High-resolution uranium-series dating of Norwegian-Greenland Sea sediments: 230Th vs. δ18O stratigraphy

MARINE G OLOGY INTERNATIONAL JOfJNNAL O f MARINE

GEOLOGY, GEOCIfEIA~TRY AND GEOPNVSIC8

ELSEVIER

Marine Geology 121 (1994) 77-85

High-resolution uranium-series dating of Norwegian-Greenland Sea sediments: 23°Th vs. 180 stratigraphy J.C. Scholten a, R. Botz a, H. Paetsch b, p. Stoffers a, M. Weinelt" " Geologisch-Pal?lontologisches Institut und Museum, Universitat Kiel, Olshausenstrafie 40, 24118 Kiel, Germany b GCA-Geochemische Analysen, Wilhelmstrafle 40, D-31275 Lehrte, Germany

Received 15 December 1992; revision accepted 14 December 1993

Abstract

The ages and average sedimentation rates of four long sediment cores from the Norwegian-Greenland Sea were determined by using various 23°Th dating models. The results were then related to an independent time control (6180 stratigraphy) which was available for the cores under consideration. Average sedimentation rates were calculated from 23°Thexvs. depth profiles. These rates are similar to rates obtained by the 6~sO stratigraphy if the cores cover the past 300,000 years. A high time resolution is achieved by a 23°Th constant-flux model for those sediment cores which are not influenced by lateral sediment transport. A powerful tool for chronostratigraphic studies of sediment cores appears to be the combination of 23°Th measurements and ~180 stratigraphy, which allows the determination of individual sedimentation rates for each core layer.

1. In~oducfion

Ocean sediments give information on the paleoclimatic evolution in the geological past. The oxygen isotope composition of calcareous foraminifera depends on climatic conditions and can be used as a stratigraphic tool (Martinson et al., 1987; Shackleton and Opdyke, 1973). The ~lso method is restricted to areas where foraminifera are common constituents of the sediments. Due to the relatively cold water conditions in high latitude areas, sediments from those oceans often contain little or no calcareous foraminifera and sediment ages rely, therefore, on paleomagnetic, biostratigraphic and radionuclide dating (Herman et al., 1990; Bleil and Gard, 1989). There is clear evidence for a close relation between deep-water formation in high-latitude areas (Norwegian-Greenland Sea) and climate changes. As ocean sediments reflect paleo0025-3227/94/$7.00© 1994ElsevierScienceB.V. All rights reserved SSDI 0025-3227(94)00082-4

ceanographic conditions, age determinations on sediment cores are of great importance for understanding paleoclimate evolution. The excess 23°Th method (23°Thcx, nonsupported 23°Th in sediments) allows age determinations up to 300,000 yr of carbonate-poor sediments from high latitude areas, e.g. NorwegianGreenland Sea, Fram Strait, Arctic Ocean (Scholten et al., 1990; Eisenhauer et al., 1990). The 23°Thcx method relies on a constant production rate of 23°Th from the radioactive decay of dissolved 234U in the water column. Because 23°Th is highly particle-reactive it is removed within 10-40 years from the water column into the sediments (Nozaki et al., 1987). Within the sediment 23°Th decays (half-life of 75,200 years), a process which can be used for age dating purposes. In order to obtain a detailed chronostratigraphy for high latitude sediments from the NorwegianGreenland Sea, 23°Zhex was measured in four

78

J. C. Scholten et al./Marine Geology 121 (1994) 77-85

Arctic Ocean

Nansen Basin

Greenland

Fram Slrait

• 17728 Barents Sea

Greenland Sea

23259 • Jan Mayen

~_~.~S I Nor~ 23059 23065 • V16ring Plateau

1Oo O*

Fig. 1. Locations of investigated sediment cores.

79

J. C. Scholten et al./Marine Geology 121 (1994) 77-85

sediment cores (Fig. 1). For these cores an independent time control (6180 stratigraphy) was available (Vogelsang, 1990; Henrich et al., 1989; Weinelt, 1993), so that the quality of the derived 23°Th ages and sedimentation rates can be examined. Furthermore, the 23°Th dating method can be improved by comparing the various isotope dating techniques.

2. Methods

The sediment cores 23059, 23065, 17728 and 23259 were taken during R/V M e t e o r cruises 2/2, 7/2 and 13/2 to the Norwegian-Greenland Sea. Continuous samples of the sediment cores were taken for 23°Th measurements. Each sample represents a homogenized 5-10cm vertical sediment (core) slice. The chemical procedure for Th and U measurements follows the description of Ku (1965) and Mangini (1984). The nuclides 2a2Th, 23°Th, 23aU and 234U were measured by alpha counting in the presence of a 22aTh and 232U spikes, which were added as yield tracers before chemical separation. Calibration of the spike was performed with DL-la certified reference ore. Each sample was counted for two days in the alpha-spectrometer. Due to the counting statistics of 23°Th measurements, the precision of 23°Th analyses varies between 5-10% for samples younger than approximately 130,000 yr and 10-50% for older samples. The 23°Th~ activities (dpm/g) were calculated by using the following equation: 23°Thex = 23°Th - 234U

( 1)

where 2a°Th and 234U are the activities analysed in the sample. The 23°Thcx activities of all cores investigated including the 1 error bars are shown in Fig. 2.

incorporated into the sediments with a constant activity (Ku and Broecker, 1966; DeMaster, 1979; Osmond et al., 1979). If this assumption is correct, the 23°Th~x activity in sediment cores should decrease exponentially with depth, and the average sedimentation rate can be calculated by the following equation: (2)

S=Vb

where S = sedimentation rate (cm/kyr); = decay constant (9.24 x 10 -6 yr-1); b = "best-fit" slope of the exponential regression curve for all the 23°Thcx data (cm- 1). In the marine environment, however, it is likely that sedimentation rates (and the 23°The~ activities) have varied considerably during the geological past. Thus, the constant activity model may not give valuable information on short-term oceanic events which then determine the sedimentation. When the average sedimentation rates, as derived from the 23°Tl%x method (cores 23059, 23065, 17728; Fig. 2), are compared with the rates determined by the 6180 stratigraphy, both data sets yield average sedimentation rates in the same order of magnitude (Table 1). For core 23259, however, the average sedimentation rate using the 23°Thex data differs significantly from the rate derived by the 6180 method. The poor correlation coefficient between depth and 2a°Tl%x activity for core 23259 ( r - - - 0 . 6 0 ) already indicates that the exponential regression model used for calculation of the average sedimentation rate does not precisely describe the distribution of 2a°Thex data in this core. As can be seen in Fig. 2, the 23°Th~x activities in the cores investigated show variations which are superimposed on the general decay curve Table 1 Comparison of average sedimentation rates. Core

3. Discussion

Average sedimentationrate (cm/kyr) deduced from 23°Tl~x

~tsO

1.9 2.1 1.1 2.7

1,7 2.0 1.2 4,3

3.1. Average sedimentation rates

The determination of average sedimentation rates from 23°Thex VS. depth profiles in sediment cores is based on the assumption that 23°Thcx is

23059 23065 17728 23259

80

J. C. Scholten et al./Marine Geology 121 (1994) 77-85

23059

23065

17728

230 Thex [dpm/g]

230 Thex [dpm/g]

0,1 0,2 0,5 1,02,0 5,0 0

0,1 0,2 0,5 1,0 2,0 5,0

2~o Thex [apmlg] 0,1 0,2 0,5 1,02,0 5,0 0

230 Thox [aWalgl 0,1 0,2 0,51,02,0 5,0 0

1(

100 200

200 2C

200

3OO

23259

400

31:

400

4(

40O

600 5OO

Depth [om] 600

5(

Depth [cml 800

t~ptt [cm 6f~

Depth [eml 800

Fig. 2. 23Wl'~x versus depth in all the cores investigated. Isotope stages are derived from the 6180 stratigraphy. The solid line represents the best expontential fit for all the 23°Thex data from which an average sedimentation rate (S) was calculated (r is the regression coefficient, 95% confidence level).

(regression line). These variations are, as discussed above, due to relatively short-term changes in sedimentation rates. For core 23259, these variations are so high that the calculation of a precise average sedimentation rate from a 23°Th,x versus depth profile is not possible. However, as shown by Scholten et al. (1990) variations in 23°Tl~x activities can be used to establish a 23°Thox stratigraphy. This stratigraphy is based on characteristic variations of agecorrected 23°Thex(23°Thex0, the initial 23°Thex activities after correction for their radioactive decay using the average sedimentation rate obtained from the 23°Thox data), which can be correlated between cores from Norwegian-Greenland Sea, Fram Strait and Arctic Ocean (Botz et al., 1989; Scholten et al., 1990; Eisenhauer et al., 1990; Paetsch, 1991). All the cores investigated show relatively high 23°Th~x0 activities during stage 5 (Fig. 3). In core 23059, for instance, the maximum of 23°ThCx0

activities has, according to the 61so stratigraphy, an age of 111,000 years (Vogelsang, 1990). When transferring this age to the equivalent maxima of cores 23065, 17728 and 23259 (Fig. 3; Scholten et al., 1990) average sedimentation rates can be calculated. The results correspond to those derived from the 61so stratigraphy (Table 2).

3.2. Ages of individual sediment layers (23°Th constant-flux model) Several authors (DeMaster, 1979; Kominz et al., 1979; Osmond et al., 1979; Mangini et al., 1982; Mangini and Stoffers, 1990) used 23°Th constantflux models to calculate the age of individual sediment layers. These models are based on the assumption, that the flux of/3°Th into the sediments is constant through time. If the /3°Thex inventory [the cumulative 23°Thex activity of the total sediment core, Art (dpm/cm2)] is related to

J. C Scholten et al./Marine Geology 121 (1994) 77-85 23059 230 0

2

4

17728

23065

Thex0 (dpm/g) 6

230 Thex0 (dpm/g)

8

10

0

2

4

6

81

230

10

8

0

0

0 LI

23259

Th ex0 (dpm/g)

2 4 I LII

.

.

.

.

230

8 8 10 I I I I~

.

.

_

.

.

.

0 0

ThexO (dpm/g)

2 4 I I I I~l

6 8 10 t I I I I

,

20O

4OO

Depth (cm)

6O0

800

Del:

Depth (cm) Depth (cm)

Fig. 3. Age-corrected 23°Thex(23°Thex0) versus depth of all cores investigated. Isotope stages are derived from the 61sO stratigraphy. The age of 23°Th,x0 maximum of core 23059 (111,000 yr) was obtained from the 61sO stratigraphy and was transferred to the 23°Th.~0 maxima of the other cores (dashed line).

Table 2 Comparison of average sedimentation rates obtained from the 2a°Thox0 stratigraphy and the 61sO stratigraphy Core

Depth (cm)

Average sedimentation rate (era/year) deduced from

23°Th,xO maximum

23065 17728 23259

198.5 135 492

2a°Th~x stratigraphy

t~taO

1.8 1.2 4.4

1.8 1.2 4.1

the 23°Thexinventory [Nx (dpm/cm2)] of the surface sediments down to a depth x (cm), the age of x [Tx (yr)] can be calculated (Kominz et al., 1979): Tx= -

1/2 x In(1 - N x / N , )

(3)

In contrast to the average sedimentation rate

obtained from Eq. 2 the constant-flux model accounts for short-term changes in sedimentation rates. The assumption of a constant 23°Th flux into the Norwegian-Greenland Sea sediments, however, describes an ideal situation, as it is assumed that lateral sediment transport (e.g. sediment winnowing, erosion and/or sediment focusing) has not influenced the 23°Th inventory. This is not a very likely assumption for at least parts of the sea areas under investigation (see below). Fig. 4 compares the ages derived from the 23°Th constant-flux model with those obtained from the 3180 stratigraphy. For cores 23059 and 23065 the ages of both data sets (constant-flux model and 6180) correspond reasonably well within 20%. For core 17728, however, the constant-flux model produces ages generally higher (compared to 6aso) for the last

82

J. C. Scholten et al./Marine Geology 121 (1994) 77-85

0 0

23059

23065

17728

Age (ky)

Age (ky)

Age (ky)

1O0 IIt

III

200 ~ I I

300

0

I J i I III

0

1O0 I

I I I

I t I

200 I I

0

300

I I I J d i

0

1O0 I I I I L

23259

200 I I

I i

Age (ky)

0

300 I I I I

0

40 80 120 I I I I I I I II i I I

tl. '

100

100

100

',

200 200

200-

120 300

3OO

180

300-

400

200 4OO

4 0 0

500

24(1 5O0

6OO

600

28O

7OO

320

Depth (cm) __

230 Th constant-flux model

Depth (cm)

Depth Icm) average sedimentationrate derived from 23o Th ex

Depth Icm) ....

0-18 stratigraphy

Fig. 4. Ages versus depth plot for the cores investigated. Ages were derived from 23°Th dating models and the 6180 stratigraphy.

150 cm to the bottom of the sediment core. In this core section 23°The~ activities are relatively low (Fig. 2), most likely due to lateral sediment transport (sediment winnowing and/or erosion). Hence, the difference between Nt and Nx is rather small and that is why the constant flux model tends to produce ages which are too large. Furthermore, in core 23259 6180 and 23°Th constant-flux ages differ significantly (the latter being higher--except for the depth interval 450-500cm; Fig. 3). The 23°Thex versus depth profile of this core shows relatively constant 23°Th~x activities between 200 and 400 cm which can be explained by lateral sediment supply. Core 23259 was recovered near the Barents Sea continental shelf, an area, where sediment focusing from the continental margin is a major factor controlling sedimentation (Honjo et al., 1988; Blaume, 1992; Rumohr, in press).

3.3. Combination of t~lSo stratigraphy and 230Thex In former studies, a combination of 23°Thex activities in sediments and 6180 stratigraphy (and/or 14C) was used to calculate sedimentation rates, which then were corrected for the lateral sediment supply (Bacon, 1984; Suman and Bacon, 1989; Francois et al., 1990; Francois and Bacon, 1991). This method requires the knowledge of the 23°Thex activities of the laterally supplied sediments. The 23°Thex activities depend on the origin of the transported sediments. Since the origin of the sediments in the Norwegian-Greenland Sea has varied considerably in the geological past (Henrich, 1990; Paetsch, 1991), this technique cannot be applied here. Variable 2a°Th fluxes into the sediments which are due to lateral sediment transport result in wrong ages. Such 23°Th flux changes can be quantified if an independent time

J. C Scholten et al./Marine Geology 121 (1994) 77-85 x2

control (biostratigraphy, ~4C, 6180 event stratigraphy) or 23°Tlhx stratigraphy is available. In the present study for various core sections the 6:sO stratigraphy and ~4C ages (for core 23259) enables us to quantify the 2a°Th flux into the sediments (differences in time scales between 6180 event stratigraphy and ~4C ages are negligible for the present study). This 23°Th flux [F, (dpm/cm2/kyr)] allows the calculation of sedimentation rates for each core horizon by using the following equations:

S = FJ23°Thex0 x p

83

where S

23°Thcx0 =

average

23°Zhex0

activity bet-

ween d~pths xl and x2; p (g/cm3)=bulk dry density between xl and x2 (data from Kassens, 1990); xl (XE)=dcpth of dated (by 61SO, 14C) horizon tt (t2). In Fig. 5 a comparison is made between the sedimentation rates obtained by Eq. 4 and the rates which were derived from the 61sO stratigraphy. Whereas the 6~aO stratigraphy gives average sedimentation rates for core intervals only, the combination of 23°Thex data with the 6180 stratigraphy results allows the determination of sedimentation rates for each horizon analysed for 23°Zh (within the given precision of 23°Th and bulk dry density analyses).

(4)

and X2 /*

F a = _1 (23°Thex 0 × p) X (X2--X1/t 2 - - t l ) XI

0

23065

Sedimentation rate (cm/ky)

Sedimentation rate (cm/ky)

2 I

0

23059

4

I

~ .

t

t

6 I

t

8 I

I

10 I

I

I

17728

0 2 4 6 8 10 12 14 16 0

23259

Sedimentation rate (cm/ky)

.=..

0

1

2

3

4

Sedimentation rate (cm/ky) 5

6

04

8 12162024283236

0 •

2 *

,

3

3

,

4

100 ¸ =

. . . . . ....=.=.

i

400

z

i

] , . . . -..:-, 8

e

600

300 ¸

/,

l

f

i 5

7

t: =

1~ . . . . . . . . . . . . . . . . Depth (cm)

6

Depth (cm)

De )th (cm)

Depth (cm)

Fig, 5. Sedimentation rates calculated from the combination of 2a°Th=~ and 61aO stratigraphy (solid line) and sedimentation rates obtained from 6laO only (dashed line). No sedimentation rates according to the combination model are given for those core sections having very low 2a°yl~x activities (large error). Isotope stages are derived from the t$1sO stratigraphy.

84

J. C. Scholten et al./Marine Geology 121 (1994) 77-85

4. Conclusions

References

Average sedimentation rates can be obtained for sediments f r o m the N o r w e g i a n - G r e e n l a n d Sea by calculating a best fit exponential regression line based on 23°Thex versus depth profiles. This procedure results in reliable average sedimentation rates as shown by a c o m p a r i s o n with the 6180 stratigraphy. However, the cores have to be old e n o u g h (about 300,000 years) to compensate for d o w n c o r e variations in the 23°Thex activities. These variations in 23°Thex activities are caused by climatically-induced changes in the sedimentation rates o f the N o r w e g i a n - G r e e n l a n d Sea during the late Quaternary, as proposed by Henrich et al. (1989). W h e n the 23°Whex activities are presented as agecorrected data, a 23°Tthx stratigraphy can be established. Hence, average sedimentation rates can again be determined for the sediment sequence, similar to ~lso. The quality o f the ages obtained by applying the 23°Th constant-flux model depends on the extent to which lateral sediment transport has influenced sedimentation. For sediments from pelagic areas o f the oceans, where lateral sediment transport is o f minor importance, this m e t h o d represents a reliable tool for dating carbonatep o o r sediments. F o r cores which are influenced by lateral sediment transport the results o f the constant-flux model can be improved significantly if an independent time control is available. The combination o f both 23°Thex in sediments and 6180 stratigraphy allows sedimentation rates to be calculated with a very high resolution.

Bacon, M.P., 1984. Glacial to interglacial changes in carbonate and clay sedimentation in the Atlantic estimated from 23°Th measurements. Isot. Geosci. Sect., 2: 97-111. Blaume, F., 1992. Hochakkumulationsgebiete am norwegischen Kontinentalhang. Bet. Sonderforschungsbereich 313, Univ. Kiel, 36: 1-150. Bleil, U. and Gard, G., 1989. Chronology and correlation of quaternary magnetostratigraphy and nannofossil biostratigraphy in Norwegian-Greenland Sea sediments. Geol. Rundschau, 79:1173-1187. Botz, R., Bohrrnann, H., Mangini, A., Paetsch, H., Scholten, J.C., Stoffers, P. and Thiede, J., 1989. 23°Th stratigraphy of high latitude sediments: A transect from the NorwegianGreenland Sea to the Arctic Ocean. Terra Abstr., 1: 1-334. DeMaster, D.J., 1979. The marine budgets of silica. Ph.D. Thesis, Yale Univ., New Haven, 308 pp. Eisenhauer, A., Mangini, A., Botz, R., Walter, P., Beer, J., Bonani, G., Suter, M., Hofmann, H.J. and Wtlfli, W., 1991. High resolution 1°Be and 23°Th stratigraphy of late Quaternary sediments from the Fram Strait (core 23235). In: U. Bleil and J. Thiede (Editors), Geological History of the Polar Oceans: Arctic versus Antarctic. Kluwer, Dordrecht, pp. 475-487. Francois, R. and Bacon, M.P., 1991. Variations in terrigenous input into deep equatorial Atlantic during the past 24,000 years. Science, 251: 1473-1476. Francois, R., Bacon, M.P. and Suman, D.O., 1990. 23°Th profiling in deep-sea sediments; High-resolution records of flux and dissolution of carbonate in the equatorial Atlantic during the last 24,000 years. Paleoceanography, 5: 761-787. Henrich, R., 1990. Cycles, rhythms and events in Quaternary Arctic and Antarctic glaciomarine deposits. In: U. Bleil and J. Thiede (Editors), Geological History of the Polar Oceans: Arctic versus Antarctic. Kluwer, Dordrecht, pp. 231-244. Henrich, R., Kassens, H., Vogelsang, E. and Thiede, J., 1989. Sedimentary facies of glacial-interglacial cycles in the Norwegian Sea during the last 350 ka. Mar. Geol., 86: 283-319. Herman, Y., Osmond, J.K. and Somayajulu, L.K., 1989. Late neogene Arctic paleoceanography: Micropaleontology, stable isotopes, and chronology. In: Y. Herman (Editor), The Arctic Seas: Climatology, Oceanology, Geology and Biology. Van Nostrand Reinhold, New York, pp. 581-655. Honjo, S., Manganini, S.J. and Wefer, G., 1988. Annual particle flux and winter outburst of sedimentation in the northern Norwegian Sea. Deep-Sea Res., 35: 1223-1234. Kassens, H., 1990. Verfestigte Sedimentlagen und seismische Reflektoren: FrOhdiagenese und Pal~io-Ozeanographie in der Norwegischen See. Ber. Sonderforschungsbereich 313, Univ. Kiel, 24:1 117. Kominz, M.A., Heath, G.R., Ku, T.-L. and Pisias, N.G., 1979. Brunhes time scales and the interpretation of climate change. Earth Planet. Sci. Lett., 45: 394-410. Ku, T.-L., 1965. An evaluation of the 234U/238Umethod as a

Acknowledgements We thank A. Spies and I. D o l d for their assistance with the laboratory work. The English was improved by C. Vogt. Two referees from Marine Geology (M. Segel and G. M c M u r t r y ) reviewed the manuscript and provided additional helpful comments. This w o r k was supported by the Deutsche Forschungsgemeinschaft and is contribution no. 179 o f the Sonderforschungsbereich 313, University o f Kiel.

J.C Scholten et al./Marine Geology 121 (1994) 77-85 tool for dating pelagic sediments. J. Geophys, Res., 70: 3457-3474. Ku, T.L. and Broecker, W.S., 1966. Atlantic deep-sea stratigraphy: extension of absolute chronology to 320,000 years. Science, 151: 448--450. Mangini, A., 1984. Datierung von Sedimenten und andere Anwendungen der Radionuklide 23°Th, 2atpa and t°Be in der marinen Geologic. Habil.-Schrift, Univ. Heidelberg, pp. 1-63. Mangini, A. and Stoffers, P., 1990. A high resolution 23°Th depth profile in a piston core from the Southern Lau Basin. Geol. Jahrb., Reihe D, 92: 255-261. Mangini, A., Dominik, J., Miller, P. and Stoffers, P., 1982. Deep Pacific circulation: A velocity increase at the end of interglacial stage 5? Deep-Sea Res., 29: 1517-1530. Martinson, D.G., Pisias, N.G., Hays, J.D., Imbrie, J., Moore, T.C. and Shackleton, N.J., 1987. Age dating and the orbital theory of the ice ages: Development of a high-resolution 0 to 300,000-year chronostratigraphy. Quat. Res., 27: 1-29. Nozaki, Y., Yang, H.-S. and Yamada, M., 1987. Scavenging of thorium in the ocean. J. Geophys. Res., 92(C1): 772-778. Osmond, J.K., 1979. Accumulation models of 23°Th and 231pa in deep-sea sediments. Earth Planet. Sci. Lett., 15: 95-150. Paetsch, H., 1991. Sedimentation im Europ~iischen Nordmeer:

85

Radioisotopische, geochemische und tonmineralogische Untersuchungen an sp~itquartaren Ablagerungen. Ber. Sonderforschungbereich 313, Univ. Kid, 29: 1-82. Rumohr, J., in press. A high accumulation area on the continental slope off northern Norway and the conception of winter cascades. Deep-Sea Res. Scholten, J.C., Botz, R., Mangini, A., Paetsch, H., Stoffers, P. and Vogelsang, E., 1990. High resolution 2a°Tl~ stratigraphy of sediments from high-latitude areas (Norwegian Sea, Fram Strait). Earth Planet. Sci. Lett., 101: 54-62. Shacldeton, N.J. and Opdyke, N.D., 1973. Oxygen isotope and paleomagnetic stratigraphy of Equatorial Pacific core V28-238: Oxygen isotope temperatures and ice volumes on a 105 and 10 6 year scale. Quat. Res., 3: 39-55. Suman, D.O. and Bacon, M.P., 1989. Variations in Holocene sedimentation in the northern American Basin determined from Z3°Thmeasurements. Deep-Sea Res., 36: 869-878. Vogelsang, E., 1990. Palao-Ozeanographie des Europ~iischen Nordmeeres an Hand stabiler Kohlenstoff- und Sauerstottisotope. Ber. Sonderforschungsbereich 313, Univ. Kiel, 23: 1-136. Weinelt, M., 1993. Ver~inderung der Obertt~ichenzirkulation im Europaischen Nordmeer w~ihrend der letzten 60,000 Jahre-Hinweise aus stabilen Isotope. Ber. Sonderforschungsbereich, Univ. Kiel, 41: 1-106.