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

Abstract

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 32Si 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 32Si 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 32Si and its daughter isotope 32P 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.