Marine sediments diffusion

In seawater, physical processes that transport water can also cause mass fluxes and, hence, are another means by which the salinity of seawater can be conservatively altered. The physical processes responsible for water movement within the ocean are turbulent mixing and water-mass advection. Turbulent mixing has been observed to follow Pick s first law and, hence, is also known as eddy diffusion. The rate at which solutes are transported by turbulent mixing and advection is usually much faster than that of molecular diffusion. Exceptions to this occur in locations where water motion is relatively slow, such as the pore waters of marine sediments. The effects of advection and turbulent mixing on the transport of chemicals are discussed further in Chapter 4. 

Reaction rates of nonconservative chemicals in marine sediments can be estimated from porewater concentration profiles using a mathematical model similar to the onedimensional advection-diffusion model for the water column presented in Section 4.3.4. As with the water column, horizontal concentration gradients are assumed to be negligible as compared to the vertical gradients. In contrast to the water column, solute transport in the pore waters is controlled by molecular diffusion and advection, with the effects of turbulent mixing being negligible. 

Schematic diagram of the coupled iron and phosphate cycles during early diagenesis in marine sediments. Light gray ovals and circles represent solid phases black arrows are solid-phase fluxes. White outlined black arrows Indicate reactions white arrows are diffusion pathways. Source From Ruttenberg, K. C. (2003). Treatise on Geochemistry, Elsevier Ltd. pp. 585-643.

A conceptual model of sedimentary nitrogen cycling. Dashed arrows represent pore water diffusion and advection. Dotted arrows represent sedimentation. Source-. After Burdige, D.J. (2006). Geochemistry of Marine Sediments. Princeton University Press, p. 453. 

A third approach was developed by Aller (1980a, b) who studied solute fluxes in near-shore marine sediments showing seasonal variation. In this approach, the geometry of the burrow-sediment system is allowed for explicitly and transport in the sediment between the burrows is described with appropriate diffusion equations. It is assumed that the burrows are oriented normal to the sediment surface and distributed uniformly or randomly in the horizontal plane (Figure 2.11). Thereby a cylindrical zone of influence is ascribed to each burrow with a radius... 

Figure 8.25. Some chemical and diagenetic properties of organic-rich marine sediments as a function of depth based on DSDP interstitial water profiles. A. Schematic gradients of SO42-, total alkalinity, Ca2+ and Mg2+ in pore waters, and zones of sulfate reduction, methanogenesis and fermentation. Magnesium diffuses into the sediment and organogenic dolomite forms at depth. B. Logarithm of calculated saturation states of interstitial waters with respect to dolomite. Dolomite saturation=0. All these pore waters are oversaturated with respect to dolomite. (After Compton, 1988.)...

Iverson, N., and Jprgensen, B.B. (1993) Diffusion coefficients of sulfate and methane in marine sediments Influence of porosity. Geochim. Cosmochim. Acta 57, 571-578. 

Krom, M.D., and Berner, R.A. (1980a) The diffusion coefficients of sulfate, ammonium, and phosphate ions in anoxic marine sediments. Limnol. Oceanogr. 25, 327-337. 

Ullman, W.J., and Aller, R.C. (1982) Diffusion coefficients in nearshore marine sediments. Limnol. Oceanogr. 27, 552-556. 

In recent years various workers f1-7J have successfully developed models based on the mathematics of diffusion (8) to describe vertical profiles of selected chemical parameters in marine sediments dominated by sulfate reduction. Several papers 9, 10) have also proposed models for nitrogen diagenesis in the upper aerobic zone of such sediments. Most of these models, however, deal with only one or two relatively well behaved parameters, such as SO5" or CO2, which do not interact strongly with other components of the sediment besides organic matter. A truly comprehensive model for such sediment should deal simultaneously with all of the major chemical parameters of the system and ideally should be formulated as an initial value prob-... 

This paper proposes a system of 10 non-linear, simultaneous differential equations (Table I) tdiich upon further development and validation, may serve as a comprehensive model for predicting steady state, vertical profiles of chemical parameters in the sulfide dominated zones of marine sediments. The major objective of the model is to predict the vertical concentration profiles of H2S, hydrotriolite (FeS) and p3nrite (FeS2). As with any model there are a number of assumptions involved in its construction that may limit its application. In addition to steady state, the major limiting assumptions of this model are the assumptions that the sediment is free of CaC03, that the diffusion coefficients of all dissolved sulfur species are equivalent and that dissolved oxygen does not penetrate into the zone of sulfate reduction. 

These studies of particle mixing in marine sediments make it clear that particle transport in the upper 10 cm of the sediment column has important elfects on the distribution of reactive solids in the sediment column. While simple, diffusive mixing models explain many features of particle mixing, it is important to allow for more complex features of infaunal behavior selective transport of fine-grained, fresh sediment particles over coarser, older particles and transport over longer depth scales than can be explained by simple, diffusive mixing models. These transport... 

Isotopic Processes in the Upper (Bioturbated) Sediment Regime. 7.3 Isotopic Processes in the Deeper Diffusion-dominated Sediment Regime ANCIENT MARINE SEDIMENTS... 

Colman A. S. and Holland (1994) Benthic phosphorus regeneration the global diffusive flux of phosphorus from marine sediments into the oceans. EOS, Trans., AGU 75, 96. 

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