Material Fluxes on the Surface of the Earth (1994) / Chapter Skim
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Seafloor Diagenetic Fluxes
Seafloor Diagenetic Fluxes
Pages 143-164
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From page 143... ...
Diagenetic fluxes exceed burial rates for the major bioactive elements: about 90 percent of the organic carbon rain to the sea floor degrades during early diagenesis; over 60 percent of the biogenic silica, and about 80 percent of the CaCO3 reaching the sea floor dissolve. Thus, variations in diagenetic reaction rates can have disproportionately large effects on sedimentary concentrations and accumulation rates of these components.
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From page 144... ...
In continental margin sediments, solute transport is enhanced over that due to diffusion by biologically and physically driven irrigation, and models applied to pore water profiles must include independent estimates of the effects of these processes (Aller, 1980b; Christensen et al., 1984~. Benthic flux estimates that are less dependent on transport models are obtained through the use of benthic flux chambers.
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From page 145... ...
Because of the difficulty of making artifactfree measurements of dissolved inorganic (and organic) carbon in pore waters, benthic organic carbon degradation rates are often estimated based on fluxes of nitrate and oxygen across the deep sea sediment-water interface, and on fluxes of sulfate in nearshore sediments.
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From page 146... ...
Atlantic Coast Continental Margin Hatteras Continental Rise Hatteras Abyssal Plain Bermuda Rise Sargasso Sea Equatorial Atlantic Pacific Gyre B Pore Water AO2//iNO3- and -/\O2//\ICO2 = 138:106 Central Equatorial Pacific Eastern Equatorial Pacific Subtropical South Pacific Eastern Equatorial Pacific, 10°S to 11°N 7.6 to 8.3 8.6+0.1, 8.4+0.3 7.0+0.7,5.4+ 1.9,6.6+0.8 6.6 + 0.8 5200 m trap: 7.4; sediment: 8.0 5000 m trap: 10.5; sediment: 12 5600 m trap: 9.6; sediment: 11 8.1 +2.2 7 12 7.6 + 1.9 C
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From page 147... ...
Sayles (1979; 1981) has estimated benthic fluxes from in situsampled pore waters, using model fits to the pore water concentration versus depth data, at a large number of stations in the temperate north and south Atlantic, the Caribbean Sea, and the southern ocean between Africa and Antarctica.
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From page 148... ...
p// 0 200 400 0 1.0 2.0 7.0 8.0 , ,1 , _ _ ~ a\ Z0 0 100 200 0 1.0 Z0 70 8.0 \ r 0 1.0 Z0 -0.20 0 0.20 -20 0 2.0 0 200 400 - be 0 1.0 Z0 7.0 8.0 \ ~ ' . FIGURE 10.2 Profiles of the major components of seawater in the pore waters of marine sediments from subtropical South Atlantic.
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From page 149... ...
We have estimated the involvement of early diagenesis by comparing its rate to the rate of formation of the solid phase in the surface ocean (Table 10.6~. For carbon, we have calculated the early diagenetic flux as the difference between the rain rate to the sea floor and the burial rate, with the rain rate estimated from surface water productivity and compilations of benthic flux measurements for the deep sea (Smith and Hinga, 1983; Jahnke and Jackson, 1987)
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From page 150... ...
Quantitative evaluations of the spatial variability of benthic fluxes must take into account many factors affecting reactivity, which we will consider later. With the possible exception of CaCO3, however, there is an underlying correlation between the rain rate to the sea floor and the benthic flux (Reimers, 1989~.
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From page 151... ...
Temporal variability in the concentrations and accumulation rates of the biogenic components of marine sediments are a key source of information on changes in oceanic and atmospheric conditions on the thousand to ten thousand year time scale of the events governing glacial-to-interglacial transitions. Because early diagenetic reactions at the surface of sediments may affect the relationship of observed concentrations and accumulation rates to oceanic and atmospheric conditions, correct interpretation of the sedimentary record requires an understanding of the relationships between rain rates to the sea floor, conditions at the surface of the sediments, and the ultimate concentrations and accumulation rates of sediment components.
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From page 152... ...
The result of the rapidity of CaCO3 dissolution in undersaturated pore waters is that transport processes within the sediment column are of secondary importance in detaining calcite burial efficiency. Dissolution in sediments occurs for two reasons: because the sediments lie underneath undersaturated bottom water, and in response to acids released by the oxidation of organic matter by O2 (see the stoichiometry in Table 10.1~.
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From page 153... ...
~Dymond and Lyle (1985~; sediment trap data, 1-yr duration; burial rates from radioisotope dating. eBender and Heggie (1984~; estimates of TCO2 produced are from models of NO3 profiles in the pore water.
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From page 154... ...
If the sediment trap based estimates of rain rates do not underestimate the true rain rate of CaCO3, then we must conclude that, even at these sites below the lysocline, dissolution due to metabolic acids is an important process. SiO2 The driving force for biogenic silica dissolution in marine sediments is similar to that for CaCO3: pore waters are undersaturated with respect to the solid phase present.
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From page 155... ...
The importance of sulfate reduction lies in the fact that it occurs to a significant extent on continental margins, where the bulk of organic carbon burial in marine sediments takes place (Berner, 1982~. Organic carbon burial efficiency varies because of varying composition of the degrading organic matter pool, and also because of variations in electron acceptor availability.
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From page 156... ...
Even here, however, there is a complication: from the compilation presented here, we can conclude that reac tions within the sediments account for 40 to 75 percent of CaCO3 dissolution at the sea floor, and a substantial fraction must be due to the acid introduced into pore waters by oxic degradation of organic matter. Thus, quantitative interpreta tions of CaCO3 preservation depend on an understanding of organic matter diagenesis.
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From page 157... ...
The results are preliminary: the model requires that pore water nutrient, ICO2, and/or O2 data and solid phase organic carbon data be available for the determination of the degradation rate constant (pore water data to furnish the rate, solid phase data to convert the rate into a first-order degradation rate constant) ; and that sedimentation rate and bioturbation mixing rate data (we have used 2~0Pb measurements)
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From page 158... ...
, using data from these sites and others: the rain rate of organic carbon to the sea floor increases with the bulk sedimentation rate, and the degradation rate constant varies with the rain rate. Thus, the model confirms the importance of variations in reactivity in determining the relationship between organic carbon burial efficiency and bulk sedimentation rate.
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From page 159... ...
Early diagenetic reaction rates are greater than burial rates: thus, they may exert important control over the rates of burial of the biogenic sediment components, organic carbon, calcite, and opal. The burial efficiency of the biogenic sediment components that is, the fraction of the rain to the sea floor that is preserved to become part of the sedimentary record varies substantially in the oceans for each component.
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From page 160... ...
In the top figure, xlk' is held constant at 2.0 and the mixing rate coefficient, DBO is allowed to vary; in the bottom figure, DBO is constant at 0.16, while x/k, is varied. For both, the rain rate to the sediment-water interface is lO,umol C/cm2/yr; the organic carbon degradation rate at x = 0 (ko)
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From page 161... ...
. Carbonate chemistry in marine pore waters: MANOP sites C and S, Earth and Planetary Science Letters 61, 220-232.
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From page 162... ...
Maynard (1979~. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: Suboxic diagenesis, Geochimica et Cosmochimica Acta 43, 1075-1090.
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From page 163... ...
The role of sedimentary organic matter in bacterial sulfate reduction: The G model tested, Limnology and Oceanography 29~2)
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