# Deglacial stratification of the polar Southern Ocean

## TL;DR

This study shows how meltwater from Antarctica during past warming events affected ocean circulation and CO2 levels, influencing global climate patterns.

## Contribution

The study provides new evidence of how Antarctic meltwater caused ocean stratification, impacting deep ocean ventilation and CO2 release.

## Key findings

- Meltwater from the Antarctic Ice Sheet temporarily stratified the upper ocean during deglaciations.
- This stratification weakened the Southern Ocean's lower cell while strengthening its upper cell.
- The findings suggest the open Antarctic Ocean can directly ventilate the deep ocean during warming events.

## Abstract

The effect of global warming on the circulation of the Southern Ocean is complicated by the potential for interactions between wind-driven upwelling and surface buoyancy fluxes. Here, we study changes in the Southern Ocean’s surface conditions during the last two deglaciations, when Earth warmed rapidly. We demonstrate that during each of these events, meltwater from the Antarctic Ice Sheet temporarily stratified the upper ocean near Antarctica. The findings point to specific mechanisms for the early rise in atmospheric CO2 concentrations during deglaciations and the climatic “seesaw” behavior between the hemispheres, highlighting the potential impacts of the Antarctic Ice Sheet on the ocean’s ability to absorb heat and carbon dioxide in a warming world.

It is widely accepted that meltwater from northern ice sheets drove changes in the Atlantic Meridional Overturning Circulation, impacting global climate. In contrast, data are lacking on the Antarctic Ice Sheet’s (AIS) effect on Southern Ocean overturning, despite the Southern Ocean’s central role in ventilating the ocean interior. Here, we present diatom-bound nitrogen isotope evidence indicating that during deglaciations, as wind-driven upwelling in the open Antarctic Ocean increased, AIS melting strengthened density stratification in the polar Antarctic Ocean. This change coincides with reconstructions of meltwater discharge and resembles model simulations of global warming including AIS meltwater forcing. The transient input of freshwater and, thus, buoyancy weakened Southern Ocean’s “lower cell” that outcrops in the polar Antarctic Ocean even as open Antarctic Ocean’s “upper cell” strengthened. Given evidence that deep-ocean ventilation and CO2 release were ongoing early in deglaciation, our data suggest that the open Antarctic Ocean, and not solely the most polar Antarctic Ocean, can directly ventilate the deep ocean. During the last deglaciation, the strongest density stratification in the polar Antarctic Ocean occurred just before the Antarctic Cold Reversal, when southern hemisphere warming paused and the northern hemisphere warmed sharply (i.e., during the Bølling–Allerød period). We propose that when combined with stronger wind-driven upwelling, the freshwater-driven redirection of upwelled water away from deep water formation and toward the upper cell deepened the global pycnocline, working to restart North Atlantic deep water formation in the Bølling–Allerød period. If so, polar ocean freshening in both hemispheres drove the “bipolar seesaw” events of deglaciation.