Nutrient cycling and the biological pump in the Southern Ocean across the last deglaciation
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Date
03/07/2019Author
Dumont, Matthew
Metadata
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.