WHAT CAN WE SEE: SENSITIVITY OF SILICON ISOTOPES TO PERTURBATIONS OF THE SILICA CYCLE

The appearance and radiation of the opal biomineralizing diatoms resulted in a precipitous drop in oceanic concentrations of silicic acid, possibly on the order of more than 1000 mM.  This is known from changes in the degree of silicification of radiolarian tests and from the occurrence and fabric of cherts in the geologic record.  The silicon isotopic composition (d³⁰Si) of diatoms has recently been shown to reflect the percentage of available silicic acid that was converted to biogenic opal.  It may be possible to use silicon isotopes to identify and reconstruct an abrupt drawdown of silicic acid such as that following the rise of the diatoms.  It is useful to consider how large a shift in d³⁰Si to expect for a drop in silicic acid concentrations over a particular interval of time.  To this end a two-box (surface and deep) model was constructed of d³⁰Si and silica cycling in the oceans.  Inputs of silicon are fixed at present day values. The ratio of silicon flux in and out controls the biological production of opal that occurs in the surface box.  At steady state, when concentrations of silicic acid are very high (e.g. 1400 mM), the d³⁰Si of the surface box equals the d³⁰Si of the inputs minus the isotopic fractionation factor (in permil) associated with biomineralization, and the d³⁰Si of opal equals that of the inputs.  The resulting d³⁰Si values of silicic acid in the surface box are 1 to 2 permil higher than that of the modern day ocean.  When silicic acid concentrations drop due to an imbalance between the input and output fluxes of silicon, the isotopic composition of opal and silicic acid increases to a degree controlled by the extent of the imbalance and by the fractionation factor.  When the falling silicic acid concentrations approach the lower values of the modern ocean, the d³⁰Si of silicic acid quickly drops towards that of the inputs while that of the opal increases due to Rayleigh distillation.  These results suggest variations in the d³⁰Si of opal over several hundred thousand years could be useful for reconstructing perturbations in the silica cycle, provided that the samples have been selected at a temporal resolution appropriate to the (changing) residence time of silicon in the oceans.