Evolution of the Norwegian Current and the Scandinavian Ice Sheets during the past 2.6 m.y.: evidence from ODP Leg 104 biogenic carbonate and terrigenous records

Palaeogeography, Palaeoclimatology, Palaeoecolog.v, 108 (I 994): 75 94

7"~

Elsevier Science B.V., Amsterdam

Evolution of the Norwegian Current and the Scandinavian Ice Sheets during the past 2.6 m.y." evidence from ODP Leg 104 biogenic carbonate and terrigenous records R. H e n r i c h a a n d K.-H. B a u m a n n b ~University (~/Bremen, Department of Geosciences, PostJach 330440, D-28944 Bremen, German)' bGEOMAR, Research Center[or Marine Geosciences, Wischho[;s'tr. 1-3, D-2300 Kiel 14. Germany (Received March 1, 1993; revised and accepted July 22, 1993)

ABSTRACT Henrich. R. and Baumann, K.-H., 1994. Evolution of the Norwegian Current and the Scandinavian Ice Sheets during the past 2.6 m.y,: evidence from ODP Leg 104 biogenic carbonate and terrigenous records. Palaeogeogr.. Palaeoclimatol,, Palaeoecol., 108: 75- 94. Records of biogenic and terrigenous components have been obtained from the interval corresponding to the last 2.6 m.y. of ODP Sites 643 and 644 in order to reconstruct surface and deep water regimes in the Norwegian Sea. Surface water regimes record long lasting moderate glacial conditions during the interval 2.6 1.0 Ma. Small intrusions of Atlantic water episodically penetrated into the Norwegian Sea forming a narrow tongue along the eastern margin, which is documented at Site 644. The polar front was most probably situated between the Site 644 and 643 locations on the outer Voring Plateau during these time intervals. Deep water regimes reflect long-term persistent corrosive bottom waters, most probably due to a weakly undersaturated water column and a low rate of carbonate shell production in surface waters. Deep water production in the Norwegian-Greenland Sea may have operated in a different way, e.g. brine formation during winter sea ice growth. Bottom waters were oxygenated throughout the entire period, and deep water was exchanged per.~;istently with the North Atlantic. Increased glacial/interglacial enviromental contrasts are documented, reflecting a strengthening of the Norwegian Current and intensified glaciations on the surrounding land masses during the interval 1.0 0.6 Ma. During this time a major shift in the mode of deep water production occurred. Tile onset of large amplitudes in glacial/interglacial environmental conditions with maximum contrasts in surface water regimes, different modes of deep water production, and intensified exchange with the North Atlantic marks the last 0.6 Ma. A broad development of the Norwegian Current is observed during peak interglacials, while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions.

Introduction A m a j o r e l e m e n t in the e v o l u t i o n o f C e n o z o i c e n v i r o n m e n t s has been the t r a n s f o r m a t i o n from w a r m E o c e n e to L o w e r M i o c e n e oceans into the later type o f oceans c h a r a c t e r i z e d by s t r o n g thermal gradients, oceanic fronts, cold deep oceans and cold high latitude surface w a te r masses (Thiede et al., 1989; B o h r m a n n et al., 1990; J a n s e n et al.., 1990; M u d i e et al., 1990). This t r a n s f o r m a t i o n is linked with the climatic t r a n s i t i o n into cold high latitude climates and the s u r f a c e - w a t e r c o n n e c t i o n between higher a n d l o w e r latitude oceans in the 0031-0182/94/$07.00 © 1994 SSDI 0031-0182(93)E0129-H

N o r t h e r n H e m i s p h e r e . A m a j o r threshold o f the climate system was passed with the inception o f glaciers and ice-sheets in the N o r t h e r n H em i sp he r e . Th e best l o c a t i o n to m o n i t o r the onset o f glaciations in the N o r t h e r n H e m i s p h e r e w o u l d be in the Arctic and n o r t h e r n and western N o r w e g i a n - G r e e n l a n d Sea. So far, O D P has not yet entered that region. Nevertheless, d a t a from O D P Leg 104, situated at the s o u t h e a s t e r n entrance to the Arctic o n the V o r i n g Plateau, d o c u m e n t m i n o r input o f ice-rafted detritus ( I R D ) into the N o r w e g i a n - G r e e n l a n d Sea in the late M i o c e n e an d t h r o u g h the Pliocene (Jansen et al., 1990: W o l f

Elsevier Science B.V. All rights reserved.

76

R. HENRICHAND K.-H.BAUMANN

and Thiede, 1991). This points to the existence of periods when glaciers were able to form and reach the shelves somewhere in the Sub-Arctic. A major shift to repeated strong glaciations occurred at about 2.6 Ma, when huge quantities of IRD were delivered by icebergs into the Norwegian Sea documenting the first development of ice sheets in Scandinavia (Jansen et al., 1988; Henrich et al., 1989). Glaciations were further amplified at about 1.0 Ma. Today, the Norwegian-Greenland Sea is characterized by strong meridional gradients in the sea surface environment. The present day eastern Norwegian Sea margin is ice-free due to the Norwegian Current, a relatively warm (6 10°C), saline (35.1-35.3%o) northward flowing branch of the North Atlantic Drift (e.g. Swift, 1986). The modern surface current system in the western part of the area is characterized by the East Greenland Current which carries the cold (< 0°C), less saline (30-34%o) polar water southward along the East

20°W

Greenland coast (Fig. 1). Between the regions dominated by the Polar and the Atlantic water masses, the so-called Arctic Surface Water forms as a mixture of both (0-4°C, 34.6-34.9%0). The Arctic surface water is contained in two cyclonic gyres, e.g. the Jan Mayen Current and the East Iceland Current, fed by Polar and Atlantic waters. The system of warm and cold currents results in the formation of distinct oceanographic fronts (Johannessen, 1986). Thus, this region is important for the climate because of the existence of ice and because of the formation of deep water masses, which fill the rest of the world ocean. Major paleoceanographic and paleoclimatic changes evidenced by shifts in facies successions and sediment parameters in Leg 104 drill sites have already been proposed from the long-term record in the Norwegian Sea during the last 2.6 m.y. (Jansen et al., 1988; Henrich, 1989; Krissek, 1989). However, qualitative and quantitative examinations of certain terrigenous sediment components

0*

20°E

Fig. 1. Surface water circulation in the Norwegian-Greenland Sea (EGC=East Greenland Current, NC=Norwegian Current), EIC= East Iceland Current, JMC= Jan Mayen Current) and location of ODP Leg 104 Sites 643 and 644 on the Voring Plateau, mid-Norwegian continental margin (depth in km).

N ( ) R W [ G I A N ( ' U R R [ i N T A N D S C A N I ) I N A V I A N ICE S H E E R S D U R I N G PAST 2.6 M "~: E V I D E N C E F R O M O D P LEG 104

as well as sedimentological and isotopic investigations have mostly been used to document the oceanographic and climatic evolution of the Norwegian Sea. The main objective of this study is to provide detailed paleoceanographic interpretations from the biogenic calcareous and terrigenous records of Leg 104 Holes 643A and 644A covering the section of large-scale glacial/interglacial climatic fluctuations of the past 2.6 m.y. A high resolution data set on carbonate depositional patterns and compositional differences was used to interpret climatically induced changes in surface regimes during the past 2.6 m.y. Specifically, abundance variations of planktic foraminifers and coccoliths were determined to display shifts in surface water temperature regimes and environmental gradients. By comparing planktic and benthic foraminifer records we will comment on the efficiency of pelagic/benthic coupling, a process known from studies of the Recent system, in which an ahnost immediate reaction of the benthic fauna to a seasonal pelagic food pulse was observed (Graf 1989). We will estimate the intensity and temperature regime of inflowing surface water from the North Atlantic during warm periods and compare it with variable cold water regimes in glacials and deglaciations. Today, the Atlantic water infow contributes the major heat flux to the north European subarctic, which is responsable for the favorable climate in Scandinavia. A decline in the strength of Atlantic water inflow would promote cooling, but it would also, as long as it ever weakly persists, still ~:ransfer heat, moisture and salt to the north and thus promote glaciation on the surrounding continents. From the terrigenous records, and a quantification of ice rafting of fossil carbonates, especially of nannofossil chalk, we will deduce patterns of iceberg drift and density under variable surface water regimes. Finally, we will provide statements on deep water properties based on carbonate preservation studies on planktic fora-minifers and benthic foraminifer contents and species occurrences. Material and methods

Samples were collected from ODP Leg 104 (Sites 643 and 644). Both sites belong to a transect of three sites across the Voring Plateau (Fig. 1). Site

77

643 is situated on the outer Voting Plateau at 67 43'N 01°02'E in 2764 m water depth, while Site 644 is on the inner Voting Plateau at 66:41'N 04"35'E in 1227 m water depth (Thiede et al., 1989). Calcium carbonate contents were measured using a LECO CS-125 infrared analyzer (Heinrich et al., 1989: Wolf, 1991). Both total carbon (TC) and total organic carbon (TOC) contents were determined by infrared measurements of CO2 absorption. Organic carbon was calculated from CO2 that was released by treatment with hydrochloric acid. The bulk calcium carbonate content was calculated in weight percentages of the bulk sediment sample as: C a C O 3 % = ( T C % - T O C % ) × 8.33 Since most traditionally applied, methods to characterize carbonate preservation (e.g. plankton/ benthos ratios, contents of dissolution sensitve taxa, conventional fragmentation indices) fail to produce reliable data in the Nordic Seas, a different approach to quantify the state of perservation has been applied (Henrich, 1989). The SEM (Scanning Electron Microscope) approach to quantify carbonate preservation is based on the recognition of four stages of structural breakdown during progressive dissolution of two morphotypes of the planktic foraminifer species Neoglohoquadrina pachyderma sin. and its conversion into a numerical dissolution index on a statistically sufficient number of tests. The dissolution index 0 accounts for best preservation, while the index 4 defines strongest dissolution (for details see Henrich, 1989; Henrich et al., 1989). Samples, usually taken at 10 20 cm intervals, were analyzed for the composition of their coarse fraction (Heinrich et al., 1989: Wolf, 1991). The sediment samples were dried, weighed and washed on 63 gm sieve. The coarse fraction was further split into 63 125 lam, 125 250 gm, 250 500 I~m, and >500 ~tm subfractions. The 125 500 pm fraction seems to be most representative for coarse fraction composition with respect to biogenic calcium carbonate particles. Thus, a split ( > 5/)0 grains) of this size fraction was studied and counted for biogenic, terrigenous and volcanic components. The grain counts were calculated as weight percentages of the bulk sediment (for details see Henrich et al., 1989). In this study we use the

78

weight-pecentages of terrigenous grains as an IRD-proxy. The processing technique for coccolith analysis using a SEM was described by Baumann (1990). This method attempts to separate qualitatively the clay size fraction by applying a modified Atterberg method. The sediment was suspended in 0.01N NH4 solution to prevent coccolith carbonate dissolution. Every 24 hours the suspension was sucked off, new solution added and homogenized with the sample. This process was repeated until the supernatent was nearly clear. SEM mounts were prepared from each sample. For quantitative analysis, photos of an arbitrarily selected part of the scanned sample were taken and all particles (including up to 500 coccolith specimens) counted. The data were recorded as particle percent (grain%) for the Quaternary and reworked coccoliths, and relative abundances were calculated for Quaternary species. Baumann (1990) showed that these results are comparable to smear slide estimates of coccoliths as described by Backman and Shackleton (1983). Results

Stratigraphy and determination of age models Henrich (1989) provided a first detailed stratigraphic interpretation of ODP Holes 643A and 642B sediment sections of the last 1.0 m.y. on the basis of glacial and interglacial stages. In this study, the Hole 643A record is reinterpreted on the basis of a significantly higher sample density and a high resolution stratigraphic concept for the Hole 644A sediment record is introduced. In the section from 2.6 to 1.0 Ma the stratigraphic framework of both sites is based only on the paleomagnetic interpretation given by Bleil (1989). No paleomagnetic events are missing within Hole 644A, showing a rather continuous and undisturbed section. However, occasionally sections with low recovery and also problems of gas ruptures are documented in the initial reports (Eldholm, Thiede et al., 1987). In the paleomagnetic record of Hole 643A the Olduvai is obviously missing, indicating a hiatus from approximately 1.9 to 1.6 Ma. No additional biostratigraphic

R. HENRICH AND K.-H. BAUMANN

control is available in these sections at both sites, most probably because of an almost persistent strong carbonate dissolution. In the section from 1.0 Ma to Recent the stratigraphy is mainly based on a detailed oxygen isotope record (Wolf, 1991; Wolf et al., 1992) and paleomagnetic data (Bleil, 1989) of Hole 643A. The carbonate and the nannofossil records were compared to this data (Fig. 2a). Glacial/interglacial stage boundaries were established within Hole 644A by a correlation of the typical curve shape patterns of carbonate and the nannofossil abundance patterns, respectively. The stratigraphic distribution of Quaternary calcareous nannofossils from the Norwegian-Greenland Sea are summarized in biozonation schemes (Gard, 1988; Gard and Backman 1990; Baumann, 1990). The zones are well defined by the quality and quantity of typical coccolith species (Fig. 2b). Additional control is supplemented from abundance peaks of autochthonous coccoliths and epibenthic calcareous foraminifers (Figs. 5, 6). However, the best stratigraphic control is attained in the youngest section covering the past 600 ky (oxygen isotope stages 15-1), where a detailed carbonate record, high resolution studies on nannofossil abundances, and a stable isotope record has been determined in the Hole 643A sediment section. The stage boundary levels, all other stratigraphic fix points and the calculated linear sedimentation rates (LSR) are summarized in Table 1. We were able to define specific interglacial events (in particular within isotope stages 7 and 5) on the basis of the carbonate record at both holes. At glacial/interglacial stage boundaries the early interglacial increase in carbonate usually lags after the depletion in 6180 content by a few k.y. Independent confirmation of this stratigraphic concept arises from investigations of benthic foraminifer assemblages in the past 1.0 m.y. section of Hole 643A (Osterman, pers. comm.). High abundances of epi- and shallow endobenthic calcareous foraminifers have been found only in interglacial sediments, e.g. the epibenthic species Cibicidoides wuellerstorfi during stages 19, 17, 7 and 5, and high abundances of the species Eponides sp. and Cassidulina reniforme during stages 15, 13, 11 and 9. A stratigraphic check of the published, widely

NORWEGIAN CURRENT AN[) SCANDINAVIANICE SHEETS DURING PAST 2.6 MY.: EVIDENCE FROM ODP LEG 104 a.

H o l e 643A

Iso (0/~) vs. PDB

Carbonatebulk

(N. pachyderma~in.) 5

4

(wL-%)

3

2

0

H o l e 644A

Isotope

Carbonatebulk

Stages

(wt-%)

10 20 30 40 50 60

10

20

30

79

b.

Isotope

Isotope

Stages

Coccolith Abundance

Stages

Depth (mbsf) in Hole 643A 644A

40

0

I

I 6

[] 8

250

~

..........................................

lO

7.72

25.65

41.19 49.35

13.19

50.75

14.45 15.29

63.25 84.04

16.07

72.73

16.99

74.48

5oo

~"iii~

IxsI

iiiii~ !ii ~iii!i~i::::i '- ::":": '~:'~:'~:;:~~

18.34 20.38

750 Age (ka)

J

21.41

10,12

I

30 Depth (m)

14.26

6.46

11.90

16

~

3.18

12 I 14

B

10.33

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20

0.33

1.98

8.40

l

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0.20

Jo,J Depth (m)

-J

~225LS.:

~

"diverse"

I

C. pelagicus E. huxleyi

:;;':=~

~ ~ ~

83.61 87.29

G. rnuellerae G. caribbeanica Gephyrocapsaspp.

Fig. 2a. Oxygen isotope record of Hole 643A (Wolf, 1991; Wolf et al., 1992) and carbonate records of Holes 643A and 644A. Stage boundaries and specific isotopic events identified in the oxygen isotope and carbonate records are indicated. Paleomagnelic boundaries (Bleil, 1989) are indicated (B/M= B r u n h e s / M a t u y a m a , J = Top Jaramillo). b. ldealised age scheme of nannofossil abundances (arbitrary scale!) and dominating species/species groups compared to ages of oxygen isotopc stages I 19 (the last 750 k.y.). Depths of nannofossil event boundaries in Holes 643A and 644A are indicated.

spaced sample record on benthic foraminifer species distribution in Hole 644A (Osterman and Qvale, 1989) displays high abundances of Cibicidoides wuellerstorfi only during the interglacial stages 19, 15, 9 and 5 and high abundances of Cassidulina laevigata only during stages 19, 15, 13, I1, 9, 7, and 5. Bulk carbonate records, carbonate composition and patterns o['carbonate preservation during the past 2.6 m.v. Carbonate records of both holes will be discussed in their stratigraphic context and trends in the basinward site (643) will be compared with the shelf-proximal site (644). Compositional differences will be deduced from a comparison of bulk carbonate and planktic foraminifer records. Patterns of carbonate preservation are derived

from the SEM-dissolution studies on N. pachvderma sin. Carbonate records in the periodJ?om 2.6 to 1.0 Ma The entire section from 2.6 to 1.0 Ma is almost free of carbonate at the basinward Site 643. Relictic tests of planktic foraminifers almost exclusively belong to the cool adapted species N. paehyderma sin.; most of them are extremely dissolved (Fig. 3). The carbonate record of the shelf-proximal Site 644 is more varied, thus it will be discussed in more detail. The carbonate is predominately composed of coccoliths throughout the whole section. Planktic foraminifers generally contribute less than l wt.-% of total sediment. In the 2.6-1.9 Ma period maximum carbonate values are below 5 wt.-% and long intervals are almost free of carbonate. At about 1.9 Ma the first major carbon-

80

R. HENRICH AND K.-H. BAUMANN

TABLE 1 Age determinations and linear sedimentation rates (LSR) in Holes 643A and 644A since 1.0 Ma. Age determinations in the older section (2.6-1.0 Ma) are mainly based on the paleomagnetostratigraphy (Bleil, 1989) Stage

2/3 3/4 4/5 5/6 6/7 7/8 8/9 9/10 10/11 11/12 12/13 13/14 14/15 15/16 16/17 17/18 18/19 19/20 20/21 21/22 T Jaramillo B Jaramillo

Age 643A (k.y.) Depth (mbsf)

LSR Depth (cm/k.y.) (mbsf)

LSR (cm/k.y.)

27 59 71 128 186 245 303 339 362 423 478 524 565 620 659 689 726 736 763 790 910 980

7.50 2.81 3.75 2.89 4.48 3.69 3.31 2.50 6.52 1.97 2.27 1.74 2.20 1.84 6.10 2.77 6.81 17.90 10.19 1.41 1.78 2.17

15.25 13.75 6.67 12.67 10.34 4.83 14.72 41.30 8.93 3.00 11.09 32.68 6.00 4.10 5.00 17.57 25.00 9.63 8.89 10.33 10.40

0.90 1.80 2.25 3.90 6.50 7.68 9.60 10.50 12.00 13.20 14.45 15.25 16.15 17.16 19.54 20.37 22.89 24.68 27.43 27.81 29.94 31.46

644A

9.00 10.65 14.45 21.80 27.90 30.70 36.00 45.50 50.95 52.60 57.70 71.10 74.40 76.00 77.50 84.00 86.50 89.10 91.50 103.89 111.17

ate peak exceeding 20 wt.-% is recorded followed by moderate carbonate contents ( 5 - 1 0 w t . - % ) during 1.9 1.6 Ma. In the period 1.6-1.3 Ma high amplitude shifts in carbonate contents can be observed with m a x i m u m values exceeding 20-40 wt.-%. Nevertheless, only low contents of well preserved planktic foraminifers were observed, especially within carbonate maxima. The section from 1.3 to 1.0 M a shows a pattern similar to the section from 1.9 to 1.6 Ma, e.g. carbonate contents between 5 wt.-% and 10 wt.-%. Carbonate records in the section from 1.0 Ma to Present Carbonate records during the last 1.0 m.y. (Fig. 5) display high amplitude fluctuations at both sites. Based on changes in carbonate composition, an interval spanning from !.0 to 0.6 M a has been distinguished from the uppermost section. At Site

644 the record from 1.0 to 0.6 M a shows a clear predominance of coccoliths (Fig. 5), although the proportion of planktic foraminifers is greater than during the older period (Fig. 3). In contrast, high amounts of planktic foraminifers and very low contents of coccoliths were observed in Hole 643A. Carbonate peaks in Hole 644A during 1.0 M a to 600 ka (Fig. 5) occur roughly every 40 k.y. M a x i m u m values exceeding 20-30 wt.-% occur at 970 ka and 840 ka. Elevated carbonate contents (around 20 wt.-%) are found during the interglacial stages 21, 19 and 17. The carbonate content between the maxima never exceeds 10 wt.-%. A similar trend is seen in the basinward Hole 643A. M a x i m u m carbonate values of 20-30 wt.-% occur at 970 ka and 840 ka, as well as carbonate maxima of around 20 wt.-% during the interglacial stages 21, 19, and 17. Otherwise, the carbonate contents are below 10 wt.-%. Carbonate preservation in the period from !.0 to 0.6 Ma is much better than in the older parts. Nevertheless, short-term dissolution peaks occur, mostly related to diamicton deposition. In the section 0.6 M a - P r e s e n t an overall much better carbonate preservation is recorded than in the earlier periods (Fig. 4). SEM-dissolution values are lowest in the high carbonate peaks. Nevertheless, a number of short-termed dissolution peaks with indices of 2 4 occur, mostly combined with diamicton deposition. Amplitude shifts of the carbonate records (Figs. 3 and 5) are much larger during the past 0.6 m.y. than observed during the earlier periods. By visual inspection, the succession of m a x i m u m peaks follow roughly every 100 k.y., while minor peaks seem to occur at 20 k.y. intervals, especially during interglacial stages. Highest carbonate contents are recorded consistently at both sites during the peak interglacials of stages 15, 13, 11 and the uppermost stage 5. The other interglacial stages reveal only moderately high carbonate contents at Hole 643A (15-30 wt.-%). At Hole 644A interglacial carbonate values are low within stages 9 and 7 ( < 1 0 wt.-%), and intermediate within stage 5 (15-20wt.-%). In addition, intermediate to high carbonate values were also observed for the lower part of glacial stages 8 and 6, thus giving evidence of strong Atlantic water inflow in the southern

N O R W I , G I A N C U R R E N T ANI) S C A N | ) I N A V I A N ICE S H E E T S D U R I N G PAS] 2.6 M . Y : EVIL)ENCE F R O M ODP LEG 104

Hole 643A Carbonate b u l k

(wt-%)

0

1020304050600

F, [

Hole 644 A Planktic foraminifers

Dissolution Index

(wt,-%) 10

20

Carbonate b u l k

(wt.-%)

30

40

0

1

2

3

0 ,~,o

,~, ,~ ,~,

Planktic foraminifers

D ~ o l u t | o n Index

(wt.-%)

0

10 I

0

20 0 I

1

2

3

5OO

F P 10~0 .b

1500 D

2000

2500

b

Age (kap

Fig. 3. Bulk carbonate, planktic foraminifer and dissolution records from Holes 643A and 644A (2.6 Ma-Present). Bulk carbonate and planktic foraminifer contents are given as weight percentages of bulk sediment weight; dissolution indices are determined on the planktic foraminifer species N. pachyderma sin. (index 0 = best preservation, index 4 = strongest dissolution).

Norwegian Sea during those periods. Glacial stages 14, 12, 10, the uppermost part of 8, most of stages 6 and 4 - 2 generally reveal carbonate values below 10 wt.-%. Within diamictons only very low values of less than 1 wt.-% were measured at both sites. A drastic decrease of carbonate values is also seen within parts of interglacial stages 7 and 5 (Fig. 5). With decreasing bulk carbonate contents consistent changes in carbonate composition are observed. High carbonate values consist either of adequate amounts of coccoliths and planktic foraminifers or display a predominance of coccoliths. Intermediate carbonate contents (e.g. stages 9, lower 8, 7, and 6, and within stage 5) depict considerably higher proportions of planktic foraminifers than coccoliths, while low values are almost exclusively composed of planktic foraminifers. Parallel to the shift from coccolith-dominated,

carbonate-rich sediments to planktic foraminiferdominated, intermediate- to low-carbonate sediments the proportions of subpolar planktic foraminifers decrease and a change from warm-adapted to cool-adapted coccolith species associations is recognized. Variability of coccolith abundances and species distribution during the past 1.0 m. v.. The downcore variations in the total amount of coccoliths show large fluctuations in Holes 643A and 644A (Fig. 5). Generally, interglacial stages are characterized by relatively high abundances, whereas glacial sediments are mostly barren of coccoliths. The coccolith flora at both sites is characterized by a low diversity assemblage. In general, a total of 12 calcareous nannoplankton

82

R. H E N R I C H

Site 643

Site 644

Planktie foraminifers

Dissolution Index

(wt.-%) 0 0

10 Lb._ ~

AND K.-H. BAUMANN

20 ,

'

30 •

'

40

0

1

2

3

4

Occurrence of Diamictons

Planktie foran~nifers (wt.-%) 0

10

~ i

20

Dissolution Index 0

1

2

3

Occurrence of 4

Diamictons

|

/

|

--| /

/

,

,]!

'

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p-

8OO

I. P

i

looo ~._

1200

Age

(ka)

Fig. 4 Planktic foraminifer (wt -%) and dissolution records and diamicton occurrences from Holes 643A and 644A (1.2 Ma-Present). Note that good carbonate preservation (low dissolution indices) is highly correlated with peaks in planktic foraminifer contents, while strongest dissolution is mostly connected with diamicton deposition and minimum planktic foraminifer contents.

species have been identified from the two sites investigated. However, a single species mostly dominates the assemblage, often constituting up to more than 80% of the total nannoflora. Prior to 1.0 Ma coccoliths are found only in sediments of Hole 644A. Sparse occurrences characterize most of the samples investigated from this time interval. Relatively high abundances of calcareous nannofossils were seen at around 1.7 Ma, 1.5 Ma, 1.4 Ma, and 1.35 Ma. The assemblages usually consists of the cold-water species C. pelagicus, but also of the warm-temperate and eurythermal C. leptoporus, P. lacunosa and Reticulofenestra spp. together with minor amounts of other species. The entire section at Site 643A during the interval 2.6-1.0 Ma is almost free of carbonate and barren in coccoliths. Within the interval 1.0-0.6 Ma only minor

amounts of coccoliths were observed in interglacial stages (Fig. 5). However, a single peak with high abundance of calcareous nannofossils was found in Hole 644A representing stage 19 sediments. The assemblage is dominated by G. caribbeanica, associated by minor occurrences of C. leptoporus. The much lower amounts of these species which were observed at the same time interval of Hole 643A record a marked discrepancy between the two sites investigated. Generally, there is a positive correlation between the coccolith abundance, the amount of planktic foraminifers, and the bulk calcium carbonate content (Fig. 5). High values of coccoliths and planktic foraminifers generally equate with high carbonate values, and low biogenic values with correspondingly low carbonate contents. However, the degree of coherency of these data sets is not consistent, especially during

NORWEGIAN CURRENT AND SCANDINAVIAN ICE SHEETS DURING PAST 2.6 MY.: EVIDENCE FROM ODP LEG 104

Hole 643A Carbonate bulk 10

20

0 . .I. .

0

.

30

40

50

Hole 644A Coccoliths bulk (grain-%)

(wt.-%) 60

0

.

~3

10

20

30 40

50

Coccoliths bulk (grain- %)

Carbonate bulk

(wL-%) 60

0

~

10

20

30

40

'1

0

10

20

30

Isotope Stages 40

i

L_

!1 6 8

lm

,o.

lllllll ,

m 12

"

....22i12 .iill2111il.iilli22ill22111

..il.22il.2ili.illl

4

6OO

.....

1._~_8

800

1000

1200

Age (ka)

Fig. 5. Bulk carbonate contents versus bulk coccolith grain percentages (1.2 Ma Present). Note synchronous peaks in both records during peak interglacial conditions, e.g. especially in stages 15, 13, 1I and 5.

the time interval 1.0-0.6 Ma. In Hole 643A high carbonate contents are due to high amounts of planktic foraminifers ( > 10wt.-%), whereas only minor amounts of planktic foraminifers ( < 5 wt.-%) are observed for the same time interval in Hole 644A. Thus, relatively high carbonate values are mostly derived from calcareous nannofossils. During the last 600 k.y. coccoliths are abundant and moderately well preserved in interglacial stages 15, 13, 11, lower 7, and 5 of both sites, whereas stages 9, 8, and upper 7 contain only low amounts of coccoliths. A comparison of the both sites investigated shows a very similar coccolith abundance patterns. The assemblages in stages 15, 13, 11, and 5 are not only marked by the highest amounts of coccoliths (up to 50 grain-%, Fig. 5) but are also characterized by the dominant occur-

fences of species of the genus Gephyrocapsa, such as G. aperta, G. caribbeanica, G. margereli and G. muellerae. Besides, these species are associated with minor occurrences of transitional-subtropical species (C. leptoporus, Helicosphaera carteri, Oolithotus fragilis). In contrast, the assemblages in isotopic stages 8 and lower 7 are dominated by subpolar species C. pelagicus with only minor occurrences of other species. The Holocene sections of both sites are also dominated by C. pelagicus. However, the assemblage also includes the eurythermal species E. huxleyi and minor occurrences of C. leptoporus and S. pulchra. These floral data suggest relatively high carbonate productivity and the presence of warmer surface water masses during most of the interglacial stages of the past 600 k.y. However, the records from both sites also show a clear evolutionary

84 trend within the genus Gephyrocapsa. The variation in the abundance of distinct species is therefore not only based on climatically induced changes, but is also the result of evolutionary processes.

Patterns of pelagic/benthic coupling during the past 1 m.y. Benthic/pelagic coupling, a process recently discovered during synoptic studies of modern pelagic and benthic ecosystems in the Norwegian Sea (Graf, 1989) describes the almost simultaneous transfer of food from the photic zone (export production) and its almost complete consumption at the sea floor by benthic organisms. The benthic communities of the Norwegian Sea seem to be specifically adapted to these seasonal pulses of food supply and develop a high standing stock. With respect to a potential documentation of this phenomenon in the geologic record the observed high abundances of epibenthic and shallow endobenthic foraminifers are most important. Such contemporaneous peaks of planktic and benthic foraminifers might be considered as an indication of a high pelagic/benthic coupling efficiency. Synchronous fluctuations of planktic and benthic foraminifer records are a particular common feature in the Hole 643A record. Elevated contents of planktic and benthic foraminifers (Fig. 6) are recognized at 970 ka and 840 ka as well as during all the interglacial stages, and in addition during lower glacial stage 8. A completely different pattern is recorded in Hole 644A (Fig. 6). Highest peaks of benthic foraminifers occur during glacial stages within sections of low to barren planktic foraminifer contents. Identification of benthic foraminifers down to the species level reveals, that most of the benthics belong to typical shelf assemblages. In particular, the species Elphidium excavatum appears in high abundances (Osterman and Qvale, 1989). Besides, the coarse fraction in the same intervals is dominated by IRD, giving evidence that the benthic foraminifers were also transported by ice rafting. After filtering out these ice-rafted benthic shelf foraminifer peaks, simultaneous peaks of planktics and benthics can also be recognized in the 644A record, especially during intergla-

R. HENRICH A N D K.-H. BAUMANN

cial stages. This indicates a high efficiency of benthic/pelagic coupling during these periods.

Terrigenous ( IRD ) records and reworked nannofossils dur&g the past 2.6 m.y. In the section prior to 1.0 Ma at Hole 643A a fluctuating IRD input (5-15 wt.-%) is generally observed (Fig. 7). Higher values above 20 wt.-% are recorded only at 2.45 Ma, 2.1 Ma, and 1.9 Ma. During the same period IRD contents are also generally low in Hole 644A. Only some IRD pulses exceed 15 wt.-% in the section between 2.2 Ma and 2.1 Ma. During most of the period the IRD record of Hole 644A depicts only minor peaks (generally below 5 wt.-%), a feature which correlates with maximum carbonate contents in the same sections. In contrast, a continuously higher IRD input is observed at Site 643 during the same periods. The IRD records of both holes display much higher amplitudes during the last 1 m.y. (Fig. 7) with a further increase in amplitude at about 0.6 Ma, most obviously seen at Site 644. The highest IRD pulses (20-40 wt.-%) occur within stages 14, 12, 10, lowermost 9, 6, the 6/5 transition, and lowermost 3. Moderate IRD input (5-15 wt.-%) is reflected in both holes during stages 18, 16, lower 8 and within stage 4, as well as in Hole 644A during stages 22 to 15 with the exception of two pulses exceding 20 wt.-% within stages 20 and 18. Reworked specimens of older nannofossils are present in both holes throughout the last 1.0 m.y. (Fig. 8). They are almost continuously present in the sediments with a low content at all levels containing calcareous nannofossils. However, reworked nannofossils become much more abundant in glacials than in interglacials. Maximum contents are recorded within the interval 0.9-0.7 Ma, and within stages 8, 6, and 4-2. The reworking mostly consists of Cretaceous nannofossils but Paleogene and some early Neogene redeposited species are also found. In glacial sediments these nannofossils often comprise up to 100% of the total assemblage.

N O R W E ( J l A N ( ' L R R E N F A N D S C A N I ) I N A V I A N ICE SHEFTS D U R I N G P e S T 2.6 MXt : E V I D E N ( E F R O M ODP LEG 1!)4

Hole 643A Planktic

foraminifers

(wt.-%) 0

lo

20

3o

40

o

~

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Benthic foraminifers

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t

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o

s

lo

xs

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o

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~

--

--

Age (ka)

Fig. 6. Planktic and benthic foraminifer records (wt.-%) from Holes 643A and 644A (I.2 Ma Present). Oxygen isotope stages are indicated. Note synchronous peaks of planktics and benthics during interglacial stages in both holes indicative of most efficient pclagic/bcnthic coupling. In glacial stages the shelf-proximal Site 644 record displays episodic high inputs of glaeiall;~ reworked shell" benthic foraminifcrs, a pattern which is not seen in the basinward Site 643.

Discussion

Reconstruction of surface water regimes and ice drift patterns during the past 2.6 m.y. A broad view of the paleoceanographic evolution oF the Norwegian Sea during the past 2.6 m.y. already has been outlined in a number of previous studies (Jansen et al., 1988, 1989; Henrich, 1989). The principle argumentations presented in these earlier papers are followed here, but with a much closer focus on the evolution of the Atlantic water intrusions and on the different environmental patterns of glacial surface water regimes. We will relate the paleoceanographic reconstructions to the glaciation history of surrounding land masses and discuss them in context with the paleoclimatic

evolution of the Northern Hemisphere. Recently, our data base has been significantly extended and the stratigraphic resolution considerably improved. In addition, we have included high resolution studies on coccolith assemblages in Holes 643A and 644A which will be described in detail elsewhere (Baumann, in prep.). Thus, we are now able to extract much more detailed paleoceanographic information from these records. Generally, the paleoceanographic evolution of the Norwegian Sea can be subdivided into three phases: - - P h a s e 1 (2.6-1.0 Ma), characterized by overall long-persistent moderate glacial conditions and a long-term stability of rather small ice caps over Scandinavia. Phase 2 (1.0 0.6 Ma), revealing intensified

86

R. HENRICH

T e r r i g e n o u s d e t r i t u s ( w t . - %) Hole 643A 10

20

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R e w o r k e d Nannofossils (grain- %)

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03

644A 2 1

0

40

200

200

400

400

600

600

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800

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b 1000

Y

1500 ~'~:'..!~!:..'~!:!:?~~:'.:':.:f<.'.~~?.::i:x:~::?&'.:-::~:~:~:?~:~:~::'.~:~ ~.::.'~'~'6~:::::'~:~:::~(:~::~ ~:::~!~4 :~:::::~:?~:::::::~ ~?'>~"~-::"67~'& ". "~4:::::~':'~ ~:~~ ~!~ ~Y>V:~..~$" :x. . . . . . . . . . . . ~ ~:?~:"~x:::>:-.>~:~ .. ~... =====================

1000

10o0

2OOO ±

i.--2500

r---

1200 Age (ka)

1200 Age (ka)

Fig. 8. Variability of grain percentages of rcworked nannofossils from Holes 643A and 644A between 1.2 Ma-Present. Note peak occurrences during glacial stages.

Age (ka)

Fig. 7. IRD-records (wt.-%) of Holes 643A and 644A (2.6 Ma-Present).

contrasts between glacial and interglacial conditions, where the Norwegian Current gained much more strength during interglacials and more severe glaciations affected the Scandinavian mainland. - - P h a s e 3 (0.6 Ma-Present), with an overall increased meridionality, b r o a d Atlantic water intrusions reaching far up into the high Arctic and strong glaciations on the land masses surrounding the Nordic Seas.

Phase 1 (2.6-1.0 Ma) During phase 1 long term persistent moderate glacial conditions prevailed. Evidence comes from the almost continously low amplitude fluctuating

input of IRD at Site 643 throughout the entire period (Fig. 7). A generally low carbonate production in surface waters is evidenced by sparse occurrences of strongly dissolved planktic foraminifer tests, all belonging to the cool adapted species N. pachyderma sin., while autochthonous coccoliths are missing. Similar conditions are also recorded at Site 644 over long periods (Fig. 7). Thus it may be assumed that the polar front was situated south of the studied area, presumably south of the Greenland-Scotland Ridge. Surface water regimes may have been dominated by a closed sea ice cover in winter season, while during summer some opening of the ice cover occured and icebergs drifted around. So far, this picture is consistent with earlier interpretations (Jansen et al., 1988; Henrich, 1989). However, the relatively high contents of nannofossils at about 1.7 Ma, 1.5 Ma

N O R W E G I , ~ N C U R R E N T A N D S C A N D I N A V I A N ICE S H E E T S D U R I N G PAST 2.6 M Y :

and 1.35 Ma, as well as generally higher carbonate levels in parts of the remaining section indicate at least episodic intrusions of Atlantic water covering the eastern Voting Plateau, but not its margin. Silicoflagellate assemblages in the carbonate rich horizons of Hole 644A display warm-associated species (Locker and Martini, 1989). The nannofossil assemblages also indicate at least short term intrusions of relatively warm Atlantic surface water. Hence, we have to assume, that the polar front was situated between Site 644 and 643 on the Voting Plateau with a small tongue of northward-bound Atlantic water close to the shelf margin (Fig. 9). Such a situation would also help to explain the continuous supply of IRD and the episodic low amplitude IRD pulses during diamicton deposition recorded at both holes (see Fig. 4) because there is need for small-scale waxing and waning of the inland ice sheet in order to compensate for the loss of ice volume calved off by icebergs. Furthermore, a seasonal weak underflow of Atlantic water could cause small scale openings during the moderate glacial conditions and would thus promote both IRD dispersal by drifting icebergs and moisture supply needed for the increase of ice sheet volume. The ,overall extraordinary light 6180 isotope values of planktic foraminifers in the entire section prior to 1.0 Ma in Hole 644A (Jansen et al., 1988, 1989) would therefore display the lowered salinities in the summer configuration. Hence. the period 2.6 1.0 Ma is characterized by a glacial/interglacial mode of changes in surface water conditions operating at the lower boundary conditions of ocean/atmosphere response in the Norwegian Sea. Comparing these rather restricted environmental conditions in the Norwegian Sea with the North Atlantic records a strong zonal gradient becomes evident. The North Atlantic records display strong large scale glacial/interglacial shifts in surface water regimes during the entire period from 2.6 to 1.0 Ma (Shackleton et al., 1984; Ruddiman et al., 1986; Aksu et al., 1989; Raymo et al., 1989). As a result, the circulation followed a much more zonal pattern and the overall exchange between the North Atlantic and the Norwegian Sea was much more restricted than during the younger periods. How do these paleoceanographic results fit with

E V I D E N C E F R O M O D P LEG 1(14

g7

land records of early glaciations? According to the proposed model ice sheets over Scandinavia and Greenland which extended onto the shelves would be expected. There is no evidence of such early glaciations in Scandinavia because the history of older glaciations have been completely removed by younger glacial advances. However. these early glaciations are preserved nicely on the peninsula of Tjornes on Iceland. At least 7 glacial adavances and retreats are documented in the glaciomarine section during the period from about 2.0 to 0.7 Ma (Einarrson and Albertsson, 1988). Intercalated marine mollusc beds record intermittent phases with open Arctic water conditions.

Phase 2 (1.0-0.6 Ma) The onset of phase 2 at about 1.0 Ma is marked by a series of strong environmental shifts in surface water regimes, i.e. the first major increase in carbonate shell production. Contemporaneous maxima of planktic foraminifers and calcareous benthic foraminifers were observed, giving evidence of a high efficiency of pelagic/benthic coupling. Rather warm-adapted coccolith species and silicoflagellate assemblages (Locker and Martini, 1989) reveal relatively high surface water temperatures. The nannoflora observed are generally dominated by various species and varieties of the genus Gephyrocapsa during the last 1.0 Ma. However, remarkable low abundances of these species were observed within the interval 1.0-0.6 Ma. In addition, the assemblages are characterized by an almost complete absence of medium and large Gephyrocapsa, while C. leptoporus, P. lacunosa, and Reticulofenestra spp. are the prominent members of the flora. These assemblages generally correspond to the flora in the upper portion of the "small" Gephyrocapsa zone of Gartner (1977). "Small" Gephyrocapsa are the dominant nannofossil group in the middle Pleistocene (Gartner, 1977, 1988; Pujos, 1988). The long acme of this group is often used stratigraphically, although its spatiotemporal distribution shows a shift from high (1.3 1.0 Ma) to low latitudes (1.0-0.65 Ma) and "small" Gephyrocapsa species are generally much more abundant at high latitudes (Pujos, 1988). Thus, the low contents of coccoliths in the assemblages of ODP Leg 104 Sites are mostly due to the

88

R. HENRICH AND K.-H. BAUMANN

20oW

10ow

0o

0o

20OE

IOOE

Continental Ice Sheet Margin t=~=... -~`~1, Assumed position of Polar Front (summer situation)

,'i::i','?~'! sea ice cover

Ice Drift Currents ~,~

Atlantic Surface Water

Masses

Fig. 9. Tentative model of surface water circulation during weak interglacial periods in the interval from 1.6 to 1.3 Ma. Also indicated is the inferred configuration of continental ice sheets and the position of the polar front. Note the northward penetration of a small tongue of Atlantic water close to mid Norwegian continental margin.

ecological factors, limiting the paleo-biogeographic occurrence of the worldwide dominating species. Furthermore, nannofossil occurrences were limited to interglacial periods. Ruddiman and McIntyre (1976) already correlated the total nannofossil abundance with the influence of glaciations. The barren intervals are correlated with glacial periods and caused by low productivity in surface waters. However, much consistent evidence for a first major and broad Atlantic water intrusion and the development of a far north-reaching Norwegian Current can be seen centered around 1.0 Ma. Simultaneously, this also marks a major threshold

in the Northern Hemisphere climatic oscillations with the onset of large scale climatic shifts and strong environmental contrasts in glacial and interglacial surface water regimes. A major increase of glaciation intensity testified by the Leg 104 sediment sections by the onset of high amplitude shifts in the IRD records (see Fig. 7, compare similar results in Krissek, 1989; Henrich, 1989; Jansen and Sjoholm, 1991) is in good accordance with a general increase of glaciomarine sediment input into the deep ocean. Bohrmann et al. (1990) have also observed a major change in accumulation rate of bulk sediment at about 1.0 Ma. Despite the

N O R W E G I A N C U R R E N I ' A N D S C A N D I N A V I A N ICE S H E E T S D U R I N G PAST 2 6 M.Y.: E V I I ) E N ( E F R O M O D P I,EG 104

clear environmental shift at the onset, phase 2 bears in many ways a transitional character between the zonal mode of circulation during phase 1 and the strong meridionality of phase 3. The transitional character of phase 2 is reflected by numerous trends, e.g.: carbonate is still dominated by coccoliths in Hole 644A, while in Hole 643A planktic foraminifers are much more abundant (Figs. 5, 6). In contrast to Phase 1 minor but significant amounts of pelagic carbonates were also observed. the interglacial high carbonate zones contain minor but significant amounts of subpolar planktic foraminifers (Spiegler and Jansen, 1989). a trend to less depleted 6 ~ O and 6~3C values in planktic foraminifer tests in Hole 644A (Jansen et al., 1989). higher frequencies of chalk dropstone occurences and reworked nannofossils (Fig. 8) in glacial and deglacial sediments, giving evidence both of glaciations reaching further south and intensified drift of icebergs towards the north. In conclusion, the meridional circulation mode was probably developed during phase 2. An efficient exchange of surface water masses with the Atlantic occurred during interglacials while a more effective seasonal opening of the sea ice cover during glacial summers allowed a transfer of heat and moisture to the Scandinavian ice sheets along the eastern side of the Norwegian-Greenland Sea. Despite the overall increased surface water exchange with the North Atlantic, the Norwegian Current attained only moderate temperatures during the inllerglacial stages. Phase 3 (0.6 Ma Recent) During phase 3 the highest efficiency in oceanographic/atmospheric response to changing orbital parameters was observed. This is indicated by large amplitude and high frequency shifts of carbonate and terrigenous records. As a consequence, a very high variability of surface water regimes is found. Following is a compilation of the wide spectrum of glacial and interglacial circulation patterns which have been identified during the past 0.6 m.y. The spectrum of different surface water regimes in the Norwegian Sea includes: - - " v e r y warm" interglacials (stages 15, 13, 11. 5.5, and 1) with a broad inflow of Atlantic waters

Nt)

evidenced by maximum carbonate contents (Figs. 5 and 6), high amounts of subpolar planktic foraminifers (Spiegler and Jansen, 1989: Henricb et al.. 1989) and high abundances of coccoliths (Fig. 5), including subtropical species. ~'temperate" interglacials (stages 9, parts of stages 7 and 5) with a rather small extension of Atlantic waters evidenced by intermediate to high carbonate values with a predominance of planktic foraminifers (Fig. 6), relatively low contents of subpolar planktics and high abundances of the cool adapted coccolith species C. pelagicus. --pronounced cool phases during interglacials (substages 7.4, 5.2) indicated by a drastic decrease of carbonate contents (see Fig. 2a) as well as increased IRD contents. The carbonate distribution indicates only a very narrow inflow of Atlantic waters into the southeastern Norwegian Sea. while the coastal areas off Norway were still affected by iceberg drift. a weak inflow of cool-temperate Atlantic waters during glacial stages (lower stages 8 and 6) characterized by relatively high carbonate contents dominated by planktic foraminifers (Fig. 6) with only minor proportions of subpolar planktics. --glacial circulation patterns (e.g. episodically within stages 14, 12, 10, 6, 4, and 2) characterized by episodic high inputs of IRD and a wide coverage of diamicton deposition (Fig. 4), which are characterized by sections nearly free of carbonate. Surface water regimes during the IRD events are characterized by intensive iceberg drift and devclopment of a meltwater lid proximal to the tidewater ice margins. The diamictons mark the episodic phases of highest instability of the Scandinavian ice sheets, when the ice margin was far advanced on the shelf or had already overridden the shelf edge (see Henrich et al., 1989" Henrich, 19901. Causes for the instability of the marginal ice front on the shelf may be variable, e.g. large scale surges, ice margin instability because of small scale sea level oscillation, and, in cases when the ice margin overrides the shelf edge, directly feeding of icebergs into the deep-sea. Deep water properties durinjz the past 2.6 m.v.

Today, deep water formation in the Norwegian Greenland Sea is a major element of the

90

global conveyor belt (Broecker et al., 1988). Changes in deep water properties are evident for global deep water circulation and contribute very essential information on overall ocean/atmosphere responses during glacials and interglacials. However, Leg 104 sites are located in an area in which no modern deep convection is observed. Thus, only indirect evidence on deep water formation can be drawn from these sites. Nevertheless, the Leg 104 planktic and benthic stable isotope and dissolution records indicate drastic changes of deep water properties in the Norwegian Sea throughout the past 2.6 m.y..

Phase 1 (2.6-1.0 Ma) In Hole 643A persistent strong dissolution is recorded throughout the entire period 2.6-1.0 Ma (Fig. 3). In sediments of Hole 644A very bad carbonate preservation was also observed over long periods. However, intermittently good carbonate preservation is seen, especially within sections of high carbonate contents from 2.0 to 1.9 Ma and 1.6 to 1.3 Ma. Carbonate preservation is always depending on the rate of carbonate shell production in surface waters and the degree of carbonate saturation in the water coloumn, respectively. Severe dissolution may also be observed if water column and bottom waters were only weakly undersaturated with respect to carbonate but the rain of carbonate tests from surface waters was very low. Increasing the supply of carbonate tests would result in a considerable improvement of preservation without any change in deep water properties. In contrast to earlier views (Jansen et al., 1988, 1989; Henrich, 1989), we now favor the above outlined interpretation based on the following argumentation: - - A low production of tests of the cool adapted planktic foraminifer species N. pachyderma sin. in surface waters under moderate glacial conditions would result in a low carbonate rain to the sea floor. Only during periods of Atlantic water inflow would the carbonate rain (mostly coccoliths) increase and thus improve the preservation. --Frequent occurences of sponge spicules in Hole 644A throughout the period 2.6-1.0 Ma (see Henrich et al., 1989) also suggest well-oxygenated bottom waters. Benthic foraminifers are recorded

R. HENRICH AND K.-H. BAUMANN

in the entire section of Hole 644A from 2.6 to 1.0 Ma, but so far only a single occurrence of the epibenthic species Cibicidoides wuellerstorfi has been reported (Osterman and Qvale, 1989). Therefore, the data on benthic organisms suggest oxygenated waters throughout the section, but different environmental properties than at present. --The benthic foraminifer 6180 records in the Norwegian Sea (Jansen et al., 1989) display generally heavier values than those determined in the North Atlantic (Site 610) throughout the period from 2.6 to 1.0 Ma. This indicates colder bottom water temperatures in the Norwegian Sea and a persistent exchange of deep water with the Atlantic. The mostly more negative 613C of the Norwegian Sea benthics indicates an increased nutrient level. A similar depletion is reflected in the 613C record of the planktic foraminifers together with very light 6180 values, suggesting a stabilized surface water mass with lowered salinities. These negative 613C signatures of planktics and benthics might be taken as evidence for sluggish deep water exchange with nutrient enrichment. However, the heavier ~180 of the Norwegian Sea benthics suggest a persistent deep water exchange with the North Atlantic. A solution of this problem may be a different mode of deep water formation, for example by brines formed during winter sea-ice growth. By such a process the 613C of the surface water mass could be transfered to the benthics (Jansen and Veum, 1990). The volume of deep water formed by these processes should be much lower than those of today.

Phase 2 (1.0 Ma-0.6 Ma) During phase 2 the dissolution records display a transitional pattern with increased improvement of preservation through the section (Fig. 4). This trend goes together with a general increase of planktic foraminifers and the first occurences of significant amounts of subpolar planktics (Spiegler and Jansen, 1989). In the younger part of the section an overall good preservation also during glacial stages is observed, while short termed dissolution peaks appear to be strictly confined to phases of diamicton deposition. During interglacials synchronous maxima of planktic and benthic foraminifers (Fig. 6) were observed, indicating a

N O R W E G I A N C U R R E N ' I A N D S C A N D I N A V I A N ICE S H E E T S D U R I N G PAST 2 6 M Y :

high efficiency of pelagic/benthic coupling with deep water properties and exchange rates more similar to those of today.

Phase 3 (0.6 Ma Present) During phase 3 mostly good carbonate preservation (Fig. 4) was observed during glacial and interglacial stages. Dissolution maxima record short termed episodes with corrosive bottom waters developed under a meltwater lid (Henrich and Thiede, 1991). Synchronous maxima of planktic and benthic foraminifers during peak interglacials and frequent occurrences of epibenthic species Cibicidoides wuellerstor~ in most interglacial sections indicate that deep water properties and exchange rates were similar to those of today. The generally good carbonate preservation, which is observed also during glacials, points to an effective exchange of deep water. However, a less diverse and less abundant benthic fauna during glacials (Fig. 6; see also Struck, 1992) evidences different environmental parameters compared with interglacial conditions. This may display changes in the degree of seasonal food export to the deep ocean due to the generally less productive surface waters during glacials. Considering the obvious changes in surface water circulation with a generally lower salt import because of less efficient Atlantic water inflow during glacials, a different mode of deep water formation is required to explain all observed features. One possible mode would be large scale brine formation by winter sea ice growth in areas along the eastern margin of the Norwegian Sea, in which open water conditions developed during the summer season due to a weak inflow of Atlantic waters. Factors o['external and internal forcing in northern hemi~v~here climate evolution during the past 2.6 tti. I'.

In the ODP Leg 104 records as well as in the North Atlantic records we recognize glacial/interglacial shifts throughout the entire period of the past 2.6 m.y., which seems to reflect the cyclic: fluctuations of earth orbital parameters on the Milankovitch frequencies. In the North Atlantic a strong power of the 41 k.y. obliquity cycle is

E V I D E N C E F R O M O D P k E G 11)4

(~]

obvious in the planktic and benthic oxygen isotope and carbonate records during the period from 2.5 Ma to about 0.8 Ma, while during the past 0.8 m.y. the 100 k.y. cycle as well as the 19 23 k.y. precession cycle dominates (Shackleton et al., 1984: Ruddiman et al., 1986). A similar change is reflected in the Norwegian Sea records (Jansen et al., 1989), with a transitional zone ranging from about 1.2/1.0 Ma to about 0.6 Ma. Prior to about 1.0 Ma the glacial/interglacial changes in the oceanic response of the Norwegian Sea Io the Milankovitch forcing appear to operate at its lower limit with only very small amplitude changes. Such a pattern is evident, both for shifts in surface water regimes as well as in changes in deep water properties. Only temporally and spatially limited Atlantic water intrusions into the Norwegian Sea occurred (e.g. first weak interglacials). Long lasting carbonate dissolution occurred as a result of very low carbonate production in surface waters and a lower rate and different mode of deep water exchange with the North Atlantic. During the period 1.0 0.6 Ma the oceanic response in the Norwegian Sea to the Milankovitch forcing increased with time. The pattern of strong, large amplitude glacial/interglacial shifts with maximum peaks occuring roughly every 100 k.y. and minor peaks at about 20 k.y. spacings evolved. Again, these changes were recognized contemporaneously in variable glacial/ interglacial surface and deep water regimes. Ruddiman et al. (1989) and Ruddiman and Raymo (1988) have proposed that changes in uplift rates and bypassing of critical levels of altitude of high mountain ranges excerted a strong internal forcing on the fluctuations of Northern Hemisphere climate patterns. Specifically, the high atmospheric circulation would have been influenced by increased mountain uplift, so that storm tracks of westerlies would be deflected southward and carry polar air far into the interior parts of the North American and Asian continents, which in turn increased growth rates of mountain glaciers. Kutzbach et al. (1989) and Ruddiman and Kutzbach (I 989) discussed the effects ot" these changes based on an atmospheric circulation model. They claim a strong decrease of temperature and an increase of aridity in the interior of the northern continents coupled with increased aridity of the Mediterra-

92

nean and the southwestern North American states. Following this concept, the observed shift from strongly zonal to increased meridional circulation patterns between the North Atlantic and Norwegian-Greenland Sea could monitor the oceanic responses caused by the increased uplift rates of high mountain ranges. Nevertheless, because the Norwegian Sea paleoceanographic records document very drastic and simultaneous changes in surface and deep water properties, efficient oceanic feedback mechanisms could play a much more important part in the responses of the climate system to orbital forcing.

Conclusions Reconstructions of surface and deep water regimes in the Norwegian Sea based on evidence from biogenic and terrigenous records of ODP Holes 643A and 644A reveal a phasewise evolution in oceanic response of the Norwegian Sea to climatic forcing. The oceanic response was only weak in the early phase and increased progressively throughout the past 2.6 m.y.. During phase 1 (2.6 1.0 Ma) surface water regimes record long lasting moderate glacial conditions and long-term stable small ice caps on the surrounding land masses. Episodically, small dimensioned Atlantic water intrusions penetrated into the Norwegian Sea within a narrow tongue along the eastern margin (Hole 644A). The polar front was most probably situated on the outer Voring Plateau between Site 644 and 643 during these time intervals. Deep water regimes reflect long-term persistent corrosive bottom waters most probably due to a weakly undersaturated water coloumn and a low rate of carbonate shell production in surface waters. Deep water production in the Norwegian-Greenland Sea may have operated on a different mode, e.g. brine formation during winter sea ice growth. Bottom waters were oxygenated throughout the entire period, and deep water was exchanged persistently with the North Atlantic. In phase 2 (1.0 0.6 Ma) increased glacial/interglacial environmental contrasts are documented reflecting an overall gain in the strength of the Norwegian Current and intensified glaciations in

R. HENRICH AND K.-H. BAUMANN

Scandinavia. During this time interval a shift in the modes of deep water production and exchange rates occured with the transition to characteristic glacial and interglacial shifts in deep water properties and modes of deep water formation. Phase 3 (0.6 Ma present conditions) marks the onset of large amplitudes in glacial/interglacial environmental conditions with maximum contrasts in surface water regimes, different modes of deep water production, and exchange rates with the North Atlantic. A broad development of the Norwegian Current is observed during peak interglacials, while during glacials seasonally variable sea ice cover and iceberg drift dominate surface water conditions.

Acknowledgements We gratefully acknowledge the very constructive reviews by D. Fiitterer (Bremerhaven) and E. Jansen (Bergen). G. Bohrmann read an early version of this paper and made valuable suggestions. P. Goldschmidt improved the English of the manuscript. For technical assistance we thank S. Schulz and T. Reincke. This research was financially supported by the Deutsche Forschungs Gemeinschaft (grant "He 1671/2").

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