Studies invoking the silicic acid leakage hypothesis to explain events of high biogenic opal deposition at low latitudes did not consider variations in formation rate of Southern Ocean intermediate waters. Comparison of delta Si-30 records from the Southern Ocean to records of biogenic opal accumulation from low-latitude upwelling systems indicates a strong negative correlation between the silicic acid content of Southern Ocean surface waters and opal deposition at low latitudes during the last 80 kyr BP. This implies that Si leakage was somehow rendered ineffective when the silicic acid content of Southern Ocean waters was high. Comparison of opal accumulation records with records of intermediate water formation rates suggests that lower rates of intermediate water formation offset the higher silicic acid content of these water masses during glacial periods. This indicates that physical processes that control intermediate water transport dominate over biological processes that set the silicic acid content of Antarctic surface waters in determining the magnitude of silicic acid leakage from the Southern Ocean.
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