Variations in surface water mass conditions in the Norwegian Sea: Evidence from Holocene coccolith and dinoflagellate cyst assemblages

Marine Micropaleontology, 20 ( 1992 ) ! 29-146

t29

Elsevier Science Publishers B.V., Amsterdam

Variations in surface water mass conditions in the Norwegian Sea: Evidence from Holocene cocco!ith and cyst assemblages K.-H. Baumann and J. Matthiessen GEOMAR, Research Center for Marine Geosciences, Wischhofstrafle I-3, D-2300 Kie114. Germany (Received January 31, 1992; revision accepted September 9, 1992 )

ABSTRACT Baumann, K.-H. and Matthiessen, J., 1992. Variations in surface water mass conditions in the Norwegian Sea: Evidence from Holocene coccolith and dinoflagellate cyst assemblages. Mar. MicropaleontoL, 20:129- ! 46. Coccolith and dinoflagellate c;/st assemblages have been investigated in five sediment cores from the Norwegian Sea and Fram Strait. Both fossil groups~are characterized by similar patterns of composition. The assemblages contain high proportions of single species. The coccolith flora is of low diversity and consists almost entirely of Coccolithus pelagicus and Emiliania huxlevi. The dinoflagellate cysts are generally dominated by Operculodinium centrocarpum and Nematosphaeropsis labyrinthus. Other species, especially Bitectatodinium tepikiense, Peridinium faeroense and hnpagidinium pallldum, sometimes contribute considerably to the assemblages. Based on the abundance of the assemblages and the ratio change between the dominating species it has been possible to establish three intervals of distinct major changes in surface water mass conditions. Sparse occurrences of coccoiiths and dinoflagellate cysts have been observed before 10,000 yrs B.P., indicating harsh environmental conditions with a distinct influence of meltwater and temporarily very slight inflow of Atlantic water. The modern surface-water circulation pattern was reinitiated during Termination IB. The assemblages suggest slightly cooler and probably less saline surface water conditions than are present today until 7500 yrs B.P. Solar insolation may have caused a first temperature peak which is responsible for the early Holocene productivity maximum. A considerable change in the composition of dinocyst and coccolith assemblages occurs corresponding approximately to the onset of the Holocene climatic optimum. This change was most probably linked to an almost synchronous reorganization of the hydrographic properties in the entire North Atlantic realm after the ice sheets had vanished. Since 6000 yrs B.P. the Norwegian Current with its modern oceanographic and ecological properties has been fully established.

Introduction The Norwegian-Greenland Sea is an area of remarkable meridional and zonal hydrographical contrasts in surface water conditions. Relatively warm and saline North Atlantic water penetrates across the Iceland-Scotland Ridge forming the Norwegian Current, which cools substantially on its way north and then sinks Correspondence to: K.-H. Baumann, GEOMAR, Research Center for Marine Geosciences, Wischhofstra3e 13, Building 4, D-2300 Kiel 14, Germany.

and flows as a subsurface current into the Arctic Ocean. Cold, less saline, seasonally ice-covered polar waters intrude from the Arctic Ocean through the Fram Strait (Fig. 1 ), forming the East Greenland Current. Arctic surface water masses cover the central part of the Greenland and Iceland Sea, forming the Arctic domain (Swift, 1986), which consists of Atlantic and Polar waters. Phytoplankton, especially coccolithophorids and diatoms, are a sensitive indicator of these environmental conditions (cf. Paasche, 1960; Samtleben and Bickert, 1990; Samtle-

0377-8398/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

130

K.-H. BAUMANN AND J. MATTHIESSEN

30*

0*

30"E

75'

70c

65'

60' Fig. !. Site locations (indicated by dots) and schematic distribution of hydrographic features as well as present major surface water circulation in the Norwegian-Greenland Sea (EGC= East Greenland Current; NC= Norwegian Current; Vo. PI. = Voring Plateau; Jan M. = J a n Mayen ).

ben and SchriSder, 1992) because they are primary producers in the photic zone directly dependent on temperature, salinity and nutrients and, thus, refect the properties of surface waters. It has been shown that the distribution of major fossilizable phytoplankton groups (coccoliths, diatoms and dinoflagellate cysts) in recent sediments from the Norwegian-Greenland Sea is related to the properties of surface water masses (Eide, 1990; Ko~ Karpuz and Schrader, 1990; Schrader and Ko~ Karpuz, 1990; Matthiessen, 1991; Samtleben and SehrSder, 1992). Therefore, these groups are potentially sensitive proxy indicators of past changes in surface water mass conditions. This has already been proven for diatoms in Holocene sediments from the Norwegian Sea (Stabell, 1986, 1987; Ko~ Karpuz and Schrader, 1990), whereas coccoliths and dinoflagellate cysts have mainly been used for stratigraphic

purposes and to depict a more general view of oceanographic changes in high latitude deepsea sediments (cf. Belanger, 1982; Bujak, 1984; Harland, 1984; Gard, 1988; De Vernal and Mudie, 1989a,b; Mudie, 1989; Baumann, K.H., 1990; Baumann, M., 1990; Gard and Backman, 1990; Mudie et al., 1990). To fill this gap a joint study of coccoliths and dinoflagellate cysts has been conducted on sediment cores located in an area influenced by the Norwegian Current (Fig. 1 ). Cores were selected for this study which have a well-constrained chronology for the last 15,000 years, thus enabling us to make a detailed investigation of the last deglaciation and the Holocene. Special emphasis is placed on the Holocene record because this period has not yet been analyzed in much detail in sediments from the Norwegian-Greenland Sea. The paleoceanographic implications of these analyses will be discussed and the causes of floral changes will be evaluated. it must be kept in mind that coccoiiths and dinoflagellate cysts are different fossil states of living coccolithophores and dinoflagellates. Coccoliths form the coccosphere, the outer calcareous wall of a single algal cell. In contrast, many species of dinoflagellate cysts are most probably hypnozygotes in the sexual life cycle of dinoflagellates (Dale, 1983). Therefore, the numbers and biological responses of coccoliths and dinoflagellate cysts cannot be directly compared. Nevertheless, a general comparison in a paleontological context seems very appropriate. Material and methods

Regional setting of cores The late Pleistocene and Holocene sections of six sediment cores which were collected from five stations by RV Polarstern in 1983 (Augstein et al., 1984) and in 1984 (Gersonde, 1986) and by RV Meteor in 1986 (Gerlach et al., 1986) have been studied (Table 1 ). A composite section of two cores has been used

SURFACEWATERMASSCONDITIONSIN THE NORWEGIANSEA

131

TABLE 1 Site locations and water depths of sediment cores investigated in !his study Cruise

Meteor 2/2

Polarstern ARK I/3 Polarstern ARK Ill/3

GKG GKG GKG/KAL GC GKG

Core

Longitude

Latitude

Water depth

23059 23062 23071 23199 21295

70:18,3'N 68°43,7'N 67°05,1'N 68:22,6'N 77:59,5'N

03:07.3'W 00:I0,1'E 02:54,4'E 05:13,5'E 02:25,2'E

2283m 2244m i306m 1968 m 3112m

G K G =box core, GC = gravity corer, KAL= large box corer.

from site 23071. Four sites are located along a transect from the Voring Plateau to Jan Mayen influenced by surface waters of the Norwegian Current, whereas core 21295 was taken in the southern part of the Fram Strait (Fig. 1 ). The Holocene section of the cores usually consists of brown to grey-brown slightly silty foraminiferal mud and foraminiferal-nannofossil ooze with abundant Pyrgo spp. and low amounts of coarse terrigenous t~,ebris. This sediment is often underlain by grey to greyishbrown sandy to silty mud. Occasionally, coarse lithic dropstones are observed in this sediment (Henrich, 1986; Ramm, 1988; Henrich et al., 1989; Kassens, 1990). For comparison, we have attempted to analyze samples from the same stratigraphic level for both coccoliths and dinoflagellate cysts. This could be achieved for all of the sediment cores except for 23199. Sediments were sampled at an average spacing distance of 2 to 5 cm depending on sedimentation rates.

Stratigraphy Detailed oxygen isotope analyses on tests of the planktic foraminifera Neogloboquadrina pachyderma sin. (Jones and Keigwin, 1988; Ramm, 1988; Vogelsang, 1990) provided the chronostratigraphic basis, indicating that Terminations L. and Ia (Bmecker and Van Donk, 1970; Duplessy et al., 1981 ) are recorded in all of the sediment records (Fig. 2). Additionally, absolute age control is given by accelerator

mass spectrometry (AMS) ~4C-datings on sediment cores 21295 (Jones and Keigwin, 1988 ), 23071 (Vogelsang, 1990)and 23199 (Ramm, 1988 ) and conventional 14C measurements on records of 23059 and 23062 (Vogelsang, 1990). Contamination by detrital carbonate cannot be excluded when using the conventional radiocarbon dating method (Vogelsang, 1990). The same holds true for the 14C-AMS ages taken from Ramm (1988) which were measured on samples "'--'*~;"~"- d ~ r , ~ n t ,-~1. careoos foraminifera species. However, it is assumed that conventional ~4C-datings and ~4C-AMS datings are comparable despite different samples and methods of measurement. The age model of 21295 has been adopted from Jones and Keigwin (1988). The calculations of sedimentation rates in sediment cores 23199 and 23071 are based on the t4C-AMS ages and an assumed core top age of 2000 years, which is in good agreement with conventional ~4C-ages measured for the upper centimeter of cores from V~ring Plateau and Jan Mayen fracture zone (Vogelsang, 1990). The age models for cores 23059 and 23062 were taken from Vogelsang (1990) for the late glacial, whereas the Holocene section was dated exclusively with the conventional ages. Ages between these measurements have been calculated from sedimentation rates by linear interpolation. This method may be problematic when applied to Terminations because variations in sedimentation rates must be assumed. However, sediment core 21295 from ~,UJLIt

L I ~ . A I L I I J I , t , I I ~ I) ~ A A

A~,A

~,,A,It t,. -....~,*~

132

I ~ - H . B A U M A N N A N D J. M A T T H I E S S E N

o~1'0 (%o vs. PDB) 212N 5,0 4,0 3,0 2,0 o "" ~ i

t i

10 t

I<

!*

230~ S,0 4.0 3,0 a,0

3410yrs.

._.

23062 5.0 4.0 3.0 2,0

23071 05,0 4,0 3.0 2.0

23199 5.0 4.0 3.0 2.0

10

A

40

•

: ,= 40

144e0yrs,I 1S2~30YrS'i

40

1O0"~"~zr•

~ 9 0 yrs.

1ooo "'~Ju yrs'l

Fig. 2. Oxygen isotope records of iV. pachyderma sin. in Voting Plateau and Fram Strait sediment cores. Isotopic data for sediment cores are from Jones and Keigwin ( 1988 ): 21295 (only few AMS ~4C ages are indicated); from Ramm ( 1988 ): 23199; and from Vogeisang ( 1990): 23059, 23062, 23071. Termination IA is indicated by lower shaded scctions; upper shaded sections represent Termination Is.

the Fram Strait, which has been dated by ~4CAMS measurements throughout the whole record (Jones and Keigwin, 1988), shows only small variations in sedimentation rates throughout the last 15,000 years.

Preparation methods The processing technique for coccolith analysis using the Cambridge Stereoscan S150 Scanning Electron Microscope (SEM) was described by K.-H. Baumann (1990). This method attempts to separate qualitatively the clay size fraction by applying a modified Atterberg method. The sediment was suspended in 0.01 N 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 are recorded as panicle percent (grain-%) for the Quaternary and reworked coccoliths and

relative abundances were calculated for Quaternary species. K.-H. Baumann (1990) showed that these results are comparable to smear slide estimates of coccoliths as described by Backman and Shackleton (1983). The samples for dinoflagellate cyst studies were processed using standard palynological methods involving cold HCI and HF to remove carbonates and silicates from the < 150 /~m fraction (Barss and Williams, 1973; Doher, 1980; Phipps and Playford, 1984). After each treatment with acids the residue was sieved with 10 s m gauze. Oxidation was not used in order to prevent the loss of more fragile protoperidinoid cysts (Dale, 1976). Strewn slides of the residues were prepared and analyzed under the light microscope. The nomenclature of the species has been taken from Lentin and Williams (1989). Absolute concentrations (cysts/gram dry sediment) were calculated for all Quaternary dinoflagellate cysts according to the Stockmarr (1971) method and the percentage abundances of selected Quaternary dinoflagellate cyst species have been plotted. These species usually comprise 85-90% of the total assemblages. Species which are only present in minor amounts and which have rarely

SURFACE V,~ATERMASS CONDITIONS IN THE NORWEGIAN SEA

] 33

been seen in the cores are not discussed in this paper. Reworked pre-Quaternary dinoflagellate cysts, pollen and spores are plotted as percentages of the total amount of Quaternary dinoflagellate cysts.

In the late Holocene its abundance is usually less than 60% in every sample. However, core 21295 has a relatively uniform flora with constantly more than 80% of C. pelagicus. The second increase in the amount of coccoliths is primarily due to an increase of Emiliania huxleyi. This is clearly documented in sediment cores 23062 and 23199, with 23059 and 23071 showing only a small increase in this species. The percentages of E. huxleyi remain less than 25% in early Holocene records and reach up to 40% in the late Holocene strata (Fig. 3). Other species recorded consistently in the samples are Calcidiscus leptoporus, Gephyrocapsa muellerae and Syracosphaera pulchra (summarized as "varia" ). Together they form less than 20% of the flora and never exceed 30% in most of the samples. In the early Holocene they reach maximum abundances in all sediment cores, most obviously seen in 23062 and 23071. This increase is related to a slight decline in the abundance of C. peiagicus at this

Results

Coccoliths In general, late glacial sedimems investigated are characterized by rare occurrences of coccoliths (Fig. 3). Termination IB is marked by a distinct increase in absolute numbers of Quaternary coccoliths (grain-°) reaching a maximum in early Holocene sediments. All records show a significant minimum at the transition from early to late Holocene, whereas highest abundances of coccoliths are found in the late Holocene. The coccolith floras are dominated by Coccolithus pelagicus, which usually comprises more than 60% in the early Holocene samples.

Relative Abundance (%) 21295 0

i

23059

50

0 .

m

100 0 .

50

23071

23062 100 0

50

100

m

0

50

23199 100 0

50

100

0

20 10

40 ..C:

~.20 60

T 0

o ~

80

•. - . . ¢ p - - . .---0-

Coo~ltthus pelagcus Emilwnia huxleyi Varm

Coocol~s bu~k(gram- %) lC)

20 0

10

20

30 0

10

20

30

1011

.

•

10

'

r

~

20 0

.

10

=

20

30

Coccoliths / grain - % (< 63 pro)

Fig. 3. Coccolithabundances (grain-%) and relative abundances of single speciesversus depth in all cores investigated. Termination IAis indicated by lowershaded sections;upper shaded sectionsrepresentTerminationIs.

134

K.-H. BAUMANN AND J. MATTHIESSEN

pure and Nematosphaeropsis labyrinthus are

level, although their absolute numbers are clearly lower than observed in the late Holocene.

s,.fll the dominant species. Sediment cores 22,062 and 23071 contain higher amounts of Bitectatodinium tepikiense in the late glacial section. Peridinium faeroense has been recorded with minor percentages from all records (Matthiessen, 1991 ) but it is more abundant only at site 2307 I. In Holocene sediments the assemblages are usually dominated by two or three species comprising 85-90%. O. centrocarpum and N. labyrinthus are the important species on the Voring Plateau and the Jan Mayen Fracture Zone, whereas in the Fram Strait Impagidinium pallidum is important, as well. All sediments record a changeover in dominances between these species or significant peaks of one of those species which makes it possible to recognize major floral events (Fig. 4). The early Holocene assemblages are dominated by N. labyrinthus in sediment cores 2307i, 23i99 and 2i295, whereas in cores 23062 and 23059 N. labyrinthus revealed only

Dinoflagellate cysts The record of dinoflagellate cysts in the sediments is characterized by a more or less continuous increase in their absolute numbers from the late glacial to the late Holocene. Abundances increase by at least half an order of magnitude during Termination Ia and at the transition from the early to the late Holocene (Fig. 4). Each core shows a distinct succession in relative abundances of single species with only small variations between sites from the V~ring Plateau to the Jan Mayen Fracture Zone. Sediment core 21295 revealed a different pattern most probably due to its northernmost location. The late glacial record appears to be more variable, though Operculodinium centrocar-

Relative Abundance (%) 0 0~

21295 50

15 0

23059 50 .

1

100 0 i

23062 50 |

I

100 |

0 0,0

| I

23071 50

100

0

23199 50

100

,

lo] A

E

40.

o

50. -

..=-

Nemalasphaeropsis labyrinthus

-.-.0-..-m.

o

I.~O

-

.J

Operculodinium cen~'ocarpum

~

Impagidinium pallidurn

..A -

Splnifentes etongatus

~

Bitecta~linium fepikiense •

Peridinium faeroense

Dlnocysts/gsad.

3oooq

)

.

I

I

I

2OOOO4OOO0O "10(~00 20000

Dinocysts/g Sediment

Fig. 4. Dinocystabundances (dinocysts per gram sediment) and relative abundances of single species versus depth in the sediment cores investigated. Termination I A is indicated by lower shaded sections; upper shaded sections represent Termination IB.

SURFACE WATER MASS C O N D I T I O N S IN T H E N O R W E G I A N SEA

a significant peak. This may be caused by a distortion of the orginal signals through intense bioturbation within these sediments of low sedimentation rate. L pallidum contributes significantly to the assemblages in early Holocene sediments of 21295 with a distinct peak at Termination IB whereas in all other cores it has been found with less than 5%. However, in these cores L pa!!idum also reaches a maximum content in early Holocene. A changeover in dominance from N. labyrinthus to O. centrocarpum marks the transition to the late Holocene in sediment cores 23071, 23199 and 21295, whereas in cores 23059 and 23062 only a decrease ofN. labyrinthus and an increase of O. centrocarpum has been observed. The late Holocene records contain the most uniform assemblages of all sediment cores. Assemblages from the Voting Plateau and the Jan Mayen Fracture Zone are nearly monospeI.,;lll~, ~UII~I31,111.~ U . I . LJ. L { ~ t l £ t t l ~ t l J u t t t . t i t a a species dominates the dinocyst record of 21295, as well, but N. labyrinthus and I. pallidum are also numerically important (Fig. 4).

Comparison of coccolith and dinoflagellate cyst records Coccoliths and dinoflagellate cyst assemblages in the Holocene sections obviously show similar trends in species composition and abundances. Both groups show a two-step increase in absolute abundance. The first step was reached after Termination Ia at around l 0,000 yrs B.P. and the second major step of increased abundance is documented at the transition from early to late Holocene around 7500 to 6000 yrs B.P. (Fig. 5 ). The highest numbers of individuals have been observed in the late Holocene. The assemblages are always dominated by a few species (Fig. 6). The downcore distribution of single coccolith species appears to be more variable in all cores than the distribution of dinoflagellate cysts. Changes in composi-

] 35

Dinoflagellate C y s t s (g Sediment) 0

2OOO0

Coccoliths (Grain - %)

40O0O 0

10

2O

3O

t

10 ========+===============+=======+ z:=+=======z~: = :=:=.:+ == +:= +'~.=~:.~:,~:~::::,;+~:~::::~.+::~::z -.,']

|

15

~

~

[ ---~ -

~

23( -

21;

Fig. 5. Concentrations of bulk coccoliths and dinoflagellate cysts in sediments at five core sites during the last ! 5,000 years. Shaded sections indicate major changes of ecological conditions.

tion of the assemblage of both groups occur synchronously, thus enabling us to establish two distinct steps of major changes in surfacewater mass conditions. These changes are indicated by bars in Figs. 5 and 6.

Late glacial The late glacial sequence is characterized by scattered occurrences of coccoliths and a variable dinoflagellate cyst record. This may be caused by a variable input of reworked organic matter resulting in a dilution of authochthonous dinoflagellate cysts (Matthiessen, 1991 ). Therefore, a direct comparison of both fossil groups seems to be difficult and has not been attempted yet. In the late glacial sequence of sediment cores 23062 and 23071 higher amounts of O. centrocarpum, N. labyrinthus, B. tepikiense have been observed. In sediment core 23071 P. faeroense is additionally important.

136

K.-H. BAUMANN AND J. MATTHIESSEN

Coccoliths

Dinoflagellate Cysts O. centrocarpum

N. labyrinthus

0

40

80 0

50

I. pallidum

100 0

5

C. pelagicus

25

50

40

60

80

E. huxleyi

100 0

25

50

0 %

5.~ Q)

10

t~:~:i

:i:i:i:i:i:~

)

15

]~

~

22062

~

23071

m -

I

I)

23199

&_. -A-

I

-

21295

I

Fig. 6. Relative abundances of selected species in sediments at five core sites during the last 15,000 years. Shade<] sections indicate major changes of ecological conditions.

N. labyrinthus and O. centrocarpum have been found throughout the last 15,000 years, whereas B. tepikiense only occurred in higher amounts in the late glacial than in the Holocene section of all sediment cores. Higher abundances of the latter species have frequently been recorded from latest Pleistocene and Holocene sediments of the northeastern North Atlantic and the Norwegian Sea (Harland, 1988, 1989; Stoker et al., 1989), the shelf areas around the British Isles (Harland, 1988; Duane and Harland, 1990; Graham et al., 1990) and the Norwegian coast (Dale, 1985; Bakken and Dale 1986). Single small peaks have also been recorded from the western Labrador Sea (De Vernal and Hillaire-Marcel, 1987; Hillaire-Marcel and De Vernal, 1989) as well as from the western Mediterranean (Turon and Londeix, 1988 ). Today B. tepikiense is found in temperate to cold-temperate regions and it has been recorded either from neritic and estuarine environments or outer ne-

ritic and oceanic environments (Reid, 1975; Wall et al., 1977; Turon, 1980; Miller et al., 1982; Dale, 1983; Harland, 1983; Mudie et al., 1990). Higher relative abundances have especially been reported from cold neritic environments which are characterized by slightly reduced surface water salinities ( ~ 30-35%0) and strong variations in annual sea-surface temperatures. Therefore, B. tepikiense appears to be adapted to a wide temperature range whereas it only tolerates slight deviations from normal marine salinities. Peridinium faeroense occurs as a single, important species only in sediment core 23071 showing generally higher abundances in late glacial times. It is described as a boreal to temperate species (Dale, 1976, 1985) which is typically found in sheltered bays and fjords today (Dale, 1985 ). ?. faeroense dominates the dinoflagellate cyst assemblages on the North Icelandic shelf and contributes with higher amounts to assemblages along the Norwegian

SURFACEWATERMASSCONDITIONSIN THE NORWEGIANSEA

Reworked Dinocysts

137

Reworked

Pollen (Rel. Abundance) (Rel. Abundance) 400 800 0 2000 4000 0

Reworked Coccoliths

Pediastrum

(Grain - %) 1

(ReL Abundance) 0 25

spp. 2

•

i

I I ~' )

I

L

i

o-.-~ ~ -- A--~-

50

i

23059 23062 23071 2319 c 2129 E

•|11 ============ ==================================================i q~ ~:::::::::~ !P

"-'-C,~

I @

I

@

Fig. 7. Abundances of reworked dinocysts, pollen and coccoliths, and relative abundance of Pediastrum spp. at five core sites during last 15,000 years. Shaded sections indicate major changes in ecological conditions.

shelf and off the Barents Sea (Matthiessen, 1991 ). Therefore, higher abundances may indicate a stronger influx of shelf waters and sediments to the depositional area, which is in good agreement with the higher sedimentation rates of core 23071. The peak input of reworked coccoliths, dinoflagellate cysts and pollen may be interpreted as a result of rather restricted marine conditions with a dominating influx of terrigenous and coastal material (Fig. 7). This could also indicate massive reworking of Quaternary dinoflagellate cysts from the shelfs into the deep sea during the transgression and associated sea-level rise. However, the late glacial dinocyst record suggests that reworking had only a minor influence. The maximum input of reworked material never correlates with peaks of individual dinocyst species, such as B. tepikiense and P. faeroense. Additionally, the maximum content of reworked, ice-rafted Cretaceous (and Tertiary) coccoliths, which have their origin south of the Scandi,navian penin-

sula, indicate a northward drift and melting of icebergs. Chalk as a facies of the Cretaceous is only observed south of about 60°N, whereas the facies far north is clastic (Hancock, 1984). The freshwater algae Pediastrum spp. (Fig. 7) has sporadically been reported in deep-sea sediments from the eastern Arctic Ocean (Mudie, 1992) and in sediments underlying the Arctic domain (Matthiessen, 1991 ). Mudie ( 1992 ) suggested that these occurrences indicate a melt out of ice-rafted sediments. This seems to be plausible because the Arctic domain is today seasonally ice-covered and melting of this ice may also result in sedimentation of Pediastrum spp.

Early Holocene C. pelagicus and N. labyrinthus are the most important species in the assemblage from 10,000 to 7500 yrs B.P., each comprising more than 60% and 40%, respectively, (with the exception of 23059) in the southern sediment

138

cores. N. labyrinthus is a cosmopolitan species which is restricted to the outer neritic and oceanic environment (Wall et al., 1977; Harland, 1983). However, it is only found in higher abundances in the subpolar gyre west of Ireland (Harland, 1983), in the Greenland Sea in sediments underlying the Arctic surface water (Matthiessen, 1991 ) and in the eastern Arctic Ocean (Mudie, 1992). These observations suggest a preference for colder oceanic environments. The relative abundances of N. labyrinthus and O. centrocarpum agree extremely well in cores 23071, 23199 and 21295 (Fig. 6 ), indicating similar paleoceanographic conditions at the different site locations during this time interval. Among the species generally considered as good temperature indicators, C. pelagicus is well-known as a cold-water species. It has been shown that its temperature range includes even negative temperature values (Smayda, 1958; Braarud, 1979). C. pelagicus dominates surface sediments underlying Arctic surface water masses (Eide, 1990; Samtleben and Schrtider, 1992). Sediment core 21295 deviates from this trend containing nearly monospecific assemblages of C. pelagicus and higher abundances of N. labyrinthus together with L pallidum. Slightly higher abundances of L pallidum have also been observed in all the other cores within this interval. This species appears to be endemic to boreal to arctic regions (Head et al., 1989; Matthiessen, 1991; Mudie, 1992). Higher abundances ofL pallidum in sediments underlying the Arctic domain and the eastern Arctic Ocean suggest that it is well adapted to cold, seasonally ice-covered water masses with a reduced salinity.

Transition to late Holocene The transition from early to late Holocene (7500 to 6000 yrs B.P. ) is more clearly seen in the dinofl~_gellate cyst record. Sediment cores 21295, 23071 and 23199 record a change in

K.-H. BAUMANN AND J. MATTHIESSEN

dominance of N. labyrinthus to O. centrocarpum within this interval. This change is not clearly seen in the other cores because bioturbation has certainly blurred this event. O. centrocarpum is a cosmopolitan species (Dale, 1976; Wall et al., 1977; Harland, 1983) which appears to tolerate wide ranges of temperatures and salinities (Dale, 1985) and is found in greater abundances even in estuarine environments (Wall et al., 1977). In the Atlantic Ocean it seems to be associated with the North Atlantic Drift (Harland 1983) and it dominates the assemblages in sediments underlying the Norwegian Coastal Current (Harland, 1983; Dale, 1985 ) and the Norwegian Current (Matthiessen, 1991 ). O. centrocarpum appears to be a good indicator for the modern, relatively warm North Atlantic water masses. A ratio change between E. huxleyi and C. pelagicus can be observed, as well. Before 7500 yrs B.P.C. pelagicus makes up more than 65% of the assemblages, whereas after 6000 yrs B.P. its amount is consistently below 65%. An increase in E. huxleyi and in subpolar species ("varia") also indicate changes in hydrographic properties, probably a strengthening of the inflow of relatively warm Atlantic surface water until 6000 yrs B.P. In addition to O. centrocarpum, E. huxleyi is also known as a cosmopolitan species which can tolerate great variations in salinity and in temperature (Bukry, 1974). In all oceans, particularly at mid-latitudes, it is the most abundant living coccolithophorid, forming gigantic blooms and probably being the most productive lime-secreting organism on earth (Westbroek et al., 1986). In surface sediments in the area below the Norwegian Current it makes up more than 50% of the assemblages, whereas it is less abundant in the western part of the Norwegian-Greenland Sea (Eide, 1990; Samtleben and Schr/Sder, 1992).

Late Holocene In the late Holocene (6000 yrs B.P. to Recent) the assemblage is composed of C. pelag-

SURFACE WATER MASS CONDITIONS IN THE NORWEGIAN SEA

icus together with E. huxleyi and O. centrocarpum. The paleoecological information on these species reveals that E. huxleyi and O. centrocarpum are more tolerant of changes in salinity and temperature. Additionally, C. leptoporus, S. pulchra and H. carteri ("varia'" ) have a minor but consistent contribution to the assemblage. This shows that the Norwegian Current with its modern oceanographic and ecological properties has been fully established. In contrast, the situation in the Fram Strait differs from the southern cores because the species which are adapted to colder and less saline environments, L pallidum and N. labyrinthus are still present in high amounts and C. pelagicus clearly dominates the coccolith assemblages. Therefore, the ecological conditions of this region are still different from conditions in the Voring Plateau region.

Palaeoceanographic implications Deglaciation phase Micropaleontological and stable isotope analyses of cores from the Norwegian Sea indicate that the last deglaciation occurred in two distinct steps (Jansen et al., 1983; Sejrup et al., 1984; Jansen and Bjorkhnd, 1985; Jansen and Erlenkeuser, 1985; Vogelsang, 1990; Weinelt et al., 1991; Lehman et al., 1991; Veum et al., 1992). The first warming episode with the onset of the inflow of relatively warm Atlantic water into the Norwegian Sea started at about 14,000 yrs B.P. and lasted until about 11,000 yrs B.P. At about 13,000 yrs B.P. the Norwegian Sea became seasonally ice-free, whereas the final warming of the eastern part of this area started shortly after l 0,000 yrs B.P. Numerous records (Jones and Keigwin, 1988; Jones, 1991; Weinelt et al., 1991 ) from throughout the Norwegian-Greenland Sea demonstrate that the late glacial interval was punctuated by several melt-water events due to the decay of the surrounding ice-sheets. The strong input of reworked coccoliths, pollen., and dinoflagellate

| 39

cysts (Figs. 7 and 8) between 15,000 and 11,000 yrs B.P. brackets the interval of most intense ice-sheet disintegration at about 12,000 yrs B.P. (Fairbanks, 1989, 1990) although it slightly predates this event. However, Jones ( 1991 ) showed that the Norwegian-Greenland Sea exhibits the earliest evidence of decay of the Northern Hemisphere ice-sheets, beginning at approximately 15,000 yrs B.P., most probably caused by increasing insolation and global sea-level rise. In general, the time interval before Termination IB ( 15,000 to 10,400 yrs B.P.) is not well documented by the coccolith and dinoflagellate cyst records studied here. Nevertheless, the scarce input ofdinoflagellate cysts indicate relatively cold, seasonally ice-free water. The occurrence of P. faeroense and B. tepikiense suggests environmental conditions which are characterized by slightly reduced salinities compared to normal marine conditions. Peaks of B. tepikiense have also been associated with mixing of colder brackish surface waters with warmer waters (Dale, 1985) which may be caused by melting events (De Vernal and Hillake-Marcel, 1987; Hillaire-Marcel and De Vernal, 1989). Therefore, brief inflows of relatively warm Atlantic surface water would have been diluted by the massive input of meltwater leading to a reduction in salinity and providing optimum ecological conditions for the two dinoflagellate cyst species. Lehman and Keigwin (1992) have shown that sudden changes in the inflow of Atlantic water since 13,500 yrs B.P. changed ice-melting rates and caused variations of summer sea-surface temperatures (SST) of >i 5°C. Environmental conditions have significantly improved during this interval allowing small numbers of planktic foraminifers and diatoms to enter the southeastern Nom, egian Sea (Jansen and Bjorkhnd, 1985), whereas on Voting Plateau an almost monospecific planktic fauna composed ofcold water species Neogloboquadrina pachyderma sin. is indicating persistence of polar conditions (Ramm, 1988).

140

K.-H. BAUMANNANDJ; MATTHIESSEN Sea Level Curve

Coccollths

0

50

100 (m)

. ~ few

S u m m e r SST (DiatomTransferFunclJon)

Dinoflagellste C y s t s

Reworked Dinoflagellate Cysts, O. cen/rocarpum ~ ~ . s , Pollen - , ~ few more ! ~ ~ few more I ~ .

E. huxleyi C. petagicus

(i~oados)

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more ~

10

12

14 (°C)

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zx

/ I i! 1

,

---

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~

~,~.~,x~~,~

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"' ' ~ ~ ~ . - : - i

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- - a - - SST ----e.--- Abundance 15 (Fatrt~mNs,19e9,1g~O)

(KOCKarpuzand Schrader.1990)

~o q~ 0

10

20

30

(valves x 10s)

Diatoms

Fig. 8. Schematic illustration of absolute abundances of main coccolith and dinoflagellate cyst species together with Barbados sea-level curve (Fairbanks, 1989, 1990), diatom valve abundances, and summer SST estimates (diatom transfer function). The record from Karpuz and Schrader (1990) comes from sediment core HM52-43 collected at 2600 m water depth in the southeastern Norwegian Sea; age model based on ~4C AMS-ages of the same core published in Veum et al. (1992). Shaded sections indicate major changes in ecological conditions observed in this study.

Faunal and floral evolution in early Holocene The increase in absolute numbers of Quaternary coccoliths and dinoflagellate cysts at about lO,O00 yrs B.P. indicates that the inflow of relatively warm Atlantic water became permanently established. A first maximum in absolute plankton numbers which is more prominent for coccoliths than for dinoflagellate cysts occurred between about 9500 and 8000 yrs B.P. This is in good agreement with faunal and floral data of previous studies in the southeastern Norwegian Sea (cf. Jansen et al., 1983; Sejrup et al., 1984; Jansen and Bj~rklund, 1985; Stabell, 1986; Kog Karpuz and Schrader, 1990). This period ofmost rapid environmental change coincides with maximum Northern Hemisphere summer insolation. At about 9000 yrs B.P. average solar re-

diation over the Northern Hemisphere was about 8% higher in July than it is today (COHMAP-Members, 1988). Experiments with global atmospheric circulation (GCM) models indicate that the increased insolation caused summer air temperatures similar or slightly higher than at present (Kutzbach and Guetter, 1986; COHMAP-Members, 1988). Mean annual temperature estimates also reveal a maximum in temperatures around 9000 yrs B.P. similar to those at present (Folland et al., 1990). However, the still smaller ice sheets continued to influence the climate and close to the ice sheets it was still colder than at present. Besides, based on a stronger seasonality and a still existing permanent influx of meltwater, summer warming could not heat up the surface layer of the Norwegian-Greenland Sea as much as today.

SURFACE WATER MASS CONDITIONS IN THE NORWEGIAN SEA

SST estimates which have been reconstructed by means of planktic foraminifera (Haake and Pflaumann, 1989; Bard et al., 1990) and diatoms (Ko¢ Karpuz and Schrader, 1990) also confirm our a~sumption of slightly reduced temperatures in early Holoeene (Fig. 8). The dominance of the cold water coccolith species C. pelagicus indicv.tes conditions similar to those of the present day Arctic domain. A slight increase in the relative abundances of coccoliths of the "varia" group reaching a first peak at about 9000 to 8000 yrs B.P. can also be attributed to a first productivity maximum caused by the generally increasing temperatures, although their absolute numbers are much lower than in the late Holoeene. The composition of the dinoflagellate cyst assemblages, especially the higher abundances of L pallidum, suggests that Arctic surface waters have penetrated into the western Norwegian Sea (Matthiessen, 1991 ). This is also supported by planktic foraminifera assemblages from Voring Plateau which record colder surface waters than during the late Holocene (Ramm, 1988). In addition, dinocyst records from Rockall Channel indicate that relatively cold surface waters from the Labrador Sea penetrated into the northeast Atlantic in early Holocene (Turon, 1980). Generally colder conditions were recorded for the eastern Canadian continental shelf between 10,000 and 7000 yrs B.P. (Scott et al., 1984). Glacial ice covering the Hudson Bay completely disappeared at 8000 yrs B.P. (Josenhans and Zevenhuizen, 1990).

Change in surface water conditions during the Holocene The major shift in coccolith and dinoflagellate cyst assemblages from 7500 to 6000 yrs B.P. is most probably associated with the Holocene climatic optimum. A second increase in absolute numbers of coccoliths, above all in the number of E. huxleyi and the "varia"-group, and a drastic increase in dinoflagellate cysts

|4]

indicate increasing sea-surface temperatures. Summer SST obtained by means of faunal and floral transfer functions (Bard et al., 1990; Ko¢ Karpuz and Schrader, 1990) are also characterized by a temperature maximum between 6000 and 4000 yrs B.P. (Fig. 8). GCM experiments also emphasize that temperatures at about 6000 yrs B.P. were higher than at present (COHMAP-Members, 1988), at least in summer. Additionally presence of subtropical radiolarians in sediment cores from the southeastern Norwegian Sea point at warmer conditions between 7000 and 4000 yrs B.P. (Jansen and Bjorklund, 1985; Jansen, 1987). A significant change in planktic foraminifera fauna has been observed on the Voring Plateau as well, paralleling the transition in coccolith and dinocyst floras. This faunal change has been associated with the transition from cold to temperate surface waters at about 7500 yrs B.P. (Ramm, 1988). Similar synchronous changes in p!anktic and benthic foraminifera assemblages have also been observed along the Norwegian coast and on the northwest European continental shelf around 7800 to 8000 yrs B.P. (Nagy and Ofstad, 1980; Nagy and Qvale, 1985; Hald and Vorren, 1984, 1987; Thiede, 1985; Nordberg, 1991 ). These changes were probably linked to variations in the hydrographic properties of the Norwegian Coastal Current. Data compiled from literature suggest that the observed change between 7500 and 6000 yrs B.P. may be seen in the entire North Atlantic region and adjacent seas. Ruddiman and McIntyre ( 1981 ) and Vilks and Mudie (1978) have already shown or. the basis of an extensive data set obtained from sediment cores in the northern North Atlantic and Labrador Sea that a fully interglacial ocean configuration was established around 6000 yrs B.P. Several welldated records from the North Atlantic and the Norwegian-Greenland Sea support this evidence (Bard et al., 1987a,b; Jones and Keigwin, 1988; Jansen and Veum, 1990; Lehmann

142

et al., 1991; Veum et al., 1992), showing that present-day conditions were established between 8000 and 6000 yrs B.P. This hydrographic change was accompanied by an almost synchronous reorganization of the surface water masses, as documented by sedimentolo~cal and micropaleontological studies in the western Arctic Ocean (Hein and Mudie, 1991 ), the Labrador Sea (Vilks and Mudie, 1978; Scott et al., 1984; De Vernal and Hillaire-Marcel, 1987; Hillaire-Marcel and De Vernal, 1989) and in the northwestern North Atlantic (Aksu et al., 1989, 1992). In the Labrador Sea interglacial climatic conditions were established diachronously until about 4000 yrs B.P. (De Vernal and Hillaire-Marcel, 1987; Hillaire-Marcel and De Vernal, 1989). It appears that the mid-Holocene hydrographic changes likely occurred synchronously in the entire North Atlantic. Dickson et al. (1988) have shown that a small salinity anomaly was transported from the Iceland Sea into the Labrador Sea and along the subpolar gyre again into the Norwegian-Greenland Sea. Therefore, changes in hydrographic conditions in one area may have influenced past surface water conditions in the entire region. In late glacial and Holocene the glacial-eustatic sealevel rise may have contributed to changes in ocean circulation (Fairbanks, 1989). A variable meltwater discharge has been recorded with a distinct decrease towards slow rates around 7000 to 6000 yrs B.P. (Fig. 8). This decrease, as well as the contribution of meltwater to surface-water masses, may have affected the circulation and the hydrographic properties in that region despite the effect of increasing insolation in the early Holocene. Possible events in latest Holocene

The well-dated dinoflagellate cyst record of sediment core 21295 has even in the late Holocene a greater variability than those observed in other cores. This may be caused mainly by better age control, especially with a

K.-H. BAUMANN AND J. MATTHIESSEN

more equal distribution of age assignments in the Holocene and a relatively young core top age (520 yrs B.P.) in comparison with the other sediment cores. Nevertheless, similar trends have also been observed in the cores from the V~ring Plateau area. However, the following conclusions are considered as rather tentative as they are mainly based on this single core. O. centrocarpum attained its highest values around 4000 to 2000 yrs B.P. decreasing again in the surface sample, whereas I. pallidum gained its lowest values in the late Holocene around 4000 yrs B.P. Cooling trends have been recorded from the northeastern North Atlantic (Bard et al., 1990) as well as from the eastern continental shelf of Canada (Vilks and Mudie, 1978; Scott et ai., 1984) since 4000 yrs B.P. Ko~ Karpuz and Schrader (1990) also concluded that a decrease in sea-surface temperature occurred since about 4000 yrs B.P. Subtropical radiolarian assemblages disappeared in sediments from the southern Norwegian Sea at about 4000 yrs B.P. (Jansen and Bj6rldund, 1985; Jansen, 1987). At about the same time modern oceanographic conditions in the Kattegat were established (Mikkelsen, 1985; Nordberg, 1991 ). It may be speculated that the entire Norwegian-Greenland Sea as well as the North Atlantic were effected by these changes in hydrographic properties. Conclusions ( 1 ) Calcareous nannofossil and dinoflagellate cyst assemblages are good indicators of paleoenvironmental changes, although they are only of low diversity and contain high proportions of single species. Based on the concentration and diversity of the assemblages and the ratio change between the dominating species it has been possible to establish distinct steps of major paleoceanographic changes in Holocene sediments. (2) The first step of deglaciation is characterized by an influx of detrital material into the

143

SURFACE WATER MASS CONDITIONS IN THE NORWEGIAN SEA

Voring Plateau area and into the Fram Strait. These influxes may be interpreted as the result of ice melting and/or increased sediment transport from the shelfs. Sparse occurrences of eoccoliths and dinoflagellate cysts have been observed before 10,000 yrs B.P., also indicating harsh environmental conditions with a dominant influence of meltwater and very slight inflow of Atlantic water. (3) The surface water circulation pattern was reinitiated after Termination lB. The assemblages suggest slightly cooler conditions than we have today with a similar surface circulation pattern until 7500 yrs B.P. Solar insolation may have caused a first temperature maximum which is responsible for the early Holoeene productivity maximum. (4) A considerable change in the composition of dinocyst and coccolith assemblages occurred corresponding approximately to the onset of the Holocene climatic optimum. This change was most probably linked to an almost synchronous reorganisation of the hydrographic properties in the entire North Atlantic realm after the ice sheets had vanished. (5) Since 6000 yrs B.P. the Norwegian Current with its present-day oceanographic and ecological properties has been fully established. However, some of the records indicate a slight decrease in sea-surface temperatures since about 4000 yrs B.P. Acknowledgements We want to express out thanks to G. Bohrmann, W. Brenner and D. Niirnberg who critically read an early version of the manuscript and suggested many improvements. Thanks to N. Biebow, who did most of the figure drawings, and J. Welling, who carefully improved the English of the text. We thank E. Jansen and an anonymous reviewer for their careful review and helpful comments. We would also like to thank the officers and crews of RV Meteor and PRV Polarstern for their excellent support at sea. This investigation was financially sup-

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