Recent benthic foraminifera as tracers of water masses along a transect in the Skagerrak, north-eastern North Sea

Journal of Sea Research 35 (1-3): 111-121 (1996)

RECENT BENTHIC FORAMINIFERA AS TRACERS OF WATER MASSES ALONG A TRANSECT IN THE SKAGERRAK, NORTH-EASTERN NORTH SEA HELENE BERGSTEN 1, KJELL NORDBERG 1 and BJ(]RN MALMGREN 2 1Department of Oceanography, Earth Sciences Centre, GSteborg University, Guldhedsgatan 5a, S-413 81 GSteborg, Sweden 2Department of Marine Geology, Earth Sciences Centre, GSteborg University, Guldhedsgatan 5a, S-413 81 G5teborg, Sweden

ABSTRACT Recent benthic foraminifera have been investigated in surface samples (0-8 cm) from thirteen sites along a profile in the Skagerrak, between Norway and Denmark. The investigated transect is sampled in an oceanographically well investigated area. Principal component analysis based on the 32 most abundant taxa is used to recognize similarities within the large data set of benthic foraminifera (a total of 93 samples), which is shown to cluster into four assemblage groups (A-D). The spatial distribution of these foraminiferal groups indicates that they inhabit areas that correspond with the delimitations of different water masses. We have identified areas where it is possible to investigate effects on the sea floor of different tracers, natural or anthropogenic contaminants, related to specific water masses.

Key words: Skagerrak, North Sea, foraminifera, principal component analysis, PCA, water masses, tracers

1. INTRODUCTION Benthic foraminifera have been widely used for environmental reconstructions for over one century. Even so, the knowledge of their response to environmental factors is still limited. The understanding of the modern distribution patterns of benthic foraminifera and their relation to the environment is therefore very important. Planktonic foraminifera in the sediments are often used as markers for different surface water masses of the open ocean. Shelf environments generally lack good planktonic assemblages but are often rich in diversified benthic foraminiferal faunas. However, the complexity and variability of shelf environments make studies of foraminifera as tracers more difficult here. The present investigation was carried out along a transect in the Skagerrak (Fig. 1) with well investigated, but complex hydrography. Several regularly occurring specific water masses can be differentiated along the transect (e.g. Rodhe, 1987, 1992; Rydberg, 1993; Aure & Dahl, 1994; Rodhe & Holt, 1996; Rydberg et al., 1996), but their geographic delimitations vary depending on, for example, weather conditions and season. Both surface and deep water masses characterize the area and the

main current system is shown in Fig. 2. The objective of this study is to search for relationships between the recent benthic foraminiferal fauna and water masses along this hydrographically well-investigated transect. Such relationships would allow us to investigate the validity of various tracers, natural or anthropogenic, that are related to specific water masses and, hence, to establish their provenance or transport path. Such studies could be performed either on different benthic organisms or the sediment itself depending on the tracer. This study is part of the programme 'Large-scale environmental effects and ecological processes in the Skagerrak-Kattegat' (Rosenberg et aL, 1991). When using large data sets useful information is gained by employing multivariate ordination techniques. In the present study we have used R-mode principal component analyses to assess the natural assemblages of species and the strength of the relationship between clusters of samples and water masses. Recent benthic foraminifera of the Skagerrak have previously been studied by HSglund (1947), Lange (1956), Van Weering & Qvale (1983), Murray (1991), Corliss & Van Weering (1993), Moodley etal. (1993),

H. BERGSTEN K. NORDBERG & B. MALMGREN

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10" Fig. 1. Map of the North Sea, Skagerrak, Kattegat area with bathymetric depth contours. The section K-K' represents the investigated transect, while section L-L' shows salinity and velocity profiles (Fig. 6). The 13 investigated sites are marked by symbols (for legend see Fig. 7). Geographic positions and water depths of these sites are given in Table 1. Seidenkrantz (1993), Conradsen et aL (1994) and Alve & Murray (1995). Conradsen et aL (1994) presented a compilation study of the distribution of recent benthic foraminifera in different parts of the Kattegat and Skagerrak. The heterogeneous net of stations and variations in sampling and processing procedures resulted in distribution maps of a general character, which were compared to the hydrography of this area. 2. STUDY AREA The investigation was carried out along a NW-SE transect in the Skagerrak between Arendal (Norway) and Hirtshals (Denmark) (Profile K-K'; Fig. 1). The Skagerrak has a maximum depth of about 700 m in the Norwegian Trench and the sill depth between the Skagerrak and Atlantic Ocean is 270 m. The Kattegat and Skagerrak form the connection between the low-salinity Baltic Sea and the normal-salinity North Sea.

The current system in the Skagerrak-Kattegat area is dominated by the large-scale atmospheric and oceanic circulation patterns and the outflowing Baltic water (e.g. Svansson, 1975; Stigebrandt, 1983). The tidal range is very small, approximately 20 cm, and has little influence on the current system. The prevailing current system in the region has been described by Svansson (1975, 1984), Rodhe (1987) and Nordberg (1991) and is illustrated in Fig. 2. Water masses entering the Skagerrak generally flow at relatively constant depths. The South Jutland Current (SJC), which flows north-eastwards along the Danish west coast is part of the cyclonic circulation of the North Sea. It continues to form part of the North Jutland Current (NJC), which flows into the Skagerrak-Kattegat. The NJC is dominated by the Southern Trench Current (STC), which supplies water from the northwestern and western North Sea via the Dooley Current (DC) and the large quantity of North Atlantic water (Tampen Bank Current, TBC) flowing along the southern rim of the Norwegian Trench. The intermit-

RECENT BENTHIC FORAMINIFERA IN THE SKAGERRAK

113

tioned into 1 cm slices and stored. For this study the uppermost 6-8 cm of the sediment cores have been used (Table 1). The samples were freeze dried, washed over a 125 #.m sieve and investigated for their foraminiferal content in a dry state. In connection with the preparation of foraminifera the size fractions >63 p.m were dried and weighed to obtain an estimate of the sand content. Foraminiferal analyses were performed on 93 samples (Table 1), where at least 450 specimens of the total fauna were identified in each of the samples. In total, 115 taxa were recognized and relative abundances for various species were computed. Hyperammina spp. and Rhabdammina spp. are excluded from the total sum of benthic foraminifera, since specimens of these branching taxa tend to break, thus producing an anomalously high number of, more or less, countable parts. Planktonic specimens were counted but not identified at the species level. Relative abundance data, adding up to a unit value in individual samples, have long been known to be subject to the so-called constant-sum constraint (Pearson, 1897). In this study percentage data were log-ratio transformed to relieve this constraint (cf. Aitchison, 1986). Thirty-two taxa had relative abundances of more than 2% in at least five samples or more than 5% in at least one sample and were included in the R-mode principal components analysis (PCA) (Table 2). The principal components were extracted from the 32x32 matrix of pairwise correlaFig. 2. Present current system of the North Sea, Skagerrak tion coefficients between individual taxa (correlation and Kattegat area. Data compiled from Dooley (1974), matrix, R). We also used the covariance matrix as a Svansson (1965, 1975), Lee (1980) and Fumes et aL basis for a PCA. That, however, gave a result which (1986). North East Atlantic Current (NEAC), Tampen Bank was very similar to that of the correlation matrix, so Current (TBC), Dooley Current (DC), Southern Trench Cur- we decided to show only the results based on R. The rent (STC), South Jutland Current (SJC), North Jutland Cur- significance of each of the selected taxa to each of rent (NJC), Norwegian Coastal Current (NCC) and the Baltic the principal components was determined using a Current (BC). From Nordberg (1991). bootstrap variety of PCA (Diaconis & Efron, 1983). Ninety-five percent confidence intervals were detertently outflowing hyposaline water from the Baltic Sea mined using bootstrap PCA based on 1000 replicates forms the Baltic Current (BC), which together with the of the PCA. For 21°pb analyses 2 cm sediment slices NJC make up the Norwegian Coastal Current (NCC). were subsampled at 11 intervals (0-22 cm) in both This current flows out of the Skagerrak along the Nor- core OSl and OS15, and were directly analysed as wegian coast. pressed pellets by gamma spectrometry. The deep parts of the Skagerrak (>300 m) are characterized by stable water masses with salinities 4. RESULTS that exceed 35%o and temperatures that range between 4 and 7°C. The long-term patterns of cyclic The plot of the first and second PC axes based on the temperature variations and the fact that a clear oxy- correlation matrix is shown in Fig. 3. The first PC gen depletion exists (Ljoen & Svansson, 1972; Aure & accounts for 46.7% and the second for 20.1% of the Dahl, 1994) suggest that the Skagerrak deep-water variation in the 32-dimensional species space. (below sill-depth) is periodically stagnant. Hence, these two PCs explain 66.8% of the variability in the relative abundance of these 32 species. The 3. MATERIAL AND METHODS plot of the scores of the samples along the first and second PCs gives a pattern where the different sites Sediment cores from thirteen sites were collected show cluster patterns according to their similarities during the summers of 1992 and 1993 using a multi- (Fig. 3). Four assemblage groups were identified, corer, which gives a virtually undisturbed sediment denoted A-D. Figs 4 and 5 show plots of the species surface. After collection, the sediments were sec- Ioadings along the first and second PC axes, respec-

114

H. BERGSTEN, K. NORDBERG & B. MALMGREN

TABLE 1 Location, water depth, number of subsamplesfor each core, mean and range of sand content and of planktonic foraminiferal frequency, and referabilityto benthic foraminiferalgroup for the 13 investigatedsites.

site no

position

OS6 N 58°21'6/E 8°51'0 OS5 N 58°20'0/E 8°53'0 OS4 N 58°18'5/E 8°55'0 OS3 N 58°12'0/E 9°05'0 ©$2 N 58°11'I/E 9°06'0 OS1 N 58°08'0/E 9°11'0 OS15 N 58°03'2/E 9°18'2 9201 N 57°59'8/E 9°21'2 OS16 N 57°59'4/E 9°24'1 9205 N 57°58'4/E 9°24'0 9202 N 57°56'2/E 9°27'3 9203 N 57°50'5/E 9036'5 9204 N 57°41'9/E 9046'8

sand content (%>63 p~m) planktonic foraminifera assembl. water depth No. of (m) sub-samples (no.g-1 dried sediment) group mean range mean range 177 252 325 411 525 637 525 450 350 294 177 70 58

6 8 7 7 3 8 8 8 8 8 8 6 8

tively, together with 95% bootstrap confidence intervals. Those species that have confidence intervals that do not overlap with zero values significantly contribute to the given PC. Thus samples from sites within group A and B with high scores along the first axis are controlled by those species that have significantly positive Ioadings of the first PC axis (Fig. 4),

e.g. Haplophragmoides bradyi, Ammoglobigerina globigeriniformis, Pullenia bulloides and others. On the other hand, the samples of group C and D with high negative scores are controlled by species with significantly negative Ioadings of the first axis, e.g. Elphid-

ium magellanicum, Gavelinopsis praegeri, Ammonia beccarii and others. These statistically significant species accordingly also show comparatively higher percentages within the group of sites that they characterize (Table 2).

49.9 2.3 0.4 0.6 3.9 0.6 0.8 2.7 5.2 19.0 61.5 65.2 67.7

42.9-55.6 1.6- 3.6 0.2- 0.6 0.4- 0.7 3.1- 5.0 0.3- 1.2 0.5- 1.0 2.1 - 4.0 4.8- 6.0 13.5-28.2 55.5-64.1 61.6-69.1 50.1-76.3

0.2 0.2 0.1 0.4 0.4 0.2 0.3 0.3 1.2 1.0 1.6 0.3 0.0

0.0-0.4 0.0-0.4 0.0-0.5 0.0-1.0 0.2-0.6 0.0-0.7 0.0-0.6 0.0-0.6 0.3-3.2 0.8-1.5 0.5-2.8 0.0-1.0 0.0-0.0

B B B A A A A B C C C D D

Group A is characterized by many species that are significant for both the first and second axes (Figs 4 and 5), which means that the foraminiferal assemblage includes many species that do not occur as abundantly within the other three groups (Tables 2 and 3), e.g.H, bradyi, A. globigeriniformis, R bul-

Ioides, Glomospira charoides, Pullenia subcarinata, Trochammina pusilla and Bolivina skagerrakensiso Group B has many species that are frequent also within the other groups, especially group C, but it also displays species significant for both or either of the PC axes, e.g. Uvigerina peregrina, Bulimina margin-

ata, Trifarina angulosa, Liebusella goesi, Melonis barleeanus and Hyalinea balthica. Group C is characterized by the species Stainforthia fusiformis, Cribrostomoides jeffreysi and G. praegeri. Group D displays a low-diversity fauna where Elphidium excavatum is very abundant in many samples. This spe-

cies is significant for this group together with E. magellanicum and A. beccarii. C~ ~ OS6 i 0• 0S5 Agglutinated foraminifera are most abundant in • 0S4 OS1 and OS15 (up to 78%) where H. bradyi is the • OS3 most abundant agglutinated species (10-38%). For ~2 [3 0S2 the other sites the agglutinated foraminifera never reach a total of 50% and commonly show total values 0 9201 between 5 and 25%, with Verneulina media as an important species for all groups. The concentration of x 9205 foraminifera is lowest at two of the deepest sites of D 0 ~202 the transect, QSl and OS15 (30-80 specimens per g 0 92O3 sediment), while the remaining sites of the transect + 9204 show higher and quite consistent values (80-200 ~ -;, ~ . . . .o. . . . . . 2. . . . . . . FirStPrincipalCempot~nt specimens per g). Elliptic, potato-like particles of manganese-rich Fig. 3. Distribution of scores of the 93 benthic foraminiferal material (<1 mm) are very common at sites OSl and samples from 13 investigated sites along the first and sec- OS15, but present also in low amounts in OS2 and ond principal component axes. The assemblagegroups are OS3. Generally, the sand content (% >63 #m) lies marked A-D. within three classes along the transect with very high

l

RECENT BENTHIC FORAMINIFERA IN THE SKAGERRAK

115

TABLE 2 Means and ranges of relative abundances (percentages) of the 32 benthic foraminiferal species included in the PCA. The percentage values are calculated on the total fauna of each subsample and the means and ranges are based on all subsampies included in each of the four assemblage groups A-D.

species

A 0S3, 0S2, 0S1, 0S15 mean

Ammodiscusspp. Ammoglobigerina globigeriniformis Ammoniabeccarii Biloculinella inflata Bofivinaskagerrakensis Buliminamarginata Cassidulinalaevigata Cribrostomoidesjeffreysi Cribrostomoides nitida Cribostomoides subglobosus Elphidiumexcavatum Elphidiumgerthi Elphidiummagellanicum Gavefinopsispraegeri Globobulimina turgida Glomospiracharoides Haplophragmoides bradyi Hyalineabalthica Liebusellagoesi Melenisbarleeanus Planorbulinadistoma Pullenia bulloides Pulleniaosloensis Pulleniasubcarinata Saccamminaspp. Stainforthia fusiformis Textulariabocki Trifarinaangulosa Trochamminapusilla Uvigerinaperegrina Valvufina conica Verneufinamedia

range

0.68 0.0- 2.9 2.37 0.2- 6.4

B 0S6, 0S5, 0S4, 9201 mean

C 0S16, 9205, 9202

range

0.00 0.0- 0.0 0.04 0.0- 0.7

mean

range mean

range

A-D all sites mean

range

0.00 0.00

0.0- 0.1 0.0- 0.0

0.00 0.00

0.0- 0.0 0.0- 0.0

5.26 0.00 0.18 1.27 1.47 0.49 0.00 0.00

2.2-12.8 0.0- 0.0 0.0-0.6 0.0- 2.6 0.2-2.7 0.0-1.0 0.0- 0.0 0.0- 0.0

1.19 0.09 11.69 4.76 14.03 0.77 0.50 0.39

0.0-12.8 0.0- 5.8 0.0-64.7 0.0-18.6 0.2-51.3 0.0-10.7 0.0- 3.7 0.0- 9.0

60.11 35.4-75.4 0.68 0.0- 1.8 8.13 4.2-11.0 1.33 0.3- 2.8 0.08 0.0- 0.4 0.00 0.0- 0.0 0.00 0.0- 0.0

13.58 0.41 2.15 1.39 8.71 0.76 4.51

0.0-75.4 0.0- 3.0 0.0-11.0 0.0- 9.9 0.0-62.0 0.0-13.1 0.0-38.2

3.85 1.88 4.20 1.01 0.91 1.19 0.92 0.39 1.17 1.00 0.58 0.88 2.37 0.31 7.42

0.0-17.1 0.0-49.5 0.0-19.2 0.0-10.2 0.0- 5.0 0.0- 4.6 0.0-13.0 0.0-10.7 0.0- 8.8 0.0-14.4 0.0- 6.1 0.0- 8.6 0.0-22.7 0.0- 3.5 0.2-22.9

0.00 0.06 26.84 2.18 12.85 0.06 1.51 0.57

0.0- 0.0 0.0- 0.6 4.2-64.7 0.0- 4.8 1.6-28.7 0.0-0.7 0.2- 3.7 0.0- 2.5

0.01 0.01 7.15 11.19 16.83 0.08 0.11 0.64

0.0- 0.2 0.0- 0.2 1.0-22.5 3.5-18.6 1.7-28.7 0.0-0.5 0.0- 1.2 0.0- 9.0

1.65 0.30 5.21 1.94 20.28 2.81 0.03 0.08

0.0- 6.5 0.0- 5.8 0.0-25.1 0.7- 4.2 7.6-51.3 0.0-10.7 0.0- 0.4 0.0- 0.4

0.58 0.01 0.00 0.00 0.44 2.43 13.36

0.0-1.5 0.0- 0.2 0.0- 0.0 0.0- 0.0 0.0- 1.9 0.0-13.1 3.6-38.2

3.48 0.02 0.07 0.04 17.27 0.05 1.28

0.0-14.4 0.0- 0.2 0.0- 0.5 0.0- 0.2 2.1-62.0 0.0-0.6 0.0-5.6

13.37 1.28 3.80 5.08 14.07 0.00 0.00

3.0-33.2 0.0- 3,0 1.3- 7.2 0.4- 9,9 0.5-52.3 0.0- 0,0 0.0- 0.0

3.88 0.13 4.39 0.00 2.67 1.74 2.94 1.17 0.51 0.01 0.14 2.58 0.51 0.93 9.40

0.4-13.1 0.0- 1.2 1.3-10.3 0.0- 0.0 0.0- 5.0 0.2- 3.1 0.3-13.0 0.0-10.7 0.0- 3.6 0.0- 0.2 0.0- 0.4 0.0- 8.6 0.0- 2.5 0.0- 3.5 3.3-22.9

6.51 5.22 8.51 0.02 0.22 1.47 0.04 0.08 0.26 0.13 1.63 0.29 6.68 0.07 7.33

0.2-17.1 0.2-49.5 2.3-19.2 0.0- 0.6 0.0- 1.7 0.0- 4.6 0.0- 0.2 0.0- 0.9 0.0- 2.5 0.0- 1.0 0.0- 6.1 0.0- 5.6 0.0-22.7 0.0- 1.2 2.9-22.8

2.75 1.03 1.00 3.44 0.08 0.86 0.02 0.01 3.36 3.25 0.15 0.00 0.70 0.00 4.88

0.2- 7.3 0.0- 4.3 0.0- 4.1 0.0-10.2 0.0- 0.6 0.0- 2.7 0.0- 0.2 0.0- 0.2 0.0- 8.8 0.5-14.4 0.0- 0.6 0.0- 0.0 0.0- 2.1 0.0- 0.0 0.2-10.5

amounts in cores 9202, 9203, 9204 and OS6 (Table 1). Here, the mean values lie around 60% but the individual samples range between 43 and 76%. A second level of sand is 3-5%, which is seen in cores OS16, 9201, OS2 and OS5. Site 9205 forms a separate group with a mean of 19% sand. Fine-grained sediments with sand contents below 1% are found at sites OS15, OS1, OS3 and O S 4 . 2 1 ° p b derived accumulation rates in the deep central part of the Skagerrak at sites OS1 and OS15 give mean values of 2 mm.y -1.

D 9203, 9204

0.15 0.00 0.02 1.33 0.00 0.02 0.00 0.00 1.00 1.39 0.02 0.00 0.00 0.00 7.48

0.0- 0.8 0.0- 0.0 0.0- 0.3 0.0- 2.3 0.0- 0.0 0.0- 0.2 0.0- 0.0 0.0- 0.0 0.2- 1.8 0.3- 2.7 0.0- 0.2 0.0- 0.0 0.0- 0.0 0.0- 0.0 1.0-18.4

0.21 0.0- 2.9 0.74 0.0- 6.4

5. DISCUSSION The surface sediments in the Skagerrak have been investigated by e.g. Olausson (1975), Van Weering (1981), Denneg&rd & Kuijpers (1992), Van Weering et al. (1993) and Stevens et aL (1996). According to these investigations sand or sandy sediments are found on the shallower southern flank of the Trench. Van Weering (1981) also found coarser sediments in places close to the Norwegian coast related to

116

H. BERGSTEN, K. NORDBERG & B. MALMGREN

-0.4 -0.3 -0.2 -0.1 ,,.., .... , . . . . , . . . .

Haplophragmoides bradyi Ammoglobigerina globigeriniformis Pullenia bulloides Glomospira charoides Pullenia subcarinata Cribrostomoides nitida Trochammina pusilla Bolivina skagerrakensis Melonis barleeanus Valvulina conica Saccammina spp. Pullenia osloensis Ammodiscus spp. Cribrostomoides subglobosus Hyalinea balthica Verneulina media Cassidulina laevigata Uvigerina peregrina Trifarina angulosa Bulimina marginata Globobulimina turgida Liebusella goesi Biloculinella infiata Stainforthia fusiformis Cribrostomoides jeffreysi Elphidium excavatum Elphidium gerthi Textularia bocki Planorbulina distoma Ammonia beccarii Gavelinopsis praegri Elphidium magellanicum

0 ,

0.1 ....

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0.2 ....

0.3

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Haplophragmoides bradyi A mmoglobigerina globigeriniformis Pullenia bulloides Glomospira charoides Pullenia subcarinata Cribrostomoides nitida Trochammina pusilla Bolivina skagerrakensis Melonis barleeanus Valvulina conica Saccammina spp. Pullenia osloensis Ammodiscus spp. Cribrostomoides subglobosus Hyalinea balthica Verneulina media Cassidulina laevigata Uvigerina peregrina Tri farina angulosa Bulimina marginata Globobulimina turgida Liebusella goesi Biloculinella inflata Stainforthia fusiformis Cribrostomoides jeffreysi Elphidium excavatum Elphidium gerthi Textularia bocki Planorbuli na distoma Am monia beccarii Gavelinopsis praegri Elphidium magellanicum

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Fig. 4. Species Ioadings along the first principal component axis and 95% confidence intervals determined using bootstrap principal components analysis (1000 replicates).

Fig. 5. Species Ioadings along the second principal component axis and 95% confidence intervals determined using bootstrap principal components analysis (1000 replicates).

morainic sediments here or the winnowing of these. Fine-grained sediments dominate in the deeper parts of the Skagerrak. This pattern is in accordance with the sediment types found along the investigated transect where the sand-rich sediments (43-76% sand, Table 1) are found along the southern slope and at one site close to the Norwegian coast, while the central parts of the Skagerrak generally display fine-grained sediments (0-4% sand). Van Weering et aL (1993) presented sedimentation rates for the Skagerrak with the highest rates in the eastern and northeastern parts, and the lowest rates in the deep basin. Our 21°pb derived sedimentation rates from the deepest parts of the Skagerrak show values of about 2 mm.y- 1 , which is similar to those presented by Van Weering et aL (1993) for this area. This accumulation rate indicates that the uppermost 8 cm of the central Skagerrak cores of this study represent approximately 40 years. On the slopes of the investigated transect, accumulation rates vary but are generally lower than in deeper parts (Denneg&rd et aL, 1992). The sea-floor environment changes drastically over the profile due to bathymetrical differences, current velocities, different substrates and the influence of dif-

ferent water masses (Figs 2 and 6). These variables influence the habitats of the benthic foraminifera, directly or indirectly. Percentage abundances are generally used to distinguish different foraminiferal assemblages. However, with the use of PCA we have been able to statistically and graphically illustrate the interrelationships between the 93 samples and the 13 different sites. The combination with bootstrap analysis makes it possible to determine significant species for each of the four groups along the transect. In this way we can point out not only species that occur abundantly along the profile but, more importantly, also those that are statistically significant for a specific area without necessarily being very common. The samples within groups C and D form a tighter grouping than those of A and B, reflecting a higher variability between the different samples of the latter two groups. Plotting the four statistically determined groups (A-D) along the transect shows that they occupy different and specific geographic areas (Figs 1 and 7) with A in the deepest parts of the Skagerrak, B and C on the Norwegian and Danish slopes respectively, and D in the area close to the Danish coast. Conradsen et al. (1994) produced distribution maps of the most abundant recent benthic foraminifera in

RECENT BENTHIC FORAMINIFERA IN THE SKAGERRAK TABLE 3 Significant species for groups A-D given by the bootstrap PCA analysis (Figs 3-5). GROUP A (0S3, 0S2, 0S1, 0S15) Haplophragmoides bradyi Ammmoglobigerina globigeriniformis Pullenia bulloides Glomospira charoides Pullenia subcarinata Trochammina pusilla Bofivina skagerrakensis GROUP B (0S6, 0S5, 0S4, 9201) Uvigerina peregrina Bulimina marginata Trifarina angulosa Liebusella goesi Hyalinea balthica GROUP C (0S16, 9205, 9202) Stainforthia fusiformis Cribrostomoides jeffreysi Gavelinopsis praegeri GROUP D (9203, 9204) Elphidium excavatum Elphidium magellanicum Ammonia beccarii different parts of the Kattegat and Skagerrak, and demonstrated the correspondence between the fauna and the general hydrography of this vast area. The distribution of species in the present study is generally in agreement with these previously published maps. However, a clear difference occurs compared to Conradsen et al. (1994), where the central, deep parts were characterized by a widespread B. skagerrakensis assemblage. In our study, we can distinguish a significant assemblage group (A) in the deepest parts of the Skagerrak (>400-500 m depth), which is characterised by e.g.H, bradyi (Table 3). This feature is also seen by AIve & Murray (1995), who have analysed recent benthic fauna in surface sediments from the Norwegian slope and the deep Trench. Due to different size fractions studied by us and Conradsen et al. (1994) (125 p) compared to Alve & Murray (1995) (63 pm) there are higher abundances of smaller species in the latter study and their slope assemblages are, therefore, partly different from ours. Infaunal and epifaunal species inhabit different depth intervals in the sediment and deep-infaunal species may influence the vertical abundance distribution within the investigated 0-8 cm. When evaluating the data no such effects were found, with the exception of G. turgida. It generally has a sub-surface maximum and the percentage difference between this

117

interval and the over- and underlying intervals can be quite high. 5.1. AREA A-D The cores in the shallow, sandy area off Denmark (9203 and 9204; group D) are situated in a transport bottom area, where the accumulation rates are irregular, but generally quite low (Denneg&rd et al., 1992). Here variable conditions prevail with varying salinities, temperatures, nutrient supply and current velocities including their direction (Rodhe, 1987, 1996; Rydberg, 1993; Rydberg et aL, 1996). Also, the erosion of the nearby coast and the transport of sediments along the coast are characteristic features. The high sand content in area D (>60%; Table 1) is partly due to the transport of sediments, high current velocities and the great dominance of sandy sediments within the region, including the Danish mainland. Both seasonal and short-term changes of the physical and chemical variables induce an unstable bottom environment in area D. The faunas of group D are dominated by Elphidium excavatum, E. magellanicum and A. beccarii (Tables 2-3). E. excavatum is an extremely tolerant, eurytopic species that can withstand high variability in environNORWAY Stn nr

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118

H. BERGSTEN, K. NORDBERG & B. MALMGREN

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Fig. 7. Depth profile of the 13 investigated sites between Arendal (Norway) and Hirtshals (Denmark). The location of the profile K-K' is given in Fig. 1. Positions and water depths of the sites are listed in Table 1.

mental factors such as temperature and salinity. This species dominates along the Danish Skagerrak coast down to approximately 200 m water depth and at marginal, shallow areas of the Kattegat (Conradsen et aL, 1994). Site 9204 is situated in a shallow trough off the Danish coast. Here, more or less stagnant conditions can occur occasionally, which is confirmed by the black, sulphide-coloured, sediments and the low-diversity fauna. In group D many of the foraminiferal tests, including those of the dominant species E. excavatum, occur in a worn state suggesting redeposition of a large part of the fauna. The redeposition is probably related to the continuous internal transport of sand within the area (Kuijpers et aL, 1993) and most specimens are probably transported shorter or longer distances repeatedly. In this part of the transect the water masses flow into the Skagerrak and are dominated by the South Jutland Current (SJC) (Fig. 2), which is characterized by waters from the southern North Sea (Rodhe & Holt, 1996; Rydberg et al., 1996). Site 9203 is also partly influenced by shallow waters from the NW and W North Sea. The sites of group C (9202, 9205 and OS16) are situated along the Danish slope of the Norwegian Trench. Site 9202 contains equally large amounts of sand (56-65%) as the sites of group D (9204 and 9203). For the other two sites within group C the sand content decreases with increasing water depth (9205, 14-28% and OS16, 5-6%). The dominating species for group C are C. laevigata, G. turgida and E. excavatum. The comparatively high percentages of E. excavatum at site 9202 (8-33%) probably reflect the transport and redeposition of these tests together with sand, which is supported by the comparatively high amounts of worn

tests of this species. However, neither of the two other main species nor the remaining fauna display many worn tests and most species are probably found in situ in these sandy sediments, as well as in the finer sediments at greater depths of the group C area. As E. excavatum most probably inhabits the sandy sediments of the group D area (cf. Conradsen et aL, 1994), but not the sediments of the equally sandy site 9202, we suggest that the sediment texture is of minor importance for the species habitat here, at least for the most abundant species. Planktonic foraminifera are generally very sparse at all investigated sites but show their highest frequencies within group C (Table 1). These higher frequencies of planktonic foraminifera suggest a comparatively stronger influence of oceanic surface water along this part of the transect. The sites of group C are influenced by large water masses of oceanic salinities flowing into the Skagerrak, with year-based maximum mean velocities at depths of about 200-400 m (Fig. 6; Rodhe, 1987). Generally, the water masses influencing the sites of group C are dominated by the STC (Fig. 2), which, in turn, is dominated by water masses from the NW and W North Sea (DC) and the North Atlantic Ocean (TBC). Site 9202 is influenced by the DC and the upper part of the TBC, both affecting the water masses down to approximately 200 m. The deeper sites of group C, 9205 and OS16, are mainly influenced by the oceanic water from the North Atlantic, via the TBC. The sites of group B are found at water depths that are quite similar to those of group C. Three of the four sites are situated along the Norwegian slope of the Trench and the fourth along the Danish slope. The sand contents of group B vary a great deal (0-56%)

RECENT BENTHIC FORAMINIFERA IN THE SKAGERRAK

with the sandiest sediments close to the Norwegian coast (OS6). The Holocene record of OS6 is only 20 cm, indicating that this site is situated within a transport or erosion area on the northern flank of the Trench, where a lot of winnowed material from older deposits could be part of the sediment load (cf. Van Weering, 1981). The sites of group B are characterized by C. laevigata, H. balthica, B. marginata and U. peregrina that occur with varying abundances at the different sites. Especially the latter two species, which are more abundant in OS6 and OS5, show signs of recrystallization, discoloration and transportation. Site 9201 is the only site within group B that is situated on the Danish slope, between site OS16 of group C, and site OS15 of group A (Fig. 7). Inspection of the PCA plot (Fig. 3) shows that 9201 is a transitional site with scores close to, especially, OS16, OS15 and OS3. Despite similar water depths and sediment texture variations along the slopes on both sides of the Trench, the PCA shows that the samples cluster into two groups, B and C, indicating different faunal characteristics. The sandy sediments at the Norwegian slope, especially OS6, do not have the characteristic high abundances of E. excavatum as seen in the other sites with high sand contents on the southern flank of the transect (9202, 9203 and 9204). However, C. laevigata is the most abundant species of most sites within both group B and C, independently of sand content. For the Skagerrak and Kattegat region there is a general relationship between fine-grained sediments and high nutrient and carbon contents (e.g. Olausson, 1975; Van Weering, 1981 ). Accordingly, the very variable sediment texture of the sites within groups B and C probably gives rise to a different nutrient supply for the benthic foraminifera. In spite of this, the PCA groups B and C as two separate clusters (Fig. 3), which suggests that nutrient supply is not a major steering factor for the most abundant species of these assemblages. The presence of sandy sediments on both sides at similar water depths with different characteristic species suggests that both the sediment texture and the bathymetry are of minor importance for the dominant foraminiferal species along the transect. Within the group B area, the environment is characterized by outflowing water, provided by the previously inflowing water along the Danish slope described above. These water masses have had a residence time in the Skagerrak of about 100 days (Rodhe, 1987). The Norwegian Coastal Current (NCC) with its Baltic component, causing lower salinities as a diagnostic feature, can be traced down to about 100 m depth and has therefore only a minor effect on the sites of group B. The sites of group A are situated at the deepest parts of the transect, where the sediments are generally fine-grained. Agglutinated species are frequent,

119

with H. bradyi as the abundant characteristic species. Several of the abundant species generally live at greater water depths, e.g.H, bradyi, G. charoides, T. pusilla and Valvulina conica. The concentration of foraminifera is lowest at the two deepest sites of the transect (30-80 specimens per g sediment), while the remaining sites of the transect show higher and quite consistent values (80-200 specimens per g sediment). At least partly, this feature could be explained by lower sediment accumulation rates at the slopes. Rosenberg et al. (1996) explain lower numbers of benthic macrofaunal species per square metre in the deeper stations by higher accumulation rates and higher clay contents. A lower concentration of meiofauna in the deepest parts of the Skagerrak was found also by De Bovee et al. (1996), who studied the quantitative distribution of metazoan meiofauna. They found a decreasing trend of the animal abundances with increasing water depths, while we found more consistent values of foraminifera above 450 m water depth. De Bovee et aL (1996) explained the lower abundance of animals at greater depths by high accumulation rates and high concentrations of dissolved manganese in the pore water of these sediments. We found abundant manganese-rich particles in the sediments of group A, especially at sites OS1 and OS15. The water mass characterizing the deepest parts of the Skagerrak is of north Atlantic origin and flows into the area by the TBC (Fig. 2). The residence time of the deep water here is about 25 months and the mean oxygen depletion rate is 0.04 cm3-dm -3 per month (Aure & Dahl, 1994). These features together with the long-term temperature curves published by Ljoen & Svansson (1972) suggest that the deep Skagerrak basin can be described as stagnant in a broad sense. This environment together with the comparatively higher accumulation rates and the possibly inhibiting concentrations of manganese contribute to lower concentrations of meiofauna here, including the benthic foraminifera. 5.2. ENVIRONMENTAL INFLUENCE Crucial factors that influence the benthic foraminiferal composition and distribution are e.g. salinity, temperature, water depth, sediment texture and composition, food availability and dissolved oxygen concentration. Most of these variables are directly or indirectly linked to the characteristics of the overlying water mass. In this study, sites located at similar water depths and with comparable temperatures and salinities, on both sides of the Norwegian Trench, show different foraminiferal assemblages. Also sites with very different substrates display similar faunas when affected by the same water mass. When looking at the spatial distribution of the foraminiferal groups defined here it can be seen that they inhabit areas that correspond

120

H. BERGSTEN, K. NORDBERG & B. MALMGREN

with the briefly known extensions of the different water masses. Our results further extend the knowledge of the different water mass delimitations along the sea-floor, and we suggest that the four foraminiferal groups can be used as distinct tracers of the different water masses along the investigated transect. For on-going and future studies of different tracers, natural or anthropogenic contaminants, related to specific water masses we suggest that all areas apart from that of group A are applicable. The group A area is known to be influenced by many different water masses and by the sedimentation of different particles with various provenance from the overlying water masses, which produce a mix of signals. This could be explained by the fact that the Skagerrak deep basin is the main depositional area for fine sediments for the entire North Sea region, including the outflowing Baltic water. The other areas, however, represent, more clearly, the influence of specific water masses. 6. CONCLUSIONS Foraminiferal samples along a transect between Denmark and Norway, across the Skagerrak, have been subdivided into four assemblage groups, termed A-D, by the use of principal components analysis. Well-defined foraminiferal assemblages (A-D) seem to be related to specific water masses. The sites of assemblage group D, close to Denmark, are linked to the South Jutland Current, which flows into the Skagerrak and is characterized by water masses from the southern North Sea. Significant species are: Elphidium excavatum, Elphidium magellanicum and Ammonia beccarii. Assemblage group C, at the Danish slope of the Norwegian Trench, is characterized by inflowing water masses derived from the NW and W North Sea and the Atlantic Ocean. Significant species are: Stainforthia fusiformis, Cribrostomoides jeffreysi and

-

-

-

Gavelinopsis praegerL - Assemblage group B, at the Norwegian slope of the Norwegian Trench, is influenced by the outflowing water masses, given under C above, which are here affected by a residence of about 100 days in the Skagerrak. Significant species are: Liebusella goesi,

Hyalinea balthica, Uvigerina peregrina, angulosa and Bulimina marginata.

Trifarina

- Assemblage group A, in the deepest parts of the Norwegian Trench, is influenced by the Skagerrak deep water which can be described, in a wide sense, as stagnant oceanic water. Significant foraminiferal species are: Haplophragmoides bradyi, Ammoglo-

bigerina globigeriniformis, Pullenia bulloides, Glomospira charoides, Pullenia subcarinata, Trochammina pusilla and Bofivina skagerrakensis. Within this study we have identified areas where it is possible to investigate effects on the sea floor of different tracers, natural or anthropogenic contami-

nants, related to specific water masses. Acknowledgements.--The authors are grateful to the crews of RV 'Argos' and RV 'Ocean Surveyor' for valuable cooperation during sampling. Robert Relfson and Mikael Gustafsson, GSteborg, Sweden, are thanked for field and laboratory assistance. The manuscript benefited from the comments by Associate Prof. Karen Luise Knudsen,/~rhus, Denmark and Dr Elisabeth Alve, Oslo, Norway. We are grateful to Prof. I. Nick McCave and Dr Andy Buckley, Cambridge, U.K., for 21°pb determinations. The study was financially supported by the National Swedish Environmental Protection Agency (SNV), the Futura Foundation and the Wilhelm and Martina Lundgren Science Foundation. 7. REFERENCES Aitchison, J., 1986. The statistical analysis of compositional data. Chapman & Hall, London: 1-416. Alve, E. & J.W. Murray, 1995. Benthic foraminiferal distribution and abundance changes in Skagerrak surface sediments: 1 9 3 7 (HSglund) and 1992/93 data compared.--Mar. Micropaleont. 25" 269-288. Aure, J. & E. Dahl, 1994. Oxygen, nutrients, carbon and water exchange in the Skagerrak Basin.--Cont. Shelf Res. 14; 965-977. Conradsen, K., H. Bergsten, K.L. Knudsen, K. Nordberg & M.S Seidenkrantz, 1994. Recent benthic foraminiferal distribution in the Kattegat and the Skagerrak, Scandinavia.--Cushman Foundation Sp. Publ. 32: 53-68. Corliss, B.H. & T.C.E Van Weering, 1993. Living (stained) foraminifera within surficial sediments of the Skagerrak.--Mar. Geol. 111; 323-335. De Bovee, F., P.O.J. Hall, S. Hulth, G. Hulthe, A. Landen & A. Tengberg, 1996. Quantitative distribution of metazoan meiofauna in continental margin sediments of the Skagerrak (northeastern North Sea).--& Sea Res. 35" 189-197. Denneg&rd, B., A. Jensen, A. Kuijpers & T.C.E Van Weering, 1992. Quaternary sediment accumulation and recent sedimentary processes in the Skagerrak and northern Kattegat, an overview. In: B. Denneg&rd & A. Kuijpers. Marine geological environmental investigations in the Skagerrak and northern Kattegat. Univ. GSteborg, Dept Marine Geology, Report 7; 6-14. Denneg&rd, B. & A. Kuijpers 1992. Marine geological environmental investigations in the Skagerrak and northern Kattegat. Univ. G~teborg, Dept Marine Geology, Report 7: 1-100. Diaconis, P. & B. Efron, 1983. Computer-intensive methods in statistics.--Scient. Am. 248; 96-108. Dooley, H., 1974. Hypothesis concerning the circulation of the northern North Sea.--& Cons. perm. int. Explor. Mer 36: 54-61. Fumes, G.K., B. Hacket & R. Saetre, 1986. Retrofiection of Atlantic water in the Norwegian Trench.--Deep-Sea Res. 33: 247-265. HSglund, H., 1947. Foraminifera in the Gullmar Fjord and the Skagerak.--Zoologiska Bidrag fr•n Uppsala 26: 1-189. Kuijpers, A., F. Werner & J. Rumohr, 1993. Sandwaves and other large-scale bedforms as indicators of non-tidal surge currents in the Skagerrak off northern Denmark.--Mar. Geol. 111" 209-221. Lange, W., 1956. Grundproben aus Skagerrak und Katte-

RECENT BENTHIC FORAMINIFERA IN THE SKAGERRAK

gatt, mikrofaunistisch und sedimentpetrografisch untersucht.--Meyniana 5; 51-86. Lee, A.J., 1980. North Sea: Physical oceanography. In: F.T. Banner, M.B. Collins & K.S. Massie. The North-West European shelf-seas: the sea bed and the sea in motion. II. Physical and chemical oceanography, and physical resources. Elsevier Oceanographic Series 24B, New York: 467-493. Ljoen, R. & A. Svansson, 1972. Long-term variations of subsurface temperatures in the Skagerrak.--Deep-Sea Res. 19" 277-288. Moodley, L., S.R. Troelstra & T.C.E van Weering, 1993. Benthic foraminiferal response to environmental change in the Skagerrak, northeastern North Sea.--Sarsia 78" 129-139. Murray, J., 1991. Ecology and palaeoecology of benthic foraminifera. Longman, Avon: 1-397. Nordberg, K., 1991. Oceanography in the Kattegat and Skagerrak over the past 8000 years.--Paleoceanogr. 6: 461-484. Olausson, E., 1975. Man-made effects on sediments from Kattegat and Skagerrak.--Geologiska F6reningens i Stockholm F6rhandlingar 97." 3-12. Pearson, K., 1897. Mathematical contributions to the theory of evolution. On a form of spurious correlation which may arise when indices are used in the measurements of organs.--Proc. R. Soc. London 60: 489-498. Rodhe, J., 1987. The large-scale circulation in the Skagerrak: interpretation of some observations.--Tellus 39A: 245-253. - - , 1992. Studies of currents and mixing in the Skagerrak. Univ. G6teborg, Dept Oceanography, PhD Thesis: 1-110. - - , 1996. On the dynamics of the large-scale circulation of the Skagerrak.--J. Sea Res. 35: 9-21. Rodhe, J. & N. Holt, 1996. Observations of the transport of suspended matter into the Skagerrak along the western and northern coast of Jutland.-~J. Sea Res. 35" 91-98. Rosenberg, R., I. Cato, L. F6rlin, J. Rodhe, A. Thuren & K. Grip, 1991. Large-scale environmental effects and ecological processes in Skagerrak-Kattegat. National Swedish Environmental Protection Agency (SNV),

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Solna, ISBN 91-620-3922-9: 1-79. Rosenberg, R., B. Hellman & A. Lundberg, 1996. Benthic maerofaunal community structure in the Norwegian Trench, deep Skagerak.--J. Sea Res. 35:181-188. Rydberg, L., 1993. On the Skagerrak circulation and the supply of water from the southern North Sea to the Skagerrak. Univ. G6teborg, Dept Oceanography, Report 53: 1-13. Rydberg, L., J. Haamer & O. Liungman, 1996. Fluxes of water and nutrients within and into the Skagerrak.--J. Sea Res. 35" 23-38. Seidenkrantz, M.-S., 1993. Subrecent changes in the foraminiferal distribution in the Kattegat and Skagerrak, Scandinavia: anthropogenic influence and natural causes.--Boreas 22: 383-395. Stevens, R., H. Bengtsson & A. Lepland, 1996. Textural provinces and transport interpretations with fine-grained sediments in the Skagerrak.--J. Sea Res. 35:99-110.

Stigebrandt, A. 1983. A model for the exchange of water and salt between the Baltic and the Skagerrak. -J. Phys. Oceanogr. 13:411-427. Svansson, A., 1965. Some hydrographic problems of the Skagerrak and the Kattegat.--Progr. Oceanogr. 3: 355-372. --, 1975. Physical and chemical oceanography of the Skagerrak and the Kattegat, I: Open sea conditions. Fishery Board Sweden, Insitute of Marine Research, Report 1: 1-88. , 1984. Hydrographic features of the Kattegat.--Rapp. P.-v. Reun. Cons. perm. Int. Explor. Mer 185: 78-90. Van Weering, T.C.E., 1981. Recent sediments and sediment transport in the northern North Sea: surface sediments of the Skagerrak.--Spec. Publ. int. Ass. Sedimentol. 5" 335-359. Van Weering, T.C.E. & G. Qvale, 1983. Recent sediments and foraminiferal distribution in the Skagerrak, northeastern North Sea.--Mar. Geol. 52: 75-99. Van Weering, T.C.E., G.W Berger & E. Okkels, 1993. Sediment transport, resuspension and accumulation rates in the northeastern Skagerrak.--Mar. Geol. 111" 269-285.