210PB AND 137 CS CHRONOLOGIES: WHEN IT IS GOOD, IT IS VERY GOOD, WHEN IT IS BAD IT IS HORRID

For the last fifty years, isotopes ²¹⁰Pb and ¹³⁷Cs have been used to determine the chronologies of environmental change. Although these isotopes are produced by different processes, both are particle reactive and are readily absorbed on suspended matter within the aquatic environment. ²¹⁰Pb chronologies are based on mathematical age models relating the decrease in activity with depth. ¹³⁷Cs chronology is based dominantly on the peak production in 1963. However, the use of these isotopes to evaluate sedimentation rates, is by no means a totally routine procedure, but full of 'pitfalls'.

Most analytical problems have been solved over the past fifty years thanks to technical advances in measuring radioactivity. However, major pitfalls that remain in the selecting of the sampling site, in sampling the material at the site and in choosing which algorithm would best model the deposition dynamics. There are two common algorithms which have been employed: the constant initial concentration and the constant rate of supply. These algorithms assume that there is a constant flux of the isotopes and that the sediment deposited is homogenous. In reality, these conditions are rare. Interpretations based on a single depth profile alone are not sufficient as the activity of an isotope may be affected by factors such as: sediment source(s), episodic or event-related deposition, post-depositional disturbance, and similar causes. Therefore, basic knowledge of the depositional controls is necessary prior to interpretation of sediment isotopic profiles. for example, the sites within the Everglades, the ²¹⁰Pb started out low at the surface, rise to a peak mid-core then decreased with logarithmic decay. A standard explanation for this profile would be that the upper portion of the core had been physically or biologically disturbed. The ¹³⁷Cs distribution showed no indication of disturbance and dated the change in ²¹⁰Pb at approximately 1960. Further analysis of this site, demonstrated that the ²¹⁰Pb peak occurred corresponding to a transition from a saw-grass dominated to a cattail dominated marsh. This and like examples demonstrate the importance in using both isotopes coupled with knowledge of the sedimentary history to properly ascertain a valid chronology.