Orbital to millennial-scale atmosphere-ocean-cryosphere interactions and interhemispheric teleconnections in the Southeast Pacific (IODP Site U 1542)

Abstract

From orbital (10 to 100 thousand years or kyr) to millennial (1 to 10 kyr) timescales, the Southern Ocean is thought to substantially modulate global climate and ocean variability. Most critical factors include the Southern Ocean’s impact on surface, intermediate, and deep-water circulation affecting global heat, salt, and nutrient distribution and the processes influencing storage and outgassing of atmospheric CO2. This PhD thesis is focused on the reconstruction of past climatic variability in the Southeast Pacific spanning the past 800,000 years, based on the analysis of different organic paleoclimatic tracers, in a sedimentary archive from International Ocean Discovery Program (IODP Site U1542, situated on the southern Chilean margin. This work has generated a series of millennialscale resolution paleoclimatic records of sea surface temperature (SST), bottom current velocity, accumulation of terrestrial organic compounds from leaf waxes and soil bacteria, and ice-rafted debris. Results show that SST covaries with the strength of the Antarctic Circumpolar Current (ACC). The millennial-scale variability appears to be a persistent and inherent feature of the climate over the last eight glacial cycles. The recurrence of millennial-scale events is independent of the glacial state, but the amplitude increases when the glacial state lasts longer. The sediment record at Site U1542 underlines a persistent close link between millennial-scale climatic events from both hemispheres and that the millennial-scale fluctuations of the ACC covary with atmospheric CO2 release over the past 800,000 years. In a second application of the proxy datasets, the location of Site U1542 near the former Patagonian ice sheet (PIS), enabled the reconstruction of the past variability of the PIS on multiple glacial cycles using terrestrial proxies leaf waxes compounds and ice-rafted debris. The timing of terrestrial sediment deposition aligns notably with the extent of PIS moraines. These data reveal a close connection between sediment export and SST in the Southeast Pacific, suggesting that the PIS extent is controlled by ocean temperature forcing. The third study of this thesis aims to explore the meridional sea surface changes of the Southern Ocean using a biomarker tracer for polar water masses. A compilation of Southern Ocean records highlights a long period of retreated polar water between 410 and 310 kyr, following the Mid-Brunhes Transition. This retreat notably mirrors low atmospheric CO2 concentrations, emphasizing the role of the Southern Ocean in the global carbon budget. Overall, the records developed from the sedimentary archive Site U1542 provide an unprecedented picture of orbital and millennial-scale Southern Ocean variability over the past 800,000 years, highlighting the close atmosphere-ocean-cryosphere interactions of Earth’s climate.