GLACIAL-INTERGLACIAL PERTURBATIONS IN THE GLOBAL SILICA CYCLE AND IMPLICATIONS FOR VARIABILITY IN ATMOSPHERIC CO2 LEVELS OVER THE LATE QUATERNARY

Silicic acid (H₄SiO₄) is a key nutrient in the ocean, one potentially limiting to the growth of an important phytoplankton group (diatoms) dependent upon its availability for the construction of opaline frustules. Conceptually, through the regulation of diatom productivity, changes in the availability of H₄SiO₄ might be expected to produce an antagonistic response in the overall productivity of the other (i.e., non-diatom) phytoplankton species. That non-diatom species include those responsible for the export of calcium carbonate (CaCO₃) to the deep sea has important implications for the operation of the global carbon cycle. On this basis, it has previously been hypothesized that greater dissolved Si supply to the world ocean during glacial times could give rise to a reduction in global CaCO₃ export sufficient to account for the concurrently low observed levels of atmospheric CO₂ (xCO₂).

Here we present a model of the global ocean carbon cycle optimized for use on glacial-interglacial time scales, in which processes controlling the biogeochemical cycling of Si within the ocean are explicitly represented. In contrast to predictions made by assuming a simple linear response of ocean H₄SiO₄ inventory to changes in dissolved Si supply, results obtained with this model demonstrate that a realistic reduction in Si supply is able to account for little more than 2 ppmv of the observed rapid initial deglacial xCO₂ rise (some ~70 ppmv). This muted response is a consequence of a combination of the highly non-linear nature of the sedimentary sink for H₄SiO₄ and the relatively long e-folding time of atmospheric composition with respect to perturbations in the global silica cycle. However, increasing Si supply has the potential to explain as much as 18 ppmv of the declining trend in xCO₂ apparent between Stages 5a and 2 as the Earth System descends towards its full glacial state.