Sensitivity of silicon isotopes to whole-ocean changes in the silica cycle

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 (δ 30Si) 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 δ 30Si of seawater and the average δ 30Si of opal outputs by a few hundredths to a few tenths of permil. This would be largely tied to a change in the δ 30Si 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 δ 30Si, but a sustained halving of river inputs could interfere with use of δ 30Si 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 δ 30Si of seawater from greater than + 1.9‰ down to about + 0.8‰. The isotopic composition of diatom opal, however, has an isotopic composition that sticks close to the + 0.8‰ 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 δ 30Si of opal produced in the deep sea (for example, by sponges) should document this transition.