Abstract
Relief of iron (Fe) limitation in the Southern Ocean during ice ages, with potentially increased carbon storage in the ocean, has been invoked as one driver of glacial–interglacial atmospheric CO2 cycles. Ice and marine sediment records demonstrate that atmospheric dust supply to the oceans increased by up to an order of magnitude during glacial intervals. However, poor constraints on soluble atmospheric Fe fluxes to the oceans limit assessment of the role of Fe in glacial–interglacial change. Here, using novel techniques, we present estimates of water- and seawater-soluble Fe solubility in Last Glacial Maximum (LGM) atmospheric dust from the European Project for Ice Coring in Antarctica (EPICA) Dome C and Berkner Island ice cores. Fe solubility was very variable (1–42%) during the interval, and frequently higher than typically assumed by models. Soluble aerosol Fe fluxes to Dome C at the LGM (0.01–0.84 mg m−2 per year) suggest that soluble Fe deposition to the Southern Ocean would have been ≥10 × modern deposition, rivalling upwelling supply.
Highlights
Relief of iron (Fe) limitation in the Southern Ocean during ice ages, with potentially increased carbon storage in the ocean, has been invoked as one driver of glacial–interglacial atmospheric CO2 cycles
Our data are much more representative of dissolution of atmospheric dust in the ocean than previous ice-core studies, avoiding both unrepresentative leaching by strong acids[19,20,22,23] or precipitation of insoluble Fe(III) hydroxides during melting. These methods are relevant for the possible modes of deposition of dust into the ocean, either deposition within rain, where dust has already been exposed to liquid at pH B5.3, or deposition of dry dust or solid ice/snow containing dust to the ocean where the first liquid encountered is seawater. We find that both total aerosol Fe fluxes and aerosol Fe solubility were very variable throughout the Last Glacial Maximum (LGM) in EDC ice (1–42% soluble; pH B5.3 mean 10%, median 6%) leading to highly variable soluble aerosol Fe fluxes of 0.01–0.84 mg m À 2 per year at Dome C throughout the interval
At Berkner Island, aerosol Fe solubility was lower than Dome C, higher dust fluxes resulted in similar mean soluble aerosol Fe fluxes to those at Dome C during the LGM
Summary
Relief of iron (Fe) limitation in the Southern Ocean during ice ages, with potentially increased carbon storage in the ocean, has been invoked as one driver of glacial–interglacial atmospheric CO2 cycles. Our data are much more representative of dissolution of atmospheric dust in the ocean than previous ice-core studies, avoiding both unrepresentative leaching by strong acids[19,20,22,23] or precipitation of insoluble Fe(III) hydroxides during melting These methods are relevant for the possible modes of deposition of dust into the ocean, either deposition within rain, where dust has already been exposed to liquid at pH B5.3, or deposition of dry dust or solid ice/snow containing dust to the ocean where the first liquid encountered is seawater. Our measured aerosol Fe solubilities and soluble aerosol Fe fluxes are higher than typically used in models of ocean biogeochemistry, and we suggest that models should consider higher and more spatially and temporally variable soluble atmospheric Fe fluxes to the Southern Ocean during glacial intervals
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