Glacial Southern Ocean Expansion Recorded in Foraminifera‐Bound Nitrogen Isotopes From the Agulhas Plateau During the Mid‐Pleistocene Transition.

The emergence of 100‐Kyr glacial cycles (The Mid‐Pleistocene Transition [MPT]) is attributed in part to slower global overturning circulation and iron stimulation of biological carbon drawdown in the Southern Ocean. We present foraminifera‐bound nitrogen isotope values and polar planktic foraminifera abundances from the Agulhas Plateau that show that increases in biogenic sediment accumulation coincide with northward migrations of the Subtropical Frontal Zone (STFZ) and elevated foraminifera‐bound nitrogen isotope values during MPT glacial episodes. The nitrogen isotope values of two planktic foraminifera species, Globigerina bulloides and Globorotalia inflata show remarkable coherence amongst the sea surface temperature gradient between the STFZ and SAZ, and polar foraminifera abundances, indicating a strong relationship between nitrogen isotope dynamics above the Agulhas Plateau and migrations of the STFZ. Northward migration of the STFZ may have been essential to prolonging glacial intervals by increasing deep ocean carbon storage via a northward shift of the South Westerly Winds and a reduction in upwelling, delivery of fresher surface waters into the upper limb of global overturning circulation, or inhibiting heat and salt delivery to the Atlantic as Agulhas Leakage. Plain Language Summary: The Mid‐Pleistocene Transition (MPT) is a unique period of time, during which the natural rhythm of Earth's climate shifted, the pace of glacial and interglacial cycles changed from 40,000 years to 100,000 years as ice sheets expanded, carbon accumulated in the deep ocean, and the planet cooled. The exact cause of these changes is unclear, but internal system feedbacks were likely at play. As climate changes today and the oceans increasingly take up carbon, the MPT provides a unique case study to investigate Earth's natural climate system and important carbon cycle feedbacks. In this paper we use the nitrogen isotopic composition of plankton shells and other biogeochemical records to show that leading up to the MPT the Southern Ocean underwent major physical changes and expanded as 100,000 years glacial cycles emerged. Our data indicates that the expansion of the Southern Ocean delivered cold, nutrient rich waters northward where they could mix with warm, salty Indian and Atlantic Ocean waters as hypothesized by other studies. Key Points: Foraminifera‐bound nitrogen isotope values from the Agulhas Plateau record latitudinal migrations of the Subtropical FrontElevated foraminifera‐bound nitrogen isotope values after 900 ka are consistent with Southern sourced nitrateSouthern Ocean contraction during a "super interglacial" brings more subtropical nitrate to the Agulhas Plateau [ABSTRACT FROM AUTHOR]