Application of low-level beta counting of ~(32)Si for the measurement of silica production rates in aquatic environments

摘要

Silicon is an important element for certain plankton groups in aquatic systems. In marine systems there is a strong coupling between the biogeochemical cycles of carbon and nitrogen with that of silicon due to the high proportion of primary productivity carried out by diatoms. Currently ~(32)Si is the most used radiotracer for the measurement of biogenic silica production rates in aquatic systems with isotope activity typically quantified by Cherenkov or liquid scintillation counting. Here we describe a new method for quantifying ~(32)Si activity using a gas-flow proportional counter optimized for low-level beta radiation analysis. The background activity is two-orders of magnitude lower and more stable than that of liquid scintillation counters, leading to consumption of 67-75% less isotope per sample. Sample preparation is non-destructive enabling reanalysis of samples indefinitely. Methods for counting samples under conditions of secular equilibrium and non-equilibrium between ~(32)Si and its daughter isotope ~(32)P are described. Analysis of equilibrated samples yields the highest sensitivity and accuracy, but requires that sample analysis be delayed for 120 days for secular equilibrium. Non-equilibrium methods are less accurate, but provide results within 2 days enabling experimental designs and protocols to be evaluated and optimized in near real time.