Silicic acid dynamics in the glacial sub-Antarctic: Implications for the silicic acid leakage hypothesis

The silicic acid leakage hypothesis (SALH) purports that changes in silicon and nitrogen depletion ratios in the glacial Antarctic created a large pool of unused silicic acid that was transported to lower latitudes in subantarctic mode water (SAMW) where it enhanced diatom productivity lowering atmospheric pCO(2). However, increased opal accumulation beneath the sub-Antarctic during glacial times implies significant consumption of silicic acid in subantarctic surface waters that may have significantly diminished or eliminated Si leakage. To test how nutrient dynamics in the sub-Antarctic affected the Si( OH) 4 content of SAMW during the last glacial period, we produced delta(30) Si opal records for cores from the subantarctic and subtropical zones of the Indian Ocean spanning the last 50,000 years. Comparison with diatom-bound delta N-15 records shows that subantarctic surface waters were enriched in Si relative to N during the last glaciation consistent with the SALH. The record from the subtropics does not show this enrichment in Si during the last glacial period suggesting that subantarctic surface waters were mainly incorporated into SAMW rather than being transported across the Subtropical Front. Isotope mass balance calculations were used to test for Si leakage from the sub-Antarctic. The results show that silicic acid concentration in SAMW would more than double during the last glaciation if upwelling and northward Ekman drift in the Antarctic were similar to the present-day circulation. Calculations that assume increased stratification in the glacial Antarctic eliminate Si leakage, but they do not produce the known increase in opal burial in the glacial sub-Antarctic. Reconciling the isotope data and opal burial records with a highly stratified Antarctic requires the addition of a large local source of silicic acid in the glacial sub-Antarctic that is inconsistent with present-day circulation and nutrient distributions. Including such a source in our calculations results in significant opal burial in the sub-Antarctic, but it does not enhance Si leakage over that occurring in the Holocene. Resolving past changes in the circulation of the Southern Ocean is clearly vital to future tests of the SALH.