Geochimica n Cosmochimica Acto Vol. 57, pp. 1419-1432 Copyright 0 1993 Pergamon Press Ltd. Printed in U.S.A.

CUI16-7037/93/$6.00 + Ml

Trace-metal distributions in seawater and anoxic brines in the eastern Mediterranean Sea

PAUL M. SAAGER,’ JOHAN SCHIJF,“* and HEIN J. W. DE BAAR~ ‘Free University, Dept. of Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands ‘University of Utrecht, Dept. of Earth Sciences, PO Box 80.021, 3508 TA, Utrecht, The Netherlands

‘Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands

(Received September 25, 199 1; accepted in revised form December 15, 1992)

Abstract-The vertical distribution of dissolved and particulate trace metals (Mn, Fe, Ni, Cu, Cd) is reported for seawater and the anoxic brines of Tyro and Bannock Basins in the eastern Mediterranean. In normal Mediterranean seawater, the distribution of dissolved trace metals is governed by water mass circulation, as well as by local input sources. For Cd and possibly Cu, steady-state behaviour is not a viable assumption, probably as a result of anthropogenic sources. In agreement with previous reports, no correlation was observed between dissolved Ni, Cu, Cd, and the major nutrients. At the seawater- brine interface, all trace metals show dramatic changes in concentration. In the brines, the concentrations of dissolved Fe and Mn are strongly elevated compared to overlying waters, yet probably limited by formation of solid sulfide and/or carbonate phases. The distributions of dissolved Fe and sulfide show a remarkable mirror image of those found previously in the Orca brine (Gulf of Mexico), and the ionic concentration products are approximately the same in all three basins. The difference in absolute Fe and sulfide concentrations is thought to be caused by a high supply of Fe to Orca Basin. Concentrations of dissolved Ni, Cu, and Cd in the brines are at least one order of magnitude lower than in overlying waters, most likely due to formation of solid sulfide phases. We believe these data provide the first indication of substantial nickel-sulfide (co)precipitation within an anoxic basin. Particulate metal concentrations are elevated at the seawater-brine interface and are probably associated with the presence of solid sulfide phases. In addition, the extreme density gradient at the interface will slow the sinking rate of particles and lead to accumulation of particles at the interface due to accumulation.

INTRODUCTION

PREVIOUS INVESTIGATIONS of dissolved trace metals in the Mediterranean Sea ( KREMLING and PETERSEN, 198 1; LAU- MOND et al., 1984; BOYLE et al., 1985; STATHAM et al., 1985; COPIN-MONTEGUT et al., 1986; SHERRELL and BOYLE, 1988; MORLEY and BURTON, 199 1) showed that, in contrast with open ocean waters, there generally exists only a modest cor- relation ( Alboran Sea) or no correlation at all (western central Mediterranean) between dissolved trace metals and nutrients. Concentrations of dissolved trace metals in nutrient-depleted Mediterranean surface waters are elevated compared to sim- ilarly nutrient-depleted Atlantic surface waters. Sluggish re- cycling of nutrients (BOYLE et al., 1985) and non-steady- state behaviour due to anthropogenic inputs ( BETHOUX et al., 1990) and other external sources (atmospheric, coastal; VAN GEEN et al., 1988) have been invoked to explain these observations. Since the formation areas of important water masses such as Levantine Intermediate Water reside in the eastern Mediterranean, this is an interesting area to study the distribution of trace metals. For the eastern Mediterranean thus far, only limited information was available about trace- metal distributions ( KREMLING and PETERSEN, 198 1; SPI- VACK et al., 1983; COPIN-MONTEGUT et al., 1986).

Recently, two anoxic, brine-filled depressions have been discovered in the eastern Mediterranean Sea (JONGSMA et al., 1983; SCIENTIFIC STAFF CRUISE BANNOCK 1984-12,

* Present address: Woods Hole Oceanographic Institution, Chemistry Department, Woods Hole, MA 02543, USA.

1985). Bannock Basin is located west of Crete and actually consists of a suite of basins, most of which are interconnected, Tyro Basin is located south of Crete (Fig. 1) . At both stations, an extremely sharp interface (Bannock Basin 3323 dbar, Tyro Basin 3380 dbar) separates oxygenated Mediterranean waters from anoxic brines with salinities an order of magnitude higher than normal seawater. Across this interface, many chemical and physical parameters decrease or increase dra- matically over less than 1 m (DE LANGE et al., 199Ob). The sharp density gradient effectively prevents downward mixing of oxygen, so that the decomposition of organic matter has led to permanently anoxic conditions in the brine. Sulfide concentrations in the brine are 2-3 mmol/L (LUTHER et al., 1990)) second in magnitude only to Framvaren Fjord, Nor- way ( SK& 1983). In Tyro Basin, the brine has a homogenous composition, but Bannock Basin is vertically stratified in two brine layers ( hereafter brine 1 and brine 2 ) that differ slightly in composition. The brines probably originated from disso- lution of Messinian evaporites underlying the Mediterranean seafloor. The brine composition in Tyro Basin reflects dis- solution of early stage evaporites, mainly consisting of halite; whereas in Bannock Basin, dissolution of later stage evaporites is likely to have occurred (DE LANGE et al., 199Oc). The combination of anoxia and extreme salinity in these brines may yield interesting distributions of dissolved trace metals. Thus far, only limited reliable observations exist concerning the distribution of dissolved trace metals in anoxic brines (Bannock and Tyro basins, Mn: DE BAAR et al., 1987; DE LANGE et al., 1990a,b,c. Bannock and Tyro basins, Ba: DE LANGE et al., 1990~: Al: HYDES et al., 1988. Orca Basin, Fe and Mn: TREFRY et al., 1984). Additional information about

1419

1420 P. M. Saager, J. Schijf, and H. J. W. De Baat

the origin and chemistry of the brines was publish previ- ously (VAN HINTE et al., I987; CITA et al., 1989; CITA, 1990; VAN DER WEIJDEN et al., 1990a).

The vertical distributions of dissolved trace metals have also been studied in various anoxic basins of approximately seawater salinity. In anoxic waters, dissolved Cu, Zn, and Cd reach very low concentrations as a result of sulfide precipi- tation (JACOBS et al., 1985, 1987; HARALDSSON and WES- TERLUND, 1988; LANDING and LEWIS, 1991). For Ni, no evidence of sulfide precipitation exists, and concentrations are mostly constant throughout the entire water column. Concen~ations of dissolved Fe and Mn increase dmmati~~ly at the oxic/anoxic interface, with Fe con~ntmtions decreas- ing downward below the interface as a result of sulfide pre- cipitation. Manganese concentrations remain high in anoxic waters or decrease gradually as a result of sulfide and /or car- bonate precipitation.

In this paper, the vertical distributions of dissolved and particulate Fe, Mn, Ni, Cu, and Cd are presented.

SAMPLING AND METHODS

Samples were collected on board RV Tyro at three stations in the eastern Mediterranean between 21 May and 5 June 1987 (Fig. I ). We used modified GoFIo-bottles ( BRULAND et al., 1979) mounted on a new stainless steel frame with Neil-Brown CTD-Rosette. Upon recovery, the bottles were immediately connect& to Nz-pressure fines

to prevent oxidation of the anoxic samples. Inside the clean air con- tainer, seawater was filtered through dean 0.4 pm Nuclepore filters using clean Teflon tubing and filtration units. The samples were acid- ified to pH 2-3 with quartz-distilled (Q-) HCI and stored in hot-acid leached polyethylene bottles until analysis in the shore laboratory. The filters were also analysed in the shore laboratory.

This sampling procedure is often used but does not necessarily collect 100% of the particulate matter. During filtration, large particles may settle to the bottom of the bottle and thus escape filtration. Since Iarge settling particles usually make up a very small percentage of the total particle concentration, the difference is likely to be smah though. Because of the small samp1e sizes, particle composition could not be determined and we only determined total particulate tmce- metal con~ntrations per voiume of water. VAN DER SLXKX et at. ( 1990) also sampled particles at these stations using a shipboard con- tinuous Bow centrifuge. In the Results Section, the differences between the two techniques wil1 be discussed. All analytical procedures were performed inside a class-100 laminar flow hood situated in a clean air container or laboratory. Nutrient and oxygen data were presented by HYDES et al. ( 1988) and will not be discussed here. Sulfide data are from LUTHER et al. ( 1990).

Manganese

Brine samples were diluted with Mini-Q water (Millipore) and analysed with direct injection graphite-furnace-AAS using standard addition (Perkin Elmer 5000 atomic absorption spectrophotometer; HGA 400 heated graphite furnace; AS 40 autosampler). The detection limit was 100 nmol/kg, and precision of the rn~~rnen~ was about 5% at 5 pmol/kg Mn. Seawater samp1es were pre~oncentra~d with Cheiex- 100 chromatography f KINGSTON et at., 1978 f and analysed

30

40

30

it-J ?il FIG. 1. Chart showing locations of the three stations sampled during the ABC cruise: Station 5 (35”09’N ZO”07’E).

Bannock Basin (34OI7’N 2O”Ol’E), and Tyro Basin f33?4’N 26”02’E).

Trace metals in seawater and anoxic brines 1421

with GFAAS using standard caiibration curves i6 tl, 1 N HNO: since a comparison with standard addition showed no matrix effects. The detection limit was 35 pmo!/kg, and precision was about 5% at 2 nmo!/kg. Blanks were below detection limit.

Iron, Nickel, Copper, and Cadmium

For both normal seawater and anoxic brines, samples were pre- concentrated with APT-DD~/chlorofo~ solvent extraction ( BRULAND et a!., 1979) and analysed with GFAAS (Perkin Elmer I 100 AAS) using extracted standard calibration curves to correct for matrix effects. Detection limits were 2 pmo!/kg for Cd, 0.2 nmo!/ kg for Ni, 0.1 nmol/kg for Cu, and 0.2 nmol/kg for Fe. Overal! procedural blanks were 2 pmo!/kg for Cd, 0.45 nmol/ kg for Ni, below detection limit for Cu, and 0.5 nmo!/kg for Fe. Samples were corrected for btanks, as tiey were low and reproducible. Analytica! precision was about 5% at 100 pmo!/kg Cd and 2 nmol/kg Cu, Ni, and Fe.

Particles

Filters were leached in hot 7 N Q-HN03 for 2 h. The lea&ate was evaporated to dryness, and the residue taken up in 0.1 N Q-HN4. Dissolved residues were then buffered to pH 4.3 with 1 N Q-am- monium acetate and extracted with APDC-DDDC/ch!oroform sol- vent extraction ( BRULAND et al., 1979). Extraction efficiency was zero percent for Mn. The original leachates were analysed directly for Mn with GFAAS using standard addition to compensate for the salt matrix. The extracts were analysed for Fe, Ni, Cu, and Cd using 0.1 N Q-HN03 standard vibration curves since no matrix effects were observed. Con~nt~tions were at least twenty times hi&er than estimated detection limits except for Cd (0.1 nmolfkg for Fe; Mn 2 pmo!/kg; Cu 3 pmol/kg; Cd 0.1 pmol/kg; Ni 2 pmo!/kg). Overall procedural blanks were below detection limit. Analytical precision of the data was about 5%. Accuracy of the particulate trace-metal concentrations is only about IO-20% due to uncertainties in the amount of seawater that was filtered.

RESULTS

Hydrography

Hydrographical data are taken from HYDES et al. ( 1988) and shown summarized in Fig. 2. Quality of the salinity and tem~rature data did not allow the calculation of potential density ( cre). This poses restrictions to comparison of the data between stations.

In reviewing the hydrography ofthe eastern Mediterranean, MALANOTTE-RIZZOLI and HECHT (1988) showed that few

facts have been reliably assessed as yet. Contradictory obser- vations exist in the past literature, notably on deep water masses and deep-water formation areas. Yet some common trends can be discerned. First, due to the presence of the sills at the Strait of Sicily (sill depth, 500 m), there is virtually no exchange of water between the deep western Mediterranean and the deep eastern Mediterranean. The Strait of Crete (sill depth, 2000 m) further divides the eastern M~ite~an~n into the western Ionian basin and the eastern Levantine basin, but exchange between both basins is believed not to be impeded to a large extent.

At our stations, three well-defined water masses can be identified. At the surface, Atlantic Surface Water (ASW) penetrates as far as the easternmost M~ite~n~n. Due to extensive heating of surface waters during summer a lid of warm water is formed, preventing early dissipation of ASW in the western Mediterranean. In winter, ASW can no longer be identified east of the Straits of Sicily. At our stations, sa- linity of these waters varies between 38.4 and 38.9 psu. ASW

Pot. Temperature (“C)

-i 13 14 15 16 17 16 19 20

Salinity (p.s.u.)

38.4 38.5 38.6 38.7 38.8 38.9 38.0 39.1

FIG. 2. Vertical profiles of temperature and salinity at Tyro Basin and Bannock Basin.

extends between 20 and 50 m depth. Imm~iately below, Levantine Intermediate Water (LIW) is characterized by a maximum saiinity of 38.9-39.1 psu at 80-200 m depth. LIW extends between 60 and 400-600 m depth and generally flows westward. Density surfaces probably shoal eastward, as the core of LIW at Tyro Basin is located shallower than at Ban- neck Basin. There is stilt much debate as to the exact for- mation areas of this water mass, but they all reside in the easternmost Mediterranean. Below 600 m Mediterranean deep water is present with a temperature of 13.6’C and sa- linity of 38.66 psu, hereafter called Eastern Mediterranean Deep Water (EMDW ) . Various contradictory observations point to different locations as formation areas for this deep water mass. For both the Ionian and the Levantine basin, the Adriatic Sea as well as the Aegean Sea have been claimed as source areas. Neither the general flow direction nor the ‘renewal time of EMDW has been assessed unequivocally. Best estimates of deep water residence times are around 100 y ( MALANOTTE-RIZZOLI and HECHT, 1988).

Trace metals

Vertical profiles of dissolved and particulate trace metals are presented in Figs. 3-7 for Tyro and Bannock basins; data for all stations are given in Table I.

1422 P. M. Saager, J. Schijf, and H. J. W. De Baar

(a) Dissolved Mn (nM) (W Dissolved Mn (MA) Particulate Mn (PM)

FIG. 3. (a) Vertical profiles of dissolved Mn for Tyro Basin and Bannock Basin: Complete water column. (b) Dissolved Mn: Expansion of the brine. (c) Particulate Mn concentrations in Tyro Basin and Bannock Basin.

Dissolved Metals

Manganese

In normal Mediterranean seawater, the vertical distribution of dissolved Mn (Fig. 3a,b) is characteristic of open ocean waters, indicating particle scavenging as the major removal process. Concentrations are strongly elevated in surface waters (2-3.5 nmol/kg), decrease exponentially with depth to very low values (0.06-0.4 nmol/kg) between 1000 and 2000 m depth and increase below 2500 m depth towards the seawater- brine interface. The profiles are in very good agreement with recent data for the Mediterranean (MORLEY and BURTON, 199 1). Earlier data are higher ( KREMLING and PETERSEN, 198 1 ), presumably as a result of some residual contamination (BOYLE et al., 1985).

At the seawater-brine interface, the concentration shows a dramatic gradient of three to four orders of magnitude over a depth interval of less than 10 m. The first report of this (DE BAAR et al., 1987) was confirmed by DE LANGE et al. ( 1990b), who by high-resolution sampling at the seawater- brine interface found this increase to take place over less than 1 m. In Bannock Basin, there is a distinct minimum at the second interface between brine I and brine 2, with concen-

(a) Dissolved Fe (nhl)

0

Fe (nM)/salinity ([-34x10)

trations in brine 2 being about 30% lower than in brine 1. Concentrations are intermediate between the Cariaco Trench (-600 nmol/kg, JACOBS et al., 1987) and the Black Sea

(-5-8 pmol/kg, LEWIS and LANDING, 199 1) and are two orders of magnitude lower than at Orca Basin ( x 300 pmol/ kg, TREFRY et al., 1984).

Iron

Dissolved Fe concentrations vary along with the presence of different water masses (Fig. 4a-c). At Bannock Basin, concentrations in ASW are high, and Fe minima and maxima ( l-6 nmol/kg) coincide with minima and maxima in salinity. At Tyro Basin, surface water concentrations are higher than at Bannock Basin down to 100 m depth. At Bannock Basin, sampling resolution is sufficient to reveal a gentle maximum in intermediate waters more or less coinciding with a similar salinity maximum related to LIW. In both basins, EMDW has low but variable concentrations with slightly higher levels at Tyro Basin.

Both in the Tyro and Bannock basins, a very pronounced dissolved Fe maximum is observed at the seawater-brine in- terface ( x 100 nmol / kg), more than two orders of magnitude

(W Dissolved Fe (nM)

FIG. 4. (a) Vertical profiles of dissolved Fe for ‘Tyro Basin and Bannock Basin: ‘Total water column. (b) Dissolved Fe: Expansion of the brine. (c) Dissolved Fe profile in Bannock Basin with salinity for comparison. Salinity scale has been modified in order to match trace-metal scale. (d) Particulate Fe concentrations in Tyro Basin and Bannock Basin.

Trace metals in seawater and anoxic brines 1423

(a) Dirrolvod Ni (nM) ( W Dissokod MI (nN)

0 2 4 6 B

0.3 Ni (nl)/salinity ((-381x4) (4 Particulate Ni (PM)

FIG. 5. (a) Vertical profiles of dissolved Ni for Tyro Basin and Bannock Basin: Total water column. (b) Dissolved Ni: Expansion of the brine. (c) Dissolved Ni profile in Bannock Basin with salinity for comparison. Salinity scale has been modified in order to match trace-metal scale. (d) Particulate Ni concentrations in Tyro Basin and Bannoek Basin (see Fig. 4a). Vertical profiles of dissolved Ni for Tyro Basin and Bannock Basin: Total water column.

higher than in the overlying water column. In the brines, concentrations decrease strongly downward. In Bannock Ba- sin, a concentration minimum was observed at the interface between brine 1 and brine 2, in good agreement with similar minima for Mn, BEE ( SCHIJF, 1992), and concentration steps for several other parameters (DE LANGE et al., 199Oc; BRE- GANT et al., 1990; LUTHER et al., 1990). Actual concentra- tions in the brine are of the same order as for the Black Sea (LEWIS and LANDING, 199 1) and the Cariaco Trench (JACOBS et al., 1987) but almost three orders of magnitude lower than in Orca Basin (TREFRY et al., 1984).

(8 Dlasolved Cu (n&t)

0 2 4 6

(cl Cu (nM)kallnity ([-38)x4)

Nickel

At Tyro Basin and Station 5, the vertical distribution of Ni is fairly uniform throughout the water column (Fig. Sa- c ) . Average concentrations are about 0.5 nmol/ kg higher at former station (5.1 vs. 4.6 nmol/kg). At Bannock Basin, better sampling resolution showed that concentrations dif- fered in different water masses (Fig. 5~).

Our data are slightly higher than reported for the deep Alboran and Ligurian Sea (BOYLE et al., 1985; HUYNH-NGQC and WHITEHEAD, 1986; MORLEY and BURTON, 1991). These

04 Dissolved Cu (nM)

---I l - I

(4 Paftlculate Cu (PM)

Tml

FIG. 6. (a) Vertical profiles of dissolved Cu for Tyro Basin and Bannock Basin: Total water column. (b) Dissolved Cu: Expansion of the brine. (c) Dissolved Cu profile in Bannock Basin with salinity for comparison. Salinity scale has been modified in order to match trace-metal scale. (d) Particulate Cu concentrations in Tyro Basin and Bannock Basin.

1424 P. M. Saager, J. Schijf, and H. J. W. De Bar

(4 Dissolved Cd (PM)

0 -. - 0.

. 0 0

Cd (pM)lsallnlty ([-3a]xlOO)

03 Dissolved Cd (PM)

(4 Particulate Cd (PM)

FIG. 7. (a) Vertical profiles of dissolved Cd for Tyro Basin and Bannock Basin: Total water column. (b) Dissolved Cd: Expansion of the brine. (c) Dissolved Cd profile in Bannock Basin with salinity for comparison. Salinity scale has been modified in order to match trace-metal scale. (d) Particulate Cd concentrations in Tyro Basin and Bannock Basin.

western Mediterranean deep waters contain about 4-4.3

nmol/ kg Ni. Deep water Ni concentrations tend to increase

towards the eastern Mediterranean.

In the brine, concentrations sharply fall to values of 0.4-

2.0 nmol/kg. In Bannock Basin, the concentration in brine

I is about a factor of two higher than in brine 2. In contrast

with Mn and Fe, there is no distinct feature at the interface

between brine I and brine 2. To the best of our knowledge,

the anoxic brine waters of Bannock and Tyro basins constitute

the first observations of Ni concentrations being much lower

than in the overlying oxygenated water column.

centrations in ASW and LIW are elevated (70-100 pmol/

kg) and decrease to uniform deep water values of 40-60

pmol/ kg, with higher values at Tyro Basin, as was also found

for the other metals.

At the seawater-brine interface, a very steep concentration

gradient is observed. In the brine, concentrations are more

than one order of magnitude lower than in the overlying

water column. Similarly low values have previously been re-

ported for other anoxic basins (JACOBS et al., 1985, 1987;

HARALDSSON and WESTERLUND, 1988; LANDING and LEWIS.

1991).

coppet

The vertical distribution of dissolved Cu is also strongly

related to the presence of different water masses (Fig. 6a-c).

At Bannock Basin and Station 5, dissolved Cu concentrations

are elevated in ASW and LIW and decrease below the salinity

maximum at about 200 m depth. At Tyro Basin, concentra-

tions show considerable scatter. Average deep water concen-

trations are about 0.5 nmol/kg higher than at former stations.

Suspended Particles

Munguncrc

In the brines, concentrations are about one order of mag-

nitude lower than in overlying waters. At the interface be-

tween brine 1 and brine 2, scatter in the dataset obscures any

trends. In brine 2, the concentration is almost a factor of two

lower than in brine I. Copper concentrations in oxygenated waters are similar

to those previously reported for the Mediterranean ( LAU- MOND et al., 1984; BOYLE et al., 1985; MORLEY and BURTON,

199 1). The low concentrations in the anoxic brines are in good agreement with values reported for Cariaco Trench

(JACOBS et al., 1987), Framvaren Fjord (JACOBSON al., 1985).

and the Black Sea ( HARALDSSON and WESTERLUND, 1988 )

In the oxygenated water column, the concentration ofpar-

ticulate Mn (20- 100 pmol/ kg) is almost one order of mag-

nitude lower than that of dissolved Mn (Fig. 3~). Concen-

trations are of similar magnitude as data for the Sargasso Sea

and northeast Pacific Ocean ( SHERRELL, 1989). In the brine,

an increase of more than one order of magnitude is observed, two orders of magnitude less than the increase of dissolved

Mn. At Bannock Basin, the particulate Mn concentration at

the seawater-brine interface (20-50 pmol/kg) is much lower

than at Tyro Basin, and in the brine, the concentrations are

about half those in Tyro Basin. For Bannock Basin, sampling

resolution is too poor; but for Tyro Basin, a distinct partic- ulate Mn maximum is observed at or just above the interface. Our brine data compare well with levels found in the Black Sea ( LEWIS and LANDING, I991 ), but the concentration at

the seawater-brine interface is considerably lower. Concen- trations in overlying waters agree fairly well with pre.vious

data for these basins ( VAN DER SLOOT et al., 1990); however, in the brine. our values are lower by one order of magnitude.

The vertical distribution of dissolved Cd seems to be related In Tyro Basin, particulate Fe concentrations above the to the presence of various water masses (Fig. 7a-c). Con- brine are of the same order as dissolved concentrations, then

Trace metals in seawater and anoxic brines 1425

TABLB 1 Dived tmx mtal cmcenmcicm, ** CO”- “.a l10t uulyred, bmdcea indicate dm mpected ofcmmhdm

STATION 5 (WWNIzo”ar’B) dismkd cuwmntkms

apul M” Cu cd Ni Fe

(dbar) (m) (“m&x) (Pmdnc8) (“mlk) (““KM@ 1 “.a 1.89 83 4.75 .*

:: “* n:* 2.89 3.12 4.70 4.57 .* l .

“.a :z “.L tz :: z: l * .I

“.a P, “.8

2:57 1.59 ;I

s:13 l * 4.18 . .

loo2 “.a 1.92 46 5.26 . . 1497 “.B 3.12 46 4.61 I. 2ooo “.B 2.36 47 4.76 . . 2808 “.a 1.98 5.01 l * 2966 n.a 2.32 4.14 .*

TYRO BASIN (33’54’N/26”02’E) dissolved cmccnmions

1:: 2.4 “.a n.a 1.77 “.8 70 4.60 “.a la.8 “.a

% 0.6 n.a 2.33 2.56 Soo “.a 2.63

(1;) 5.09 5.34 2.33 4.61 5.13 2.58

1010 0.4 2.44 56 5.43 5.66 1300 “.B 2.57 56 5.04 1.30 1700 0.2 2.95 61 491 2 56

2ooQ 2666 2996 3200

“.B 0.25

0:;)

3.41 2.50

0:;;)

‘.- 4.44 4.84

w;)

2.80

3268 0.4 “.a n.a “.a “.a 3350 1.3 2.67 46 5.33 1.40 3360 0.4 2.20 46 5.78 0.70 3370 “.a 2.44 40 5.89 1.31 3390 4320 0.42 “:a 1.53 13o 340 4450 “.a 1.22 121 3425 4200 “.a “.a 1.09 19.5 3450 4350 0.27 1.91 59.8 3475 4340 0.48

4” 1.33 28.7

BANNOCK BASIN (34°18’Nn0”olE) diSsOlv+d c~“rratio”S

w hln C Cd N F (dbar) (nmolkg) (“m&g) @molkg) (“motkg) (“moeWg)

0 2.86 2.07 “.B 457 4176

5 a4 1:07 2.67

:E 1500 1750

z% 2999 3100

3126 3150 3300 3307 3310 3315 3323 3323 3329 3359 3376 3403 3420

3430

3468

3470

3491 3528 3580 3629

3.45 “.a 3.50 “.a “.a “.B “.a “.a “.a 0.66 “.a “.a “.a “.a 0.06 “.B “.a “.a 0.09 “.a 0.19 “.a “.a n.a 0.54 0.79 0.87 2.17 2.16

46u) 4350 5290 4640

I% 4820

“.a 3790

n.a 2870

“.a 2570

“.a 2910 3780 3750 3480

1.49 2.64 2.36 2.83 2.97 2.40 2.26 3.17 3.04 2.89 2.88 1.71 2.17 1 .a6 1.63 1.79 1.76 1.52 1.80 “.a 0.94 3.34 2.49 1.76 “.8 I .05 0.86 1.00 0.83 “.B 0.60 “.B 0.38 “.a 0.33 0.46 0.41 0.15 0.14 0.19 0.21 0.35 0.40 0.19 0.58 0.17 0.19

n.a 88 “.B “.a 99 “.a 92 “.a “.a a3 “.a 62

:; “.a

:: 45

:

:; 68

:; 68 39 “.B 36 “.a

3 “.a

2 “.a

2 2

“.a 2

n.a 2

“.a 2

“.a

:

:

“.a

4.97 4.74 5.12 4.98 “.a 5.01 5.17 “.a “.B 4.85 “.a 4.63 4.90 4.50 4.60 “.B “.B “.a 4.40 “.a 4.56 4.20 “.8 4.37 4.26 4.5 1 4.75 “.a 4.91 “.iI 0.90 “.a 0.99 0.94 0.73 0.71 0.46 1.44 0.46 0.66 1.13 0.58 0.35 0.36 0.27 0.39 0.65

6.04 1.35 2.49 0.87 1.24 1.62 1.63 2.26 1.88 “.B 0.72 0.40 0.67 “.B 0.60 1.53 “.a “.B 1.30 2.49 n.a “.B 0.62 0.47 0.70 1.97 0.95 “.a

lo6 82.8 23.3

“.a 16.2 29.4 27.5 10.8 10.1 12.3 10.4 10.0 10.2 9.59

22.8 20.5 12.9 19.8 16.5

3679 4070 0.22 5

3730 3870 0.27 3784 3620 0.32 ; 0.39 19.5

Table 1, continued. Particulate concentrations. n.a. not analysed. Brackets indicate data suspected of contamination.

TYRO BASIN (33°S4’N126W2’E) ~utidatc am~cnoationr

ocph M” Cu Cd N

(pnovlrg) (P”Mw @md/kg) (P”Gw ( F

e tMWkg) 98 032

3350 7”;

134 0:46 39

3:4 3360 0.47 :: 1.8 3370 3375 :3”

%!I 0.70 2.0 373 0.20 2 14.6

3380 “.P “.P “.a “.B 3400 G 272 0.64 27.3 3425

z

231 0.28 (2) 26.5 3450 262 0.66 3475 217 0.23 z:

17.1 29.4

BANNOCK BASIN (34°18’N/20WE) particulate cwce”!mtio”s

Deph C Cd N F (dbar) (pm%g) (pm&g) (pm&kg) (pm&kg) (nmoeVkg) 3322 18 433 0 29 3322 15 589 0:16

54 13 0 2218

3323 2:;

677 1.38 ; 47.7

:z! 171 1.17 16.6

237 65 0.68 (I$) 7.6 3420 224 0.25 4.6 3470 240 ‘8: 0.30 :: 6.6

increase by a factor of ten in the brine, still of the same order as dissolved concentrations (Fig. 4d). In Bannock Basin, concentrations are high and variable at the interface and de- crease in the brine to values about half those in Tyro Basin. Concentrations are of the same magnitude as dissolved con- centrations. Particulate Fe concentrations in the brines are in good agreement with those reported for anoxic waters of the Black Sea (LEWIS and LANDING, 199 1). For overlying waters and the interface, our data further agree fairly well with previous reports for these basins by VAN DER SLOOT et al. ( 1990), but brine values are lower by one order of mag- nitude, the same difference as observed for Mn. Their data represent total particulate concentrations measured by neu- tron activation, whereas our data represent a leached fraction with 7N HN03. This might explain part of the discrepancy, but not all since it is unlikely that more than about 70% of particulate Fe will be refractory (LEWIS and LANDING, 199 1 ), resisting the acid-leach with 7N HN03. The difference may further be attributed to oxidation of their anoxic brine samples during ondeck centrifugation, leading to precipitation of Fe and Mn oxyhydroxides ( TROUP et al., 1974), since apparently no precautions had been taken to avoid oxidation both during sampling and centrifuging (VAN DER SLOOT et al., 1990).

Nickel

At Tyro Basin, particulate Ni concentrations in oxygenated water are about two orders of magnitude lower than dissolved concentrations (Fig. 5d) and are similar to those measured for the Sargasso Sea and the Pacific Ocean ( SHERRELL, 1989). Concentrations increase by about a factor of 2-4 across the seawater-brine interface and are only one order of magnitude lower than dissolved concentrations. In Bannock Basin, a sharp maximum at the seawater-brine interface is observed. Concentrations decrease sharply into the brine much like particulate Fe, Cu, and Cd and are about twice as low as in Tyro Basin.

Copper

In the oxygenated waters at Tyro Basin, particulate Cu concentrations are an order of magnitude below dissolved

1426 P. M. Saager, J. Schijf, and H. J. W. De Baar

levels (Fig. 6d). Both in Tyro and Bannock Basin, particulate Cu concentrations show a pronounced maximum at or above the seawater-brine interface, decreasing in the brines to values of the same order as dissolved Cu. Concentrations at Tyro Basin are twice those at Bannock Basin. They are slightly higher than reported for the Sargasso Sea and the northeast Pacific Ocean (SHERRELL, 1989).

For oxygenated waters, our data are more than one order of magnitude lower than data by VAN DER SLOOT et al. ( 1990), and in the brine, almost two orders of magnitude. Their particulate Cu and Zn data are very high in comparison with open ocean inanitions ( SHERRELL, 1989) or aeolian dust from the central M~te~n~n ( TO~WADIN et al., 1984 ) . Such a large difference is unlikely and hints at contamination, where the rubber strings of the Niskin bottles used for ob- taining deep water samples are well-known sources of Cu and Zn (SPENCER and BREWER, 197 1). Latter explanation is supported by the fact that the difference was much less for other elements (Al, Mn, Fe, Co). This is consistent with the much lower concentrations of particulate Cu and Zn for sur- face waters that had been sampled (VAN DER SLOOT et al., 1990) with a membrane pump from the bow of the ship. For brine samples, the centrifuging method may have led further to oxidation of dissolved Mn and Fe and resulted in precip nation of FeMn~xyhydroxides. These, in turn, could have scavenged considerable amounts of trace metals, thus ex- plaining the discrepancy between their data on the one hand vs. our data and results from the Black Sea and Framvaren Fjord on the other hand ( HARALDSSON and WESTERLUND, 1988; LANIXNC and LEWIS, 199 1).

Cadmium

In oxygenated waters, particulate Cd concentrations are two orders of magnitude lower than dissolved concentrations (Fig. 7d) and near the detection limit. Therefore, the data show considerable scatter and must be considered as maxi- mum values. Otherwise, ~n~entmtions are in fair agreement with results for the Sargasso Sea and Pacific Ocean (SHER- RELL, 1989). In Bannock Basin, concentrations decrease in the brine below a maximum at the seawater-brine interface. In Tyro Basin, particulate Cd concentrations scatter much and show no trend in the brine. Actual concentrations are similar to those reported for anoxic waters of Framvaren Fjord and the Black Sea ( HARALDSSON and WESTERLUND, 1988).

DISCUSSION

Oxygenated Waters

As in the western basin, the vertical distributions of Ni, Cu, and Cd in the eastern Mediterranean are not correlated with those of the nutrients. Ineffective recycling of nutrients, as well as non-steady-state behaviour, seem likely explana- tions (BOYLE et al., 1985; SHERRELL and BOYLE, 1988; BETHOUX et al., 1990). This being said, we will focus here on the distinctly different metal concentrations in various water masses.

Surface water concentrations are higher than reported for the western and central Mediterranean, consistent with pre-

vious observations ( SP~VACK et al., 1983; HYDES et al., 1988). Continuous atmospheric input is at present the most plausible explanation. Atmospheric Cu and Cd fluxes exceed river input by at least one order of magnitude (MARTIN et al., 1989). For Cd, most of this flux is attributed to anthropogenic sources ( DULAC et al., 1987). Sediment-derived metal fluxes as sug- gested by KREMLINC~ and PETERSEN ( 198 1) cannot be ruled out but are probably of minor importance. Concentrations of Al increase considerably from west to east ( HYDES et al., 1988), emphasizing the importance of atmospheric input ( GANOR and MAMANE, 1982; BUAT-MFNARD et al., 1989). However, Mn concen~ations in ASW do not increase east- ward. Apparently, Mn is subject to more intensive particle scavenging than Al, leading to fast removal of dissolved Mn from surface waters, as is also inferred from the vertical pro- files. The less reactive nature of dissolved Al in the Mediter- ranean Sea has previously been inferred from its strong cou- pling with dissolved silica (MACKENZIE et al., 1978; HYDES et al., 1988 1. The differences in surface water Mn, Fe, and Al concentrations between Bannock and Tyro stations are probably a local and temporal phenomenon. KREMLING and PETERSEN ( 198 I ) reported high surface water Mn concen- trations for a station close to the coast of Libya, but they found no significant maxima at a station close to Tyro Basin, just south of Crete. Their samples were taken near the end of the year. These observations are in good agreement with meteorological data ( MAY, 1982 ) , indicating that prevailing wind directions change from predominantly northern winds during summer to mainly west and southwestern winds in winter. Apparently, the elevated dissolved surface water con- centrations are quickly removed by intensive particle scav- enging. As all surface water enrichments are consistent with atmospheric input sources, there is no need to invoke sedi- mentary fluxes.

Higher metal concentrations are observed in LIW relative to ASW. Concentration maxima in intermediate waters in the western M~ite~nean (BOYLE et al., 1985; COPIN- MONTEGUT et al., 1986; HUYNH-NGOC and WHITEHEAD, 1986; SHERRELL and BOYLE, 1988 ) are probably also related to the presence of LIW. The formation areas of LIW are not well established but are believed to be located in the eastern- most Mediterranean ( MALANOTTE-RIZZOLI and HECHT,

1988 ). No trace-metal data are available for this part of the Mediterranean, but high concentrations due to the general concentration increase in eastward direction would be ex- pected ( SPIVACK et al., 1983). For Cd and Ni, LIW largely retains its preformed signal on its way to the western Medi- terranean, and trace-metal concentrations remain more or less constant. For Cu, levels decrease somewhat, indicative of continued particle scavenging. In contrast, deep water concentrations between both basins differ considerably. Due to the presence of the sill at the Strait of Sicily, deep water exchange is largely impeded, as demonstrated by the different hydrographical conditions in both basins ( PLAKHIN, 197 1, 1972).

Trace-metal concentrations are lower in EMDW than ASW and LIW for Mn, Fe, and Cd at both stations and for Cu only at Bannock Basin. Nickel concentrations are fairly uni- form throughout the water column. The strong concentration decrease of dissoIved Mn, Cu, and Fe could be explained by

Trace metals in seawater and anoxic brines 1427

continuous particle scavenging. It is difficult to explain the concentration decrease of Cd by scavenging processes, since open ocean profiles of Cd show no indication of strong scav- enging behaviour. Nickel does not show any evidence of scavenging, and there is no reason to believe that Cd would be significantly more particle reactive than Ni. Control of the dissolved Cd distribution by scavenging is not supported by the very low particulate Cd concentrations either (see be- low ) . Also, particulate-dissolved ratios are equal for Cd and Ni and an order of magnitude lower than for Cu, Mn, and Fe. If deep water Cd covcentrations reflect a preformed signal, surface water concentrations at the deep water formation areas must be well below average surface water levels at our stations. Both the southern Adriatic Sea and the Aegean Sea, believed to be deep water formation areas for the Ionian and the Levantine basins, are close to populated areas. Although no data have been published for these areas, it is highly un- likely that surface water concentrations will be as low as our deep water values. Preliminary data seem to confirm this notion ( BOYLE and LEA, 1992). It therefore seems most likely that at present the eastern Mediterranean is not at steady state and that the high surface water concentrations, at least of Cd and possibly Cu, are a recent feature, caused by an- thropogenic inputs (MARTIN et al., 1989; DULAC et al., 1987). Apparently, anthropogenic sources are less significant for Mn, Fe, and Ni.

Lastly, deep water Fe, Ni, and Cd concentrations in EMDW are higher at Tyro Basin than at Bannock Basin and Station 5. This is in support of theories suggesting different deep water formation areas for the Levantine and Ionian basins and further suggests that exchange of deep water be- tween both basins is impeded by the presence of the sill at the Strait of Crete, in contrast with the literature report (cf. MALANOTTE-RIZZOLI and HECHT, 1988).

The Brines

General trends are very similar to previous observations in Framvaren Fjord (JACOBS et al., 1985; HARALDSSON and WESTERLUND, 1988), the Cariaco Trench (JACOBS et al., 1987 ) , and the Black Sea ( HARALDSSON and WESTERLUND, 1988; LANDING and LEWIS, 1991; LEWIS and LANDING, 1991). Dissolved Fe and Mn are extremely elevated at the seawater-brine interface as a result of reductive dissolution of Fe- and Mn-oxyhydroxides adsorbed on settling particles and then decrease downward in the brines due to sulfide pre- cipitation. Dissolved Cu and Cd decrease sharply due to sul- fide precipitation, probably by incorporation into Fe-sulfides (LUTHER et al., 1980, 1986). In reducing environments, Mn- precipitation with carbonates has often been proposed as limiting dissolved Mn concentrations (cf. MIDDELBURG et al., 1987 ). This process could be important for Bannock and Tyro basins; yet preliminary modelling (unpublished results) indicates that carbonate anions are complexed extensively by Mg and Ca, rendering the free carbonate concentration low. It is possible that dissolved Mn concentrations are kept low (cf. Orca Basin, TREFRY et al., 1984; Black Sea, LEWIS and LANDING, 199 1) by precipitation as Mn-sulfides or co- precipitation with pyrite. However, there are some contra- dictory observations arguing against this option, which shall be discussed in a following section.

The depletion of Ni in the brines is most intriguing and to the best of our knowledge has not been witnessed before. Nickel concentrations in anoxic waters of Cariaco Trench (JACOBS et al., 1987) and Framvaren Fjord (JACOBS et al., 1985; HARALDSSON and WESTERLUND, 1988) are much the same as in the overlying oxygenated waters. In anoxic waters of the Black Sea, LANDING and LEWIS ( 199 1) measured Ni concentrations which were at most 40% lower than overlying waters. Yet such a decrease was not found by HARALDSSON and WESTERLUND ( 1988), who studied virtually the same Black Sea location about four years earlier. On the other hand, these anoxic seawaters presumably are all supersatu- rated with respect to a solid Ni-sulfide phase (JACOBS et al., 1985 ) . Also, Ni concentrations in Fe-sulfide minerals in salt marsh sediments can be as high as a few percent (LUTHER et al., 1980). One should also keep open the possibility that Ni concentrations are regulated by coprecipitation with cal- cium-carbonate. Total alkalinity at Bannock Basin and Tyro Basin is much higher than at the anoxic basins of normal salinity. No Ni analyses of local carbonate phases are avail- able, however, to support this theory.

Just like Ni, dissolved U also showed strong depletion in the brines (VAN DER WEIJDEN et al., 199Ob) but not in anoxic basins of seawater salinity. This was attributed to the very long residence times of the brine water, in combination with the extremely high sulfide levels of the brine waters. The pre- cipitation of a solid Ni- (or U-) phase would in general be kinetically hindered and only be possible if the residence time of the water is long enough to enable precipitation of a solid phase. It must be noted that, in that case, reaction rates would be extremely slow. There are some hydrographic consider- ations to this theory in the case of Ni. In Framvaren Fjord, the residence time of the water is on the order of 1000 yr, and sulfide levels of deep waters are even higher than at Ban- neck and Tyro basins ( SKEI, 1983), yet no Ni depletion was observed at this site. If kinetic considerations are indeed a key factor in determining the Ni- and U-depletions, this would imply very high residence times for Bannock and Tyro Basins and be in support of age estimates, based on oxygen isotope measurements, of several thousands of years for these waters ( STENNI and LONGINELLI, 1990).

Alternatively, due to the high ionic strength and the ionic composition, solubility products of metal-sulfides may be entirely different from those in anoxic seawater of normal ionic strength. Probably also, stability constants for most metal-anion complexes change by several orders of magnitude at these high ionic strengths (F. Millero, pers. commun.). Unfortunately, the exact thermodynamic calculation of ionic activity products and state of saturation with respect to the least soluble solid phase is not trivial. First, the assessment of activity coefficients in such a hypersaline medium is dif- ficult. Secondly, thermodynamic formation constants of the dissolved metal complexes in the brine are only partly known, and thus calculation of the speciation of the metals in the brine is highly speculative. Also, pH is an $-defined concept in a solution with such a high ionic strength, making spccia- tion calculations of sulfide quite challenging (S*-, HS-, H$, various polysulfides) . Lastly, the selection of proper pure solid phases or solid solutions from the literature data is not an unequivocal issue. With the present dataset, it is not possible

1428 P. M. Saager, J. Schijf, and H. J. W. De Baar

to distinguish between kinetic hindrance, sulfide precipitation, or carbonate precipitation as the mechanism responsible for the observed N&depletion in the brines.

Comparison of our Fe data and sulfide data of LUTHER et al. ( 1990) with those reported for Orca Basin ( WIESENBURC et al., 1985) yields some remarkable features. Apart from exact concentrations, the profiles of dissolved Fe and sulfide in Bannock and Tyro basins are the mirror image of those in Orca Basin (Fig. 8). Given these opposite trends, the ionic concentration products are actually very similar in all three basins (Table 2 ). At comparable total dissolved salts content (Table 2), the ionic activity products may also be similar; hence, it is conceivable to postulate solubility control by one and the same solid iron-sulfide phase, most likely FeS, as proposed by LUTHER ( 199 1). As with Ni, it is difficult to provide a solid theoretical framework to determine the ther- modynamic state of the dissolved species with respect to solid

Bannock Basin

1 2 3

sulfide (mmol/l) or Fe x 0.02 (nmol/kg)

Orca Basin

-2500 -II 0.0 10 20 3.0

sulfide (Fmol/l) or Fe x 0.08 (pmol/kg)

FIG. 8. Vertical distributions of dissoved Fe and H2S in Bannock Basin and Orca Basin (data from WIESENBLJRG et al., 1985).

TABLE 2. Upper panel: Ionic compositions of Bannock and Tyro Basin (suIti& dataof m et al.. 1990) compared with data from Chca Basin (data of WIESENBIJRG et al.. 1985 ). Lower panel: Major ionic composition (mostly Na and Cl) of Orca (SHOKES et al.. 1977). Bannock and Tvro basins (DE LANGE et al..

r

199Oc).

species

(mmw Bannock Tym Orca Basin Basin Basin

I Na+ 4200 5300 4720

K+ 125 19 19

cat 16.22 35 32

MC 613.644 71 51

Cl- 5400 5350 5000

SO42- 104.135 53 47

CC+- 4.1-4.2 4 45 5 salinity 322-326 ? 320 cond. @S/cm) 177 191 ?

phases. Nevertheless, the initial formation of (amorphous) FeS as a direct control on dissolved concentrations may be postulated (LUTHER, 1991 ), likely followed by transforma- tion to pyrite framboids (JACOBS et al., 1985).

The low sulfide concentrations in Orca Basin have been interpreted as the result ofgreatly decreased bacterial activity in the brine due to the high salt content ( WIESENBURG et al., 1985). In light of the high sulfide levels found in Bannock and Tyro basins (with similarly high salt contents as Orca Basin), this explanation now has to be abandoned. The dif- ference must probably be sought in differences in Fe input into the brine basins. In that case, sulfide concentrations in Orca Basin are not limiting the Fe concentration, but rather are limited by the high Fe concentration. Figure 8 suggests that the input of Fe into Orca Basin is much larger than into Bannock and Tyro basins. Possible sources of Fe are disso- lution of Fe-oxides on settling particles and influx of sedi- mentary Fe by slumping and turbidites. However, such a large difference will not be the result of a great difference in atmospheric Fe-input, in view of the enormous dust input into the Mediterranean, which would create an opposite effect ( GANOR and MAMANE, 1982). In the Orca Basin, turbidites are a common phenomenon ( ADDY and BEHRENS, 1980), as well as in Bannock and Tyro basins (CAMERLENGHI, 1990). The difference would then have to be ascribed to dif- ferences in the frequency of turbidite deposition and the leachable Fe content of the turbidites. Recently, intense mud flows were observed for Orca Basin (BROOKS et al., 1990). The Orca Basin brine probably originates outside of the basin, flowing over the sea floor into the basin (BROOKS et al., 1990). It is possible that this brine has become enriched in Fe before entering the basin. In contrast, Bannock and Tyro brines are formed by dissolution of evaporites outcropping on the walls of the basins themselves ( CAMERLEINGHI, 1990).

Interestingly, concentrations of Fe and Mn and, to a lesser extent Ni and Cu, in the two brines of Bannock Basin differ significantly, along with many other chemical parameters (cf. DE LANGE et al.. 1990~). The differences in Mn concentra-

Trace metals in seawater and anoxic brines 1429

tions cannot be explained by alteration of the brines prior to outflow into the basin as in Eqn. 1:

Mg2+ + ( 1 + x)SO:- + (2 + x)CaCOs

+ (2 + 2x)HzO + CaMg(CO&

+ (1 + x)CaS04~2H20/ + xCO:-, (1)

where dolomite incorporates large quantities of Mn. DE LANGE et al. ( 199Oa) showed that the upper brine has un- dergone more extensive dolomitization than the lower brine. As dolomite was found to incorporate large amounts of other ions such as Mn (DE LANGE et al., 199Oa; ZENGER et al., 19801, this would tend to decrease the Mn concentrations in brine 1 relative to brine 2. In fact, the opposite was found (Fig. 3b). Coprecipitation of Mn with calcium-carbonate may account for the difference. Alkalinity increases from 2.7 peq/ kg in seawater to 4 peq / kg in brine 1 to 4.2 peq / kg in brine 2 (from DE LANGE et al., 199Ob). Latter increase normalized to Mediterranean seawater is of approximately similar mag- nitude as the decrease of Mn. Although carbonate data were not published, CO:- concentrations seem very similar in both brines (De Lange, pers. commun.; unpubl. i-es.), thus arguing against carbonate precipitation as the mechanism responsible for the difference in Mn concentrations between the two brines.

As mentioned earlier, it would be possible that Mn con- centrations in the brine are limited by precipitation of sulfides. This is supported by the fact that the apparent ion concen- tration products of Mn and sulfide are similar in brine 1 and brine 2 (10.1 * IO-“, 11.4. 10-‘2, respectively). Also, the dissolved profiles of Mn and sulfide show an opposite trend in the two brine layers. Actual activity products may be either more different or more similar in view of small differences in ionic strength and composition between the two brine lay- ers ( DE LANGE et al., 199Oc). Particulate Mn concentrations in the brine are one order of magnitude higher than in ox- ygenated overlying waters but are still very low, and Fe-sul- fides in the water column do not seem to contain high con- centrations of Mn. Higher particulate Mn concentrations may be caused by incorporation into sulfides but may also partly be caused by much lower sinking rates in brine water due to the high density ( = 1.2 kg/L). Unfortunately, we have no info~ation about particle com~sition. Other anoxic basins of normal salinity often appear to be just saturated with re- spect to MnS-phases (JACOBS et al., 1987). Thus, water col- umn data do not seem to support in situ precipitation of Mn- sulfide phases as the major process determining the Mn con- centrations in the brines. On the other hand, the sink for Mn may well be located in the sediment. Due to their restricted size, horizontal mixing along isopycnals may be of consid- erable importance in the basins, and the seawater composition could well be in steady state with respect to porewaters at the sediment-water interface. Dissolved Mn and sulfide would then be transported along isopycnals to the sediment-water interface and there be removed in the sediment. This need not be a pure Mn-S phase but could well be FeS which con- tains Mn (LUTHER et al., 1986).

The distribution of dissolved Fe is probably determined by FeS and subsequent formation of pyrite (LUTHER, 199 1). Apparent concentration products of [Fe] [ HS-j in the two

brines are very similar (Table 2) + This process may also ac- count for the difference in Ni and Cu. Pyrite framboids have been observed in the water column (VAN DER SLOOT et al., 1990). The dramatic concentration decrease below the sea- water-brine interface argues for in situ removal of Fe.

Also, Mn and Fe exhibit a minimum at the interface be- tween brine 1 and brine 2. The mi~mum of Mn and maxima of phosphate and aikalinity at the interface of brine 1 and brine 2 have been explained as resulting from recent dolomite precipitation and carbonate recrystallization at the interface, according to Reaction 1 (DE LANGE et al., 1990a). The min- ima for Fe and REE (SCHIJF, 1992) may be related to the same process.

The temperature profile in the upper brine of Bannock Basin is characterized by a multilayered structure ( BOLDRIN and RABITTI, 1990)) indicating it is less well mixed than the lower brine and the brine in Tyro Basin. This small-scale structure is also revealed by F- and HzS and to a lesser extent is still shown by the ~st~butions of dissolved Fe, Mn, and REE ( SCHIJF, 1992). From the interface to about 3335 dbar depth, the temperature is constant and this layer is charac- terized by the very distinct maxima for Mn (DE LANGE et al., 1990b), Fe, and REE at the seawater-brine interface with concentrations dropping dramatically to the base of this layer. Apparently, removal processe s, such as pr~pi~tion of sul- fides (DE LANGE et al., 199Oc), occur much faster than mix- ing. At the interface between brine 1 and brine 2, concen- trations of dissolved Mn, Fe, and REE show a distinct min- imum, where the strong temperature gradient also argues against any vertical mixing across this interface. Also, the con~ntmtion differences between the various layers are not eroded by mixing, indicating that mixing between the iayers is limited. These results imply that vertical advection diffusion models to model trace-metal profiles in the brines, such as used for the Black Sea (SPENCER and BREWER, 197 1) , would be invalid in the brines or at the seawater-brine interface. This type of model only applies when potential tem~mture and salinity are linearly related and horizontal advection is negligible. The former condition is not valid, and the latter may not be valid either. Due to the restricted size of the brines, lateral transport along isopycnals into and from the sediment may in fact dominate transport.

Particulate Metals

Particulate metal concentrations in the overlying oxygen- ated waters were all lower than dissolved concentrations. The particulate-dissolved ratio varies from element to element. Taking the particulate con~ntration at Tyro Basin at 3300 m depth as best representing the overlying oxygenated water column, the ratio decreases in the order Fe > Mn F+: Cu > Cd(?) = Ni, the uncertainty of Cd reflecting the limited quality of particulate concentrations near the detection limit. This order reflects both the relative particle affinity of each metal and partly the amount present in the refractory fraction. Only for Fe, this fraction is thought to be large ( SHERRELL, 1989). Pyrite dissolves in hot nitric acid. The ratio for Fe and Mn is about one and reflects the well-known particle reactivity of both elements (cf. LANDING and BRULAND, 1987). The ratio for Cu is also significantly higher than for

i430 P. M. Saager, J. Schijf, and H. J. W. De Baar

Cd and Ni and is in accordance with the scavenging behaviour of this element as reflected in the dissolved vertical profiles in the open ocean (BOYLE et al., 1977). Cadmium and Ni are least particle-reactive. This further supports our previous remark that scavenging does not account for the strong con- centration difference of Cd between ASW and LIW relative to EMDW.

In the brine, this order changes to Fe > Cu( XCd?) > Ni B Mn. The major difference with the oxygenated waters is the position of Mn. Manganese is much more soluble in a reducing environment, as clearly shown by the very low par- ticulate-dissolved ratio of about 10m4. This would not support an in-situ sink for Mn, also because the turn-over time of particles would have to be very rapid in order to quantitatively remove dissolved Mn from the water column. In view of the high density of water in the brines, this is not very likely. Particulate Mn concentrations just above the seawater-brine interface do not show a pronounced maximum and are lower than reported for the Black Sea (LEWIS and LANDING, 199 1). Besides the possibility that this is just an artefact due to the poor sampling resolution in relation to the sharp and small interface (DE LANGE et al., 1990b), it may be the result of the strong density gradient. This prevents turbulent exchange of dissolved Mn across the interface, which upon oxidation would have precipitated as Mn oxides. At Orca Basin, par- ticulate Mn concentrations above the seawater-brine interface are much higher, in keeping with the orders of magnitude higher (eddy) diffusion coefficients calculated for Orca Basin ( WIESENBURG et al., 1985).

In the brines, the particulate-dissolved Fe ratio still is about one. The higher particulate concentrations are probably brought about by precipitation as Fe-sulfides, which, however, have a higher solubility than Fe-oxyhydroxides, thus allowing higher dissolved Fe concentrations in the brine than in ox-

ygenated water. The presence of Fe-sulfide phases and pyrite

framboids was demonstrated by SEM (VAN DER SLOOT et al., 1990). Only a very small part of the Fe will be related to the alumino-silicate (detrital) fraction, namely approximately the particulate Fe concentration of the overlying waters, thus less than lO% ofthe total partjcufate fraction. Fe-monosulfid~ incorporate significant quantities of Mn (LUTHER et al., 1980), possibly as a solid solution with MnS, but lose most of it during conversion to pyrite.

Particulate-dissolved ratios of Cu and Cd are higher than that of Ni, in accordance with the observation that Ni-de- pletion is seldom observed in anoxic basins. Nickel appears to be less particle-reactive than Cu and Cd in anoxic waters of Bannock and Tyro brines. Pure sulfides of Ni, Cu, and Cd are hardly ever present in anoxic marine environments, and it is thought that pyrite may account for the removal of these elements from the water column (LUTHER et al., 1980, 1986). This still does not explain the dist~bution of Ni, since pyrite has been demonstrated in other anoxic basins as well ( SKEI, 1983), although Ni depletion was not observed. Lastly, it is possible that Ni, Cu, and Cd are associated with particulate organic matter found near the interface ( WENNEKE and DE

LANGE, 1990). VAN DER SLOOT et al. ( 1990) found Fe and Zn to be associated with particles of biological origin, also containing P, S, and Ca. Cu, however, appeared to be asso-

ciated exclusively with Fe-phases, most likely pyrite. Nickel and Cd were not measured by them.

There is a striking difference in particulate trace-metal dis- tribution between the Tyro and Bannock basins, both qual- itatively and quantitatively. ERBA (1989) showed that pel- liculate particles at the seawater-brine interface of both basins differ in colour, in aspect, and in concentration density, being most abundant in Bannock Basin. She attributed this to fre- quent disturbance of the interface at Tyro Basin by turbidites. In Bannock Basin, particulate Fe, Ni, Cu, and Cd concen- trations in the brine are in fact not significantly higher than in normal Mediterranean seawater. Only at the interface are levels very high, but in the brine they again decrease sharply with depth. In situ formation of solid phases such as pyrite at the interface and stagnation of sinking particles due to the strong density gradient may account for such a particle max- imum (e.g., ERBA, 1989; DE LANGE et al., 1990a; VAN DER

SLoor et al., 1990). Finally, the concentration of particles in the brine will be

greatly influenced by their sinking rate in the brine. The ratio of sinking particles to suspended particles in the brines may be completely different from the overlying seawater and may also be different between the Tyro and Bannock basins. Fu- ture studies should pay careful attention to the particle cycle in the brines, preferably by combination of in situ filtration techniques ( SHERRELL, 1989) and sediment traps.

1)

2)

3)

4)

CONCLUSIONS

The distributions of dissolved Ni, Cu, and Cd in the east- ern Mediterranean are governed by the general water cir- culation and local input. There is no correlation with the nutrients. This is in agreement with previous reports for the western Mediterranean. The vertical dist~bution of Cd indicates that the eastern Mediterranean is probably not in a steady-state situation, likely as a result of anthropogenic input. At the seawater-brine interface, reductive dissolution of Fe and Mn occurs. In the brines, the concentrations of al1 trace metais appear to be limited by formation of in- soluble sulfide phases. Precipitation with carbonates may be of importance for Mn and possibly Ni. For Ni, this is the first report of such dramatic depletion in an anoxic basin. Possible mechanisms include kinetic hindrance of sulfide precipitation, different thermodynamic conditions with respect to solid phases as a result of the high ionic strength, and ionic com~sition of the brines compared to normal salinity anoxic basins and precipitation by car- bonate phases. The profiles of dissolved Fe and sulfide are the mirror image of those found previously in the Orca brine (Gulf of Mexico), yet the ionic concentration products are strikingly similar in all three basins.

Acknowledgments-We thank officers and crew of the RV Tyro for their assistance during sampling. Jeroen de Jong is thanked for the dissolved Mn analyses of the brine samples. Comments by G. Luther and an anonymous reviewer considerably improved the manuscript. This research project was supported by the Netherlands Council of Oceanic Research (NRZ ) .

Ediforinl bundling: F. J. Miller0

Trace metals in seawater and anoxic brines 1431

REFERENCES

ADDY S. K. and BEHRENS E. W. ( 1980) Time of accumulation of hypersaline anoxic brine in Orca Basin (Gulf of Mexico). Mar. Geol. 37,241-252.

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