Sediment mixed

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).

Short-lived U-series nuclides such as °Pb and " Th thus provide key information about the rate and depth of sediment mixing in marine records. This information is critical if the resolution of down-core paleoceanographic records are to be assessed. 

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. 

The Meurthe river in North-Eastern France has two major tributaries, the Fave and Mortagne rivers. Let R be the concentration of an element in the main Meurthe river and r that of the same element in its tributaries (Table 1.1, columns 2 and 3). 65 percent of fine-grained sediments from the Upper Meurthe (R0) mix with 35 percent sediment from the Fave (r0) river. At the next confluent, 80 percent of the Meurthe fine-grained sediments mix with 20 percent Mortagne (r2) sediment. Find the composition of the sediments in the Meurthe down each tributary. 

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). 

Benninger, L.K., Aller, R.C., Cochran, J.K., and Turekian, K.K. (1979) Effects of biological sediment mixing on the 210Pb chronology and trace metal distribution in a Long Island Sound sediment core. Earth Planet. Sci. Lett. 43, 241-259. 

Krishnaswami, S., Benninger, L.K., Aller, R.C., and Von Damm, K.L. (1980) Atmospherically-derived radionuclides as tracers of sediment mixing and accumulation in near-shore marine and lake sediments evidence from 7Be, 210Pb, and 239,240pu Earth planet Sci Lett 47> 307-318. 

Olsen, C.R., Simpson, H.J., Peng, T.H., Bopp, F., and Trier, R.M. (1981) Sediment mixing and accumulation rate effects on radionuclide depth profiles in Hudson Estuary sediments. J. Geophys. Res. 86, 11020-11028. 

DuBois L. G. and PreU W. L. (1988) Effects of carbonate dissolution on the radiocarbon age structure of sediment mixed layers. Deep-Sea Res. 35, 1875-1885. 

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. 

Turbidity currents—Local, rapid-moving currents that result from water heavy with suspended sediment mixing with lighter, clearer water. Causes of turbidity currents are earthquakes or when too much sediment piles up on a steep underwater slope. They can move like avalanches. 

Sediment cores from the deep Baltic basins are distinctly laminated on top, which indicates that for the past 100 years the layers have formed unaffected from sediment mixing caused by bioturbation. Since oxygen concentrations less than 2 ml/1 are life-threatening to marine macrofauna and meiofauna, it can thus be concluded that hypoxic to anoxic conditions dominated over this period (Chapter 14). 

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