Deglacial Si remobilisation from the deep-ocean reveals biogeochemical and physical controls on glacial atmospheric CO2 levels

Abstract

During the last glacial period, the sluggish deep Ocean circulation sequestered carbon into the abyss leading to the lowering of atmospheric CO2. The impact of this redistribution on biologically essential nutrients remains poorly constrained. Using sedimentary δ30Si of diatoms and biogenic accumulation rates in the Eastern Equatorial Pacific (EEP), we present evidences for the remobilisation of dissolved Silica (DSi) along with carbon from the deep ocean during the Last Deglaciation. Because DSi is essential for diatoms growing in the surface ocean, its concentration in the abyss during the glacial periods amounts to a negative feedback on the oceanic CO2 uptake. However, this effect can be muted by the increased Fe inputs during glacial periods which reduces diatom Si requirements in Fe limited regions such as the EEP. Our results from the EEP suggest that the efficiency of the biological CO2 pump and the size of the local CO2 source is tightly controlled by changes in DSi utilisation driven by Fe availability across the last glacial-interglacial transition.We use a modified PANDORA box model to illustrate that the inventory of DSi in the global ocean surface is controlled by Fe availability in HNLC areas rather than by straightforward Si supply though upwelling. The Holocene is characterised by a fast mode of Si cycling driven by high biological requirement for Si under conditions of iron limitation and efficient overturning, promoting CO2 outgassing and an inefficient biological C pump via the rapid exhaustion of DSi in the surface. The last glacial period saw slower marine Si cycling as a result of decreased DSi biological requirement under Fe-replete conditions in the sea surface and increased Si and CO2 sequestration in the abyssal ocean. The switch between the two modes of Si cycling happened at 15 ka BP, i.e. mid-deglaciation, and resulted in contrasting biological carbon drawdown responses in the EEP and globally between both phases of the deglacial CO2 rise. This illustrates that in addition to deep-sea CO2 storage and overturning, the efficiency of the biological pump also plays a crucial role in determining ocean-atmosphere CO2 exchange and shows the dual controls of ocean circulation and Fe-Si availability in this process.