Modes of deep-water circulation in the subtropical South Atlantic over the last 770 kyr revealed by a zonal δ13C gradient

Glacial-interglacial (G-IG) cycles are closely linked to deep ocean circulation and carbon storage, with the Atlantic Meridional Overturning Circulation (AMOC) playing a critical role in Earth's climate system. Long-term changes in the balance between North Atlantic Deep Water (NADW) and Southern Sourced Waters (SSW) have likely provided feedbacks amplifying orbital forcing, partly by influencing atmospheric CO₂ through the modulation of deep-ocean carbon storage via Southern Ocean ventilation. Understanding the evolution of this deep-water interplay requires long paleoceanographic records, which remain scarce in the subtropical western South Atlantic. Here we present a new 770 ka benthic foraminifera carbon stable isotope (δ 13 C) record from sediment core GL-854 retrieved from the western South Atlantic (WSA) at 2200 m water depth. We compared this record with published δ 13 C data from the eastern South Atlantic to investigate the zonal δ 13 C gradient variability (∆δ 13 C w-e ) of NADW. Our results reveal that ∆δ 13 C w-e G-IG variability responds to a “deep-water seesaw” driven by increased influence of SSW at mid-depths promoted by shallower deep-water cells during intense glacial stages. RAMPFIT change-point analysis reveals four distinct periods representing long-term phases of zonal ventilation over the Late Pleistocene. Phases characterized by near-zero average gradients (Phases II and IV) encompass periods of extreme glacial conditions when the zonal gradient reversed in response to increased SSW influence, contrasting with phases with stronger well-ventilated NADW presence in the WSA (Phases I and III). We hypothesize that these phases are controlled by orbitally-triggered variations in Antarctic sea ice extent modulating the deep-water seesaw. Spectral power in the obliquity and eccentricity domains identified in our record suggests that the orbital forcing on sea ice extent is propagated toward subtropical regions through controls over the deep-water seesaw. Our interpretation proposes a framework connecting sea-ice and ocean-atmosphere dynamics to deep-water geometry within the South Atlantic basin, which ultimately contributed to the climate changes during the Late Pleistocene. • South Atlantic zonal d13C gradient tracks the shifting NADW/SSW balance over the last 800 kyr. • Four distinct modes of mid-depth zonal ventilation asymmetry identified in the South Atlantic over 800 kyr. • Orbitally-paced Antarctic sea-ice variability is proposed as a key modulator of the Atlantic deep-water seesaw. • Eccentricity thresholds may modulate climate sensitivity to insolation, influencing sea ice and long-term circulation modes.