Sediment deposition, radionuclide

Measurements of radionuclides and metals in marine sediments and particulate matter are conducted for a variety of purposes, including the determination of sedimentation rates, trace metal and radionuclide fluxes through the water column, enrichment of metals in specific phases of the sediments, and examination of new sedimentary phases produced after sediment deposition. Such studies address fundamental questions concerning the chronology of deep-sea and near-shore sedimentary deposits, removal mechanisms and cycling of metals in the ocean, and diagenesis within deep-sea sediments. 

Particle-reactive radionuclides provide a tracer with which to evaluate the net lateral redistribution of sediments. In some cases, inventories or accumulation rates of these nuclides can be compared to their known rate of supply to estimate the net extent of lateral redistribution of sediments. In other cases, inventories (or accumulation rates) of nuclides can be compared among several sites to assess the relative rate of sediment deposition among the sites, even if the rate of supply of the tracer is not known accurately. Inherent in this approach is the assumption that sediments deposited at all of the sites have a uniform initial radionuclide content. Deposition of sediments must be roughly at steady state over the... 

By combining the findings of Cacchione, Drake and the results reported here, a coherent model can be proposed to explain the deposition inventory of the radionuclides. The down-canyon current transports large quantities of sediment toward the radioactive waste disposal site at 4000 m. Within the upper canyon, fine material is transported the furthest. Near the mouth of the canyon, sediment erosion of the walls occurs due to the down-canyon currents meeting a proposed opposing on-shore bottom current. The eroded material from the walls is transported and the finer material is deposited in eddies formed where the two currents meet. 

Solution of equation (10) which involves sedimentation in the presence of mixing and that of equation (11) which contains the sedimentation term only, are exponential in nature. The major conclusion which arises from this is that the logarithmic nature of the activity-depth profiles by itself is not a guarantee for undisturbed particle by particle sediment accumulation, as has often been assumed. The effects of mixing and sedimentation on the radionuclide distribution in the sediment column have to be resolved to obtain pertinent information on the sediment accumulation rates. (It is pertinent to mention here that recently Guinasso and Schink [65] have developed a detailed mathematical model to calculate the depth profiles of a non-radioactive transient tracer pulse deposited on the sediment surface. Their model is yet to be applied in detail for radionuclides. )... 

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 ... 

Reference materials that represent the primary deep-sea and coastal depositional environments and biological materials would solve many of the problems that radiochemists face in analysis of sediments from these settings. Radiochemists require reference materials comprising the primary end member sediment and biological types (calcium carbonate, opal, and red clay from the deep-sea and carbonate-rich, silicate-rich, and clay mineral-rich sediments from coastal environments and representative biological materials). Additional sediment reference material from a river delta would be valuable to test the release of radionuclides that occurs as riverine particles contact seawater. 

Harvey, B.R. (1981) Potential for the post depositional migration of neptunium in Irish Sea sediments. In Proceedings of the International Symposium on Impacts of Radionuclide Releases into the Marine Environment Vienna (1980). IAEA, Vienna, pp. 93-103. 

Aquatic sediments are formed in all surface waters by the settling of coarse and fine inorganic and organic particles. They are present in rivers, in lakes and in the oceans, and radionuclides deposited on the surface of the earth will sooner or later come into contact with these sediments. They may enter the sediments by sorption of molecularly-dispersed species (ions, molecules), by precipitation or coprecipitation, by coagulation of colloids (in particular carrier colloids) followed by sedimentation of the particles formed, or by sedimentation of coarse particles (suspended matter). By desorption, the radionuclides may be remobilized and released again into the water. 

As is the case with deep-sea sediments, all of the dating methods based on decay of unsupported radionuclides assume that N(,— the initial amount of Th, Pa, or Be per unit of deposit— remains constant over time. In general, this requires that both the accretion rate of the deposit and the uptake rate of the radionuclide from seawater have remained constant. The various ratio methods are used primarily so that the latter assumption can be relaxed, the theory being that the variations in the uptake rate of two isotopes,... 

Table V shows some transuranic radionuclide concentrations found in near shore sediment close to Cape Cod, Mass. The total transuranic content of these shallow sediments agrees well with that predicted as being delivered to the latitude, arguing that the core segment represents part of the period of high 23sp delivery from SNAP 9A fallout. The implication is that all of the delivered transuranic element is rapidly deposited in the sediment in contrast to the "soluble fallout radionuclides.

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. 

Estuaries are complex and dynamic environments, with sediments subject to both erosion and deposition at different times and to varying water chemistry, particularly in salinity, pH and suspended sediment load. These all influence the behaviour of radionuclides (Clifton Hamilton, 1982 Kelly et al., 1991). A single estuary is likely to contain sedimentary deposits ranging from coarse sands to fine muds with a correspondingly large range of radionuclide concentrations. Most actively... 

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