Paired δ18O signals of Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba show thermal stratification structure in Nordic Seas

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Marine Micropaleontology 48 (2003) 107^125 www.elsevier.com/locate/marmicro

Paired N18 O signals of Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba show thermal strati¢cation structure in Nordic Seas Johannes Simstich a;1;
, Michael Sarnthein b , Helmut Erlenkeuser c a GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1^3, 24148 Kiel, Germany Institut fu«r Geowissenschaften, Christian-Albrechts-Universita«t, Olshausenstr. 40, 24118 Kiel, Germany Leibniz-Labor fu«r Altersbestimmung und Isotopenforschung, Christian-Albrechts-Universita«t, Olshausenstr. 40, 24118 Kiel, Germany b

c

Received 10 April 2002; received in revised form 10 November 2002; accepted 30 November 2002

Abstract The vertical density gradients in the Nordic Seas are crucial for the preconditioning of the surface water to thermohaline sinking in winter. These gradients can be reconstructed from paired oxygen isotope data in tests of different species of planktonic foraminifera, the isotopic signatures of which represent different calcification depths in the water column. Comparison of N18 O values from foraminiferal tests in plankton hauls, sediment traps, and nearby core top samples with the calculated N18 Ocalcite profile of the water column revealed species-specific N18 O vital effects and the role of bioturbational admixture of subfossil specimens into the surface sediment. On the basis of core top samples obtained along a west^east transect across various hydrographic regions of the Nordic Seas, N18 O values of Turborotalita quinqueloba document apparent calcification depths within the pycnocline at 25^75 m water depth. The isotopic signatures of Neogloboquadrina pachyderma (s) reflect water masses near and well below the pycnocline between 70 and 250 m off Norway, where the Atlantic inflow leads to thermal stratification. Here, temperatures in the calcification depth of N. pachyderma (s) differ from sea surface temperature by approximately 32.5‡C. In contrast, N. pachyderma (s) calcifies very close to the sea surface (20^50 m) in the Arctic domain of the western Nordic Seas. However, further west N. pachyderma (s) prefers somewhat deeper, more saline water at 70^130 m well below the halocline that confines the low saline East Greenland Current. This implies that the N18 O values of N. pachyderma (s) do not fully reflect the freshwater proportion in surface water and that any reconstruction of past meltwater plumes based on N18 O is too conservative, because it overestimates sea surface salinity. Minimum N18 O differences ( 6 0.2x) between N. pachyderma (s) and T. quinqueloba may serve as proxy for sea regions with dominant haline and absent thermal stratification, whereas thermal stratification leads to N18 O differences of s 0.4 to s 1.5x. 7 2002 Elsevier Science B.V. All rights reserved. Keywords: stable isotopes; planktonic foraminifera; habitat; Norwegian Sea

1 Former address: Institut fu«r Geowissenschaften, Christian-Albrechts-Universita«t zu Kiel, Olshausenstr. 40, 24118 Kiel, Germany * Corresponding author. Fax: +49-431-600-2941. E-mail addresses: [email protected] (J. Simstich), [email protected] (M. Sarnthein), [email protected] (H. Erlenkeuser).

0377-8398 / 02 / $ ^ see front matter 7 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0377-8398(02)00165-2

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1. Introduction The regions where deep water formation and ventilation takes place in the Nordic Seas (Fig. 1) belong to the most important convection sites of the total ocean. At the same time they act as a salinity pump in the system of the Atlantic thermohaline circulation, and as a ‘heat pump’ for northern Europe (Killworth, 1983; Schmitz, 1995). The cooling of warm, highly saline water masses of low-latitude origin facilitates winter convection (Schott et al., 1994; Send and Marshall, 1995). On the other hand, a dynamic sea

ice cover and the advection of lower saline Polar Water from the Arctic Ocean increases the strati¢cation in the upper water column. The southward £ow of newly formed intermediate and deep water masses from the Nordic Seas into the North Atlantic closes the circulation loop and, in turn, promotes the renewed in£ow of warm surface water (Schmitz, 1995). Geological records show how susceptible this system is to disturbances (Sarnthein et al., 1995, 2001). Fairly small amounts of freshwater added to the sea surface may weaken or even interrupt thermohaline overturn (Rahmstorf, 1995; Dick-

Fig. 1. Surface hydrography and locations of plankton tows, sediment traps, and sediment surface samples. Convection centers indicated by encircled C’s (Aagaard and Carmack, 1994; Schott et al., 1994). Broken lines envelope core top samples used for the transect of Fig. 6.

MARMIC 912 2-4-03

J. Simstich et al. / Marine Micropaleontology 48 (2003) 107^125

son et al., 1996). Thus, the mode of the Nordic Seas’ heat pump depends on the preconditioning of the surface water for deep convection. This process is intimately linked to the strati¢cation of the upper 50^150 m of the water column, as de¢ned by the temperature and salinity gradients across the pycnocline. Such gradients can be reconstructed for past climatic scenarios from comparison of paired oxygen isotope ratios of di¡erent planktonic foraminifera species with differential depth habitats. This approach was successfully applied to the tropical Atlantic (Ravelo and Fairbanks, 1992; Kemle-von Mu«cke, 1994; Mulitza, 1994; Mulitza et al., 1997), the Labrador Sea (Hillaire-Marcel et al., 2001), the Southern Ocean (Niebler, 1995; Mortyn et al., 2002), and the California Current (Weinheimer et al., 1999), however, not yet to the Nordic Seas. The present study establishes the water depths that are characteristic of the formation of isotopic signals of the planktonic foraminifera Neogloboquadrina pachyderma (sinistral coiling) (Ehrenberg 1861) and Turborotalita quinqueloba (Natland 1831). In a second step, these depths are employed as proxy for reconstructing the modern near-surface structure of this ocean region from core top sediments.

2. Samples and isotope data 2.1. Isotope data To reconstruct the modern strati¢cation of the Nordic Seas, sediment surface samples were analyzed from the top centimeter of giant box cores or multicores from the Nordic Seas and northwestern North Atlantic (Fig. 1; Table 1). The samples were wet sieved over a 63-Wm sieve. Approximately 30 specimens of (encrusted) Neogloboquadrina pachyderma (s) and 60 Turborotalita quinqueloba were picked for stable isotope analysis in the size fraction 125^250 Wm which contains a su⁄cient specimen number of both species. A potential small ‘intra-species’ N18 O variability within this broad size fraction (Berger et al., 1978 ; 0.4x for N. pachyderma (s) according to Hillaire-Marcel and Bilodeau, 2000) is regarded as

109

insigni¢cant for our approach, since the common distribution pattern of species-speci¢c grain sizes led us to preferentially select N. pachyderma (s) specimens in the upper part and of T. quinqueloba specimens in the lower part of the 125^250-Wm size fraction. Thus the species-speci¢c grain size range for our stable isotope data was narrowed down. The foraminifera specimens were cracked open, cleaned with methanol in an ultrasonic bath and dried at 40‡C. To calibrate the oxygen isotope signature, specimens of Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba were analyzed from six plankton hauls (Table 2) and two sediment trap series (Table 3) which are located along a northwest^southeast transect across the major water masses of the Nordic Seas (Fig. 1). ‘Living’, i.e. the plasma-¢lled foraminiferal tests were identi¢ed by the use of Rose Bengal (Lutze, 1964). All tests (125^250 Wm) were cleaned from organic tissue under an atmosphere of pure oxygen and low temperature, using a ‘cold’ plasma asher of type EMITECH 1050. Stable isotopes were measured on a Finnigan MAT 251 mass spectrometer with the automated Carbo-Kiel preparation line (KIEL DEVICE I) at the Leibniz Laboratory of Kiel University. All N18 O values are reported vs. the PeeDee Belemnite (PDB) scale established via the NBS 20 (National Bureau of Standards) carbonate stable isotope standard. The analytical reproducibility for N18 O is P 0.07x. However, multiple measurements of single core top samples revealed a standard deviation reaching P 0.11x N18 O (n = 10) for Neogloboquadrina pachyderma (s) and P 0.17x (n = 10) for Turborotalita quinqueloba (Simstich, 1999). This comparatively high within-sample scatter is likely to re£ect a signi¢cant isotopic variance within the samples due to bioturbational admixture of subfossil specimens and is employed as standard deviation relevant to the paired core top data we discuss in this study. 2.2. Calculation of equilibrium N18 O The theoretical N18 O values of inorganic calcite (N Ocalcite ) which is precipitated in isotopic equilibrium with the temperature (T‡C) and the N18 O 18

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Table 1 Locations and N18 Oforam values of sediment surface samples Core

217 258 264 271 PS2613-1 PS2616-7 PS2627-5 PS2638-6 PS2641-5 PS2644-2 PS2645-5 PS2646-2 PS2647-5 PS2656-2 31-02 16301 16302-1 16304-1 16305-1/2 16306-3 21291-3 23000-2 23008-1 23016-1 23019-1 23022-1 23027-1 23037-1 23038-1 23039-1 23040-1 23041-1 23042-1 23043-1 23055-1 23058-1 23059-1 23060-2b 23062-2 23063-1 23065-1 23066-1 23068-1 23071-1 23074-3 23126-1 23138-1 23142-1 23144-1 23227-1 23228-1

Longitude

Latitude

Water depth

N18 O (x PDB)

(‡E/W)

(‡N)

(m)

Neogloboquadrina pachyderma (s)

313.63 313.00 35.96 310.78 30.48 37.34 315.68 322.75 319.48 321.76 321.39 321.21 321.06 34.07 3.99 11.03 11.18 12.34 12.48 12.30 8.07 8.04 7.92 9.77 9.58 9.79 6.69 30.12 5.91 5.80 7.78 30.24 30.05 33.35 4.01 3.00 33.12 32.99 0.16 30.01 0.83 1.00 1.50 2.91 4.92 13.80 15.28 15.72 16.40 11.57 12.84

80.45 79.52 79.90 78.79 74.18 75.00 73.12 72.09 73.16 67.87 68.40 68.56 68.78 65.85 70.00 67.69 67.75 68.26 68.23 68.19 78.01 66.90 66.93 67.76 67.79 67.76 66.54 65.51 67.73 67.65 67.00 68.70 70.00 70.26 68.42 69.50 70.31 70.00 68.73 68.75 68.50 68.24 67.83 67.08 66.77 70.78 71.10 71.22 70.99 70.10 70.50

326 320 334 356 3259 3390 2009 428 469 778 1001 1114 1376 3758 3260 175 286 237 191 240 2400 668 840 614 997 606 601 3063 1243 1426 967 2247 3293 2133 2298 3276 2283 3229 2239 2302 2796 2792 2234 1306 1160 2351 1921 1622 1414 2827 2584

2.89 2.83 2.71 2.66 3.86 3.91 3.40 3.54 3.35 3.41 3.42 3.51 3.56 2.83 2.44 1.88 1.82 1.79 1.73 3.15 2.45 2.49 2.05 2.23 2.33 2.91 2.72 2.39 2.29 2.37 2.50 2.55 2.74 2.37 2.58 2.35 2.65 2.40 2.20 2.43 2.59 2.42 2.26 2.28 2.56 2.70 2.38 2.53 2.38

MARMIC 912 2-4-03

Source Turborotalita quinqueloba

3.74 3.64 3.24

2.93 3.38 3.45 1.95 2.17 1.59 1.37 1.12 1.43 1.22 2.44 1.30 1.47 1.26 1.40 1.61 1.41 1.75 1.59 1.95 2.03 2.20 1.74 2.02 2.18 2.04 1.69 1.39 1.78 1.96 1.88 1.73 1.50 1.73 1.62 1.55 1.88 1.96

Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Horwege (1987) Horwege (1987) Horwege (1987) Horwege (1987) Horwege (1987) Horwege (1987) Horwege (1987) Simstich (1999) Horwege (1987) Horwege (1987) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Horwege (1987) Simstich (1999) Horwege (1987) Simstich (1999) Horwege (1987) Horwege (1987)

J. Simstich et al. / Marine Micropaleontology 48 (2003) 107^125

111

Table 1 (Continued). Core

23229-1 23230-1 23231-2 23232-1 23233-1 23235-1 23237-1 23238-1 23239-1 23240-1 23241-2 23242-1 23243-2 23244-1 23246-2 23247-2 23249-1 23254-3 23258-3 23259-3 23260-2 23261-2 23262-2 23270-2 23277-1 23294-3 23312-2 23343-4 23347-4 23348-2 23349-4 23350-3 23351-4 23352-2 23353-2 23354-4 23400-3 23411-5 23424-3 23506-1 23507-1 23508-1 23509-1 23512-2 23514-3 23515-4 23516-3 23517-3 23518-2 23519-4 23522-2 23523-3

Longitude

Latitude

Water depth

N18 O (x PDB)

(‡E/W)

(‡N)

(m)

Neogloboquadrina pachyderma (s)

Turborotalita quinqueloba

11.53 4.84 33.99 31.62 6.88 1.39 8.05 7.75 8.36 9.59 3.64 34.42 36.51 38.67 312.92 317.12 319.47 9.74 13.95 9.30 11.45 13.11 14.42 30.82 30.63 310.59 7.74 313.00 316.08 318.95 320.19 319.35 318.21 312.42 312.72 310.63 37.81 3.51 30.07 37.60 39.25 39.40 313.50 313.42 325.95 325.20 328.28 329.09 328.20 329.60 328.66 330.22

75.84 78.87 78.90 79.03 79.41 78.87 66.90 66.99 67.50 68.16 68.30 69.35 69.38 69.37 69.41 69.49 69.50 73.06 75.00 72.02 72.13 72.18 72.23 73.16 72.04 72.38 66.94 72.21 70.44 70.42 70.39 70.40 70.36 70.01 70.57 70.33 72.35 65.80 70.04 72.39 73.83 73.86 73.83 72.94 66.67 66.96 65.03 64.69 64.54 64.80 63.76 62.25

3.05 2.84 3.06 3.37 2.90 3.03 2.29 2.27 2.34 2.20 2.36 2.50 3.01 3.43 3.70 3.48 3.49 2.79 2.65 2.50 2.56 2.55 2.41 3.79 3.47 3.19 2.20 3.42 3.59 3.25 3.33 3.40 3.46 3.69 3.48 3.51 3.80 2.62 2.46 3.53 3.64 3.35 3.41 3.37 3.48 2.97 2.30 2.53 2.17 2.30 3.07 3.24

2.32

2043 1215 1979 2642 1264 2500 644 981 1529 1851 1844 3420 2721 2085 1867 1375 856 9999 1762 2518 2089 2224 1135 2768 2688 2224 974 2400 1239 737 309 400 1637 1823 1404 1747 2630 2849 3247 2670 3150 3202 2576 2610 713 1055 1034 1058 1081 1903 1560 2156

MARMIC 912 2-4-03

Source

2.89 2.73 2.37 2.79 1.52 1.72 1.87

2.48 2.77 3.56 3.34 2.27 1.81 2.06 2.15 1.89 1.78 3.83 2.95 2.99 1.45 3.54 3.64 3.58

3.38 3.30 2.99 3.00 3.64 2.06 1.88 3.38 3.37 3.52 3.30 3.33 3.53 1.25 1.36 1.05 1.38 1.36 1.36

Horwege (1987) Horwege (1987) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Horwege (1987) Horwege (1987) Horwege (1987) Horwege (1987) Horwege (1987) Horwege (1987) Simstich (1999) Horwege (1987) Horwege (1987) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Hohnemann (1996) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999)

112

J. Simstich et al. / Marine Micropaleontology 48 (2003) 107^125

Table 1 (Continued). Core

23524-2 23525-3 23526-3 23527-3 23528-3 23536-1 23537-1 23538-1 23539-1 23540-2 23541-1 23542-1 23543-1 23545-1 23547-4 23549-9 23550-10 23552-8 23554-9 BS88-6-4 BS88-6-8 BS88-6-10 BS92-7-1

Longitude

Latitude

Water depth

N18 O (x PDB)

(‡E/W)

(‡N)

(m)

Neogloboquadrina pachyderma (s)

Turborotalita quinqueloba

329.91 327.60 328.06 329.89 328.84 325.74 32.51 32.17 32.45 32.51 5.77 7.08 7.22 8.31 4.00 34.60 318.72 319.00 320.30 331.07 329.68 330.66 312.70

63.00 63.25 63.40 62.79 63.16 63.74 62.05 62.01 62.80 62.77 67.62 67.68 67.58 67.35 69.99 75.06 67.88 69.53 68.92 67.37 66.45 66.20 68.00

2029 1178 1670 2144 1632 370 1689 1667 1210 1126 1441 1373 1409 1155 3280 3624 910 1280 1570 626 299 498 unknown

3.47 3.40 3.23 3.09 3.30 2.12 2.35 2.50 2.76 2.61 2.45 2.37 2.30 2.34 2.61 3.88 3.49 3.53 3.54 2.66 2.63 2.94 3.79

1.31 1.27 1.54 1.51 1.51 1.79 1.34 1.18 1.45 1.42 1.78 1.87 1.85 1.50 2.17 3.93 3.84 3.38 3.50

Source

2.31

Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich Simstich

(1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999) (1999)

Most samples are from the top 0^1 cm sediment in giant box cores and multicores. The last four samples are from the top 0^1 cm of gravity cores.

values of the ambient sea water (N18 Owater ) are calculated after O’Neil et al. (1969 ; cited in Shackleton, 1974): 18

0:5

N Ocalcite ¼ ½21:933:16
ð31:061 þ TÞ  þ N w

ð1Þ

N 18 Owater; vs: VSMOW ¼ 312:17ð 1:18Þ þ 0:36ð 0:03Þ
S ðr2 ¼ 0:72; n ¼ 346Þ

ð3Þ

18

Conversion of N Owater from the Vienna Standard Mean Ocean Water (V^SMOW) scale to Nw (PDB scale) followed : N w ¼ 0:9998
N 18 Owater; vs: VSMOW 30:2x

ð2Þ

We adopted the conversion factor 30.2x, which is tied to early empirical Eq. 1 (compare table 1 of Bemis et al., 1998). Using 30.27x for conversion (Hut, 1987) would hardly change N18 Ocalcite , only by the order of the analytical error in our data set. The relationship between N18 Owater and salinity (S) was deduced from a new set of water pro¢les (Fig. 2 ; Simstich, 1999; Weinelt et al., 2001). Accordingly, N18 Owater in the eastern and central Nordic Seas is now calculated using Eq. 3 :

Numbers in brackets indicate P standard deviation, correlation coe⁄cient (r2 ), and number of measurements (n). In the East Greenland Current (EGC), west of 19‡W, N18 Owater is now calculated with Eq. 4 : N 18 Owater; vs: VSMOW ¼ 317:11ð 0:37Þ þ 0:49ð 0:01Þ
S ðr2 ¼ 0:96; n ¼ 15Þ

ð4Þ

The steeper regression slope of Eq. 4 as compared to Eq. 3 (0.49 vs. 0.36) results from the joint meltwater input of Arctic sea ice and the Greenland ice sheet into the EGC.

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113

Table 2 Locations, dates, catch intervals, and N18 Oforam values of plankton hauls Depth interval

N18 O of Neogloboquadrina pachyderma (s) (x PDB)

N18 O of Turborotalita quinqueloba (x PDB)

mean N18 Ocalcite

(m)

Living, not encrusted

Living

Dead

(x PDB)

MN116 75.00‡N 7.31‡W 21 August 1994

0^50 50^100 100^150 150^300

2.68 3.32 3.35 3.41

3.89

3.72 4.33 4.44 4.57

MN54 75.00‡N 0.18‡E 31 July 1994

0^50 50^150 150^300 300^500 500^1000

2.51 3.08 3.63

3.60 3.78

3.81 4.57 4.67 4.72 4.75

MN323 69.69‡N 0.47‡E 7 July 1992

0^20 20^40 40^60 60^80 80^100 100^200 200^300 300^500 500^700 700^1000 1000^1500 1500^2000 2000^2500

1.73 1.82 2.15 1.85

Plankton haul

3.51 3.48

2.11 1.87

MN319 67.23‡N 2.92‡E 5 July 1992

0^20 20^40 40^60 60^80 80^100 100^200 200^300 300^500 500^700

1.28 0.96

0^20 20^40 40^60 60^80 80^100 100^200 200^300 300^500 500^700

1.37 1.37 1.57

2.27 1.91

3.36 3.39 3.73

3.35 3.64 3.72

3.52 3.62

1.88 2.17 2.25 2.51

2.09 2.19 2.32

0^50 50^100 100^500 500^1000 1000^2000

Dead, encrusted

2.39

1.92 1.89 2.07

MN2 70.00‡N 3.40‡E 10 July 1994

MN314 67.54‡N 5.58‡E 28 June 1992

Living, encrusted

2.03 2.04

1.43 2.10 2.14 2.21 2.09 2.10 2.13

2.31 2.29

1.58 1.48

1.52 1.50 2.57 2.14 1.77 1.81 1.86 1.82 1.84 1.90 1.88 1.91 1.63 1.79 1.79 1.84 1.79 1.04 1.27 1.38 1.32 1.39 1.36 1.44

1.69

1.70 2.03 1.86 1.77 1.92

2.52 2.96 3.75 4.16

1.67

1.45

1.73 1.82 1.81

1.28 1.23 1.51 1.60 1.48 1.44 1.49 1.33

The expected N18 Ocalcite values are averaged across the catch intervals.

MARMIC 912 2-4-03

2.19

2.05 1.92 1.91 1.99 1.88 2.00

2.56 2.72 2.90 3.02 3.06 3.17 3.28 3.35 3.52 4.13 4.60 4.72 4.75

1.76 1.84 1.82

2.67 2.95 3.25 3.52 3.92

1.42 1.41 1.32 1.44

2.19 2.27 2.45 2.75 2.83 2.92 3.04 3.48 4.33

1.55 1.70 1.48 1.40

2.16 2.38 2.59 2.81 2.86 2.92 3.06 3.49 4.30

114

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Table 3 Locations, mooring intervals, depths, and N18 Oforam values of sediment traps N18 O of Neogloboquadrina pachyderma (s) (x PDB)

Sediment trap

Trap interval Start

End

NB6 69.69‡N 0.46‡E 3292 m 1991/1992

06 20 03 17 01 15 14 14 13 05 19 26

August August September September October October November December January June June June

20 03 17 01 15 14 14 13 12 19 26 07

15 30 14 29 13 13 28 28 30 10 30 19 09 29 18 02 17

July July August August September September October September November May May June July July August September September

30 July 14 August 29 August 13 September 28 September 28 October 18 February 28 October 09 January 30 May 19 June 09. July 29 July 18 August 02 September 17 September 02 October

2.43 2.09 2.06

06 20 03 17 01 14 14 03

August August September September October November December July

20 03 17 01 15 14 13 10

3.31 4.23 3.16 2.85

NB7 69.69‡N 0.48‡E 3254 m 1992/1993

OG5 72.38‡N 7.71‡W 2624 m 1991/1992

August September September October October November December January February June June July

August September September October Oct. December January July

500 m

1000 m 3000/2200 m

N18 O of Turborotalita quinqueloba (x PDB) 500 m

1000 m 3000/2200 m

0.76 1.37

1.57 1.13 1.19 1.39 1.36 1.37 1.43 1.39 1.31

1.11 0.29 1.53 1.31 2.58 2.49 2.31

2.20

1.97 1.66 1.27 0.94 0.79

2.30 2.18

2.01 2.23 2.07

2.25 1.99 1.56 1.57 1.68 1.62 1.79

1.92

2.07 1.86

1.50

1.63

2.49 2.55 2.46 2.39

2.59 2.52 2.33 2.47

3.39 3.39 3.12 2.93

3.35 3.33 3.49 3.15 2.99

2.85 3.03 3.39

Modern reference data for salinity and temperature data were obtained from the ‘HydroBase’ climatological data base (Lozier et al., 1995; Curry, 1996) as summer average over July^September. This time span represents the main production season of planktonic foraminifera in the Nordic Seas. In contrast to the North Atlantic south of Iceland, where production culminates

1.60 2.27 2.46 2.36 2.16 2.01

2.45 2.20 1.93 1.86 1.75 1.57 3.21 3.23

3.17 3.07 3.22 3.11

twice a year in spring and fall (Tolderlund and Be¤, 1971), all sediment trap studies in the Nordic Seas only reveal a single major production spike of planktonic foraminifera during summer (Kohfeld et al., 1996; Jensen, 1998; Ostermann et al., 1998; von Gyldenfeldt et al., 2000; Schro«der-Ritzrau et al., 2001; Weinelt et al., 2001). The same meridional trend from a bimodal to a unimodal

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115

Table 4 14 C-ages of sediment surface samples Core 23008-1 23071-1 23247-2 23270-2 23277-1 23343-4 23347-4 23351-4 23352-3 23354-4 23415-8 23537-1 23541-1 PS2613-1 PS2644 PS2644 23074 23074 PS2647 PS2647 23522 23071 23400 23411 23424 17730 23259 PS1926 88-3 23055 23056 23059 23062 23065 PS1243 14

Longitude (‡E/W) 7.92 2.91 317.12 30.82 30.63 313.00 316.08 318.21 312.42 310.63 319.15 32.51 5.77 30.48 321.76 321.76 4.91 4.91 321.06 321.06 328.66 2.91 37.81 3.51 30.07 7.39 9.27 318.28 331.07 4.01 3.84 33.12 0.16 0.83 36.35

Latitude (‡N)

Depth in core (cm)

Species

Age (corrected

66.93 67.08 69.49 73.16 72.04 72.21 70.44 70.36 70.01 70.33 53.18 62.05 67.62 74.18 67.87 67.87 66.67 66.67 68.78 68.78 63.76 67.09 72.35 65.80 70.04 72.12 72.04 71.49 67.41 68.42 68.50 70.31 68.73 68.50 66.37

0^1 0^1 0^1 0^1 0^1 0^1 0^1 0^1 0^1 0^1 0^1 0^1 0^1 0^1 0.5 0.5 0.5 0.5 0.5 0.5 0^1 0.75 0.5 0.5 0.5 0.1^1.9 0.1^1.9 1 0 0^1 0^1 0^1 0^1 0^1 1.5

G. bulloides N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma G. bulloides G. bulloides G. bulloides N. pachyderma N. pachyderma N. pachyderma G. bulloides N. pachyderma N. pachyderma C. wuellerstor¢ G. bulloides N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma Not given N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma N. pachyderma

(s) (s) (s) (s) (s) (s) (s) (s) (s)

(s) (s) (s) (s) (s)

(s) (s) (s) (s) (s) (s) (s) (s) (s) (s) (s) (s) (s)

Source 14

C yr BP)

1120 P 30 1420+50/-40 420 P 30 3190 P 30 380 P 30 330 P 30 3200 P 30 20 P 30 4200 P 40 2990 P 40 1330 P 30 430 P 40 600 P 30 450 P 30 3580 P 30 3630 P 20 290 P 30 2410 P 30 3320 P 30 100 P 40 3440 P 40 1610 P 70 640 P 130 1610 P 70 370 P 75 140 P 60 130 P 0 310 P 110 1530 P 65 1815 P 55 890 P 65 1365 P 55 1690 P 55 2145 P 45 1290 P 65

Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Simstich (1999) Voelker et al. (1998) Voelker et al. (1998) Voelker et al. (1998) Voelker et al. (1998) Voelker (1999) Voelker (1999) Hohnemann (unpubl.) Kohly (1994) Kohly (1994) Kohly (1994) Kohly (1994) Trauth (1995) Trauth (1995) Stein et al. (1996) Andrews et al. (1996) Vogelsang (1990) Vogelsang (1990) Vogelsang (1990) Vogelsang (1990) Vogelsang (1990) Bauch and Weinelt (1997)

C dates are corrected for a 400-year reservoir age (Bard et al., 1994).

productivity maximum is known from the North Paci¢c (Kuroyanagi et al., 2002). 2.3. Age control To verify the modern age of core top samples in giant box cores, 14 samples were dated by the Accelerator Mass Spectrometry (AMS) 14 C method at the Leibniz Laboratory, in addition to using dates from published sources (Table 4). Most core top dates (corrected for a 14 C reservoir e¡ect of 400 yr) are much younger than 2500 yr BP (Be-

fore Present) (Table 4) and accordingly represent the Latest Holocene. Thus, we tentatively correlated the isotope data of the core top samples to modern hydrographic data, assuming that ocean circulation, water mass distribution, positions of the oceanic fronts, and sea surface temperature (SST) did not basically change over the time span of the last 2500 years in the Nordic Seas (KocT et al., 1993; Sarnthein et al., 2001), an assumption that ¢nally did not fully hold true. Four samples with an age of 3000^4200 14 C years may also re£ect Late Holocene sedimentation process-

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Fig. 2. Relationships between salinity and N18 Owater in the central and eastern Nordic Seas (a), and in the EGC west of 19‡W (b).

Fig. 3. Scheme of the ontogenetical growth of planktonic foraminiferal tests and the concurrent development of oxygen isotope signatures in the water column, summarizing data from sediment traps, plankton tows, and core top samples (Fig. 4), including previous ¢ndings of Kahn and Williams (1981), Arikawa (1983), Hemleben et al. (1989), Kohfeld et al. (1996), Berberich (1996), Volkmann and Mensch (2001), and Stangeew (2001).

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Fig. 4. Equilibrium N18 Ocalcite pro¢les and measured N18 Oforam values in plankton hauls and traps (bold numbers) and in nearby core top samples (regular numbers) from seven sites in the Nordic Seas, as labelled at the base of the graphs. 14 C ages of core top samples are corrected by 3400 years.

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Fig. 5. N18 O vital e¡ects of Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba (thick gray, vertical lines), as deduced from the di¡erence (vN18 O) between N18 Oforam from plankton tows and calculated mean N18 Ocalcite for various individual catch intervals (thin vertical bars). Horizontal bar is bulk error range. The N18 O vital e¡ects were only deduced for catch intervals above the local pycnokline and intervals further below, where the abundance of specimens reached a maximum (Simstich, 1999).

es and therefore were not removed from the data set. Some negative ages are linked to the ‘bomb e¡ect’ (Levin and Hesshaimer, 2000) and illustrate that these samples are truly modern.

3. Derivation of the original N18 O signal from foraminifera in the sediment Our approach in the Nordic Seas is focused on the paired N18 O signal of the polar species Neogloboquadrina pachyderma (s), which in some regions is a deep dweller, and the subpolar species Turborotalita quinqueloba, which is presumed to inhabit the near-surface water (Be¤, 1977; Carstens and Wefer, 1997). Since the two species occur all over the Greenland and Norwegian seas, they provide su⁄cient CaCO3 tests for an isotopic survey of the entire region. Plankton net and sedi-

ment trap samples document how the isotopic signatures of N. pachyderma (s) and T. quinqueloba evolve in harmony with ontogenetical growth and sedimentation processes of the calcite tests (Fig. 3). Both species inhabit the mixed layer and settle below the pycnocline for reproduction. Here N. pachyderma (s) ¢nally wraps its initial test with a thick secondary calcite crust. It is the cold deep subsurface water, where the encrustation of N. pachyderma (s) takes place, which is the main origin for the oxygen isotopic di¡erence to T. quinqueloba, a species hardly a¡ected by secondary encrustation. After reproduction, the dead foraminifera settle to the sea £oor, bearing a cumulative isotope signal from the whole depth range they inhabited. Isotope analysis of any sediment sample just provides a single value which pretends to present a single distinct, but only ‘apparent calci¢cation depth’ (Emiliani, 1954; Kel-

Fig. 6. Apparent calci¢cation depths (match of N18 Oforam and N18 Ocalcite ) of Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba from sediment surface samples projected meridionally onto the transect line across the Nordic Seas, de¢ned in Fig. 1. Lines in (a) indicate N18 Ocalcite isolines calculated from (b) temperature (isothermes) and (c) salinity (isohalines) according to the Eqs. 1^4; (d) shows isopycnals reduced to sea surface pressure. N18 Oforam values were corrected for the vital e¡ect (Fig. 5) and the ‘water-column-to-sediment-surface anomaly’ (Figs. 3 and 4). The statistical scatter of these corrections combined with the measurement errors de¢ne the 1c-ranges (vertical bars) shown in (a), averaging to 0.16x for N. pachyderma (s) and to 0.20x for T. quinqueloba (Simstich, 1999).

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logg et al., 1978). To constrain this depth, the oxygen isotope ratio measured on monospeci¢c foraminifera samples (N18 Oforam ) is compared with the calculated theoretical N18 Ocalcite pro¢le

119

as deduced through Eqs. 1^4. The obvious N18 O o¡set between N18 Oforam and N18 Ocalcite (Fig. 4) requires a correction of N18 Oforam for two di¡erent factors.

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In a ¢rst step, the N18 Oforam values need to be corrected for the vital e¡ect which diminishes the N18 Oforam value relative to the theoretical N18 Ocalcite value (Urey et al., 1951; Wefer and Berger, 1991). Plankton hauls from various hydrographic regions in the Nordic Seas revealed that average N18 Oforam values of encrusted Neogloboquadrina pachyderma (s) are 30.9x ( P 0.08, n = 9) lower than the mean N18 Ocalcite value of the pertinent depth intervals, likewise, the vital e¡ect of not encrusted N. pachyderma (s) lies near 31.0x ( P 0.06, n = 13) and that of Turborotalita quinqueloba at 31.1x ( P 0.07, n = 17) (Figs. 3^5). In general, these o¡sets con¢rm previously published values of 30.7x in plankton tows o¡ California (Ortiz et al., 1996) and of 31x in the Arctic Ocean (Kohfeld et al., 1996; Bauch, 1997). Vital o¡sets of 31.3x were reported for not encrusted N. pachyderma (s) and for T. quinqueloba in the Fram Strait and Laptev Sea (northern Siberia) (Volkmann and Mensch, 2001). A second correction considers an average 0.3x di¡erence found between the N18 Oforam values for planktonic foraminifera settling in the deep water column and the N18 Oforam values of Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba from most core top sediments (Figs. 3 and 4). Core top 14 C ages of core top samples, which reach 450^1610 years BP (Fig. 4), document that the N18 O enrichment by 0.3x may be ascribed to bioturbational admixture of subfossil ‘cold’, that are slightly 18 O enriched foraminiferal tests from the Little Ice Age. On the other hand, the o¡set is zero at Site MN323/NB6 and 7, where the 14 C age of the nearby core top sample 23424 is ‘truly modern’, and thus con¢rms the concept of bioturbational admixture of ‘colder’ tests.

4. Results N18 Oforam data from sediment samples on a transect from Norway to Greenland were projected on a vertical, roughly east^west running pro¢le at 67^70‡N, which crosses the principal surface currents and hydrographic structures in

this region (Fig. 1). In the west, low sea surface salinity ( 6 34.4 psu; Fig. 6c) and a temperature minimum ( 6 0‡C; Fig. 6b) between 50 and 100 m water depth mark the Polar Water of the East Greenland Current. Further east a somewhat higher salinity (34.6^34.9 psu) and temperatures above 0‡C characterize the Arctic Domain, where doming isopycnals (Fig. 6d) indicate a potentially unstable strati¢cation of the surface water (Killworth, 1983). East of 5‡W, Atlantic Water keeps the temperature above 3‡C. Salinity reaches a maximum of 35.1 psu in the Norwegian Current and only drops below 34 psu in the Norwegian Coastal Current east of 10‡E. Temperature and salinity de¢ne the distribution pattern of calculated N18 Ocalcite (Fig. 6a). Maximum values ( s 4x) occur below the pycnocline in the western Arctic Domain, where highly saline Atlantic Water mixes with cold Polar Water (Swift, 1986). Near to the surface N18 Ocalcite decreases west of the Arctic Domain because of the low salinity in the EGC. N18 Ocalcite also decreases to the east with increasing temperatures in the Norwegian Current. The warm and low saline waters of the Norwegian Coastal Current result in local minimum N18 Ocalcite values of 6 1.5x. The apparent calci¢cation depths (Fig. 6) of Turborotalita quinqueloba approximately match the pycnocline between 25 and 75 m water depth on most of the pro¢le. Apparent calci¢cation of Neogloboquadrina pachyderma (s) occurs between 25 and 70 m water depth in the western Arctic domain. The calci¢cation depth of N. pachyderma (s) sinks to 70^250 m o¡ Norway and to 70^130 m below the EGC.

5. Discussion Summer SSTs are 3^10‡C east of 15‡W, that is east of the Polar Front, in the Arctic Domain and in the Norwegian Current. Here, the apparent calci¢cation depths of Neogloboquadrina pachyderma (s) occupy water depths where temperatures are generally 2^3‡C lower than the actual SST (Fig. 6b). This ¢nding con¢rms the concept of an ‘optimum temperature range’, ¢rst proposed

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by Duplessy et al. (1991). They claimed for N. pachyderma (s) a systematic N18 O o¡set by 0.75x, equivalent to 32.5‡C, from N18 Oforam values that directly compare to the actual SST. All together, however, the apparent calci¢cation depths of N. pachyderma (s) follow a pattern which more closely resembles the isopycnal pattern (Fig. 6d) than that of the isotherms (Fig. 6b). Accordingly, density changes may also contribute to the control of the depth distribution of foraminiferal calci¢cation, as ¢rst assumed by Savin and Douglas (1973) and more recently by Weinheimer et al. (1999). In the same sea region east of the Polar Front, Turborotalita quinqueloba shows apparent calci¢cation depths which are as shallow as 50 P 25 m (Fig. 6). The depth distribution reconstructed for the N18 O values of N. pachyderma (s) and T. quinqueloba is also recovered from the maximum specimen numbers in a series of plankton tows from the Nordic Seas and the North Paci¢c (Arikawa, 1983; Ortiz and Collier, 1995; Carstens and Wefer, 1997; Jensen, 1998; Kohfeld, 1998; Simstich, 1999). However, in the domain of the EGC, the apparent calci¢cation depths of both Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba appear di¡erent. They follow the 34.0^34.5 psu isohalines from an average water depth of 25 m in the east to approximately 70^130 m in the west (Fig. 6c). The westward deepening of the habitats below the EGC is corroborated by specimen numbers in plankton hauls from the sea ice covered western and northwestern Fram Strait, where the maximum abundance of both species drops down to as far as 100^200 m water depth (Carstens and Wefer, 1997; Volkmann, 2000). The shallow calci¢cation depths of Turborotalita quinqueloba in the central and eastern Nordic Seas may be linked in part to the shallow euphotic habitat preferred by this symbiont bearing species (Be¤, 1977; Hemleben et al., 1989). Further west, however, in the range of the EGC, the in£uence of the symbionts on T. quinqueloba may be subdued by the in£uence of a narrowly con¢ned maximum in nutrient concentrations which are characteristic along the well-de¢ned pycnocline below the Polar Water of this low-salinity EGC and likewise control the depth range of Neo-

121

globoquadrina pachyderma (s). In this region, the two planktonic foraminifera species thus follow the same rules and cover the same depth range. A similar nutrient controlled deep habitat may also a¡ect or bias any reconstruction of the past, for example glacial and deglacial meltwater lids (Duplessy et al., 1991; Sarnthein et al., 1995). Today and in the past, the N18 Oforam patterns of Neogloboquadrina pachyderma (s) ^ where almost identical with that of Turborotalita quinqueloba ( 6 0.2 vN18 O; Figs. 6a and 7) ^ will never re£ect the full salinity reduction at the immediate sea surface, because most tests have attained their isotopic signals in more salty, deeper water along the halocline that forms the base of the surface layer (Fig. 6c). Accordingly, past N18 O reductions of N. pachyderma (s) in a meltwater-a¡ected region will only provide a very conservative record of surface water freshening. The actual salinity reduction in a meltwater lid may have by far exceeded the estimated value. This o¡set is clearly depicted in our modern data set (Fig. 6), where local sea surface salinity of the low-salinity EGC is overestimated by up to 2.5 psu. Likewise, modern salinity is overestimated by N18 Oforam values of N. pachyderma (s) in the narrowly con¢ned Norwegian Coastal Current (single value east of 10‡E in Fig. 6). However, di¡erent from the EGC and Arctic Domain, where interspecies N18 O di¡erences decrease to 6 0.2x, this low-salinity water mass in the east is more di⁄cult to distinguish from surface water with regular salinity by means of low interspecies N18 O di¡erences. Because of marked thermal strati¢cation (Fig. 6b) they only decrease down to 0.36^0.70x (Fig. 7). In between the two low-salinity boundary currents, the joint signal of absolute N18 O values and interspecies N18 O di¡erences may help to distinguish two further prominent water masses in the Nordic Seas. (1) The pronounced thermal strati¢cation of the Atlantic in£ow, which is represented by the Norwegian Current in the east and the Irminger Current in the far southwest, is well recorded by most prominent N18 O di¡erences between Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba, reaching more than 1.5x (Fig. 7). (2) The unique site of deepwater convection, thus well mixed surface layer of the Arctic

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Fig. 7. Spatial distribution pattern of N18 O di¡erences between Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba. High values delineate the in£ow of Atlantic water into the Norwegian and Irminger Seas. Low values document in£uence of lowsaline surface water of polar origin. Large numbers label vN18 O isolines.

Domain in the central Nordic Seas is clearly represented by a combination of two signals. On the one hand, both species under discussion show the absolute N18 O maxima in this sea region ( s 3.5x; Table 1). On the other hand, they reveal extremely low, if not negative interspecies N18 O di¡erences of 0.2 to 30.35x (Fig. 7).

6. Conclusions Though N18 O values of planktonic foraminifera form within a broad depth interval, they provide oceanographic records that can be assigned to distinct depth levels. In particular, apparent calci¢cation depths of Turborotalita quinqueloba occur

at 25^75 m and hence are generally indicative for conditions along the thermocline. In contrast, N18 O values of Neogloboquadrina pachyderma (s) mostly represent deeper water masses below the thermocline, at 25^250 m, where isotopic temperatures di¡er from the actual SST by 32.5‡C (sensu Duplessy et al., 1991). This applies to the region east of the Polar Front at 15‡W. Further west, however, beneath the low-salinity EGC, both N. pachyderma (s) and T. quinqueloba prefer the deeper and nutrient-enriched habitats of the particularly well developed halocline. Accordingly, the N18 O values of N. pachyderma (s) do not re£ect the full freshwater proportion in surface water. Today, they overestimate actual sea surface salinity by up to 2.5 psu. In the past,

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they will provide a record of meltwater plumes that is extremely conservative. In summary, paired foraminiferal N18 O values may form a proxy to distinguish the following major surface structures in the Nordic Seas: (1) the freshwater-enriched EGC in the west and the Norwegian Coastal Current in the east, although less clearly identi¢ed due to distinct thermal strati¢cation ; (2) the Arctic Domain with its deepwater convection, which is marked by the combined signal of partly negative N18 Oforam di¡erences reaching up to 30.35x and high absolute N18 Oforam values ; and (3) the Atlantic in£ow, which is characterized by thermal strati¢cation in the Norwegian and Irminger seas and is recorded by most prominent N18 Oforam di¡erences between Neogloboquadrina pachyderma (s) and Turborotalita quinqueloba, reaching up to 2.16x.

Acknowledgements We are grateful to many technicians and students for tedious but careful sample preparation. We thank Stefan Jensen, Ralf Schiebel, Christoph Hemleben, William B. Curry, and Dorinda R. Ostermann for samples and access to hydrographic data. This study received generous support from the Deutsche Forschungsgemeinschaft, Bonn, through the Sonderforschungsbereich 313 at Kiel University.

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