Nutrient cycling and the biological pump in the Southern Ocean across the last deglaciation

Abstract

Three high resolution multi-proxy records from the Indian sector of the Southern Ocean are presented that reconstruct nutrient cycling and the biological pump across the last deglaciation. Three new diatom silicon isotope records were constructed that demonstrate a major redistribution of the silicic acid and silicon isotopes in the world's oceans between the last glacial maximum and the present driven by changes in iron availability and oceanic circulation. Such changes have important implications for the power of diatoms to efficiently export carbon to the deep ocean. New records of diatom-bound nitrogen isotopes (d15NDB), paleo-productivity proxies and planktic foraminiferal radiocarbon together demonstrate the role of Antarctic strati cation in restricting deep ocean ventilation and promoting the drawdown of atmospheric CO2 across glacial-interglacial cycles. The presence of 14C-depleted surface waters in the Antarctic during the deglaciation invokes a strong link between the upwelling of aged deep waters in the Southern Ocean and the rise in atmospheric CO2. Anomalously heavy d15NDB observations during the deglacial upwelling intervals were interpreted as evidence for enhanced denitrification outside of the known oxygen minimum zones, driven by the deoxygenation of the deep ocean during glacial periods. The development of new denitrification regions has important implications for the marine nitrogen cycle and may have modulated the atmospheric CO2 rise across the last deglaciation by weakening the biological pump. Because Southern Ocean surface waters impart chemical properties on Southern Ocean intermediate waters, the new proxy records predict that intermediate waters became deoxygenated, 14C-depleted and silica-rich during the deglaciation. This hypothesis helps explain numerous lower latitude proxy records from regions fed by intermediate waters, including the enhancement of denitri cation in oxygen minimum zones, anomalously 14C-depleted intermediate waters in the Arabian Sea, and the enhancement of diatom productivity in low latitude upwelling regions. Consequently, this work highlights the role of Southern Ocean hydrographical and biogeochemical processes in regulating nutrient cycling and the biological pump at low latitudes.