Abstract

The Southern Ocean plays a fundamental role in regulating global atmospheric CO2 levels, yet the underlying processes and feedbacks that control the carbon cycle during climate transitions remain unclear. Following the last glacial, the rapid rise in atmospheric CO2 was interrupted by an enigmatic 1,900-year plateau during a period of pronounced mid- to high-latitude Southern Hemisphere cooling called the Antarctic Cold Reversal (ACR, 14,600-12,700 years ago or 14.6-12.7 kyr BP). Here we report the first biomarker and ancient DNA analysis of a highly-resolved Antarctic ‘horizontal’ ice core, which combined with marine sediment records reveals a coherent signal of high-latitude Southern Ocean marine productivity and microbial diversity across the South Atlantic at the ACR . Transient climate modelling shows this period coincided with the maximum seasonal variability in sea-ice extent, suggesting perhaps that sea-ice feedbacks enhanced CO2 sequestration, making the high-latitude Southern Ocean a significant carbon sink that contributed to the sustained plateau in CO2 levels during the ACR .