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Sources and distribution of trace and minor elements in the western Dead Sea surface sediments
Applied Geochemistry,
Pergamon
Vol. 12,pp. 497-505,
1997 1997 Elsevier Science Ltd All rights reserved.Printed in Great Britain 088&2927/97$17.00+ 0.00
0
Pn: S08&2927(97)0002%0
Sources and distribution of trace and minor elements in the western Dead Sea surface sediments Barak Herut* National Institute of Oceanography, Israel Oceanographic and Limnological Research, Haifa 31080, Israel
and Ittai Gavrieli and Ludwik Halicz Geological Survey of Israel, Jerusalem 95501, Israel (Received 5 July 1996; accepted in revisedform 1 February 1997)
Abstract-Twenty Dead Sea surface sediment samples were analyzed for their major, minor and trace element compositions. The samples represent muddy sediments along the western parts of the lake, from water depths of 8-250 m. These sediments were deposited after 1983, under oxic conditions, following the overturn of the water column in 1979, which ended about 300 years of meromictic stratification with an anoxic lower water mass. The changes in their metal concentrations are discussed in view of the different brine oxidation state. The sediments consist of detrital minerals-carbonates, quartz and clays and authigenic mineralsaragonite, halite and traces of gypsum. Calculations indicate that all mud samples contain more than 3.6% authigenic aragonite, which was found to precipitate preferentially in near shore sediments. An increase in Ca and a decrease. in Al concentrations with decreasing water depths and in a transect from N to S were observed. These are attributed to differential settling of detritus and authigenic carbonates close to the shore and fine Al-silicates in the deep waters, and to the southward decrease in the contribution of clay minerals, mostly derived from the Jordan river. Fe, Ce, Be and Eu were found to exhibit conservative behavior with respect to Al during the transition from stream sediments in the drainage basin to lake sediments. When compared to normal marine sediments, the Dead Sea sediments have similar Cu, Ni, Zn, Be, Ce and Eu concentrations, whereas Cd is enriched by nearly 1 order of magnitude. A good correlation exists between Cd and P, suggesting that the Cd enrichment arises from outcrops of Cd-rich phosphate rocks that are found in the Dead Sea basin. The somewhat depleted Pb concentrations in the lake muddy sediments might be explained by the somewhat high Pb concentrations (normalized to salinity) in the Dead Sea water column, as compared to seawater and by its removal, mainly through halite precipitation. The unusual distribution of Mn concentrations in the
surface sediments and its association with authigenic aragonite imply, as has already been suggested, that Mn co-precipitates with aragonite. 0 1997Elsevier Science Ltd
INTRODUCTION
The Dead Sea is the lowest natural surface on earth (410 m below MSL), located in the deepest (730 m below MSL) romb-shaped graben in the Dead Sea rift valley (Garfunkel, 1981). This terminal lake is characterized by a unique brine composition (Ca-chloride brine), a high ionic strength (> 9), and a relatively low pH (-6.5). Because of the increased catchment of water from its drainage basin (40000 km2) over the last decades, the lake has experienced a negative water balance. This led to a drop in water level, which, over the last decade, has averaged 70 cm/a (Anati and Shasha, 1989). In 1979, the negative water balance resulted in an overturn of the Dead Sea water column (Beyth, 1980; Steinhorn and Gat, 1983), which, until then, was stratified with an anoxic lower water body at depths below 40 m (Neev and Emery, 1967). At present, the Dead Sea is saturated to over-
*To whom correspondence should be addressed: Tel.: 9724-851-5202; Fax: 972-4-851-1911; e-mail: [email protected]
saturated with respect to aragonite, gypsum and halite. Massive halite precipitation began in 1982, forming a modern halite layer several tens of centimeters thick, interbeded with detritus material (Gavrieli, 1996). The latter consists mainly of clays, carbonates and quartz (Neev and Emery, 1967; Garber, 1980; Levy, 1987), which reach the lake in flood events or are carried into the lake as airborne particles. The clay minerals are homogeneously distributed within the Iake and are representative of the detritus carried into the lake by the surrounding rivers (Nathan et al., 1992). Based on experimental and field studies, Nathan et al. (1992) established that clay minerals experienced no diagenesis over a time period of up to 16 000 a and thus maintain their original mineralogy, which consists mainly of kaolinite and illite/smectite in similar amounts. This behavior, which is different from that in other saline lakes, was attributed to the relatively low pH of the Dead Sea brine. Early diagenetic reactions within the sediments involve decomposition of organic matter and sediment-water interaction (Nissenbaum et al., 1990).
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498
These involve aragonite precipitation and organic matter mineralization, which is responsible for the release of nutrients into the pore water. Bacterial SO, reduction leads to the formation of Fe sulfides and gypsum dissolution (Nishri, 1982). Manganese diffusing out of the bottom sediments was suggested to be the most important source of dissolved Mn in the Dead Sea (Nishri, 1984). The trace metal content of the Dead Sea sediments was studied by Nissenbaum (1974), who concluded that no special enrichment of metals existed in the Dead Sea sediments. Most of the above studies were conducted while the Dead Sea was stratified with an anoxic lower water body at depths below 40 m. The overturn in 1979 led, among other phenomena, to the penetration of 02 into the entire water mass and resulted in a dramatic decrease in the concentration of dissolved Fe2+, from in 1978 to its total disappearance upto9Smmolkg-’ from the water column after the 1979 overturn (Nishri and Stiller, 1984). A decrease in the concentrations of Cd and Pb in the Dead Sea brine was observed between 1980 and 1985. This phenomenon was attributed to co-precipitation with halite, which began to precipitate in 1983 (Stiller and Sigg, 1990; Herut et al., 1995; Stiller et al., 1996). The present study was carried out in view of the changes that the Dead Sea has undergone over the last two decades. It was intended to determine the concentrations of some elements in the Dead Sea surface muddy sediments that were deposited after the 1979 overturn and to examine the factors that dictate their concentrations. Measurements of some major elements were used to normalize and to examine the trace and minor elements’ behavior. Finally, the distribution patterns of some trace metals in the surface sediments of the western parts of the Dead Sea, including the river outlets, are presented.
METHODS AND SAMPLES
that these sediments had been deposited since 1983. On board, whenever possible, halite and mud layers were cut apart and sampled separately. Mud samples were transferred to pre-cleaned 250-ml centrifuge vessels into which Ar gas was introduced to prevent oxidation.
Mud and mixed halite and mud samples were re-suspended in deionized distilled water (DDW) and centrifuged in order to dissolve salts and remove interstitial brines. The DDW/ sediment volume ratio was about 1:2, and thus only limited dissolution of the less soluble authigenic salts (gypsum and aragonite) was possible ( < 0.1% weight). In order to evaluate the effect of the above procedure, several unwashed samples, sampled in the earlier stages of the study, were analyzed as well. All washed samples were frozen and lyophilized for 48 h. XRD analyses were carried out on selected washed
samples with a Philips diffractometer using a Cu tube operating at 35 kV and 40 mA at a scanning speed of 1.5”[email protected]’ . Kb radiation was filtered out by a Ni filter. Organic C concentrations were determined on l-2 g of dry sediment sample by the potassium dichromate method following the procedure of Walkley (1947) and Gaudette er al. (1974). Calcium, Mg, P, S, Sr, Pb, Cd, Zn, Cu, Ni, Be, Ce and Eu determinations were carried out on l-2 g of dry sediment digested with concentrated HNOs (65 wt%) for 3 h at 140°C inUnisea1, Teflon-lined, high pressure decomposition vessels (Hemt et al.. 1993). The digests were diluted to 25 ml in volumetric flasks with DDWand filtered through Whatman no. 2 filter paper. Aluminium, Fe and Mn concentrations were determined by total digestion of 0.2 g of dry sediments with HF, diluted to 1OOml and treated following the procedure of Jeffrey (1975). Each sample was digested and analyzed in duplicate. Lead., Cd., Zn., Cu. Ni. Mn. Al and Fe were determined using an IL-951 or a Perkin Elmer 1lOOB Flame Atomic Absorption Spectrometer (FAAS). Calcium, Mg, P, S, Sr, Be, Ck and Eu were determined on a JY-48 Inductively Couuled Plasma-Atomic Emission Snectrometer (ICPAESj. The accuracy and precision of the methods are estimated on the basis of analyses of Estuarine Sediment 1646 (NIST) Standard Reference Material for which near complete recoveries were found for all certified metals, except Ni (83%) and Mn (90%) (Table 1).
Sampling Surface sediment samples were collected during a 2-day cruise. on 22-23 Auzust 1994. on the R/V Tiolit. from the northern to the southern parts of the western Dead Sea, at
water depths ranging from 8 to 250 m (Fig. 1). The samples were collected with a stainless steel Van Veen grab sampler, from which only the top sediments, not deeper than 3 cm, were collected by lifting the screens of the grab. All the mud samples were collected from above the modem halite layer or from detritus material interbeded within this layer, implying
RESULTS
Transparent coarse halite aggregates commonly formed the surface sediments in the deeper ( > 50 m) parts of the lake. Often, the aggregates were partly sunken in brown sediment. XRD analyses of representative samples indicate that the mud consists of Al-
Table 1. Concentrations determined in Standard Reference Material Standard Reference Material Estuarinesediment 1546 (NIST) *HF digestion.
Value
Al* %
Fe* %
Cd ppm
Pb ppm
cu ppm
Zn ppm
Ni ppm
Mn* ppm
Certified Found
6.25f0.2 6.01 iO.02
3.35kO.l 3.18k0.0
0.36kO.07 0.33kO.02
28.2k1.8 28kO.2
1853 16.4kO.2
138*6 125*1
32*3 26.4kO.S
375_+20 338
Trace and minor elements in the western Dead Sea surface sediments
I
499
Massada Fig. 1. Dead Sea bathymetric map (after Hall and Neev, 1978)with location of sampling stations (numbers).
brines (Gavrieli, 1996) which are introduced into the lake at its southern shore (Epstein et al., 1975). The chemical compositions of 20 suficial Dead Sea mud sediment samples are presented in Table 2. In general, the heavy metal concentrations are similar to those found in most shelf and deep-sea ocean sediments (Salomons and Forstner, 1984). However, Cd is enriched by about an order of magnitude, and the concentrations of Pb are somewhat lower than in most deep-sea ocean sediments. At the mouth of the Kidron river, through which partially treated sewage is introduced into the lake, the sediments are relatively enriched by organic C, Cd, Pb, Cu, and Zn (Table 2). The concentrations of Al (Fig. 2) and Fe were found to increase with water depth, and thus distance from the shore, a trend similar to that found in sediments from the Mediterranean coastal region of Israel (Herut et al., 1993) and in other normal coastal
silicate minerals (mainly clays), calcite, dolomite, quartz and sometimes aragonite. Laminated aragonite sediments were encountered only at shallow depths, close to the shore where fresh to brackish waters emerge as springs. No mud sediments were collected at some of the deep sites where the surface was composed of coarse halite layer with no mud on top, and preventing the grab from penetrating to below the surface. In shallow waters (< 50 m), the surface sediments consisted of brown mud, which commonly contained a few idiomorphic single halite crystals, 0.5-l cm in size. In the southern parts of the lake, this mud was interbedded with fine to medium grained halite (hundreds of microns to a few millimeters in size) which formed layers l-10 cm thick. Several studies have indicated that this halite is the product of the outsalting that occurs upon the mixing between the Dead Sea brine and the industrial end-
Table 2. Chemical composition of twenty surface sediments from the Dead Sea Percentage dry weight
ppm dry weight
Sample Water depth(m) CaO MgO P205 SO3 A&O3 Fez03 Org. C 11 12 13 14 17 18 19 20 22 23 24 25 26 27 28 29 30 31 32 33
100 30 8 12 200 55 10 140 8 250 220 15 12 65 185 50 10 250 110 45
nd: not determined.
28.93 29.26 30.10 22.97 27.63 38.07 31.89 23.25 34.33 25.15 23.86 26.29 33.79 25.89 20.63 22.06 23.09 20.82 20.46 20.99
2.81 3.43 3.66 2.55 2.05 2.86 4.31 2.21 2.61 2.31 2.36 2.59 1.90 2.37 2.05 2.41 2.38 1.91 1.77 1.83
0.40 0.29 0.26 0.28 0.68 0.95 0.40 0.55 0.91 0.68 0.55 0.82 1.63 0.79 0.36 0.36 0.38 0.41 0.35 0.24
1.74 2.57 1.18 0.57 0.55 0.82 2.15 0.83 4.57 0.82 0.69 1.23 1.63 0.92 0.60 0.72 0.88 0.54 0.47 0.49
5.86 5.63 5.27 6.69 4.63 2.32 3.72 7.41 3.33 7.50 7.82 5.82 3.25 5.50 7.90 7.24 6.41 8.45 8.16 7.61
3.02 3.33 2.95 3.60 2.50 1.33 2.07 3.87 1.73 4.10 4.70 3.56 2.09 3.30 4.66 4.46 4.05 5.40 5.06 4.82
Sr
Mn
Pb Cd Cu Ni Zn
Be Ce Eu
0.77 1386 638 11.0 1.0 22 45 78 1.22 38 1.21 0.89 1804 905 8.4 1.0 19 40 66 1.31 41 1.03 0.78 2390 1332 7.3 0.6 20 41 63 0.99 37 0.94 615 396 10.4 0.8 19 37 65 1.08 43 1.28 0.65 365 208 7.6 1.5 16 36 72 0.84 34 0.99 1.03 0.48 442 133 6.0 2.9 12 24 52 0.41 23 0.74 0.80 2904 630 13.8 2.3 24 59 103 1.02 29 0.72 nd 660 439 12.8 1.6 21 54 98 1.48 51 1.49 0.88 2052 665 7.9 3.4 18 44 78 0.79 31 0.70 nd 725 426 12.7 1.7 25 58 106 1.66 51 1.48 nd 580 396 13.5 1.3 23 55 94 1.69 54 1.49 1.42 1005 524 10.5 1.8 23 52 94 1.25 41 1.22 1.54 1025 485 14.4 4.3 32 53 172 0.82 31 0.97 nd 882 513 11.5 2.5 27 55 100 1.20 40 1.19 0.70 549 444 9.5 1.4 21 53 88 1.48 44 1.30 0.83 820 567 9.8 1.5 25 61 94 1.46 46 1.51 0.64 955 642 9.6 1.5 25 58 89 1.14 38 1.35 0.46 475 431 12.5 0.8 24 67 99 1.66 51 1.71 0.53 377 359 12.7 1.4 23 52 91 1.62 49 1.49 0.50 485 500 8.5 1.0 25 57 90 1.30 48 1.53
B. Herut ef al. DEAD
SEA-AI
(% dry wt.)
Fig. 2. Concentration distribution maps of (a) Al and (b) Cd in the surface sediments of the western Dead Sea.
sediments. The Al concentrations in sediments from the Jordan river delta (stations 30, 32 and 33), at the northern part of the lake, are similar to those found in the central deeper parts of the lake (stations 23,24,28 and 31). An opposite trend was found in the distribution patterns of Ca, Cd (Fig. 2), Pb and Mn, i.e. their concentrations decrease with increasing water depth or distance from the shore. No specific trend between organic C and depth was found.
DISCUSSION The common practice used to identify the changes in trace metal content in sediments as they are transported from the drainage basin to their deposition basin is by normalizing their concentrations to a recognized conservative element, such as Al in aluminum-silicates. A non-conservative behavior may exist because of water-sediment interactions or as a result of anthropogenic impacts. In the present study, the scatter plots of Fe, Ce, Be, Eu, vs Al concentrations fall within the scatter of the linear correlations found in 448 stream sediments collected from parts of the Dead Sea drainage basin (Fig. 3, Gil et al., 1992). This similarity attests to their common origin (Al-silicates) and conservative behavior in the Dead Sea. Unlike the situation in many marine and stream sediments (Herut et al., 1993), no correlation
exists between Al or Fe with Ni, Cu, Zn, Cd and Pb in the stream sediments of the western Dead Sea (Gil et al., 1992); nor were such correlations found in the Dead Sea sediments. Furthermore, no correlation exists between the trace metal concentrations and organic C. The correlation in the stream sediments between Mn and Al (r = 0.84) is not maintained in the Dead Sea sediments, implying a non-conservative behavior of Mn, as will be discussed hereafter. A negative correlation between the AlzOs and CaO concentrations (Fig. 4) and a general trend of decreasing CaO concentrations and increasing AlzOs concentrations with water depths (Fig. 2) were observed. These are explained by the differential depositions of detritus carbonates close to the shore, and fine Al-silicates in the deep waters. Additional Ca is deposited in the form of authigenic aragonite, which, as will be discussed later, preferentially crystallizes and settles in shallow waters. A trend of increasing CaO and decreasing Al203 concentrations was also observed in transacts from N to S at water depths close to 50 and between 100 and 250 m (Fig. 5). This is probably due to the southward decrease in the contribution of the fine river-borne clay minerals, mostly derived from the Jordan river. Cadmium was found to be enriched in the Dead Sea sediments by nearly 1 order of magnitude as compared to normal marine sediments (Salomons and Forstner, 1984) and it has a distribution pattern of decreasing
501
Trace and minor elements in the western Dead Sea surface sediments 40
24 2
0
8
4 6 A1203 (%)
20 0
2
4
6
8
A1203 (%) Fig. 4. CaO vs AlzOs in surface sediments of the Dead Sea. The regression line calculated from the data is included.
OO
2
4 6 AI203 (%)
8
10
2
4
8
10
60 50
40 2 2 -
30
5 20 10
/ 0
0
AI203
6 (%)
Fig. 3. FesOs, Be and Ce vs AlsOs in surface sediments of the Dead Sea. The regression lines and the 99% confidence bands of 448 stream sediments from the Dead Sea drainage basin are included.
concentration with increasing water depth (Fig. 2). This trend cannot be attributed to association with organic material, as is often the case (Salomons and Forstner, 1984), since no such correlation was found (Table 2). These observations, however, are attributed to Cd-rich phosphate detritus, which is derived from phosphate outcrops in the drainage basin of the Dead Sea. The common origin of Cd and P (as P205) is
suggested by their positive linear correlation (r = 0.89, P
B. Herut et al.
502
9 about 50 m
2
20
N
Southward (not to’ scale) ------->
S
Fig. 5. North-south transects of CaO and A1203concentrations at water depths close to 50 and above 100 m. Note the increase of CaO and decrease of AlaOs concentrations southward, with increasing distance from the Jordan river mouth.
Trace metals in authigenic minerals Since the sediments were washed prior to their analysis to remove their salt contents, halite is excluded from the following discussion. In view of the small water/mud ratio in the washing procedure, the dissolution of the other authigenic minerals, gypsum and aragonite, was limited. Thus, the main authigenic minerals to be considered are aragonite and gypsum. The significant Sr enrichment in the Dead Sea sediments as compared to normal marine sediments is attributed to carbonate minerals and authigenic gypsum, each of which is characterized by a typical
Sr/Ca ratio. Taking the Sr distribution coefficient between gypsum and brine to be 0.19 (Butler, 1973), a Sr/Ca weight ratio of 0.0188 in the Dead Sea brine (Herut, 1988), and assuming that all the sulfur in the sediment is present as gypsum, the mass of Sr present in the sediment in the form of gypsum can be determined: Srs = 0.19 x 0.0188 x Ca,,
40 ,...,.,,...,. .............. _..................
$ c
? E
.,.,
0
32
; a.. %
? r
5
24
t
,. ,,. ,... ,.,
!I a
_..,_...,
4 ;
z
3
.
16
... ... . ... . . . . .
3
2 2
(1)
%
I
.._....... .. .... ..
..
_.. ... .... .. .. ...cl _ _...
f
2 I
0'
Dead Sea
drainage basin stream sediments
I
OL 0
0.4
0.8
1.2
1.6
P,O,(%)
Fig. 6. Cd vs PsOs in surface sediments of the Dead Sea. The regression line of 448 stream sediments from the Dead Sea drainage basin is included.
I
Dead Sea
surface sediments
Fig. 7. Box plots of Pb/Al (wt/wt) in (a) surface sediments of the Dead Sea and (b) 448 stream sediments of the Dead Sea drainage basin. The bottom and the top edge of each box are located at the sample 25 and 75 percentiles. The center horizontal line is drawn at the sample median. The central vertical lines (whiskers) extend from the box as far as the data extend to a distance of at most 1.5 interquartile ranges. Only values more extreme than this are marked with a zero.
Trace and minor elements in the western Dead Sea surface sediments
503 0
400
0
0.2
0.4
0.8
0.6
1
Fig. 8. Plot of Sr/Ca in the sediments vs Ca,,&a (the fraction of Ca derived from authigenic aragonite, see eqn. (2) in text). The line represents a mixing between two end members: (1) authigenic aragonite with -7800 ppm Sr (Katz et al., 1977) and (2) detritic carbonates with -200 ppm Sr (Sass and Katz, 1982). The data points are plotted, based on their Sr/Ca ratios.
Ca, is the weight of Ca in the gypsum, calculated from the S content. Applying eqn. (1) it was determined that Sr in authigenic gypsum accounts for no more than 4% in all samples, and no more than 2% in most, of the Sr found in the sediments, implying that most of the Sr is concentrated in the carbonate minerals. Indeed, the average Sr/S ratio of all our samples (0.21 dry wt/dry wt) is about two orders of magnitude higher than the calculated Sr/S ratio in authigenic gypsum (about 0.005 dry wt/dry wt). It is assumed, therefore, that the Sr concentrations in the sediments are mostly determined by the relative contribution of aragonite and detrital carbonates. These can be estimated by a mixing model (Faure, 1986): where
(Sr/Ca), = [Srd + (1
-f)srdcl/[%f
+
(1
-flcadc]9
(2) where the subscripts ‘s’, ‘a’ and ‘dc’ designate sediment, aragonite and detrital carbonate, respectively, and f the aragonite fraction (f= Ca,/(Ca, + Cad&
Fig. 8 is a plot of eqn. (2) that describes a mixing line between two end-members where the x-axis is the aragonite fraction v) and the y-axis is the Sr/Ca in the sediment ((Sr/Ca),). The two end members are: (1) detritus with no aragonite cf=O), which contains w 200 ppm Sr (Sass and Katz, 1982); and (2) Sr-enriched authigenic aragonite (f= l), which contains _ 7800 ppm Sr. The latter value was calculated from the distribution coefficient of Sr between aragonite and Dead Sea brine (-0.9) and is close to the concentrations found in aragonite laminae from the lake Lisan sediments (Katz et al., 1977), which was the Dead Sea precursor.
Ca,,, (s) Fig. 9. Mn vs calculated %Ca., (the percentage of Ca bound to authigenic aragonite) in surface sediments of the Dead Sea. The regression line calculated from the data is included.
The data points from the present study are plotted on the mixing line according to their Sr/Ca ratio. The contribution of authigenic aragonite to the Ca budget can thus be determined, and it ranges from 6% (station 18) to 64% (station 19). The va’lues correspond to aragonite concentrations of 4.2 and 36.2% dry wt, respectively. Similar calculations for all the surface sediment samples indicate that aragonite content ranges between 3.6 and 36.2% dry wt. Thus, although aragonite was detected by XRD only in the shallow water samples that contain relatively high concentrations of aragonite, it can be concluded that all samples contain some authigenic aragonite. A correlation was found between the Mn concentration and the calculated authigenic fraction of Ca (Fig. 9). No such correlation was found for any other heavy metal. This suggests that Mn is removed from the Dead Sea brine by coprecipitation with aragonite. Such a mechanism has been suggested by Garber et al. (1980) and Nishri and Nissenbaum (1993), following the finding of Mn-rich aragonite in near shore sediments. Our finding of the unusual Mn distribution of increased concentrations close to the shore, and the preferential aragonite precipitation close to the shore line, further supports this removal mechanism. This Mn co-precipitation explains why no positive linear correlation etists between Mn and Al in the Dead Sea sediments, in contrast to the correlation found in the stream sediments of the Dead Sea drainage basin (r = 0.84).
Trace element concentrations in surface sediments under anoxic (pre-1979) and oxic (recent) conditions The oxygenation of the lower water mass after the overturn in 1979 resulted in a gradual depletion of Fe
B. Herut et al.
504
Table 3. Average concentration of metals in surface muddy sediments during anoxic and oxic conditions in the Dead Sea (ppm dry wt)
Year Depth Number of stations Fe Sr Mn Pb Cd cu Ni Zn
Present studyoxic water column
Nissenbaum (1974)anoxic lower water mass
1994 100-250
1972 195-300 2 16300~3000 1186*590 1576*938 10*3 12.5kO.5 32+11 44.5* 1.5 5Ok2
a
29 lOOk6994 640 f 327 418;118 11.5+2 1.34kO.3 21.9+2.8 52.5k9.1 90.8k11.3
sulfides and Fe oxy-hydroxides (Nishri and Stiller, 1984), whereas the content of dissolved Mn has remained constant (Nishri, 1984). The massive halite precipitation that began in 1983 removed large amounts of Cd and Pb from the water column (Stiller and Sigg, 1990; Herut et al., 1995). A comparison of the average concentrations of some trace elements in surface sediments under pre-1979 reducing conditions (Nissenbaum, 1974) and post-1979 oxic conditions (this study) is presented in Table 3. A decrease of about 1 order of magnitude is observed in the Cd concentrations between 1972 and 1994. The higher Cd concentrations in 1972 are attributed to CdS, which precipitated under reducing conditions (Nissenbaum, 1974) that no longer exist in the Dead Sea. Furthermore, massive halite precipitation that began in 1983 significantly scavenged dissolved Cd from the water column (Stiller and Sigg, 1990; Herut et al., 1995). The increased Fe concentration by about a factor of 2 in the surface sediments collected in 1994 is attributed to its precipitation from the water column as Fe-oxide coating particles. This situation did not exist in 1972, when the dominant Fe species in the anoxic water column was dissolved Fe*+, which decreased drastically after the oxidation of the whole water column (Nishri and Stiller, 1984). The removal of Mn through its oxidized species (MnOOH or Mn02) was probably minor as observed by its constant dissolved content (Nishri, 1984). The lower Mn concentration in 1994 may be due to the decrease in aragonite precipitation and the associated co-precipitated Mn. Such a decrease may also explain the lower Sr concentrations. The increase in Zn concentrations is hard to explain and needs to be further studied.
CONCLUSIONS (3) Iron, Ce, Be and Eu have a conservative behavior with respect to Al in the Dead Sea drainage basin and lake sediments.
(4) The concentration ranges of Cu, Zn and Ni in the Dead Sea surface sediments are similar to those in the stream sediments of its drainage basin and other marine sediments. (5) Cadmium-rich detritus from the phosphate rocks in the Dead Sea drainage basin is the main source of Cd in the sediments, and leads to Cd concentrations of nearly 1 order of magnitude greater than in normal marine sediments. (6) The concentrations of Pb in the Dead Sea surface sediments are somewhat lower than in the surrounding stream sediments, even after normalization to Al. The mechanism that leads to this depletion is still to be studied. However, it may account for the relatively high Pb concentration in the Dead Sea brine and to its removal mainly by coprecipitation with halite. (7) All the Dead Sea surface sediment samples contained more than 3.5% dry wt of authigenic aragonite, which was found to precipitate preferentially in near shore sediments. (8) The unusual distribution of Mn concentrations in the surface Dead Sea sediments is mainly attributed to its co-precipitation with aragonite. Acknowledgements-This work was funded by the Israeli Ministry of Energy and Infrastructure, Grant 94-17-007. The authors are grateful to captain Moti Gonen and his crew on the R/V Tiolit, without whom this study could not have been carried out. Editorial Handling: Dr Olle Selinus.
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Trace and minor elements in the western Dead Sea surface sediments Holocene Carbonate Sedimentation and Diagenesis in Shallow Epicontinental Sea (ed. B. H. Purser), Springer,
Berlin, pp. 423-452. Epstein J. A., Zelvianski B. and Ron G. (1975) Manganese in sodium chloride precipitating from mixing Dead Sea brines. Isr. J. Earth Sci. 24, 112-113. Faure G. (1986) Principles of Isotope Geology. Wiley, New York. Garber R. A. (1980) The sedimentology of the Dead Sea. Ph.D. thesis, Rensselaer Polytechnic Inst., Troy, NY, 169 pp. Garber R. A., Nishri A., Nissenbaum A. and Friedman G. M. (1980) Modem deposition of manganese along the Dead Sea shore. Sediment. Geol. 30, 261-274. Garfunkel Z. (1981) Internal structure of the Dead Sea leaky transform (rift) in relation to plate kinematics. Tectonophysics 80, 81-108.
Gaudette H. E., Wilson R. F., Lois T. and David W. F. (1974) An inexpensive titration method for the determination of organic carbon in Recent sediments. J. Sediment. Petrol. 44, 249-253.
Gavrieli I. (in press) Halite deposition in the Dead Sea: 1960-1993. In The Dead Sea (eds T. Neimi, Z. BenAvraham and Y. Gat), Oxford University Press. Gil D., Shirav M. and Halicz L. (1992) Geochemical survey in the southern Judean Desert and the Dead Sea beach. Geol. Surv. Dr. Rep., TG-25-92, 22pp. (in Hebrew). Halicz L., Gavrieli I., McLaren J. W. and Lam W. H. (1994) Trace metal concentrations in the Dead Sea brine-a preliminary study. Geol. Surv. Isr. Curr. Res. 9, l-3. Hall J. K. and Neev D. (1978) The Dead Sea geophysical survey, 19 July-l August 1974. Geol. Surv. Isr. Rep., MG/ l/78,-28 pp. Herut B. (1988) The behaviour of brines at low temperatures. Geol. Surv. Isr. Rep. GSI/29/88. Herut B., Gavrieli I. and Halicz L. (1995) Heavy metals in the sediments of the Dead Sea. Final report, Ministry of Energy and Infrastructure, ES-49-95. Herut B., Horning H., Krom M., Kress N. and Cohen Y. (1993) Trace metals in shaIIow sediments from the Mediterranean coastal region of Israel. Mar. Pollut. Bull. 26,675-682.
Jeffrey P. G. (1975) Chemical Merhodr ofRock Analysis, 3rd edition, Pergamon, Oxford. Katz A., Kolodny Y. and Nissenbaum A. (1977) The geochemical evolution of the Pleistocene Lake Lisan-
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Dead Sea system. Geochim. Cosmochim. Acta 41, 16091626. Levy Y. (1987) The Dead Sea-Hydrographic, geochemical and sedimentological changes during the last 25years (19591984). Geol. Surv. Isr. (in Hebrew), 61 pp. Nathan Y. (1984) The mineralogy and geochemistry of phosphorites. In Phosphate Minerafs (eds J. 0. Nriagu and P. B. Moore), Springer, Berlin, pp. 275-291. Nathan Y., Shoval S. and Sandler A. (1992) Clays in the Dead Sea. Geol. Surv. Dr. Rep., GSI/21/92. Neev D. and Emery K. 0. (1967) The Dead Sea: depositional processes and environments of evaporation. Geol. Surv. Isr. Bull, 41, 147 pp. Nishri A. (1982) The geochemistry of manganese and iron in the Dead Sea. Ph.D. thesis, Weixmann Institute of Science, Rehovot, 149 pp. Nishri A. (1984) The geochemistry of manganese in the Dead Sea. Earth Planet. Sci. Lett. 71, 415-426. Nishri A. and Nissenbaum A. (1993) Formation of manganese oxyhydroxides on the Dead Sea coast by alteration of Mn-enriched carbonates. Hydrobiologia 267, 61-73.
Nishri A. and Stiller M. (1984) Iron in the Dead Sea. Earth Planet. Sci. Lett. 71, 405-414.
Nissenbaum A. (1974) Trace elements in Dead Sea sediments, Israel. J. Earth Sci. 23, 11l-l 16. Nissenbaum A. (1977) Minor and trace elements in the Dead Sea water. Chem. Geol. 19. 99-l 11. Nissenbaum A., Stiller M. and Nishri A. (1990) Nutrients in pore waters from the Dead Sea. Hydrobiologia 197, 8389. Salomons W. and Forstner U. (1984) Metals in the Hydrocycle. Springer, Berlin, 349 pp. Sass E. and Katz A. (1982) The origin of platform dolomites: new evidence. Am. J. Sci. 281, 1184-1213. Stiller M. and Sian L. fl990) Heaw metals in the Dead sea and their coprecipimtion ‘with halite. Hydrobiologia 197, 23-33. Stiller M., Gat J. R. and Kaushansky P. (in press) Halite precipitation and sediment deposition as measured in sediment traps deployed in the Dead Sea: 1981-1983. In The Dead Sea (eds T. Neimi, Z. Ben-Avraham and Y. Gat), Oxford University Press. Walkley A. (1947) A critical examination of a rapid method for determining organic carbon in soils-effect of variations in digestion conditions and of inorganic soil constituents. Soil Sci. 63, 251-264.
Pergamon
Vol. 12,pp. 497-505,
1997 1997 Elsevier Science Ltd All rights reserved.Printed in Great Britain 088&2927/97$17.00+ 0.00
0
Pn: S08&2927(97)0002%0
Sources and distribution of trace and minor elements in the western Dead Sea surface sediments Barak Herut* National Institute of Oceanography, Israel Oceanographic and Limnological Research, Haifa 31080, Israel
and Ittai Gavrieli and Ludwik Halicz Geological Survey of Israel, Jerusalem 95501, Israel (Received 5 July 1996; accepted in revisedform 1 February 1997)
Abstract-Twenty Dead Sea surface sediment samples were analyzed for their major, minor and trace element compositions. The samples represent muddy sediments along the western parts of the lake, from water depths of 8-250 m. These sediments were deposited after 1983, under oxic conditions, following the overturn of the water column in 1979, which ended about 300 years of meromictic stratification with an anoxic lower water mass. The changes in their metal concentrations are discussed in view of the different brine oxidation state. The sediments consist of detrital minerals-carbonates, quartz and clays and authigenic mineralsaragonite, halite and traces of gypsum. Calculations indicate that all mud samples contain more than 3.6% authigenic aragonite, which was found to precipitate preferentially in near shore sediments. An increase in Ca and a decrease. in Al concentrations with decreasing water depths and in a transect from N to S were observed. These are attributed to differential settling of detritus and authigenic carbonates close to the shore and fine Al-silicates in the deep waters, and to the southward decrease in the contribution of clay minerals, mostly derived from the Jordan river. Fe, Ce, Be and Eu were found to exhibit conservative behavior with respect to Al during the transition from stream sediments in the drainage basin to lake sediments. When compared to normal marine sediments, the Dead Sea sediments have similar Cu, Ni, Zn, Be, Ce and Eu concentrations, whereas Cd is enriched by nearly 1 order of magnitude. A good correlation exists between Cd and P, suggesting that the Cd enrichment arises from outcrops of Cd-rich phosphate rocks that are found in the Dead Sea basin. The somewhat depleted Pb concentrations in the lake muddy sediments might be explained by the somewhat high Pb concentrations (normalized to salinity) in the Dead Sea water column, as compared to seawater and by its removal, mainly through halite precipitation. The unusual distribution of Mn concentrations in the
surface sediments and its association with authigenic aragonite imply, as has already been suggested, that Mn co-precipitates with aragonite. 0 1997Elsevier Science Ltd
INTRODUCTION
The Dead Sea is the lowest natural surface on earth (410 m below MSL), located in the deepest (730 m below MSL) romb-shaped graben in the Dead Sea rift valley (Garfunkel, 1981). This terminal lake is characterized by a unique brine composition (Ca-chloride brine), a high ionic strength (> 9), and a relatively low pH (-6.5). Because of the increased catchment of water from its drainage basin (40000 km2) over the last decades, the lake has experienced a negative water balance. This led to a drop in water level, which, over the last decade, has averaged 70 cm/a (Anati and Shasha, 1989). In 1979, the negative water balance resulted in an overturn of the Dead Sea water column (Beyth, 1980; Steinhorn and Gat, 1983), which, until then, was stratified with an anoxic lower water body at depths below 40 m (Neev and Emery, 1967). At present, the Dead Sea is saturated to over-
*To whom correspondence should be addressed: Tel.: 9724-851-5202; Fax: 972-4-851-1911; e-mail: [email protected]
saturated with respect to aragonite, gypsum and halite. Massive halite precipitation began in 1982, forming a modern halite layer several tens of centimeters thick, interbeded with detritus material (Gavrieli, 1996). The latter consists mainly of clays, carbonates and quartz (Neev and Emery, 1967; Garber, 1980; Levy, 1987), which reach the lake in flood events or are carried into the lake as airborne particles. The clay minerals are homogeneously distributed within the Iake and are representative of the detritus carried into the lake by the surrounding rivers (Nathan et al., 1992). Based on experimental and field studies, Nathan et al. (1992) established that clay minerals experienced no diagenesis over a time period of up to 16 000 a and thus maintain their original mineralogy, which consists mainly of kaolinite and illite/smectite in similar amounts. This behavior, which is different from that in other saline lakes, was attributed to the relatively low pH of the Dead Sea brine. Early diagenetic reactions within the sediments involve decomposition of organic matter and sediment-water interaction (Nissenbaum et al., 1990).
497
B. Herut et al.
498
These involve aragonite precipitation and organic matter mineralization, which is responsible for the release of nutrients into the pore water. Bacterial SO, reduction leads to the formation of Fe sulfides and gypsum dissolution (Nishri, 1982). Manganese diffusing out of the bottom sediments was suggested to be the most important source of dissolved Mn in the Dead Sea (Nishri, 1984). The trace metal content of the Dead Sea sediments was studied by Nissenbaum (1974), who concluded that no special enrichment of metals existed in the Dead Sea sediments. Most of the above studies were conducted while the Dead Sea was stratified with an anoxic lower water body at depths below 40 m. The overturn in 1979 led, among other phenomena, to the penetration of 02 into the entire water mass and resulted in a dramatic decrease in the concentration of dissolved Fe2+, from in 1978 to its total disappearance upto9Smmolkg-’ from the water column after the 1979 overturn (Nishri and Stiller, 1984). A decrease in the concentrations of Cd and Pb in the Dead Sea brine was observed between 1980 and 1985. This phenomenon was attributed to co-precipitation with halite, which began to precipitate in 1983 (Stiller and Sigg, 1990; Herut et al., 1995; Stiller et al., 1996). The present study was carried out in view of the changes that the Dead Sea has undergone over the last two decades. It was intended to determine the concentrations of some elements in the Dead Sea surface muddy sediments that were deposited after the 1979 overturn and to examine the factors that dictate their concentrations. Measurements of some major elements were used to normalize and to examine the trace and minor elements’ behavior. Finally, the distribution patterns of some trace metals in the surface sediments of the western parts of the Dead Sea, including the river outlets, are presented.
METHODS AND SAMPLES
that these sediments had been deposited since 1983. On board, whenever possible, halite and mud layers were cut apart and sampled separately. Mud samples were transferred to pre-cleaned 250-ml centrifuge vessels into which Ar gas was introduced to prevent oxidation.
Mud and mixed halite and mud samples were re-suspended in deionized distilled water (DDW) and centrifuged in order to dissolve salts and remove interstitial brines. The DDW/ sediment volume ratio was about 1:2, and thus only limited dissolution of the less soluble authigenic salts (gypsum and aragonite) was possible ( < 0.1% weight). In order to evaluate the effect of the above procedure, several unwashed samples, sampled in the earlier stages of the study, were analyzed as well. All washed samples were frozen and lyophilized for 48 h. XRD analyses were carried out on selected washed
samples with a Philips diffractometer using a Cu tube operating at 35 kV and 40 mA at a scanning speed of 1.5”[email protected]’ . Kb radiation was filtered out by a Ni filter. Organic C concentrations were determined on l-2 g of dry sediment sample by the potassium dichromate method following the procedure of Walkley (1947) and Gaudette er al. (1974). Calcium, Mg, P, S, Sr, Pb, Cd, Zn, Cu, Ni, Be, Ce and Eu determinations were carried out on l-2 g of dry sediment digested with concentrated HNOs (65 wt%) for 3 h at 140°C inUnisea1, Teflon-lined, high pressure decomposition vessels (Hemt et al.. 1993). The digests were diluted to 25 ml in volumetric flasks with DDWand filtered through Whatman no. 2 filter paper. Aluminium, Fe and Mn concentrations were determined by total digestion of 0.2 g of dry sediments with HF, diluted to 1OOml and treated following the procedure of Jeffrey (1975). Each sample was digested and analyzed in duplicate. Lead., Cd., Zn., Cu. Ni. Mn. Al and Fe were determined using an IL-951 or a Perkin Elmer 1lOOB Flame Atomic Absorption Spectrometer (FAAS). Calcium, Mg, P, S, Sr, Be, Ck and Eu were determined on a JY-48 Inductively Couuled Plasma-Atomic Emission Snectrometer (ICPAESj. The accuracy and precision of the methods are estimated on the basis of analyses of Estuarine Sediment 1646 (NIST) Standard Reference Material for which near complete recoveries were found for all certified metals, except Ni (83%) and Mn (90%) (Table 1).
Sampling Surface sediment samples were collected during a 2-day cruise. on 22-23 Auzust 1994. on the R/V Tiolit. from the northern to the southern parts of the western Dead Sea, at
water depths ranging from 8 to 250 m (Fig. 1). The samples were collected with a stainless steel Van Veen grab sampler, from which only the top sediments, not deeper than 3 cm, were collected by lifting the screens of the grab. All the mud samples were collected from above the modem halite layer or from detritus material interbeded within this layer, implying
RESULTS
Transparent coarse halite aggregates commonly formed the surface sediments in the deeper ( > 50 m) parts of the lake. Often, the aggregates were partly sunken in brown sediment. XRD analyses of representative samples indicate that the mud consists of Al-
Table 1. Concentrations determined in Standard Reference Material Standard Reference Material Estuarinesediment 1546 (NIST) *HF digestion.
Value
Al* %
Fe* %
Cd ppm
Pb ppm
cu ppm
Zn ppm
Ni ppm
Mn* ppm
Certified Found
6.25f0.2 6.01 iO.02
3.35kO.l 3.18k0.0
0.36kO.07 0.33kO.02
28.2k1.8 28kO.2
1853 16.4kO.2
138*6 125*1
32*3 26.4kO.S
375_+20 338
Trace and minor elements in the western Dead Sea surface sediments
I
499
Massada Fig. 1. Dead Sea bathymetric map (after Hall and Neev, 1978)with location of sampling stations (numbers).
brines (Gavrieli, 1996) which are introduced into the lake at its southern shore (Epstein et al., 1975). The chemical compositions of 20 suficial Dead Sea mud sediment samples are presented in Table 2. In general, the heavy metal concentrations are similar to those found in most shelf and deep-sea ocean sediments (Salomons and Forstner, 1984). However, Cd is enriched by about an order of magnitude, and the concentrations of Pb are somewhat lower than in most deep-sea ocean sediments. At the mouth of the Kidron river, through which partially treated sewage is introduced into the lake, the sediments are relatively enriched by organic C, Cd, Pb, Cu, and Zn (Table 2). The concentrations of Al (Fig. 2) and Fe were found to increase with water depth, and thus distance from the shore, a trend similar to that found in sediments from the Mediterranean coastal region of Israel (Herut et al., 1993) and in other normal coastal
silicate minerals (mainly clays), calcite, dolomite, quartz and sometimes aragonite. Laminated aragonite sediments were encountered only at shallow depths, close to the shore where fresh to brackish waters emerge as springs. No mud sediments were collected at some of the deep sites where the surface was composed of coarse halite layer with no mud on top, and preventing the grab from penetrating to below the surface. In shallow waters (< 50 m), the surface sediments consisted of brown mud, which commonly contained a few idiomorphic single halite crystals, 0.5-l cm in size. In the southern parts of the lake, this mud was interbedded with fine to medium grained halite (hundreds of microns to a few millimeters in size) which formed layers l-10 cm thick. Several studies have indicated that this halite is the product of the outsalting that occurs upon the mixing between the Dead Sea brine and the industrial end-
Table 2. Chemical composition of twenty surface sediments from the Dead Sea Percentage dry weight
ppm dry weight
Sample Water depth(m) CaO MgO P205 SO3 A&O3 Fez03 Org. C 11 12 13 14 17 18 19 20 22 23 24 25 26 27 28 29 30 31 32 33
100 30 8 12 200 55 10 140 8 250 220 15 12 65 185 50 10 250 110 45
nd: not determined.
28.93 29.26 30.10 22.97 27.63 38.07 31.89 23.25 34.33 25.15 23.86 26.29 33.79 25.89 20.63 22.06 23.09 20.82 20.46 20.99
2.81 3.43 3.66 2.55 2.05 2.86 4.31 2.21 2.61 2.31 2.36 2.59 1.90 2.37 2.05 2.41 2.38 1.91 1.77 1.83
0.40 0.29 0.26 0.28 0.68 0.95 0.40 0.55 0.91 0.68 0.55 0.82 1.63 0.79 0.36 0.36 0.38 0.41 0.35 0.24
1.74 2.57 1.18 0.57 0.55 0.82 2.15 0.83 4.57 0.82 0.69 1.23 1.63 0.92 0.60 0.72 0.88 0.54 0.47 0.49
5.86 5.63 5.27 6.69 4.63 2.32 3.72 7.41 3.33 7.50 7.82 5.82 3.25 5.50 7.90 7.24 6.41 8.45 8.16 7.61
3.02 3.33 2.95 3.60 2.50 1.33 2.07 3.87 1.73 4.10 4.70 3.56 2.09 3.30 4.66 4.46 4.05 5.40 5.06 4.82
Sr
Mn
Pb Cd Cu Ni Zn
Be Ce Eu
0.77 1386 638 11.0 1.0 22 45 78 1.22 38 1.21 0.89 1804 905 8.4 1.0 19 40 66 1.31 41 1.03 0.78 2390 1332 7.3 0.6 20 41 63 0.99 37 0.94 615 396 10.4 0.8 19 37 65 1.08 43 1.28 0.65 365 208 7.6 1.5 16 36 72 0.84 34 0.99 1.03 0.48 442 133 6.0 2.9 12 24 52 0.41 23 0.74 0.80 2904 630 13.8 2.3 24 59 103 1.02 29 0.72 nd 660 439 12.8 1.6 21 54 98 1.48 51 1.49 0.88 2052 665 7.9 3.4 18 44 78 0.79 31 0.70 nd 725 426 12.7 1.7 25 58 106 1.66 51 1.48 nd 580 396 13.5 1.3 23 55 94 1.69 54 1.49 1.42 1005 524 10.5 1.8 23 52 94 1.25 41 1.22 1.54 1025 485 14.4 4.3 32 53 172 0.82 31 0.97 nd 882 513 11.5 2.5 27 55 100 1.20 40 1.19 0.70 549 444 9.5 1.4 21 53 88 1.48 44 1.30 0.83 820 567 9.8 1.5 25 61 94 1.46 46 1.51 0.64 955 642 9.6 1.5 25 58 89 1.14 38 1.35 0.46 475 431 12.5 0.8 24 67 99 1.66 51 1.71 0.53 377 359 12.7 1.4 23 52 91 1.62 49 1.49 0.50 485 500 8.5 1.0 25 57 90 1.30 48 1.53
B. Herut ef al. DEAD
SEA-AI
(% dry wt.)
Fig. 2. Concentration distribution maps of (a) Al and (b) Cd in the surface sediments of the western Dead Sea.
sediments. The Al concentrations in sediments from the Jordan river delta (stations 30, 32 and 33), at the northern part of the lake, are similar to those found in the central deeper parts of the lake (stations 23,24,28 and 31). An opposite trend was found in the distribution patterns of Ca, Cd (Fig. 2), Pb and Mn, i.e. their concentrations decrease with increasing water depth or distance from the shore. No specific trend between organic C and depth was found.
DISCUSSION The common practice used to identify the changes in trace metal content in sediments as they are transported from the drainage basin to their deposition basin is by normalizing their concentrations to a recognized conservative element, such as Al in aluminum-silicates. A non-conservative behavior may exist because of water-sediment interactions or as a result of anthropogenic impacts. In the present study, the scatter plots of Fe, Ce, Be, Eu, vs Al concentrations fall within the scatter of the linear correlations found in 448 stream sediments collected from parts of the Dead Sea drainage basin (Fig. 3, Gil et al., 1992). This similarity attests to their common origin (Al-silicates) and conservative behavior in the Dead Sea. Unlike the situation in many marine and stream sediments (Herut et al., 1993), no correlation
exists between Al or Fe with Ni, Cu, Zn, Cd and Pb in the stream sediments of the western Dead Sea (Gil et al., 1992); nor were such correlations found in the Dead Sea sediments. Furthermore, no correlation exists between the trace metal concentrations and organic C. The correlation in the stream sediments between Mn and Al (r = 0.84) is not maintained in the Dead Sea sediments, implying a non-conservative behavior of Mn, as will be discussed hereafter. A negative correlation between the AlzOs and CaO concentrations (Fig. 4) and a general trend of decreasing CaO concentrations and increasing AlzOs concentrations with water depths (Fig. 2) were observed. These are explained by the differential depositions of detritus carbonates close to the shore, and fine Al-silicates in the deep waters. Additional Ca is deposited in the form of authigenic aragonite, which, as will be discussed later, preferentially crystallizes and settles in shallow waters. A trend of increasing CaO and decreasing Al203 concentrations was also observed in transacts from N to S at water depths close to 50 and between 100 and 250 m (Fig. 5). This is probably due to the southward decrease in the contribution of the fine river-borne clay minerals, mostly derived from the Jordan river. Cadmium was found to be enriched in the Dead Sea sediments by nearly 1 order of magnitude as compared to normal marine sediments (Salomons and Forstner, 1984) and it has a distribution pattern of decreasing
501
Trace and minor elements in the western Dead Sea surface sediments 40
24 2
0
8
4 6 A1203 (%)
20 0
2
4
6
8
A1203 (%) Fig. 4. CaO vs AlzOs in surface sediments of the Dead Sea. The regression line calculated from the data is included.
OO
2
4 6 AI203 (%)
8
10
2
4
8
10
60 50
40 2 2 -
30
5 20 10
/ 0
0
AI203
6 (%)
Fig. 3. FesOs, Be and Ce vs AlsOs in surface sediments of the Dead Sea. The regression lines and the 99% confidence bands of 448 stream sediments from the Dead Sea drainage basin are included.
concentration with increasing water depth (Fig. 2). This trend cannot be attributed to association with organic material, as is often the case (Salomons and Forstner, 1984), since no such correlation was found (Table 2). These observations, however, are attributed to Cd-rich phosphate detritus, which is derived from phosphate outcrops in the drainage basin of the Dead Sea. The common origin of Cd and P (as P205) is
suggested by their positive linear correlation (r = 0.89, P
B. Herut et al.
502
9 about 50 m
2
20
N
Southward (not to’ scale) ------->
S
Fig. 5. North-south transects of CaO and A1203concentrations at water depths close to 50 and above 100 m. Note the increase of CaO and decrease of AlaOs concentrations southward, with increasing distance from the Jordan river mouth.
Trace metals in authigenic minerals Since the sediments were washed prior to their analysis to remove their salt contents, halite is excluded from the following discussion. In view of the small water/mud ratio in the washing procedure, the dissolution of the other authigenic minerals, gypsum and aragonite, was limited. Thus, the main authigenic minerals to be considered are aragonite and gypsum. The significant Sr enrichment in the Dead Sea sediments as compared to normal marine sediments is attributed to carbonate minerals and authigenic gypsum, each of which is characterized by a typical
Sr/Ca ratio. Taking the Sr distribution coefficient between gypsum and brine to be 0.19 (Butler, 1973), a Sr/Ca weight ratio of 0.0188 in the Dead Sea brine (Herut, 1988), and assuming that all the sulfur in the sediment is present as gypsum, the mass of Sr present in the sediment in the form of gypsum can be determined: Srs = 0.19 x 0.0188 x Ca,,
40 ,...,.,,...,. .............. _..................
$ c
? E
.,.,
0
32
; a.. %
? r
5
24
t
,. ,,. ,... ,.,
!I a
_..,_...,
4 ;
z
3
.
16
... ... . ... . . . . .
3
2 2
(1)
%
I
.._....... .. .... ..
..
_.. ... .... .. .. ...cl _ _...
f
2 I
0'
Dead Sea
drainage basin stream sediments
I
OL 0
0.4
0.8
1.2
1.6
P,O,(%)
Fig. 6. Cd vs PsOs in surface sediments of the Dead Sea. The regression line of 448 stream sediments from the Dead Sea drainage basin is included.
I
Dead Sea
surface sediments
Fig. 7. Box plots of Pb/Al (wt/wt) in (a) surface sediments of the Dead Sea and (b) 448 stream sediments of the Dead Sea drainage basin. The bottom and the top edge of each box are located at the sample 25 and 75 percentiles. The center horizontal line is drawn at the sample median. The central vertical lines (whiskers) extend from the box as far as the data extend to a distance of at most 1.5 interquartile ranges. Only values more extreme than this are marked with a zero.
Trace and minor elements in the western Dead Sea surface sediments
503 0
400
0
0.2
0.4
0.8
0.6
1
Fig. 8. Plot of Sr/Ca in the sediments vs Ca,,&a (the fraction of Ca derived from authigenic aragonite, see eqn. (2) in text). The line represents a mixing between two end members: (1) authigenic aragonite with -7800 ppm Sr (Katz et al., 1977) and (2) detritic carbonates with -200 ppm Sr (Sass and Katz, 1982). The data points are plotted, based on their Sr/Ca ratios.
Ca, is the weight of Ca in the gypsum, calculated from the S content. Applying eqn. (1) it was determined that Sr in authigenic gypsum accounts for no more than 4% in all samples, and no more than 2% in most, of the Sr found in the sediments, implying that most of the Sr is concentrated in the carbonate minerals. Indeed, the average Sr/S ratio of all our samples (0.21 dry wt/dry wt) is about two orders of magnitude higher than the calculated Sr/S ratio in authigenic gypsum (about 0.005 dry wt/dry wt). It is assumed, therefore, that the Sr concentrations in the sediments are mostly determined by the relative contribution of aragonite and detrital carbonates. These can be estimated by a mixing model (Faure, 1986): where
(Sr/Ca), = [Srd + (1
-f)srdcl/[%f
+
(1
-flcadc]9
(2) where the subscripts ‘s’, ‘a’ and ‘dc’ designate sediment, aragonite and detrital carbonate, respectively, and f the aragonite fraction (f= Ca,/(Ca, + Cad&
Fig. 8 is a plot of eqn. (2) that describes a mixing line between two end-members where the x-axis is the aragonite fraction v) and the y-axis is the Sr/Ca in the sediment ((Sr/Ca),). The two end members are: (1) detritus with no aragonite cf=O), which contains w 200 ppm Sr (Sass and Katz, 1982); and (2) Sr-enriched authigenic aragonite (f= l), which contains _ 7800 ppm Sr. The latter value was calculated from the distribution coefficient of Sr between aragonite and Dead Sea brine (-0.9) and is close to the concentrations found in aragonite laminae from the lake Lisan sediments (Katz et al., 1977), which was the Dead Sea precursor.
Ca,,, (s) Fig. 9. Mn vs calculated %Ca., (the percentage of Ca bound to authigenic aragonite) in surface sediments of the Dead Sea. The regression line calculated from the data is included.
The data points from the present study are plotted on the mixing line according to their Sr/Ca ratio. The contribution of authigenic aragonite to the Ca budget can thus be determined, and it ranges from 6% (station 18) to 64% (station 19). The va’lues correspond to aragonite concentrations of 4.2 and 36.2% dry wt, respectively. Similar calculations for all the surface sediment samples indicate that aragonite content ranges between 3.6 and 36.2% dry wt. Thus, although aragonite was detected by XRD only in the shallow water samples that contain relatively high concentrations of aragonite, it can be concluded that all samples contain some authigenic aragonite. A correlation was found between the Mn concentration and the calculated authigenic fraction of Ca (Fig. 9). No such correlation was found for any other heavy metal. This suggests that Mn is removed from the Dead Sea brine by coprecipitation with aragonite. Such a mechanism has been suggested by Garber et al. (1980) and Nishri and Nissenbaum (1993), following the finding of Mn-rich aragonite in near shore sediments. Our finding of the unusual Mn distribution of increased concentrations close to the shore, and the preferential aragonite precipitation close to the shore line, further supports this removal mechanism. This Mn co-precipitation explains why no positive linear correlation etists between Mn and Al in the Dead Sea sediments, in contrast to the correlation found in the stream sediments of the Dead Sea drainage basin (r = 0.84).
Trace element concentrations in surface sediments under anoxic (pre-1979) and oxic (recent) conditions The oxygenation of the lower water mass after the overturn in 1979 resulted in a gradual depletion of Fe
B. Herut et al.
504
Table 3. Average concentration of metals in surface muddy sediments during anoxic and oxic conditions in the Dead Sea (ppm dry wt)
Year Depth Number of stations Fe Sr Mn Pb Cd cu Ni Zn
Present studyoxic water column
Nissenbaum (1974)anoxic lower water mass
1994 100-250
1972 195-300 2 16300~3000 1186*590 1576*938 10*3 12.5kO.5 32+11 44.5* 1.5 5Ok2
a
29 lOOk6994 640 f 327 418;118 11.5+2 1.34kO.3 21.9+2.8 52.5k9.1 90.8k11.3
sulfides and Fe oxy-hydroxides (Nishri and Stiller, 1984), whereas the content of dissolved Mn has remained constant (Nishri, 1984). The massive halite precipitation that began in 1983 removed large amounts of Cd and Pb from the water column (Stiller and Sigg, 1990; Herut et al., 1995). A comparison of the average concentrations of some trace elements in surface sediments under pre-1979 reducing conditions (Nissenbaum, 1974) and post-1979 oxic conditions (this study) is presented in Table 3. A decrease of about 1 order of magnitude is observed in the Cd concentrations between 1972 and 1994. The higher Cd concentrations in 1972 are attributed to CdS, which precipitated under reducing conditions (Nissenbaum, 1974) that no longer exist in the Dead Sea. Furthermore, massive halite precipitation that began in 1983 significantly scavenged dissolved Cd from the water column (Stiller and Sigg, 1990; Herut et al., 1995). The increased Fe concentration by about a factor of 2 in the surface sediments collected in 1994 is attributed to its precipitation from the water column as Fe-oxide coating particles. This situation did not exist in 1972, when the dominant Fe species in the anoxic water column was dissolved Fe*+, which decreased drastically after the oxidation of the whole water column (Nishri and Stiller, 1984). The removal of Mn through its oxidized species (MnOOH or Mn02) was probably minor as observed by its constant dissolved content (Nishri, 1984). The lower Mn concentration in 1994 may be due to the decrease in aragonite precipitation and the associated co-precipitated Mn. Such a decrease may also explain the lower Sr concentrations. The increase in Zn concentrations is hard to explain and needs to be further studied.
CONCLUSIONS (3) Iron, Ce, Be and Eu have a conservative behavior with respect to Al in the Dead Sea drainage basin and lake sediments.
(4) The concentration ranges of Cu, Zn and Ni in the Dead Sea surface sediments are similar to those in the stream sediments of its drainage basin and other marine sediments. (5) Cadmium-rich detritus from the phosphate rocks in the Dead Sea drainage basin is the main source of Cd in the sediments, and leads to Cd concentrations of nearly 1 order of magnitude greater than in normal marine sediments. (6) The concentrations of Pb in the Dead Sea surface sediments are somewhat lower than in the surrounding stream sediments, even after normalization to Al. The mechanism that leads to this depletion is still to be studied. However, it may account for the relatively high Pb concentration in the Dead Sea brine and to its removal mainly by coprecipitation with halite. (7) All the Dead Sea surface sediment samples contained more than 3.5% dry wt of authigenic aragonite, which was found to precipitate preferentially in near shore sediments. (8) The unusual distribution of Mn concentrations in the surface Dead Sea sediments is mainly attributed to its co-precipitation with aragonite. Acknowledgements-This work was funded by the Israeli Ministry of Energy and Infrastructure, Grant 94-17-007. The authors are grateful to captain Moti Gonen and his crew on the R/V Tiolit, without whom this study could not have been carried out. Editorial Handling: Dr Olle Selinus.
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