SLICS – Short Lived Isotope Counting Services
Principles of SLIC
SLIC (short lived isotope counting) is the process of using multiple short-lived naturally occurring isotopes to assess age and dynamics within sedimentary systems. SLIC is a natural progression from the sole use of lead-210 made possible with the evolution of low background gamma spectrometers capable of simultaneous isotope detection.
By using multiple isotopes, each acting as unique environmental indicators and each with separate qualifying characteristics, provides for in-depth and credible assessment of sedimentary dynamics.
Berillium-7, cesium-137, and lead-210 are the most common used isotopes for SLIC. They satisfy criteria required for establishing chronologies based on use of half-life. A half-life is defined as the time it takes for half a given number of atoms in a sample to decay to another element. The age of sediment is calculated by comparing the original isotopic concentration to the percent of the remaining in the sample. The criteria for a radioisotope to be a candidate for dating are:
With these criteria met, the age of a substance can be calculated by the following formula:
T age = ln(A 0/ A s) x 1/λ
Where A0 is the isotopic activity at time zero (the present) and A s is the activity of the unknown, λ is the decay constant for the isotope.
Short Lived Isotopes
Berylium-7 : 7 Be is formed in the atmosphere by the interaction of cosmic rays with nitrogen and oxygen, has a very short half-life (≈54 days). It is a very reactive element and attaches instantaneously to particulate material.
Because of its short half-life and geochemistry, 7Be is useful for measuring sediment transport on less than an annual time scale. In most sedimentary systems, where sediment deposition is on the order of millimeters to centimeters per year, the isotope is confined to the surface 1 to 2 cm. The best used of this isotope is to determine the total inventory or flux (g/cm2/yr) at a site and comparing this value with surrounding sites. The mapped result defines the loci of deposition. Its value has been demonstrated in a variety of studies, for example Holmes and Marot, 1999)
Cesium-137 : 137Cs, with a half-life of 30.3 years, is a thermonuclear byproduct. Its presence is directly related to the atmospheric testing of nuclear devices during the latter half of the 1950’s and early 1960’s. With the exception of the Chernobyl failure and the Indian/Pakistani tests, there has been no cesium-137 released to the atmosphere since the cessation of atmospheric nuclear testing. These events produced minor amount of cesium-137 when compared to the nuclear testing of the sixties. Many investigators use cesium-137 exclusively as a dating tool. They measure cesium-137 in sediment and assume highest measured value is result of the maximum fluxes between 1962 and 1965. Due to difficulties in sample and sampling resolution the peak value is assumed be correspond to 1963 ± 2 years. They use this as a “dated” horizon and calculate a sedimentation rate from this value. In some marsh sediments, this isotope has been demonstrated to be mobile and thus not a good isotope for “dating” purposed. However, in clay systems, it is conservative and meets criteria #5.
Lead-210 : 210 Pb, with a half-life of 22.8 years, is ideal for most ecosystem studies, where changes have occurred within the last century. A member of the 238U series, lead-210 is subject to disequilibria with its distant relative radium-226 (226Ra) due to the physio-chemical activity of the intermediate gaseous progenitor radon-222 (222Rn). Radioactive disequilibrium arises when the gaseous radon-222 escapes into the atmosphere. With a half-life of 3.8 days, the radon-222 decays through a series of very short half-life isotopes to lead-210. This process produces excess lead-210 in the atmosphere and subsequently the hydrosphere. Like berylium, the highly reactive lead is rapidly adsorbed to or incorporated on particulate material. Precipitation of this material produces excess lead-210 over lead-210 in equilibrium with ambient 226Ra already in sediments. This excess lead-210 provides the mechanism for age and depositional assessment. (More : Understanding Lead-210 dating)