Ca-carbon isotope relationship in foraminifera of the Greenland-Iceland-Norwegian Seas

Marine Geology 140 (1997) 61-73

Ground truthing the Cd/Ca-carbon isotope relationship in foraminifera of the Greenland-Iceland-Norwegian Seas K. McIntyre a,b3*,A.C. Ravelo b, M.L. Delaney b, L.D. Anderson b, T. Johannessen ’ a Earth Sciences Department, University of California, Santa Cruz, CA 95064, USA b Ocean Sciences Department and Institute of Marine Sciences, University of California, Santa Crux, CA 95064, USA ’ Geophysical Department, Allegt. 70, University of Bergen, Bergen, N-5007, Norway Received 9 May 1996; received in revised form 18 December 1996; accepted 19 December 1996

Abstract In order to examine whether the paleoceanographic nutrient proxies, 613C and cadmium/calcium in foraminiferal calcite, are well coupled to nutrients in the region of North Atlantic Deep Water formation, we present data from two transects of the Greenland-Iceland-Norwegian Seas. Along Transect A (74.3”N, 18.3cE to 75.O’N, l2..5”W, 15 stations), we measured phosphate and Cd concentrations of modern surface sea water. Along Transect B (64.5”N, 0.7”W to 70.4”N, 18.2”W, 14 stations) we measured Cd/Ca ratios and F13C of the planktonic foraminifera Neogloboquadrina pachyderma sinistral in core top sediments. Our results indicate that Cd and phosphate both vary with surface water mass and are well correlated along Transect A. Our planktonic foraminiferal 613C data indicate similar nutrient variation with water mass along Transect B. Our Cd/Ca data hint at the same type of nutrient variability, but interpretations are hampered by low values close to the detection limit of this technique and therefore relatively large error bars. We also measured Cd and phosphate concentrations in water depth profiles at three sites along Transect A and the S13C of the benthic foraminifera Cibicidoides wuellerstor$ along Transect B. Modern sea water depth profiles along Transect A have nutrient depletions at the surface and then constant values at depths greater than 100 meters. The 613C of planktonic and benthic foraminifera from Transect B plotted versus depth also reflect surface nutrient depletion and deep nutrient enrichment as seen at Transect A, with a small difference between intermediate and deep waters. Overall we see no evidence for decoupling of Cd/Ca ratio and 613C in foraminiferal calcite from water column nutrient concentrations along these transects in a region of North Atlantic Deep Water formation.

0 1997 Elsevier

Science B.V.

Keywords: C-l 3/C-12; cadmium;

Neoglohoquadrina pachyderma;

Norwegian

Sea; Greenland

Sea; Iceland Plateau;

seawater

1. Introduction Paleoceanographers use nutrient proxies, such as cadmium/calcium (Cd/Ca) ratios and the carbon isotope signatures (S13C) of foraminiferal calcite, * Corresponding author.

0025-3227/97/$17.000 1997Elsevier Science B.V. All rights reserved. PII SOO25-3227(97)00004-2

to reconstruct

surface and deep water nutrient variability over geologic time. These proxies are often used to investigate gradients in deep water

nutrient concentrations as a measure of deep water circulation patterns. For example the 613C of benthic foraminifera is used to interpret the varying intensity of North Atlantic Deep Water (NADW)

62

K. McIntyre et al. / Marine Geology 140 (1997) 61-73

formation over the last 3.0 m.y. (e.g., Oppo and Fairbanks, 1990; Raymo et al., 1990; Mix et al., 1995). Any change in the relationships between Cd/Ca ratio or 6i3C in foraminiferal calcite and phosphate concentrations in the water column could affect interpretations of past ocean circulation which rely on our knowledge that nutrient proxies are calibrated to modern nutrients with sufficient confidence to reconstruct past nutrient variability. This link motivated us to examine the modern and recent evidence about nutrient proxies in a region of North Atlantic deep water formation, the Greenland-Iceland-Norwegian (GIN) Seas. Deep and intermediate water formation occurs in the central gyres of the Greenland and Iceland Seas resulting from the mixing of warm, saline North Atlantic Surface water (NASW) with cold, fresh Polar Surface Water (PSW) (Fig. la) and the subsequent super cooling of these waters during the winter season (Hopkins, 1991). The intermediate waters then spill over the shallow ridges that separate the GIN Seas from the North Atlantic, ultimately mixing with other deep water masses in the North Atlantic to form North Atlantic Deep Water (NADW). The low nutrient concentrations imparted from GIN Seas surface waters make NADW recognizable in nutrient maps of the modem ocean, and nutrient proxies are the primary way in which its changing presence can be recognized in the paleoceanographic record. In this study, we measured surface water phosphate and Cd concentrations along Transect A (Fig. lb) and we measured planktonic foraminiferal 613C and Cd/Ca in coretops along Transect B (Fig. 1b) of the GIN Seas. Our measurements along each transect reflect surface water chemistry in each of the major surface water masses of the modern GIN Seas. Along Transect A, phosphate measurements were made to characterize the nutrient distribution and Cd measurements were made to characterize the distribution of this paleoceanographically important trace metal. Along Transect B coretop Cd/Ca ratios and Fi3C of planktonic foraminiferal calcite provide the paleoceanographic record of recent surface water nutrient distribution. We use these data sets in two ways to examine nutrient-proxy coupling. First, we use

80

65

60

-20

0

20

Longitude (degrees)

(a)

80

-20

0 I

Transect

20

A

85

60

(b)

65

-20

0

20

60

Longitude(degrees)

Fig. 1. Regional circulation of the GIN Seas and location of sites in this study. (a) Surface circulation of the GreenlandIceland-Norwegian Seas (After Johannessen et al., 1994). The North Atlantic Surface Water (NASH’) is advected from the North Atlantic and flows northward and westward. The Polar Surface Water (PSW) flows southward and eastward from the Arctic Ocean. The Arctic Surface Water (ASW) is located between the NASW and PSW and forms by the mixing of these two water masses. The location where the NASW and ASW meet is the Arctic Front (AF). The location where the PSW and ASW meet is the Polar Front (PF, not shown). (b) The location of surface water (open circles) and deep water (f&d squares) samples along Transect A, box cores (open diamonds) along Transect B, and sea water Sr3C depth profiles (filled squares).

K. Mcintyreetal./Marine Geology140 (1997)61-73

the measurement of multiple proxies at the same location to examine their coupling to each other. Thus, we assess the Cd-phosphate relationship along Transect A and we assess the relationship of foraminiferal Cd/Ca ratios and 613C in recent sediments along Transect B. Second, we can compare Cd and phosphate distributions along Transect A to Cd/Ca ratios and 613C distributions along Transect B because these east to west transects encounter the same surface water masses and frontal systems. This comparison allows us to assess the extent to which modern nutrient distributions in this region are reflected by the paleoceanographic proxies in the most recent sediments. To extend these nutrient-proxy comparisons from surface to depth, we measured seawater phosphate and Cd concentrations with depth at three sites, 655, 664 and 668, along Transect A (Fig. 1b) and we measured benthic foraminiferal 613C along Transect B. We use the water column profiles from Transect A to see if the coupling of Cd to phosphate observed at the surface continues to depth. We reconstruct the surface to deep water nutrient profile at Transect B using the coretop benthic and planktonic foraminiferal 613C values. Finally, we compare this coretop-based 613C depth profile with the sea water nutrient depth profiles along Transect A, as well as with nearby modern seawater 613C water column data, in order to assess the extent to which foraminiferal F13C accurately records modern nutrient variability.

2. Samples and methods We collected water samples for Cd and phosphate analyses along Transect A in August of 1993 using a Plexiglas surface sampler with hot acid leached (1 N HCl at 60°C for a minimum of 1 week) HDPE Nalgene bottles attached. Samples were “fished” off the bow of the ship as it slowed down approaching station, or just as it stopped, to avoid contamination from the ship. We collected water samples from vertical profiles at three stations using Niskin bottles as supplied commerically which had been cleaned with 3 N HCl at room temperature for a minimum of 24 hours and took aliquots into hot acid leached HDPE Nalgene

63

bottles. We filtered a subset of our samples shipboard using a 0.45 uM Calyx filter in an enclosed acid-leached filtration system to assess whether there was any particulate metal contribution to total Cd. Seawater samples were acidified to pH < 1 with Optima HCI typically within 15 min of collection. At U.C. Santa Cruz (UCSC) we pre-concentrated Cd from sea water samples using an ammonium l-pyrrolidinedithiocarbamate-diethyldithiocarbamate ( APDC-DDC) extraction into chloroform (Bruland et al., 1979; Bruland, 1980). We analyzed Cd by standard additions on the concentrated solutions using standard operating conditions for Cd on a Perkin Elmer model 5000 Atomic Absorption Spectrophotometer equipped with a model 500 Heated Graphite Analyzer and an AS40 autosampler (GFAAS). Cd reagent blanks averaged 1.6 f 1.5 pM (1s standard deviation), for a detection limit (3 x the standard deviation of the reagent blanks) of 4.6 pM; sample concentrations were at least 4 times and up to 40 times the detection limit. Cd measured in a reference material treated as a sample, Continenental Atlantic Standard Seawater (CASS), averaged 218 + 26 pM compared to the certified value of 267 f46. We do not have any internal addition experiments on individual samples to confirm the observed recovery efficiency (82%). We have not made any recovery efficiency corrections to measured concentrations. No substantial difference was noted between results from filtered and unfiltered seawater samples. In two cases a seawater Cd analysis was rejected because the concentration in the filtered sample exceeded one standard deviation of replicates of the unfiltered sample. Phosphate concentrations were determined in samples and standard solutions by colorometric techniques using standard operating conditions on a Shimadzu UV-2 10 1PC spectrophotometer (Strickland and Parsons, 1972) in the UCSC, Institute of Marine Sciences Marine Analytical Labs. The detection limit was 0.03 PM; sample concentrations were at least 4 times and up to 36 times the detection limit. Error bars on all analyses we present are one standard deviation (Is) on two or more replicate samples. Box core samples along Transect B were col-

64

K. McIntyre et al. j Marine Geology 140 (1997) 61-73

lected as described in Johannessen et al. (1994), using the top 0.5 cm of the core for these core top analyses. Based on an average bioturbation depth of 4 cm, the ages of these core tops are thought to vary between roughly 400 yr in the eastern part of the transect where sedimentation rate is high to 2000 yr in open ocean sediments (Johannessen et al., 1994). Isotope measurements were made at the University of Bergen on grouped Neogloboquadrina pachyderma sinistral using a Finnegan MAT 25 1 mass spectrometer. Errors ( 1s) on replicate internal lab standards were 0.06%0 for 613C and 0.07%0 for 6180. We measured 613C and #*O on grouped Cibicidoides wuellerstorfi at UCSC using a VG Prism mass spectrometer. Errors ( 1s) on replicate internal lab standards were 0.05%0 for 613C and 0.08%0 for #*O. Results are reported in %O relative to VPDB and calibrated via NBS-19. We cleaned planktonic foraminifera for Cd/Ca analyses at UCSC using established sample preparation methods including recent modifications (Boyle, 1981; Boyle and Keigwin, 1985/1986; E.A. Boyle, pers. commun., 1993; Oppo and Rosenthal, 1994) as applied in our lab (Delaney, 1990). Cleaned foraminiferal calcite was then dissolved in 0.075 N HNO,. We analyzed Cd and manganese (a check for diagenetic overgrowths) concentrations in these solutions using standard operating conditions for these elements on a GFAAS. We analyzed Ca dilutions, with lanthanum as an ionization suppressant, on a Perkin Elmer model 2380 flame atomic absorption spectrophotometer (FAAS) with a Gilson model 231 autosampler. For Cd, Mn, and Ca, standards were made so that standard and sample matrices matched. All Mn/Ca ratios were < 100 pmol,/mol, reflecting no post depositional overgrowths, and therefore are not reported. Errors (1s) on replicate determinations over multiple analytical runs of Cd/Ca in solutions with compositions similar to foraminiferal calcite range from 0.004-0.010 umol/ mol, or 5-8% relative standard error. Replicate measurements over multiple analytical runs of a planktonic foraminiferal sample, V30-40 68 cm Neogloboquadrina dutertrei, averaged 0.017 f 0.007 (n= 10). Error bars on all analyses we present are one standard deviation ( 1s) on two or more replicate samples.

Temperature and salinity data used to designate surface water mass along Transect A were measured by CTD at each site. For the depth profiles along Transect A, CTD measurements were made at the same depths as the Niskin bottle samples were taken. The temperature and salinity data for Transect B were chosen from the Norwegian Oceanographic Data Centre and Levitus Data Base to match the depth and season in which N. pachyderma sinistral most likely calcified (Johannessen et al., in prep). Use of a more general set of summer temperature and salinities in determining surface water masses along Transect B would not substantially change the location of frontal systems. Little published data exist on the seasonal variation of surface water nutrients and foraminiferal fluxes in this region. Previous studies (e.g. Danielsson and Westerlund, 1983) have predicted surface nutrients should vary seasonally because of springtime inputs of freshwater from melting sea ice, spring-summer increases in productivity, and winter season homogenization of the upper water column. Seasonal data on foraminiferal fluxes (Bathmann et al., 1990) and organic matter fluxes (Bodungen et al., 1995) indicate that the maximum fluxes to the sediments occur in the summer (June and July) and continue at heightened levels into fall.

3. Results and discussion 3.1. Surface water masses along Transect A and Transect B in the GIN Seas

The surface circulation of the GIN Seas is characterized by advection of surface waters from the North Atlantic and Arctic oceans and the mixing of these end members (Fig. 1a). In the east the warm, high salinity North Atlantic Surface Water (NASW) flows northward along the Norwegian Coast and westward. In the west, the cold, low salinity Polar Surface Waters (PSW) flow southward along the Greenland Coast, and eastward. In the center of the GIN Seas the Arctic Surface Water (ASW) is formed by mixing of the NASW and the PSW, and has intermediate temperatures and salinities. The interfaces between

K. McIntyre

et al. / Marine

these three water masses are denoted by the Arctic Front (AF) between the NASW and ASW, and the Polar Front (PF) between the ASW and PSW. The southwest-northeast trends of these water masses and frontal systems are cross cut by the two transects, A and B, examined in this study. Thus samples along each transect encountered the same water masses, despite the latitudinal difference between the transects. In order to reconstruct the distribution of surface water masses, we examined the temperature and salinity along our transects. Temperature and salinity from Transect A (Fig. 2a, Table 1) and

a

ASW

PSWPF

NASW

AF

NCW

36.0 35.0 34.0 ; E: 33.0 q 2s 32.0

Geology 140 (1997)

61-73

65

Transect B (Fig. 4a, Table 2) generally decrease from east to west. Our data indicate that all three water masses are encountered in both of these transects. The Arctic and Polar fronts appear as small shifts in the slope of these trends at 336”E and - 1OS”W in Transect A (Fig. 2a) and 67”N 7” E and 69.5”N 14.5”W in Transect B (Fig. 4a), respectively. At Transect A the phosphate and Cd data (Fig. 2b, c) both indicate the transition between ASW and PSW more clearly than the temperature and salinity data and the location of the PF is reconstructed to accommodate this geochemical data. The location of the AF and PF assigned here are consistent with the locations of these fronts as described in Pohl et al. (1993) for Transect A and with the locations described by Johannessen et al. ( 1994) for Transect B. The association of temperature and salinity with specific water masses is also similar to that described in Hopkins (1991). 3.2. Phosphate variation along Transect A

31.0

b

-15 W

-10

-5

0

5

10

Longitude(')

15

20 E

Fig. 2. Surface water sample composition versus site longitude for Transect A. (a) Temperature (dots) and salinity (circles). (b) Phosphate (filtered and unfiltered samples shown), mean* 1s standard deviation. (c) Cd (filtered and unfiltered samples shown), mean* Is standard deviation. Water mass abbreviations as given in Fig. 1 caption. Shaded areas indicate frontal systems, AF is defined by surface water temperature and salinity, PF is defined by surface water temperature and salinity and by geochemical gradient.

Nutrient distribution, as characterized by dissolved phosphate concentration, correlates with water mass along Transect A. Phosphate concentrations range from low to high in the NASW, are low in the ASW, and are highest at the one PSW site (Fig. 2b, Table 1). The Arctic and Polar Fronts are characterized by phosphate concentrations intermediate between those of the adjacent water masses. NASW samples have an average phosphate concentration of 0.27 &-0.10 uM, with concentrations grading from 0.14 uM in the East to 0.36 uM toward the Arctic front. The ASW is characterized by the most invariant and lowest phosphate concentrations with an average of 0.16 + 0.02 PM. The phosphate concentration in the PSW, indicated by one site, is generally the highest in the entire transect at 0.38 PM. Primary productivity is the process modifying the phosphate concentration of the ASW. The temperature and salinity data along Transect A indicate that the ASW is intermediate between the NASW and PSW, presumably because the ASW is made of a mixture of these two water masses. In contrast to the temperature and salinity data, phosphate concentrations are lowest in the ASW,

K. McIntyre et al. / Marine Geology 140 (1997) 61-73

66

Table 1 Cadmium and phosphate data for surface waters along Transect A Longitude (“E)b

Phosphate MM)

Cadmium (PM)

74.30 74.30

18.30 16.10

74.30 74.30 74.30 74.30 74.30

15.00 14.01 12.31 11.01 7.01

75.00 75.00 75.00 75.00

5.00 3.01 - 3.00

75.00 75.01 75.00

-7.00 - 9.00 - 11.oo

75.02

- 12.51

0.15*0.01 0.14*0.04 0.09_+0.02 0.27 +0.02 0.27 +0.03 0.40 +0.02 0.28 + 0.01 0.36 0.35 kO.05 0.27 0.24+0.03 0.20 0.13 0.17+0.04 0.17 0.15_+0.02 0.29 +0.07 0.31+0.04 0.34kO.03 0.41 io.01

69.0 kO.8 52.1k24.8 60.5k9.5 64.7k3.0 75.7k21.9 103.2f21.2 73.511.8 61.656.8 83.6+ 1.2 47.5k9.0 28.2 k4.6 32.2i 10.8 27.1k8.5 29.0 25.5 + 1.6 20.5 & 3.6 41.4kO.4 63.2k1.8 146.0 162.2f13.2

Site

Sample treatmenta

Latitude (“N)

645 650

uf f uf uf uf uf uf f uf uf uf f f uf f f f uf f uf

652 653 654 655 657 659 660 661 664 666 667 668 669

1.oo

Salinity

Watermass*

1.92 9.22

33.91 34.79

NCW NASW

8.56 8.41 7.25 8.35 8.07

34.97 34.97 35.06 35.00 35.04

NASW NASW NASW NASW NASW

6.15 5.19 4.83 4.38

34.84 34.73 34.64 34.58

AF AF ASW ASW

3.76 3.81 3.89

34.57 34.50 34.40

ASW ASW ASW

31.18

PSW

Temperature (“C)

-0.55

“uf indicates an unfiltered sample, f indicates a filtered sample. b(-) indicates “W. “Values represent two replicates, if no error is reported only one analysis was made. dThe water masses are described in Fig. la, except NCW = Norwegian Coastal Waters.

and therefore, could not be a product of mixing. Instead, as might be expected in a summer transect, biological activity must be lowering phosphate concentrations in the ASW relative to its surface water sources. 3.3. Cd concentrations in surface waters along Transect A

Cd concentrations vary with water mass and covary with phosphate concentrations along Transect A (Fig. 2c, Table 1). Cd concentrations are high in the NASW, with an average of 72 k 15 pM. The lowest Cd concentrations are in the ASW with an average of 27+4 pM. The highest concentrations are in the PSW at 154 pM. Thus trends in Cd with water mass are similar to those in phosphate, with high values in NASW and PSW and lower values in ASW. Within the confidence limits the slope of the Cd-phosphate relationship is identical to the slope reported by Pohl et al.

(1993) for a surface water transect along 75”N in June 1989, and, more significantly, to the global slope of Boyle (1988) of 0.2 nM/uM for waters with phosphate < 1.3 uM. Plotting our data along with the global Cd-phosphate data set (Fig. 3) better demonstrates the strong correlation of Cd and phosphate within the low ranges seen in the North Atlantic and Sub-arctic. As with phosphate, the low Cd concentrations of the ASW cannot be a product of mixing PSW and NASW end members, and likely reflect the removal of nutrients by biological productivity. In this case the ACd/APO, slope may be related to the ratio in which these two biologically mediated chemical species are being taken up by phytoplankton in this region. Overall there is clear coupling between Cd and phosphate, with no indication of substantial preferential removal or addition of Cd to surface waters relative to phosphate, and our data fall along the global Cd-phosphate relationship.

64.55 65.10 65.42 65.60 67.03 67.07 67.12 67.45 68.52 68.42 69.13 69.45 70.00 70.35

(“N)

Latitude

)

0.72 2.40 3.22 4.62 6.58 7.30 8.28 11.65 10.65 13.87 13.12 14.53 13.02 18.20

(“W

Longitude

2798 3182 2863 3905 2604 2093 1617 1662 2126 1633 1892 1458 1460 1632 0.10*0.18 0.26+0.21 0.32+0.19 0.53+0.09 0.5El+o.o5 0.54*0.11 0.86kO.10 0.79kO.16 0.92+0.07 0.88+0.12 0.95+0.04 l.OOkO.03 0.68kO.16 0.61 kO.04

(4) (9) (20) (3) (5) (3) (3) (3) (3) (2) (4) (6) (4) (2)

along

2.11+0.09 2.39kO.06 2.46kO.18 2.55 kO.08 2.75iO.14 2.85kO.13 3.28 k 0.06 3.37kO.12 3.37 f 0.09 3.60+0.15 3.55 kO.03 3.7OkO.24 3.48fO.19 3.40*0.00

sinistral 6180 fi”)b

B

(4)

0.016+0.003 0.029 0.023 k 0.005 0.013~0.004 0.027 + 0.008 0.038 &O.OOl

C. w_tellerstorJi

C. wueliersforf

3.89 3.88 f 0.04 3.85 f 0.00 3.84f0.01 3.84 f 0.07 3.77 3.79 kO.02

I .06 1.08kO.17 1.00~0.04 0.95 f 0.07 I .02 f 0.04 0.90 1.30 f 0.03

1.33

1.68 1.63 1.22

8.04 6.93 6.64 6.37 5.38 5.38 3.97 3.16 2.80 2.18

(“C)

Temperature

was made no error

3.84 k 0.02

1.24kO.01

If only one analysis

3.84 3.75 3.72

6’80 (%)b

1.10 1.25 1.20

8°C (X0)

samples.

(4) (3) (4) (2)

(2) (4) (4) (3) (3) (5)

0.013 & 0.000 0.026 + 0.010 0.019+0.003 0.015~0.003 0.016iO.001 0.014~0.007 0.018

sinistral Cd/Ca a (pmol/mol)

N. pachydmma

Transect

N. pachyderma

foraminifera

N. pachyderm sinistral 6’Y (X0)

and benthic

Site Water Depth (m)

and 613C for planktonic

“If a value represents more than two replicate analyses then (n) is the number of replicate “The number of replicate analyses of 6l*O is the same as that of 6% for that species. “The water masses are described in Fig. la. dNB=Norwegian Basin, IP=Iceland Plateau.

16132 16130 49- 14 52-41 57-13 57-12 57-11 57-09 57-04 71-12 57-05 57-06 71-17 23351

Site

Table 2 Cd/Ca ratios

is reported.

35.21 35.18 35.15 35.12 35.00 35.00 34.95 34.83 34.84 34.63 34.54 34.42 34.45 33.94

Salinity

NB NB NB NB NB NB IP IP IP IP IP

NASW NASW NASW NASW NASW NASW AF ASW ASW ASW ASW PFIP PFIP PSW

IP

GIN Sea Basind

Surface Water Mass”

K. McIntyre et al. J Marine Geology 140 (1997) 61-73

68

PSW

0

PF

A6w

AF

NASW 35.4

2 E

6

5

6

E

4

35.0 34.6

E z z

34.2

0

b 0

0.5

1

1.5 Phosphate

2

2.5

3

3.5

33.8

-0.4

4

(pmoi/lrg~

Fig. 3. Global cadmium versus phosphate in seawater. This study (dots) and global data (grey diamonds). Global data compiled by Boyle (1988) from a variety of sources; see Boyle ( 1988) for original references. Our data were converted from molar concentrations as measured to moles/kg by density.

3.4. 6i3C and CdjCa ratios ofplanktonic foraminifera N. pachyderma sinistral along Transect B The distribution of 613C of core top N. pachyderma sinistral is related to water mass distribution, with low values in the NASW and PSW, and highest values in the ASW (Fig. 4b, Table 2). Within the NASW, N. pachyderma sinistral 6r3C values range from a low of 0.10%0 to a high of 0.54%0 at the AF, with an average value of 0.45 + 0.25%” (Fig. 4b). N. pachyderma sinistral 613C values in the ASW are more consistent than those in NASW, with a mean of 0.88+0.06%0. Foraminifera from the one site representing the PSW have a value of 0.61%0. This distribution of 6i3C suggests a nutrient distribution that is consistent with the relationship between nutrients and water mass seen along Transect A (Fig. 2). Cd/Ca ratios of N. pachyderma sinistral at these sites are extremely low and difficult to measure using current analytical protocols. Cd/Ca ratios appear to have a similar pattern of variation compared to 6r3C in planktonic foraminifera along Transect B, however, the large error bars on Cd/Ca ratios make this difficult to verify (Fig. 4c, Table 2). T-tests comparing individual sites demonstrate that Cd/Ca ratios from sites in the ASW are not statistically different than Cd/Ca ratios from sites in the NASW at the 95% confidence interval.

Distance along Transect B

Fig. 4. Core top sample composition versus distance along Transect B. (a) Temperature (dots) and salinity (circles) of surface waters above sediments along Transect B. (b) 6r3C of planktonic foraminifera N. pachyderma sinistral from core tops, mean* 1s standard deviation. (c) Cd/Ca ratio of planktonic foraminifera N. pachyderma sinistral from core tops, mean k 1s standard deviation. Water mass abbreviations as given in Fig. 1 caption. Shaded areas indicate frontal systems defined by surface water temperature and salinity.

We can predict ranges in phosphate concentration along Transect B from the range of 613C values and the range of Cd/Ca ratios of the core top N. pachyderma sinistral samples along Transect B. Dividing the measured N. pachyderma sinistral 613C range of 0.9%0 by the global 6i3C-phosphate slope, A613C/AP0, = 1.1 (Broecker and MaierReimer, 1992), yields a predicted phosphate concentration range of 0.81 uM. In order to make the same prediction from the Cd/Ca data requires that we know the distribution coefficient (0) between the ratio in foraminiferal calcite and the seawater ratio (Cd/Ca,,,, = D*Cd/Ca,,,,,,,). Boyle (1992) demonstrated that benthic foraminiferal

K. Mclntyreetal./Marine Geology 140 (1997) 61-73

distribution coefficients vary with depth from 1.3-2.9 and that benthic distribution coefficients are not species specific. While the distribution coefficient is not known for N. pachyderma sinistral, a comparison of our planktonic foraminiferal data with published Antarctic planktonic foraminiferal data implies a distribution coefficient within a similar range as observed for benthic foraminifera. Assuming that D = 1, dividing the measured N. pachyderma sinistral Cd/Ca range, 0.025 ymol/ mol, by a global average calcium concentration, 10.3 mM, yields a range for Transect B of 257 pM Cd; this is divided by the Cd/phosphate slope of Boyle (1988) 0.2 nM/uM, to yield a predicted phosphate concentration range of 1.29 FM. Assuming D=2, the predicted phosphate concentration is 0.64 uM. If you assume the phosphate range predicted by the 613C of N. pachyderma sinistral of 0.81 uM is correct, and apply the global Cd/phosphate slope to estimate water Cd concentration, the calculated D is 1.6. In any of the above cases, both the 6r3C and Cd/Ca of N. pachyderma sinistral predict a phosphate range along Transect B higher than the observed phosphate concentration range, 0.32 PM, of modern surface waters along Transect A. In order to get a phosphate range of this magnitude along Transect B the 613C/phosphate slope would have to be unreasonably high (2.81%&M) and the distribution coefficient would have to be much higher than expected ( - 4.0). Differences in the location and timescale of the two transects could account for the difference between the range of phosphate concentrations predicted along Transect B and the range of phosphate concentrations observed at Transect A. Taking the differing phosphate ranges at face value, higher productivity could be causing greater nutrient depletion in the southerly Transect B. On the other hand these two transects reflect very different timescales, and surface water conditions could certainly have changed over time. For example, the addition of isotopically light carbon to the ocean and atmosphere from fossil fuels could have affected the isotopic composition of N. pachyderma sinistral in those sites with a higher sedimentation rate. Overall these two spatially and temporally disparate data sets yield a cohesive view of nutrient variation with surface water mass in the GIN Seas.

69

Both indicate nutrient concentration depletion in the ASW relative to the NASW and PSW. 3.5. Phosphate and Cd depth pro$les along Transect A

Water depth profiles at three sites along Transect A, 655, 664 and 668, indicate depleted phosphate and Cd at the surface and uniformly enriched concentrations below 100 m water depth (Fig. 5a, Table 3). Deep water phosphate concentrations are - 1 nM and the difference between surface and deep water phosphate is -0.8 uM at all three sites. Cd deep water concentrations range from 155-210 pM, and the difference between surface and deep water Cd concentrations ranges from 124-188 pM. In general, phosphate and Cd concentrations in deep waters are four times those for surface waters. Phosphate and Cd correlate well with a slope that is not significantly different from the 0.2 nM/uM slope reported by Boyle (1988) for intermediate and deep waters with phosphate < 1.3 PM. Like our surface water data the depth profile data plot on the global cadmium-phosphate relationship (Fig. 3). A simple profile of surface nutrient depletion, a shallow regeneration zone, and then constant values to depth exists at all three sites for both phosphate and Cd. The surface phosphate and Cd depletion is the result of nutrient export to deep waters via biological productivity during the summer. Overall, the profiles at these sites are similar to previously published Cd and phosphate profiles for this region in both in both structure and absolute values (Danielsson and Westerlund, 1983). From a paleoceanographic perspective this data indicate that Cd and phosphate are not decoupled with depth (along Transect A) by any process in the water column, either in their uptake by phytoplankton or in their regeneration from decaying organic matter. Thus changes in Cd concentrations independent of phosphate concentrations can be ruled out in the modern GIN Seas region. 3.6. S13C of benthic foraminifera C. wuellerstorfl along Transect B

A reconstructed depth profile of foraminiferal 613C versus the depth at which the foraminiferal

K. McIntyre et al. / Marine Geology I40 (1997) 61-73

70

a

b

c 250

‘1

0 0

0

0.2 0.4 0.6 0.8 Phosphate ( pM )

1

1.2 0

50

100

0.2

0.4 0.6 0.8 Phosphate (PM)

1

1.2

150 200 250

Cadmium (PM)

Fig. 5. Results from water samples from vertical profiles from three stations along Transect A. (a) Phosphate, meanf 1s standard deviation, for sites 668, 664 and 655 (see Fig. lb for station locations). (b) Cd, mean+ 1s standard deviation, for sites 668, 664, and 655 (see Fig. lb for station locations). (c) Cd versus phosphate for 668, 664 and 655. Both filtered and unfiltered phosphate and Cd data are shown.

shell calcified (Fig. 6a) is similar to that of the phosphate and Cd depth profiles at Transect A (Fig. 5). Near the surface planktonic foraminiferal 613C is high, indicative of low nutrient concentrations, and is variable reflecting our grouping of planktonic values from across the GIN Seas. At depth benthic foraminifera C. wuellerstorfi 613C values are relatively constant, averaging 1.10 + 0.13%0 (Table 2). There is a small but statistically significant difference (t-test at the 95% C.I., comparing individual sites in each basin) of -0.2%0 between 613C in C. wuellerstor- from the deep waters of the Norwegian Basin (> 2600 m depth) and 613C in C. wuelferstorfi from the intermediate waters of the Iceland Plateau (~2600 m depth). Sea water dissolved inorganic carbon 613C profiles from GEOSECS Station 19 (Kroopnick, 1980a,b), Sites HM 94-6 and HM 79-23 (Table 3)

in the Norwegian Basin and from GEOSECS Station 15 (Kroopnick, 1980a,b) on the Iceland Plateau (Fig. 1b) show the same basic structure as the C. wuellerstorj 613C profile, high and variable surface values and low and relatively constant values at depth (Fig. 6b). The difference between deep waters and intermediate waters seen in the C. wuellerstor- 613C profile is not apparent in the modern seawater 613C data. This discrepancy must result from processes that affect benthic foraminifera independently of seawater, either due to biological effects or due to the longer timescales represented by sediments. Two kind of processes could be called upon to explain the difference between the C. wuellerstorfi 613C profile and the modern sea water 613C profiles, processes in which the microenvironment in which the foraminifera grew is isotopically different from the regional average value (e.g., the

K. McIntyre et al. / Marine Geology 140 (1997) 61-73

71

Table 3 Cadmium and phosphate data with depth for water samples from vertical profiles, Transect A stations Site

Sample treatmenta

Latitude (“N)

655

uf uf Llf uf uf

74.30

uf f f uf f uf uf f uf

74.30

f uf f uf f uf f uf f uf uf f uf uf

75.00

Longitude (“E)b

Depth (m)

Phosphate (PM)

11.01

0 20 50 1000 1500 2000

0.28+0.01 0.26iO.02 0.80 + 0.00 0.99 io.01 1.04_+0.03 I .07 + 0.04

73.5_+1.8 74.1 * 4.0 197.4i3.6 182.6+_21.4 187.5k27.3 192.9

- 3.00

0 20 300 1000 1500

0.17+0.04 0.15+0.01 0.83 k 0.05 0.91 io.04 0.98 + 0.07 0.92+0.01 0.97 io.01 0.93 f 0.02 0.97 + 0.04

29.0 22.2 i2.5 166.0 * 14.9 179.1+45.2 210.4k3.1 177.7k39.0 199.lL5.5

0.29 kO.07 0.31+0.04 0.66_fO.O6 0.52kO.03 0.82_+0.01 0.9OkO.02 0.85 k 0.06 0.89 kO.04 0.99 +0.04 1.04+_0.05 1.02+ 0.02 0.93 +0.02 1.05+0.01 1.03*0.01

41.4kO.4 63.2+ 1.8

Uf

664

668

2000 3000

- 11.00

10 30 50

100 1200 1500 1700

Cadmium (PM)

165.9-t 15.3

83.4k8.8 149.7+ 17.1 160.5k21.4 160.4i 15.1 157.2k6.4 155.7k24.6 154.8 +_19.5 171.8k12.4 138.4k23.6 172.Ok28.1 180.3k8.8

“uf indicates an unfiltered sample, f indicates a filtered sample. b(-) indicates “W. ‘All values represent two replicates, if no error is reported only one analysis was made.

“Mackensen” effect) and processes in which the longer, averaged timescale of coretops includes larger changes in environment conditions than the modern seawater. The “Mackensen” effect on C. wuellerstorf (Mackensen et al., 1993) would make mid-depth foraminifera lighter in 6r3C if there is a well developed benthic fluff layer at this depth, a concept that seems at odds with the very well oxygenated waters in these basins. The preferential mixing of glacial age benthic foraminifera into coretops on the Iceland Plateau would have the same effect but would also affect benthic foraminiferal VO (Table 2), which is essentially the same

across the transect (3.82 -L-0.05%/00) and close to modern values (Kroopnick, 1980a,b). If sedimentation rates on the Iceland plateau were high enough (relative to the Norwegian Basin) to include C. wuellerstor- that grew after the addition of isotopically light fossil fuel carbon to the oceanatmosphere system, then the mid-depth C. wuellerstorji would have a lighter carbon isotopic value. This would also affect the planktonic foraminiferal carbon isotopic values, dampening the effect of the greater productivity in this region. Although none of these ideas can be well explored using the data in this paper, the minima in the foraminiferal

K McIntyre et al. / Marine Geology 140 (1997) 61-73

12 a 0I

b

d 1 Cl

.

1 it+ B +f

.

500

4

Oa’ .

o

1000

O.

0

6 1500 s B 1; 2000

u*

0.

B

I

101

.

0

101

:

P Q 2500

‘0

0

0

00

3000 0

0 GEOSECS19 l GEOSEt-315 OHM944 . HM79-23

3500 I

2.2

I

I

1.8

I

I

I

1.4

a”c (4bo)

I

I

1

2.2

III

II

1.8

:

.

0

yield a better understanding of surface frontal movements over geologic time. Depth profiles of phosphate and Cd in modem waters indicate depletion of nutrients at the surface and then higher and constant values with depth. Depth profiles of foraminiferal 6i3C reconstruct a similar pattern but hint at a variation with depth not seen in 613C depth profiles of modem GIN Seas waters. Finally there does not appear to be any decoupling between nutrients and proxies in the modem GIN Seas. If decoupling is found in down core records of this region, they most likely stem from processes that occur only over geologic timescales and therefore must be researched and explained in that realm.

I I

1.4

1

Acknowledgements

a"ccm

Fig. 6. Planktonic and benthic foraminiferal 6r3C versus water depth along Transect B. (a) The 6r3C of benthic foraminifera C. wuellerstorfi (open diamonds) and the Fr3C of planktonic foraminifera N. pachyderma sinistral (filled diamonds) plotted versus the depth at which they grew (depth of the core tops for C. wuellerstorf, the surface for N. pachyderma sinistral). N. pachyderma sinistral data are shifted by l.OO%ato account for the biological offset between the 6r3C of pachyderma and the 8r3C of the dissolved inorganic carbon (Kahn and Williams, 1981). (b) The 613C of seawater at GEOSECS Station 15, 19, HM 79-23 and HM 94-6 (see Fig. lb for station locations).

613C depth profile is at the depth at which GIN Seas overflows feed into the North Atlantic (Hopkins, 1991) potentially making it oceanographically significant. A more negative preformed 613C value for NADW would reduce the Atlantic-Pacific 613C gradient thus changing interpretations of deep water aging.

4. Conclusions The surface water/coretop data presented in this study confirm a relationship between nutrient concentrations and water mass in modern GIN Seas that is robust and consistent over modem and recent geological timescales. While it may be difficult to use this relationship in paleoceanographic reconstructions that focus on one site in the GIN Seas, approaches that employ multiple sites may

We acknowledge support for this work from Calspace CS-34-94 (Ravelo, MC Intyre) and support from NSF OCE-9416593 (Delaney). This paper was greatly improved by revisions suggested by Y. Rosenthal, E.A. Boyle and M.A. Arthur. K. MC Intyre and A.C. Ravelo thank the University of Bergen, the CARDEEP program, and Eystein Jansen for enabling us to participate in the Johan Jhort cruise. We thank Laura Linn and Sergio Sanudo-Wilhelmy for technical assistance and Ken Bruland for scientific advice. Special thanks go to Rob Franks for maintaining the UCSC IMS Analytical Labs and for his assistance in making location maps.

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