Antarctic Circumpolar Current dynamics, and terrigenous sediment provenance variations in the Drake Passage during the last 140,000 years

Abstract

The Antarctic Circumpolar Current (ACC) is the largest ocean current system on Earth. Through promoting deep water upwelling and new water masses formation, the ACC plays a crucial role on global ocean circulation and climate changes. The Drake Passage is the narrowest constriction for the ACC and exerts a strong control on the physical, chemical, and biological exchange between the Pacific and Atlantic Ocean. Resolving changes in the ACC through this specific channel is, therefore, important for elevating our knowledge of the Southern Ocean’s role in global ocean circulation and climate variability. However, previous studies showed a significant disagreement of the ACC flow speed changes and its potential impacts on ocean circulation and climate variability remain elusive. In Wu et al., (2019), we identified southern Patagonia and the Antarctic Peninsula as the main sources for terrigenous sediments in the modern Drake Passage region, based on a comprehensive set of surface sediment samples. We found the variability of the ACC shows a clear bottom current speed pattern in the Drake Passage responding to the dynamics of ocean fronts, in agreement with modern observation. Understanding present-day sediment provenance and transport processes is crucial for studies about the dynamics of ocean circulation, as well as for paleoclimate reconstructions in the Drake Passage. Further, we reconstruct changes in the ACC strength in the central Drake Passage over the past 140,000 years. We found substantial reductions in ACC bottom flow speeds during the glacial periods and increased bottom currents during interglacials. The amplitude was larger during Termination II compared to Termination I. Superimposed on these long-term changes, we found strong millennial-scale fluctuations in ACC intensity, increasing in amplitude toward the Last Glacial Maximum (LGM). We hypothesize that the central ACC reacts highly sensitive to the Southern Hemisphere millennial-scale climate oscillations, likely related to westerly’s wind stress, oceanic fronts and Antarctic sea ice extent during the LGM. This strong variation of ACC regulates Pacific-Atlantic water mass exchange via the “cold water route” and could significantly affect the Atlantic Meridional Overturning Circulation (AMOC). In a third study, the mineralogical, magnetic and geochemical properties of terrigenous sediments reveal that the fine-grained materials mainly derived from western Patagonia and the Antarctic Peninsula over the last 140 ka. The ACC might have severed as a major driver for the sediment transport in the Drake Passage region. Expansion of ice sheets in Patagonia and on the Antarctic Peninsula together with relative sea-level lowstands enhanced the efficiency of terrigenous input during glacial maxima. Our high-resolution records reflect the waxing and waning of glaciers in southern Patagonia and on the Antarctic Peninsula. In the last study, authigenic Nd and Pb isotopic records from the central Drake Passage deciphered past water mass mixing in the Southern Ocean during the last 140,000 years. We found enhanced Pacific-derived deep waters into the deep Southern Ocean at the expense of the North Atlantic-derived waters during glacial times. A pronounced gradient between mid-depth and deep waters suggests a stratified deep ocean during glacial periods. Enhanced stratification together with a stronger biological pump would support an enhanced storage of CO2 in the deep Southern Ocean during glacial maxima. Finally, this thesis improves our understanding of terrigenous sediment sources, changes in the ACC dynamics and ocean circulation in the Pacific sector of the Southern Ocean over orbital to millennial time-scales. The studies provide new insights into the evolution of the ACC dynamic changes in the central Drake Passage and its potential influences on global thermohaline circulation over the past 140,000 years. Future sedimentological and palaeoceanographic work should reconstruct Quaternary changes of the ACC across a meridional transect in the Drake Passage to better quantify the ACC and throughflow transport and velocities.