Sediment mixing processes

Figure 12. An example of use of to assess the rate and depth of sediment mixing from a core on the slopes of the Bahamas (Henderson et al. 1999b). The exponential decrease in Pbxs seen in the upper 6 cm of the sediment reflects decay of °Pb as it is mixed downward. The diffusional model of mixing described in the text indicates a mixing rate, D, of 51 cm kyr for this core. The two circled points at greater depth reflect rapid injection of surface material to depth in a process known as conveyor-belt feeding (Robbins 1988 Smith et al. 1997).

Once the radionuclides reach the sediments they are subject to several processes, prime among them being sedimentation, mixing, radioactive decay and production, and chemical diagenesis. This makes the distribution profiles of radionuclides observed in the sediment column a residuum of these multiple processes, rather than a reflection of their delivery pattern to the ocean floor. Therefore, the application of these nuclides as chrono-metric tracers of sedimentary processes requires a knowledge of the processes affecting their distribution and their relationship with time. Mathematical models describing some of these processes and their effects on the radionuclide profiles have been reviewed recently [8,9,10] and hence are not discussed in detail here. However, for the sake of completeness they are presented briefly below. 

Once these nuclides deposit on the ocean floor they are likely to be subjected to particle mixing processes. In the following we discuss attenuation due to a simple case of mixing, in which the sedimentary particles are mixed to a constant depth, L, from the sediment-water interface [72,73]. For such a case the temporal variation in the standing crop (atoms/cm2) C, of the radionuclide in the mixed layer is given by ... 

In this chapter, we will review various solution techniques for the diffusion equation, which is generally dehned as the mass transport equation with diffusive terms. These techniques will be applied to chemical transport solutions in sediments. There are also a number of applications to chemical transport in biohlms. There are many other applications of the diffusion equation, including most of the topics of this text, but they require more background with regard to the physics of mixing processes, which will be addressed in later chapters. 

Figure 23.4 Surface mixed sediment layer (SMSL) of thickness zmjx above a permanent sediment. The processes are A = particle settling, B = transfer into permanent sediment, C = diffusive exchange, D = resuspension, E = chemical or biochemical degradation.

During mixing of the lake in November-December, the accumulated Mn(II) is reoxidized, and the manganese oxides formed are eliminated from the water column by sedimentation. This process results in the precipitation... 

Similar to 234 Th, downcore profiles of 7Be can also be used to determine seasonal changes in sedimentation and sediment mixing rates in estuaries (Canuel et al., 1990). The basic assumption here, as described earlier, is that the nuclide (e.g., 7Be) traces movements of particles during sediment accumulation and that the delivery and trapping of the nuclide to surface sediments is uniform across habitats within an estuary. The three basic processes controlling the depth distribution are (1) supply rate from sedimentation (2) radioactive decay and (3) postdepositional particle mixing processes. Finally, it should be noted that using 7Be for the aforementioned purposes also requires concurrent measurement of 7Be in atmospheric fallout (Canuel et al., 1990). 

Under oxic conditions all sediments are mixed as the result of the actions of a variety of types of biota. Depending on the depth of biological activity, the sedimentary record will reflect this mixing process in the distribution of radionuclides. The full equation describing the distribution of a radioactive species in a sediment pile is... 

Figure 1 Particle cycling in the surface mixed layer of marine sediments. The processing of the rain of particles to the seafloor results in exchanges of solutes across the sediment-water interface and alteration of the particulate reactants, so that the composition of accumulating sediment is signiflcantly different from that of the particulate rain...

Natural lacustrine and estuarine sediments whose accumulation rates are low, generally below 0.25cmyr , often do not satisfy the above requirements. The biophysical term biotur-bation refers to surficial sediments mixed by the actions of deposit feeders, irrigation tube dwellers, and head-down feeders (Boudreau, 1999). In general, these bioturbation processes do not occur in reservoirs where sediment accumulation rates exceed land often Scrnyr (Callender, 2000). At these rates, the sediment influx at the water-sediment interface is too great for benthic organisms to establish themselves. 

This sequence of reactions indicates that conversion of I to I03 in seawater is not likely or is certainly not a significant chemical process, particularly in the presence of organic matter, which converts the intermediates (I2 and HOI) into organic iodine or I. Thermal or dark chemical reactions are not likely in deep waters indicating that any I detected in deep waters occurs primarily from mixing processes and/or the decomposition of particulate organic matter sinking to the sediments. 

The net effect of the above processes is that the bulk of the Cs inventory is transported with water movements with a small fraction being adsorbed to suspended particulate and deposited in bed sediments, whereas a large fraction of the Pu and Am inventory is adsorbed to suspended sediments and deposited in the bed sediments of the eastern Irish Sea. The behaviour of Pu and Am deposited in bed sediments is then determined by the processes of sediment mixing (in which bioturbation is very important), resuspension, and remobilisation of adsorbed radionuclides into the solution phase. 

Atmospheric reactions have been successfully represented as a sum of molecular reactions and mixing processes. Rate constants for a large number of atmospheric reactions have been tabulated by Baulch et al. (1982, 1984) and Atkinson and Lloyd (1984). Rates of reactions on solid surfaces have been examined in selected cases for solids suspended in air or water or in sediments. Reactions for the atmosphere as a whole and for cases involving aquatic systems, soils, and surface systems are often parameterized by the methods of Chapter 4. That is, the rate is taken to be a linear function or a power of some limiting reactant - often the compound of interest. As an example, the global uptake of CO2 by photos5mthesis is often represented in the empirical form d[C02]/df = - [ 02] . 

The depths and rates of particle mixing in Long Island Sound sediments have been studied by Aller and Cochran (1976), Benninger et al. (1979), Aller et al. (1980), and Krishnaswami et al. (1980). Depth profiles of both Th and Be show that the top 4-5 cm of Long Island Sound sediments must be mixed rapidly in order to distribute these short-lived nuclides throughout this zone. The mixing process can be quantitatively described... 

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