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

Opal mass accumulation rates from IODP Site 361-U1475

Biogenic silica accumulation was obtained by analyzing approximately 200 mg of homogenized and freeze dried sediment for each sample. Cleaning, chemical treatment, and measurement followed protocols outlined in (Mortlock & Froelich, 1989). Samples were measured with a UV Vis spectrophotometer at 812 nm wavelength. Full procedural replicates were performed on 163 of the 435 samples yielding an average standard deviation of 0.2%. Samples were referenced to RICCA VerSpec SiO32- in 1% NaOH for intercomparison. Opal mass accumulation rates were calculated by multiplying the fraction of opal by dry bulk density and sedimentation rates from Starr et al. (2021)

Abundance of Neogloboquadrina pachyderma sinistral from IODP Site 361-U1475

N. pachyderma abundance was obtained by washing ~ 10 cm³ of sediment through a 150µm sieve and drying at ~ 50 ºC for 24 h. This dried fraction was split until a total of 300-400 individuals remained. From this amount, we identified the relative abundance of Neogloboquadrina pachyderma tests according to Kennett and Srinivasan (1983) and Loeblich and Tappan (1988)