The silicic acid leakage hypothesis (SALH) attempts to explain part of the large and regular atmospheric C02 changes over the last glacial-interglacial cycles. It calls for a reduction in the carbonate pump through a growth in diatoms at the expense of coccolithophorids in low-latitude surface waters, driven by a "leakage" of high-Si:N waters from the Southern Ocean. Recent studies that present low opal accumulation rates from the glacial eastern equatorial Pacific have challenged SALH. In a corollary to SALH, we argue that the key to SALH is the dominance of diatoms over coccolithophorids, and this does not depend on the magnitude of diatom production per se. In support of our claim, we show in a numerical model that atmospheric C02 can be lowered with even a reduced absolute flux of silicic acid leakage, provided that Si:N in the leakage is elevated and that the excess Si can be used by diatoms to shift the floral composition in their favor.
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