Southern Ocean Vertical Carbon Stratification Across the Mid‐Pleistocene Transition

Abstract Paleoceanographic evidence detailing the evolution of ocean carbon sequestration at high resolution across the Mid‐Pleistocene Transition (MPT; 1.2–0.7 Ma, million years ago) remains scarce. Here, we present a continuous record of the stable carbon isotope gradient, referred to as the “chemocline,” between the sub‐surface and deep ocean (Δδ 13 C (sub‐deep) ) from a Subantarctic Zone (SAZ) sediment core site located south of Africa. We demonstrate that periodic breakdowns of Southern Ocean chemocline occur at multiple glacial terminations extending back to at least 1.55 Ma, coinciding with intensified upwelling in the Antarctic Zone (Jaccard et al., 2013, https://doi.org/10.1126/science.1227545 ). This indicates that the accumulation and (rapid) release of respired carbon from the deep Southern Ocean is a recurring feature of glacial‐interglacial cycles in both the pre‐MPT and post‐MPT world. A comparison of Δδ 13 C (sub‐deep) and records of atmospheric CO 2 levels and SAZ iron accumulation reveals a potential relationship between a more efficient Southern Ocean biological pump and declining glacial CO 2 levels across the Early to Middle Pleistocene. This apparent coupling breaks down after ∼0.3 Ma, wherein glacial Δδ 13 C (sub‐deep) levels revert to pre‐MPT levels, while glacial CO 2 remains low. One explanation is that an expansion of the SAZ during glacials between these two intervals modifies Δδ 13 C (sub‐deep) at our study site without necessitating a change in CO 2 level or iron fertilization. However, further investigation is required to reconstruct the extent of Southern Ocean frontal migrations before, during, and after the MPT, while also distinguishing other potential complicating factors, such as alterations in deep ocean stratification and water‐mass geometry.