22. DISTRIBUTION OF BROMINE, Cl/Br RELATIONSHIPS, AND IODINE ININTERSTITIAL WATER OF THE BLACK SEA, BASED ON DSDP LEG 42B

O.V. Shishkina,1 Institute of Oceanology, USSRAcademy of Sciences, Sadovaya, Lublino, Moscow

INTRODUCTION

The purpose of the present work was to study thedistribution of bromine, chloride/bromide ratios, andiodine in interstitial waters of three holes in the BlackSea. The holes were drilled in the course of Leg 42B byD/V Glomar Challenger in May 1975.

Owing to their similar ionic radius, chloride andbromide ions resemble each other in chemical andgeochemical behavior, occurrence, and migrationpatterns in the earth's crust. The larger ionic radius ofiodine influences its pattern of dissemination andresults in low Clarke values. Owing to their solubility,chloride and bromide are largely found in solution,principally in the sea and saline lakes.

Separation of chloride and bromide is achieved as aresult of the greater solubility of bromide than chlorideduring the evaporation of seawater to the point ofprecipitation of NaCl. Bromide remains in solution,being precipitated only at the very last stages ofcrystallization. The precipitate of NaCl contains onlytraces of Br, whereas the residual liquid is enriched inbromide. Consequently, the Cl/Br ratio diverges fromits normal level of about 300 chiefly in two specialcases: (1) as a result of volatilization of NaCl, as occursin meteorites, or leaching of salts having a Cl/Br ratiogreater than 300; (2) in waters mixed with residualbrines rich in bromide with a Cl/Br ratio less than 300.Lesser separation occurs as a result of the differentialsorptive properties, and the greater concentration ofbromine in living matter. These processes lead to greaterconcentrations of bromine in soils, peat, clays enriched withorganic matter, and interstitial waters.

The Cl/Br ratio in the seas and oceans equals 300, ormore precisely 293. It remains constant (Vinogradov,1948) in spite of introduction of river salts, coastalerosion, or even evaporation of fresh waters orseawater to the point of precipitation of NaCl.Accordingly, deviations of Cl/Br ratios from the normserve as clues to the genesis of natural waters, andnature of interstitial processes.

As a result of the studies the enrichment of marinesediments in iodine was established, as well as theconditions under which iodine was transferred to porewaters. Iodine, along with other products, is enriched ininterstitial waters in the course of breakdown oforganic matter. The process is particularly prevalent inmarine sediments in regions of high biologicalproductivity. Iodine in such sediments is the source of

iodine of formation fluids, and has been used as a tracerelement for potential presence of petroleum.

METHODSSamples containing several ml of pore fluid were

provided for the purpose of determining bromine andiodine. In samples from the highly mineralized sectionsof Holes 380/380A chlorine was redeterminedargentometrically. For samples from Sites 379 and 381,shipboard values of chloride were used to calculateCl/Br ratios. These appear to be somewhat too high,but insufficient fluid was available to permitredeterminations.2 True chloride values are obtained bysubtracting the sum of bromine and iodine from totalhalogens, with appropriate corrections for atomicweight.

Bromine was determined volumetrically, throughoxidation by hypochlorite in acid medium, followed byiodometric titration. The method gives the sum ofbromine and iodine. Corrected bromine is obtained bysubtracting iodine with correction for atomic weight.For calculating Cl/Br ratios data are taken in units ofg/kg(°/oo).

Iodine is determined by the volumetric method, afterseparation of organic matter, by oxidation withbromine water and subsequent iodometric titration(Shishkina et al., 1969). In a number of instances themore sensitive method of Miller and Schneider (1963)was employed. Data are given in mg/kg.

All analyses were performed by G.A. Pavlova andV.S. Bykova under the supervision of the author.Results are shown in Tables 1 to 4 and in Figures 1 to 3.

DISCUSSION OF RESULTSSite 379 is located in the eastern central basin of the

Black Sea, where the surficial layer of sediments isenriched in organic matter (Shishkina, 1959, 1962).Interstitial chloride, salinity, alkalinity, calcium, andmagnesium reveal the history of development of theBlack Sea basin and diagenetic interactions between thesediments and waters.

In Hole 379A, at 3.94 meters depth, the interstitialwaters already show a lower chlorinity and salinity thanBlack Sea bottom water (11.44 °/oo and 20.20 °/oorespectively). With depth, chlorinity drops to 2.7 °/oo at

translated by F.T. Manheim.

2Translator's note: The shipboard data of Manheim and Sotelo aregiven in g/1, not g/kg as assumed by Shishkina. This makes thechloride values appear too high, as the author notes. The data citedby Shishkina have therefore been replaced by either laboratory data,or shipboard values corrected for specific gravity, as noted. Cl/Brratios have been recalculated wherever the "USA" data were used

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O. V. SHISHKINA

TABLE 1

Chloride, Bromide, and Cl/Br for Interstitial Waters at Site 379

TABLE 3

Chloride, Bromide, and Cl/Br for Interstitial Waters in Hole 380A

Sample(USA)

Sample(Interval in cm)

Cl(7 o)USA a

Br

Hoc

I) Q/Br (mg/kg)

Depth(m)

Hole 379A

1234678

1113141617181927

Hole 379B

235

1-3, 144-1504-3, 144-1507-3, 140-150

12-2,140-15019-5, 140-15021-5, 140-15023-5, 137-15029-4, 140-15034-5, 140-15036-3, 140-15040-3, 140-15043-2, 140-15045-2, 140-15047-3, 140-15064-4, 128-138

3-1, 145-1504-1, 140-1505-2, 144-150

11.44 0.039 2932.65 0.013 2043.28 0.016 2053.24 0.020 1622.00 0.018 1111.96 0.019 1031.75 0.018 973.00 0.021 1423.67 0.026 1414.15 0.025 1663.62 0.033 1105.19 0.035 1485.92 0.041 1447.42 0.046 161

19.71 0.099 199

3.03° 0.015 202

3.43b 0.013 264

3.3Oc 0.017 194

2.108.81

12.2612.7918.4019.5419.5419.72

21.7524.2726.6528.3230.4932.9641.60

3.9432.4460.9

110.3166.9185.9204.9268.4

318.9336.4372.9380.9399.9429.9583.2

8.2112.0014.03

17.550.081.0

aData of Manheim and Schug (this volume)."Shipboard data in grams per liter.cChloride determined argentometrically by author.

TABLE 2

Chloride, Bromide, and Cl/Br for Interstitial Waters in Hole 380

α ciSample Sample C ‰ ) (%o) Br I Depth(USA) (Interval in cm) USAa SSSR (%„) Cl/Br (mg/kg) (m)

384042444850

c

52

1-5,140-150 9.63° 9.26 0.028 3316-3, 140-15010-3, 140-15114-5, 140-15021-3, 140-15025-3, 140-150

16.7 17.0320.3 21.6125.027.2 26.86

23.1129-1, 140-150 33.1

0.0420.0590.0450.0590.0480.081

405

366

555

455

481

409

5.5219.6318.9742.9325.2417.2525.11

7.955.492.0

133.5194.9232.8273.9

aData of Manheim and Schug (this volume)."Shipboard data corrected for specific gravity (g/kg).cProbably contaminated (translators note).

32.4 meters and maintains low salinities to 268 metersdepth, fluctuating between 1.8 and 3.3 °/00 Lowchlorinity and correspondingly low salinity characterizefreshening of the basin during the Neoeuxinian epoch,when it was isolated from the Mediterranean Sea. Atthis time the sea was also aerated from top to bottom,as indicated by the positive sediment redox potentials atcore depths of 6-8 meters from the sea floor (Shishkina,1961). Further corroboration is provided by studies ofsulfur isotopes (Vinogradov et al., 1962).

From 268 to 621 meters chlorinity graduallyincreases to values exceeding those in the modernoceans, 20.92 °/oo (Table 1, Figure 1). The more salinefluids in the interval 268 to 621 meters provide theconcentration gradients for upward diffusion of salts inthe section.

The distribution of bromine in pore waters in Hole379A follows closely that of chloride. In the uppermostsediment absolute concentration of bromide ininterstitial waters is 0.039 °/°° and the Cl/Br ratio is

Sample(USA)

Sample(Interval in cm)

(/oo) ( % o )

USAa SSSRI

,) Cl/Br (mg/kg)Depth

(m)

i59606263646568717374767778798287

1-2, 140-1505-4, 140-1507-2, 0-10

11-4, 140-15013-1, 140-15015-7, 140-15017-5, 140-15023-5, 140-15029-5, 140-15033-2, 140-15036-4, 140-15043-5, 0-1546-5, 140-15049-5, 135-15052-2, 000-01563-2, 140-15073-3, 130-150

37.538.439.842.539.941.141.142.243.7

- •

43.848.547.750.650.655.0b

57.3

37.3638.3240.3341.5740.4640.7341.3142.6943.6344.0644.0448.4050.1250.4352.4855.1857.88

—0.0720.0740.0610.094

-0.0740.0690.0820.069

-

0.0800.101

_0.0950.105

-

—532544681430

-558619532638

-605496

_552526

-

—27.96

_31.5227.4128.6127.2230.3128.1228.93

27.8029.25

_28.8427.95

336376.9406.9434.9450475490.9549.9598.5630.5652.5728.5758.4787.0810.2918.4

1014.3

aData of Manheim and Schug (this volume)."Shipboard data correcred for specific gravity (g/kg).

TABLE 4

Chloride, Bromide, and Cl/Br for Interstitial Waters at Site 381

ClSample Sample (%o) Br I Depth(USA) (Interval in cm) USAa ( % o ) Cl/Br (mg/kg) (m)

919293959798

100103104105106107108109110

1-2, 144-1502-2, 145-1503-3, 140-1507-5, 140-150

12-4, 140-15014-5, 135-15016-5, 135-15023-1 (top)25-3, 140-15027-2, 140-15029-5, 000-1531-4, 140-15033-4, 140-15035-5, 140-15037-2, 140-150

15.716.223.536.030.2b

35.6b

37.737.737.738.439.539.438.739.440.1

0.0420.0550.0810.0900.1090.1190.1170.1100.1310.1130.1030.1260.1110.1540.113

374295290400277299322342288340383312349256355

5.867.81

15.6526.6520.8027.4126.4226.4229.2829.3029.6530.2029.4132.6830.45

3.0017.4027.0065.00

111.00133.00152.00209.00225.00244.00264.50282.00301.00321.50336.00

aData of Manheim and Schug (this volume)."Shipboard data corrected for specific gravity (g/kg).

293, similar to that in normal seawater (293 ±3). Withdepth the absolute concentration of bromide decreaseswith the general freshening of the section, but the Cl/Brratio decreases sharply and fluctuates between 97 and205 meters. Below 336.4 meters the concentration ofbromide increases in absolute terms and the Cl/Br ratiofluctuates between 100 and 199, significantly lower thannormal values for ocean waters (Cl/Br = 300). Thesource of chloride and bromide in pore fluids at depthsbelow 600 meters appears to be brines formed throughevaporation of seawater up to the stage of precipitationof NaCl, i.e., relatively richer in bromide than chloride.This is suggested by the significant lowering of theCl/Br ratio in the pore fluids of the site. Fluctuation ofCl/Br ratios from layer to layer may be explained byvariable rates of diffusion for ions of the elements inquestion and varying sediment properties with depth,such as permeability, water content, etc.

The enrichment of bromide and the significantenrichment of iodine in the pore fluids attests to the theprocess of mineralization of organic matter, but forbromide this factor plays a lesser role than brine as asource.

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BROMINE, Cl/Br RELATIONSHIPS AND IODINE IN INTERSTITIAL WATER

10 20

Br (%o) Cl/Br I (mg/kg)0 0.02 0.04 0.06 0.08 0.10 0 100 200 300 0 20 40

100

200

£ 300

α

400

500

600 L

Figure 1. Distribution of Cl, Br, Cl/Br, and I in Sites 379A/B.

Accumulation of iodine in the pore fluids of Site 379is intensive. The iodine concentration in pore fluids ofthe uppermost horizon of sediments (2.1 mg/kg) isalready 50 times greater than in seawater. Iodinefurther increases consistently with depth and reaches41.6 mg/kg at 583.2 meters. Such enrichment in porewaters of the Black Sea, peripheral areas of the PacificOcean (off Peru, Chile, Japan) and in the AtlanticOcean off Southwest Africa may be attributed tomineralization of sediments rich in reactive organicmatter. Such mineralization causes increases inconcentrations of iodine, ammonia, alkalinity, andother properties (Pavlova and Shishkina, 1973). Porewaters having large iodine concentrations are usually inthe process of, or have undergone, major diagenetictransformations, and become chloride-alkaline waterslacking in sulfate (in continental margin areas) orchloride-calcium waters (in the Black Sea).

Interstitial waters found in the course of drilling Site379 represent chloride-calcium waters of variousconcentrations. The process of consolidation ofsediments with depth and increasing temperature (to32°C) contributes to the process of iodine enrichmentin pore fluids. In general, these processes, includingenrichment in iodine, bromine, and ammonia areanalogous to those observed in corresponding types offormation fluids found in oilfield basins.

At Site 380, in the Bosporus region, interstitial waterswere obtained from two columns: Hole 380 (7.9 to 360

m) and Hole 380A (336 to 1068 m). Pore waters of lowsalinity, corresponding to the fresh-water stage of theBlack Sea were obtained at 7.9 meters, where achlorinity of 9.26 °/oo was obtained (Tables 2 and 3,Figure 2). The nature and changes in pore fluid in theuppermost strata of sediment (to 10-12 m depth)delineates the fresh-water stage of the basin, knownfrom our previous work (Shishkina, 1967, 1972).

A major feature of interest in this site is the increasein chlorinity salinity with depth; the latter reaches 98°/oo at 1050 meters, or three times greater than the saltcontent of present-day seawater. Its ionic compositionalso changes.

Study of bromide and Cl/Br ratio in this site isespecially significant, in that it provides an answer tothe question of the source of the brines and mechanismof their emplacement, while elaborating information onthe history of development of the Black Sea basin.

Greater increase in chloride concentration thanbromide with depth is indicated by Cl/Br ratios havingvalues greater than 300. The ratios fluctuate in mostcases around 500, with some isolated horizons havingratios of 600 and higher (681; Sample 62, Hole 380A).The absolute concentrations of chloride increasegradually and evenly with depth, following thediffusion laws, and providing an indication of the lackof reactivity of chloride in sediment-water systems. Incontrast, bromide fluctuates, presumably in response tomineralization of organic matter, and perhaps also in

633

O. V. SHISHKINA

100-

Figure 2. Distribution of Cl, Br, Cl/Br, and I in Holes 380/380A.

I (mg/kg)0 10 20 30 40 50

Cl (%o) Br (°/oo) Cl/Br I (mg/kg)0 10 20 30 40 50 0 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0 100 200 300 400 0 10 20 30 40

100

£200

300

400 L

Figure 3. Distribution of Cl, Br, Cl/Br, and I in Site 381.

response to varying diffusional conditions. The keyfeature is that the Cl/Br ratio is well above 300. Thisindicates that the pore fluids are influenced by watersthat have leached salt deposits, or by deeper pore fluids(not reached by the drill) that have leached salt layers.Data on dolomite and other minerals in the coresconfirm the existence of an evaporite basin. The data

on the Cl/Br ratios indicate that evaporation reachedthe stage of halite precipitation. Leaching of haliteproduces waters having high concentrations of chloridebut low concentrations of bromide.

Iodine concentrations in the pore waters increasewith depth from 5.52 mg/kg at 7.9 meters to 42.9mg/kg at 133.5 meters depth. Within the upper 300

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BROMINE, Cl/Br RELATIONSHIPS AND IODINE IN INTERSTITIAL WATER

meters iodine concentrations fluctuate from horizon tohorizon within the limits of 17.25 to 42.9 mg/kg. Indeeper layers concentrations remain relatively constantat 28-31 mg/kg. Accumulation processes for thiselement have been described previously.

At Site 381, drilled near Site 380 in the Bosporusregion, the upper sediment layers containing fresh-water sequences are absent. Pore fluids at a depth of 3meters already show chlorinity of 15.7 °/oo, appreciablyhigher than the chlorinity of near-bottom waters of theBlack Sea. With greater depth interstitial salinitiesincrease to 40.1 °/oo at 336 meters (Table 4, Figure 3).The concentration of bromide also increases withdepth, but the Cl/Br ratio fluctuates around values notfar from 300, which is noteworthy considering theincrease in total chlorinity. Local decreases in Cl/Brratios below 300 may be attributed to liberation ofbromide in the course of mineralization of organicmatter at specific horizons. Comparison of data on thedistribution of chloride and bromide and Cl/Brbetween Holes 380/380A and 381 shows the appreciablyless, if any, influence on the latter of waters that mayhave leached salt deposits.

At Site 381 the distribution of Cl/Br in the porefluids does not give as clear a picture as in the precedingsites. One must also note that varying rates ofsedimentation, and consequent differences in age of thedeposits and time available for diffusion, lead todifferences in the distribution of elements in porewaters at different depths. Distribution of iodine inpore fluids is similar to that observed at Holes38O/38OA. Its concentration increases from 5.86 mg/kgat 3 meters to 32.68 mg/kg at 321.5 meters, and ingeneral, is governed by previously discussedmechanisms.

CONCLUSIONS1. The results of studies of bromide concentrations

and Cl/Br ratios in interstitial waters show asprincipal features Cl/Br ratios < 300 at Holes 379/379A,and Cl/Br > 300 at Holes 38O/38OA and 381 in theperipheral parts of the Black Sea.

These data provide information on the sources andmechanisms of distribution of chloride and bromide inthe pore fluids. At Holes 379A/B increasedconcentrations of chloride and bromide at depths to600 meters below sea floor, and relative enrichment inbromine are associated with hypersaline brines. Theseformed as a result of evaporation of seawater in basinsreaching the stage of deposition of NaCl.

At Holes 38O/38OA pore waters are more stronglyenriched in chloride than in bromide, as indicated byCl/Br ratios in the range of 500 to 600. This indicatesinfluence of waters (possibly deeper lying waters) thatmay have leached deposits of solid NaCl. Theremaining distribution of the elements in question areexplained by upward diffusion of salts.

2. Pore waters of all three holes are enriched iniodine. Its concentration ranges from 2 to 5 mg/kg to41.6 mg/kg in Hole 379A, to 28.8 mg/kg at 810 metersdepth in Hole 38OA and to 30.45 mg/kg at 336 metersat Site 381. Accumulation of iodine in interstitial waters

occurs as a result of mineralization of organic matterand simultaneous liberation of other products ofdecomposition (ammonia). The large concentrations ofiodine in pore waters of Hole 379A are explained by thedeposition of these sediments in the region of"halistasis" or high organic enrichment. As a result ofdiagenesis and consolidation, iodine is transferred fromthe sediment to the pore fluid. This process is facilitatedby higher temperatures to 32°-40°C in the deeperlayers. Concentration of iodine in pore waters in therange 30 to 40 mg/kg is comparable to the range in oilfield brines. Observation of iodine at suchconcentrations may be used as an exploration tool foroil prospecting.

REFERENCES

Miller, A.D. and Schneider, L.A., 1963. Opredeleniemikrokolichestv ioda v pr i rodnykh vodakhkataliticheskim metodom (Determination ofmicroquantities of iodine in natural waters by the catalyticmethod): Zh. Analit. Khimiya, v. 18, no. 3.

Pavlova, G.A. and Shishkina, O.V., 1973. Raspredelenie iodav osadkakh Tikhogo okeana i nakoplenie ego v ilovykhvodakh v protesse ikh metamorfizatsii (Distribution ofiodine in sediments of the Pacific Ocean and accumulationin pore waters as a result of their metamorphism):Geokhimiya, no. 7.

Shishkina, O.V., 1959. Metamorfizatsiya khimicheskogosostava ilovykh vod Chernogo morya. Soobschenie I. Sb."K poznaniyu diageneza morskikh osadkov"(Metamorphism of the chemical composition of porewaters of the Black Sea). In Strakhov, N.M. (Ed.), Towardunderstanding diagenesis of marine sediments: Izdatel'stvoAkad. Nauk, SSSR, Moscow.

, 1961. OkislitePno-vosstanovitel'nyi potentsialverkhnei desyatimetrovoi tolschi Chetyertichnykhotlozhenii Chernogo morya (Oxidation-reductionpotential in the upper 10 m sediment layers of the BlackSea): Doklady Akad. Nauk, SSSR, v. 139, no. 5.

1962. Nekotorye rezultaty issledovaniya ilovykhvod Cherno morya (Some results of studies of pore fluidsin the Black Sea): Trudy Institute of Oceanology, Akad.Nauk, SSSR, v. 54, contrib. 2.

., 1967. Otlichitel'nyei cherty khimicheskogo sostavailovykh vod Chernogo morya (Special features of chemicalcomposition of pore fluids in the Black Sea). InGeofizicheksie i Gidrokhimicheskie Issledovaniya vChernom more: Izatel'stvo "Nauka," Moscow.

, 1972. Geokhimiya morskikh i okeanicheskikhilovykh vod (Geochemistry of marine and oceanic porefluids): Izdatel'stvo *Nauka," Moscow.

Shishkina, O.V., Pavlova, G.A., and Bykova, V.S., 1969.Geokhimiya galogenov v morskikh okeanskikh osadkakhi ilovykh vodakh (Geochemistry of halogens in marinesediments and pore waters): Izdatel'stvo "Nauka,"Moscow.

Vinogradov, A.P., 1948. Rasseyannye khimicheskie elementyv podzemnykh vodakh raznogo proiskhozhdeniya(Distribution of chemical elements in underground watersof various origin): Trudy Lab. Gidrogeol. Probelm. Akad.Nauk, SSSR, v. 1.

Vinogradov, A.P., Grinenko, V.A., and Ustinov, V.I., 1962.Izotopnyi sostav soedinenii sery v Chernom More (Isotopiccomposition of sulfur compounds in the Black Sea):Geokhimiya, no. 10.

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