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U.S. Department of the Interior
Short-Lived Isotopic Chronometers
A Means of Measuring Decadal Sedimentary Dynamics
The best method of assessing rates of change in ecosystems is by long-term monitoring. However, such information is unavailable for most ecosystems, and other means must be employed. In sedimentary environments, chronological scales can be determined by the distribution of radioactive isotopes in the sediment. These timescales are developed by using a known property of radioactive material, the "half-life." The half-life of an isotope is the amount of time it takes for half a given number of radioactive atoms to decay to another element. The age of the sediment containing a radioactive isotope with a known half-life can be calculated by knowing the original concentration of the isotope and measuring the percentage of the remaining radioactive material.
The requirements for a radioisotope to be a candidate for "dating" are that:
(1) the chemistry of the isotope (element) is known; (2) the half-life is known;
(3) the initial amount of the isotope per unit substrate is known or accurately estimated;
(4) the only change in concentration of the isotope is due to radioactive decay; and
(5) in order to be useful, it must be relatively easy to measure. If all these conditions
are met, the effective range for each isotope is about eight half-lives. Four radioisotopes
(137Cs, 7Be, 14C, and 210Pb) satisfy these criteria, and are useful for measuring sedimentary
dynamics over the last 100 to 150 years. The following summarizes the uses and potential uses
of these four radioisotopes in dating recent sediment.
7Be is a naturally produced radioisotope that is formed by cosmic ray bombardment of atmospheric nitrogen (N) and oxygen (O). It is transferred through precipitation from the atmosphere to earth. Beryllium is a highly reactive element and becomes rapidly and tightly associated with a sedimentary substrate. 7Be has a half-life of 53 days, which makes its effective range of applicability for dating sediment about 1 year. Thus, detection of its presence is a reliable indicator that the substance was in contact with the atmosphere within the past year. This information is important, as it is used to calibrate other isotope-based geochronometers and define sedimentary sinks.
14C is produced in the Earth's atmosphere by the interaction of cosmic ray particles with nitrogen (N), oxygen (O), and carbon (C). Of these elements, nitrogen is the most important in terms of the amount of 14C produced. 14C was also produced by thermonuclear activity (bomb testing), which contributed significantly to the atmosphere, reaching its peaks in 1963 (Northern Hemisphere) and 1964 (Southern Hemisphere). All 14C produced is rapidly oxidized to CO2 and is assimilated into the carbon cycle. As CO2 becomes incorporated in all carbon-based materials, balance is established between intake, respiration, and decay. 14C has a half-life of 5,730 years and has an effective range of applicability of 100 to 70,000 years for dating organic material. The amount of bomb-produced carbon is determined by comparing present radiocarbon activity to 1950 carbon activity, the date established by convention as the baseline for all radiocarbon dating. Post-2952 carbon values are reported as a percentage of modern (that is, 1950) carbon, and denoted as 14C.
137Cs, with a half-life of 30.3 years, is a thermonuclear byproduct. Its presence in natural systems is directly related to atmospheric thermonuclear activity. The curve below shows that 137Cs fallout production (and deposition) began about 1952; deposition peaked during 1963 and 1964. Under ideal conditions, the sediment profile should mimic the 137Cs production. However, the inability to accurately sample small intervals, and the mixing of the sediment by organisms, often cause deviations from the ideal profile.
210Pb, with a half-life of 22.3 years, is ideal for most
ecosystem studies. A member of the 238U series, 210Pb
forms by the decay of its intermediate gaseous parent, radon-222. 222Rn, formed by the decay
of radium, escapes into the atmosphere by recoil or by diffusion, and rapidly decays to form 210Pb.
This isotope has a residence time in the atmosphere of about 10 days before it is removed by precipitation. The highly reactive
lead is then rapidly adsorbed to and incorporated into the depositing sediment. This flux
produces a concentration of "unsupported" 210Pb (lead whose activity in the sediment is higher
than that of its radium grandparent, 226Ra). Dates of sediment deposition are calculated by
determining the decrease in 210Pb activity at each selected sediment interval; this decrease
is a function of time. If the initial concentration of 210Pb is known, or is estimated using
7Be data, then the "age" of a sediment interval is calculated by the following:
substituting the constants,
where A210Pb0 is the unsupported 210Pb activity in disintegrations per minute at time zero (the present), A210Pbh is the activity in disintegrations per minute at depth h, and 0.03114 is the decay constant for 210Pb. Ideally, a plot of 210Pb activity and depth will be an exponentially decreasing curve asymptotically approaching the supported value.
Lead-210 ExamplesCore 19 C (below) taken from a mudbank in Florida Bay, demonstrates the distribution of 210Pb and 226Ra. Dates, calculated by using the 210Pb method, were corroborated by comparing the distribution of total lead measured in the core to the total lead in a nearby coral in which the ages had been determined by annual banding. Dates determined by the 210Pb method in cores from throughout Florida Bay are now being used to construct the paleoecological history of the region.
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For more information contact:
Charles W. Holmes
U.S. Department of the Interior, U.S. Geological Survey
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Last updated: 04 September, 2013 @ 02:03 PM(TJE)
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