Diffusive flux sediments

In coastal sediments where organic carbon concentrations are high, the redox boundary is at or near the sediment-water interfece. Under these conditions, denitrification acts as a sink for nitrate. In some settings, the rate of sedimentary denitrification is fast enough to drive a diffusive flux of nitrate from the bottom waters into the sediments. Remineralization of organic matter imder suboxic and anoxic conditions releases... 

The diffusive flux of the aqueous species of a chemical between the open water (op) and the pore water of the sediment column (sc) can be described by the linear expression ... 

The amount of P supplied by resuspension was relatively small compared with water-column standing pools and major flux vectors. Thus, resuspension of bottom sediments may not be a major mode of phosphorus resupply. The pool of resuspendable P is finite. The deposition-resuspension cycle will not increase the amount of P in this pool unless P is added from another source (e.g., by diffusion of P from lower sediment levels). However, the diffusive flux would be relatively small. The resuspendable particulate P can be recycled during spring mixing by repeated deposition and resuspension, but this cycle does not increase the amount of P in the resuspendable pool. Eadie et al. (24) reported a resuspended P flux (sediment-trap-based) of3200 mg of P/m2, 66 times our estimate here. However, this large P flux would require the resuspension of over 2.0 cm of surface sediment and much higher suspended Al levels than were measured in the water column. 

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. 

A major factor governing diffusive fluxes of sulfate into sediments is lake sulfate concentration. A linear relationship exists between lake sulfate concentrations and diffusive fluxes calculated from pore-water profiles (Figure 5). The relationship extends over a range of 3 orders of magnitude in sulfate... 

Despite the strong relationship between sulfate concentrations and diffusive fluxes, there is no universal relationship between lake sulfate concentrations and concentrations of S in sediments (Figure 1A cf. 24, 26). Concentrations of S in sediments are the net result of inputs from seston, diffusive inputs, recycling to the water, and dilution by other materials. Mathematically this quantity may be expressed as... 

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... 

The subsurface maximum in pore-water HgT (Figure 3) suggested that diffusion from the profundal sediments to the overlying water column could be important. Fickian diffusive flux calculations (eq 2) were used to estimate Hg loading from pore waters. Diffusion coefficients for mercury in pore waters were not available. However, free-water diffusion coefficients for monovalent anions (see Table I) averaged about 5 X 10"6 cm2/s (53, 55) and... 

Internal cycling was examined by measuring rates of accumulation in sediment cores, seston deposition rates, and diffusive fluxes to the sediments. 

Seston-S deposition probably is a more important process than dissimilatory reduction in lakes with low [SO42 ]. As lakewater sulfate concentrations increase, seston deposition reaches a plateau limited by the overall primary production rate and the maximum algal S content, but diffusive fluxes continue to increase in direct proportion to [SO42 ]. Thus, in highly acidic lakes (pH 3 5 [SOjt2 J > 100 peq/L), such as McCloud Lake, Florida and Lake 223, Ontario, dissimilatory sulfate reduction probably is the major sulfate sink. Nriagu and Soon (131 concluded that endproducts of dissimilatory reduction and elevated sediment S content would not be observed below S mg/L (240 / eq/L), but we see clear evidence of dissimilatory reduction in Little Rock Lake at concentrations of approximately SO /teq/L. 

In-lake processes remove approximately half of the sulfate inputs from the water column of Little Rock Lake. Two processes, seston deposition and dissimilatory reduction, are responsible for sulfate retention. For the preacidified lake, seston deposition probably is the dominant sink, accounting for 70% of net retention. Preliminary data and theoretical considerations suggest that the diffusive flux of sulfate to sediments will increase during experimental acidification, and we believe that dissimilatoty reduction is the dominant sulfate sink in lakes with elevated sulfate concentrations. 

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... 

For lakes which have undergone significant acidification, it has been suggested that heavy metals could be released from surface sections by pH-dependent dissolution, resulting in sub-surface maxima in sedimentary heavy metal concentrations. In two Canadian acid lakes, however, Carignan and Tessier found that downward diffusive fluxes of dissolved zinc from overlying waters into anoxic pore waters were responsible for the pronounced sub-surface sediment maxima in solid phase zinc, presumably as the insoluble sulfide. 

The thermal motion of particles in a suspension opposes their sedimentation by creating a counteracting diffusive flux. This thermal motion is called Brownian motion. 

Mass balance calculations for the Amazon shelf seabed provide important insights into N cycling in the study area. If all of the riverine particulate N regeneration is assumed to take place in the water column, the total supply of N to the seabed (including marine PON deposition and the diffusive flux across the sediment-water interface) equals 5 7 x 10 mol d . This value is 4.4 x 10 mol d i greater than the burial term for N on the Amazon shelf (1.3 x 10 mol d O. Based on the observation of denitrification reactions in the seabed (Aller et al. 1986,... 

---