New isoprenoid GDGT index as a water mass and temperature proxy in the Southern Ocean

Abstract. The Southern Ocean plays a crucial role in the global carbon cycle, ocean heat transport, and Antarctic ice dynamics. Investigating past variability in the Southern Ocean, including temperature and water mass distribution, can improve understanding of how this system may respond to current climate change. Isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) can be used as an ocean temperature proxy and have been applied to sediments in the Southern Ocean to reconstruct past temperature variability. However, applications of current isoGDGT-based temperature indices are subject to substantial uncertainty in the Antarctic Zone. In this study, we propose a new isoGDGT-based index, the Antarctic IsoGDGT Zonal (AIZ) index, composed of GDGT-0, GDGT-1 and GDGT-2, developed through statistical reanalysis of Southern Ocean core-top data. We interpret that the AIZ index captures shifts in archaeal community composition across the Polar Front (PF). South of the PF, cold-adapted archaea, which are characterized by high relative abundances of GDGT-0, dominate, whereas more diverse archaeal communities occur north of the PF. Because these community shifts are tied to water mass boundaries, the AIZ index serves as an effective tracer for reconstructing past PF movements. Furthermore, the AIZ index exhibits a significant correlation with subsurface temperature (subST) south of the PF, suggesting that it can be used as a temperature proxy in the Antarctic Zone (subST=24.17×AIZ-1.45 (R2 = 0.81, n = 134, p < 0.0001)). Applying the AIZ index to late Pleistocene sediment cores collected around the ACC confirms its reliability as a water mass tracer and temperature proxy in the Antarctic Zone. Our study highlights the high potential of isoGDGTs for reconstructing palaeoceanographic conditions in the Southern Ocean.