Sediment-water Solute Fluxes

Mean values of flux magnitudes and ratios exceeded median values (Table 18.4) there were a small number of large values for each of these fluxes (Fig. 18.16A). 

Type of flux or flux ratio Minimum Maximum Mean Median ft 

AH measurements were made in environments where the sediments were aphotic and all sediment incubations were made in the dark. Flux units are umol O2, N or P h N P and O N ratios atomic basis. Negative values indicate fluxes into sediments. 

We also examined flux data with respect to water temperature at the time of measurement using the sediment-water flux data set developed by Bailey (2005 Fig. 18.16D). In aU cases there were sharp increases in rates with increased temperature. Estimated Qio (0—30°C) values for NH4, PO4 and SOC fluxes were 2.9, 3.0, and 1.8, respectively. It is also useful to note there is considerable bias in the temperature range in which these measurements are made. Less than 10% of the reported rates were from temperatures 5°C, and only a sHghdy higher percentage were from temperatures of 5—10°C. About 50% of aU measurements 

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To apply Fick s laws to solute fluxes in sediments, adjustments have to be made to these equations to account for the negative interference effects that sediment particles have on the diffusion of solutes in pore waters (Lerman, 1979 Berner, 1980). For example, tortuosity, defined as the length of the tortuous path that a solute travels around particles across a distance across a certain depth interval can be described by the following equation (Berner, 1980 Krom and Berner, 1980a) ... 

Janssen, F., Faerber, P., Huettel, M., Meyer, V., and Witte, U. (2005a). Pore-water advection and solute fluxes in permeable marine sediments(I) Calibration and performance of the novel benthic chamber system Sandy. Limnol. Oceanogr. 50, 768—778. 

An observer balanced on the sediment-water interface (z = 0) as sediment f>articles continue to arrive from above and pile up will see the particles and pore water flow by in the downward direction. In this sense one can always speak of the fluxes of solids, waters, and solutes as moving up or down (Lerman, 1979). 

The study has been broken into two parts the first concentrates on describing diagenetic processes involving organic-matter decomposition and production or consumption of the nutrients SO ", NH4, alkalinity, and HP04. The second emphasizes the associated chemical behavior of Fe and Mn (Part II). Several types of measurements were made (1) seasonal pore water and solid-phase analyses, (2) direct measurement of solute fluxes out of the sediment, (3) rates of reaction as a function of depth and temperature, and (4) the abundance and composition of the fauna at each station. Taken together, these measurements provide one of the most detailed descriptions of controls on diagenesis near the sediment-water interface that is presently available. 

Condition (a) specifies a constant concentration of solute along the sediment-water interface and within the burrow core. Condition (b) requires pore-water solute concentrations to go through a maximum or minimum value half-way between any two burrows. The last condition, (c), matches the bioturbated zone to the underlying unburrowed zone by requiring continuity in flux across the lower boundary at depth L. 

Several kinds of chemical measurements were made in each of three distinct depositional environments from the Sound. These are (1) seasonal variation of pore-water solute profiles over 1- or 2-year periods, (2) solid-phase analyses of total Fe and Mn, and (3) direct flux measurements of Fe and Mn released from bottom sediments. In addition, several laboratory experiments were performed to help substantiate or disprove interpretations of field data. The availability of Th- or Pb-particle reworking rates in surface sediments (Benninger et al., 1979 Aller et al.,... 

Condition (6.13a) specifies that the solute concentrations along the sediment-water interface and within the ideal burrow are equal and constant. The second condition, (6.13b), requires that concentrations go through a maximum or minimum halfway between individual microenvironments. Condition (6.13c) matches the bioturbated zone with the underlying unburrowed zone by requiring a continuity of flux between the two regions. In this case, the gradient is taken as —0 because of the requirement that... 

At FOAM the minimum flux predicted on the basis of concentration gradient is always larger than the measured flux. Similar overestimates occur for winter cores at both NWC and DEEP and the fall core at DEEP. These discrepancies are consistent with a partial loss of Mn from solution in the top centimeter or at the sediment-water interface in each case. It should be noted that although the calculated flux estimates are approximate and either overestimate or underestimate the flux for the reasons just discussed, they do predict the measured flux to within a factor of 2-6 in all cases. 

The diffusive boundary layer plays an important role for the exchange of solutes across the sediment-water interface (Jorgensen 2001). For chemical species which have a very steep gradient in the diffusive boimdaiy layer it may limit the flux and thereby the rate of chemical reaction. This may be the case for the precipitation of manganese on iron-manganese nodules (Boudreau 1988) or for the dissolution of carbonate shells and other minerals such as alabaster in the deep sea (Santschi et al. 1991). For chemical species with a... 

In the last decade, however, in-situ techniques have been developed to overcome these problems. Profiling lander systems were deployed to record the pore water microprofiles of oxygen, pH and pCOj, and Ca whereas benthic chambers were deployed to measure solute fluxes across the sediment-water interface directly. Very often, reactive-transport models are used to explain the interrelation between measured microprofiles, to predict overall calcite dissolution rates by defining the dissolution rate constants, and to distinguish between dissolution driven by organic matter oxidation and by the undersaturation of the bottom water. 

Advective flux refers to the bulk flow of solids or pore water relative to an adopted frame of reference such as the soil-water interface of wetlands (Berner, 1980). Advection is associated with the flow of material with either the velocity of its own or the velocity of medium (water) through which the material is transported (Lerman, 1979). During advective transport, solutes are typically transported at the same velocity as water or air. Advective fluxes can include sediments and solutes carried in surface water flows, or solutes in groundwater and pore water flow. The flow of material of a given density p(M L ) with velocity f/(L T ) results in the advective sediment flux J as described in the following equation ... 

The net flux of solutes due to diffusion across the soil or sediment-water interface can then be described as follows ... 

In general, the total solute flux from sediments to overlying water column is the result of the sum of several mixing processes, which include molecular diffusion (Dg), biodiffusion (Dg), diffusion due to irrigation (D,), and mixing due to wave and current mixing The sum of all these effects... 

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