SLIC – WHEN IT IS GOOD IT IS VERY VERY GOOD AND WHEN IT IS BAD, IT’S HORRID

Short-Lived Isotopic Chronologies using Pb-210 and Cs-137 to assess surface processes has come into common use over the last four decades. Although produced by different processes, both isotopes are adsorbed to sediment in the same way. Pb-210 chronologies are based on mathematical models relating the decrease in activity with depth. Cs-137 did not come into wide use until the late 1960’s when it was recognized that atmospheric nuclear testing had produced detectable amounts. Cs-137 chronologies are based on the identification of the 1963/64 activity peak.

Most chronologic investigations rely on only one of these isotopes. Because the activity of an isotope may be affected by many factors such as: sediment source(s), episodic or event-related deposition and post-depositional disturbance, using only one isotope often produces a faulty chronolgy. Thus,a basic knowledge of system physical sedimentology is necessary to interpret the isotopic profiles. For example, in a core off the Bonnet Carré spill-way in Lake Pontchartrain, the Pb-210 profile using the isotopic age models yielded a date of greater than 100 years at 70cm depth. However, Cs-137 was present below this horizon. Since Cs-137 did not exist prior to 1954, its presence in the lower portion of the core negates the date obtained using Pb-210. Upon further analysis of the core, it was determined that the Pb-210 profile was an artifact of the chaotic flow regime during the opening of the spill way. A similar situation was found in the northern Everglades. In this case, the Pb-210 started out low at the surface, rose 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 Cs-137 distribution showed no indication of disturbance and dated the change in Pb-210 at approximately 1960. The Pb-210 peak occurred at transition from a saw-grass dominated to a cattail dominated marsh. As the cattail peat accumulated at a higher rate, the lower concentration of Pb-210 was attributed to the dilution of the isotopes due to the vegetative changes. These and like examples demonstrate the importance in using both isotopes coupling with knowledge of the sedimentary dynamics to properly ascertain a valid chronology.