Sulfate depletion, pore waters

Dissolved sulfate normally is added to the sediment column by two mechanisms (1) burial of sulfate-containing seawater with the sediment and (2) diffusion of dissolved sulfate from overlying seawater or sediments into sulfate-depleted pore waters (here, ignoring addition of sulfate by advection or bioturbation). The flux of sulfate due to burial of seawater is (Berner, 1980)... 

Occurrence and Rates of Sulfate Reduction. Sulfate reduction is widespread in lakes, as evidenced by depletion of sulfate in sediment pore waters. Pore-water profiles showing depletion of sulfate have been published for more than 35 lakes (e.g., 98, 99). An absence of sulfate depletion in pore waters does not indicate an absence of sulfate reduction. Sulfate depletion was not evident in pore waters of McNearney Lake, but stable isotope measurements indicated that low rates of sulfate reduction must occur (1). Sulfate depletion was noted in 15 of 17 lakes in northeastern North America and Norway (80, 98), but uptake of 35S042 occurred even in two lakes in which no sulfate depletion was observed. Sulfate production and reduction can occur concurrently, and the former may exceed the latter. 

Sediments deposited in Flodelle Creek spring pool and the Great Lakes have similar and relatively uncomplicated sulfur geochemistry that is controlled by two processes. These processes are the assimilation of sulfur into living biota and its subsequent deposition as organosulfur when the organism dies, and the complete reduction of the pore-water sulfate to H2S that forms sulfide minerals. Low dissolved sulfate concentrations limit the amount of sulfide minerals formed. The 834S value of most of the Smin is essentially the same as the dissolved sulfate. The possible exceptions are minerals formed in sediment from which some 34S-depleted H2S had diffused. 

Berner (3J5.) attributes monosulfide preservation in Black Sea sediment to insufficient elemental sulfur (polysulfides) to completely convert FeS to FeS2 (Equation 3). Both sulfide minerals in sediment from Walker Lake are typically depleted in 34S with a 834Smin average of -15%o. At a few depths, however, 834S values are similar or enriched in 34S relative to the sulfate in the modern lake (834S +10%o calculated from pore-water sulfate data). 

The profile of Mg2+ in Figure 8.25 indicates downward diffusion of this constituent into the sediments. Mass balance calculations show that sufficient Mg2+ can diffuse into the sediments to account for the mass of organogenic dolomite formed in DSDP sediments (Baker and Bums, 1985 Compton and Siever, 1986). In areas of slow sedimentation rates, the diffusive flux of Mg2+ is high, and the pore waters have long residence times. Dolomites form under these conditions in the zone of sulfate reduction, are depleted in 13c, and have low trace element contents. With more rapid sedimentation rates, shallowly-buried sediments have shorter residence times, and dolomites with depleted 13C formed in the sulfate-reduction zone pass quickly into the underlying zone of methanogenesis. In this zone the DIC is enriched in 13C because of the overall reaction... 

The study of sulfur diagenesis has a long history. Scientists have long accepted that microorganisms play a major role in geochemical sulfur transformations (Baas Becking, 1925). They also recognized at an earlier time that removal of sulfate occurs in the pore waters of marine mud (Murry, 1895). Subsequent work established that depletion of sulfate from marine pore water is a... 

Dissolved sulfide in this zone builds up because of its bacterial production. Its maximum concentration is less than the total sulfate reduced because a portion of the H2S reacts with iron and organic matter to form insoluble products. At any depth, the concentration gradient of H2S is kinetically controlled and reflects the balance between the rate of these removal processes, the rate of gain or loss by diffusion, and the rate of its formation by reduction. One of these processes, the production of H2S, must cease when all S04 has been consumed. The net result is a concentration maximum that falls in a range from 1 pM to >10 mM. The depth of maximum pore-water H2S commonly correlates closely with the depth of total S04 depletion. In most environments, H2S persists at measurable concentrations (i.e., greater than a few micromolar) in pore waters to depths of a few centimeters to several meters below the point at which S04 is removed. The essentially total removal of pore-water H2S is a reflection of the availability of excess iron over sulfide sulfur in most sediments (see below). Pyrite content may increase gradually within zone III, but the rate of this increase is most rapid at the top of this zone. Frequently, increases in pyrite cannot confidently be distinguished from scatter in the data within this zone. 

With suitable anoxic conditions and abundant organic matter, the pore waters in sediments may be totally depleted of sulfate within a depth of 1 to 2 m. In pore waters of Saanich Inlet, British Columbia, where organic carbon contents reached 5%, Nissenbaum et al. (1972) found essentially total sulfate depletion at depths of less than 0.5 m. This depth corresponds to a time span of hundreds of years. In contrast, some off-shore cores from the JOIDES Deep Sea Drilling Programme show that, in more slowly accumulating sediments, there is negligible sulfate depletion despite apparent continuation of sulfate reduction for millions of years, and to depths of several hundred meters. A number of these extreme, as well as intermediate cases are summarised by Goldhaber and Kaplan (1974). 

Sulfate reduction rates in the deep basin sediments of the Saguenay Fjord are relatively low in comparison to other coastal environments (32 nmol cm-2 d-1 integrated over the top 30 cm), and the pore water is only weakly depleted in sulfate. Sulfate reduction is more intense, however, in the rapidly accumulating... 

Fig. 8.10 Geochemical data for core GeoB 1023-4 recovered off north Angola (17°09.6 S, 10°59.9 E, 2047 m water depth). Barium and sulfate pore-water concentration profiles as well as the distribution of solid-phase barium indicate the precipitation of authigenic barite at a front slightly above the depth of complete sulfate consumption. Below the sulfate/methane transition barite becomes undersaturated and is thus subject to dissolution due to the total depletion of pore-water sulfate. Dissolved barium diffuses upwards into the sulfate zone where the mineral barite becomes supersaturated and so-called authigenic or diagenetic barite precipitates at a front at the base of the sulfate zone. Modified from Gingele et al. (1999), after Kolling (1991).

Microbes residing in sediment beneath oceans and lakes derive energy by oxidizing organic matter. 02 is available as the oxidant at the sediment-water interface, but it is depleted within millimeters below the interface. Nitrate and Fe(III) oxidants are available in the first few centimeters of sediment. When they are exhausted, sulfate becomes the predominant oxidant for a distance of 1 m. The sulfate reduction product, HS-, is released in millimolar concentrations into solution in the sediment pores. 

Details of sulfur isotope geochemistry are presented elsewhere in this volume (see Chapter 7.10) and are only highlighted here as related to paleo-environmental interpretations of finegrained siliciclastic sequences. Formation of sedimentary pyrite initiates with bacterial sulfate reduction (BSR) under conditions of anoxia within the water column or sediment pore fluids. The kinetic isotope effect associated with bacterial sulfate reduction results in hydrogen sulfide (and ultimately pyrite) that is depleted in relative to the ratios of residual sulfate (Goldhaber... 

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