Chronometers for Sediment Accumulation

The use of radionuclides as chronometers to reconstruct estuarine sedimenthistory advanced significantly in the decade from 1975 to 1985. During that time, researchers from Scripps Institution of Oceanography, Yale University, Lamont-Doherty Geological Observatory and elsewhere measured distributions of natural and anthropogenic ra-dionuclides in Chesapeake Bay, Narragansett Bay, Long Island Sound, and the Hudson River in an effort to determine sediment chronologies and contaminant histories (e.g., Goldberg et al., 1977, 1978; Thomson, Turekian, and McCaffrey, 1975; Benninger et al., 1979; Turekian et al., 1980; Bopp et al., 1982; Olsen, Simpson, and Trier, 1981b). Broadly speaking, the radionuclides that can be used as sediment chronometers fall into two groups: Those that are continuously supplied to the estuarine waters and those that arrive in discrete events or pulses. The former group includes the natural radionuclides such as those of the uranium and thorium decay series while the latter comprises anthropogenic radionuclides such as 137Cs and Pu (Plutonium) isotopes that are added to the estuary from globalfallout associated with atmospheric testing of atomic weapons, or, in the case of the Hudson, supplied by releases from nuclear power facilities.

In deciding which radionuclide(s) to use for sediment chronometry, an important consideration is the half-life. This parameter, indicating the time required for a given population of radioactive atoms to decrease to one-half its original value, can be matched to the time scale of sediment history being considered. Thus, a radionuclide such as 210Pb, which has a half-life of 22 years, is appropriate for considering deposition over multi-decadal time scales (e.g., the past ~100 years) and is useful in looking at sediment deposition over the period of intense industrialization and human perturbation of an urban estuary such as the Hudson River.

The sediment geochemist's 'tool kit' includes chronometers with half -lives ranging from twenty-four days to thousands of years. The shorter-lived nuclides are useful for measuring short-term rates of deposition or the mixing effects of organisms on the sediment record, while the longer-lived ones can be used to look at long-term deposition rates. The natural radionuclides constitute one of the most important groups of sediment chronometers and comprise those produced in the atmosphere by the interactions of cosmic rays with atmospheric gases (and thus termed 'cosmogenic') and those in the uranium and thorium decay series. In order of increasing half-life this group includes:

• 234Th (half-life = 24.1 days): 234Th is produced from decay of dissolved 238U (Uranium) in the water column and is rapidly scavenged to sediments. Early work in Long Island Sound by Aller and Cochran (1976), subsequently confirmed by many researchers studying other estuaries, showed that 234Th is scavenged extremely rapidly (time scales of hours to days) in the shallow es-tuarine environment. 234Th scavenged from the overlying water column to the bottom sediments is referred to as "excess" 234Th to differentiate it from 234Th that is supported by 238U contained in the constituent minerals of the sediments. In practice, excess 234Th is calculated by subtracting the 238U activity from the measured 234Th activity. The short half-life of 234Th makes it suitable for determining particle mixing rates in estuar-ine sediments and it is typically confined to the upper few centimeters of the sediment column. A complication in the use of 234Th in estuaries is the fact that its source, dissolved 238U, increases with salinity and thus is not constant throughout the estuary. Indeed, 234Th is a useful tracer of particle transport across salinity regimes in a partially mixed estuary such as the Hudson (Feng, Cochran, and Hirschberg, 1999a, b).

• 7Be (half-life = 53 days): 7Be is produced naturally in the atmosphere from nuclear reactions of cosmic rays with atmospheric gases. It is added to the estuary by precipitation (wet and dry) and, like 234Th, is rapidly scavenged onto particles. Its distribution in the sediments is similar to that of 234Th and it also a useful tracer of particle mixing rate and sediment accumulation rates in areas where deposition rates are rapid.

• 210Pb (half-life = 22.3 years): 210Pb is produced in the atmosphere from decay of 222Rn (Radon) gas that has emanated from rocks and soils. An important source of 210Pb to many estuaries is direct deposition from the atmosphere and this radionuclide is often used to determine accumulation rates of estuarine sediments. This application is complicated by the fact that excess activity of 210Pb (relative to its grandparent 226Ra (Radium)) is often confined to the upper 15-20 cm, a zone in which particle mixing by organisms is likely to be important.

• 14C (Carbon) (half-life = 5,730 years): Radiocarbon is added to the estuary by rivers, gas exchange of CO2 with the atmosphere, and mixing of waters from the open ocean as well as in association with terrestrial organic matter. It is produced naturally in the atmosphere, but has

Figure 6.1. Releases of 137Cs and Pu isotopes to the Hudson River estuary. These anthropogenic radionuclides have been added from the atmosphere and watershed as a consequence of atmospheric testing of atomic weapons. Such testing and the fallout flux decreased markedly after the Nuclear Test Ban Treaty (19 63). 137Cs also has been added byreleases from the nuclear power plant at Indian Point. These inputs provide'marker horizons' in sediment cores and enable sediment accumulation rates to be determined (from Bopp et al., 1982).

Figure 6.1. Releases of 137Cs and Pu isotopes to the Hudson River estuary. These anthropogenic radionuclides have been added from the atmosphere and watershed as a consequence of atmospheric testing of atomic weapons. Such testing and the fallout flux decreased markedly after the Nuclear Test Ban Treaty (19 63). 137Cs also has been added byreleases from the nuclear power plant at Indian Point. These inputs provide'marker horizons' in sediment cores and enable sediment accumulation rates to be determined (from Bopp et al., 1982).

also been produced in association with atmospheric testing of atomic weapons. Radiocarbon is present in both sedimentary organic carbon and calcium carbonate shells of organisms, and by relating the radiocarbon content of a sample to that of a pre-industrial, pre-bomb standard, it is possible to determine the radiocarbon age of the sample. These ages may be offset from the true age by the 'age' of carbon in the reservoir from which the sample carbon was derived, but comparison of radiocarbon ages at several depth horizons in a sediment core often produces reliable indications of the accumulation history of the sediment (e.g., Benoit, Turekian, and Benninger, 1979).

Among the group of anthropogenic radionuclides, the one most extensively applied to Hudson River sediments is 137Cs (half-life = 30 y). The input of this radionuclide to the Hudson is from several sources (Chillrud, 1996): atmospheric fallout from the global atmospheric testing of atomic weapons augmented by discharges from the Knolls Atomic Power Laboratory and the Indian Point Nuclear Power Station. The 137 Cs signal from global fallout first appeared in the sediment record around 1954. While global fallout of 137Cs peaked in 196364, releases from Indian Point were at a maximum in 1971 (Fig. 6.1). 137Cs inputs associated with the nuclear power industry may be distinguished from those of global fallout by the presence of additional short-lived radionuclides (e.g., 60Co (Cobalt) and 134Cs) produced in reactors. However the half-lives of 60Co and 134Cs preclude their being detected in sediments older than about thirty years.

Determining sediment chronologies using natural radionuclides requires use of the gradient of the radionuclide with depth in the sediment. In the ideal case, the sediment-water interface has the highest radioactivity because it has recently been supplied with particles containing freshly scavenged radionuclide. Penetration of the radionu-clide to depth in the sediments is a function of the half-life and sediment accumulation rate. Because radioactivity decreases exponentially with time, the familiar decay equation can be applied to sediments by assuming that depth in the sediment column is related to time via the sediment accumulation rate (S):

where A is the radioactivity (or simply activity) at depth x, A0 is the activity at the sediment-water interface andX is the decay constant (= 0.693/half-life). Sediment accumulation rates can be determined by plotting ln(activity) vs. depth; the slope of the resulting line is -X/S. In practice, plots of activity versus depth often show scatter related to variations in accumulation rate or A0 with time or to other processes such as physical or biological disruptions to the sediment column that are not represented in equation (1). These complications must be evaluated for each site (see below).

In contrast to the more-or-less continuous supply of natural radionuclides to the accumulating sediments, anthropogenic radionuclides such as 137Cs are added sporadically depending on their source and are used to indicate specific time horizons in the sediment associated with inputs of these radionuclides to the system. For example, the 1963-64 peak arising from global fallout is a principal time horizon in applications of 137Cs or 239,240Pu to sediment chronologies (Bopp et al., 1982; Fig. 6.1). One caveat associated with the use of 137Cs as a chronometer is that this radionuclide is subject to release or desorption from particles as they are transported from regions of low to high salinity through the estuary. Indeed, mass balances of anthropogenic radionuclides (238Pu, 239,240Pu, 134Cs, 60Co and 137Cs) in the Hudson River estuary have shown that nearly all of Pu was trapped in the estuary, but only 10-30 percent of 137Cs, 134Cs and 60Co were retained on the fine particles and trapped in the estuary (Olsen et al., 1981b; Chillrud, 1996). The distribution patterns of sediment inventories of these radionuclides are influenced strongly by the transport of fine particles, variations in sediment accumulationrates and desorption (of Cs and Co) from particles as they encounter higher salinities (Olsen et al., 1981b).137Cs also may be mobile in the sediment column in the more saline reaches of the estuary, although the effect of Cs mobility on 137Cs profiles is less important in areas of rapid sediment accumulation (Olsen et al., 1981a).

An important consideration in application of natural or anthropogenic radionuclide chronometers to estuarine sediments is the characterization of the effects of other processes on the profiles. As indicated above, these can include post-depositional physical disturbances (ranging from the influence of waves and tidal currents to dredging andshipping) andbiological mixing of the sediment by organisms. The measurement of multiple radionuclides on a single core can help sort out the effects of these processes, but it is also desirable to consider the radionuclide data in the context of other information such as sediment grain size or mineralogy, water content, benthic faunal data, bottom morphology, and sediment structure as revealed by X-radiography of cores. Indeed, as we shall see, X-radiography of a sediment core often reveals information on benthic faunal activities as well as sediment structure and composition.

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