Christopher Fogwill, Chris Stewart MacGregor Turney , Laurie Menviel, Andy Baker , Michael Weber, Bethany Ellis, Zoë Thomas, Nick Golledge, David Etheridge, Mauro Rubino

Determining the feedbacks that modulate Southern Ocean carbon dynamics is key to understanding past and future climate. The global pause in rising atmospheric CO2 during the period of mid- to high-latitude southern surface cooling known as the Antarctic Cold Reversal (ACR, 14,700-12,700 years ago) provides an opportunity to disentangle competing influences. We present highly-resolved and precisely-aligned ice and marine reconstructions that capture a previously unrecognized increase in microbial diversity and ocean primary productivity during the ACR. Transient climate modeling across the last glacial suggests this period corresponds to a maximum seasonal difference in sea-ice extent. Our results indicate that this increased seasonal sea-ice variability drove changes in high-latitude light, temperature and nutrient availability, turning the southern seasonal sea-ice zone into a globally significant carbon sink.