A 2-box model has been used to assess the impact of both long- and short-term budgetary imbalance in the silica cycle on the average silicon isotopic composition (y30Si) of the ocean and marine sediments. Over a 100-ky time span, such as a Quaternary glacial cycle, a sustained change in the riverine flux of silicon to the oceans could alter the average y30Si of seawater and the average y30Si of opal outputs by a few hundredths to a few tenths of permil. This would be largely tied to a change in the y30Si of silicon entering the ocean due to a shift in the proportion of riverine and non-riverine sources of silicon. A doubling of the riverine flux of silicon would have little impact on average marine y30Si, but a sustained halving of river inputs could interfere with use of y30Si as a tracer of nutrient utilization. Studies on the longer term focussed on the transition from a high silicic acid to low silicic acid ocean associated with the rise of the diatoms. This transition is marked by drop in the average y30Si of seawater from greater than +1.9x down to about +0.8x. The isotopic composition of diatom opal, however, has an isotopic composition that sticks close to the +0.8x of the inputs and is thus unlikely to provide information about the transition to the low silicic acid ocean of the modern day. However, the y30Si of opal produced in the deep sea (for example, by sponges) should document this transition.