Sediment reworking (or mixing) by biological and physical processes can make the sedimentary record of environmental change difficult to interpret. Because of the reworking, sediments do not remain permanently in the same stratigraphic position. Rather, the feeding habits of sediment-dwelling organisms can move freshly deposited sediment to different depths below the sediment surface. This activity results in sediment horizons that contain material of a variety of ages. This phenomenon is observed and interpreted in Massachusetts Bay sediments, using radionuclide profiles from sediment cores.
The radionuclides 210Pb, 239+240Pu, and 234Th can be used to interpret processes of sediment reworking, because each of these isotopes is strongly adsorbed to particles. The different source functions and half lives of each isotope ensure that unique insight about sediment reworking processes can be gleaned from the sedimentary records. 210Pb and 234Th are both naturally-occurring uranium-series radionuclides with relatively constant inputs to the sediments. 210Pb originates primarily (in shallow waters) from decay of atmospheric 222Rn, while 234Th is a decay product of uranium in seawater. Each isotope reveals information about particle reworking spanning a time scale somewhat greater than its half life (22.3 year for 210Pb, 24 day for 234Th). The longer-lived 239+240Pu (> 6500 year half life) is derived from atmospheric testing of nuclear weapons. If sediments are not reworked, the earliest detection of 239+240Pu in sediments corresponds to its first significant production in the 1950s, while the peak activity corresponds to the fallout peak in 1963.
In slowly accumulating sediments typical of the continental shelf, bioturbation is the primary process driving vertical transport of these radionuclides, rather than sedimentation. Under these conditions, the depths of penetration of 239+240Pu, and of "excess" 210Pb (that present in excess of their U-series parent isotopes) are strongly dependent on the depth of sediment mixing, or "bioturbation". The shapes of the nuclide profiles can also be used to infer rates of bioturbation. In some cases, mixing can be interpreted as a simple diffusive process. However, in a number of sediment cores from Massachusetts Bay, subsurface maxima in radionuclide profiles provide evidence of rapid removal of sediments from the sediment surface (ingestion) followed by injection of this material (defecation) a few centimeters below the sediment surface. This process has significant implications for the age of surficial sediment and for discerning change in sediment contaminant concentrations following change in contaminant inputs.
The primary evidence for ingestion of surficial sediment by organisms, followed by defecation at a depth of a few cm, stems from the profiles of excess 210Pb. In the absence of such a process, the maximum excess 210Pb activity would be expected to be observed at the sediment surface, which typically is thought of as the youngest sediment. However, in roughly half of the cores collected from 8 locations in Massachusetts Bay, there is a clear 210Pb maximum present at a depth of ~3 cm (Crusius and others, 2004; Figure 9.1b). This subsurface maximum is most readily explained if surficial sedimentary material is being consumed by organisms in the sediment and deposited at depth. This process has been identified during previous work in Massachusetts Bay and elsewhere (Wheatcroft and others. 1994; Shull and Yasuda, 2001). The organisms thus far identified in nearby sediments that carry out this sort of mixing are cirratulid polychaetes, including Tharyx acutus (Myers, 1977) and Cirriformia grandis (Shull and Yasuda, 2001).
With the aid of a sediment mixing model we sought to quantify the rate of this sediment reworking process. A mathematical description of this mixing process is presented in Crusius and others (2004) and will not be repeated here. However, we will present some model simulations of this process. When the ingestion rate constant is on the order of 0.5 yr-1 or greater, a well-defined subsurface maximum is observed in the 210Pb profile (Figure 9.2). In other words, the surficial sediment must be fully ingested and subsequently defecated at least once every two years in order to reproduce this feature in the sediment. The 239+240Pu data also reveal a subsurface maximum in many cases (e.g. Figure 9.1 d), although this feature could conceivably be caused by the fallout maximum of 239+240Pu, hence we de-emphasize the Pu data for our interpretation. The 234Th data that are available show well-defined maxima at the surface, which is most likely due to the much shorter half life of 234Th. Over the time scale recorded by this isotope (a few months), there is little downward transport of sediment. However, there is one thing that must be borne in mind when evaluating the 234Th data. Samples from a few centimeters below the surface and deeper were analyzed many months after collection, after "excess" 234Th would have decayed, which means that we cannot be certain there was not significant downward transport of 234Th. Taken together, however, the 210Pb and 234Th data suggest that downward transport by this process of subsurface defecation occurs on a time scale of a few months to a couple of years.
The interpretations of the radiochemical profiles contribute two main points to our understanding of new contaminant additions to coastal sediments. First, any new sediment-bound contaminant that settles to the sea floor can be rapidly diluted within the upper sediment column at a rate and to a depth that is dependent on the kinds of sediment dwelling organisms present. For some low-toxicity contaminants the natural dilution process can be an environmental benefit. However, for contaminants that can be dangerous at extremely low concentrations, mixing downward can be a disadvantage because the contaminant is less likely to be resuspended and flushed from the nearshore environment. Second, this mixing process implies that surficial sediments may not accurately or sensitively record recent changes in contaminant deposition. These results thus indicate that in slowly accumulating sediments impacted by this sort of bioturbation, newly deposited contaminants may be interspersed at a range of depths in the sediment, making it difficult to infer the temporal history of contamination.
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