rhenium in the black sea: comparison with molybdenum and...

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EPSL Earth and Planetary Science Letters 131 (1995) 1-15

Rhenium in the Black Sea: comparison with molybdenum and uranium

Debra Colodner a, John Edmond b, Ed Boyle b

a Lamont-Doherty Earth Observatory of Columbia University, NCL-104, Palisades, NY 10964, USA

b Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, E34-201, Cambridge, MA 02139,

USA

Received 26 August 1994; accepted after revision 13 January 1995

Abstract

Rhenium concentrations were measured in the Black Sea to determine the behavior of this redox-sensitive element in an anoxic basin. Under reducing conditions, Re is removed from solution, leading to its depletion from Black Sea deep waters. Compared with uranium, water column Re and MO profiles show evidence of removal from mid-depths, suggesting that these elements are scavenged either in the water column or slope sediments to a greater degree than U. Black Sea surface waters are enriched in Re, reflecting Re in rivers draining into the northern Black Sea that are enhanced up to 80-fold over other large world rivers. This enrichment is most likely anthropogenic and may be due to coal burning in the region. A simple two-box model is used to demonstrate that Re concentrations in the Black Sea are not in steady state with respect to current riverine inputs and authigenic fluxes. Riverine inputs of U and MO are not as significant, perhaps reflecting the fractionation of Re from the other elements during a high-temperature combustion process (Re,O, is volatile above 270°C).

1. Introduction

Rhenium, a rare element in the earth’s crust, has earned the scrutiny of geochemists due to its applicable redox behavior and radiochemistry. The Black Sea is the largest anoxic basin in the world, and is commonly considered an archetype in geochemical studies of anoxic waters and sediments. This first study of Re in an anoxic basin was therefore undertaken in this sea. Early studies of Re geochemistry discovered its strong enrichment in organic-rich shales [ll and more recent work has determined that this enrichment occurs through Re reduction and removal from

anoxic (and perhaps suboxic) pore waters [2]. The affinity of Re for reducing sediments may be used to study the redox environment in which sediments were deposited. In addition, lS7Re is unsta- ble and decays with a halflife of 46 x lo9 yr to lg70s. This gives rise to natural variation in OS isotopic ratios, and provides a new geochronome- ter [3] and geochemical tracer of long-term global change [4,5]. Weathering of Re-enriched black shales, with their high lg70s/ lg60s ratios, may play an important role in controlling the OS isotopic composition of seawater [6]. This study of Re in the Black Sea was undertaken to better understand the mechanism of its removal from

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2 D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) l-15

solution under anoxic conditions. Profiles of dis- solved Re in the Black Sea have been reported by us previously [2], but were not discussed in detail.

In seawater, Re occurs as the unreactive per- rhenate anion (ReO;) that attains high concentrations (40 pmol/kg ‘1 relative to other trace elements of similar crustal abundance [2,7,8]. Its residence time with respect to river inflow has been estimated to be 750,000 yr [2]. In organic-rich sediments, Re is reduced and removed from pore waters, with anoxic sediments comprising about 40% of the sink for Re in the oceans [2]. Suboxic sediments and hydrothermal circulation most likely account for the remaining 60% of Re removal [9]. (Here, ‘anoxic’ refers to sediments with no oxygen which contain sulfide, whereas ‘suboxic’ sediments contain no oxygen and no sulfide. ‘Reducing’ is used more generally to refer to both anoxic and suboxic sediments.) The marine geochemistry of Re is most similar to that displayed by molybdenum and uranium. MO and U are present in seawater as a stable oxyanion (Moo:-) and oxyanionic complex 0_JO,(CO&->, have long residence times (N 780 and N 500 ky respectively), and show little involvement in biological cycling [lo-121. Like Re, these elements are strongly removed to reducing sediments.

All three elements are added to anoxic sediments principally by diffusion along a redox and concentration gradient from overlying seawater to the depth where the elements are reduced and removed from pore waters [2,10,111. For both MO and U it appears that removal from solution occurs only at or below the sediment-water interface, even in anoxic basins such as the Black Sea [10,13,14]. One goal of this study is to determine whether this is true for Re as well. In addition, comparison of Re, MO and U in the Black Sea may reveal differences in their geochemistries (e.g., removal rates) that are difficult to resolve from sediment pore water profiles.

1 Colodner et al. [2] reported a discrepancy with Anbar et al. [8] for the concentration of Re in seawater. Subsequent work has revealed that the data of Colodner et al. were in error by + 10% due to miscalibration of the “‘Re spike and that the value of Anbar (39.8 + 0.2 pmol/kg) is correct.

2. Geographic Setting

The Black Sea and the major rivers supplying fresh water to the basin are depicted in Fig. 1. The Black Sea is permanently devoid of oxygen below about 100 m due to sluggish renewal of deep waters. Stable density stratification arises from the excess of fresh water inputs over evapo- rative losses, and the inflow of saline Mediter- ranean waters. Exchange with the Mediterranean occurs via the Sea of Marmara and is restricted by the narrow Dardanelles and Bosporus Straits. Warm, saline waters (- 38%0) from the Sea of Marmara spill over the Bosporus sill and entrain colder, less saline waters from intermediate depths to create Black Sea deep water with a salinity of about 22%0 [15]. While surface waters (< 65 m> are mixed yearly [16], the deep water replacement time is estimated to be between 500 and 2000 yr, with more recent estimates toward the shorter end of this range [17-201. Water column profiles were collected at two stations in the central Black Sea during Cruise 3 of the 1988 joint US/Turkish Black Sea expedition of the R.V. Knorr. Station BS3-2 (42”50’N, 32”OO’E) is located in the center of the western gyre, and BS3-6 (43”04’N, 34”OO’E) is between the western and eastern gyres. A profile in the sea of Mar- mara was collected in August, 1988.

Seven major rivers draining into the northern Black Sea were sampled in the summer of 1990. The Prut, Dnestr, Bug and Dnepr Rivers princi-

46’

42’

I

260 32* 36O 40’

Fig. 1. Map of the Black Sea and surrounding rivers with station locations.

D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) I-15 3

pally drain the low-relief Russian platform. The eastern and western drainages of the Black Sea are dominated by the Kuban and Danube Rivers respectively. The geological feature most important to note for this study is the absence of black shale deposits in descriptions of the area [21]. All of the rivers have been significantly affected by hydroelectric, industrial and agricultural use [ 15 I.

3. Methods

Black Sea water column samples were collected in 30 1 Go-F10 bottles and pressure-filtered

through 0.4 pm Nuclpore filters under nitrogen using stringent trace-metal clean procedures. They were acidified onboard to approximately pH 2 using Ultrex 6N HCl. River waters were filtered through 0.1 pm Nuclepore filters into acid-leached high-density polyethylene bottles and acidified with triple-distilled HCl to pH 2 in the field [22].

Re was determined using isotope dilution inductively coupled plasma mass spectrometry, as detailed in [23]. Briefly, a spike enriched in lssRe was added to between 10 and 30 ml of sample, depending on sample availability. After allowing at least 24 h for spike-sample equilibration the

Table 1 Re in the Black Sea, June, 1988 sd = Standard deviation for duplicate samples; [I = not plotted; a, and salinity from 1171.

Station Sample

2 DH2 2 DHl 2 DH4 2 DH6 2 DH8 2 DH7 2 DH11

; DH13 DH15

2 DH16 2 DH17 2 DH18 2 DH19 2 DH.20 2 DH21 2 DH22 2 DH23 2 DH24

; DH25 DH26

2 DH27 2 DH28

22 G1115 DH29

6 DH35 6 DH36 6 DH32 6 DH33

G1092 DH34

z G1093 DH30

z G1116 DH31

20 30 40 50 60 70 80 90

100 105 110 115 120 130 140 170 200

z 800

1200

lE 2100

13.752 14.348 14.671 14.912 15.452 15.880 16.030 16.108 16.257 16.281 16.276 16.355 16.391 16.440 16.490 16.598 16.690 16.972 17.104 17.171 17.212 17.222 17.223 17.223

26.0 18.465 27.3 18.675 26.9 19.189 25.5 19.992 24.8 20.479 24.5(0.2) 20.727 20.0(0.3) 20.887 23.0 21.031 22.5 21.130 22.2(0.04) 21.2 21.0 22.1 20.6 19.3 18.0 16.0 11.6 8.0 7.8 7.6

21.164 21.200 21.248 21.290 21.384 21.422 21.556 21.652 22.003 22.162

[‘;.;I

[16:1]

22.250 22.310 22.318 22.330 22.330

150 180 500

E.z 1500 1500 2153 2153 2174 2174

16.432 16.546 17.023 17.157 17.157 17.219 17.219 17221 17.221 17.221 17.221

20.8 21.321 21.1 21.416 11.6 8.9

10.9 8.8

::: 8.6

21.962 22.222 22.222 22.313

8.1 9.0

22.313 22.320 22.320 22.320 22.320 6 G1089

6 G1090 2185 17.221 8.5 22.320

309 285 253 111

3.4 2.1 1.4

0.6 4.9 9.8

12.9 14.4 16.9 21.0

4 D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) l-l 5

samples were purified and preconcentrated using anion exchange of ReO, on Biorad AGl X -8

resin. The Re fraction was eluted from the column with 8N HNO,, evaporated to a small drop, and taken up in 0.5 ml of 0.8N HNO,. The sample was introduced to the ICPMS (VG Plas- maquad at MIT) using a flow injection valve placed downstream from a peristaltic pump. The Data were collected by peak jumping on the 185 and 187 peaks for approximately 30 s. The precision of the results is estimated to be 7% for samples taken from the same Go-F10 bottle by different investigators (based on four paired samples from Station 6). Improved precision (1.1%) is obtained for duplicate measurements of the same subsample (three duplicates, Station 2, Table 1).

4. Results

The Re concentrations in the Black Sea are listed in Table 1. At the time these samples were collected the oxic-anoxic interface was marked

Re (pmol/kg)

5 10 15 20 25 30 0 ,“““““““““~_

..o@: :

0 500- 0 . -

0 . _

000

- lOOO- g

S

5 - 0

&500- (33

,

.

2000

1 “@

a 2500 1 I, I,, s 7, r t r,, r 1, I, ‘I I,!

: Re cons. 0 Re meas.

by a suboxic zone of low oxygen and low sulfide from 60 to 100 m. Re concentrations decrease smoothly through the surface waters and this transition zone, to low and constant values below 600 m, with the exception of one point (run in duplicate) at 80 m (Table 1 and Fig. 2a). This point, with Re concentrations below the general trend, is also lower in MO and V [lo], and occurs at one of two Mn particulate maxima [24]. This indicates some uptake of the three elements onto Mn phases which might consist of Mn-oxides or bacterial matter [25,26]. The depletion for Re and MO represents about 15% of the value expected from the adjacent points, with a 20% depletion for V. In both the oxic and anoxic regions Re concentrations covary with salinity (with the exception of the point at 80 m).

Within the Sea of Marmara, Black Sea surface waters flow southward above more saline, north- ward flowing Mediterranean waters. Re concentrations in Marmara surface waters are slightly enriched compared to Black Sea surface waters, but maintain a constant relationship with salinity

MO (nmollkg)

0 10 20 30 40 50 60 70

50;rw]

I i MO cons. 0 MO meas. I

U (nmol/kg)

5 6 7 8 9 10 0 I I , I I I I I I 1 “gii; I I

k 0 : -

500- 0 1 - 0 . _

0

lOOO-

0

1500- 0 :

:

2000 - * -

m” I

c - 2500 ~‘~~,~~,~,~~~~,~~,,I~,,~

: , U cons. 0 U meas.

Fig. 2. Observed Re, MO and U concentrations in the Black Sea water column compared to concentrations one would predict if the elements behaved conservatively in the basins and (i.e., no authigenic removal to sediments). MO data are from [lo] and U data are from [28]. M,,, was calculated by mixing seawater (Re = 40 pmolkg, MO = 105 nmol/kg and U = 14 nmol/kg) and average river water (Re = 2 pmol/kg, MO = 5 nmol/kg and U = 1 nmol/kg) in a proportion determined by the salinity at each depth. All of the elements are depleted from deep water by removal into reducing sediments. Only Re is enriched in surface waters.

D. Colodner et al. /Earth and Planetary Science Letters I31 (1995) 1-15 5

Table 2 Re in the Sea of Marmara (Station 3, August, 1988)

Depth (m) Re (pmovkg) Salinity

5 30.8 21.5 30 43.8 38.6 100 44.6 38.6 450 42.8 38.6 750 43.9 38.6 1200 43.5 38.6

(Re/salinity = 1.4 pmol Re/g salt in the surface waters of both basins (Table 2). High Re concentrations in Marmara deep waters reflect the evap- orative concentration of seawater as it passes through the Mediterranean. The Re/salinity ratio in Marmara deep waters is identical to that of open ocean seawater (1.1 pmol Re/g salt).

All of the rivers draining into the Black Sea which we have analyzed are unexpectedly enriched in Re, with concentrations ranging from 40 to 109 pmol/kg (Table 3). For comparison, seawater contains 40 pmol/kg Re, and global average river water contains 2.0 pmol/kg, based on concentrations in the Amazon, Orinoco and Ganges-Brahmaputra Rivers [2]. (These are the only large rivers in which Re has been measured.) Concentrations similar to those in Black Sea rivers have been observed only in small rivers draining black shales in the Venezuelan Andes [2]. The origin of this enrichment will be discussed below.

5. Discussion

5.1. Rhenium behauior in the anoxic water column

Black Sea deep waters are depleted in Re with respect to concentrations expected from a simple mixture of Black Sea surface water and Marmara deep waters (Fig. 2a). Re has therefore been removed from Black Sea deep waters either by scavenging or precipitation within the anoxic water column and/or within sediment pore waters. The ultimate sink for Re is the sediments, as is evident from their high Re concentrations. Re- cent (Unit 1) Black Sea sediments contain between 20 and 60 ppb Re, with an average value of about 40 ppb Re [27]. For comparison, oxic marine sediments contain < 0.1 ppb Re [2,7].

The relationship between Re and salinity in the deep Black Sea is fairly linear, suggesting that the distribution of Re may be controlled by conservative mixing of high-Re-low-salinity surface waters with low-Re-high-salinity deep waters [2]. However, the potential temperature-salinity relationship in this region is not linear, and a number of processes, such as lateral injection of Bosporus water and double diffusion, have been called on to explain the T-S features [17]. The complex hydrography of the deep Black Sea makes it difficult to determine if Re is conservative in this realm. It is possible that Re is scavenged onto particles within the water column, with its concentration controlled by an equilibrium chemical reaction. The nature of this reaction is not known, but it is not simply precipitation of a Re-sulfide, as sulfide concentrations increase with depth throughout this zone.

In addition to potential removal within the anoxic water column, Re is likely added to sediments by diffusion into sedimentary pore waters, as noted previously for U [13] in the Black Sea. Based on low concentrations of U in sediment trap material [28] it appears that U removal occurs only at or below the sediment-water interface. Compared with U, Re and MO exhibit steeper gradients in the upper water column (Fig. 21, indicating that water column scavenging may be relatively more important for these two elements. This inference contrasts with the results of Francois [14], who showed that MO concentrations in trapped material from the anoxic waters of Saanich Inlet, British Columbia were not enriched. It is possible that differences in temporal and spatial scales as well as chemistry between Saanich Inlet and the Black Sea can account for this discrepancy. Alternatively, mid-depth Black Sea sediments may be a more efficient sink for MO and Re, although an equilibrium process that discriminates between the elements under sul- fidic conditions is not obvious. Further comparison between the three elements is made below.

5.2. Anthropogenic Re in rivers draining into the Black Sea

As noted above, rivers discharging into the northern Black Sea all have very high Re concen-

6 D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) I-15

trations compared to other world rivers. This is most likely due to industrial activity in the river basins. Although similarly high Re concentrations were observed in a localized area of black shale deposits in the Venezuelan Andes, the geology of the Russian platform bares few similarities to the mountains of Venezuela. The relief of the northern Black Sea region is low, resulting in slower weathering rates, and widespread exposures of black shales are not indicated [21]. In addition the rivers are known to carry high levels of many metals introduced anthropogenically, including Cu, V, U and MO [30].

Because Re, U and MO behave coherently in many geological settings most potential sources of Re should furnish the other elements to the environment as well. U and MO are also elevated in Black Sea rivers compared to average river water by a factor of 2-10, although neither element shows the factor of 20-50 enrichment seen for Re (Table 3). The relative enrichment of Re compared to MO and U indicates that the pro- posed anthropogenic source must fractionate the elements. Any high-temperature process with oxygen present could have this effect, as the heptoxide (Re,O,) is volatile above 270°C [311. In comparison, MOO, is the most volatile molybdenum oxide, with a sublimation temperature of 1155”C, and U has no volatile oxides [32]. One possible source is the roasting of molybdenite and copper porphyry ores [33], although these sources are probably too localized to contaminate the entire region with Re. In addition, Re can be recovered in economic quantities from the flue dusts of molybdenite and copper smelters, so

Table 3 Re, U and MO in rivers in the northern Black Sea drainage

efforts are made to prevent loss to the environment. Oil burning is also a potential source of Re, but the Re concentrations of petroleum are not known.

An alternative source of Re contamination is coal combustion. Coal is the source for much of the power in the region, and emissions from power plants and smaller users are generally un- treated. In addition much of the coal burned is a low-grade brown coal, or lignite, with generally higher concentrations of trace elements than higher grade coals [34]. Kuznetsova and Saukov [35] report Re concentrations in Russian brown coals of between 95 and 325 ppb, and Bertine and Goldberg [36] give an average value for Re in lignite and hard coal of 50 ppb. For comparison, the concentration of Re in average crustal rock is about 0.05-0.5 ppb [371. Because of the high mobility of Re in oxic waters any Re released to the environment can be easily leached from ash fallout and carried to the rivers.

Some rough calculations can help to determine whether coal burning is sufficient to produce the high Re concentrations observed in the rivers. In these calculations we will use an average concentration for Re in coals of 50 ppb. According to a United Nations report [38] 8 X 1Or4 g coal were burned annually in the former USSR. Assuming that half of this is burned within the heavily populated Black Sea drainage area, and including coal burned in Eastern Europe which might add Re to the Danube, we estimate that a minimum of 4 x 1014 g coal are burned in the region each year. In a study of MO emissions from a coal-fired power plant equipped with scrubbers and precipi-

Station River Re (pmol/kg) U (nmol/kg) U (nmovkg) MO (m&/W MO WoVkg) :990 1990 [22] 1977 [47] 1960’S [30]

E Dnepr Prut @ @ Reni Kherson

RR; Danube Danube @ Vilkovo Reni @

EO Dnestr @ @ Majaki Odessa Bug Novaja Rll Kuban @ Temruk World Average River Water Seawater

108.6 40.4 3.19 5.38 7.1 :; 17

2.7-9.2 19 72.8 66.6 4.08 3.63

107.6 71.5 13.5 4.97 4.2 :: 62.8 4.17 13.9 ;x

:: 26

2 0 [2] 40’181

0.78-1.3 [12] 5 [461 14 [481 105 [49]

[n] = Reference number.

D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) I-15 7

tators, Kaakinen [39] found that 15% of the MO entering the plant as coal was found in outlet particulate matter. If 15% of the Re from coal combustion in the Black Sea region makes its way into the waterways, and with a river input to the Black Sea of 300 km3/yr [30], about 50 pmol/kg Re is added to these rivers. Given the higher volatility of Re relative to MO and the lack of emission controls in the Black Sea region, 50 pmol/kg is likely to be a minimum value.

A second approach to calculating the potential Re source from coal burning is via estimates of SO, deposition in the region. Amann [40] has collected SO, emission data from various inter- governmental sources which indicate that about 10 X lOi g of sulfur were emitted in the Euro- pean part of the former USSR annually (for the

year 1985). Assuming that half of this comes from coal burning [41], and an average Re/S ratio in coals of 2.5 X lop6 (50 X lop9 g Re/0.02 g S) 1361, up to 200 pmol/kg Re in the rivers could be explained by coal burning. Admittedly these calculations contain many poorly constrained estimates, but they do support the idea that coal combustion may be an important source of Re contamination. Unfortunately measurements of trace elements in Black Sea aerosols are limited and do not include elements which are typical products of coal burning, although the influence of anthropogenic sources is apparent [42]. Other elements which are released by coal combustion, such as As and Se, have not been measured in the Black Sea rivers. Their profiles in the surface Black Sea show no evidence of anthropogenic

f Upw;ell!lg (U)

Bospoms (bi) 176

Deep Box

Fauthigenic

(F4

Fig. 3. Two box-model of the Black Sea. The numbers indicated are water fluxes (Q) in km3/yr [191. Cold surface waters (cold-intermediate layer of [17]) are entrained by warm, salty Mediterranean waters as they flow into the Black Sea through the Bosporus, producing Black Sea deep water. This input is balanced by upwelling. F, is the flux of metals into deep Black Sea sediments by diffusion from overlying water. Mass balances for the surface and deep boxes are as follows:

Surface: Q,c, + QkiCaz + Q,C, = QkoCs + Qb& + Q,c,

Deep: Q,C, + F,, = QbiCb + Q,C,

where C = metal concentration, ‘s’ = surface, ‘d’ = deep, ‘az’ = Azov. ‘b’ = Bosporus and other subscripts are as defined in the figure.

Model inputs: C, and C,, F, (mol y-‘)

Re 80 pmol kg-’ 1.5-7 x 103

MO 20 nmol kg ’ 4.6-15 x lo6

U 4 nmol kg-’ 0.84-1.8 x lo6

8 D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) I-15

enhancement, but unlike Re these elements are rapidly removed by biological and inorganic processes in the upper water column [431.

5.3. Re enrichment in the Black Sea

The influence of contaminated rivers on Re concentrations in the Black Sea is seen most clearly in surface waters. In Fig. 2 Re, MO and U concentrations measured in the Black Sea are compared with the concentrations one would ex- pect if the elements behaved conservatively in the basin (i.e., if they were not removed to sediments). The ‘conservative’ profiles are constructed from a mixture of Marmara deep water and world average river water (based on the Amazon, Orinoco and Ganges-Brahmaputra Rivers, and assumed to represent pre-anthropogenic concentrations; Table 3). The mixing ratio of these two end members is determined from the salinity at each depth. As seen in Fig. 2a Re is enriched in surface waters relative to this conservative, pre- anthropogenic, prediction. Thus, there is more Re in Black Sea surface waters than can be accounted for by riverine concentrations such as

Re pmolikg MO nmol/kg

0 20 40 60 80 100120

"m 0

soo- 0 0

-CD

lOOO-

-0

1500- 0

2000- ."g

those observed in other major rivers, even if there were no removal of Re from the basin.

Because there are no pre-industrial measurements of Re in river waters, direct proof that the rivers have been contaminated by human activity will always be lacking. However, given water renewal times in the deep Black Sea of between 500 and 2000 yr 117-201 an increase in Re inputs due to industry may be observable as an imbalance in the Re budget for the basin. In order to determine whether Black Sea Re concentrations are consistent with a recent increase in riverine Re, a simple two-box model was constructed fol- lowing Falkner et al. [19] (Fig. 3). The model indicates a substantial imbalance in the Re distribution in the Black Sea, as described below.

The model consists of surface and deep boxes separated at the pycnocline. Inflow of salty Mediterranean water through the Bosporus enters the deeper box, entraining cold near-surface waters (the cold-intermediate layer [171) to form Black Sea deep water. Outflow through the Bosporus originates in the surface box. It is assumed that inflow through the Kerch Straits from the Sea of Azov is essentially riverine. Riverine

0 1020304050607080 0

U nmobkg

OW

500 -i

0 0

0 1

zsooi

Fig. 4. Predicted steady-state concentrations for surface and deep waters (ranges indicated by horizontal bars) compared with

measured values (0). Surface and deep water concentrations were calculated using the two-box model, with present-day (1990)

river concentrations and F, fixed. The range in predicted concentrations results from the range in authigenic fluxes (see Table 4).

Present-day authigenic fluxes are not sufficient to produce the depletion of MO and U in deep water with current river inputs. Re is

farthest from steady state, due to very high riverine fluxes of this element.

D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) 1-15

Table 4 Authigenic flux of Re, MO and U to deep Black Sea sediments

Element s &d Fa mg cm** y-l mol y-t

Re min ;:: ;:z;

20 ppb ~271 1500 max 60 ppb 7000

MO min 3.5 33 ppm I501 4.6 x 106 max 5.5 66 PPm 15x106

u min 3.5 15 ppm U31 0.84 x 106 max 5.5 20 mm 1.8 x 106

F,=AXSX{M,,, - Alsed04/A[)shale]r where

A = area of the Black Sea = 3.95 X 10” cm2 [19]

S = sediment accumulation rate

Used = Re, MO or U concentration in mol/g

Al,,, = Al in Black Sea sediments = 5.5 X 10e4 mol g-’ [lo]

@f/A%,a,e = ratio in average shale (Re/Al = 2.4 X 10. lo; MO/AI = 9 x lo-‘+ U/Al = 3.8 x 10-h) [51]

concentrations are taken to be a flux-weighted [45], and Re concentrations in Black Sea sedi- average of the rivers analyzed (80 pmol/kg Re). ments [271 (Table 4). With current riverine inputs These rivers represent 80% of the river input to and removal rates to sediments, predicted the Black Sea. The authigenic flux (F,) of Re to steady-state Re concentrations in both the sur- the sediments was calculated using sediment ac- face and deep Black Sea are much higher than cumulation rates determined by 14C [44] and *l’Pb observed. Using a maximum estimate of authi-

Table 5 Model-predicted steady-state surface and deep water concentrations with current river inputs

Re>mol/kg) MO (nmol/kg) U (nmollkg)

F, - fixed

Surface 90-100 30 - 60 6-9

70 - 80 30 - 70 6-9

FP - variable

Surface 50- 80 10-20 6-8

20-60 5- 10 5-7

k (lo’* L y-l) 200- 1000 1500 - 5000 140 - 350

Observed

Surface 26 40 8.4

8 3 5.1

Fa-fiied: Authigenic flux does not change with changing bottom water concentrations. F,-variable: Authigenic flux is allowed to respond to changing bottom water concentrations through a first-order rate constant k, where F, = k[ M Ideep; k is estimated from calculated authigenic fluxes (Table 4) and 1988 deep water concentrations. For Re, where current deep water concentrations may be contaminated, a range for k is calculated using model-derived pre-anthropogenic concentrations as well as 1988 concentrations. The large range in values predicted by the models results from the range in authigenic accumulation rates calculated in Table 4.

10 D. Colodner el al. /Earth and Planetary Science Letters I31 (1995) I-IS

genie Re removal, steady-state concentrations are calculated to be about 70 pmol/kg Re in deep waters and 90 pmol/kg in surface waters (Table 5 and Fig. 4). It is clear that measured concentrations of 29 pmol/kg in surface waters and 9 pmol/kg in deep waters are not at steady state and will continue to increase as long as riverine concentrations remain high. It is also possible that anthropogenic Re is carried to surface waters through the atmosphere [42], but this additional source is not necessary to explain the observed enrichment.

If Re is added to bottom sediments via diffusion from overlying water or by scavenging onto sinking particles, then as the Re concentration of deep water (Re,) increases F, may also increase. A second version of the two-box model allows F, to vary as a function of Red, such that

F,=kXRe,

A range of values for the first-order rate constant (k) may be estimated using measured sedimentary concentrations (Table 41, and current (8 pmol/kg), or model-derived pre-anthropogenic deep water concentrations (7 pmol/kg, see below). When this is done, steady-state Re concentrations are somewhat lower than predicted by the fixed-);, models, but still much higher than observed (Table 5). Even using a maximum value for k (maximum F,), steady-state deep water concentrations are predicted to be about 20 pmol/kg, and surface waters 50 pmol/kg. The conclusion remains: The distribution of Re in the Black Sea is not at steady state and is consistent with a recent increase in riverine inputs. With current riverine concentrations, over four times as much Re enters the Black Sea through rivers than through the Bosporus Straits.

The predicted evolution of Re concentrations in both surface and deep boxes as a function of

Black Sea evolution (100 years) Re(rivers) = 80 pmollkg

surf.

35

3 30

E 25 a 2 20

15

10 deep

5 1950 1970 1990 2010 2030 2050

year

Fig. 5. Model-predicted increase in Re concentrations in surface and deep waters of the Black Sea over 100 yr. The deep water residence time for this model is about 700 yr, and authigenic fluxes increase in proportion to deep water concentrations. Initial surface and deep concentrations were derived assuming that pre-industrial rivers had similar metal concentrations to world average river water. Riverine concentrations were increased to modern-day levels all at once in year 1, and Black Sea water allowed to evolve with time. Even with this extreme case of a step function increase in riverine inputs, deep water concentrations increase by only 50% over 100 yr. Assuming that most industrialization occurred after World War II (40-50 yr ago), the predicted surface water concentration is 37 pmol/kg. This is a maximum value, as Re inputs certainly did not increase to modem levels in a single year.

D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) l-15 11

time is illustrated in Fig. 5. Pre-anthropogenic riverine concentrations can be estimated from world average river water as approximately 2 pmol/kg Re (Table 3). Using this value to calculate pre-anthropogenic Black Sea surface and deep water concentrations, the model gives initial steady-state values of 7.0 pmol/kg in deep waters and 7.5 pmol/kg in surface waters. This is very similar to the deep water concentration observed today (8 pmol/kg). Assuming that Re river fluxes increased to present levels in a step function at the beginning of regional industrialization after World War II, concentrations predicted for surface and deep waters today are 8 pmol/kg in deep waters and 37 pmol/kg in surface waters. These are maximum estimates, as the increase in Re flux was most likely more gradual. In this model framework steady-state Re concentrations are not reached for at least 600 yr.

concentration in deep waters is lower than the model predicts, even with riverine concentrations set to 0 nmol/kg, and the highest authigenic flux consistent with deep sediment concentrations. This suggests that we may have underestimated the authigenic MO flux by about 20%. There may be an additional sink for MO on the shelves, with authigenic accumulation at substantially higher rates than in abyssal sediments. In either case it appears that any industrial increase in riverine concentrations has not elevated deep water MO concentrations. Surface water MO concentrations are within the ranges predicted by the model for either pre-anthropogenic or current riverine inputs. Inflow through the Bosporus continues to dominate MO in the Black Sea, with twice as much MO entering via this pathway than through rivers.

5.4. Comparison of Re, U and MO

A similar exercise can be pursued for U and MO in the Black Sea. Uranium enrichment in surrounding rivers is less pervasive than that for Re, and the flux-weighted average concentration (4 nmol/kg, [221) is only four times the average concentration in world rivers and well within the range of concentrations seen in many large rivers [12]. As might be expected, U in the Black Sea appears to be balanced within uncertainties (Ta- ble 5). The degree of enrichment for MO in surrounding rivers is also four times its concentration in world average river water (5 nmol/kg, [46]), with a flux-weighted average concentration in Black Sea rivers of 20 nmol/kg. However, its

If authigenic MO burial is not accurately repre- sented by the abyssal sediment data, it is possible that this is a problem for Re as well. A 20% underestimate of the authigenic Re flux does not change the predicted surface and deep water concentrations significantly, however. The lowest predictions continue to be 20 pmol/kg in deep waters and 50 pmol/kg in surface waters. These concentrations are still significantly higher than those measured.

The above calculations suggest that the three elements are removed from Black Sea deep waters at different rates. Although it is possible that Re deep water concentrations are anthropogeni- tally enhanced, MO and U concentrations are probably less disturbed. The MO/U ratio of deep Black Sea water is lower than in any of the sources to the basin (Table 6). Assuming that MO

Table 6 MO/U and Mo/Re ratios in Black Sea sources, water column and sediments

Mo/Re (1OhoVmol)

Average Black Sea rivers

Bosporus inflow (seawater)

Black Sea Deep Water

Average Black Sea sediments [13,27, 501

5 0.25

1.5 2.6

0.6 0.3

7 2.5

12 D. Colodner et al. /Earth and Planetary Science Letters 131 (1995) l-15

and U deep water concentrations represent steady state, one can estimate residence times for the elements with respect to authigenic removal by the two methods detailed in Table 7. In the first method one simply divides the total inventory of the element in the deep water column by its flux to sediments. Because previous calculations indicated that we may be underestimating the authi- genie flux of MO, another estimate of residence time can be made which is independent of authi- genie flux. This calculation uses the depletion of the elements from their predicted conservative (i.e., no authigenic removal) distributions, as illustrated in Fig. 2. Given a range of water renewal times for the basin, one may calculate residence times for the elements. Using either method, residence times estimated for MO are significantly less than those for U.

Based on elemental ratios, a two-box model and simple residence time calculations it appears that MO is removed from the Black Sea at a greater rate than U. This removal rate difference

is most likely explained by kinetic factors, as Black Sea deep waters and sediments are every- where reducing enough to remove both MO and U. As noted above, it is possible that MO is scavenged within the water column to a greater extent than U. Although sediment trap data from the anoxic Saanich Inlet of British Columbia ar- gue against water column removal of MO [14] it is possible the longer residence time of particles in the deeper Black Sea allows more significant scavenging of MO to occur. In contrast, the factors which catalyze U removal from the Black Sea appear to be limited to the sediments [28].

Residence time calculations for Re can be misleading, as deep water concentrations may not be in steady state. Given the slow evolution of deep water concentrations predicted by the model (Fig. 51, and the deep water concentration predicted with pre-anthropogenic riverine inputs (7 pmol/kg), it is likely that Re concentrations have not increased by more than 15% however. With this range of Re concentrations residence times

Table 7 Residence time calculations for Re, MO and U in the Black Sea

Re MO U

Deep water concentration 0 Pmovkg

8 3000 5100 (7)

F, 1.5 - 7 x 103 4.6 - 15 x 106 0.84 - 1.8 x 106 moVY

Deep water conservative concentxation (Mc) pmol!kg

26 68000 9200

Residence time, ZM (y) 200 300 20 - 30 500-1000

method 2 (2&300) I

Methodl: 7~- V = volume of deep Black Sea = 500,000 km3 [ 191 a

Method 2: ZM = 7~ M 7M c d

~w=waterrenewaItime=400-7OOyears[17,19]

M, is the 1988 deep water concentration of each metal. A4, is the metal concentration one would predict for deep water from its salinity, assuming conservative mixing of seawater and average river water, and no authigenic flux. Numbers in braces correspond to model-derived pre-anthropogenic deep water concentrations.


appear to be intermediate between those of MO relatively more Re and MO removal occurring and U (Table 7) (500-2500 yr using method 1 and within the water column and in shelf and slope 200-300 yr using method 2). The relative removal sediments, whereas U removal is relatively re- rates suggested by this study (MO > Re > I-I> will stricted to abyssal sediments. More sluggish re- need to be confirmed in studies of uncontami- duction/adsorption kinetics for U might explain nated basins. this pattern.

Renewal times derived by method 2 are consis- tently lower than those calculated with method 1. This discrepancy would be eliminated if the residence time of Black Sea deep waters were at the higher end of previous estimates, or 2000 yr [20]. This inconsistency between recently published residence times and U removal in the Black Sea was also noted by Anderson et al. [13].

Acknowledgements

We thank James Murray, Bill Landing, Brent Lewis and Erik Brown for careful sample collec- tion and Flip Froelich and Steve Emerson for making their Black Sea samples available to this study. Kelly Falkner and Sarah Herbelin collected samples of the Ukrainian, Eastern Euro- pean and Russian rivers with logistical and scien- tific support from the Institute of Biology of the Southern Seas at Sevastopol. We also thank K. Falkner for making unpublished data available to this study. This paper was improved greatly by the thoughtful reviews of Bob Anderson, Jim Murray and Greg Ravizza. D.C.C. gratefully ac- knowledges support for this work from NSF grant OCE-92-17558 and an NSF graduate fellowship. This is LDEO contribution 5318. [MK]

6. Conclusions

Re, MO and U all display similar behavior in the Black Sea. They decrease from surface to deep waters, and are removed to reducing Black Sea sediments. Re concentrations in Black Sea surface (and possibly deep) waters are anthropogenically enhanced due to high Re loads in the rivers flowing into the northern Black Sea. A potential source for this anthropogenic enrichment is coal combustion, as Re occurs in high concentrations in coals. Riverine inputs of U and MO are not enhanced to the same degree, imply- ing that the anthropogenic source fractionates the three elements, perhaps through a high-temperature process in which Re,O, is volatile (> 270°C). Based on a simple two-box model of the Black Sea, U and MO concentrations are close to steady state, whereas Re concentrations are not.

Assuming that elemental concentrations in the deep Black Sea have not been anthropogenically perturbed, residence times for the three elements may be calculated. This exercise reveals that the authigenic removal rates for the three elements differ, with the highest rates for MO, followed by Re and U. The reasons underlying this difference are not yet clear. Redox conditions in the deep Black Sea water column and Black Sea sediments are more than adequate to reduce all three elements, suggesting that kinetic factors are respon- sible for the varying removal rates. It is possible that element removal is spatially separated, with

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