Palaeogeography, Palaeoclimatology, Palaeoecology, 73 (1989): 85 106 85 Elsevier Science Publishers B.V., Amsterdam Printed in The Netherlands

LATE QUATERNARY PALEOCHEMISTRY OF HIGH-LATITUDE SURFACE WATERS

L. D. K E I G W I N 1 a n d E. A. B O Y L E 2

~ Woods Hole Oceanographic Institution, Woods Hole, MA 02543(U.S.A.) 2Massachusetts Institution of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge,

MA 02139 (U.S.A.)

(Received May 27, 1988; revised and accepted January 5, 1989)

Abstract

Keigwin, L. D. and Boyle, E. A., 1989. Late Quaternary paleochemistry of high-latitude surface waters. Palaeogeogr., Palaeoclimatol., Palaeoecol. 73:85 106.

Recent studies have stressed the role of high latitude nutrient levels and productivity in controlling the carbon isotopic composition of the deep sea and the CO: content of the atmosphere. We undertook a study of the chemical composition of the polar planktonic foraminifer Neogloboquadrina pachyderma (s., sinistral coiling) from 30 late Holocene samples and 49 down core records from the high-latitude North and South Atlantic Oceans to evaluate the history of sea surface chemical change from glacial to interglacial time. Stable isotopic analysis of coretop samples from the Atlantic, Pacific and Southern Oceans shows no significant correlation between the 513C of N. pachyderma and either 513C or PO 4 in seawater. Conversely, Cd/Ca ratios in planktonic foraminifera are consistent with the PO 4 content of surface waters. The level of maximum glaciation (18,000 yr B.P.), identified by CLIMAP and 51sO, was chosen for mapping. Isopleths of 51sO on N. pachyderma (s.) in the North Atlantic reveal a pattern largely influenced by sea surface temperature (S.S.T.) and generally support the S.S.T. reconstruction of CLIMAP. Differences between the two suggest significantly lower salinity in North Atlantic surface waters at high latitudes than in lower latitudes.

Down core ~3C records of N. pachyderma confirm that low 5~3C values occurred in the northeast Atlantic during the latest glacial maximum (Labeyrie and Duplessy, 1985). However, a map of 513C for the 18,000 yr B.P. level for a much larger region in the North Atlantic shows that minimum N.pachyderma 51~C occurred in temperate waters. N. pachyderma ~13 C decreased toward the southwest, reaching a minimum of -l°/0o at 37°N. Despite the variability seen in &~3C records of N. pachyderma, none of our cores show significant temporal variability in Cd/Ca. From the combined Cd/Ca and 513C data we can see no evidence for an upwelling gyre in the eastern North Atlantic during the latest glacial maximum, nor evidence that the southern and northern oceans had significantly different levels of preformed nutrients than today.

Introduct ion

I n d e p e n d e n t o b s e r v a t i o n s o f e l e v a t e d a tmo-

s p h e r i c C O : l e v e l s d u r i n g t h e l a t e s t g l a c i a t i o n

( B e r n e r e t al., 1979; D e l m a s e t al. , 1980;

B a r n o l a e t al., 1987; N e f t e l e t al., 1988) h a v e

f o c u s s e d t h e a t t e n t i o n o f p a l e o c l i m a t o l o g i s t s

on t h e p o t e n t i a l of h i g h l a t i t u d e b i o l o g i c a l

p r o d u c t i v i t y a n d n u t r i e n t l e v e l s fo r c o n t r o l -

l i n g a t m o s p h e r i c CO2 in l a t e Q u a t e r n a r y

t ime . T h r e e g r o u p s o f c l i m a t e m o d e l e r s h a v e

s u g g e s t e d t h a t m o r e ef f ic ien t w i t h d r a w a l of

s u r f a c e w a t e r n u t r i e n t s in t h e p o l a r o c e a n

o c c u r r e d , in o r d e r to e x p l a i n t h e app rox .

70 ppm d e c r e a s e in a t m o s p h e r i c C O : d u r i n g

t h e l a t e s t g l a c i a l m a x i m u m ( K n o x a n d McE1-

roy , 1984; S a r m i e n t o a n d T o g g w e i l e r , 1984;

S i e g e n t h a l e r a n d W e n k , 1984). P a l e o c e a n o g r a -

p h e r s s t u d y i n g t h e deep sea r e c o r d h a v e k n o w n

for t h e l a s t t e n y e a r s t h a t t h e n a t u r e o f deep

0031-0182/89/$03.50 q~ 1989 Elsevier Science Publishers B.V.

86

sea circulation changed from glacial to inter- glacial time, and that this must reflect changes in the high lati tude surface ocean. Speculation about the history of the North Atlantic Ocean has ranged from a complete shutdown of North Atlantic Deep Water (N.A.D.W.; Streeter and Shackleton, 1979) to a diminishment of N.A.D.W. relative to Antarctic Bottom Water (A.A.B.W.; Boyle and Keigwin, 1982, 1985/6) to a changed chemical composition of northern source water owing to increased "preformed" nutrients (Labeyrie and Duplessy, 1985; Mix and Fairbanks, 1985).

Considering the importance of the paleo record of chemical change in the high latitude surface ocean to both the record of deep sea circulation and the record of atmospheric CO2, we undertook a study of the chemical com- position of the planktonic foraminifer Neo- globoquadrina pachyderma. This is the only planktonic foraminiferal species adapted to polar surface waters and as such it is our best hope for interpreting the record of high latitude chemistry during glacial time. We have added to the core top oxygen and carbon isotopic data base of Labeyrie and Duplessy (1985) for the sinistral (s.) coiling (polar) phenotype of this species and have studied the isotopic and Cd/Ca chemistry of both dextral (d.) and sinistral specimens in cores spanning the latest glacial to interglacial transition from the high lati tude North and South Atlantic oceans. In the following sections we present results and discuss them according to the way our investigation evolved, after ini- tially testing the observations of Labeyrie and Duplessy (1985).

C h e m i s t r y o f the sur face N o r t h At lant i c 18,000 years ago

Labeyrie and Duplessy (1985) studied carbon isotopic records of N.pachyderma (s.) from North Atlantic and Indian Ocean sediment cores and concluded that low carbon isotopic ratios off Ireland were evidence for divergence similar to that found in the modern Weddell Sea. This observation lead those authors to

hypothesize that deep water with a higher content of preformed nutrients was formed in the northeast Atlantic, providing a mechanism for enhanced transfer of CO2 from the atmo- sphere to the deep ocean. Their conclusions were based entirely on the assumption that 513C of N.pachyderma (s.) is a reliable proxy for surface ocean nutr ient levels. In support of this assumption Labeyrie and Duplessy (1985) showed an apparent constant offset between the carbon isotopic ratio in Holocene N. pachy- derma (s.) and the ~13C of ZCO/ in near-surface water.

The carbon isotope ratio of total CO 2 in seawater is known to correlate with the nutr ient content (Kroopnick et al., 1972) and this relationship has been used by some authors to infer that the 513C in planktonic foraminifera is a tracer for/~13C of total CO 2 and that it may be a proxy for upwelling intensity (Berger et al., 1978). However, studies of planktonic forminiferal 5~3C from sediment traps in upwelling areas (Curry et al., 1983) and from core tops underlying upwelling areas (Prell and Curry, 1981) reveal no systematic relationship with upwelling intensity. Fre- quently cited reasons for this lack of relation- ship include poorly understood "vital effects", for example, the inclusion of metabolic CO 2 in the test and the rapid depletion of surface- water nutrients by phytoplankton. This latter process would tend to enrich in ~3C the total CO/ pool from which planktonic foraminifera build their shells.

Since Cd/Ca is also thought to be a proxy for oceanic nutrients (Boyle, 1988b) we investi- gated the distribution of surface nutrients during glacial time using this technique as well as stable isotopes. Guided by the map of North Atlantic glacial ~13C produced by La- beyrie and Duplessy (1985), we chose two cores with high rates of sedimentation for detailed comparison (all core locations are in Table I and Fig.l). The first, KN714-15, is located on the east flank of the Reykjanes Ridge, to the northeast of the region of light carbon de- scribed by Labeyrie and Duplessy (1985). Fau- nal and lithological studies of this core have

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TABLE I

Sample locations used in this study

Sample Name Key to Lat i tude Longitude Water Fig. 1 depth

(m)

Sample Name Key to Lati tude Longitude Water Fig.1 depth

(m)

31 33 1 63°38'N 01°46'E 1580 31-36 2 64°15'N 0°31'E 2620 AII107-19GGC 3 54°55'S 08°43'W 3255 AII107-22GGC 4 54°48'S 03°20'W 2768 BLW 2 5 77°40'S 165°00'E 370 CH67-19 6 45°45'N 03°57'W 1800 CH69-32 7 45°24'N 05°10'W CH69-69 8 43°51'N 04°30'W CH72-101 9 47°38'N 08°29'W 1925 CH72-104 10 45°56'N 08°47'W 1755 CH73-110 11 59°30'N 08°56'W CH73-136 12 55°34'N 14°28'W CH73-139C 13 54°38'N 16°21'W 2209 CH73-144 14 50°09'N 17°24'W CH77-07 15 66°36'N 10°31'W 1487 CH82-4PC 16 41°40'N 32°50'W 3427 CHN82-15 17 43°22'N 28°14'W 2151 CHN82-20PC/PG 18 43°30'N 29°52'W 3020 CHNl15-26PG 19 53°36'S 00°06'E 2643 CHNl15-27PC 20 54°42'S 02°05'W 3140 CHN115-36PG 21 52°20'S 06°30'E 3692 DSDP-606 22 37°21'N 35°30'W 3007 FRAM-I/4 23 84°30'N 08°59'W 3820 FRAM-I/7 24 83°53'N 06°57'W 2990 HU75-41 25 62°39'N 53°52'W 2381 HU75-42 26 62°39'N 53°54'W 2403 K l l 27 71°47'N 01°36'E 2900 KN708-1 28 50°00'N 23°45'W 4053 KN708-6 29 51°34'N 29°34'W 2469 KN714-15 30 58°46'N 25°57'W 2598 KNR64-16BC 31 63°50'N 00°51'E 2217 M269-965 32 60°54'S 57°06'W MD73-025 33 43°49'S 51°19'E MD73-026 34 44°59'S 53°17'E MD80-304 35 51°04'S 67°44'E MD82-424 36 54°06'S 00°22'W

MD84-551 37 55°00'S 73°16'E MG138 38 79°16'N 5°20'W NO79-06 39 54°31'N 36°53'W PAPA-II 40 50°00'N 145°00'W 4300 PS21-295-4 41 78°00'N 02°25'E 3112 RAMA-44PC 42 53°00'N 164°39'E 2980 RC9-225 43 54°59'N 15°24'W 2334 SU81-47 44 44°53'N 03°18'W V23-22 45 54°12'N 45°58'W 3669 V23-23 46 56°04'N 44°33'W 3292 V23-42 47 62°11'N 27°56'W 1514 V23-81 48 54°15'N 16°50'W 2393 V23-82 49 52°35'N 21°56'W 3974 V23-83 50 49°52'N 24°15'W 3871 V27-17 51 50°05'N 37°18'W 4054 V27-19 52 52°06'N 38°48'W 3466 V27-20 53 54°00'N 46°12'W 3510 V27-60 54 72°11'N 8°35'E 2525 V27-86 55 66°36'N 01 °07'E 2900 V27-114 56 55°03'N 33°04'W 2532 V27-116 57 52°50'N 30°20'W 3202 V28-14 58 64°47'N 29°34'W 1855 V28-38 59 69°23'N 04°24'W V28-56 60 68°02'N 06°07'W 2941 V29-177 61 41°32'N 25°43'W 3391 V29-178 62 42°51'N 25°09'W 3448 V2 c 179 63 44°01'N 24°32'W 3331 V29-180 64 45°18'N 23°52'W 3179 V29-183K 65 49°08'N 25°30'W 3629 V29-206 66 64°54'N 29°17'W 1624 V30-96 67 39°57'N 33°08'W 3188 V30-101K 68 44°06'N 32°30'W 3519 V30-108 69 56°06'N 38°44'W 3171 V180-P3 70 38°26'N 123°48'W 1600 V4-8 71 37°14'N 33°08'W 1655 V30-97 72 41°00'N 32°56'W 3371

been discussed elsewhere (Ruddiman and McIntyre, 1981 and Ruddiman and Bowles, 1977) and a detailed isotopic stratigraphy and radiocarbon chronology is presented by Keigwin and Jones (in press). A second core (CH82-20) was chosen from a region to the southwest of the proposed divergent gyre, to extend the geographic range of Labeyrie and Duplessy's observations. An isotopic stratigra- phy of this core was discussed previously

(Boyle and Keigwin, 1985/6) and is extended here.

Stable isotope results on KN714-15 and CH82-20 show the expected glacial to intergla- cial decrease in (~lsO and increase in fi13C (TableII; Figs.2, 3). The northernmost core, KN714-15, fits the pattern found by Labeyrie and Duplessy (1985) with relatively heavy carbon in N. pachyderma (s.) during the glaciation (Fig.2). Surprisingly, (f13C to the

T A B L E IIa

S t a b l e i so tope r e s u l t s (%0, PDB) a n d C d / C a r e s u l t s (gmol mol ~) for co re CHN82-20

89

t'. wuellerstorfi N. p. (s.) N.p. (d.) N . p . (s.)

I )ep th ~ f i lso 613 C 61so 61zC 6180 613C

(cm)

C d / C a

4.00 2.48 1.26

5.00 2.45 1.22

9.00 2.19 1.22

15.00 2.50 1.24

19.00 2.43 1.24

21.00 2.42 1.00

25.00 2.21 1.14

27.00 2.49 1.15

29.00 2.37 1.02

33.00 2.69 1.01

35.00 2.72 1.02

37.00 2.52 0.82

39.00 2.72 0.98

43.00 2.87 0.77

47.00 3.12 0.95

53.00 3.05 0.84

57.00 2.98 1.07

59.00 3.07 0.91

60.00 3.12 0.99

66.00 2.85 0.58

72.00 2.98 0.74

76.00 3.48 0.65

80.50 3.69 0.85

84.25 3.66 0.19

86.00 4.20 0.99

88.00

92.00

94.00 4.30 0.41

98.00 4.11 0.78

i00.00 4.23 0.40

104.00 4.27 0.64

106.00 4.05 0.65

108.00 4.30 0.76

] 10.00 4.24 0.58

] 14.00 4.24 0.84

116.00 3.73 0.80

]20.00 3.92 0.59

122.00 3.93 1.00

126.00 3.93 0.74

128.00 3.83 0.92

130.00 3.83 0.75

132.00 3.91 0.73

134.00 136.00 3.77 1.00

t38.00 3.90 0.86

t40.O0 3.84 0.93

142.00 3.75 0.95

t44.00 3.78 0.64

I46.00 3.59 1.16

t48.00 3.77 0.96

150.00 3.68 1.07

2.48 - 0.37

2.56 - 0.23

2.39 - 0.36

2.39 -- 0.25 2.37 0.24

3.29 - 0.43

3.43 - 0 . 3 3

3.48 - 0.76

3.20 - 0 . 7 4 2.55 0.10

2.84 0.68

3.05 - 0.56 2.40 0.18

2.75 -- 1.13

2.86 -- 0.89

3.19 --0.65 2.56 0.12

3.24 0.48

2.99 --0.41 2.16 0.05

3.01 0.27 2.02 0.09

0.028

0.040

0.031

0.018

0.037

0.038

0.023

90

T A B L E IIa (continued)

P. wueUerstorfi

Dep th a J l sO 613 C

(cm)

N. p. (s.) N.p. (d.) N. p. (s.)

61s0 J l sC 61s0 61sC Cd/Ca

152.00 3.72 0.98 154.00 3.64 1.03 156.00 3.68 1.16 158.00 3.68 1.13 159.75 3.61 1.11 163.00 3.46 1.11 165.00 3.57 1.13 167.00 3.45 1.30 169.00 3.44 1.35 171.00 3.28 1.13 173.00 3.47 1.19 175.00 3.46 1.28 177.00 3.46 1.28 191.00 3.53 1.13

"Samples above 59 cm a re f rom t he g r av i t y core. Samples below t h a t level a re f rom the p i s ton core and dep ths a re ad ju s t ed by + 25 cm.

T A B L E IIb

Stable i so tope r e su l t s ( ~ , PDB) and Cd/Ca r e su l t s (pmol mol -1 ) f rom Core KN714-15

Dep th N . p . (s.) N . p . (d.) N . p . (s.) N . p . (d.) (cm)

61s0 61aC ¢~1s0 61sC Cd/Ca Cd/Ca

1.00 1.50 0.53 0.016 25.00 1.39 0.54 0.023 49.00 1.55 0.53 0.033 75.00 1.42 0.44 0.024 95.50 0.028 97.50 0.039

104.00 1.23 0.28 0.044 109.00 0.034 124.00 1.13 0.31 0.013 149.00 1.42 0.30 0.013 174.50 0.023 189.00 1.53 0.21 0.032 205.00 0.028 209.00 1.59 0.04 0.025 215.00 0.030 229.00 1.56 0.29 0.021 235.00 1.46 0.09 0.028 242.O0 0.029 249.00 1.56 0.30 0.033 259.00 1.78 0.39 0.024 275.00 1.76 0.16 279.00 0.024 285.00 0.028 295.00 0.025 299.00 3.43 0.14 1.81 0.25 0.035 0.024 3O5.OO O.O28 309.00 0.028 315.00 0.026

0.037

0.024 0.049 0.023 0.042

0.032 0.031

TABLE IIb (continued)

91

Depth N.p. (s.) N.p. (d.) (cm)

N. p. (s.)

(~180 (~13C (~180 (~13C Cd/Ca

N. p. (d.)

Cd/Ca

3]9.00 325.00 1.87 0.34 329.00 334.00 349.00 3.13 - 0.07 1.60 0.21 359.00 3.64 0.16 365.00 3.37 0.38 2.02 0.36 3(;9.00 3.50 0.32 2.22 0.23 3']'4.00 3.44 0.19 3']'9.00 3.44 0.32 2.75 0.18 385.00 3.78 0.16 389.00 3.54 0.19 394.00 398.00 3.78 0.18 398.50 405.00 3.96 0.05 -109.00 4.07 0.05 3.61 0.36 4] 5.00 4.43 0.18 4] 9.00 4.39 0.04 425.00 4.23 0.14 429.00 4,23 0.01 3.59 0.16 429.00 3.66 0.22 438.50 439.00 4.27 0.12 445.00 4.15 - 0.03 448.50 449.00 4.11 -0 .12 454.00 3.72 0.27 458.50 459.00 4.39 - 0.01 463.00 4.01 0.16 468.00 3.80 0.16 3.73 0.19 473.00 3.84 0.13 479.00 3.74 0.28 489.00 3.62 0.40 3.47 0.30 499.00 4.01 0.44

0.030 0.027 0.019

0.025 0.024 0.030 0.025 0.022 0.028 0.046 0.038 0.026

0.039 0.032 0.038 0.028 0.020 0.017 0.022 0.023 0.019

0.012 0.016

0.027

0.027

0.019 0.030

0,025 0.026 0.028 0,025 0.027 0,039 0.035

0.037

0,034 0,027

0.021

0.013 0.036

0.016

0.032

0.019

0.024 0.020 0.016

0.039 0.038

0.041

0.043

0.036

0.014

0.038

TABLE IIc

Stable isotope resul ts (%0, PDB) and Cd/Ca (pmol mol ~) for Core AII-107 22GGC

Depth N . p . (s.) N . p . (s.) N . p . (s.) (cm)

6180 613C (~180 (~13C Cd/Ca

1.00 3.38 0.99 3.48 1.00 0,070 3.00 3.29 0.84 3.45 0.90 0,057

21.00 3.40 0.72 3.34 0.91 0,060 25.00 3.57 1.00 0.061 31.00 3.51 0.66 0.078 35.00 3.61 0.72 0.067 41.00 3.68 0.77 0.074 45.00 3.98 0.61 47.00 4.00 0.73 0,065 48.00 3.92 0.35

0.072 0.060 0,065

92

TABLE IIc (continued)

Depth N . p . (s.) (cm)

N. p. (s.) N. p. (s.)

51s0 513C 51s0 51sC Cd/Ca

49.00 3.99 0.24 50.00 4.00 0.30 51.00 4.62 0.76 52.00 4.00 0.29 53.00 55.00 4.29 0.47 61.00 4.77 0.08 65.00 4.86 0.21 71.00 4.70 0.25 75.00 4.64 0.32 81.00 4.43 0.27 85.00 4.49 0.32

4.16 0.47 0.060

0.062

0.046 0.058 0.069

0.058 0.071

0.063

0.045

0.056 0.054

0.051 0.044

TABLE IId

Stable isotope resul ts (%0, PDB) from cores V27-114 and V30-108

V27-114 V30-108

Depth 51 s o 513C Depth (cm) (cm)

618 0 5'3C

259 4.30 - 0.22 271 3.94 + 0.13 281 4.70 4.26 0.12 291 4.42 -0 .17 300 4.17 -0 .17 352 3.27 0.11 400 3.78 0.22

0.05

85 4.52 - 0.08 90 4.44 - 0.24 95 4.42 - 0.12

100 4.26 - 0.16

southwest is about 0.25%o lower than that found previously off Ireland (Fig.3). The car- bon isotope contrast in N.pachyderma (s.) between these two cores during glacial time is about 0.75%o.

Unlike the carbon isotope results, Cd/Ca shows no significant glacial/interglacial change, and no change between the two cores, even where 513C differs substantial ly (Table III, Figs.2, 3). Cadmium distribution in the ocean bears strong similarity to phosphate distribu- tion (Boyle et al., 1976; Bruland, 1980) and the Cd/Ca ratio in benthic foraminifera has been observed to follow the phosphate concentra- tion in the deep sea (Boyle, 1988a). Cd/Ca in thick-shelled tropical planktonic foraminifera has been shown previously to increase with the

decreasing isotopic temperature of deeper dwelling species, which is consistent with increasing Cd concentrations with depth in the water column (Boyle, 1981).

Considering the carbon isotopic contrast found by Labeyrie and Duplessy (1985) in the northeast Atlantic, and the greater depletion in 13C we have found to the southwest (Fig.3), we decided to map as wide a range of environ- ments as possible in the glacial North Atlantic using stable isotopes in N.pachyderma (s.). Samples from the CLIMAP 18 kyr time slice (Ruddiman and McIntyre, 1981) were provided by W. F. Ruddiman, from which N. pachyderma (s.) were picked from the 150-250 ~m size fraction and analyzed for stable isotopes using standard procedures. Additional analyses were

93

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t 6 -

2 0 -

2 4 -

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4 . 5 0 4 . 0 0 3 . 5 0 3 . 0 0 2 , 5 0 2 . 0 0 I I I I 1

"• 2 ~

/ /

/

ACHY L

PACHY R

f /

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. 5 0 . 0 0 - . 5 0 . 0 0

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.02 .04 .06

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Fig.2. Stable isotope and Cd/Ca results on N. pachyderma from piston core KN714-15 in the northern North Atlantic Ocean (59N, 26 'W, 2598 m). Results are plotted vs. age using 12 accelerator radiocarbon dates on mixed planktonic foraminifera and N. pachyderma (Keigwin and Jones, in press). Although 313C of N. pachyderma increases by 0.5%0 from glacial to interglacial time, Cd/Ca values do not change significantly, maintaining low values typical of nutrient-depleted surface water. Note that the Cd scale is greatly expanded; Cd/Ca changes on this scale are equivalent to a 50% expansion of the 513C scale.

TABLE III

Additional 1 Cd/Ca results (pmol tool- )onN.pachyderma(s.)

Core Depth Cd/Ca (cm)

AII107-19GGC 0 1 0.073 CHN115-27PC 0.4 0.061

2 4 0.155" CHN115-36PG 0 3 0.103 CHN115-26PG 0 2 0.095 KNR54-16BC 0 1 0.013 K708-6 60 0.023 RAMA44P 136 0.081 RC9-225 135 0.012 V23-42 70 0.031 V23-82 110 0.018 V27-17 35 0.010 V27-19 35 0.014 V27-20 80 0.027 V29-183 38.5 0.017 V23-23 8 9 0.029

0.104 0.158" 0.209* 0.091

*Suspected contamination

p e r f o r m e d for o x y g e n i so top ic s t r a t i g r a p h y

f rom se l ec t ed cores (V27-114, V30-108; T a b l e IId)

w h e r e C L I M A P was u n a b l e to i d e n t i f y the 18

k y r leve l due to d o m i n a n c e of t he f a u n a by

N. pachyderma (s.).

O x y g e n i so tope r a t i o s of N. pachyderma (s.)

a re m a p p e d for c o m p a r i s o n w i t h the C L I M A P

(1981) t e m p e r a t u r e e s t i m a t e s (Fig.4). We chose

the C L I M A P s u m m e r r e c o n s t r u c t i o n , a s sum-

i n g t h a t N. pachyderma (s.) was m o r e l i k e l y to

h a v e l ived in t h a t s e a s o n d u r i n g the g l ac i a l

m a x i m u m , b u t cho i ce of s e a s o n does n o t affect

the r e su l t . I t is e v i d e n t t h a t ~ l sO i s o p l e t h s c a n

be c o n t o u r e d wel l i n t o t he N o r w e g i a n Sea, i n

w a t e r s w h e r e C L I M A P was n o t ab le to esti-

m a t e t e m p e r a t u r e s u s i n g t he t r a n s f e r f u n c t i o n a p p r o a c h . C o n t o u r i n g of ~ lsO shows the com-

p r e s s i o n of v a l u e s t o w a r d l ow e r l a t i t u d e s ,

m a r k i n g the p o l a r f ron t , t he s o m e w h a t l ower

v a l u e s off t he B r i t i s h Isles, w h i c h p r o b a b l y

94

~ 0 - i 8

4 . 5 0 4 . 0 0 3 . 5 0 3 . 0 0 o I I I

2 0 -

4 0 -

SO-

SO-

l O 0 -

i 2 0 -

t 4 0 -

~.60-

i 8 0 -

200

• &

HUELLERS

2 . 5 0 I *&

"i &A

¢ .

t

t . 50

~C-13

t . O 0 .50 . 0 0 - . 5 0 - t . O 0 f I i I I

WUELLERS

JA

&&&

* t

~ k tA A

CHN82 CORE 20

Fig.3. Stable isotope results on the benthic foraminifera P. wuellerstorfi and on N. pachyderma from central North Atlantic core CHN82-20 (44°N, 30°W, 3020 m). Data from the gravity core and piston core are spliced together at 59 cm (see Table IIa). The carbon isotope ratio in N. pachyderma increases from glacial to interglacial time, but Cd/Ca results (Table IIa) do not change significantly and are the same as our more northern core (Fig.2).

ref lect w a r m e r water , and some ev idence of cooler su r face w a t e r in the n o r t h w e s t At lan t ic .

Ca rbon i so tope resul ts , wh ich a re not as eas i ly i n t e r p r e t e d as 51 sO, ind ica te t h a t lowes t 513C in N. pachyderma occu r r ed in the cen t r a l N o r t h A t l an t i c a t a b o u t 40 ° N (Fig.5). Al- t h o u g h the shape of the ~13C isople ths shows lower va lues to the sou thwes t , the gene ra l p a t t e r n is not cons t r a ined by da t a to the s o u t h e a s t and could also be some func t ion of la t i tude . Glac ia l age samples sou th of 40 ° N r a r e ly h a v e enoug h N. pachyderma (s.) for s tab le i so tope ana lys i s so it will be ve ry difficult to ex tend the m a p to lower l a t i tudes to define the reg ion of m i n i m u m ~13C values .

Addi t iona l Cd/Ca d a t a f rom the 18 ky r t ime slice of the N o r t h A t l an t i c conf i rm the r e su l t f rom cores KN714-15 and CH82-20. The r a n g e of ~Cd/Ca in samples f rom the g lac ia l m a x i m u m is a b o u t 0.02 ~mol /mol (Table III), close to the

ana ly t i c a l p rec i s ion of the me thod a t low Cd concen t r a t ions . I f the a p p r o x i m a t e l y 0.75 %0 r ange in ~13C was due en t i re ly to va r i ab i l i t y in sur face w a t e r n u t r i e n t s (equal to a r a n g e of approx. 1 u M / k g PO4), we would h a v e expected the r a n g e of Cd/Ca to be l a rge r by more t h a n a f ac to r of 10.

&13C and Cd/Ca in S o u t h e r n O c e a n sur face w a ter s

To c o m p l e m e n t our N o r t h At l an t i c t ime ser ies of n u t r i e n t proxies we inves t iga t ed the h i s to ry of i so topic and Cd/Ca change in a core f rom the h igh l a t i tude Sou th At lant ic . Core AII107-22GGC was t a k e n f rom 2768 m on the wes t f lank of the Mid At l an t i c ridge, jus t n o r t h w e s t of the Bouve t Tr iple J u n c t i o n (Fig.l) . Oxygen isotope s t r a t i g r a p h y of N. pa- chyderma (s.) f rom this core shows a fa i r ly

95

\ / ... J • ~ / 0 4 7 0 ° 4 9 5 \ \

4 3 7 0 . . . . 4 4 4

/ X

/ . ~ . 446"~42 , / / ~ : ~ - . ' : : . ' : ~ . ' . , - , . , . ~ * ~ ~ , ~ / 0 ~ ~ , : . ~ . , : , . . - - " .

4 . 4 , - ~ v / 1 0 ~ 4 51 ~ " ~ 4 2 0 : " , " , .

3 " t ~ ~ ( ~ . ~ ~ I ~ ~ ~ ' . ~ , ' . • 3 . 4 '.' .... ? -. -',t" •

2.9~ 243W 3 3 9 (.~.';, , .-~J I,~ ~ - ~.~'. ..

Fig.4. Comparison of CLIMAP 18 kyr Sea Surface Temperature (summer) reconstruction (A) and map of ~sO of N. pachyderma (s.) (B). The general pattern of ~ls O isopleths is similar to the pattern of reconstructed isotherms, suggesting that/i ~ sO in N. pachyderma is some first-order function of S.S.T. An oxygen isotopic range of about 2% across about 30 ~ of latitude, if entirely due to temperature effects, would indicate only about an 8°C range in S.S.T. This contrasts with the larger CLIMAP range of inferred temperature, which may indicate an effect on ~sO due to lower salinity at higher latitudes. Lower salinity or higher S.S.T's may also be indicated off the British Isles at 18 kyr.

t y p i c a l d e c r e a s e in 81sO of a b o u t 1.5%o f rom

65 cm ( g l a c i a l m a x i m u m ) to t h e c o r e top ,

i n d i c a t i n g a r a t e o f s e d i m e n t a t i o n o f 3 to

4 c m / 1 0 0 0 y r s (Fig.6; T a b l e II). T h e o x y g e n

i s o t o p e s t r a t i g r a p h y is s u p p o r t e d b y i n c r e a s e d

p e r c e n t a g e in t h e r a d i o l a r i a n C. davisiana d u r i n g t h e g l a c i a l , a r e l a t i o n s h i p w h i c h is wel l -

e s t a b l i s h e d in l o w e r l a t i t u d e s ( H a y s e t al . , 1976) b u t h a s n o t b e e n d e m o n s t r a t e d p r e v i o u s l y fo r a

l o c a t i o n s o u t h of t h e P o l a r F r o n t . F r o m t h e

g l a c i a l to t h e i n t e r g l a c i a l 513C i n c r e a s e s by 1%o,

b u t C d / C a does n o t c h a n g e s i g n i f i c a n t l y (Fig .6) , a p a t t e r n w h i c h h a s j u s t b e e n s h o w n for t h e

N o r t h A t l a n t i c . T h i s r e s u l t is in e s s e n t i a l

a g r e e m e n t w i t h a s e c o n d core , f r om t h e I n d i a n

O c e a n s e c t o r o f t h e S o u t h e r n O c e a n (Boy le ,

1988b). A t t h a t l o c a t i o n (51°S, 68°E) L a b e y r i e

a n d D u p l e s s y (1985) s h o w e d a 1%o g l a c i a l i n t e r - g l a c i a l r a n g e in t h e 513C of N. pachyderma (s.)

a n d C d / C a a n a l y s e s (Boy le , 1988b) s h o w e d v a l u e s of a b o u t 0.08 p m o l / m o l , w i t h no s igni f i -

c a n t g l a c i a l / i n t e r g l a c i a l d i f f e r e n c e .

96

l 0 2 0 01108 ~ 17

- 0 8 0 . 0 6 0 -

. t40 .OlO

0-06

Q-24

- o70

• 05 002

0.58 1.06

i4% z4

'-.89 63 / t / / • -.60 - . 5 0 / - ' 5 0

Fig.5. Map of carbon isotope ratios in N. pachyderma (s.) during the latest glaciation. All data are listed in Table IV. The major sources of data are this paper, based on samples of the CLIMAP 18 kyr time slice provided by W. Ruddiman, and Labeyrie and Duplessy (1985). All new data in this figure come from analyses of specimens in the 150 250 ~tm size fraction. This detailed map confirms the isotopically light carbon in N. pachyderrna (s.) discovered by Labeyrie and Duplessy (1985) off Ireland, but it also shows that the lightest carbon in the Atlantic occurred far to the southwest. The extensive area of light carbon and the lack of supporting Cd/Ca evidence argue against upwelling of nutrient-rich subsurface waters as the cause of the (513C pattern.

Surface water chemistry and modern N. pachyderma

Our records of late Quaternary Cd/Ca and 613C in N.pachyderma suggest that they can- not both be accurate proxies for near surface water nutrients. This contradict ion has lead us to reevaluate the relat ionship between Holo- cene N. pachyderma (s.) (~13C and the 613C and PO4 of near-surface waters. Labeyrie and Duplessy (1985) found a constant offset be- tween 513C in the foraminiferal calcite and the 13C of total CO2 (from GEOSECS data), which they interpreted to mean that the carbon isotope ratio in N. pachyderma recorded past variat ions in ZCO 2 613C. However, as Labeyrie and Duplessy pointed out, the fit of the foram-seawater relationship was improved by eliminating Southern Ocean data, which is curious since these authors considered the Weddell Sea as a modern analog for the

Northeast Atlantic during the latest glacia- tion.

Our compilation of Holocene data on N. pa- chyderma (s.) draws on many sources, especi- ally that published by Labeyrie and Duplessy (1985). Data from their table I and appendix 1 have been amended by L. Labeyrie (pers. comm., 1988). Their data and our additional sources are listed in our Table V. We made every effort to sample as wide a spectrum of marine environments as possible, but this was difficult due to the restricted occurrence of N.pachyderma (s.) to the higher latitudes in the modern ocean. Phosphate data were taken from GEOSECS, where there were sites near our sample locations; otherwise we referred to Reid (1962). Carbon isotope results on the total CO 2 in sea water were taken from Kroopnick (1984) where there were GEOSECS sites nearby. Because of uncer ta inty about exactly where N. pachyderma (s.) lives in the water

5. O

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98

TABLE IV

18 Kyr t ime s l ice d a t a on N o r t h A t l a n t i c N. pachyderma (s.) and T e s t ima te s

Core Depth 51s0 513C TA,g TFe b Source of Cd/Ca (cm) i so tope d a t a t (~mol mol 1)

V27-19 35.0 4.46 - 0.15 6.70 2.20 KB

V27-17 35.0 4.42 0.01 5.90 1.20 KB V27-20 80.0 4.33 - 0.08 7.40 3.00 KB V23-82 110.0 4.34 - 0 . 0 7 6.00 1.30 KB

V29-179 73.0 3.63 - 0.52 10.50 8.00 KB V27-116 60.0 4.52 0.03 6.40 1.80 KB V29-180 57.5 3.80 - 0.63 9.20 5.70 KB

V23-81 295.0 3.78 - 0 . 3 3 6.10 1.60 KB

KN708-1 110.0 4.21 - 0.15 6.20 1.70 KB V23-83 100.0 4.31 - 0.12 6.00 1.40 KB RC9-225 135.0 3.99 - 0.45 6.20 1.60 KB

V29-178 48.5 3.46 - 0.58 12.20 8.40 KB

V30-101K 45.0 3.83 - 0.62 10.00 7.00 KB V23-42 70.0 4.52 - 0.06 6.50 1.70 KB

V29-183K 38.5 4.10 - 0.19 KB KN708-6 60.0 4.46 0.14 6.00 1.50 KB

V29-177 41.5 3.05 - 0 . 6 3 15.10 9.40 KB KN714-15 425.0 4.23 - 0.24 KB

CHN82-20 106.0 3.48 - 0.76 KB V30-97 75.5 - 0.89 KB CHN82-4 72.0 2.92 - 1.04 KB V29-206 170.0 4.37 - 0.14 4.50 - 0.80 KB

V30-108 85.0 4.52 - 0.08 KB V28-14 170.0 4.64 4.50 - 0.80 KDS V27-114 291.0 4.42 - 0.17 KB CHN82-15 31.0 3.39 - 0.48 KB V23-23 115.0 4.34 - 0.07 6.80 2.30 MF V30-96 63.0 2.73 - 0.62 17.80 12.30 KB V28-56 70.0 4.70 2.80 - 2.00 KDS DSDP606 46.0 2.65 - 0.60 18.50" 13.30" KB

V27-86 55.0 4.64 0.06 3.40 - 1.60 LD V27-60 190.0 4.84 0.05 3.90 - 1.20 LD K l l 55.0 4.93 0.02 LD CH77-07 200.0 4.66 0.38 LD CH72-101 280.0 3.51 - 0.10 7.00 LD HU75-41 114.0 4.98 0.24 LD HU75-42 60.0 4.61 0.18 LD CH73-110 140.0 4.40 - 0.30 LD CH73-136 120.0 4.26 - 0.42 LD

CH73-139C 170.0 4.02 - 0.37 8.00 LD CH69-32 150.0 4.20 0.19 LD CH69-69 160.0 3.98 0.03 LD

SU81-47 220.0 3.44 - 0.03 LD CH72-104 380.0 3.73 0.00 7.00 LD CH67-19 380.0 3.71 - 0.05 7.00 LD NO79-06 290.0 4.45 0.06 LD F R A M I/4 16.0 4.6 0.30 ZMT F R A M I/7 37 4.6 0.10 ZMT 31-33 112 4.44 0.14 S 31-33 30 4.88 0.24 S

0.014

0.010 0.027 0.018

0.012

0.017 0.031

0.023

*Tempera tu re e s t i m a t e s come from n e a r b y core V4-8. t K B = th i s paper; KDS = Kel logg, Duplessy and Sha ck l e t on , 1978; MF = M i x and F a i r b a n k s , 1985; LD = L a b e y r i e and Duplessy, 1985; ZMT = Zahn, M a r k u s s e n and Thiede, 1985; S = Se j rup et al., 1984.

99

TABLE V

Stable isotopes on Holocene N. pachyderma (s.) with PO 4 + ECO 2 data from nearest GEOSECS station

Core Geosecs ZCO 2 ZCO2 PO4 PO4 ~13C 51sO Source of station 0 m 100 m 0 m 100 m isotope data*

Kl l 18 1.81 1.17 0.25 0.88 0.49 3.02 LD V27-86 18, 19 1.80 1.16 0.25 0.94 0.27 2.68 LD HU75-41 5, 8 1.90 1.47 0.63 1.05 0.44 2.78 LD HU75-42 5, 8 1.90 1.47 0.63 1.05 0.77 2.66 LD NO79-06 24 2.12 1.14 0.44 1.00 0.18 LD CH73-110 19, 23 1.77 1.32 0.29 0.89 0.13 2.67 LD CH73-144 23, 24 1.90 1.27 0.39 0.89 0.25 LD CH77-07 19, 15 2.10 1.22 0.17 0.81 0.91 3.68 LD MG138 17 2.25 1.37 0.36 0.90 0.46 LD V23-22 3 1.59 1.17 0.17 0.73 0.41 1.76 LD V23-23 5 1.80 1.44 0.65 0.98 0.28 2.66 MF V27-60 18 1.81 1.17 0.25 0.88 0.40 2.84 LD MD73-025 428, 429 1.60 1.45 0.85 1.08 1.17 2.32 LD MD80-304 430 1.10 0.80 1.91 2.04 1.40 2.91 LD MD82-424 89 1.41 0.89 1.32 2.15 0.92 3.59 I,D, L MD73-026 429 1.50 1.25 1.51 1.62 -0.17 2.00 LD, L KNR54-16BC 19 1.76 1.10 0.30 0.90 0.47 2.32 KB CHNl15-26PG 89 1.41 0.90 1.40 1.50 1.04 3.05 KB CHNl15-27PG 89 1.41 0.90 1.40 1.50 1.05 3.28 KB CHN155-36PG 89 1.41 0.90 1.40 1.50 1.29 3.26 KB V180-P3 1.00 2.00 0.28 1.23 KB AII107-22GGC 89 1.41 0.90 1.40 1.50 0.99 3.38 KB RAMA-44PC 1.75 2.25 0.73 2.61 KB M269-965 78 1.60 2.00 0.80 3.20 W PS21-295-4 17 0.30 0.90 0.43 3.46 JK PAPA-II 1.20 1.80 0.28 3.59 KB BLW2 0.73 ',3.46 B FRAM I/4 0.8 1.9 ZMT FRAM I/7 0.7 2.0 ZMT V28-38 2.93 L MD84-551 3.15 L V28-56 2.86 KDS V28-14 2.18 KDS 31-33 19 1.76 1.10 0.14 1.64 S

*LD = Labeyrie and Duplessy, 1985; L= Labeyrie et al., 1986; D = Durazzi, 1981; MF = Mix and Fairbanks, 1985; KB = this paper; W = Wefer et al., 1982; JK = Jones and Keigwin, 1988; B = Enriqueta Barrera, pers. comm.; KDS = Kellogg, Duplessy and Shackleton, 1978; S = Sejrup et al., 1984.

c o l u m n , w e c o n s i d e r h y d r o g r a p h i c d a t a f r o m

b o t h t h e s e a s u r f a c e a n d a t 100 m d e p t h .

R e s u l t s f o r t h e (~13C o f E C O 2 ( F i g . 7 ) a n d f o r

P O 4 (F ig .8 ) i n d i c a t e t h a t 513C i n N.pachy- derma (s.) f r o m c o r e t o p s d o e s n o t a p p e a r t o b e

a f u n c t i o n o f e i t h e r . I n f a c t , t h e a v a i l a b l e d a t a

s u g g e s t t h a t t h e f o r a m c h e m i s t r y - s e a w a t e r

c h e m i s t r y r e l a t i o n s h i p m a y a c t u a l l y b e o p p o -

s i t e t o t h a t p r e d i c t e d b y t h e s o l i d l i n e i n F i g s . 7

a n d 8. A l t h o u g h t h e r e i s l i k e l y t o b e u n c e r -

t a i n t y a b o u t t h e e x a c t a g e o f o u r H o l o c e n e

s a m p l e s , a n d p e r h a p s s o m e q u e s t i o n a b o u t

w h i c h G E O S E C S s i t e s w e r e c h o s e n ( a n d

w h e t h e r o r n o t t o i n t e r p o l a t e b e t w e e n s i t e s ) ,

t h e f o r a m d a t a a n d (o r ) t h e s e a w a t e r c h e m i c a l

d a t a w o u l d h a v e t o b e i m p o s s i b l y d i f f e r e n t

f r o m o u r c h o s e n r e f e r e n c e v a l u e s i n o r d e r f o r

t h e t w o t o c o r r e l a t e a s p r e d i c t e d .

T h e s e a w a t e r c h e m i c a l d a t a m i g h t d i f f e r

s o m e w h a t w e r e t h e y c o l l e c t e d i n a d i f f e r e n t

100

C - 1 3 o f TOTAL C02 v s . PACHY LEFT C-13

2 . 0 0 -

(vl

I I.I

l" h W --I

1.3 .II Q.

! . 5 0 -

1 . 0 0 -

. 5 0 -

. 0 0 -

0 / °

A

4 .

J.

_* '&& j .

& • &

0 •

' I ' I ' I ' I ' I ' I . 00 .50 1 . 0 0 i . 5 0 2 . 0 0 2 . 5 0 3 . 0 0

C - L 3 TOTAL C02 0 0 m ( s o l i d ) & tOOm (oDsn)

Fig.7. Carbon isotope ratios in Holocene N. pachyderma (s.) plotted against the carbon isotope ratio of total CO 2 in seawater (from GEOSECS data). Solid symbols are hydrographic data from the sea surface and open symbols are data from 100 m; circles represent data from the Southern Ocean and triangles mark North Atlantic data. Regardless of whether N. pachyderma (s.) calcifies at 0 m or 100 m, its 513C does not follow the predicted relationship (solid line) within or between ocean basins.

PACHY LEFT C - 1 3 v s . P04 @ Om ( s o l i d ) ~ lOOm (open)

2 . 0 0 -

1 . 5 0 - 0 - - 0

T ~- t .O0- u. '

' I" U

a. . 5 0 -

. 0 0 -

o - - 0

' I ' I ' I ' I ' I ' I . O0 • 50 I . O0 t . 50 a . O0 2 . 5 0 3 . O0

P 0 4

Fig.8. Carbon isotope results in Holocene N. pachyderma (s.) plotted vs. PO4 content at the sea surface (solid symbols) and at 100 m (open symbols). Circles represent data from the Southern Ocean and triangles mark North Atlantic data. The solid line indicates the predicted relationship. It is evident that 613C in N. pachyderma (s.) does not follow near surface nutr ient levels.

season, but the sense of the change is insuffi- cient to change significantly the relat ionship in Figs.7 and 8. Most hydrographic casts in high latitudes are collected during the sum- mer, when a seasonal nutr icl ine develops. Since available evidence shows that N. pachy- derma (s.) reac:hes maximum abundance in the nor thern North Atlantic Ocean in the spring (Tolderlund and Be, 1971), this foram might secrete its test when the upper water column is still well mixed. However, phosphate data from Norwegian Sea. locations which were collected in late winter (Clark et al., 1984) near GEO- SECS stations 18 and 19, which were collected in late summer, show a seasonal contrast of only about 0.6 pM/kg at the sea surface and insignificant contrast at 100 m water depth.

Some concern has been expressed that gas exchange with the atmosphere might shift high lati tude surface water 613C away from its initial value during the season when a thin mixed layer forms. If such a shift were to occur, and were left-coiling N. pachyderma to grow dominantly in that season, foraminiferal 613C might be a misleading measure of surface water phosphorus levels. Atmospheric 513C is -7.2%0; at temperatures near 2°C, seawater equilibrated with the atmosphere would have 513C ~ 3.5%0 (Mook et al., 1974). Measurement of surface water 613C in the Antarct ic indi- cates that the actual ~13C is closer to 1.5%o (Broecker, 1982) because of the upwelling of deep ocean waters containing light metabolic CO 2. If a 30 meter mixed layer containing 2200 ~mol/kg ECO 2 were cut off from fur ther exchange with underlying waters for long enough, 513C of this water would increase by 2%o. This mixed layer contains 66 moles of carbon dioxide. Typically, gas exchange rates are thought to be about 20 moles/m2/yr (Broecker and Peng, 1982), so it would take several years for this mixed layer to equili- brate with the atmosphere. In the 6 months that an isolated mixed layer might exist during the warm season, 513C could shift +0.3%0. This shift could be larger only if the gas exchange rate were much higher or the mixed layer much thinner. Charles and Fairbanks (1988) find as

101

much as a 0.5%0 enrichment in 13 C which could be at t r ibuted to vigorous gas exchange in the region of the Antarct ic Polar Front. Even this extreme effect, however, is insufficient to account for the difference between our core top N. pachyderma ~13C from the North Atlantic and Southern Ocean (Fig.8).

There are a few other possible explanations for the disagreement between seawater chemis- try data and core top foraminiferal data. There might, for example, be some "vital effect" resulting from the ability of N. pachyderma to survive in Antarct ic sea ice (Lipps and Krebs, 1974). In addition, micropaleontologists have suggested, based on morphological analysis, that Arctic and Antarct ic N. pachyderma are at least two separate populations and perhaps different subspecies (Kennett, 1970). If this assertion is true, then different "vital effects" between subspecies or populations might ac- count for the results in Figs.7 and 8. It has fur ther been suggested that the morphotype differences can lead to large isotopic offsets (Healy-Williams and Williams, 1986). Finally, differences in stable isotope ratios are com- monly observed among foraminiferal speci- mens of different size. Several possible explanations for this phenomenon have been explored (Berger et al., 1978; Williams et al., 1981), including, for example, isotopic fraction- at ion with growth rate changes, or vertical migration through water of varying temper- ature. Wefer et al. (1982) found insignificant changes in ~13C or ~lsO of N. pachyderma (s.) of varying size fractions collected from sedi- ment traps in the Drake Passage, which they at t r ibuted to the ra ther homogeneous hydro- graphic conditions in the Southern Ocean.

Our own analyses of several size fractions from core AII-107 22GGC at 47 and 53 cm reveal increasing 61sO and 613C of a few tenths permil with a size increase of a few hundred microns in the Southern Ocean (Table VI). These samples were chosen for analysis be- cause of the abundance of N. pachyderma (s.) specimens, but because they lie on the isotopic terminat ion where bioturbat ion may have mixed glacial and interglacial specimens, they

102

T A B L E VI

Effect of size u p o n s tab le i so tope ra t ios and Cd/Ca in N. pachyderma (s.)

Core Dep t h Size 5180 513C Cd/Ca (cm) (jm]) (%0) (%0)

AII-107 22GGC 47 250-300 3.89 0.77 180 250 3.85 0.77 150-180 3.70 0.49 125-150 3.70 0.30

53 250-300 4.34 0.46 180-250 4.20 0.43 150 180 4.17 0.45 125-150 3.91 0.07

KN714-15 428 300-355 4.26 0.27 0.031 250-300 4.42 0.13 0.019 180-250 3.92 0.05 0.018 150-180 3.66 - 0.22 0.027 125-150 3.54 - 0.64 0.029

are not the best test of the size fraction effect. Results from a North Atlantic sample of glacial maximum age show an even stronger size fraction effect (Table VI). These new results do not shed light on the cause of this size effect, which is typical of many planktonic foramini- feral species (Berger et al., 1978). Since our analyses of N. pachyderma (s.) were routinely from the 150-250 ~m range, it is unlikely that isotopic variability of more than a few tenths permil can be explained by changing specimen size. Our inclusion of data from other studies, which were not from discrete size fractions, may introduce scatter into Figs.5, 7, 8, but it would be surprising if it would change our conclusions which are based on /i13C differ- ences of about 0.75%0. Our results indicate no effect of size on Cd/Ca (Table VI).

Paleocl imat ic impl icat ions o f the N. pachyderma chemical record

Carbon isotopes, cadmium and surface water nutrients

Although we could speculate about ways to try to reconcile the N. pachyderma (s.) record of carbon isotopes with that of Cd/Ca in our late Quaternary samples, it is fruitless without significant "ground t ruthing" in the modern

ocean. It appears this will come only from analysis of plankton tow and sediment trap material. What few Cd/Ca data we do have from Holocene N.pachyderma are consistent with what we know about surface ocean nutrient levels in the modern ocean, suggest- ing the Cd/Ca is a useful tracer for surface water nutr ient content. Southern Ocean samples have more than twice the Cd content of our one sample from the Norwegian Sea and the PO 4 content of the Southern Ocean samples is correspondingly higher (Fig.9). These results are roughly in accord with the much larger data set defining the benthic foram relationship (Boyle, 1988a). Our Norwe- gian Sea result on N. pachyderma has less Cd than benthic foraminifera from the most nutr ient depleted deep water of the North Atlantic, probably due to entrainment of southern source water with its higher nutr ient levels into N.A.D.W. Although the geographic distribution of 513C in the North Atlantic is intriguing (Fig.5) and further mapping may help us understand the significance of the pattern, we disagree in general with Labeyrie and Duplessy (1985) and Mix and Fairbanks (1985) and argue that there is no reliable evidence that preformed nutr ient levels in the North Atlantic increased during glaciations. We further argue that the upwelling gyre

0 3

~ 02 E

-6 E c

d

Q.O

• ~ o o •

o (is /o ,.'s 2'.0 zTs ~o PO 4 /zmol/kg

Fig.9. The Cd/Ca ratio in Holocene foraminifera vs. the PO~ content in ambient seawater. Small data points are benthic foraminiferal results (from Boyle, 1988b) and data with range bars are results on N. pachyderma (s.) from this paper and Boyle (1988b). The length of the bar for N. pachyderma data reflects uncer ta in ty in the depth range of calcification (0 to 100 m). The N. pachyderma data follow the general trend of benthic data, indicating the usefulness of Cd/Ca in this species as a tracer for near-surface water nut r ien t levels.

proposed by Labeyrie and Duplessy (1985) did not exist. Thus, we believe that carbon isotope ratios in N. pachyderma cannot be used to identify the locus of glacial deep water forma- tion in the North Atlantic, as proposed by Duplessy et al., (1988), although it possibly could be done with Cd/Ca.

Subject to our extremely limited geographic coverage in the Southern Ocean, we find no evidence that the levels of preformed nutrients changed significantly from glacial to intergla- cial time. Cd/Ca shows no significant change down cores from either Atlantic or Indian Ocean sectors of the Southern Ocean, but on average the :ratio is higher than in North Atlantic samples, consistent with modern dis- tributions. 613C in N. pachyderma (s.) is greatly decreased in Southern Ocean cores during glacial time, exactly opposite to the effect predicted by models explaining lower CO2 in the glacial atmosphere through reduction in preformed nutrients (Knox and McElroy, 1984; Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984). Considering that there is no

103

empirical evidence in support of these models, they will have to be a re-evaluated unless it can be shown that our geochemical measures are misleading.

Oxygen isoptopes and sea-surface salinity.

Oxygen isotope ratios in N. pachyderma (s.) may have more paleoceanographic usefulness than 51sC. When plotted vs. the CLIMAP paleotemperature estimates and those in Du- plessy et al. (1981), the oxygen isotope results show a well-defined linear pattern which must originate from the effects of temperature as well as salinity change (Fig.10). The line through the data in Fig.10 is the Epstein et al. (1953) paleotemperature equation for equilib- rium calcite precipitation, which was arbitrar- ily fixed such that the calcite of N. pachyderma (s.) from sample V29-179 was precipitated at equilibrium with the CLIMAP (1981) tern- perature estimate. Since the data in general do not parallel the predicted equilibrium line, it is clear that both the temperature of calcite formation and the local salinity varied north and south of V29-179 (44-N) at 18,000 yrs B.P. Data to the "cold" side of the equilibrium line may be affected by several sources of scatter, including uncertainty in picking the 18 kyr level, inaccuracies of the different paleotem- perature estimates, especially for low temper- atures and local salinity effects. "Warmer" data, however, display a regular departure from that point where seawater composition and equilibrium precipitation were specified. It is likely that these data fall off the; line in a systematic manner because of increasing local salinity with decreasing latitude. Conversely, higher latitude samples probably had lower salinity than those at temperate latitudes. Oxygen isotopic ratios in N. pachyderma have been used previously to infer the distribution of meltwater during deglaciation in the North Atlantic (Fillon and Williams, 1984) and during the late Quaternary in the Southern Ocean (Labeyrie et al., 1986). A more complete study of oxygen isotopes and paleotemperature esti- mates in planktonic foraminifera from middle

104

NORTH ATLANTIC PACHY LEFT 0-~8 ,vs. AUG TEHP.

OB

2 . 5 0 -

3 . 0 0 -

3 . 5 0 -

4 . 0 0 -

4 . 5 0 -

X

5 . 0 0 : I 2 20

; 4

,~ ~ ,~/, , . . gd, S -~vzg-jz9

: /

' ~ / ~ / ' i ' i ' i ' I ' I ' I ' 6 R t0 1 2 t4 t5 ~,S

TEHP (C)

Fig.10. Scat ter plot of 51sO of N. pachyderma (s.) from 18 kyr levels vs the CLIMAP temperature estimate (summer) from the same samples. Data above about 8°C, where the CLIMAP temperature estimates are thought to be most accurate, are strongly l inear but depart from the Epstein et al. (1953) relat ionship for equilibrium calcite precipitat ion (solid line). This departure (dashed line) may be an art ifact of increasing salinity with decreasing latitude.

latitudes in the North Atlantic would be an important step leading to methods of decon- volving temperatures and salinities in late Quaternary isotope records.

Conclusions

Comprehensive study of the carbon and oxygen isotope geochemistry and the cadmium geochemistry of the polar planktonic foramini- fera N. pachyderma from widely distributed samples of Holocene and late Quaternary age showed the following:

(1) Carbon isotope ratios in Holocene N. pa- chyderma (s.) do not appear to decrease with increasing nutr ient levels in near surface waters and do not appear to covary with the 513C of total CO2. Analysis of sediment trap and plankton tow samples are needed to learn what controls 513C in this species. Stable isotopic analyses on this species should be on tightly controlled size fractions. The Cd/Ca

ratio does appear to be a useful tracer for nutr ient levels, al though our data set of Holocene samples is still very limited.

(2) As recognized previously by others (La- beyrie and Duplessy, 1985) there is an interest- ing pattern in the 513C ofN. pachyderma in the glacial North Atlantic, al though the pattern is significantly different than previously thought. In the central North Atlantic at about 40°N &13C is as much as 1%o lower than in the northern North Atlantic. The significance of this pattern is unclear at this time, but available evidence does not support the existence of an upwelling gyre in the northeast Atlantic during the most recent glacial maximum.

(3) Cd/Ca does not show significant spatial variations in glacial samples from the North Atlantic. Downcore records of this species do not reveal significant glacial/interglacial change, suggesting no change in near surface water nutrients. Surface waters in the late Quaternary Southern Ocean were always en-

r i c h e d in n u t r i e n t s c o m p a r e d to N o r t h A t l a n t i c

s u r f a c e w a t e r s . C l i m a t e m o d e l s w h i c h p r e d i c t

d e c r e a s e d p r e f o r m e d n u t r i e n t s in t h e S o u t h e r n

O c e a n to a c c o u n t fo r g l a c i a l l o w e r i n g o f

a t m o s p h e r i c p C O 2 l e v e l s a r e n o t s u p p o r t e d by

o u r da ta .

(4) O x y g e n i s o t o p e r a t i o s in N. pachyderma h a v e b e e n f o u n d u s e f u l in e s t i m a t i n g pa leo -

s a l i n i t y w h e n c o m p a r e d w i t h t r a n s f e r f u n c t i o n

p a l e o t e m p e r a t u r e e s t i m a t e s in b o t h t h e

n o r t h e r n a n d s o u t h e r n h e m i s p h e r e s . T h e gla-

c ia l N o r t h A t l a n t i c s h o u l d be p a r t i c u l a r l y

i m p o r t a n t in q u a n t i f y i n g t e m p e r a t u r e s a l i n i t y

r e l a t i o n s h i p s b e c a u s e o f t h e a b u n d a n c e o f

f o r a m i n i f e r a f rom w i d e s p r e a d l o c a t i o n s ,

A c k n o w l e d g e m e n t s

T h i s w o r k b e n e f i t e d g r e a t l y f r o m f o r a m

p i c k i n g , i s o t o p i c a n a l y s e s a n d d a t a m a n a g e -

m e n t by C. E b e n F r a n k s a n d f r o m d r a f t i n g by

A m y P a l l a n t . T h e m a n u s c r i p t w a s t y p e d by J .

W i l l i a m s a n d E. E v a n s . W e a r e e s p e c i a l l y

g r a t e f u l to W. F. R u d d i m a n for p r o v i d i n g

N o r t h A t l a n t i c s a m p l e s a n d to E. B a r r e r a , W.

B. C u r r y and R. C. T h u n e l l for m a k i n g

a v a i l a b l e u n p u b l i s h e d da ta . W e a p p r e c i a t e

f r e q u e n t h e l p f u l d i s c u s s i o n s w i t h W. B. C u r r y ,

D. M c C o r k l e a n d N. S l o w e y , w i t h L. L a b e y r i e

a n d J . C. D u p l e s s y d u r i n g S e p t e m b e r 1986 a t

W o o d s H o l e a n d a r e v i e w o f t h e m a n u s c r i p t by

D. M c C o r k l e a n d L. L a b e y r i e . T h i s r e s e a r c h

w a s f u n d e d by N S F g r a n t s OCE84-10789,

OCE87-19012 a n d OCE84-11141. T h i s is W o o d s

H o l e O c e a n o g r a p h i c I n s t i t u t i o n C o n t r i b u t i o n

No. 6803.

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