Evaluation of 32Si as a tracer for measuring silica production rates in marine waters

Abstract

The radioisotope ³²Si was evaluated as a tracer for measuring silica production rates in marine waters. ³²Si is a weak β‐emitter that decays to ³²P allowing the isotope to be used to determine simultaneously the production rate of biogenic silica and organic phosphorus. High specific activity ³²Si, 42,000–52,000 Bq (µg Si)⁻¹, was produced by the proton spallation of KCl. Reasonable tracer additions result in only nanomolar increases in the ambient silicic acid and orthophosphate concentrations during production rate experiments. Protocols for quantifying the amount of ³²Si and ³²P in samples by liquid scintillation counting (LSC) and gas‐flow proportional counting (GFPC) are presented. Both protocols eliminate the 4‐month wait for secular equilibrium between ³²Si and ³²P required by earlier methods. GFPC is especially useful as the instrumentation is lightweight and portable allowing real‐time analysis of samples at sea. Real‐time analysis by GFPC at sea and postcruise analysis by LSC gave results that agreed to within 17 and 7% for ³²Si and ³²P, respectively. Subsequent intercalibrations of the LSC and GFPC methods indicated that agreement to within 0.1–4% for both isotopes can be achieved. Parallel incubations of samples from Monterey Bay, California, using the radioisotope ³²Si and the stable isotope ³⁰Si gave silica production rates that generally agreed to within 30%. No significant bias was observed between rates obtained with the two tracers. The greater sensitivity and ease of analysis of the radioisotope makes ³²Si the tracer of choice for future studies of silica production in the ocean.