Foraminifera-bound Nitrogen Isotopes Reveal Northward Migrations of the Subtropical Frontal Zone During the Mid-Pleistocene Transition

Abstract:

The Mid-Pleistocene Transition (MPT) is an interval in Earth’s history when internal feedbacks acted in concert to extend the length of glacial-interglacial cycles and reduce atmospheric CO2. The Southern Ocean is thought to play a significant role sequestering carbon during the MPT but the mechanism responsible remains ambiguous. Increases in biogenic sediment accumulation in the Subantarctic Zone and decreased carbon isotopic composition of benthic foraminifera during the MPT indicate a potential link between Southern Ocean biological pump efficiency and deep ocean carbon storage. IODP Site U1475, in the Subtropical Frontal Zone (STFZ) of the Southern Ocean, is sensitive to frontal migrations and changes to nutrient delivery that may impact biological pump efficiency and deep ocean carbon storage. Here we present new species-specific records of foraminifera bound nitrogen isotopes (𝛿15Nfb) and compare them to biogenic sediment mass accumulation rates and foraminiferal assemblages, from Site U1475. This multi-proxy comparison provides context to assess the relative significance of Southern Ocean frontal migrations in changing nutrient supply and demand during the MPT.

We present 𝛿15Nfb records from two species, Globigerina bulloides and Globorotalia inflata. Our records reveal a ~2‰ increase in the nitrogen isotope composition of surface waters over U1475 leading up to the deep ocean carbon isotope minima (~900 ka). These relatively high 𝛿15Nfb values, leading up to 900 ka and in later glacial periods, occur alongside increases in biogenic sediment accumulation and abundance of polar species. These findings support northward migrations of the STFZ in glacial and weak interglacial intervals leading up to 900 ka as well as in subsequent glacial intervals. These data are consistent with a purported northward migration of Southern Ocean fronts associated with reduced deep ocean ventilation. In addition, northward frontal migrations enhanced nutrient supply in the STFZ, potentially a region of greater iron delivery and nutrient drawdown. The physical changes in the Southern Ocean both in the surface and at depth may have worked to increase carbon storage during the MPT.