The nature of deep overturning and reconfigurations of the silicon cycle across the last deglaciation

M Dumont,L Pichevin,K Dobby,E Michel,S Moreton,X Crosta,R Ganeshram,W Geibert

doi:10.1038/s41467-020-15101-6

Abstract

Changes in ocean circulation and the biological carbon pump have been implicated as the drivers behind the rise in atmospheric CO2 across the last deglaciation; however, the processes involved remain uncertain. Previous records have hinted at a partitioning of deep ocean ventilation across the two major intervals of atmospheric CO2 rise, but the consequences of differential ventilation on the Si cycle has not been explored. Here we present three new records of silicon isotopes in diatoms and sponges from the Southern Ocean that together show increased Si supply from deep mixing during the deglaciation with a maximum during the Younger Dryas (YD). We suggest Antarctic sea ice and Atlantic overturning conditions favoured abyssal ocean ventilation at the YD and marked an interval of Si cycle reorganisation. By regulating the strength of the biological pump, the glacial–interglacial shift in the Si cycle may present an important control on Pleistocene CO2 concentrations.

Highlights

Changes in ocean circulation and the biological carbon pump have been implicated as the drivers behind the rise in atmospheric CO2 across the last deglaciation; the processes involved remain uncertain
We suggest that this proposal provides further insight into how the biological pump moderates atmospheric CO2 across glacial-interglacial cycles
The DSi supplied to these zones is sourced from circumpolar deep water[31,32], which upwells within the Antarctic Zone (AZ) and delivers DSi to the Polar Front Zone (PFZ) by Ekman transport

Summary

Introduction

Changes in ocean circulation and the biological carbon pump have been implicated as the drivers behind the rise in atmospheric CO2 across the last deglaciation; the processes involved remain uncertain. Overturning in the Southern Ocean is thought to have played a key role with respect to the deglacial rise in CO2 in part because it is where many of the world’s water masses outcrop today[4] This is a region where deep ocean ventilation is moderated[5,6,7,8] and where nutrients are redistributed via intermediate waters to the low latitudes controlling the strength of carbon drawdown into the ocean via the biological pump[9,10]. A supply of DSi-rich waters favours the proliferation of diatoms[20,21] that efficiently export organic carbon from the ocean surface[22] without exporting alkalinity This promotes a greater drawdown of CO2 into the ocean by increasing the Corg:CaCO3 rain ratio[23,24]

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Conclusion