Absence of a strong, deep-reaching Antarctic Circumpolar Current zonal flow across the Tasmanian gateway during the Oligocene to early Miocene

The vigorous eastward flow of the Antarctic Circumpolar Current (ACC) connects all major ocean basins and plays a prominent role in the transport of heat, carbon and nutrients around the globe. However, the establishment of a deep circumpolar flow, similar to the present-day ACC, remains controversial thereby obscuring our understanding of its climatic impact. Deciphering the chemical composition of Circumpolar Deep Water (CDW) within the ACC can provide critical insights about its development and evolution. Here we present new fossil fish teeth/bone debris neodymium isotope (epsilon(Nd)) records from Deep Sea Drilling Project (DSDP) Sites 278 and 274 in the southwest Pacific Ocean, with the aim to trace changes in deep water masses across the Tasmanian Gateway between the early Oligocene and early Miocene (similar to 33-22 Ma). Site 274 provides the first Nd isotope record proximal to the Ross Sea during the Oligocene (33.5-23.4 Ma). Its Nd isotope composition shows excursions to very radiogenic values, epsilon(Nd(t)=) -3.1 and epsilon(Nd(t)=) - 3.7, at 33.5 Ma and 23.8 Ma, respectively, in response to major steps in Antarctic ice sheet expansion. A shift to lower, more unradiogenic epsilon(Nd)((t)) values between 29.7 and 29.1 Ma is linked to an increased influence of proto-CDW upwelling at the site. In contrast, the Nd isotope record from Site 278 in the southern Emerald Basin shows little variability epsilon(Nd)((t)) = -6.0 to -6.7) throughout the Oligocene and early Miocene (30.9-21.8 Ma). Comparison with published data north of the ACC path, demonstrates the presence of two deep water masses in the South Pacific prior to the inferred onset of the ACC (33-30 Ma), one occupying depths between similar to 2500 and 3000 m (epsilon(Nd(t)) = -3 to -5) and a deep/bottom water mass (> 3000 m) with a more unradiogenic Nd isotope composition (epsilon(Nd(t)=) similar to -6). Site 278 located close to the proto-polar front (proto-PF) indicates that following the inferred onset of the ACC, deep waters bathing the southern Emerald Basin remained more radiogenic in the Southwest Pacific compared to sites along the proto-PF in the South Atlantic and Indian Ocean (epsilon(Nd(t)()=) similar to -8.1). This indicates a provinciality in Nd isotope compositions of deep waters along the proto-PF across the Tasmanian Gateway. Our data are incompatible with the existence of a modern-like homogenous (lateral and vertical) Nd isotope composition of CDW along the main flow path of the ACC in all oceanic basins in the Oligocene to early Miocene. We attribute distinct Nd isotope compositions of deep waters across the Tasmanian Gateway to reflect a less deep reaching and weaker ACC (proto-ACC) than today. Our findings suggest that the modern strong and deep-reaching ACC flow must have been developed at a later point in the Neogene.

Automated summary

The Antarctic Circumpolar Current (ACC) is crucial for global heat, carbon and nutrient transport. Neodymium isotope records reveal changes in deep water masses across the Tasmanian Gateway, indicating the presence of different deep water masses during the early Oligocene and early Miocene periods. The study suggests that the ACC in the past may have been less deep and not as strong as it is today, with distinct neodymium isotope compositions of deep waters across the Tasmanian Gateway.