Impact of the pH-DEPENDENT Speciation of Silicic Acid on the Silicon Isotope Composition of Diatoms

Variations in the natural abundances of the stable isotopes of silicon (δ³⁰Si) are a useful tracer of the silica cycle, tracking relative differences in silicic acid drawdown in surface waters by diatoms at times in the past. Refinement of this tracer for silicic acid utilization requires understanding the impact of environmental variables (e.g. temperature, phytoplankton growth rate, pH) on the δ³⁰Si of diatom silica. Initial investigation into the impacts of temperature and growth rate suggested that they do not significantly affect the δ³⁰Si of marine diatoms but no look has been taken at the influence of pH. At the typical seawater pH range of 7.5 to 8.5 most silicic acid is in the undissociated form, Si(OH)_4. By pH 10.5, however, SiO(OH)_3^- predominates. Silicon isotopic fractionation with speciation has never been investigated but if it were 5% of the magnitude suggested for boron isotope fractionation (as might be expected for a secondary isotope effect) then there should be a 1 ppm difference between Si(OH)_4 and SiO(OH)_3^-. Cultures of the diatom, Thalassiosira weissflogii (which utilizes Si(OH)_4 and not SiO(OH)_3^-), grown at pH 7.8 and pH 9.5 display a δ³⁰Si difference of 0.7 ppm, which is consistent with a silicon isotope fractionation on the order of 1 ppm between the different species of silicic acid. In the oceans this should not contribute significantly to the δ³⁰Si variations recorded in diatom silica, as even in dense phytoplankton blooms pH rarely exceeds 8.5. In environments, such as lakes, estuaries, and sea ice brine, where phytoplankton may grow at higher pHs the speciation of silicic acid may make a contribution to diatom δ³⁰Si that cannot be ignored.