Tracing the Migration of Fronts in the Pacific Southern Ocean Across the Mid-Pleistocene Transition

Resumen: The Southern Ocean has a strong influence on the Earth’s climate, but its changes during a major climate shift that occurred between 1.25 and 0.7 million years ago, known as the Mid-Pleistocene Transition (MPT), are not well understood. In particular, the Pacific Southern Ocean is the least studied sector. To address this gap, we used microalgae fossils known as diatoms, from four sediment cores along a transect (54°S–60°S) to show changes in the ecosystem of the surface waters. We focused on variations related to the Antarctic Circumpolar Current frontal system; this current connects the Pacific, Atlantic, and Indian sectors of the Southern Ocean. Changes in surface water conditions, like increased sea-ice extent and colder surface water temperatures, coincide with a northward relocation of the frontal system during the mentioned time interval, before shifting back to its present-day positions. These shifts in water mass boundaries and associated ecological zones coincided with increases in sea ice and cooler summer sea temperatures, contributing to milder interglacial periods during the MPT. The movement of these fronts was influenced by underwater geography, particularly the East Pacific Rise, and closely tied to changes in sea surface conditions.