Pleistocene atmospheric CO₂ change linked to Southern Ocean nutrient utilization

Biological uptake of CO₂ by the ocean and its subsequent storage in the abyss is intimately linked with the global carbon cycle and constitutes a significant climatic force¹. The Southern Ocean is a particularly important region because its wind-driven upwelling regime brings CO₂ laden abyssal waters to the surface that exchange CO₂ with the atmosphere. The Subantarctic Zone (SAZ) is a CO₂ sink and also drives global primary productivity as unutilized nutrients, advected with surface waters from the south, are exported via Subantarctic Mode Water (SAMW) as preformed nutrients to the low latitudes where they fuel the biological pump in upwelling areas. Recent model estimates suggest that up to 40 ppm of the total 100 ppm atmospheric pCO₂ reduction during the last ice age were driven by increased nutrient utilization in the SAZ and associated feedbacks on the deep ocean alkalinity. Micro-nutrient fertilization by iron (Fe), contained in the airborne dust flux to the SAZ, is considered to be the prime factor that stimulated this elevated photosynthetic activity thus enhancing nutrient utilization. We present a millennial-scale record of the vertical stable carbon isotope gradient between subsurface and deep water (Δδ¹³C) in the SAZ spanning the past 350,000 years. The Δδ¹³C gradient, derived from planktonic and benthic foraminifera, reflects the efficiency of biological pump and is highly correlated (rxy = -0.67 with 95% confidence interval [0.63; 0.71], n=874) with the record of dust flux preserved in Antarctic ice cores⁶. This strongly suggests that nutriënt utilization in the SAZ was dynamically coupled to dust-induced Fe fertilization across both glacial-interglacial and faster millennial timescales. In concert with ventilation changes of the deep Southern Ocean this drove ocean-atmosphere CO₂ exchange and, ultimately, atmospheric pCO₂ variability during the late Pleistocene.