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Geol Rundsch (1995) 84:28-48 @ Springer-Verlag 1995
R. Henrich • T. Wagner • P. Goldschmidt K. Michels
Depositional regimes in the Norwegian-Greenland Sea: the last two glacial to interglacial transitions
Received: 2 March 1994 / Accepted: 10 August 1994
Abstract Various models of surface and deep-water circulation in the Norwegian-Greenland Sea (NGS) have been proposed for the last two glacial to intergla- cial transitions. Although much progress has been made in understanding the sedimentary response to cli- matic and oceanographic changes, conflicting interpre- tations have been developed. To clarify some of these discrepancies and to test or modify the existing circula- tion concepts, a multiparameter approach is applied, combining sedimentological, micropaleontological, or- ganic- geochemical and isotopic methods. On the basis of indicative properties a combined litho- and organo- facies concept is developed and calibrated with modern depositional settings beneath different surface water masses. Sedimentary regimes are then derived for gla- cial and deglacial settings.
Atlantic water intrusions in the NGS reveal complex and highly dynamic patterns for the last two glacial and interglacial periods, with repetitive inflows during Iso- tope Stage 6 and a high variability in Isotope Stage 5. Specific facies patterns show maximum extensions of Atlantic Water intrusions during the climatic high- stands 5.5.1, 5.3 and 5.1 and narrowest intrusions in the cool phases 5.4 and most pronounced in 5.2. In con- trast, different glacio-marine depositional regimes de- pict variable sea ice coverage and supply of ice-rafted debris. Most conspicuous are short-term depositional events marked by diamictons, which are related to the high instabilities of continental ice sheets. Some of the diamictons seem to occur contemporaneously with Heinrich layers H1 and H2. The probable temporal and obvious phenomenological concidence of Heinrich layers and NGS diamictons suggests a common trigger mechanism which caused an almost simultaneous disin-
R. Henrich (1~) - T. Wagner Fachbereich 5, Geowissenschaften, Universit~it Bremen, Klagen- furter Strasse, D-28359 Bremen, Germany
P. Goldschmidt • K. Michels Sonderforschungsbereich 313, Universitfit Kiel, Heinrich-Hecht- Platz 10, D-24098 Kiel, Germany
tegration of huge continental ice masses along the shelves of North America and the eastern margin of the NGS.
A previous estuarine circulation model claims re- gional upwelling along the eastern margin of the NGS for specific periods of the last deglaciation. The organic character of sediments covering the same time intervals show a clear predominance of reworked fossil organic matter and thus does not support the estuarine model.
Key words Norwegian • Greenland Sea Glacial cycles • Depositional regimes
Introduction
Circulation patterns of modern surface waters in the Norwegian-Greenland Sea (NGS) display pronounced meridional hydrographic contrasts (Fig. 1, Johannessen 1986). Steep temperature and salinity gradients in the surface waters are reflected by a strong variability in pelagic sedimentation patterns (Peinert et al. 1989; Bathmann et al. 1990; Samtleben and Bickert 1990; Hebbeln and Wefer 1991): a high pelagic carbonate ex- port flux under the Norwegian Current; up to ten times lower pelagic carbonate export of different composition under the Arctic Surface Water; a high terrigenous par- ticle flux with strong seasonal pulses at the ice edges, often combined with productivity blooms; and a very low terrigenous and pelagic carbonate flux under the East Greenland Current.
These vertical flux patterns are modified by near- bottom transport and lateral advection due to: contour currents, especially in the western basins; turbidity cur- rents filling up the deep sea abyssal plains, in particular the Norway and the Lofoten basins; and the formation of dense brines which carry suspensions downslope and lead to specific high accumulation areas at the conti- nental slope (Blaume 1992; Rumohr in press).
These modern processes are clearly reflected by varying compositions of surface sediments (Kellogg
2 9 ¸
WO°E
' / ~ 4 Z _ _ 0L_~ ~°~g3, '
WO°E
Fig. 1A Location of investigated cores in the Norwegian-Green- land Sea (2000 and 3000 m bathymetry are shown) and B present surface water circulation and major oceanographic fronts in the Norwegian-Greenland Sea
1980; Henrich 1992; Baumann et al. 1993). Even more variable sedimentary regimes are observed during gla- cial climates, when large ice shields developed on the Nordic continents and sea ice spread from the Nordic seas far into the adjacent North Atlantic.
Previous studies have developed a number of ap- proaches to investigate glacial to interglacial paleocea- nographic changes and to determine the feedback mechanisms that promote these climatic shifts. In parti- cular: (1) the stable isotope composition of planktonic foraminifers has been used to quantify surface water properties, such as temperature and salinity (Jansen and Erlenkeuser 1985; Jones and Keigwin 1988; Vogel- sang 1990; Sarnthein et al. 1992; Stein et al. 1994); (2) surface water temperature gradients were calculated using temperature transfer techniques on planktonic fo- raminifers (Kellogg 1980; Haake and Pflaumann 1989) and diatoms (Ko~ Karpuz and Jansen 1992) - In addi- tion, temperature trends were deduced from composi- tional differences of plankton communities (Belanger 1982; Jansen and Bjorklund 1985; Baumann 1990; Gard and Backmann 1990; Baumann and Matthiessen 1992); and (3) based on the stable isotope composition and the Cd/Ca ratios of benthic foraminifers, bottom water properties were estimated and interpreted in terms of models of deep water circulation (Corliss et al. 1986; Boyle and Keigwin 1987; Jansen and Veum 1990).
Although a great deal of progress was made in these studies, major discrepancies and interpretational con- flicts arose from the increasing database, so contradict- ing models of surface and deep water circulation were developed. This is most obvious for circulation con- cepts of the last glacial maximum. The CLIMAP (1981) reconstruction of sea surface temperatures of plank-
tonic foraminifers assumes a permanent ice cover over the entire NGS, with a large but weak anticlockwise ice drift gyre. Because of slightly increased carbonate con- tents and generally good carbonate preservation, how- ever, Henrich et al. (1989) propose at least seasonally open waters in the eastern NGS caused by an areally small inflow or underflow of Atlantic waters. In con- trast, Sarnthein et al. (1992) reconstruct a clockwise cir- culation with an inflow of Atlantic water in the western sector of the NGS on the basis of isotopic measure- ments of Neogloboquadrina pachyderma sin.
Other parameters commonly used to reconstruct glacial and deglacial circulation patterns are ice-rafted debris (IRD) contents and tracer lithologies. For specif- ic glacial time slices Bischof et al. (1990) deduce from abundance variations of coal clasts a southward ice drift along the eastern margin of the NGS. In contrast, a northward ice drift along the eastern margin, occasion- ally reaching as far north as the Fram Strait, is recon- structed from the occurrence of chalk dropstones (Hen- rich 1990; Hebbeln 1991; Spielhagen 1991). As some of these time slice reconstructions reveal chalk and coal clasts in the same samples (Henrich 1992), there is also an interpretational conflict.
During specific short-term and deglacial periods pulse-like glacio-marine sedimentary events are docu- mented in NGS deposits (Henrich et al. 1989). These diamictons reveal increased IRD and total organic car- bon (TOC) contents. The widespread distribution and westward pinching out of diamictons indicate enhanced glacio-marine input from the eastern adjacent conti- nents and shelves into the NGS (Henrich 1992). Con- trasting interpretations have been proposed to explain the steeply increased TOC contents in diamictons. Based on results from coarse fraction analyses, Henrich et al. (1989) claim a predominantly terrigenous and re- worked origin of sedimentary organic matter (OM). In contrast, a predominance of marine OM is postulated in the anti-estuarine circulation model due to regional upwelling along the eastern margin of the NGS (Kass-
30
ens 1990; Vogelsang 1990; Sarnthein et al. 1992; Wei- nelt et al. 1992).
Although there are numerous models of circulation patterns and water mass movements for both the past and present, little is known about the near-bottom cur- rent intensities and lateral sediment transport. Data on near-bottom current intensities for the NGS have not yet been published. Sidescan sonar and 3.5 kHz arrays reveal sediment waves on both the Barents Sea bottom and the Greenland continental slope, indicating lateral sediment transport (Mienert et al. 1993). Contourites are also known in the glacial/intergalcial sections of se- diment cores from these areas (Thiede and Hempel 1991).
To clarify some of the above-mentioned discrepan- cies we have applied a multiparameter approach to minimize interpretational conflicts. Furthermore, we have grouped sets of indicative properties to specific li- tho- and organofacies. Modern facies patterns were traced back into the geological record and additional glacial facies types were recognised. Ice-rafted debris tracer lithologies were used to reconstruct ice drift pat- terns, whereas near-bottom (bottom water) current in- tensities were deduced from settling tube analyses. As a result, we are able to reconstruct surface and bottom water regimes in the past and to interpret paleoceano- graphic changes by mapping the spatial and temporal distribution of litho- and organofacies.
Methods
Studies were performed on glacial and interglacial de- posits from Isotope Stages 6-5 and 2-1 along two east- west transects of the NGS (transect 1 VCring Plateau - Jan Mayen Fracture Zone - Iceland Plateau - southern Greenland Basin; transect 2 Knipovitch Ridge - Boreas Basin; Fig. 1A). Sediments were collected during RV Meteor cruises M 2/2 in 1986 (Gerlach et al. 1986; Hen- rich 1988), M 7/2 in 1988 (Hirschleber et al. 1988), M 13/2 in 1990 (Gerlach and Graf 1991) and M 21/4 in 1992 (Pfannkuche et al. 1993).
The time stratigraphic framework is mainly based on stable isotope stratigraphy measured on N. pachyderma sin., supplemented by Accelerated Mass Spectrography (AMS) 14C datings and conventional 14C datings in some cores. Chronologies of cores 23 071 and 23 065 are taken from Vogelsang (1990), with minor modifications (Wagner and Henrich 1994), those of cores 23 342 and 23352 from Henrich (1992) and Henrich et al. (in prep.) and that of core 17728 from Weinelt (1993). The stratigraphy of cores 23 456 and 23 454 is based on the correlation of carbonate records with the nearby cores 17728 (Weinelt 1993) and 21910 (Schacht 1991).
Bulk, coarse and fine carbonate contents were mea- sured using a Leco CS 125 carbon/sulphur analyser. Based on microscopic analyses the amounts of subpolar planktonic and benthic foraminifers were determined. Carbonate preservation was determined using scanning
electron microscopy (SEM) dissolution indices of N. pachyderma sin. (Henrich 1989).
Total organic carbon and nitrogen contents were measured twice using a Leco CS 125 carbon/sulphur analyser and a Carlo-Erba NA 1500 instrument, respec- tively. Carbonate was removed by the repeated addi- tion of 0.25 M HC1. The data pairs obtained were aver- aged and taken for further interpretation if the devia- tion was less than 0.05 wt.% for organic carbon and less than 0.01 wt.% for nitrogen. The isotopic composition of organic carbon (~ 13Corg ) was determined using a Finnigan MAT 251 mass spectrometer [a detailed dis- cussion of the organic geochemical methods is given in Wagner and Henrich (1994) and Wagner and H01e- mann (submitted)].
Rock-Eval pyrolysis was performed on bulk sedi- ment samples using on oil-show analyser and a Rock- Eval instrument. For the immature organic character of surface sediments the modified temperature program of Liebezeit and Wiesner (1989) was applied (for de- tails, see Wagner 1993; Wagner and Henrich 1994). Hy- drogen indices (HI) were calculated using the TOC contents determined from the Leco measurements. Es- timates of mineral matrix adsorption according to Langford and Blanc-Valeron (1990) show that only sur- face samples were significantly affected (Wagner 1993).
Bulk sediment (box core) and < 63 txm fraction sam- ples (long box core) were analysed for their maceral composition using a Zeiss Axiophot with incident and ultraviolet light at 1000 x magnification. Depending on the amount of organic particles, 100-500 points were counted and calculated as a grain percentage of the whole maceral composition. Macerals were determined following the nomenclature of Stach et al. (1982), al- though minor modifications were applied (dinoflagel- late cysts and their fragments were counted as algin- ite).
Twenty-four lithological classes of IRD were deter- mined by investigating the > 125 ~m sediment fraction. At least 600 grains per sample were examined under a Leitz Wetzlar reflecting light microscope. This is well within the range of statistical significance (see Watkins et al. 1982). The grain counts were recalculated as weight percentages. In addition, the data was subjected to statistical analysis using the McCABFAC program of Imbrie and Kipp (1971) and Klovan and Imbrie (1971). Data presented in this paper exceed a factor loading (correlation coefficient) of 0.7.
Settling velocity distributions of the sand fraction of samples from core 23071 were measured using a Ma- crogranometer. Separations were carried out with the Separator 3S (Oehmig and Michels 1994) to determine the particle assemblages which form specific modes in the settling velocity distributions. On the basis of these separations, assemblages of transported grains were de- tected. As settling velocity and grain density determine the critical shear stress at which a particle is just moved, critical shear stresses and therefore current velocities
31
Watermass/ Glaciomarine
Setting
Associated Lithofacies
Corresponding l Organofac es
Modern and past interglacial environments
Atlantic Water
g~
] facies II facies
I II I I :
Marginal Sector of Atlantic Water
Organo- Organo- facies facies I - 1 I f - 1
Arctic Surface Water
Greenland Current
Organo- Organo- facies facies I - 2 I I - 1
Glacial and deglacial environments
Seasonally variable ice cover and iceberg drift
/ Facies II Facies I ~ BL_~L2
Organo- Organo- ~ ~ facies facies I facies II facies n - 1 I I - 1 I I I - 1 1II-4 I I - 2 I I - 2 I I ]
! High instability of
tidewater ice margins on the outer shelves
Diamicton Facies D, E, F
}a
Fig. 2 Correlation scheme of lithofacies and organofacies in rela- tion to modern surface water masses and to glacial depositional regimes. Diverse organofacies in modern and past interglacial en- vironments are related to different stages of early diagenetic de- gradation of autochthonous organic matter
can be calculated from the settling velocity distribution (a detailed methodological description is given in Mi- chels 1994).
Results
Definition of organofacies and lithofacies and correlation with modern and glacial oceanographic settings
Based on the sedimentological and micropaleontologi- cal properties, a lithofacies concept was developed and applied to glacial and interglacial deposits of the NGS (Henrich et al. 1989; Henrich 1992). This was expanded by an organofacies concept, considering organic geo- chemical and organic petrographic properties (Wagner 1993; Wagner and Henrich 1994). Eight lithofacies and eight organofacies types that display distinct temporal and spatial patterns were distinguished. These are cor- related with modern and glacial oceanographic regimes and the diagenetic stage of the sediment section (Fig. 2).
Under modern conditions the carbonate-rich lithofa- c iesA is deposited beneath central Atlantic Water in the eastern sector of the NGS. High amounts of subpo- lar planktonic foraminifers and very low contents of terrigenous coarse debris are indicators of this facies (Table 1). Lithofacies A is associated with three types of organofacies depending on the stage of early diagen- etic alteration (Fig. 2). The highest proportions of ma- rine OM are preserved in near-surface deposits of orga- nofacies I-1. An increased HI and heaviest 6 13Corg val- ues support the biogenic and lithological results (Ta- ble2). In relation to surface sediments, reduced amounts of autochthonous OM are recorded by orga- nofacies II-3. This is evidenced by increased propor- tions of marine macerals and a slightly increased HI.
Minimum amounts of marine OM, revealing the strong- est diagenetic overprint, are recorded in organofacies II-2 (Fig. 2). Minimum TOC contents and the highest proportions of oxidized, terrigenous OM are indicative for organofacies II-2 (Table 2).
In the marginal sector of Atlantic Water and in the transition to the Arctic Surface Water the decreasing influence of temperate waters is clearly recorded by in- termediate carbonate and subpolar planktonic contents (lithofacies B3, Table 1). Lithofacies B3 is correlated with organofacies II-2 (Fig. 2). High proportions of ter- rigenous vitrinite dominate the organic-poor to -inter- mediate character of this facies type (Table 2).
Lithofacies B2 and B1 represent the depositional en- vironment beneath Arctic Surface Water and the mod- ern Greenland Current. Terrigenous muds with low carbonate contents reflect a strongly reduced carbonate export flux. Terrigenous coarse fraction contents are low beneath Arctic Surface Water (lithofacies B2), in- dicating an occasional supply of IRD. In contrast, inter- mediate to high amounts of terrigenous coarse fraction are observed in lithofacies B1 beneath the Greenland Current, evidencing a much greater transport of IRD by icebergs from Greenland. Lithofacies B1 and B2 are associated with the terrigenous-dominated organofacies II-1 and II-2 (Fig. 2). A generally intermediate to low TOC content and significantly increased amounts of terrigenous OM determine the organic character (Ta- ble 2).
In contrast with modern conditions, glacial and de- glacial depositional regimes were dominated by season- ally varying extents of sea ice coverage and variable ice- berg densities. Lithofacies B2 was mainly deposited be- neath a rather dense sea ice cover with some drifting icebergs. However, slightly increased carbonate con- tents in lithofacies B2 may indicate at least seasonally open surface waters. In contrast, common icebergs and in general less dense sea ice coverage is recorded by lithofacies B1. Both lithofacies correlate with either or- ganofacies II-1 of II-2 (Fig. 2). Comparable glacio-ma- rine settings with an increased input of fossil OM are characteristic of lithofacies C and the associated orga- nofacies II-1 (Fig. 2). Increased amounts of terrigenous coarse fraction and low to intermediate TOC contents
32
Table 1 Characteristics of lithofacies of sediments from the study area
Lithological Facies A Facies B3 Facies B2 Facies B1 Facies C Facies D, E, F Facies
Lithology Brownish foram/ Brownish foram Brownish foram- Brownish foram- Grayish foramini- Dark diamictons: nanno oozes/mud mud iniferal silty terri- iniferal silty to feral silty to san- F = complex, D
genous mud sandy terrigenous dy terrigenous = dark gray, E mud mud = dark olive
gray
Low abundance Low abundance Few burrows, No burrows, and scattered of burrows, drop- scattered drop- abundant drop- dropstones stones, occasional stones, occasional stones, common
mud clasts mud clasts mud clasts
3-10 1-10 3-10 0-0.5
Structures/biotur- High abundance High abundance bation features and diversity of and diversity of
burrows burrows
Bulk carbonate 30-70 15-30 (wt.%)
Coarse carbonate 20-40 10-25 3-10 (wt.%)
Fine carbonate 20-50 3-15 0-3 (wt.To)
Subpolar plank- 5-30 2-5 0-1 ton (wt.%)
Benthic forams 1-5 0.3-1 0.1-0.2 (wt.%)
Organic carbon Surface: 0.5-0.8, Surface: 0.5-0.8, (wt.%) 0.1-0.3 0.1-0.3
Terrigenous 0.5 3-10 1-5 coarse fraction (wt.%)
Terrigenous fine 25-65 60-85 85-95 fraction (wt.%)
Geochemical Positive redox Positive redox properties potential, good potential, good
carbonate preser- carbonate preser- vation vation
Water mass affin- Atlantic Water Marginal sector ity/glacio-marine of Atlantic Water environment
1-10 2-8 0-0.5
1-4 1-4 0-0.5
0, trace 0, trace 0
0.1-0.2 0.1.0.3 0
Surface: 0.5-0.8, 0.1-0.3 0.1-0.3
5-30
0.3.0.5
5-30
0.5-1.5
20-40
Positive redox potential, carbon- ate dissolution at certain levels
Modern: Arctic Surface Water. Olacials: season- ally variable ice cover with few icebergs
60-85 60-85 60.80
Positive redox Good carbonate potential, carbon- preservation ate dissolution at certain levels
Modern: East Greenland Cur- rent. Glacials: seasonally varia- ble ice cover, common icebergs
Seasonally varia- ble ice cover, common icebergs, increased input of fossil OM
Strong dissolu- tion, sulphate re- duction, second- ary oxidation
High instability of ice margins with abundant icebergs and meltwater plumes
are indicative parameters (Table 1). The O M composi- tion is dominated by non-oxidized terrigenous macerals (vitrinite, Table 2). A shor t - term late deglacial sedi- mentary event is recorded by organofacies II-4 (Fig. 2). In te rmedia te to high T O C contents and predominant ly oxidized, terrigenous O M (inertinite) indicate an in- creased lateral supply of terr igenous suspensions.
Most conspicuous sediment layers are diamictons of lithofacies D, E and F which are associated with orga- nofacies III-1 and III-2 (Fig. 2). The short- term deposi- tion of diamictons is related to high instabilities of tide- water glaciers which have advanced far out onto the outer continental shelf. Maximum amounts of terrige- nous coarse fraction, very low carbonate contents and strong dissolution indices are the most typical proper- ties of these facies (Table 1). Very high T O C contents of predominant ly reworked terr igenous origin are indi-
cated by a low HI, increased Tmax, high C/N ratios and ra ther light 6 13Corg signals. The microscopic results show a predominance of non-oxidized terrigenous OM (Table 2).
Genera l pat terns of basic sedimentological propert ies along an eas t -west transect across the central NGS
The various pelagic and glacio-marine depositional re- gimes can be approximated f rom three basic bulk sedi- ment properties: carbonate, terrigenous coarse fraction and T O C contents. High pelagic carbonate contents in- dicate surface waters with Atlantic Water affinity. The amount of the terr igenous coarse fraction reflects the intensity of I R D input. Generally, variations in T O C were assumed to moni tor changes in surface water pro-
Table 2 Characteristics of organofacies of sediments from the study area. Owing to the differing significance of parameters, a ranking of individual properties was applied: high organic petro-
33
logical data; medium, HI, Tmax; low, C/N, 6 13Corg (ND = not de- termined)
Organic Facies I-1 Facies I-2 Facies II-1 Facies II-2 Facies II-3 Facies II-4 Facies III-1 Facies III-2 Facies
Organic char- Marine Terrigenous acter
Bulk TOC 0.3-0.9 0.4-0.55 (wt.%)
Alginite 30-60 20-30 (grain%)
Vitrinite 10-50 45-70 (grain%)
Inertinite 10-45 10-25 (grain%)
Hydrogen in- 80-270 40-90 dex (mg HC/ g TOC)
Tma x (° C) < 400 < 400-430
(~ 13Corg -21.5 tO ND (%0 PDB) -23.0
Corg/Ntot 3-8 7-20
Sedimentary Near-surface Input of fine- process/diag- diagenesis of grained al- enetic altera- marine OM lochthonous tion OM
Terrigenous Terrigenous Marine Terrigenous Terrigenous Terrigenous
0.15-0.5 0.14).5 0.25-0.5 0.5-0.85 0.3-1.1 0.2-0.45
10-30 5-30 30-45 25-36 20-30 15-25
30.80 10M0 15-30 25M0 40-70 45-75
5-40 30.80 30-45 35-50 15-35 5-25
20-100 40-110 80-110 150-270 30-100 70-180
< 400 < 400 < 400 < 400 420-440 450-500
- 23.5 to - 22.5 to - 24.0 to - 22.0 to < - 24.0 - 23.5 to -26.0 - 25.0 - 24.5 - 24.0 - 24.3
3-12 3-8 7 5 4-18 3-20
Almost com- Almost corn- Enhanced Latest degla- High input of High input of plete degrad- plete degrad- preservation cial winnow- coarse- coarse- ation of ma- ation of ma- of marine ing of shelf grained fossil grained fossil fine OM; rine OM; OM sediments; mature OM overmature high input of high input of high input of OM fine-grained fine-grained fine-grained non-oxidized oxidized al- oxidized al- allochthon- lochthonous lochthonous ous OM OM OM
ductivity (Sarnthein et al. 1992). However, other stud- ies have suggested a predominant deposition of terrige- nous or reworked OM (Henrich et al. 1989; Stein et al. 1989, 1993; H61emann and Henrich in press). Hence bulk sediment property records provide a first overview of paleoceanographic changes in space and time. Shifts in these sedimentological parameters over the last two glacial to interglacial transitions (Isotope Stages 6 and 5, 2 and 1) along a shelf-basin transect in the central NGS reveal consistent trends (Fig. 3).
On a glacial to interglacial scale the carbonate re- cords document highly dynamic fluctuations of Atlantic Water intrusions with pronounced spatial and temporal gradients. The steepest gradients (Fig. 3) and greatest extension (Fig. 4C) are recorded during brief peak in- terglacial periods (Isotopic Events 5.5.1, 5.3, 5.1 and Isotope Stage 1). A much smaller extension of Atlantic Water (Fig. 4D) is correlated with intermediate to low carbonate contents during cool interglacial periods (Isotopic Events 5.4 and 5.2). During glacial periods (Isotopic Stages 6 and 2) overall low carbonate values indicate a predominance of Arctic and Polar water masses (Fig. 4A). However , episodic Atlantic Water in- trusions in Isotope Stage 6 are registered by interme- diate carbonate contents along the eastern margin of
the NGS (Fig. 3B). During glacial to interglacial transi- tions (Terminations II and I) the amount of carbonate remains low (Fig. 4B). A sudden increase in carbonate marks the unhampered inflow of Atlantic waters during early interglacial periods.
During warm interglacial periods (Isotopic Event 5.5.1, Isotope Stage 1) terrigenous coarse fraction con- tents generally are low (approaching zero) under the central Atlantic Water, thus implying that minor amounts of IRD were deposited only in the western sector of the NGS (Fig. 3B). Slightly increased amounts of terrigenous coarse fraction are observed during cool interglacial periods (Isotopic Event 5.2). Distinct short- term peaks are registered during glacial and early de- glacial periods, with maximum amplitudes near the eastern margin.
Surface sediments in general display increased TOC contents, with the highest values under Atlantic Water. A pronounced downcore decrease in TOC in the up- permost port ion of the cores suggests early diagenetic degradation of labile OM. During Isotope Stage 5 mini- mum TOC values may indicate an almost complete de- gradation of labile OM, except for the high accumulat- ing core 23071 on the eastern side of the NGS (Fig. 1), where increased TOC values are observed during Iso-
34
Fig. 3A, B Calcium carbonate, A total organic carbon, and terri- genous coarse fraction (in weight per cent) for cores presently influenced by Arctic surface Water (23 342), margi- nal Atlantic Water (23 352) and the central Norwegian Current (23065 and 23071) for A Isotopic Stages 2-1 (25- 0 ka) and B Isotopic Stages 6- 5 (180-60 ka). Glacial condi- 0 tions (Stages 6 and 2) are re- corded by generally increased 5 and, towards the Scandinavian
"~ 10 continent, highly fluctuating terrigenous coarse fraction ~% 15 contents, intercalations of in- < creased TOC contents with 20 low to intermediate TOC val- ues, and very low carbonate 25 contents. In contrast, warm in- terglacial conditions (Events 5.5.1, 5.3, 5.1 and Stage 1) are displayed by increased carbon- ate contents and very low ter- rigenous coarse fraction con- tents. In interglacial sections TOC values generally remain low, except for Isotopic Event 5.5.1 at the Vcring Plateau and the last 10 ka
west
Arctic Surface Marginal Sector of [ Water Atlantic Water 1 23342 23352
Greenland Basin Iceland Plateau
Modern Environments east
Norwegian Current
1l +07, Jan Mayen Fr.z. VCring Plateau
CaCO 3 (wt.%) 0 20 40 60
, I , i , l , I B I , i ,
i . . . . . . . . . . . . . . . . . . . . . . . .
CaCO 3 (wt.%) 0 20 40 60 + ~ l ~ l , l ~ i ~ I , l ~
1 P::IIIII : . . . . . . . . . . . . . . . . . . . . . . . .
TOC (wt.%) TOC (wt.%)
CaCO 3 (wt.%) 0 20 40 60
_ , J l r l l l , l ~ I J l ,
TOC (wt.%) 0,0 0,2 0,4 0,6 0,8 1,0 0,0 0,2 0,4 0,6 0,8 1,0 0,0 0,2 0,4 0,6 0,8 1,0
0 ~_.L._~ I + I , I ,
+i g ]0 _
g .< 15
20 2
25 coarse terr. (wt.%) 0 5 10 15 20 25
0 _ ,, +,I ~, ,,I .... I,, ,,l um. ++ 10 g < i5
20 -
25
B iiii iiiiiiiiii
, I ,
coarse terr. (wt.%) 0 5 10 15 2O 25
I li I[I liJ [I I1[ [I Ill Jl Ill
I i [ t
coarse terr, (wt.% 5 10 15 2O 25
iiiiii
CaCO 3 (wt.%) Isotopc 0 20 40 60 stage/evcnt ~ J l r l I I t I P i t [ t
TOC (wt.%) 0,0 0,2 0,4 0,6 0,8 1,0
, l , l J l , I ,
r ) i 1 -1B-
- - - ] A :
2
coarse tern (wt.%) 5 i0 15 20 25
. . . . . . . . . . . . . . . . . . . . . . . . . . . ' 1 ~
21
___1 I
topic Event 5.5.1 (Fig. 3B). In the glacial and deglacial sections distinct TOC peaks often correlate with in- creased terrigenous coarse fraction supply (Fig. 3), sug- gesting that the deposition of OM is related to IRD in- put.
Facies patterns along two east-west transects across the central and northern NGS (Isotope Stages 6-5 and 2-1)
Spatial and temporal distribution patterns of litho- and organofacies along two east-west transects are present- ed for the last two glacial to interglacial transitions (Isotope Stages 6-5 and 2-1, Fig. 5). In Isotope Stage 6 deposits intercalation of diamicton deposition (lithofa- cies D, E, F and organofacies III-1 and III-2) and gla- cial background sedimentation (lithofacies C, B1/B2 and organofacies II-1 and II-2) is recorded. A major
phase of diamicton deposition occurred in the eastern and northern sectors of the NGS from 180 ka to about 170 ka. These deposits are dominated by quartz with minor dark siltstone. The number of sediment pellets deposited increased or peaked in nearly every core dur- ing this time, with strong peaks in the nor th-eastern- most NGS. The time interval 170-150 ka reveals glacial background sedimentation over wide areas of the NGS. Biogenic mat ter becomes more dominant with time over this interval. On the eastern Knipovitch Ridge (core 23 454) frequent short-term changes in lithofacies (B1/B2, C and the diamicton facies D, E and F) and IRD lithotypes (dark siltstone and quartz) indicate variable IRD deposition due to the vicinity of Spits- bergen (Fig. 5B). Dark siltstone is much more common in this area, indicating a Spitsbergen or Barents Sea ori- gin. Elsewhere the IRD lithotypes are dominated by quartz (Fig. 6). Except in the nor th-westernmost NGS, the deposition of sediment pellets decreased during this
35
180
w e s t
Arctic Surface Water 23342
Greenland Basin
CaCO 3 (wt.%)
0 20 40 60
8o
~. 100
g 120
< 140
160
60
80
160
1 0 0 2,
120
< 140
I80
60
coarse terr. (wt.%) 0 10 20 30 40
ii!i
80
100
120 <
140
160
180
Modern Environments
Marginal Sector of Atlantic Water
23352 Iceland Plateau
CaCO 3 (wt.%)
0 20 40 60
I C L ' '
TOC (wt.%) TOC (wt.%)
Norwegian Current
1l Jan Mayen Fr.z.
CaCO 3 (wt.%)
0 20 40 60
TOC (wt.%)
east
23071 VOting Plateau
CaCO 3 (wt.%) isotope 0 20 40 60 stage/event
4 - 4.2 . . . . . . i . . - 5,1
5 - 5.2
- 5.3 - 5 . 4
~ 5.5.1- - 5.5.3 - - 6.2 - 6 . 3
6 - 6,4
- 6.5
TOC (wt.%) 0,0 0,4 0,8 1,2 0,0 0,4 0,8 1,2 0,0 0,4 0,8 1,2 0,0 0,4 0,8 1,2
Fig. 3B
i iilxii:iiiiiii!i coarse terr. (wt.%)
0 I0 20 30 40 . . . . . . . .
, I , I , l , l , l l
coarse terr. (wt.%) 0 I0 20 30 40
ElI 4 4.2
i i 5.1 5 5.2
5.31
- 515.3" 6 .2 6 . 3
6 6.4
6.5"
coarse terr. (wt.%) i0 20 30 40
I l l [1111111 I;E[ r i l l 4 4,2--
L 5.L-
5 5,2-
5.3- 5.4-
_ 5.5.1_ - 5.5.3 - 6.2
6.3
6 6.4
6.5
time. During the period 167-163 ka a weak intrusion of Atlantic Water into the eastern NGS is indicated by li- thofacies B3 and A in core 23065. A second phase of repeated severe glaciation (lithofacies B2 and C) and short-term deposition of diamictons is documented from 150-128 ka (lithofacies D, E and F in core 23065, Fig. 5A). The coarse terrigenous fraction consists most- ly of quartz deposits. The domination of biogenic mat- ter in the western and nor th-western NGS indicates re- latively little IRD sedimentation there, whereas dark siltstone is more common in the eastern and nor th- eastern NGS. The decreased deposition of dark silt- stone to the west, combined with the pronounced ba- sinward decrease of diamictons, suggests that the Nor- wegian and Barents Sea shelves are the most probable source areas for this glacio-marine sediment supply. This is further supported by a contemporaneous in- crease in the terrigenous coarse fraction and bulk TOC, a pronounced terrigenous or reworked nature of the
OM and the occurrence of coal clasts in these deposits (Figs 6 and 7). Central areas of the NGS (core 23 352) were not affected by this type of deposition, as the background facies B2 and II-2 indicate (Fig. 5A). Be- cause chalk outcrops do not occur north of the North Sea (Bubnoff 1952), the frequent deposition of chalk dropstones in core 23071 shows a continuous north- ward ice drift in the eastern sector of the NGS during most of Isotope Stage 6 (Figs 6 and 7).
A steep increase in the deposition of carbonate marks the transition to the interglacial conditions of Isotope Stage 5 (Fig. 3). This trend culminates at Iso- topic Event 5.5.1 (the Eemian; Mangerud et al. 1979; Mangerud 1989), where peak interglacial conditions are recorded by the widespread occurrence of lithofacies A, except for core 23 071 and the northernmost areas of the NGS (Fig. 5). Low to intermediate carbonate and planktonic foraminifer contents in core 23071 (lithofa- cies B3) are attributed to intense lateral advection of
36 A
Fig. 4A-D A Spatial distribu- tion of calcium carbonate in the study area for Isotopic Event 6.2. Deposition of diamictons is represented by extremly low carbonate con- tents (< 1 wt.%). Carbonate 75' contents between 5 and 10 wt.% in the central sector (Iceland Plateau) monitor gla- cial pelagic background sedi- mentation. B Spatial distribu- tion of calcium carbonate in the study area for Isotopic Event 5.5.3. Low carbonate production dominates exten- sive areas of the NGS. A nar- row strip in the southern and 70' central sector (10-16 wt.%) indicates a first weak intrusion of Atlantic Water at the tran- sition to interglacial condi- tions. C Spatial distribution of calcium carbonate in the study area for Isotopic Event 5.5.1. Maximum extension of Atlan- tic Water intrusion during peak interglacial conditions is 65 recorded by high carbonate contents (>50 wt.%) in the central and eastern sectors of the NGS. D Spatial distribu- tion of calcium carbonate in the study area for Isotopic Event 5.2. During cold inter- glacial periods decreased car- bonate contents (<20 wt,%) B in the central and eastern sec- tor of the NGS display a pro- nounced decrease in the ex- tent of Atlantic Water
75'
S t a g e
50'
6.2/6.1 (1.51--135 Ka) 40' 30' 20" I O' O' 10"
20" 10" W 0. E
Stage 50"
5.5.3 (125 Ka) 40" 30" 20" 1 O" O" 10"
20" 30"
20"
CaC03 40"
10" 20"
CaCO 30" 40" 50"
N 75"
70"
65"
N 15"
70' 70'
65' 65"
20' 10" W O' IF 10" 20"
Fig. 4C-D C S t a g e 5 . 5 . 1 ( 1 2 2 K a )
50" 40' -~0" 2O" l 0" 0" 10. 20" 30.
CaC03 [~] 37
40' 50"
75'
70.
65'
N 75"
70"
65"
20" 10' W 0. E 10" 20"
D S t a g e 5 . 2 (91 K a )
50" 40" 30" 20" I 0" (7 10. 20. 30"
coco3 40" 50"
75"
70"
65"
N 75"
70"
65"
20" 10" W 0. E 10" 20.
38
the terrigenous fine fraction (Fig. 6). The bottom cur- rent intensities calculated from settling tube analyses support a high advection potential during this period. The deposition of lithofacies B3 in cores 23454 and 23456 shows the penetration of Atlantic Waters into the north-eastern areas of the NGS, whereas the depo- sition of lithofacies B1 in core 17728 still indicates gla- cial background sedimentation at the eastern side of the Boreas Basin (Fig. 5B). The meagre or absent dep- osition of sediment pellets, with the exception of peaks in the glacial sediments of core 17728, support these data. Deposition of detrital, terrigenous OM dominates over the entire NGS, as is indicated by organofacies II- 1 and II-2. Enhanced preservation of marine OM or- ganofacies II-3) during Isotopic Event 5.5.1 is recorded only over the VOring Plateau (core 23 071, Fig. 6) and in the southern Greenland Basin (core 23 342, Fig. 5A) due to increased accumulation rates. The terrigenous coarse fraction shows a distinct change throughout nearly all the NGS during Event 5.5.1. The IRD con- sists of quartz and minor dark siltstone before this event, whereas biogenic matter, indicating low to ab- sent IRD deposition, was deposited during this event and afterwards. Only cores 17728 and 23454 in the northern NGS continue to show the deposition of in= creased amounts of IRD.
During the period 123-105 ka (Isotopic Events 5.5.1-5.3) a persistent inflow of Atlantic Water (lithofa- cies A) is recorded over the Jan Mayen Fracture Zone (core 23 065, Fig. 5A). Biogenic matter deposition dom- inates throughout nearly all the NGS at this time (ex- cept for cores 17 728 and 23 454), whereas sediment pel- lets register a slow increase. The deposition of the gla- cial background lithofacies B2 occurs over the adjacent Vcring Plateau during 100-97 ka (Fig. 5A). A short- term cooling event is further evidenced by steeply de- creasing carbonate contents and slightly increased amounts of the terrigenous coarse fraction (Fig. 6). However, a renewed pronounced intrusion of Atlantic Water into the eastern and northern sectors of the NGS is indicated by the deposition of lithofacies B3 in cores 23071, 17728 and 23454 at about Isotopic Event 5.3 (103 ka, Figs 5 and 6). Western areas of the NGS were not influenced by Atlantic Water throughout this peri- od, as is evidenced by continuous deposition of lithofa- cies B2. A second cooling period between 100 and 80 ka is documented by the widespread deposition of the carbonate-poor lithofacies B2 along both transects (Fig. 5). During this time quartz was deposited in the western NGS, whereas biogenic matter continued to dominate the remaining NGS. A pronounced increase in carbonate and planktonic foraminifer contents from 80 to 70 ka (Isotopic Event 5.1) in core 23071 (Figs 6 and 7) display a renewed penetration of Atlantic Water into the eastern sector of the NGS. Central areas were also affected by Atlantic Water, as the carbonate-rich lithofacies A shows in core 23 065 (Fig. 5A). These re- peated inflows of temperate surface waters are re- corded neither by changes in the make-up of the coarse
terrigenous fraction, nor by variations in the amount and composition of OM; the uniform OM results sug- gest a pronounced diagenetic overprint.
Less dynamic sedimentary regimes than during Iso- tope Stage 6 are indicated by a monotonous facies pat- tern along the investigated transects during Isotope Stage 2 and Termination I. This is supported by the current intensity record of core 23071. During Isotope Stage 2 these intensities are generally lower than those in Isotope Stage 6 (Fig. 6). Biogenic matter often domi- nates in the Stage 2 deposits, indicating much less IRD deposition than in Stage 6. Lithofacies B2 and organo- facies II-1 or II-2 predominate during this period (Fig. 5). Highly variable amounts of the terrigenous coarse fraction are correlated with the frequent input of chalk dropstones, thus indicating intense iceberg drift from southern source areas. The deposition of diamic- tons is generally restricted to the easternmost sector of the NGS (core 23 071, Fig. 5A), which is one of the few areas in the NGS where large numbers of sediment pel- lets occur in Stage 2. As is the case with the diamictons of the penultimate glaciation, these are dominated by quartz; the most recent diamicton in core 23071, how- ever, is dark siltstone-dominated. During Termination I (14.7-14.4 and 13.5-13.1 ka) two distinct TOC peaks in diamictons are clearly dominated by terrigenous, re- worked OM. Organic petrographic results are sup- ported by the occurrence of IRD tracer lithologies, i.e. coal and chalk clasts (Figs 6 and 7). Furthermore, or- ganic geochemical data strongly suggest high propor- tions of fossil, reworked OM in these diamictons, as is indicated by increased Tmax values (435-443 ° C), low HI (<50 mg HC/g TOC), light ~ 13Corg values ( -24 .5 to -24.6%o) and increased C/N ratios (up to 11.5; Fig. 8).
The transition to Holocene oceanographic condi- tions is marked by a broad intrusion of Atlantic Water starting at 9 ka over central areas of the NGS (lithofa- cies A in cores 23 065 and 23 352, Fig. 5A). During the period 10-9 ka biogenic matter, indicative of low IRD sedimentation and generally warmer conditions, re- placed the quartz and dark siltstone deposition of the Stage 2 glaciation in most cores (Fig. 7). Almost con- temporaneously a distinct increase in TOC at the V0r- ing Plateau (9.4-6.2 ka) displays intense short-term lat- eral advection of suspended oxidized, detrital OM, most probably deriving from the adjacent Norwegian Shelf (organofacies II-4 in core 23 071, Figs 5A and 6). A delayed increase in carbonate to Holocene maximum contents is recorded since 6.5 ka over the V0ring Pla- teau (core 23071, Fig. 7) and along the northern trans- ect (Fig. 6B). The transition to a dominance of biogenic matter was also delayed in the north-eastern NGS (7 ka in core 23456 and 4.5 ka in core 23454, the core clo- sest to Spitsbergen). Sediment pellets show a brief peak at 4 ka in the eastern NGS (Fig. 7).
The most typical features of the near-surface deposi- ts are continuously increasing amounts of bulk and ma- rine OM towards the sediment surface (organofacies I-1, Figs 5A and 6) in response to the decreasing diag-
Greenland Basin OM- Lithe-
facies facies 0
~" 10 - 1 arb
"~ 1 5 - - - : 1 A //~/"
20 - 2
25 - Isotope
60stage - event
8O
I00
¢ 120 <
140
160
180 a
Boreas-Basin Lithe- facies
0 .
< 15 i 1A!
2O2 • ~
25. i ~ Isotope
stage - event 60
4 I 100:5
,~ : ~, 120 : =~ ~ K',.\\\\\\'%" O < -
-6,2
140- 6 - 6 . ~
160~_ -6.4
b 1 8 0 -
Iceland Plateau OM- Lithe- facies facies
0 _ • -¢ "/¢ - •
5-• ' , ' , ' ,
10 ::
i5 -
20 -
25 :
Knipovitch Ridge Lithe- facies
~ 1 i ~
< 15
20
25
6O
100
140 N
Jan Mayen Fr.z. OM- Litho- facies facies
0
/ 10
15
25
Vcring Plateau OM- Litho- facies facies
15
20
25 Isotope
stage - event 60
80 ,~ -5.1~,
5
100 ~ -5,3
~ .5.4
120 s ~- 5.51 553
.6.2 140 -6.3q
6
160 .6.4
180
f Organofacies ~' (Wagner 1993)
'~ I - 2 inertinite
II - 1 terrigenous/ vitrinite =
~,i [-- ~ terrigenous/ II - 2 izertinke
II - 4 ten%enous] inertinite
~ ~ terrigellous/ lII - 1 thermal rnatur~
o . terrigellous/
~ ~ tlI-2 thermal overmature OM
,.. j f Lithofacies ~"
(Henrich 1992)
Atlamic [ ] A Water [ ] B3
Arctic and palar [ ] BI/B2 Water [ ] C
Glacialsedimentary events • D/FJF
slp.=slump turb.=turbidlte
~.co nL = contarite ./2
Knipovitch Ridge Litho- facies
10
15 N- 7,
la0tope stage - event
60 ~ 4 . 2 .
80 ~ 5 5.1 5.2,
100 :5.3. -5.4.
~ 120 N 5.5.1 5532.
6.2. 140 ~ 1 6.3"
6i 160 6.4-
- 6.57
180 _2 i i
f L i t h o f a c i e s "
(Henrich 1992) Atlantic [ ] A
• W B2
Arctic and polar [ ] B 1/132 Water [ ] C
Glacial sedimentary • D/E/F events
\ J
Fig. $a, b Lithofacies for investigated cores from A the southern and B the northern trans- ect. Organofacies are shown for the southern transect. Intercalations of glacial background facies (B1/B2 and C; II-1 and II-2) and diamic- ton facies (D, E and F; III-1 and III-2) reflect highly variable glacio-marine regimes during glacial phases, especially during Isotope Stage 6. Repeated Atlantic Water intrusions into the eastern and central sector of the NGS during interglacial and specific glacial phases are recorded by lithofacies A and B3. The gen- eral decoupling of organo- and lithofacies, most obvious in Stage 5 deposits, is related to the diagenetic degradation of non-resistant or- ganic matter. The establishment of Holocene climatic conditions since about 10 ka is indi- cated by deposition of facies A, B3 and I-1
39
40
5
g 10 <
15
20
25
Core 23071 ]
CaCO 3 planktonic f0raminifer
(wt.%) (wt.%) 0 10 20 30 400 5 10 15 20 0
60
80
i00 g <
120
I40
160
marine/terrigenous terrigenous OM coarse fraction
(wt.%) (wt.%) 0,4 0,8 1,2 0 10 20 30
marine OM
!iiii 0 10 20 30 4 0 0 5 10 15 20 0 0,4 0,8 1,2 0 10 20 30
IRD tracer bottom water lithologies >500 ;m intensity
(wt.%) (cm/s) 2.5 5.O 12 14 16 18
i i i I r i r I
I s o t o p e
s t a g e / e v e n t
0 2.5 5.012 14 16 18
ark -'si,ts,one
L
4.2.
5.1-
5.2-
5.3-
-5.5.1- -5.5,3-
6 . 2
6 6 . 3 -
AnalNicalrange >12.gcm/s
Fig. 6 Comparison of parameters (CaCO3, planktonic foraminif- ers, marine versus terrigenous organic matter, coarse terrigenous fraction, IRD composition > 125 ~m and bottom current intensi- ties) for core 23071 during Isotopic Stages 6-1 and 2-1. Low car- bonate and planktonic foraminifer contents during Stage 6 con- trast with highly variable and pulse-like increased TOC, terrige- nous coarse fraction and bottom water intensity records reflecting variable glacio-marine settings. Increased IRD contents contain- ing dark siltstones support the dominant terrigenous, reworked character of the organic matter. At the transition to interglacial conditions intense winnowing causes strong terrigenous dilution, resulting in intermediate carbonate and planktonic foraminifer contents. Maximum carbonate and planktonic foraminifer con- tents are recorded at 80-75 ka and in late Holocene sections. In- creased proportions of marine organic matter are solely pre- served in Isotopic Event 5.5.1 deposits and in the uppermost sedi- ment section
enetic degradat ion of marine OM. The organic geo- chemical results support the microscopic results as the up-core increasing H I and ~ 13Corg values (up to 270 mg HC/g T O C and -22.2%0, respectively) indicate increas- ing propor t ions of mar ine OM (Fig. 8).
Discussion
Monitoring climatic variability during glacials and interglacials: The history of Atlantic Wate r intrusions
Determining the extent and duration of Atlantic Water intrusions in the NGS is essential because of: (1) its ef- fect on the climate on the Nordic continents; and (2) its
trigger function on the stability of the ice sheets with (I) weak inflows of Atlantic Water during glacials, p romot- ing an increase in ice volume by moisture supply to po- lar areas and (II) positive feedback mechanisms of At- lantic Water intrusions during ice sheet decay. As a re- sult, the dynamics of Atlantic Water intrusions has a major imprint on the deposit ional regime in the NGS.
Kellogg (1980) considered three modes of surface water circulation - that is, an interglacial mode with full deve lopment of the Norwegian Current, an interme- diate mode during weak interglacials with a rather nar- row extension of Atlantic Water in the south-eas tern sector of the NGS and a glacial mode without any At- lantic Water influence. However , later studies revealed a much more complex and dynamic history of Atlantic Water , especially during glacials and glacial to intergla- cial transitions (Henrich et al. 1989; Baumann 1990; Gard and Backmann 1990; Hebbe ln 1991; Baumann and Matthiessen 1992; Ko~ Karpuz and Jansen 1992). There is also controversy about the pathways along which these weak Atlantic Water intrusions developed. Henrich et al. (1989), Gard and Backmann (1990) and Hebbe ln (1991) registered an inflow along the eastern margin of the NGS, episodically reaching as far north as into the Fram Strait, whereas Sarnthein et al. (1992) propose intrusions on the western side entering through the D e n m a r k Strait. In the following, we dis- cuss these conflicting circulation concepts on the basis of the results of this study.
<
Dominant IRD "~ Lithology [
(Goldschrnidt, 1994) /
[ ] dark siltstone /
[ ] quartz /
[ ] biogenic matter | ank = statistically insignificant I
5
10 ~e
15-- - -_~A :
2 0 2
25
60 4 - ~.~-
80 - 5.~- - 52-
100 5 _ 5.3 - 5.4
.-M 120- _5.5a_ - 55.3
< - L 6,2 140 -- ~3
6 160- s4
180 "" Isotope stage I event
7-sga-
dora. ~D 8 ~ N ~. ~semblage ~ ~ ~
1 ~ oo,~o," ................... o o l i O ~ l i i @ l l i l
Illl ~ ~
dom. IRD g 8 ~ ~ ~'I . . . . blage ~ ~ ~ t ~ ' -
i i i i l i l
I l l l l l l i o o o ~ l l l i l i ! l l i l l l l t i l l i l ~ .. . . . . . . . . . . . . . . . .
O!!O
42
Organic Petrology
Isotope Stage/Event
& <
TOC (wt.%)
0:
5 ~
10 :~~
15: 1~
20 i 2
25:
Marine/Terrigenous Vitrinite/ OM (wt .%) Inertinite
0.2 0.6 ~ l l l n l r P r l Toe AR TOO!
1.0 0.2 0.4
i
\ ~ OM
2 4 6 8
i
m,.
I ' I , L ' I
0.02 0.040.06 AR TOC
(g/em2/ky)
Organic Geochemistry
Hydrogen Index Tmax Corg/Ntot c313Corg (mg HC/gCorg ) (C °) (PDB %0)
100 200 300 400 5 10 -25-24-23-22
Fig. 8 Organic petrological and organic geochemical results for core 23071 from the last 25 ka. Late glacial sections are domi- nated by terrigenous and reworked, thermal mature organic mat- ter, as is indicated by low HI, high Tm,× and C/N and light 6 13Corg records. This is most obvious for two TOC peaks within diamic- tons at 14.5 and 13.5 ka. Towards the sediment surface a contin- uous increase in marine organic matter is evidenced, thus moni- toring early diagenetic processes. This trend is overprinted at 9-7 ka by intense lateral advection of terrigenous organic matter
tinental margin (Stein et al. 1993). Distinct changes in the association of dinoflagellates and coccoliths support these results (Baumann and Matthiessen 1992). Mod- ern conditions had been achieved by 6 ka. Considering the differences in the stratigraphic resolution of lithofa- cies and organofacies, the time correlation is fairly good. Effective early diagenetic degradation of marine OM in the uppermost sediment section is attributed to intense biological activity. The decrease of these early diagenetic effects is clearly documented by steeply in- creasing amounts of autochthonous OM towards the sediment surface (organofacies I-1, Figs 6 and 8).
Specific glacial and early deglacial sedimentary regimes: deposition of diamictons
The most prominent features in glacial and deglacial deposits of the NGS are distinct dark coloured diamic- tons, which are intercalated within light coloured gla- cial background sediments (Fig. 5). According to their spatial distribution, their temporal frequency and their characteristic sedimentological composition (e.g. the pulse-like increased contents in TOC and temgenous coarse fraction or the abundance of IRD tracer litho- logies), differing reconstructions of depositional re- gimes and ice drift patterns have been proposed (Bi- schof et al. 1990; Henrich 1992). A conflicting circula- tion model for periods of diamicton deposition is de- duced from stable isotope evidence (Sarnthein et al. 1992; Weinelt et al. 1992).
According to Henrich (1992) the spatial distribution of diamictons in Isotope Stages 6 and 2 depict various regional glaciomarine depocenters along the eastern margin of the NGS. Diamictons pinch out in the west- ern and northern directions, suggesting a very high abundance and overall northward drift of icebergs in the eastern NGS. The coincidence of maximum TOC contents with high abundances of indicative IRD tracer lithologies, e.g. dark siltstone and coal clasts (most probably of Jurassic to Cretaceous age, H/51emann 1993; Wagner and H01emann subm.; Figs 6 and 7) sug- gest a common depositional process and consequently a predominantly allochthonous reworked origin of OM. This assumption is further supported by the frequent occurrence of chalk dropstones in diamictons along the eastern (Henrich 1990, 1992) and northern (Wagner 1993) margin of the NGS (Fig. 7) and in the Fram Strait (Hebbeln 1991; Spielhagen 1991). These unambiguous- ly prove northward ice drift patterns during the last two glacial periods (see, for example, tentative circulation models for Isotopic Events 6.2 and 5.5.3, Fig. 9). Hen- rich (1992) deduced an anticlockwise surface circula- tion combined with a reduced deep water export into the North Atlantic during the short-term periods of diamicton deposition.
Based on a southward decreasing trend in coal clast abundance in Isotope Stage 6 deposits of the NGS, the Fram Strait and the eastern Arctic Ocean, Bischof et al. (1990) reconstructed a temporary southward ice drift from the Arctic Ocean into central areas of the NGS, as far as the Vering Plateau. Owing to the low thermal maturity of coal dropstones, the postulated source ar- eas were the Franz Josef Land archipelago or the ex- tensive Siberian Shelf. However, this ice drift pattern would result in a clockwise surface circulation in the NGS. The occurrence of chalk dropstones, as discussed earlier, is not explained by this clockwise circulation model.
In contrast, an estuarine model with a temporary clockwise surface water circulation and an import of
43
Fig. 9A Reconstruct ion of surface water circulation and extension of continental and sea ice coverage for Isotopic Event 6.2. The major driving force was persistent high pres- sure areas over the continen- tal ice domes of Greenland and Scandinavia, causing strong catabatic winds which were deflected to the right by the Coriolis force. These winds resulted in an anticlock- wise surface water circulation with seasonally variable ice- free conditions and intense iceberg drift in the eastern sector (medium gray) and a permanent sea ice cover (dark gray) in the western area of the NGS. In the center cy- clonic gyres connect the two marginal ice drift streams. For comparison with spatial car- bonate distribution, see Fig. 4A. B Reconstruct ion of surface water circulation and extent of continental ice and sea ice coverage for Isotopic Event 5.5.3. At the transition to interglacial conditions a pronounced retreat of the per- manent sea ice cover to the north-western NGS is evi- denced. Extensive areas were seasonally ice-free. Contempo- ranously, a first intrusion of Atlantic Water into the south- eastern sector is recorded. For comparison with spatial car- bonate distribution, see Fig. 4B
70 °
0
65
60 °
75 °
10 °
B 7 5 °
8 0 °
20 °
8 0 °
85 ° Isotopic Event 6.2
~gCf#l"n ~ r ~ n T## ~ r ~ f t ~ l l r V p n f ~ i c e c~ve rec ] m w f s c e W s l e r m a ~ O S
,erg
70 °
0
65
60 °
10 ° W 0 ° E 10 ° 20 ° 10 o
8 5 ° I Isotopic Event 5.5.3
rg drill
7 0 ° 7 0 °
65 ° 0
65
60 ° 60 °
10 ° 20 ° 10 ° W 0 ° E 10 ° 20 ° 10 °
44
deep water over the Greenland-Scotland Ridge into the NGS is deduced from stable isotopic evidence (Jansen et al. 1983; Vogelsang 1990; Sarnthein et al. 1992; Weinelt et al. 1992). High TOC contents, heavy oxygen and light carbon isotope ratios in diamictons were proposed to indicate regional upwelling of the im- ported deep water (Vogelsang 1990). Based on high re- solution time slice reconstructions of planktonic oxygen and carbon isotope gradients, a regional clockwise sur- face water circulation during specific phases of the last deglaciation was proposed by Sarnthein et al. (1992) and Weinelt et al. (1992). Centered around 13.6 and 12.4 ka, extensive meltwater plumes are documented by extremely light oxygen isotope values along the eastern margin. These gradients in ~ 1sO depletion suggest a southward meltwater intrusion as far as the Irish Sea. According to these workers the meltwaters originated in the western Barents Sea. A narrow band of heavy oxygen and light carbon isotope values along the Nor- wegian and western Barents Sea continental margins is interpreted to be indicative of coastal upwelling. Ac- cording to this circulation model high TOC contents in diamictons are attributed to an increased deposition of authochthonous OM.
Results of this study performed on diamictons (orga- nofacies III-1 and III-2) show that allochthonous, re- worked OM dominates the organic character (terrige- nous macerals >70%, HI about 50 mg HC/g TOC, rmax 420-440°C, C/N->10, ~ 13Corg< -24%0 PDB; Figs 6 and 8). These results are further supported by organic geochemical studies performed on diamictons in ODP Leg 104 material (McDonald et al. 1989; H01e- mann 1993; H01emann and Henrich in press). No indi- cation for regional upwelling is found. Significantly in- creased amounts of fine- (< 10 p~m) and coarse-grained (> 10 p~m) terrigenous phytoclasts and abundant clasts of coal, black shale and TOC-bearing siltstone reflect the short-term deposition of laterally advected re- worked terrigenous suspensions and intense IRD supp- ly. However, a southward ice drift from Arctic regions, as was postulated by Bischof et al. (1990), appears un- likely because of the pronounced asymmetrical distri- bution patterns of IRD tracers. They display the high- est abundances along the Norwegian and Barents Sea continental margins and are lowest in central areas of the NGS. Organic-bearing Jurassic-Cretaceous strata with a broad spectrum of depositional facies outcrop at the eastern Norwegian Shelf and on the Spitsbergen Bank (thin brown coal seams, black shales and organic- poor siltstones; Bugge et al. 1984; Elverh¢i and Lauritz- en 1984; Kelly 1988). These rocks therefore appear to be the most probable sources of glacial sediments in the eastern NGS (HOlemann 1993; HOlemann and Henrich in press; Wagner and HOlemann subm.). In addition, the high frequency and abundance of chalk dropstones along the eastern margin of the NGS (Fig. 6, Henrich 1992) and their northward decrease (Spielhagen 1991; Hebbeln 1991) conflict with the basic assumption of Bischof et al. (1990). In conclusion, most of the sedi-
mentological and organic geochemical parameters con- tradict the estuarine circulation model (Sarnthein et al. 1992). To solve the conflicting evidence we propose an alternative interpretation of the isotopic data presented by Sarnthein et al. (1992). The pronounced negative 6 13C and heavy oxygen isotope values of N. pachyder- ma sin. in areas close to the eastern continental margin could indicate a deep habitat below the 'unpleasant' suspension-loaded meltwater lid (Henrich 1992). Fur- ther offshore, in suspension-free meltwaters, N. pachy- derma sin. could again thrive in surface waters and thus depict the observed very light oxygen isotope values (similar changes in habitat are found in the Fram Strait; Carstens 1988; KOhler 1992).
Sharp basal and top contacts and prominent IRD peaks suggest a short-term, pulse-like sedimentary re- gime for diamictons. Diamicton deposition is docu- mented at the V0ring Plateau from 23.2 to 23.0, 15.8 to 15.6, 14.8 to 14.5 and 13.5 to 13.1 ka (core 23071 in Fig. 5A; stratigraphy according to Vogelsang 1990). A fairly good temporal correlation with Heinrich layers H2 (21.4-19.9 ka) and H1 (14.6-13.5 ka) in the North At- lantic (Heinrich 1988; Bond et al. 1992; Grousset et al. 1993) is conspicuous (Fig. 7), although the deposition of diamictons occurred at a higher frequency than Heinrich layers. However, the composition of these two deposits is completely different as the Heinrich layers display fossil carbonate dropstones from eastern Cana- dian source areas. Bond et al. (1992) propose that surge-like events along the tidewater margins around the Hudson Strait initiated intense calving of sediment- laden icebergs and the deposition of Heinrich layers. Taking the temporal coincidence of Heinrich layers and diamictons into account, a common trigger mechanism may be suggested which caused an almost simultaneous disintegration of the North American and Scandinavian continental ice masses. The short duration and the re- peated and widespread occurrence of these deposits in- dicates very rapid climatic detoriations in northern hemisphere oceans. Nevertheless, the exact causes and the chain of feedback loops still remain enigmatic. However, the facies sequence observed in lower Stage 6 (Fig. 5A, Isotope Stage 6.5 in core 23065; Hen- rich 1992), with glacial background facies C at the base, followed by a sudden onset of diamicton deposition (fa- cies E and F) passing into facies B3 at the top, suggests that variations in Atlantic Water intrusion may have triggered the incipient instability of the tidewater ice margins (Wagner and Henrich 1994).
Early Holocene sedimentary regimes
The stepwise shift from the last glacial maximum to modern climatic and oceanographic conditions has been reconstructed with a high temporal resolution us- ing bulk carbonate and planktonic foraminifer records (Henrich 1992), changes in the assemblages of cocco- liths and palynomorphs (Combaz et al. 1974; Holtedahl
45
et al. 1974; Pelet 1974; Matthiessen 1991; Baumann and Matthiessen 1992) and stable isotope evidence (Sarn- thein et al. 1992; Weinelt et al. 1992; Weinelt 1993). The first pronounced inflows of Atlantic Water are re- corded by distinct changes in the association of dinofla- gellates and coccoliths (Baumann and Matthiessen 1992) and by a steep increase in bulk carbonate (Hen- rich 1992) at about 10 ka, whereas modern conditions were established at 6 ka. This trend is recorded by shifts in lithofacies from glacial background types B1/ B2 to Atlantic Water types A and B3 (Fig. 5). Howev- er, carbonate is diluted by laterally advected terrige- nous suspensions in cores close to the eastern margin between 9 and 6 ka (Figs 3A and 6).
This pattern is also recorded by the contempora- neous deposition of organofacies II-4 in cores 23071 and 23 065 (Figs 5A and 6). Elevated TOC contents are related to a steeply increased supply of detrital, oxid- ized OM (inertinite) far into the western deep sea ba- sin. The local distribution and the basinward pinching out of organofacies II-4 suggests source areas on the adjacent eastern Norwegian Shelf (Wagner and Hen- rich 1994). Considering the fast retreat of continental glaciers on Scandinavia and the subsequent isostatic uplift in the early Holocene, the differing hydrody- namic conditions on the inner shelf regions must have resulted in an intense winnowing by coastal currents and subsequent downslope transport of suspended gla- cial debris into the basin. This is supported by the cur- rent intensity record of core 23 071 (Fig. 6) where tur- bidites are recognized by increased bottom water inten- sities and a well defined sorting of the sediments (Mi- chels 1994).
Deep water generation during glacials
The NGS is one of the most important regions of the world's ocean due to its pronounced sensitivity to cli- matic and oceanographic changes. Under modern con- ditions, the global conveyor belt of surface and deep water circulation depends on the intensity of deep wa- ter formation in the western and northern sector of the Nordic seas and dense winter brine formation on the surrounding shelves.
Considering the lower importation of more saline, temperate Atlantic Water during glacial periods, the mechanisms of bottom water formation should be dif- ferent from modern conditions. Various models have been proposed for glacial conditions. They suggest that: (1) bottom water is generated during winter sea ice for- mation and mixed with meltwaters of continental origin (Jansen and Veum 1990) - this bottom water would therefore be less dense than present day bottom waters; (2) Norwegian Sea bottom water formation results from winter downwelling in the eastern and central NGS (Henrich 1992); (3) bottom water circulation was strongly reduced due to sea ice coverage (Kellogg
1980); and (4) deep water flowed in from the North At- lantic into the NGS over the Greenland-Scotland Ridge (estuarine circulation; Jansen et al. 1983; Vogelsang 1990; Sarnthein et al. 1992).
During phases of estuarine circulation rather old and corrosive deep waters were imported into the NGS. Hence carbonate preservation should have been very low. As preservation is generally good over long glacial periods (Henrich 1992; Wagner and Henrich 1994), such a circulation mode cannot have been estab- lished for a longer time period. Short-term dissolution spikes occur contemporaneously with diamicton depo- sition, indicating corrosive bottom waters under a sta- bilized water column (Henrich et al. 1989). Neverthe- less, as these processes cover only limited areas along the eastern margin of the NGS, an estuarine circulation model is not supported by these results. However, the mechanism of deep water formation as discussed by Jansen and Veum (1990) is in accordance with the car- bonate preservation record. For long glacial periods with good carbonate preservation, deep water have been generated, as suggested by Henrich (1992). Fur- ther indications for this assumption are the generally increased bottom current velocities over the V0ring Plateau during glacial periods (Fig. 6), which may be at- tributed to a higher supply and redistribution of downs- lope suspensions, which in turn were possibly induced by a higher rate of brine formation on the shelves.
Conclusions
Modern and past surface water regimes are displayed by specific deep sea lithofacies. Lithofacies A is deposi- ted beneath the central Atlantic water masses, whereas marginal sectors of the Atlantic Water masses are rep- resented by lithofacies B3. Glacio-marine background lithofacies (B and C) indicate surface water conditions with minor input of IRD and seasonally variable sea ice cover. Most spectacular are the glacial and deglacial dark colored diamictons evidencing short-term sedi- ment pulses due to a sudden disintegration of tidewater glaciers which have advanced far out onto the outer continental shelves.
Although a generally good correlation between li- thofacies and organofacies is recorded for near-surface sediments and glacial sections, this pattern is not ob- served for the interglacial sections of Isotope Stage 5. This is due to intense early diagenetic degradation, pre- ferentially of marine OM.
The history of Atlantic Water intrusions in the NGS reveals complex and highly dynamic patterns for the last two glacial and interglacial periods, with repeated inflows during Isotope Stage 6 and a high variability in Isotope Stage 5. Maximum extensions are registered during the climatic highstands 5.5.1, 5.3 and 5.1 and the narrowest intrusions in the cool phases 5.4 and (most pronounced) 5.2.
46
The depos i t ion of d iamictons during glacial and de- glacial per iods reflects shor t - t e rm deposi t ional events. T h e y are re la ted to high instabilities o f cont inenta l ice sheets which have advanced far out on to the ou te r shelves. Some of the d iamictons seem to occur con tem- po raneous ly with Heinr ich layers H1 and H2. Despi te a comple te ly different l i thological compos i t ion of the d iamictons in the NGS, the p robab le t empora l and ob- vious phenomeno log i ca l coincidence o f He inr ich layers and N G S diamictons suggests a c o m m o n tr igger mecha- nism which caused an a lmost s imul taneous disintegra- t ion of huge cont inenta l ice masses along the shelves of N o r t h A m e r i c a and the eas tern margin of the NGS.
The es tuar ine circulat ion mode l for specific per iods of the last deglaciat ion (Sarnthe in et al. 1992; Weinel t et al. 1992) claims regional upwell ing along the eas tern marg in o f the NGS. The organic charac te r of sediments cover ing the same t ime intervals p rove a clear p r edom- inance of r eworked , fossil O M and thus does no t sup- por t basic assumpt ions of the es tuar ine model .
La te ra l advec t ion of te r r igenous suspension is gener- ally observed in interglacial Stages 5 and 1. The intensi- ty o f these processes was s t rongest dur ing early inter- glacial per iods when extensive winnowing of glacial de- bris on the shelves took place. These p h e n o m e n a are also clearly d o c u m e n t e d in the organic record.
D e e p water was also gene ra ted dur ing glacial peri- ods in the NGS, bu t different mechan i sms and locat ions f r o m the m o d e r n sys tem have to be considered.
Acknowledgements 61~Corg analyses were performed at the In- stitute for Nuclear Geophysics, 14C-Laboratory at the University of Kiel, under the direction of Dr H. Erlenkeuser. Rock-Eval analyses were performed at the Institute of Biogeochemistry and Marine Chemistry, University of Hamburg and at the Alfred We- gener Institute, Bremerhaven. We acknowledge M. G. Wiesner and R. Stein for providing Rock-Eval data. For technical assist- ance we thank S. Schulz, B. Schlttnz, H. Meggers and numerous students. We gratefully acknowledge the instructive reviews by Drs R. Stein and D. Stow. This study was supported by a grant from the German Research Foundation. This is contribution 215 of the Joint Research Project 313 at Kiel University (SFB 313).
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