Stability of the Equatorial Atlantic mid-depth circulation across the mid-Pleistocene transition

The mid-Pleistocene Transition (1.25-0.7 Ma) marks the emergence of the 100-kyr-periodicity and more intense glacial cycles without changes in orbital forcing, requiring a fundamental shift in Earth’s internal climate system. A critical glacial Atlantic deep circulation weakening and increased Southern Ocean water masses incursion between MIS 24 – MIS 22, even during the interglacial MIS 23, has been suggested as a key driver, responsible for enhancing carbon storage, reducing atmospheric CO₂ and facilitating ice-sheet growth, and has been called as “AMOC crisis” event. However, the expression of this thermohaline disruption is not well-documented at intermediate depths in the Equatorial Atlantic, an important AMOC flow branch. To investigate the Equatorial Atlantic mid-depth water masses variability across the MPT, we applied a benthic foraminiferal δ¹³C record from a two-core composite MD23-3677Q (1988 m) and MD23-3678 (1988 m), positioned in the NADW upper layer. We built two vertical gradients (Δδ¹³C) between our record and two published data from deeper cores (DSDP 607 and ODP 925), influenced by the NADW deep layer. A close-to-zero Δδ¹³C indicates the same water mass influence at mid-depth and deep ocean. Our data suggests that the proposed Southern Ocean water masses incursion and expansion across the AMOC crisis event did not affect depths shallower than 2000 m. Moreover, no substantial changes were observed between intervals pre- and post-MPT at intermediate depths in the Equatorial Atlantic, and the variability observed in the vertical gradients is mainly driven by deep ocean changes, which were affected by the reorganization of the glacial Atlantic Ocean structure after the MPT.