Investigating oceanic carbon sequestration during glacial inception using vertical profiles of [CO32-], d13C, and d18O from the Southwest Atlantic

Despite decades of research, the cause of glacial-interglacial CO₂ cycles is not fully understood, leaving a critical gap in our understanding of Earth’s climate system. One hypothesis is that shoaling of the boundary between Northern Source Water (NSW) and Southern Source Water (SSW) enhanced oceanic carbon sequestration during glacial intervals, resulting in lower atmospheric pCO₂. To test this hypothesis, we generated vertical profiles of [CO₃^2-], δ¹³C, and δ¹⁸O using a depth transect of cores from the Brazil Margin, focusing on the two major drops in atmospheric pCO₂ during the last glacial cycle at ~115 ka and ~70 ka. Given that [CO₃^2-] is inversely related to the concentration of dissolved inorganic carbon, [CO₃^2-] should decrease if the Atlantic sequestered CO₂. We observe no significant change in the [CO₃^2-] across the first decrease in atmospheric pCO₂ and no evidence for watermass boundary shoaling in the δ¹³C and δ¹⁸O profiles.  [CO₃^2-] decreased only at ~3600 m, the core site most influenced by SSW.  During the second pCO₂ decline at ~70 ka, [CO₃^2-] decreased by ~30 µmol/kg below 2000 m water depth, coincident with marked shoaling in the δ¹³C and δ¹⁸O profiles. The lack of evidence for shoaling and deep Atlantic carbon sequestration at ~115 ka, a time of intermediate ice sheet extent and moderate global cooling, but the clear evidence for shoaling and carbon sequestration at ~70 ka, a time of near glacial maximum ice sheet extent, implies that the deep Atlantic’s capacity to store carbon depends on the Earth’s mean climate state. Our results highlight that distinct mechanisms are necessary to explain the two major drops in atmospheric pCO₂ of the last glacial cycle.