SCOPE 48 - Sulphur Cycling on the Continents

8.1 INTRODUCTION

Most of the requirements of contemporary society are satisfied at the expense of the exploitation of terrestrial ecosystems and minerals mined from the continental part of our planet. The exploitation of biological and mineral resources of the oceans is considerably less. Consequently, the potential impact of anthropogenic pollution in the oceans often draws less attention than on land. However, the oceans are more and more being turned into a pit for garbage and the cesspool of our planet. Industrial, agricultural, and household wastes are being released in the oceans in increasing amounts. The near-continental regions of the ocean such as coastal or marginal seas, and especially the continental seas-such as the Persian Gulf, Black, Baltic, Mediterranean, and Caspian Seas-are the greatest sufferers from anthropogenic impact.

The recent literature contains alarming data indicating the drastic changes in hydrochemical and biological conditions in continental and marginal seas. Hydrochemists more and more often observe uncommonly high concentrations of nitrogen, phosphorus, and heavy metals in surface horizons of littoral sediments and deficiency of dissolved oxygen in bottom waters. These oxygen deficiencies cause the death of benthic animals over enormous shelf areas. A worrisome change, at least in the deeper waters of some continental seas, has been the rise of the border of oxygen-free waters. In deep-water depressions of the Baltic Sea, H₂S has become a constant component of the bottom water.

Biologists have observed considerable changes in species composition of fauna and flora of these seas: the number of species has decreased, commercially valuable species of fish and shellfish have been displaced by medusae and other gelatinous animals. Blue-green algae have become dominant in some seas such as the Baltic, and 'red tide' blooms have become more common in many seas, including the Black Sea coast of Bulgaria and the eastern coast of the Adriatic Sea.

The above circumstances induced us to study in detail biogeochemical processes of the sulphur, carbon and oxygen cycles in the continental and marginal seas of the largest continent-Euroasia. A considerable part of the data used in this review is already published, e.g. the materials on the Baltic Sea (Lein et al., 1981; Lein and Ivanov, 1983; Lein, 1983), Black Sea (Ivanov et al., 1984; Lein et al., 1986; Lein, Ivanov and Vainshtein, 1990; Lein and Ivanov, 1989, 1990; Vainstein et al., 1985) and Caspian Sea (Rivkina, Tokarev and Gorlatov, 1985). The main part of the materials on the Far East seas and Norwegian Sea is presented for the first time.

8.2 GENERAL PROCESSES OF ELEMENT CYCLES IN THE SEAS

The sulphur cycle in seas is tied to other element cycles through organic matter cycling. In situ photosynthesis is the dominant source of organic matter, with allochthonous inputs from land being much smaller. Such allochthonous inputs of organic matter typically account for 10% of the total organic matter inputs to continental seas, somewhat less for marginal seas, and still less for open oceans (Romankevich, 1977). It should be stressed that the amounts of organic matter and nutrients flowing into the sea from land increase with the development of industry and agriculture on the land. These anthropogenic fluxes, particularly of nutrients and of toxics, disturb the natural element cycles of the marine ecosystems.

The rates of mineralization of organic matter are usually comparable to the rates of photosynthesis, although in some areas with considerable inputs of allochthonous matter, mineralization rates may exceed photosynthesis. In the oxic zone, which as a rule covers most of the water column and the upper millimetres or so of silt sediments, mineralization is carried out by aerobic organotrophic microorganisms. When constructing biogeochemical models, one usually assumes that in oxic conditions carbon-, nitrogen-, phosphorus-, and sulphur-containing organic compounds are oxidized to form inorganic oxides. However, microbial biomass containing these elements is also synthesized. This new organic matter, differing considerably in its composition from phytoplankton biomass, is also involved in subsequent element cycles. It is important to emphasize that in the course of organic matter biosynthesis, organotrophic bacteria incorporate not only organic molecules but also carbon dioxide, which is fixed through the pathway of heterotrophic CO₂ assimilation. The portion of mineral carbon assimilated by organotrophs can reach 12-18% of the total carbon of new bacteria biomass (Saralov, 1990). 

Anaerobic mineralization of organic matter occurs in anoxic zones characteristic of most of the silt sediments of continental and marginal seas. Under some conditions, anoxia is also observed in part of the water column. Anoxic water columns often occur in narrow and deep gulfs or fiords and are now found in the deep-water basins of the Baltic Sea. Anoxia characterizes the entire deep-water portion of the Black Sea, which makes up more than two- thirds of the total area of the sea.

The processes of anaerobic mineralization result in production of such reduced inorganic compounds as H₂, H₂S, and NH₄⁺ as well as various low- molecular organic compounds, such as alcohols and fatty acids, which are the products of carbohydrate fermentation and amino-acid deamination. Some of the anabrobic mineralization products are used by the so-called secondary anaerobes. The following three groups of these microorganisms are of utmost geochemical significance: acetogens, synthesizing acetate from CO₂ and H₂; sulphate reducers, reducing sulphate to sulphide through oxidation of hydrogen and low-molecular organic acids and alcohols; and methanogenes forming methane either from methyl groups of organic compounds or via CO₂ reduction by hydrogen.

A portion of the final products of anaerobic metabolism (such as sulphides and methane) remains buried in the sediments. However, the bulk of reduced products of anaerobic metabolism migrates according to concentration gradients towards the interface of the anoxic and oxic zones. Above this border the processes of H₂, H₂S, NH₄⁺ , and CH₄ oxidation are carried out by specific groups of microorganisms, such as hydrogen-oxidizing, thionic, sulphur , nitrifying, and methane-oxidizing bacteria. Most representatives of these groups  do not require complex organic compounds to synthesize their biomass. As the main carbon source, they use either only carbon dioxide ( chemoautotrophs and photoautotrophs) or carbon dioxide and intermediate products of methane oxidation (methanotrophs). Due to the intensive activity of these autotrophic and methanotrophic microorganisms, a significant formation of organic matter takes place at the interface of the oxic and anoxic zones. The role of this biomass in the nutrition of the sea organisms draws increasing attention of biogeochemists.

The above general scheme of biogeochemical processes qualitatively characterizes interactions of element cycles and the role of the main groups of living organisms. Unfortunately, except for photosynthesis, almost all of the bilogeochemical processes involving sulphur, carbon, and oxygen interactions have not received sufficient quantitative study. Therefore, the main objective of our long-term research work was to quantify these processes occurring in anoxic areas and at the oxic-anoxic interfaces. The main task of this chapter is to summarize the data obtained for continental and marginal seas of Eurasia. The details of methods used are described elsewhere (Ivanov, Lein and Kashparova, 1976; Laurinavichus and Belyaev, 1978; Lein, 1983; Galchenko, Gorlatov and Tokarev, 1986).

8.3 THE NORWEGIAN SEA

Sediments from the eastern part of the Norwegian Sea near Medvezhiy Island (the region of the wreck of the Soviet submarine Komsomolets) were studied in May 1989. Five cores were taken at stations in water depths ranging from 1670 to 1685 metres. At stations 2072 (73° 42.78 N; 13° 18.31 E), 2073 (73°43.40 N; 13° 14.70 E), and 2074 (73°42.70 N; 13° 16.10 E), only the top 15 to 20 cm were sampled. At station 2082 (73° 43.02 N; 13° 17 O E), sediments were sampled between 30 and 85 cm depth. At station 2083 (73°43.06 N; 13° 15.08 E), sediments between 90 and 310cm were sampled. The combined sediments from cores 2074, 2082 and 2083 can be considered representative of the entire upper holocene sediments, whose thickness in this region is approximately 150 cm, assuming a sedimentation rate of 0.2 mm a^-1.

Comparatively weak reducing conditions are observed in the sediments, as indicated by relatively high oxidation-reduction potentials and by insignificant decreases of sulphate with depth (Table 8.1). None the less, reduced sulphur is present even in the surface sediments; at depth reduced sulphur makes up more than 1% of the weight of the sediment (0.3 mol S g^-l; Table 8.1). The organic carbon content varies from 0.41 to 1.00% (Table 8.1).

Rates of sulphate reduction as measured with radiolabelled sulphate varied from 22 to 117 ng (S) g^-l day^-1 (0.7 to 3.7 nmol (S) g^-l day^-1) in the upper 20 cm of the sediment (Table 8.1). The highest rates were observed in the interval from 30 to 50 cm (up to 190 ng (S) g^-l day^-1; 5.9 nmol (S) g^-l day^-1). Rates were low below 80 cm. Integrating these measured rates of sulphate reduction over depth (specific gravity = 1.3), and assuming the rates do not vary seasonally, we estimate an annual rate of 45 g (S) m^-2 a^-1 (1.4mol (S) m^-2 a^-1). From the reduced sulphur data in Table 8.1, we estimate a rate of sulphur burial of 9.4 g (S) m^-2 a^-1 (0.3 mol (S) m^-2 a^-1), or 21% of the sulphate that is reduced. The rest of the reduced sulphur formed during sulphate reduction must be re-oxidized, perhaps supporting production by chemoautotrophic bacteria.

Rates of anaerobic methane oxidation (measured using radiolabelled ¹⁴C) in these sediments are fairly high (Table 8.1). Methane would appear to be a major substrate fuelling sulphate reduction in the surface sediments, if indeed sulphate-reducing bacteria are responsible for this methane oxidation. Assuming two moles of methane consumed for every mole of sulphate reduced, measured rates of methane oxidation above 45 cm at all stations are sufficient to account for an average of almost 75% of the rate of sulphate reduction at each depth (range = 17 to 178%). Below 45 cm, rates of sulphate reduction are too low to account for the measured rate of methane oxidation unless some oxidant other than sulphate is being used (Table 8.1).

Table 8.1. Chemical characteristics and bacterial processes of Norwegian Sea sediments

It is also interesting to compare the rate of methane oxidation with the rate of methane formation (also measured using radiolabelled ¹⁴C) .The rates of methane oxidation are at least two to three orders of magnitude higher than the rates of methane formation (Table 8.1). These data, together with rather high methane concentrations in the sediments (Table 8.1), suggest an important source of methane coming from deeper horizons, creating an upward flux of methane which is partially intercepted by methane-oxidizing bacteria. The origin of this deep methane remains unknown and requires further investigation.

8.4 THE SEA OF OKHOTSK

Most of the sediments of the Sea of Okhotsk are represented by pale-green diatomic silts, and are characterized by anoxia and possess high concentrations of reduced sulphur (Volkov and Rozanov, 1983). Geological information on the area is presented in Zonenshayn et al., (1987), Morozov (1987), and Chertkova, Bilichenko and Stunzas (1987). Our biogeochemical investigations were carried out on the east continental slope southwest of Alaid Island in June-July 1986 (Lein et al., 1989). The rate of sedimentation in the study region is about 0.2 mm a^-1. 

A major gas jet emitting bubbles 1 cm in diameter occurs in this region. The rate of the gas flow is up to 0.2 m S^-l (Zonenshayn et al., 1987). In the  zone of the gas-gusher outlet, the pale-green sediments common elsewhere in the Sea of Okhotsk are replaced by black sulphidic sediments, covered unevenly by a white coating of native sulphur or whitish bacterial mats, Carbonate nodules and plates of irregular form, ranging from 1-2 to 20 cm in diameter, also occur around the gas outlet.
  We sampled cores from stations underlying water of depths of 786 to 796 metres (station 1391-50° 3091 N; 155° 18.00 E; station 1395-50° 30.90 N; 155° 18.16 E; and station 1404-50°30.80 N; 155° 18.15 E). Grains and nodules of calcium carbonate and abundant carbonate cementation of sediments were observed in the sediments of all stations. Snow-like accumulations of gas hydrate were discovered at a depth of 185 to 225 cm at station 1395. Methane composed almost 98% of the volume of the hydrate. Gas hydrates were also observed at station 1404.

Sulphate reduction rates at stations 1391 and 1404 are highest near the surface and then decrease with depth over the top metre, a pattern which is quite typical for sediments of the continental slope (Table 8.2). Porewater characteristics are also typical, with sulphate and calcium ions decreasing over depth, accompanied by a gradual decrease in alkalinity. Oxidation-reduction potentials are positive in the upper horizons and moderately reduced by a depth of 1 m. The rates of methanogenesis considerably exceed the rates of anaerobic methane oxidation in the top metre at these two stations, resulting in an accumulation of methane in the upper horizons (Table 8.2).

A quite different situation is observed in the sediments of station 1395 and in the lower portions of the sediments at station 1404. In both cases the sediments contain methane gas hydrates, and dissolved methane increases sharply in the porewaters just above the hydrates, probably because of partial decomposition of the gas hydrate under the thermodynamically unstable conditions in the subsurface sediments. Anaerobic methane oxidation also sharply increases, and its intensity in sediments at a depth of 130-140 cm at station 1395 is 25-100 times higher than at depths above 60 cm (Table 8.2). The oxidation of the methane results in the porewaters being supersaturated with carbonates, leading to the calcium carbonate concretions and nodules. That the carbon of the carbonate concretions originates from methane is confirmed by stable isotopic data (Lein et al., 1989). d¹³C value of gas hydrate methane are -54.6%o, and the values of d¹³C of six samples of diagentic carbonates from the sediments of stations 1396 and 1404 varied from -47.5 to -49.2%, whereas d¹³C of shell carbonate in the same silts  ranged from 1 to -5%.

Unlike the deeper sediments of the Norwegian Sea, rates of sulphate  reduction are amply sufficient to account for the anaerobic methane oxidation observed in all of the depths at all three stations in the Okhotsk Sea (Table 8.2).

8.5 THE BALTIC SEA

The Baltic Sea is a body with a high bioproductivity and great amount of organic matter in the sediments, making it interesting for the study of biogeochemical processes. Like the, Black and Azov Seas, the Baltic Sea belongs to small continental seas (Vigliere, 1974). However, unlike these two reservoirs, the microflora of the bottom sediment and its geochemical activity in the Baltic Sea have received only occasional study. Schneider (1977) determined the number of sulphate-reducing bacteria at three stations in the Kiel fiord, and Bagander (1977) used benthic chambers to characterize the turnover of phosphorus, oxygen, and sulphur compounds in the surface horizon of sediments in the Stockholm archipelago. Our investigations, conduclued during the summer of 1978, included the study of number, distribution, and geochemical activity of different groups of microorganisms in the sediments, determination of organic carbon assimilation via bacterial processes under conditions of anaerobic diagenesis, and calculation of the carbon isotopic and material balances. At the same time we studied the biogeochemical processes of the sulphur cycle in sediments of the Baltic Sea and calculated the sulphur material-isotope balance (Lein et al., 1981; Lein and Ivanov, 1983). In this paper we characterize rates of sulphate reduction and the role of sulphate reduction in the carbon cycle in sediments of the Upper Holocene, the thickness of which varied in different areas of the sea from the first centimetres to 13 m in the region of Gdansk depression.

Table 8.2. Chemical characteristics and bacterial processes in sediments of the Okhotsk Sea 

We sampled shallow-water sediments from the Gulf of Riga (station 2589- 38m, 57°09.6; 21°01.9; and station 2601-53m, 57°43.7; 23°38.3), sediments of the open Baltic Sea (station 2611-130 m, 57° 45.8; 20° 37.3; station 2631-120 m; 57° 23.5; 19° 26.3; station 2636-160 m, 56° 43.5; 20° 00.0; and station 2656-43 m, 54° 50.4; 13° 40.6) and sediments of the deep-water Gotland depression (station 2618-215 m, 57° 24.3; 20° 00.0; and station 2622-240 m, 57° 25.2; 20° 18.1). All the sediments are rich in organic matter and have measured oxidation-reduction potentials which vary from -90 to - 340 m V. Reducing conditions are observed either just under a thin layer of oxidized sediment or at the sediment-water interface. Free hydrogen sulphide in near-bottom water was detected only in the Gotland depression at the depth of 226 m in concentration of 0.04 mg l^-1.

The lowest concentrations of total reduced sulphur are observed in the surface layers of coarse-grained sediments at the coastal stations, which contain about 2% organic carbon. In sediments of most of the other stations, organic carbon ranges from 2.1 to 5.9% (average = 4.8%), and the total content of reduced sulphur compounds reaches 1% (0.3 mmol g^-l) even in
the upper horizons. Methane concentrations range from 160 to over 500 µg (C) g^-l (wet weight; Geodekyan, Trotsyuk and Avilov, 1978). Highest concentrations are observed in the sediments of the Gulf of Riga and Gdansk depression (Geodekyan, Trotsyuk and Avilov, 1978).

The organic matter in sediments of the Baltic Sea comes both from inflow of terrestrially derived carbon with river runoff and from sedimentation of dead phytoplankton. If we assume that organic carbon in river runoff of the Baltic region has a d¹³C value of -27.1% (Wiekman, 1952) and that the d¹³C value of phytoplankton is -21.5% (Galimov, 1968), we can calculate the share of phytoplankton-derived and terrestrially derived carbon in the sediments of our stations ( also assuming no fractionation in the carbon isotopic ratio during diagenesis and no other sources of organic carbon inputs). The d¹³C values for the sediments at most of our stations range from -21.7 to -22.8%, suggesting that the main bulk of organic matter carbon is formed from phytoplankton residues (82-100%). Values for some coastal stations are as low as -24.9%, suggesting that some 60% of the carbon may be terrestrially derived.

Rates of sulphate reduction are shown in Table 8.3. Rates are moderately high at most stations, and the maximum activities are generally found in the upper 20 cm of sediment. Non-acid-volatile products (pyrite and organic sulphur) are a major product of the sulphate-reduction measurement at many of the stations (Table 8.3). As with oceanic sediments and the sediments in other seas, sulphate reduction is the major process consuming organic carbon under anoxic conditions in the Baltic Sea (Lein and Ivanov, 1980; Belyaev, Lein and Ivanov, 1980; Lein, 1984, 1989).

Our data on the rates of biogeochemical processes together with literature data allow us to calculate a material-isotope balance of the carbon cycle processes in the Baltic Sea and describe quantitatively the interaction of the carbon and sulphur cycles (Table 8.4, after Lein, 1983). In situ primary production by phytoplankton contributes some 50 x 10⁶ t (C) a^-l to the Baltic Sea, while rivers deliver some 10 x 10⁶ t (C) a^-l of terrestrially derived organic matter. In order to calculate the amount of organic carbon permanently buried in sediments (1.55 x 10⁶ t a^-l), we used the average value of organic carbon in the sediments at our stations (4.8% ) and data on the rate of sediment accumulation. The rate of organic carbon consumed through sulphate reduction (16.9 x 10⁶ t a^-l) is calculated from our average rate data (Table 8.3), assuming two moles of carbon consumed for every mole of sulphate reduced. The flux of organic carbon to the sediments from the water column (18.5 x 10⁶ t a^-l) is simply the sum of the rate of permanent burial and the rate of carbon consumption in sulphate reduction (Table 8.4).

The total flux of organic carbon to the sediments accounts for less than 30%  of the inputs of organic carbon to the Baltic Sea from phytoplankton production and terrestrial inputs from the continent. Hence, most of the organic carbon inputs (more than 70% ) are mineralized in the water column of the Baltic Sea.

It should be noted that the average carbon isotope composition of organic matter in the upper horizons of the Baltic Sea sediments from our measurements (d¹³C = -22.8%), is quite close to the estimated average value for all organic matter inputs to the Baltic Sea given in Table 8.4 (d¹³C= -22.6%).

 8.6 THE BLACK SEA

The existence of a huge zone of hydrogen-sulphide-containing water in the deep part of the Black Sea was first noted just over a 100 years ago by Academician N. J. Andrusov. Recently, the problem of sulphide in the waters of the Black Sea has again attracted the attention not only of scientists but of the public at large, because of reports that the upper border of the anoxic waters has moved up rather quickly during the last 15-20 years. There is a resulting worry that anoxic waters may reach all of the way to the sea surface.

Table 8.3. Chemical characteristics and rates of sulphate reduction in Baltic Sea sediments

Table 8.4. Mass-isotopic balance of organic carbon in Baltic Sea sediments (after Lein, 1983)

Most investigators think the Black Sea sulphide has been produced by sulphate-reducing bacteria. Since rates of sulphate reduction in marine systems are generally limited by organic matter supply, an increase in hydrogen sulphide in the water may suggest that greater inputs of organic matter are stimulating increased rates of sulphate reduction. That is, eutrophication may be leading to the increased volume of anoxic waters.

Early work on sulphate reduction in the Black Sea suggested rates of 25 g (S) m^-2 a^-l (0.8 mol (S) m^-2 a^-l; Sorokin, 1962, 1982). About half of this sulphate reduction was thought to occur in the sediments and half in the anoxic water column. More recent measurements made by Soviet microbiologists during a series of expeditions between 1980 and 1988 (Ivanov et al., 1984; Vainshtein et al., 1985; Lein and Ivanov, 1990; Lein et al., 1986, 1989; Lein, Ivanov and Vainshtein, 1990) have found higher rates. Data collected in December 1984 found rates of 84 g (S) m^-2 a^-1 (2.6 mol (S) m^-2 a^-1), with the preponderance of sulphate reduction occurring in the water column (66 g (S) m^-2a^-1; 2.1 mol (S)m^-2 a^-1) and only 18 g (S) m^-2 a^-1 (0.5 mol (S) m^-2 a^-1) in the sediments. These sediment rates are 50% higher than those originally reported by Sorokin (1962). American investigators (Albert, Taylor and Martens, 1988) working on the R. V. Knorr in March of 1988 found rates of sulphate reduction in the sediments to be virtually identical to those found by the recent Soviet studies, but their rates of reduction in the water column were much lower, only 13 to 26% of the 1984 Soviet measurements, or virtually equivalent to the water-column rates reported by Sorokin (1962).

These differences in rates of sulphate reduction in the water column are of much interest. However, the measurements are all made on the brink of the sensitivity of the radioisotopic method, and so differences may be exaggerated, and data should be extrapolated with caution. On the other hand, the rates. of all microbiological processes in the upper horizons of the hydrogen-sulphide-containing water probably undergo substantial seasonal fluctuations. Data obtained by B. B. Namsaraev and his colleagues (pers. comm.) are an indication of such a possibility. Sulphate reduction rates in samples from the upper zone of the anoxic waters from the western basin, taken in March-April 1988, are an order of magnitude higher even than our data obtained in December 1984.

The comparatively light stable sulphur isotopic composition of H₂S in the water column of the Black Sea tends to confirm that the majority of the sulphide is indeed formed from sulphate reduction in the water column (Lein, Ivanov and Vainshtein, 1990). This gives us some confidence to use our rates of sulphate reduction from 1984 in the sediments and water column (84 g (S) m^-2 a^-1; 2.6 mol (S) m^-2 a^-1) to analyse carbon consumption by this pathway. Assuming two moles of the organic carbon consumed per mole of sulphate reduced, we estimate carbon consumption by sulphate reduction in the Black Sea as 63 g (C) m^-2 a^-1 almost 80% of this occurring in the anoxic water column. This rate of carbon oxidation in the anoxic zone is significantly higher than the mass-balance calculations of Deuser (1971) and Scopincev (1975), who obtained estimates of 17 and 9 g (C) m^-2 a^-1 respectively. Both of these authors assumed anaerobic carbon consumption in the water column was less than or at most equal to the rate in the sediments.

Since the 1950s the potential for chemoautotrophic production to contribute a significant input of organic carbon to the Black Sea has been appreciated. The contact of hydrogen-sulphide-containing waters (enriched also with ammonium, methane, and other reducing compounds) with the overlying oxygenated waters creates a rather favourable environment for chemoautotrophic bacteria. Sorokin (1971) reported rates of dark CO₂ fixation, due mainly to the activity of sulphur-oxidizing bacteria, as high as 4.0 to 8.0 µg (C) l^-1 day^-1 in the zone of overlap between oxygen and sulphide, or up to 66 g (C) m^-2 a^-1. However, Scopincev (1975) noted that such high rates of chemoautotrophic production, if based on sulphur oxidation, would require a supply of dissolved sulphide of at least 200 g (S) m^-2 a^-1 (6.3 mol (S) m^-2 a^-1). This calculation, based on an assumption of three moles sulphur oxidized to fix one mole of carbon, yields an estimated supply of sulphide which is quite high compared to measured rates of sulphate reduction. Sorokin (1971) himself had measured sulphide oxidation rates averaging 250 g (S) m^-2 a^-1 (7.8 mol (S) m^-2 a^-1), but this now seems almost certainly too high.

More recent data by both American and Soviet workers on dark CO₂ assimilation in the Black Sea have suggested rates about half the value of the original Sorokin data, or 2 to 5 µg (C) l^-1 day^-1 (Jannasch, 1988; Nesterov, Namsaraev and Borzenkov, 1989,1990). These data suggest an annual rate of dark fixation of 33 g (C) m^-2 a^-1. However, Nesterov, Namsaraev and Borzenkov (1989) used different inhibitors of bacterial activity to estimate that on average only 37% of the dark CO₂ assimilation is due to production by chemoautotrophic sulphur bacteria (Table 8.5). Half was thought to be due to production by nitrifying bacteria, and the rest is fixation by heterotrophic organisms. This rate of production by sulphur-oxidizing bacteria requires a supply of sulphide of only 37 g (S) m^-2 a^-1 (1.2 mol (S) m^-2 a^-1), or 44% of our estimated rate of sulphate reduction.

The work of Nesterov, Namsaraev and Borzenkov (1989,1990) is the first showing the importance of chemoautotrophic nitrifying bacteria in the Black Sea. It is obviously of interest to estimate independently the potential for such production. Using our estimate of 63 g (C) m^-2 a^-1 for anaerobic carbon mineralization, and assuming from the Redfield ratio that the ratio of C : N in the organic matter being decomposed is 6.6: 1 (molar), annual nitrogen mineralization in the anoxic zone is estimated as 11 g (N) m^-2 a^-1. Based on a ratio of nitrogen oxidized to carbon fixed by nitrifiers of 30:1, this nitrogen flux would support a chemoautotrophic production of only 0.37 g (C) m^-2 a^-1, some 40-fold less than the estimate of Nesterov and colleagues. This discrepancy is probably the result either of an incomplete inhibition of other chemoautotrophic bacteria, or of unusually favourable hydrochemical conditions for nitrifying bacteria during the course of the study by Nesterov, Namsaraev and Borzenkov (1989) yielding rates which cannot be extrapolated to annual rates.

We suspect that the actual rate of chemoautotrophic production by sulphur-oxidizing bacteria in the Black Sea is in the range of 16 to 28 g (C) m^-2 a^-1. The maximum rate which could be supported if all of the sulphide

Table 8.5. Production of organic matter by chemoautrophy in the water column of the Black Sea (after Nesterov, Namsaraev and Borzenkov, 1990)

produced in our estimate of sulphate reduction is re-oxidized, rather than stored in the sediments, is 28 g (C) m^-2 a^-1. Karl and Knauer (1988), however, estimate a value of at least 32 g (C) m^-2 a^-1. Deuser (1971) used a value of 15 g (C) m^-2 a^-1 in his calculations.

The isotopic composition of organic carbon in the surface sediments of the Black Sea ranges from -25.5 to -26.6%. This is anomalously light compared to most marine sediments, which usually reflect the isotopic composition of phytoplankton and are isotopically more light by 1 to 2%. The average value of d¹³C of the phytoplankton in the Black Sea is -21.2% and of zooplankton is -23% (Deuser, 1971). Significant inputs of terrigenous organic matter (d¹³C = -25%) in the deep-water sediments of the Black Sea is quite unlikely. The most likely explanation for the light isotopic composition of organic carbon in the sediments of the Black Sea is an input of very isotopically light carbon produced during chemoautotrophic production.

A final statement on the carbon budget for the anoxic zone of the Black Sea will have to await further study, since it now appears that the amplitude of seasonal changes may be rather high, and there are no complete seasonal data. None the less, preparation of a carbon budget, even with the currently available data, leads to some interesting conclusions. The use of our sulphate reduction data requires a larger input of carbon to the anoxic zone than was assumed in the early budgets of Deuser (1971) and Scopincev (1975). If our sulphate reduction data are correct, the organic carbon input to the anoxic zone of the Black Sea must be approximately 72 g (C) m^-2 a^-1. Of this amount, 63 g (C) m^-2 a^-1 are mineralized in the process of sulphate reduction in water column and sediments, 5 g (C) m^-2 a^-1 are transferred in soluble form, and 4 g (C) m^-2 a^-1 are buried in bottom sediments. Anaerobic production by chemoautotrophs may be supposed to supply some 22 g (C) m^-2 a^-1 of this carbon input. The rest, 50 g (C) m^-2 a^-1, must come from  photosynthesis in the waters above the anoxic zone. In order to maintain such a downward flux of organic matter, net primary production in the Black Sea must be of the order of 200 to 250 g (C) m^-2 a^-1, since only 20-25% of the organic matter produced in primary production reaches a depth as great as 100 to 150  m as it sediments (Karl and Knauer, 1988). Our estimates of net primary production and of chemoautotrophic production agree well with the estimates of Karl and Knauer (1988), who give values of 212 and 33 g (C) m^-2 a^-1 for these two processes.

8.7 THE CASPIAN SEA

The Caspian Sea is a continental sea which lost the link with the ocean, resulting in considerable changes in the composition of water chemistry; this sea has changed considerably under the influence of river discharge and precipitation of chlorides and sulphates in Kara-Bogas-gol Bay. Compared to average ocean water, the water of the Caspian Sea is highly desalinated, depleted in chloride, and enriched with sulphate and calcium. A considerable admixture of terrigenic sulphate from river discharge is indicated by stable sulphur isotopic composition; the d³⁴S value ranges from +8.7 to + 13.8% (Mekhtieva and Rabinovich, 1975), quite unlike the typical seawater value of +21%. At the present time, all the water in the Caspian is saturated with oxygen, even in the deepest waters. In earlier work in the Caspian, an intensive smell of sulphide was noted in some near-bottom waters. However , at present it appears that hydrogen sulphide is produced only in the sediments.

Rates of sulphate reduction and other sedimentary biogeochemical processes were studied using ³⁵S and ¹⁴C tracers in the shallow sediments of the southeastern part of the Caspian Sea in 1977 (Ivanov et al., 1980) and in sediments located along the western shoreline near the estuary of the Kura river in October 1982 (Rivkina et al., 1985). Here we just summarize the sulphate reduction data. Most of the hydrogen sulphide formed in these sediments comes from sulphate reduction (Ivanov et al., 1980; Rivkina et al., 1985), as is the case for the other seas discussed in this chapter. Rates of sulphate reduction were the highest in the sediments of stations 62 and 57 (Table 8.6), where high rates were observed down to 100 and 160 cm. At station 60, in contrast, high rates of sulphate reduction were found only in the upper 10 cm of sediment. At most stations and depths, acid-volatile sulphides were the dominant short-term product of ³⁵SO₄^2- reduction, but non-acid-volatile products (pyrite and elemental sulphur) dominated in some cases (Table 8.6).

Integrating the data on rates of sulphate reduction over depth yields an average annual estimate of 50.6 g (S) m^-2 a^-1 (1.6 mol (S) m^-2 a^-1). Rates

Table 8.6. Characteristics of sediments and rates of sulphate reduction in the south part of the Caspian Sea (after
Mechtieva and Rabinovich, 1975; Rivkina et al., 1985; Ivanov et al., 1980, 1984)

may be higher elsewhere in the Caspian Sea since it may be assumed by analogy with other seas and marine systems that sulphate reduction is highest in fine-grained sediments with high organic carbon content. In the coarse and non-uniform-grained sediments which we studied, the organic carbon content ranged from 0.2 to 1% (Table 8.6) , whereas elsewhere in the Caspian Sea the organic carbon content ranges up to 3% (Bordovskiy, 1974).

The concentration of total reduced sulphur in the sediments varies from 0.03 to 0.24% (0.9 to 7.5 mmol (S) g^-l) in the sediments underlying shallow water we report on here (Table 8.6). In most of the sediment samples, pyrite is the dominant form, but in some cases, acid-soluble sulphides prevail (see station 57, horizon 25-69 cm, in Table 8.6). The rate of sedimentation on the eastern shelf of the southern Caspian is approximately 0.3 mm a^-1; in the west, sedimentation averages 0.6 mm a^-1 (Bordovskiy, 1974). Using these data, we estimate that the permanent burial of total reduced sulphur is approximately 60% of the rate of sulphate reduction. About 40% of the reduced sulphur formed during sulphate reduction is re-oxidized to sulphate in the upper layers of sediments and at the sediment-water interface.

Distribution of reduced sulphur in the deeper sediments of the Caspian Sea and its stable isotopic composition were studied by Mekhtieva and Rabinovich (1975). Using a mass-balance approach for sulphur isotopes, they estimated a permanent burial of sulphur in all of the sediments of the Caspian Sea of 2.5 x 10⁶ t (S) a^-1. Assuming that for the Caspian as a whole 60% of sulphate reduced during sulphate reduction remains buried, as in our stations, then a total of 6 X 10⁶ t (S) a^-1 of sulphate are reduced in the Caspian Sea.

8.8 CONCLUSIONS

Table 8.7 summarizes the rates of sulphate reduction and several related biogeochemical processes in anoxic zones (sediments generally, but including anoxic waters in the Black Sea) in the various seas reviewed in this chapter. Organic carbon consumption during sulphate reduction is estimated assuming that two moles of carbon are oxidized for every mole of sulphate reduced. Mineralization of ammonium is estimated from the rate of carbon oxidation, assuming stoichiometry defined the Redfield ratio. From the amount of reduced sulphur and ammonium which come from the anoxic zone into the oxic zone, it is possible to calculate the amount of oxygen consumed in the oxidation of these reduced substances, and to estimate the maximal value of secondary organic matter produced by chemoautotrophic production by sulphur- and nitrogen-oxidizing bacteria. Although the calculated values of biogeochemical processes given in Table 8.7 are only crude estimates in some
cases, they give a representation of the scale of these processes in different  regions of marginal and continental seas.

Table 8.7. Rates of different biogeochemical processes in surface sediments of some marginal and continental seas

The rate of sulphate reduction in the studied continental and marginal seas varies from 18 to 260 g (S) m^-2 a^-1 (0.56 to 8.1 mol (S) m^-2 a^-1; Table 8.7). With the exception of the shallow depths of the Black Sea, these rates are all higher than average sulphate reduction rates in the sediments of the precontinental areas of the ocean, which range from 18 to 26 g (S) m^-2 a^-1 (0.56 to 0.8 mol (S) m^-2 a^-1; Lein, 1989). The highest rates of sulphate reduction are found in the sediments of the shallow continental Baltic Sea (Table 8.7).

The intensity of oxygen consumption used up in oxidation of reduced compounds (H₂S, NH₄^-) formed in anaerobic organic matter decomposition in sediments varies from 205 to 824 g (02) m^-2 a^-1 (Table 8.7).

Sulphate reduction is responsible for most carbon oxidation in the anoxic portions of marginal and continental seas, as in most coastal marine systems. The rates of this carbon consumption can be quite significant, ranging from 14 to 195 g (C) m^-2 a^-1 in the various seas we reviewed (Table 8.7). Likewise, significant quantities of ammonium are released.

Much of the reduced sulphur formed during sulphate reduction is re-oxidized at the oxic-anoxic interface. Also, most of the ammonium released during the decomposition accompanying sulphate reduction is later oxidized. These oxidation processes-particularly the oxidation of reduced sulphur to sulphate-consume large quantities of oxygen (Table 8.7). Such consumption undoubtedly plays a major role in the potential depletion of oxygen in the bottom waters of these seas, and it is the seas with the largest rates of oxygen consumption by sulphur oxidation which in fact have anoxic waters at depth: the Baltic and Black Seas.

The potential rates of synthesis of new organic carbon by sulphur-oxidizing chemoautotrophic bacteria appear large, again particularly in the Baltic Sea. Production by nitrogen-oxidizing bacteria must certainly be smaller (Table 8.7). However, more direct measurements of the rates of these processes are required in most of the seas we reviewed.

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