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Dissolved aluminum in the Weddell Sea
Deep-Sea Research,Vol. 39. No, 3/4, pp. 537-547, 1992. Printed in Great Britain.
0198~149/92 $5,00 + 0.00 © 1992PergamonPressplc
D i s s o l v e d a l u m i n u m in the W e d d e l l Sea S. B. M O R A N , * t R. M. MOORE* and S.
WESTERLUND~
Received 7 January 1991 : in revised form 10 June 1991 ; accepted 1(/June 1991)
Abstract--Vertical profiles of dissolved aluminum (AI) at five stations in the Weddell Sea are presented. Concentrations of dissolved AI in the Weddell Sea range from 1 to 5 riM, considerably lower than AI levels in the Atlantic, typically in the 10-40 nM range, and similar to levels of 0.066 nM reported for the Pacific. The slight m a x i m u m in dissolved AI (4-5 nM) in the surface waters is suggested to be derived primarily from the partial dissolution of atmospheric dust. M i n i m u m concentrations of dissolved AI ( t - t .6 nM) are observed at intermediate depths (1500-3000 m). The slight increase in dissolved AI of about 2 nM with proximity to the sediment-water interface implies that sediments are not a significant source of dissolved AI to the deep waters. The combination of the low atmospheric input and the seasonally high rate of primary productivity that characterize the Southern Ocean can account for the low concentrations of dissolved A1 in the Weddell Sea. The results reported in this study suggest that the low A1 levels in the deep North Pacific can be explained by advection of Iow-Al source waters originating in the Weddell Sea.
INTRODUCTION
OPEN ocean vertical profiles of dissolved aluminum (A1), reported primarily for northern temperate and equatorial waters, are typically characterized by a surface maximum, a middepth minimum and increasing concentrations toward the sea floor. Several recent studies conducted in the Atlantic and Pacific Oceans suggest that the primary source of dissolved AI is from the partial dissolution of atmospheric dust in surface waters (HYDES, 1979, 1983; MOORE and MILLWARD,1984; MEASURES et al., 1984, 1986; ORIANS and BRULAND, 1985, 1986, 1988; MEASURES and EDMOND, 1990). Low dissolved A1 levels (0.5-3 nM) in the intermediate and deep waters of the Pacific relative to the Atlantic have been explained by the progressive scavenging of dissolved AI from the water column with an increase in the age of these waters (ORIANSand BRULAND,1986). Elevated concentrations of dissolved A1 (ca 20-30 nM) in the deep relative to the intermediate waters of the western North Atlantic have been attributed to the southward advection of Al-enriched North Atlantic Deep Water (MEASURES et al., 1986), which evidently propagates to about 30°S with no significant net input of A1 during transit (MEASURESand EDMOND, 1990). The slight increase in dissolved A1 of about 1-3 nM with proximity to the sediment-water interface in
*Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1. q-Present address: D e p a r t m e n t of Chemistry, W o o d s Hole Oceanographic Institution, Woods Hole, M A 02543, U.S.A. ~Department of Analytical and Marine Chemistry, Chalmers University of Technology and University of Goteborg, Goteborg, Sweden S-412 96. 537
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s.B. MORANet al.
the Pacific implies only a minor input of A1 from deep-sea sediments (ORIANSand BRULAND, 1985, 1986, 1988). The northward propagation of Antarctic Bottom Water (AABW), formed primarily in the Weddell Sea (e.g. GILL, 1973; FOSTERand CARMACK,1976; WEISS et al., 1979), has a major influence on many geophysical and geochemical properties of the deep ocean. The Weddell Sea therefore represents an important end-member component in the evolution of the deep ocean distribution of dissolved A1. In addition, the low atmospheric dust load (PROSPERO,1981) and the seasonally high rate of biological productivity (e.g. EPPLEYand PETERSON,1979) in the Southern Ocean contrast markedly with northern temperate and equatorial waters, from which the majority of oceanic data on dissolved A1 have been obtained. Thus, data on dissolved A1 from the Weddell Sea may also be useful in refining our present understanding of the major factors controlling the distribution of A1 in the global ocean. This paper presents preliminary data on the distribution of dissolved AI in the Weddell Sea; they represent the first reported measurements of dissolved AI in Antarctic waters. SAMPLING AND METHODS Seawater samples were collected for analysis of dissolved AI at five stations in the Weddell Sea on board the Stena A r c t i c a , January 1989 (Fig. 1). Samples were collected using clean 12-1 Go-Flo bottles that were coated internally with Teflon, had Teflon
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Fig. 1. Map of the WeddellSea showingsamplingStas 1-5.
Dissolved aluminum in the Weddell Sea
539
stopcocks and were mounted on a Kevlar hydrowire to reduce potential sources of Al contamination. Samples used for dissolved A1 analyses were stored frozen in 125 ml acidcleaned polyethylene bottles in the dark. Samples were packed in dry ice, shipped to Dalhousie University, where they were kept in a cold room (-20°C) and analysed for dissolved A1 within 3-4 months from the date of sampling using the fluorimetric method (HYDESand LISS,1976) modified for small sample volumes (MOOREand MILLWARD, 1984). The blank was determined to be 1 nM and was due almost entirely to the natural fluorescence of the seawater samples. The precision between repeat analyses was typically - + _ 0 . 5 n M and the minimum detectable Al concentration was 1-1.5 nM. Dissolved nutrients were determined at sea using standard colorimetric methods. Salinity was determined using an automated salinometer and potential temperature measured with a calibrated CTD. RESULTS AND DISCUSSION
Hydrography To facilitate the interpretation of the dissolved A1 results, hydrographic data for Stas 1-5 are shown in vertical profiles of salinity and potential temperature (0, Fig. 2a,b) and summarized in the plot of potential temperature against salinity (Fig. 3). Winter Water (WW) is observed in the surface layer (ca 100 m) and is characterized by the lowest potential temperature ( - 1 . 7 to -1.3°C) and salinity (34.2-34.4 psu). At a depth of about 500 miles lies the Warm Deep Water (WDW), with a potential temperature of about 0.5°C and a salinity of about 34.68 psu. W D W is advected into the Weddell Sea from the north via the Antarctic Circumpolar Current (ACC) and is a principal component from which all other water masses in the Weddell Sea are formed (GLEE, 1973). A A B W lies within the depth range of 3000-4000 m and is defined by a potential temperature of about - 0 . 4 ° C and a salinity of about 34.66 psu (Figs 2 and 3; WEISS etal., 1979). In addition, the Weddell Sea Bottom Water (WSBW) is shown (Fig. 3) with a potential temperature of - 0 . 9 ° C and a salinity of 34.65 psu, approximately intermediate between the W D W and the Western Shelf Water (WSW; WEISS et al., 1979). These hydrographic data are consistent with previous studies conducted in this region and can be explained in terms of conservative mixtures of WW, W D W and WSW (0 - - 2 ° C , S - 34.65 psu), which form the corners of a triangle in salinity and potential temperature (Fig. 3; WEISS et al., 1979).
Vertical distribution of dissolved AI Distinct differences are observed in the concentration of dissolved A1 in the WW, characterized by maximum A1 levels (4.5 nM), and the WDW, which has lower AI levels of about 3 nM (Fig. 4). These AI concentrations are similar to levels of 3-4 nM in Arctic surface waters (MOORE, 1981), a region also characterized by low atmospheric dust input. There is no indication of an Al-enrichment to about 9 nM in the shelf waters as suggested by MEASURES and EDMOND (1990), based on conservative extrapolation of AI measurements from the western South Atlantic. However, it is not possible to determine whether dissolved A1 levels are elevated in the WSBW or in the more dense WSW (from which WSBW is partly formed), as these water masses were evidently not sampled (Fig. 3, Table 1). Dissolved AI levels decrease to minimum values of 1-1.6 nM at intermediate depths (2000-3000 m) and are associated with water that has a salinity of about 34.66 psu, similar
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to A A B W but with a potential temperature ( - - 0 . 1 ° C ) intermediate between W D W and A A B W . These low A1 concentrations should be regarded with some caution, as they are virtually indistinguishable from the blank (1 nM). The precision between duplicate analyses, however, is about _+0.5 nM, indicating that the observed A1 minimum at 20003000 m is a genuine feature. The concentration of dissolved A1 in the A A B W (30004000 m) is about 2 nM (Fig. 4), which is only about 3.5 nM lower than the value predicted by MEASURES and EDMOND (1990). The concentration of dissolved AI in the WSW can be estimated roughly using A1 concentrations in the component water masses of A A B W , which is derived from a mixture (cf. WEISS et al., 1979) of about 64% W D W (3 nM A1), 26% WSW (A1 not measured) and 12% WW (4-5 nM AI). Using the concentration of dissolved A1 in the A A B W of 2 nM reported here, and assuming conservative behavior for A1, we estimate A1 levels of <1 nM in the WSW. This estimate is necessarily approximate because, with the small range in dissolved A1 concentration in the W D W and WW, the possible error in the AI concentration of WSW is increased. There is a slight increase in dissolved A1 concentration of about i nM near the sediment-water interface at Stas 3 and 5 (Fig. 4). Fluxes o f dissolved AI in the Weddell Sea
One of the most striking aspects of this study is the low concentration of dissolved A1 (1-5 nM) in the Weddell Sea. Two factors are likely to account for these low dissolved AI
542
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levels: the low atmospheric dust input and the seasonally high rate of primary productivity that characterize the Southern Ocean. Based on previous oceanographic distributions of dissolved AI (e.g. HYDES, 1979; MEASURES et al., 1984, 1986; ORIANS and BRULAND, 1986), the small dissolved A1 maximum in the surface waters of the Weddell Sea may be from the partial dissolution of atmospherically transported continental dust. Although few data on dust fluxes exist for this region, PROSPERO (1981) estimates dust deposition to the South Atlantic between 5°S and 35°S is about 85 × 10 -6 g cm -2 y-a. If this flux of dust is extended to include the Weddell Sea, then a rough estimate can be made of the input of dissolved A1 from atmospheric dust to these surface ocean waters. Assuming continental dust contains 8.23% A1 (TAYLOR, 1964) and that 5% of A1 in atmospheric dust dissolves in surface seawater (MARING and DUCE, 1987), the atmospheric flux of dissolved A1 to the Weddell Sea is 13 nmol A1 cm-2 y - 1. This is probably an upper limit because the flux of atmospheric particulates is likely to be lower in the Antarctic than in the tropical and temperate South Atlantic (PROSPERO, 1981). In addition, as recently noted in the case of Fe (MARTIN et al., 1990), the input of atmospheric dust to Antarctic surface waters, which are covered seasonally with sea ice, would be expected to occur as a pulse during the summer melt. The residence time of dissolved A1 in these surface waters can be estimated roughly using the calculated atmospheric A1 flux and the average concentration of dissolved AI within the surface 50 m of 4.4 nM (Table 1); this corresponds to a surface water residence time of dissolved A1 of about 2 years. If the average concentration of dissolved A1 in the
543
Dissolved aluminum in the Weddell Sea
Table 1.
Depth (m)
Hydrographic, nutrient and dissolved AI data from the Weddell Sea, January 1989 Pot. temp. (°C)
Salinity (psu)
Si (I,M)
PO 4 (pM)
A1 (nM)
Station 1, 73°55'S, 39°39'W, depth 1227 m 50 -1.29 34.237 67.2 100 1.55 34.316 69.7 200 - 1.45 34.413 78.8 400 1/.38 34.565 94.6 600 -1/.38 34.646 107.3 800 -0.39 34.645 105.1 1000 -0.68 34.619 92.0
2.1 2.2 2.4 2.4 2.5 2.4 2.4
4.2, 4.7 3.5 2.9, 3.2 3.4 2.2 2.6 2.6
Station 2, 73°18'S, 39°59'W, depth 2201 m 511 - 1.71 34.302 70.3 100 -1.82 34.375 71.3 200 1.74 34.402 72.6 300 - 1.26 34.451 79.6 400 -0.40 34.532 86.4 6111/ 0.61 34.666 107.7 800 11.63 34.685 114.4
2.3 2.3 2.3 2.4 2.4 2.5 2.5
4.4, 3.8 3.9,4.3 2.8, 2.3 3.1 2.9 2.2
Station 3, 72°18'S, 39°59'W, depth 3147 m 1000 0.44 34.692 115.5 151/0 1t.211 34.681 124.1 2000 0.02 34.675 126.4 2300 -1t.115 34.673 126.2 2600 -0.09 34.665 135.4 2900 -0.15 34.664 125.8 3101/ -1/.43 34.640 1//3.6
2.5 2.6 2.5 2.5 2.6 2.6 2.5
3.2 2.1 1.1 1.1, 1.5 1.1, 1.0 1.9
Station 4, 71°18'S, 38°13'W, depth 3870 m 50 - 1.74 34.313 82.5 100 - 1.73 34.441/ 81.8 150 - 1.34 34.482 85.4 200 -0.95 34.578 93.9 300 1/.32 34.651 1113.5 400 0.50 34.682 109.8 600 0.60 34.692 115.7
2.3 2.4 2.4 2.4 2.5 2.6 2.6
4.9, 4.7 3.0, 3.4 2.4, 2.6 2.9 3.1 3.3, 2.5 2.5, 3.1
Station 5, 69°39'S, 34°07'W, depth 4334 m 1000 11.37 34.677 120.0 15/)1/ 0.17 34.679 123.3 2000 -0.01 34.655 123.4 2500 -0.14 34.660 122.6 3000 -11.22 34.662 121.8 3500 -0.26 34.657 122.4 4100 -1/.29 34.650 123.5
2.5 2.6 2.5 2.5 2.5 2.6 2.5
3.1 2.7, 1.9 1.6, 1.1 1.4, 1.1 1.4 1.8 2.2, 1.7
s u r f a c e 100 m o f 3.4 n M is u s e d , t h e n t h e r e s i d e n c e t i m e o f d i s s o l v e d A1 in t h e s u r f a c e w a t e r s is a b o u t 3 y e a r s . T h i s 2 - 3 y e a r r a n g e is c o m p a r a b l e w i t h p r e v i o u s e s t i m a t e s o f r e s i d e n c e t i m e o f 2 - 6 y e a r s in t h e s u r f a c e w a t e r s o f t h e c e n t r a l P a c i f i c (ORIANS a n d BRULAND, 1986; MARING a n d DUCE, 1987).
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S.B. MORANet al.
Release of A1 from melting glacial icebergs represents another potential source of dissolved AI to Antarctic waters. The mean concentration of AI in glacial ice from the Holocene ranges from 0.093 nmol A1/g ice in the Vostok ice core (DE ANGELISet al., 1987) to 0.1 nmol A1/g ice in the D o m e C ice core from East Antarctica (PETIT et al., 1981). The flux of dissolved A1 from melting glacial ice can be estimated assuming 0.09 nmol Al/g glacial ice, a conservative value of 5% for the dissolution of AI from glacial ice (based on AI dissolution from atmospheric particulates in seawater; MARING and DUCE, 1987), and that 2 × 10 is g of glacial icebergs calve and melt per year (U.S. DEPARTMENTOF ENERGY, 1985). With these values, the flux of AI from melting glacial ice to the surface waters is calculated to be about 9 x 106 mol A1 y 1. Assuming that glacial icebergs melt within a 1.5 X 1013 m 2 area in the Antarctic (CooKE and HAYES, 1982), the corresponding flux of A1 per unit area of Antarctic surface waters from the melting glacial ice is 0.06 nmol A1 cm 2 y 1 less than 1% of the estimated atmospheric flux of A1 to the Antarctic. It is therefore concluded that the melting of glacial icebergs may not be a significant source of dissolved AI to the Southern Ocean. The calculated flux of dissolved A1 to Antarctic waters from melting glacial ice does not include the potential contribution of A1 from the partial dissolution of rock flour produced by the glacial erosion of Antarctica. In this case, it is interesting to consider that both field (EDMOND, 1973; EDMOND et al., 1979) and laboratory (HERD, 1977) studies have shown that the partial dissolution of Antarctic sediments as glacial rock flour is not a significant source of dissolved Si. However, as noted by MEASURES et al. (1986), because the concentration of dissolved A1 is much lower than dissolved Si in oceanic waters, partial dissolution of glacial rock flour could significantly increase the concentration of dissolved A1 without causing a resolvable increase in dissolved Si (assuming congruent dissolution). This suggestion is in line with recent results from sediment resuspension experiments (MORAN and MOORE, 1991). In these experiments, it was shown that the addition of low concentrations of resuspended sediments (ca 0.1-10 mg 1-1) to seawater, which had an initial dissolved AI concentration of 11 nM, caused dissolved A1 to increase by 4-19 nM, while dissolved Si levels remained constant (4 +_ 0.4/~M). Based on these experimental results, it is conceivable that there may be a significant flux of A1 from glacial rock flour to Antarctic waters. However, the low dissolved AI levels reported in this study suggest that the flux of A1 from the partial dissolution of glacial rock flour apparently does not substantially increase dissolved A1 levels at the stations occupied in the Weddell Sea (Fig. 4). However, it is also important to consider that, because the calving of icebergs is a highly spasmodic process, the flux of dissolved AI from glacial rock flour is likely to vary both temporally and regionally. More comprehensive data on the distribution of AI in Antarctic waters, particularly in the shelf waters where glaciers calve, are required to quantify the potential significance of glacial rock flour as a source of dissolved A1. The seasonally high rate of primary productivity in the Southern Ocean is likely to be an additional factor leading to the low concentration of dissolved AI observed in the Weddell Sea. EPPEEY and PETERSON (1979) report that primary production in Antarctic waters is of the order of 325 g C m 2 y- 1, compared with 102 g C m 2 y- ~ in the Atlantic and 55 g C m 2 y - 1 in the Pacific. In addition, these authors estimate new production at 146 g C m 2 y - i in the Antarctic, 26 g C m -2 y - l in the Atlantic and 8 g C m -2 y ~ in the Pacific. The associated high concentration of biogenic particles and flux of sinking particulate matter can result in enhanced scavenging of dissolved A1 from the water column. For example, several studies have reported low concentrations of dissolved A1 in biologically productive
Dissolved aluminumin the WeddellSea
545
coastal waters relative to open ocean surface waters (MEASURESet al., 1984, 1986; ORIANS and BRULAND,1986; MORAN,1990). Recent studies have clearly shown that dissolved A1 is scavenged from seawater during phytoplankton blooms (MORAN and MOORE, 1988a,b). The removal of dissolved AI is considered to be mainly passive, the A1 being associated primarily with the surface of biogenic particles (MORAN, 1990); furthermore, phytoplankton are not known to have any biochemical requirement for A1. A study of the kinetics of the removal of dissolved A1 from seawater by dead diatom particles (MORAN, 1990, submitted) provides evidence in support of a surface interaction between dissolved A1 and biogenic particles. These experimental results are consistent with the particlereactive hydrolysis chemistry of A1 in seawater and its short oceanic residence time (ORIANSand BRULAND,1985, 1986, 1988; MORAN,1990). An estimate can be made of the flux of A1 associated with sinking biogenic particulate matter from the surface waters of the Weddell Sea. This calculation assumes: (1) 0.4~tmol A1/g dry Antarctic plankton (COLLIERand EDMOND, 1984); (2) an average rate of primary productivity of 325 g C m-2 y 1(EPPLEYand PETERSON,1979); (3) 10% of the dry weight of phytoplankton is carbon (SMIxHand KALBER,1974); and (4) 45% of the total production of particulate organic carbon in Antarctic waters is exported from the surface to the deep waters (EPPLEYand PETERSON,1979). This corresponds to a flux of particulate A1 from the surface waters of the Weddell Sea of about 2 ~tmol A1 m 2 day- l (60 nmol A1 cm 2 y- 1), a factor of 5 greater than the estimated atmospheric flux of AI to the Weddell Sea. While the results of these calculations are average values, they imply that the combination of low atmospheric input and high biological productivity can account for the low concentration of dissolved A1 observed in the Weddell Sea. In contrast to distributions of dissolved A1 in the western North Atlantic (HYDES, 1979; MEASURESet al., 1986) and in the Arctic (MOORE,1981), which show a marked increase in A1 concentration from the intermediate to the deep waters, AI levels increase only slightly in the deep waters of the Weddell Sea (Fig. 4). The distribution of dissolved A1 in the deep waters of the Weddell Sea resembles AI profiles reported for the deep Pacific, which show an increase in dissolved A1 of about 1-3 nM with proximity to the sediment-water interface (ORIANSand BRULAND,1986). The slight increase in dissolved A1 concentration observed in the deep waters of the Weddell Sea may be due to a flux of A1 from the sediments, as suggested for the Pacific (ORIANSand BRULAND, 1986). A quantitative assessment of the flux of dissolved A1 from these sediments to the bottom waters will require measurements of AI in the porewaters of sediments in the Weddell Sea.
CONCLUSIONS Preliminary data on dissolved AI in the Weddell Sea further substantiate the conclusions of recent studies conducted in the Atlantic and Pacific that the oceanographic distribution of dissolved A1 is controlled by complex input and removal processes. Perhaps the most significant conclusion to be made from the Weddell Sea data is that they point to the importance of advection in controlling the deep ocean distribution of dissolved A1. Specifically, the low dissolved A1 levels in the deep waters of the North Pacific (1-3 nM; ORIANS and BRULAND,1986) relative to the North Atlantic (10-30 nM; MEASURESet al., 1986) can be explained, at least in part, by the northward advection of low-Al source waters originating primarily in the Weddell Sea.
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Acknowledgements--We thank P. A. Yeats, B. P. Boudreau, B. D. Johnson and L. Mayer for constructive criticism on an earlier draft, and C. I. Measures for useful c o m m e n t s on the final manuscript. M. Ohlson generously provided nutrient data. Reviews by K. J. Orians and J. D. Milliman improved the paper. Financial support for this work was provided by a Dalhousie University Fellowship to SBM and the National Sciences and Engineering Research Council of Canada.
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ORIANS K. J. and K. W. BROLAND (1988) The marine geochemistry of dissolved gallium: a comparison with dissolved aluminum. Geochimica et Cosmochimica Acta, 52, 2955-2962. PETIT J.-R., M. BRIAT and A. ROYER (1981) Ice age aerosol content from East Antarctic ice core samples and past wind strength. Nature, 293,391-394. PROSPERO J. (1981) Eolian transport to the world ocean. In: The sea, C. EMILIANI, editor, Wiley. New York, pp. 801-876. SMtTH F. G. W. and F. A. KALBER, editors (1974) Handbook o f marine science. Vol. 2, C R C Press, Boca Raton, 377 pp. TAYt.OR S. R. (1964) A b u n d a n c e of chemical elements in the continental crust: a new table. Geochimica et Cosmochimica Acta, 28, 1273-1285. U.S. DEPARTMENTOF ENERGY (1985) Glaciers, ice sheets, and sea level: effect o f a CO 2 change. Report from a workshop held in Seattle, Washington, 13-15 September, 1984. Contract No. DE-FG01-84ER60235. WEISS R. F., H. G. OSTLUND and H. CRAIG (1979) Geochemical studies of the Weddell Sea. Deep-Sea Research, 26, 1093-1120.
0198~149/92 $5,00 + 0.00 © 1992PergamonPressplc
D i s s o l v e d a l u m i n u m in the W e d d e l l Sea S. B. M O R A N , * t R. M. MOORE* and S.
WESTERLUND~
Received 7 January 1991 : in revised form 10 June 1991 ; accepted 1(/June 1991)
Abstract--Vertical profiles of dissolved aluminum (AI) at five stations in the Weddell Sea are presented. Concentrations of dissolved AI in the Weddell Sea range from 1 to 5 riM, considerably lower than AI levels in the Atlantic, typically in the 10-40 nM range, and similar to levels of 0.066 nM reported for the Pacific. The slight m a x i m u m in dissolved AI (4-5 nM) in the surface waters is suggested to be derived primarily from the partial dissolution of atmospheric dust. M i n i m u m concentrations of dissolved AI ( t - t .6 nM) are observed at intermediate depths (1500-3000 m). The slight increase in dissolved AI of about 2 nM with proximity to the sediment-water interface implies that sediments are not a significant source of dissolved AI to the deep waters. The combination of the low atmospheric input and the seasonally high rate of primary productivity that characterize the Southern Ocean can account for the low concentrations of dissolved A1 in the Weddell Sea. The results reported in this study suggest that the low A1 levels in the deep North Pacific can be explained by advection of Iow-Al source waters originating in the Weddell Sea.
INTRODUCTION
OPEN ocean vertical profiles of dissolved aluminum (A1), reported primarily for northern temperate and equatorial waters, are typically characterized by a surface maximum, a middepth minimum and increasing concentrations toward the sea floor. Several recent studies conducted in the Atlantic and Pacific Oceans suggest that the primary source of dissolved AI is from the partial dissolution of atmospheric dust in surface waters (HYDES, 1979, 1983; MOORE and MILLWARD,1984; MEASURES et al., 1984, 1986; ORIANS and BRULAND, 1985, 1986, 1988; MEASURES and EDMOND, 1990). Low dissolved A1 levels (0.5-3 nM) in the intermediate and deep waters of the Pacific relative to the Atlantic have been explained by the progressive scavenging of dissolved AI from the water column with an increase in the age of these waters (ORIANSand BRULAND,1986). Elevated concentrations of dissolved A1 (ca 20-30 nM) in the deep relative to the intermediate waters of the western North Atlantic have been attributed to the southward advection of Al-enriched North Atlantic Deep Water (MEASURES et al., 1986), which evidently propagates to about 30°S with no significant net input of A1 during transit (MEASURESand EDMOND, 1990). The slight increase in dissolved A1 of about 1-3 nM with proximity to the sediment-water interface in
*Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1. q-Present address: D e p a r t m e n t of Chemistry, W o o d s Hole Oceanographic Institution, Woods Hole, M A 02543, U.S.A. ~Department of Analytical and Marine Chemistry, Chalmers University of Technology and University of Goteborg, Goteborg, Sweden S-412 96. 537
538
s.B. MORANet al.
the Pacific implies only a minor input of A1 from deep-sea sediments (ORIANSand BRULAND, 1985, 1986, 1988). The northward propagation of Antarctic Bottom Water (AABW), formed primarily in the Weddell Sea (e.g. GILL, 1973; FOSTERand CARMACK,1976; WEISS et al., 1979), has a major influence on many geophysical and geochemical properties of the deep ocean. The Weddell Sea therefore represents an important end-member component in the evolution of the deep ocean distribution of dissolved A1. In addition, the low atmospheric dust load (PROSPERO,1981) and the seasonally high rate of biological productivity (e.g. EPPLEYand PETERSON,1979) in the Southern Ocean contrast markedly with northern temperate and equatorial waters, from which the majority of oceanic data on dissolved A1 have been obtained. Thus, data on dissolved A1 from the Weddell Sea may also be useful in refining our present understanding of the major factors controlling the distribution of A1 in the global ocean. This paper presents preliminary data on the distribution of dissolved AI in the Weddell Sea; they represent the first reported measurements of dissolved AI in Antarctic waters. SAMPLING AND METHODS Seawater samples were collected for analysis of dissolved AI at five stations in the Weddell Sea on board the Stena A r c t i c a , January 1989 (Fig. 1). Samples were collected using clean 12-1 Go-Flo bottles that were coated internally with Teflon, had Teflon
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Fig. 1. Map of the WeddellSea showingsamplingStas 1-5.
Dissolved aluminum in the Weddell Sea
539
stopcocks and were mounted on a Kevlar hydrowire to reduce potential sources of Al contamination. Samples used for dissolved A1 analyses were stored frozen in 125 ml acidcleaned polyethylene bottles in the dark. Samples were packed in dry ice, shipped to Dalhousie University, where they were kept in a cold room (-20°C) and analysed for dissolved A1 within 3-4 months from the date of sampling using the fluorimetric method (HYDESand LISS,1976) modified for small sample volumes (MOOREand MILLWARD, 1984). The blank was determined to be 1 nM and was due almost entirely to the natural fluorescence of the seawater samples. The precision between repeat analyses was typically - + _ 0 . 5 n M and the minimum detectable Al concentration was 1-1.5 nM. Dissolved nutrients were determined at sea using standard colorimetric methods. Salinity was determined using an automated salinometer and potential temperature measured with a calibrated CTD. RESULTS AND DISCUSSION
Hydrography To facilitate the interpretation of the dissolved A1 results, hydrographic data for Stas 1-5 are shown in vertical profiles of salinity and potential temperature (0, Fig. 2a,b) and summarized in the plot of potential temperature against salinity (Fig. 3). Winter Water (WW) is observed in the surface layer (ca 100 m) and is characterized by the lowest potential temperature ( - 1 . 7 to -1.3°C) and salinity (34.2-34.4 psu). At a depth of about 500 miles lies the Warm Deep Water (WDW), with a potential temperature of about 0.5°C and a salinity of about 34.68 psu. W D W is advected into the Weddell Sea from the north via the Antarctic Circumpolar Current (ACC) and is a principal component from which all other water masses in the Weddell Sea are formed (GLEE, 1973). A A B W lies within the depth range of 3000-4000 m and is defined by a potential temperature of about - 0 . 4 ° C and a salinity of about 34.66 psu (Figs 2 and 3; WEISS etal., 1979). In addition, the Weddell Sea Bottom Water (WSBW) is shown (Fig. 3) with a potential temperature of - 0 . 9 ° C and a salinity of 34.65 psu, approximately intermediate between the W D W and the Western Shelf Water (WSW; WEISS et al., 1979). These hydrographic data are consistent with previous studies conducted in this region and can be explained in terms of conservative mixtures of WW, W D W and WSW (0 - - 2 ° C , S - 34.65 psu), which form the corners of a triangle in salinity and potential temperature (Fig. 3; WEISS et al., 1979).
Vertical distribution of dissolved AI Distinct differences are observed in the concentration of dissolved A1 in the WW, characterized by maximum A1 levels (4.5 nM), and the WDW, which has lower AI levels of about 3 nM (Fig. 4). These AI concentrations are similar to levels of 3-4 nM in Arctic surface waters (MOORE, 1981), a region also characterized by low atmospheric dust input. There is no indication of an Al-enrichment to about 9 nM in the shelf waters as suggested by MEASURES and EDMOND (1990), based on conservative extrapolation of AI measurements from the western South Atlantic. However, it is not possible to determine whether dissolved A1 levels are elevated in the WSBW or in the more dense WSW (from which WSBW is partly formed), as these water masses were evidently not sampled (Fig. 3, Table 1). Dissolved AI levels decrease to minimum values of 1-1.6 nM at intermediate depths (2000-3000 m) and are associated with water that has a salinity of about 34.66 psu, similar
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to A A B W but with a potential temperature ( - - 0 . 1 ° C ) intermediate between W D W and A A B W . These low A1 concentrations should be regarded with some caution, as they are virtually indistinguishable from the blank (1 nM). The precision between duplicate analyses, however, is about _+0.5 nM, indicating that the observed A1 minimum at 20003000 m is a genuine feature. The concentration of dissolved A1 in the A A B W (30004000 m) is about 2 nM (Fig. 4), which is only about 3.5 nM lower than the value predicted by MEASURES and EDMOND (1990). The concentration of dissolved AI in the WSW can be estimated roughly using A1 concentrations in the component water masses of A A B W , which is derived from a mixture (cf. WEISS et al., 1979) of about 64% W D W (3 nM A1), 26% WSW (A1 not measured) and 12% WW (4-5 nM AI). Using the concentration of dissolved A1 in the A A B W of 2 nM reported here, and assuming conservative behavior for A1, we estimate A1 levels of <1 nM in the WSW. This estimate is necessarily approximate because, with the small range in dissolved A1 concentration in the W D W and WW, the possible error in the AI concentration of WSW is increased. There is a slight increase in dissolved A1 concentration of about i nM near the sediment-water interface at Stas 3 and 5 (Fig. 4). Fluxes o f dissolved AI in the Weddell Sea
One of the most striking aspects of this study is the low concentration of dissolved A1 (1-5 nM) in the Weddell Sea. Two factors are likely to account for these low dissolved AI
542
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levels: the low atmospheric dust input and the seasonally high rate of primary productivity that characterize the Southern Ocean. Based on previous oceanographic distributions of dissolved AI (e.g. HYDES, 1979; MEASURES et al., 1984, 1986; ORIANS and BRULAND, 1986), the small dissolved A1 maximum in the surface waters of the Weddell Sea may be from the partial dissolution of atmospherically transported continental dust. Although few data on dust fluxes exist for this region, PROSPERO (1981) estimates dust deposition to the South Atlantic between 5°S and 35°S is about 85 × 10 -6 g cm -2 y-a. If this flux of dust is extended to include the Weddell Sea, then a rough estimate can be made of the input of dissolved A1 from atmospheric dust to these surface ocean waters. Assuming continental dust contains 8.23% A1 (TAYLOR, 1964) and that 5% of A1 in atmospheric dust dissolves in surface seawater (MARING and DUCE, 1987), the atmospheric flux of dissolved A1 to the Weddell Sea is 13 nmol A1 cm-2 y - 1. This is probably an upper limit because the flux of atmospheric particulates is likely to be lower in the Antarctic than in the tropical and temperate South Atlantic (PROSPERO, 1981). In addition, as recently noted in the case of Fe (MARTIN et al., 1990), the input of atmospheric dust to Antarctic surface waters, which are covered seasonally with sea ice, would be expected to occur as a pulse during the summer melt. The residence time of dissolved A1 in these surface waters can be estimated roughly using the calculated atmospheric A1 flux and the average concentration of dissolved AI within the surface 50 m of 4.4 nM (Table 1); this corresponds to a surface water residence time of dissolved A1 of about 2 years. If the average concentration of dissolved A1 in the
543
Dissolved aluminum in the Weddell Sea
Table 1.
Depth (m)
Hydrographic, nutrient and dissolved AI data from the Weddell Sea, January 1989 Pot. temp. (°C)
Salinity (psu)
Si (I,M)
PO 4 (pM)
A1 (nM)
Station 1, 73°55'S, 39°39'W, depth 1227 m 50 -1.29 34.237 67.2 100 1.55 34.316 69.7 200 - 1.45 34.413 78.8 400 1/.38 34.565 94.6 600 -1/.38 34.646 107.3 800 -0.39 34.645 105.1 1000 -0.68 34.619 92.0
2.1 2.2 2.4 2.4 2.5 2.4 2.4
4.2, 4.7 3.5 2.9, 3.2 3.4 2.2 2.6 2.6
Station 2, 73°18'S, 39°59'W, depth 2201 m 511 - 1.71 34.302 70.3 100 -1.82 34.375 71.3 200 1.74 34.402 72.6 300 - 1.26 34.451 79.6 400 -0.40 34.532 86.4 6111/ 0.61 34.666 107.7 800 11.63 34.685 114.4
2.3 2.3 2.3 2.4 2.4 2.5 2.5
4.4, 3.8 3.9,4.3 2.8, 2.3 3.1 2.9 2.2
Station 3, 72°18'S, 39°59'W, depth 3147 m 1000 0.44 34.692 115.5 151/0 1t.211 34.681 124.1 2000 0.02 34.675 126.4 2300 -1t.115 34.673 126.2 2600 -0.09 34.665 135.4 2900 -0.15 34.664 125.8 3101/ -1/.43 34.640 1//3.6
2.5 2.6 2.5 2.5 2.6 2.6 2.5
3.2 2.1 1.1 1.1, 1.5 1.1, 1.0 1.9
Station 4, 71°18'S, 38°13'W, depth 3870 m 50 - 1.74 34.313 82.5 100 - 1.73 34.441/ 81.8 150 - 1.34 34.482 85.4 200 -0.95 34.578 93.9 300 1/.32 34.651 1113.5 400 0.50 34.682 109.8 600 0.60 34.692 115.7
2.3 2.4 2.4 2.4 2.5 2.6 2.6
4.9, 4.7 3.0, 3.4 2.4, 2.6 2.9 3.1 3.3, 2.5 2.5, 3.1
Station 5, 69°39'S, 34°07'W, depth 4334 m 1000 11.37 34.677 120.0 15/)1/ 0.17 34.679 123.3 2000 -0.01 34.655 123.4 2500 -0.14 34.660 122.6 3000 -11.22 34.662 121.8 3500 -0.26 34.657 122.4 4100 -1/.29 34.650 123.5
2.5 2.6 2.5 2.5 2.5 2.6 2.5
3.1 2.7, 1.9 1.6, 1.1 1.4, 1.1 1.4 1.8 2.2, 1.7
s u r f a c e 100 m o f 3.4 n M is u s e d , t h e n t h e r e s i d e n c e t i m e o f d i s s o l v e d A1 in t h e s u r f a c e w a t e r s is a b o u t 3 y e a r s . T h i s 2 - 3 y e a r r a n g e is c o m p a r a b l e w i t h p r e v i o u s e s t i m a t e s o f r e s i d e n c e t i m e o f 2 - 6 y e a r s in t h e s u r f a c e w a t e r s o f t h e c e n t r a l P a c i f i c (ORIANS a n d BRULAND, 1986; MARING a n d DUCE, 1987).
544
S.B. MORANet al.
Release of A1 from melting glacial icebergs represents another potential source of dissolved AI to Antarctic waters. The mean concentration of AI in glacial ice from the Holocene ranges from 0.093 nmol A1/g ice in the Vostok ice core (DE ANGELISet al., 1987) to 0.1 nmol A1/g ice in the D o m e C ice core from East Antarctica (PETIT et al., 1981). The flux of dissolved A1 from melting glacial ice can be estimated assuming 0.09 nmol Al/g glacial ice, a conservative value of 5% for the dissolution of AI from glacial ice (based on AI dissolution from atmospheric particulates in seawater; MARING and DUCE, 1987), and that 2 × 10 is g of glacial icebergs calve and melt per year (U.S. DEPARTMENTOF ENERGY, 1985). With these values, the flux of AI from melting glacial ice to the surface waters is calculated to be about 9 x 106 mol A1 y 1. Assuming that glacial icebergs melt within a 1.5 X 1013 m 2 area in the Antarctic (CooKE and HAYES, 1982), the corresponding flux of A1 per unit area of Antarctic surface waters from the melting glacial ice is 0.06 nmol A1 cm 2 y 1 less than 1% of the estimated atmospheric flux of A1 to the Antarctic. It is therefore concluded that the melting of glacial icebergs may not be a significant source of dissolved AI to the Southern Ocean. The calculated flux of dissolved A1 to Antarctic waters from melting glacial ice does not include the potential contribution of A1 from the partial dissolution of rock flour produced by the glacial erosion of Antarctica. In this case, it is interesting to consider that both field (EDMOND, 1973; EDMOND et al., 1979) and laboratory (HERD, 1977) studies have shown that the partial dissolution of Antarctic sediments as glacial rock flour is not a significant source of dissolved Si. However, as noted by MEASURES et al. (1986), because the concentration of dissolved A1 is much lower than dissolved Si in oceanic waters, partial dissolution of glacial rock flour could significantly increase the concentration of dissolved A1 without causing a resolvable increase in dissolved Si (assuming congruent dissolution). This suggestion is in line with recent results from sediment resuspension experiments (MORAN and MOORE, 1991). In these experiments, it was shown that the addition of low concentrations of resuspended sediments (ca 0.1-10 mg 1-1) to seawater, which had an initial dissolved AI concentration of 11 nM, caused dissolved A1 to increase by 4-19 nM, while dissolved Si levels remained constant (4 +_ 0.4/~M). Based on these experimental results, it is conceivable that there may be a significant flux of A1 from glacial rock flour to Antarctic waters. However, the low dissolved AI levels reported in this study suggest that the flux of A1 from the partial dissolution of glacial rock flour apparently does not substantially increase dissolved A1 levels at the stations occupied in the Weddell Sea (Fig. 4). However, it is also important to consider that, because the calving of icebergs is a highly spasmodic process, the flux of dissolved AI from glacial rock flour is likely to vary both temporally and regionally. More comprehensive data on the distribution of AI in Antarctic waters, particularly in the shelf waters where glaciers calve, are required to quantify the potential significance of glacial rock flour as a source of dissolved A1. The seasonally high rate of primary productivity in the Southern Ocean is likely to be an additional factor leading to the low concentration of dissolved AI observed in the Weddell Sea. EPPEEY and PETERSON (1979) report that primary production in Antarctic waters is of the order of 325 g C m 2 y- 1, compared with 102 g C m 2 y- ~ in the Atlantic and 55 g C m 2 y - 1 in the Pacific. In addition, these authors estimate new production at 146 g C m 2 y - i in the Antarctic, 26 g C m -2 y - l in the Atlantic and 8 g C m -2 y ~ in the Pacific. The associated high concentration of biogenic particles and flux of sinking particulate matter can result in enhanced scavenging of dissolved A1 from the water column. For example, several studies have reported low concentrations of dissolved A1 in biologically productive
Dissolved aluminumin the WeddellSea
545
coastal waters relative to open ocean surface waters (MEASURESet al., 1984, 1986; ORIANS and BRULAND,1986; MORAN,1990). Recent studies have clearly shown that dissolved A1 is scavenged from seawater during phytoplankton blooms (MORAN and MOORE, 1988a,b). The removal of dissolved AI is considered to be mainly passive, the A1 being associated primarily with the surface of biogenic particles (MORAN, 1990); furthermore, phytoplankton are not known to have any biochemical requirement for A1. A study of the kinetics of the removal of dissolved A1 from seawater by dead diatom particles (MORAN, 1990, submitted) provides evidence in support of a surface interaction between dissolved A1 and biogenic particles. These experimental results are consistent with the particlereactive hydrolysis chemistry of A1 in seawater and its short oceanic residence time (ORIANSand BRULAND,1985, 1986, 1988; MORAN,1990). An estimate can be made of the flux of A1 associated with sinking biogenic particulate matter from the surface waters of the Weddell Sea. This calculation assumes: (1) 0.4~tmol A1/g dry Antarctic plankton (COLLIERand EDMOND, 1984); (2) an average rate of primary productivity of 325 g C m-2 y 1(EPPLEYand PETERSON,1979); (3) 10% of the dry weight of phytoplankton is carbon (SMIxHand KALBER,1974); and (4) 45% of the total production of particulate organic carbon in Antarctic waters is exported from the surface to the deep waters (EPPLEYand PETERSON,1979). This corresponds to a flux of particulate A1 from the surface waters of the Weddell Sea of about 2 ~tmol A1 m 2 day- l (60 nmol A1 cm 2 y- 1), a factor of 5 greater than the estimated atmospheric flux of AI to the Weddell Sea. While the results of these calculations are average values, they imply that the combination of low atmospheric input and high biological productivity can account for the low concentration of dissolved A1 observed in the Weddell Sea. In contrast to distributions of dissolved A1 in the western North Atlantic (HYDES, 1979; MEASURESet al., 1986) and in the Arctic (MOORE,1981), which show a marked increase in A1 concentration from the intermediate to the deep waters, AI levels increase only slightly in the deep waters of the Weddell Sea (Fig. 4). The distribution of dissolved A1 in the deep waters of the Weddell Sea resembles AI profiles reported for the deep Pacific, which show an increase in dissolved A1 of about 1-3 nM with proximity to the sediment-water interface (ORIANSand BRULAND,1986). The slight increase in dissolved A1 concentration observed in the deep waters of the Weddell Sea may be due to a flux of A1 from the sediments, as suggested for the Pacific (ORIANSand BRULAND, 1986). A quantitative assessment of the flux of dissolved A1 from these sediments to the bottom waters will require measurements of AI in the porewaters of sediments in the Weddell Sea.
CONCLUSIONS Preliminary data on dissolved AI in the Weddell Sea further substantiate the conclusions of recent studies conducted in the Atlantic and Pacific that the oceanographic distribution of dissolved A1 is controlled by complex input and removal processes. Perhaps the most significant conclusion to be made from the Weddell Sea data is that they point to the importance of advection in controlling the deep ocean distribution of dissolved A1. Specifically, the low dissolved A1 levels in the deep waters of the North Pacific (1-3 nM; ORIANS and BRULAND,1986) relative to the North Atlantic (10-30 nM; MEASURESet al., 1986) can be explained, at least in part, by the northward advection of low-Al source waters originating primarily in the Weddell Sea.
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s . B . MORAN et al.
Acknowledgements--We thank P. A. Yeats, B. P. Boudreau, B. D. Johnson and L. Mayer for constructive criticism on an earlier draft, and C. I. Measures for useful c o m m e n t s on the final manuscript. M. Ohlson generously provided nutrient data. Reviews by K. J. Orians and J. D. Milliman improved the paper. Financial support for this work was provided by a Dalhousie University Fellowship to SBM and the National Sciences and Engineering Research Council of Canada.
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