ASSESSING POTENTIAL SOURCES OF VARIABILITY ASSOCIATED WITH 210PB AGE DATING OF SEDIMENTS

Age dating of sediment cores is critical for determining sediment accumulation rates and precise timing of events that have occurred throughout the geologic past. The determination of the short-lived radioisotope (²¹⁰Pb) by gamma emission has undergone several modifications in the past three decades in order to achieve accurate results, but there are still variables that have not yet been systematically tested. This study examined the effects of different grain sizes, sample geometry, and secular equilibrium. To test these variables a series of standards were mixed using radioactive mud with non-radioactive sand.

Errors associated with texture were initially tested by comparing the activities among three different compositions (25%, 50% and 75%) of fine-grain sediments in the mud/sand mixture in each of the four different weights, then by observing real samples from Charlotte Harbor, FL and normalizing activities to %mud and %clay. Initial findings show that sample texture influences activity, but simply normalizing dates to fine-grained sediment content may or may not help to achieve more accurate results. Errors associated with geometry were tested by analyzing and comparing four possible sample weights (10, 20, 30 and 40 grams) of the mud/sand mixture. Peak analysis and detector variability were both measured by re-running multiple samples and observing difference in the reproducibility of activities. Sample weight was also shown to affect the precision of the activities, with a threshold of ~20 grams. Errors associated with secular equilibrium deal with the assumption that a sediment sample requires 30 days for the 222Rn and 210Pb isotopes to reach equilibrium before it can be analyzed for 210Pb. To test for the variability caused by secular equilibrium standards were accurately analyzed before, during and after a 30-day period. Within statistical error, there was no difference between the initial and the 30 day count rate for each isotope with the decay chain.