Increased zonal δ13C gradient in the deep South Atlantic after the Mid-Brunhes Transition

The climate system experienced several periodic oscillations over the last ca. 800 ka known as glacial-interglacial (G-IG) cycles. Disruptions of the global carbon cycle were evident on this time scale, promoting fluctuations in the atmospheric CO₂ concentration leading to global climate variability. In the more recent interglacials, both Antarctic temperatures and atmospheric CO₂ concentrations are significantly higher than in the previous “lukewarm interglacials” (ca. 800 – 430 ka) before the Mid-Brunhes Transition (MBT). Changes in the Atlantic Meridional Overturning Circulation (AMOC) and deepwater formation rate around Antarctica have been invoked to explain a 30 ppm increase in the atmospheric CO₂ ­during post-MBT interglacial periods. Deepwater variability is tightly coupled to the ventilation of CO₂ in the Southern Ocean by atmospheric and oceanic connections, contributing to carbon storage in the deep ocean and the atmospheric CO₂. Here, we present a new 770 ka benthic foraminifera δ¹³C record from sediment core GL-854 retrieved from the western South Atlantic (WSA) at 2200 m water depth. We compare our record with published δ¹³C data from the eastern margin to investigate the zonal gradient variability of the North Atlantic Deep Water (NADW) in the deep South Atlantic basin. WSA δ¹³C variability and absolute values strongly mimic the North Atlantic mid-depth record at the NADW formation region. This similarity is interpreted as NADW preferentially carrying a modified signal through the deep western boundary current towards the WSA (rather than towards the eastern margin) after the MBT. The δ¹³C gradient based on the difference between benthic foraminifera C. wuellerstorfi from both margins (Δδ¹³C_w-e) gradually increases after a transitional period between ca. 400 ka to 300 ka towards the Holocene. We suggest that the mechanism behind this long-term increasing trend on the Δδ¹³C_w-e record post-MBT is the result of enhanced production of North Component Water due to Agulhas Leakage intensification driven by reduced sea-ice extent after the MBT. Furthermore, reduced sea-ice extent decreases the Antarctic Bottom Water density and formation in the Southern Ocean, contributing to the deepening of the AMOC during post-MBT interglacial periods. Our interpretation proposes a framework connecting sea-ice and ocean-atmosphere dynamics to deepwater geometry within the South Atlantic basin, which ultimately contributed to the climate change observed across the MBT.