite concentrations of 40-60 area-% were observed between 81 °N and 83°N in the central Arctic Ocean ice cover during two different years, which may indicate a regionally restricted and spatially stable drift-ice branch. Comparable smectite concentrations of 20 40 peak area-% within the Transpolar Drift (83°-88°N) and in both eastern Laptev Sea fast ice (anchor and turbid ice) and shelf sediments, suggest the Laptev Sea as a potential source area for the central Arctic smectite occurrence in sea ice. Source areas for smectite are very limited in the Arctic region. According to Silverberg (1972), smectite is a significant component of the clay fraction (-33%) also in western Laptev Sea sediments. In contrast, the East Siberian Sea (<5%), the Beaufort Sea and the Kara Sea exhibit only low values (Alexander, 1973; Naugler, 1967; Silverberg, 1972). Though the Barents Sea exhibits signifcant smectite concentrations within holocene sediments (Elverhoi et al., 1989; Ntirnberg et al., 1994), it is not regarded as a potential source area. The smectite is supposed to be allochthonous, derived from outside the Barents Sea (Elverhoi et al., 1989; Larssen, 1987; Larssen et al., 1987). Furthermore, Barents shelf areas are presumably too deep to allow the entrainment of significant amounts of sediment (cf. Eiverhoi et al., 1989).

4.2. Sediment incorporation processes

Processes leading to sediment entrainment into sea ice include bottom adfreezing in extensive shallows covered by bottom-fast ice (Barnes and Reimnitz, 1974; Clark and Hanson, 1983), bluff slumping onto fast ice (Kindle, 1924), flooding of fast ice by sediment-laden river water (Kulikov, 1961; Reimnitz and Bruder, 1972; Holmes and Creager, 1974), incorporation into dragging ice keels (Kindle, 1924; Reimnitz and Barnes, 1974), wind transport from land onto the ice (Kindle, 1924; Windom, 1969; Pfirman et al., 1989a), and suspension freezing (Campbell and Collin, 1958; Reimnitz et al., 1992; Dethleff et al., 1993).

Studies in the Alaskan Beaufort Sea have shown that most of the entrainment mechanisms described above are neither very effective, nor likely to result in long-range rafting at the scale of

the Beaufort Gyre (Reimnitz et al., 1992). Whether these observations can be generalized, is debatable. According to Pfirman et al. (1990), adfreezing by bottom-fast ice and bluff slumping incorporating coarse-grained beach and coastal sediments is unlikely for central Arctic conditions, since sediments observed are mainly fine-grained. Moreover, shorefast ice tends to deposit its sediment load near-shore (Barnes and Reimnitz, 1974; Reimnitz and Barnes, 1974; Clark and Hanson, 1983; Kempema et al., 1989).

Wind transports some fine-grained sediments onto Arctic sea ice (Kindle, 1924; Arnold, 1961; Reimnitz and Maurer, 1979; Clark and Noone, 1985). The annual deposition rates from eolian transport on the central Arctic ice cover, however, appear orders of magnitude too low to account for the high sediment loads observed in the Eurasian Basin (Larssen et al., 1987; Pfirman et al., 1989b, 1990). Our studies on Laptev Sea snow (Dethleff et al., 1993), additionally, indicate that this transport is insignificant for the sedimentary budget of Laptev Sea ice. The overall particle concentrations in fresh snow are low compared to those in sea ice (Table 1 ). Moreover, the expected higher concentrations in the proximity of wind- ward coasts and an offshore decrease in sediment content was not observed during recent field work (Dethleff et al., 1993). Presumably, the snow coverage of continental surfaces prevents extensive erosion. Except in the very restricted environment of the Lena River, where discoloration of the snow cover was obvious even from the air, eolian transport seems insignificant. On the central Arctic Ocean pack-ice surface, however, wind may be an important agent for redistributing particulate matter concentrated at the surfaces of multi-year floes after several freezing and melting cycles (Pfirman et al., 1990). Small grey diatom aggregates observed in 1991 probably originated from redistributional processes on the ice during melting and freezing and have subsequently been transported by wind.

According to Reimnitz and Bruder (1972), sus- pended sediments in river-ice or waters may be discharged to the sea or may flood over and under the ice when the river-ice breaks up during spring. The potential contribution of fluvial sediments to

208 D. Niirnberg et al. /Marine Geology 119 (1994) 185~14

long-range rafting, however, is still unclear, mainly due to the lack of relevant studies (Reimnitz and Bruder, 1972; Reimnitz et al., 1992). The Lena River ice breaks up in the beginning of June, about 1 month before sea ice. Since delta channels exhibit great depths (> 10 m), we suspect mainly sub-ice discharge of spring flood waters. Evidence for river overflow discharging high amounts of sediment directly onto the fast ice (Fuchs and Whittard, 1930) has not been seen on satellite images (Barnett, 1991) or by visual observations in that area (Dethleff et al., 1993). Sub-ice discharge of spring flood waters probably leads to deposition of river sediment on the shallow shelf.

This and other studies (Reimnitz et al., 1987, 1993; Kempema et al., 1989; Wollenburg, 1993; Ft~tterer et al., 1992) have shown that fine-grained, shallow marine sediment inclusions are much more characteristic than all other types of inclusions. Entrainment by anchor and frazil ice, named "suspension freezing" in this context (cf. Campbell and Collin, 1958; Reimnitz et al., 1992), thus, is suggested to be the most effective mechanism for regional entrainment of such sediments. According to Reimnitz et al. (1992), strong winds and intense turbulence in open shallow seas connected with extreme subfreezing temperatures are required. Under these conditions, the sea becomes "super- cooled" by a fraction of a degree and frazil ice forms (Martin, 1981 ). The underwater ice crystals interact and mechanically interlock with sedimentary particles in the water (frazil ice) and on the sea floor (anchor ice) and lift fine-grained particulate matter to the sea surface (Reimnitz et al., 1987, 1992). Here, a layer of sediment-laden slush ice forms, which gradually congeals into turbid ice (Barnes et al., 1982; Osterkamp and Gosink, 1984; Kempema et al., 1986, 1989; Reimnitz and Kempema, 1987; Reimnitz et al., 1987, 1990; Clayton et al., 1990). Shallow Siberian shelves with water depths less than 50 m are found favorable for these processes and thus may serve as potential source areas for the sediment load in Transpolar drift-ice (Reimnitz et al., 1992; Wollenburg, 1993).

Field observations in the Laptev Sea indicate that a several thousand kilometres long polynya (Fig. 3) is important for underwater ice formation

and consequently may also be important for sediment entrainment (see also Dethleff et al., 1993). Internal convection cells within this "ice-factory" enhance the production of frazil ice crystals, which drift in streaks away from the fast ice edge under prevailing offshore winds. During April 1992, however, the incorporation of sediment by frazil and anchor ice was not observed. This may have been due to water depths below the polynya (ca. 25 m) that are too deep for convection cells to reach the sea floor at that time of the year. Or stabilization of the water column from Lena River inflow prevented deep-reaching convection (Dethleff et al., 1993). The large, first-year turbid ice fields incorporated into fast ice in the vicinity of the polynya, we suspect formed during fall and winter 1991/1992, when conditions for suspension freezing were more favorable in the shallow inner part of the eastern Laptev Sea (see Dethleff et al., 1993). These ice masses will probably be incorporated into the drift-ice and exported to the central Arctic Ocean. Periodic observations of sediment- laden ice in the Transpolar Drift may represent seasonal variations in ice export. As a consequence, sediment release from melting ice in ablation areas is expected to fluctuate episodically.

4.3. Contribution o f sea ice sediments to central Arctic Ocean deep-sea sedimentation

Identifying the recent and fossil influence of sea ice and icebergs on deep-sea sedimentation requires knowledge of reliable indicators that distinguish between the two modes of transport. Widely distributed surface sediment accumulations on sea ice influence ablation processes since they change albedo and absorb solar energy. Cryoconites are an obvious example for this influence and have been described from the Greenland ice sheet (Drygalski and Machatschek, 1942), from alpine glaciers (Brandt, 1931a), river-ice (Brandt, 1931b) and sea ice (i.e. Barnes et al., 1988; Pfirman et al., 1989a). One effect of cryoconite formation is the aggregation of sedimentary particles. Freeze-thaw processes lead to more or less cohesive pellets (Barnes et al., 1988), which in some instances may constitute the bulk of sea ice sediments (Wollenburg, 1993). Pellets may possibly settle

D. Nfirnberg et al./Marine Geology 119 (1994) 185 214 209

intact to the sea floor after being released from the ice cover (Pfirman et al., 1990) and thus, contribute to deep-sea sedimentation. Since pellets are also formed in glacier ice, differentiation between sea ice and iceberg-rafted pellets is difficult, although sea ice sediments may tend to be more fine-grained (Goldschmidt et al., 1992).

According to Goldschmidt et al. (1992), centi- metre-sized sediment pellets found in deep-sea sediment cores from the Norwegian-Greenland Sea and central Arctic Ocean most probably reflect iceberg transport. Grain size, grain shape and mineralogical composition of the sediment aggregates are more similar to sediment found in icebergs than in sea ice under present conditions. The pellets may be formed in or under a glacier or during transport in an iceberg and may subsequently be released when the iceberg overturns or melts. If such sediment pellets are consolidated enough to survive settling through the water column and exposure on the ocean floor, they may be helpful in reconstructing the spatial and tempo- ral existence of calving glaciers and icebergs.

Presently iceberg rafting is usually interpreted from the observance of the >63 #m sediments. Sea ice sediments cannot yet be identified as such in the deep sea, because they are likely to be fine- grained and cannot be distinguished on the basis of grain size from sediments supplied by distal turbidites or oceanic currents. Sediment trap studies in Fram Strait clearly show that sea ice significantly influences particle flux to the deep sea (Hebbeln and Wefer, 1991). However, the poor correlation of clay mineral distribution patterns in central Arctic Ocean sea ice and deep-sea sediments (i.e. Stein et al., 1994-this volume) suggests that ice-rafted detritus is only one factor contribut- ing to the deep sea. In fact, clay mineral assemblages of sea-floor surface sediments seem to represent a mixture of ice-rafted detritus and material derived by various processes. Oceanic currents, reworking of material, pelagic sedimentation and sea floor topography additionally shape the depositional environment in the Arctic Ocean. The influence of sea ice sediments will presumably become more important in mature ablation areas like Fram Strait and in Greenland Sea along the axis of the Greenland Current (see also Pfirman et al., 1990).

According to Gunilla Gard from University of Goteborg/Sweden (pers. commun., 1992), sea ice sediments sampled during Arctic 91 contained no coccoliths. Reworked Mesozoic nannofossils most probably transported from the Arctic shelf areas, however, are present in central Arctic Ocean deep- sea sediments (Gard and Crux, 1992) implying that turbidity currents and/or oceanic currents contribute to deep-sea sedimentation in addition to sea ice.

4.4. Sediment concentration: A comparison between the Laptev Sea, central Arctic Ocean and Fram Strait

Calculations of sediment concentrations in Arctic sea ice have been performed by several authors. Wollenburg (1993) and Elverhoi et al. (1989) published data from Greenland Sea, Barents Sea, Fram Strait and the southern central Arctic Ocean. From studies during Arctic 91, we know the amounts of sediments in sea ice from the central Arctic Ocean up to 90°N (Table 1; Figs. 4, 6 and 7). Data are also available from potential source areas like the Laptev Sea (Table 1; Figs. 6 and 7; Dethleff et al., 1993). Thus, the presumed source areas, transport pathways and ablation areas of transpolar drift-ice have been covered. A comparison of sediment concentration data, however, is problematic since data were either sampled during different years or by different persons. Nevertheless, regional differences as shown above become obvious. Whether these differences are due to the small data base, to the annual variability of sediment transported by sea ice, or to ablation effects can not be resolved at present.

From observations in Fram Strait and the Greenland Sea during several years (Pfirman et al, 1989; Reimnitz and Saarso, 1990; Wollenburg, 1993) it can be inferred that sediment transport does not occur continuously. In 1990, the amounts of sediment sampled during RV Polarstern cruise ARK VII/1 in the Greenland Sea were very small according to Reimnitz and Saarso (1990). During previous expeditions to the same areas (Pfirman et al., 1989b) and even only a month later (Krause et al., 1991b), large percentages of the ice cover


have in contrast been reported as being sediment laden. Since suspension freezing, the postulated entrainment mechanism by anchor and frazil ice, is expected to occur only once a year or every few years (Reimnitz et al., 1992) for a few weeks, pulsating flux of ice-rafted sediments through Fram Strait may reflect episodic sediment entrainment, as well as episodic discharge of Siberian ice. Particularly in Fram Strait, ice mixing from different sources could also affect the distribution of sediments in sea ice.

Sediment concentrations found in snow and even in turbid ice of the Laptev Sea are consider- ably lower than those in surface samples of central Arctic sea ice, where extreme values were measured (Table 1; Fig. 7). The high surface accumulation in multi-year ice can be attributed to several melting/freezing cycles, which lead to redistribution and concentration of particles at floe surfaces (Sverdrup, 1938; Pfirman et al., 1989b). The patchy occurrence of surface sediment accumulations presumably results from this process. Consequently, the majority of central Arctic Ocean ice cores are either barren of sediment or sedimentary material is concentrated in layers and/or on the surface (Fig. 4).

Measured sediment concentrations in Fram Strait and Greenland Sea (1-600mg/1) fall between the Laptev Sea and the central Arctic Ocean (Table 1; Fig. 7; Wollenburg, 1993). We suggest that the ice of the central Arctic retains most of its load until released in the ablation regions, where it adds to deep-sea sedimentation. In the central Arctic Ocean, hence, contribution from sediment-laden ice may be less important than sediment contribution from gravity flows and/or oceanic currents than in the ablation areas.

5. Conclusions

Observations in Arctic regions show that its sea ice cover is heavily laden with sedimentary material. Up to 50% of the sea ice cover in the central Arctic Ocean was colored by sedimentary particles during the summer of 1991. Sediment concentrations in floe surfaces range as high as 50,000 mg/1 meltwater. Both sediment concen-

tration and distribution indicate that sediment transport through sea ice is an important geological factor and contributes to particle flux in the deep sea.

Sea ice sediments are generally dominated by silt and clay. Sand and gravel up to 5cm in diameter have also been observed. Considering the general ice drift pattern, the Siberian shelves are most probably the source areas for most of these sediments. Estimated ice export rates suggest the Laptev Sea to be the preferential site of ice production and thus, sediment entrainment.

Possible entrainment mechanisms for the sediment load have been evaluated. Eolian transport, slumping from cliffs and adfreezing at the floe base are considered to be of minor importance for the occurrence of fine grained sediments in pack ice of the Transpolar Drift. Based upon studies in the Lena River delta, we estimate the role of river discharge overflows onto the Laptev Sea ice cover as unimportant. Morphological conditions rather indicate sub-ice discharge of spring flood waters to be more probable leading to deposition of the river load on the shallow shelf areas. Sediments included within river ice are assessed to be of minor importance for the Arctic sea ice since most of the river ice melts in nearshore areas.

Sea ice sediment composition, especially the occurrence of benthic shallow water organisms, generally favors sediment entrainment on shallow shelf areas (< 50 m water depth). The occurrence of delicately structured sediment inclusions and of turbid ice within the fast ice area of the Laptev Sea suggests suspension freezing to be the dominant entrainment mechanism for sediments. Comparable smectite concentrations within Laptev Sea shelf sediments and Laptev Sea ice also provide evidence of frazil and anchor ice sediment entrainment into the ice cover. Smectite concentrations found in the Laptev Sea correlate to those in central Arctic Ocean sea ice. This pattern provides evidence for a narrow and stable ice drift branch north of Svalbard, and links source areas in the Laptev Sea to sediment-laden ice in the central Arctic Ocean.

Ice-producing polynyas such as that observed in the Laptev Sea presumably provide the main portion of sea ice sediments through suspension

D. Niirnberg et al./Mar&e Geology 119 (1994) 185-214 211

freezing. Since the occurrence o f the flaw lead is a funct ion o f a tmosphe r i c pressure pat terns , rates o f ice p r o d u c t i o n and sediment en t r a inmen t will va ry cons iderab ly . Cond i t i ons for sed iment en t r a inmen t in the shal low inner Lap tev Sea a rea m a y be op t ima l dur ing fall freeze-up, when the fast ice belt is n a r r o w and vast shal low regions are exposed to s torms. Con t inuous ice expor t , with a shor t pe r iod o f sediment en t r a inmen t fo l lowed by a long pe r iod o f clean ice p roduc t i on wou ld cause large and rhy thmica l var ia t ions in sediment flux out o f the Lap tev Sea.

C o m p a r i n g surface sediment concen t ra t ions in source areas, centra l par t s o f the T ranspo l a r Dr i f t and ab la t ion areas, we conc lude tha t con t r ibu t ion o f sea ice sediments to the deep sea is o f considerable impor t ance in ma in ab l a t i on areas. However , c lay minera l analyses in centra l Arc t ic Ocean sea f loor sediments and sea ice sediments indicate tha t here ice-raf ted det r i tus is overpr in ted by sediment suppl ied f rom gravi ty flows or oceanic currents . As yet no ind ica t ion exists which would a l low us to d is t inguish between sea ice raf ted sediments and o ther sediment sources. Ident i f ica t ion o f such an ind ica t ion is essential to recons t ruc t pa leo-sea ice cover.

Acknowledgements

N a m i n g all the persons involved in m a k i n g this s tudy poss ible can no t be done in this context . However , w i thou t the crew o f RV Polarstern and member s o f the "Arc t ic and Anta rc t i c Research Ins t i tu te" at St. Pe te rsburg we would no t have been able to ga ther such a large d a t a set. D. FOtterer is t h a n k e d for generous suppor t . Fo r da t a discussion we grateful ly t hank P. Go ldschmid t , S. Pf i rman and R. Stein. Thanks are due to M. Wahsner , who d id some clay mine ra logy analyses. W. Briggs, R. Grad inger , H. Bauch and U. St ruck identif ied b iogenous sediment componen t s . Thanks are also due to two reviewers, who improved the manuscr ip t . Research funds were p rov ided by the Bundesministerium fiir Forschung und Technologie ( B M F T - P r o j e c t 03 R 409), by D F G - L e i b n i z T H 200/5-1, and by the U.S. Office o f N a v a l Research ( O N R con t r ac t no. N 00014-88-J-1087). This is

con t r ibu t ion no. 697 o f the Al f red-Wegener - Ins t i tu te for Po la r and Mar ine Research.

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MARINE QEOLOQY - CORE · Sediment transport via sea ice is expected to contribute significantly to...

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