Silicic acid concentration of Subantarctic Mode Water (SAMW) in the South West Pacific over the last 30,000 years has been reconstructed by applying a newly developed model for silicon isotope fractionation in sponges to two records of δ³⁰Si_sponge from cores collected on the Campbell Plateau, New Zealand. These records suggest that silicic acid concentration in SAMW during the LGM was comparable to the Holocene whereas during the deglacial period it may have been higher by around a factor of two.

These results are not consistent with the theory that excess silicic acid in the glacial Southern Ocean was exported to the global thermocline through mode and intermediate waters, providing low latitude diatoms with a competitive advantage over calcifying phytoplankton, reducing the carbonate pump and contributing to lower glacial atmospheric pCO₂.

Interpretation of these results with respect to proxies for nutrient utilisation and biological productivity in the Southern Ocean suggest that the silicic acid concentration of waters upwelling in the Antarctic was up to 50% higher during the deglacial and possibly 70% lower during the LGM relative to the Holocene. These interpretations support the theory that changes in deep ocean ventilation, not silicic acid leakage, are responsible for glacial-interglacial atmospheric pCO₂ variability.

However, a critical hindrance to these interpretations of the δ³⁰Si_sponge records is the uncertainty in the influence of local variations in Si dynamics (i.e. terrestrial nutrient fluxes), which may compete with mode and intermediate water Si supply.