Sedimentation rate carbon

Figure 3. Rates of sulfate reduction (all measured with 35S) reported in the literature (references in Table I) show no obvious relationship to either sediment carbon content or carbon sedimentation rates (measured with sediment traps). The lowest reported rate of sulfate reduction occurs in the lake with the lowest carbon sedimentation rate, but there is no evidence of carbon limitation among the other lakes. Error bars indicate the range of reported sulfate reduction rates.

Relative rates of sulfate reduction and methanogenesis in lakes of varying trophic status are claimed to indicate that sulfate reduction rates are limited by the supply of sulfate (4, 5, 13). According to this hypothesis, at high rates of carbon sedimentation, rates of sulfate reduction are limited by rates of sulfate diffusion into sediments, and methanogenesis exceeds sulfate reduction. In less productive lakes, rates of sulfate diffusion should more nearly equal rates of formation of low-molecular-weight substrates, and sulfate reduction should account for a larger proportion of anaerobic carbon oxidation. Field data do not support this hypothesis (Table II). There is no relationship between trophic status, an index of carbon availability, and rates of anaerobic... 

The studies cited do not clarify what factors determine rates of sulfate reduction in lake sediments. The absence of seasonal trends in reduction rates suggests that temperature is not a limiting factor. Rates of sulfate reduction are not proportional to such crude estimates of carbon availability as sediment carbon content or carbon sedimentation rate, although net reduction and storage of reduced sulfur in sediments often does increase with increasing sediment carbon content. Measured rates of sulfate reduction are not proportional to lake sulfate concentrations, and the relative rates of sulfate reduction and methanogenesis in a variety of lakes do not indicate that sulfate diffusion becomes limiting in eutrophic lakes. Direct comparison of diffusion and reduction rates indicates that diffusion of sulfate into sediments cannot supply sulfate at the rates at which it is reduced. Neither hydrolysis of sulfate... 

The main mechanism for removal of organic carbon from the ocean is burial in sediments. This flux is equal to the average global sedimentation rate for marine sediments times their weight percent organic carbon. The total sink... 

Chapters 11 and 12 focus on the oceans. The first of these describes the use of U-series nuclides in the modern ocean, where they have been particularly useful during the last decade to study the downward flux of carbon. The second ocean chapter looks at the paleoceanographic uses of U-series nuclides, which include assessment of sedimentation rates, ocean circulation rates, and paleoproductivity. Both of these ocean chapters demonstrate that knowledge of the behavior of the U-series is now sufficiently well developed that their measurement provides useful quantitative information about much more than just the geochemistry of these elements. 

Other applications of °Thxs profiling to assess accumulation rates of sedimentary components include carbonate accumulation in the Western Equatorial Atlantic (Rtihlemann et al. 1996) biogenic and terriginous particle accumulation on the Australian continental margin (Veeh et al. 2000) sedimentation rates in the North East Atlantic (McManus et al. 1998 Thomson et al. 1993 ) (Fig. 5) sedimentation rates during key... 

These three numerical experiments show how the waters of an evaporating lagoon respond differently to the different seasonal perturbations that might affect them. Some record of these perturbations might, in principle, be preserved in the carbonate sediments precipitated in the lagoon. All three perturbations—productivity, temperature, and evaporation rate— cause seasonal fluctuations in the saturation state of the water and in the rate of carbonate precipitation. Temperature oscillations have little effect on the carbon isotopes. Although seasonally varying evaporation rates affect 14C, they have little effect on 13C. Productivity fluctuations affect both of the carbon isotopes. 

I consider a system in which organic matter is oxidized at a steady rate that is a specified function of depth in uniform calcium carbonate sediments. The oxidation of organic matter increases the total dissolved carbon in the pore water of the sediment. The resultant increase in acidity causes the dissolution of calcium carbonate and a consequent increase in alkalinity as well as another increase in total dissolved carbon. The total dissolved carbon and alkalinity are transported by diffusion between different depths in the sediment. 

This model requires an excess of sulfate over reducible carbon. Concentrations may be measured in solutions squeezed from sediment cores, diffusion coefficients are known from standard chemical data tables and sedimentation rates determined from 14C, 210Pb, or 230Th dating. Therefore, this model finds its best use in the recovery of the kinetics of organic matter decay. A discussion of this and similar equations and numerical applications may be found in Berner (1980). 

Relationship between flux of organic carbon to the sea floor and organic carbon remineralization rate. Data are a compilation of measurements made globally. Redrawn by Wakeham, S. (2002). Chemistry of Marine Water and Sediments, A. Gianguzza, E. Pelizzetti, and S. Sammartano, eds. Springer, pp. 147-164. From Heinrichs, S. (1993). Organic Geochemistry—Principles and applications, Plenum Press, pp. 101-115. 

Table 25.1 Organic Carbon Burial Rates Regimes (Pg C/y). in Different Marine Sediment ...

Relative organic carbon content of suspended particles Sedimentation rate (estimated)... 

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Figure 8A. Rates of sulfur and carbon accumulation are highly correlated in surface sediments of 11 Swiss lakes (23). The solid line is the regression line and the dotted lines represent 95% confidence intervals. Variations in the carbon accumulation rates represent differences in trophic status and...

Total S content cannot indicate whether increased carbon inputs to sediments cause increased diffusion of sulfate into sediments or restrict reoxidation and release of S from sediments, because the net effect is the same. In a survey of 14 lakes, Rudd et al. (80) did not observe a strong correlation between organic matter content per volume and net diffusive flux of sulfate. However, in English lakes the lowest C S ratios occur in the most productive lakes (24) whether this represents enhanced influx or retarded release is not clear. Among 11 Swiss lakes, ratios of C to S sedimentation rates are relatively constant and substantially below C S ratios in seston net S fluxes... 

Fontana and Thomas25) estimated LH from the sedimentation rate of carbon-black particles covered with poly(lauryl methacrylate) (PLMA) or stearyl methacrylate/N-vinyl-2-pyrrolidone copolymer (PAM/A-VP). The values of LH for PLMA were 20 to 40 A, while those for PAM-VP were 210 40 A. 

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