The balance between biogenic silica production and its dissolution in the surface ocean

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Production and dissolution rates of biogenic silica (diatom frustules) in the surface waters were determined simultaneously using silicon stable isotope tracer to investigate balance between biogenic silica production and its dissolution in the surface waters of the western subarctic North Pacific Ocean. New analytical method for the determination of silicon isotope ratios was developed using a high-resolution Inductively Coupled Plasma-Mass Spectrometry. By correcting background Si peaks, analytical precision and sensitivity similar to those of the thermal ionization mass spectrometry were obtained. This method was used for the determination of the rates of biogenic silica production and dissolution in the Oyashio surface water of the western subarctic North Pacific. Column integrated dissolution rate of biogenic silica exceeded production rate by diatoms within the euphotic layer, suggesting active silicic acid dissolved from particulate biogenic silica and its utilization by diatoms in the surface mixed layer. Batch dissolution experiments of diatom frustules obtained from the western subarctic North Pacific were also conducted in the laboratory to investigate differences in dissolution kinetics between the dominant diatom groups. Observed dissolution rates indicated that degree of solubility of diatom frustules is different between the diatom species as well as the site of the frustules. Morphological changes of diatom frustules during the dissolution process seem to be an important factor affecting surface/volume ratio of the frustules and consequently its dissolution rate. From these results, most of the biogenic silica produced by diatoms was estimated to be recycling in the surface ocean. However, existence of diatom frustules in the deep layers implies that rapid sinking of diatom containing feral pellets should be considered as another important process in the marine silicon cycles.