WHERE IS THE CESIUM???

Short-Lived Isotopic Chronologies using ²¹⁰Pb and ¹³⁷Cs has come into common use over the last five decades. Although produced by different processes, both are initially transported though the atmosphere from which they are deposited, becoming strongly attached to clayey sediment.

²¹⁰Pb is a product of radon diffusion from the earth and thus its production is related to the size of the adjacent land mass: higher ²¹⁰Pb in continental locales and low concentration in island locales. ¹³⁷Cs is a product of atmospheric nuclear testing, which began in the early 1950's and peaked in 1962. Its introduction in the atmosphere ceased, with few exceptions, in 1963 with the signing of the atmospheric test ban treaty. Its presence in sediment profiles is often used to confirm the ²¹⁰Pb-calculated chronology. Another use of this isotope is to define the rate of erosion of agricultural lands and to produce budgets of sediment input to the waterways of a region. In order to construct an accurate sedimentologic dynamic model, it is imperative that the initial ¹³⁷Cs inventory of the region be known. Fortunately, during the period of atmospheric nuclear testing, the radioactive fallout was recorded at numerous sites throughout the world. From these records, the continental fallout pattern could be mapped, showing that the highest fallout deposition exists in a band extending from near Seattle, dipping southward through the central part of the country and then trending northeastward, exiting the country in the New England region. As a result, the southwest and much of the southeast are devoid of significant amounts of ¹³⁷Cs.

During 1961-1962, the period of the most numerous and largest tests coincided with a strong La Nina. The high pressure in the Eastern Pacific produced by La Nina caused the latitudinal airflow to be pushed northward, entering the continent in the Seattle region. The ¹³⁷Cs distribution mirrors the airflow pattern. indicating that the fallout pattern is the result of La Nina conditions during the period of testing. Thus when attempting to use ¹³⁷Cs in combination with ²¹⁰Pb, or to construct erosion models, it is imperative that the location of the site being studied be assessed in reference to the fallout patterns.