Determination of the rate of production and dissolution of biosilica in marine waters by thermal ionisation mass spectrometry

A new method is described for a precise and simultaneous determination of the rate of production and dissolution of biosilica in marine waters, using isotopic dilution technique. No HF or F2 is required for chemical preparations as the change in isotopic composition is measured on silica producing SiO2− ions. The seawater sample flask is spiked with 30Si(OH)4 (<10% of increase in situ concentration) and incubated in in situ conditions. At the end of incubation, changes of the 30Si:28Si ratios in particulate and liquid phases are measured by using a thermal ionisation mass spectrometer Finnigan THQ. The relative analytical precision of the isotopic ratio measurements is <0.5%. The limit of detection of the change in isotopic ratio during incubation is 0.02 atom%. The overall repeatability determined on eight subsamples (average production: 0.23 μM day−1; average dissolution: 0.07 μM day−1) is ±0.02 and ±0.01 μM day−1 for production and dissolution, respectively. Using mass and isotopic balances of the particulate and dissolved phases in the incubation flask, the best estimates for production and dissolution rates are calculated iteratively. This method was applied to 112 samples of marine waters (production, range: 0.00-2.38 μM day−1; dissolution, range: 0.00-1.18 μM day−1).