Biogeochemical evolution of deep-sea sediments in Iceberg Alley (Scotia Sea) since the Miocene and implications for carbon and iron cycling

Major changes in Southern Ocean circulation and Antarctic ice sheet dynamics during the late Cenozoic altered marine productivity and carbon burial, yet their impact on sediment biogeochemical processes remains poorly constrained. Here, we combine geochemical data from three IODP Expedition 382 drill cores in Iceberg Alley (Pirie and Dove Basins, Scotia Sea) with a dynamic diagenetic model to reconstruct sediment biogeochemical evolution from the Miocene to the Pleistocene. A previously unrecognised early Pleistocene reorganisation of sediment biogeochemistry was identified beginning around 1.5–2 Ma, being most pronounced in the Pirie Basin. Across all sites, the reactivity of buried POC rose dramatically around this time, likely reflecting enhanced productivity and reduced oxygen exposure due to higher sedimentation rates. This led to a drawdown of dissolved sulphate, driven by intensified anaerobic degradation of organic matter, and accumulation of pyrite in the upper 100 m. A coeval increase in reactive iron content and reactivity led to anaerobic oxidation of methane using both sulphate and iron as electron acceptors in the Pirie and Dove Basins. As a result, methane concentrations in the sulphate-depleted sections downcore remain remarkably low. These sedimentological changes precede the Mid-Pleistocene Transition (1.25 to 0.7 Ma), but coincide with the expansion of the West Antarctic Ice Sheet, which likely increased the delivery of iron and poorly weathered continental material to the high-latitude Southern Ocean. The major changes observed in the Scotia Sea also correspond with shifts in the opal-to-carbonate ratio in deep marine sediments of the Pacific sector of the Southern Ocean. These findings suggest that enhanced delivery of reactive organic carbon and iron to sediments in the Southern Ocean during the early Pleistocene intensified carbon burial and pyrite formation. These processes may have strengthened long-term carbon sequestration in the Southern Ocean and contributed to declining atmospheric CO 2 during late Cenozoic climate evolution .