North Atlantic CO2 Pulses and Warm Stadials Linked to AMOC Variability

Heinrich Stadials are characterised as extreme cold episodes in the Northern Hemisphere, associated with destabilisation of the ice sheets and weak Atlantic Meridional Overturning Circulation. These occur simultaneously with recurrent intervals of atmospheric CO₂ rise, a feature typically attributed to Southern Ocean processes, in part due to the striking resemblance with Antarctic temperature records. However, elevated atmospheric CO₂ levels persist up to 500 years after the onset of southern hemisphere cooling, remaining high after the abrupt northern hemisphere transition into a warm interstadial climate.

Here we present new, high-resolution Mg/Ca and boron isotope data from the northeast North Atlantic Ocean, which provide new insights on rapid CO₂ and climate change at these times. We resolve major perturbations of surface ocean pCO₂ following Heinrich stadials as well as other non-Heinrich stadials. Pulses of high surface water CO₂ concentrationcorrespond with the sudden onset of vigorous overturning circulation in the North Atlantic Ocean. Model simulations reproduce these spikes and suggest that outgassing occurs due to carbon release from the deep ocean, coupled with high sea surface temperatures.

Further to this, our Mg/Ca data also reveal persistent warming of 1-3ºC in the shallow subsurface during Heinrich stadials. Rising atmospheric CO₂, likely augmented by an accumulation of heat in the subsurface due to weak overturning circulation and extensive winter sea ice cover may account for this warming, which could have contributed to ice sheet destabilisation. At the end of the stadial, the onset of vigorous deep overturning in the North Atlantic Ocean brought warm and CO₂-rich waters to the surface ocean, contributing to soaring northern hemisphere temperatures and causing atmospheric CO₂ to reach its peak despite the onset of Southern hemisphere cooling. Our data thus demonstrate a previously under-appreciated role for the North Atlantic in rapid CO₂ changes and highlight the prevalence of subsurface warming during North Atlantic cold events.