PNNL

Abstract

Silicon-32 has been identified as an isotope that fills the sediment geochronology gap created between Pb-210 at 1000 years. However, detector sensitivity and chemical isolation challenges of radiometric Si-32 for age dating have resulted in a fairly limited number of applications of this technique to real-world sediment studies. The significance of targeting the intermediate age bracket is to provide accurate geochronological reconstructions of paleoindicators that identify human and large-scale, climate-induced shifts in coastal areas. Therefore, we targeted Puget Sound, Washington, United States, to demonstrate the application of Si-32 age dating to determine the relative importance of climate cycles versus human activities in an in-land sea. The enhanced reconstructions will be achieved through the design of new detectors, chemical separation of biogenic silica from kilogram-scale sediment samples, and demonstration of the viable age-dating reach of Si-32. Implications of detector background on the duration of required measurements and the age-dating reach are considered. The design of a low-background, gas-proportional beta counter for measurement of the daughter isotope P-32 is discussed; detector background goals for these new detectors are based on the sensitivity study.