Cenozoic ice sheets and ocean variability controls on sedimentation in glaciated margins

Resumen

The Antarctic Circumpolar Current (ACC) connects all major ocean basins, links the deep and shallow layers of the oceans and has a strong influence on global ocean circulation, biogeochemical cycles, the stability of the Antarctic ice sheet and thereby Earth´s climate system. However, the timing of the onset of the ACC and the establishment of a vigorous, deep circumpolar flow, similar to presentday remain controversial. Moreover, the links between the ACC and the Antarctic ice sheet in past warmer than today climates are poorly known. This knowledge is essential for improving our understanding on ACC-Antarctic ice sheet interactions in the ongoing climate warming that can inform coupled ocean-ice sheet global climate models used to forecast future changes. In this context, this PhD thesis aims to advance our understanding on the evolution of the ACC since its initiation (proto-ACC) to the time when the modern deep ACC is established over the last 34 million years (Ma). In addition, we aim to relate proto-ACC dynamics offshore the eastern Wilkes Land margin to Antarctic ice sheet behaviour during the warm late Oligocene and the earliest Miocene (24-23 Ma), including the second major Antarctic glaciation (23.03 Ma). To achieve these objectives, we conducted sedimentological, geochemical, and isotopic analyses on sedimentary sequences recovered by the Deep Sea Drilling Project Leg 28 (Sites 269 and 274) and Leg 29 (Site 278) across both sides of the Tasmanian Gateway. In addition, we conducted a study in the glaciated margins of Lake Baikal (Russia). There the tectonic and sea level histories are well known allowing us to test, using bathymetric and seismic reflection data, the climate vs. sea-level changes and tectonic controls on deep-water deposition in glaciated margins. This PhD Thesis shows that between 34-30 Ma, deep waters from the South Atlantic and Indian Ocean did not flow into the Southwest Pacific via the Tasmanian Gateway. Instead, the Southwest Pacific deep water circulation was characterised by the presence of two deep water masses, one occupying depths between ~2500-4000 m (Equatorial-like Deep Water) and another one, a bottom water mass, occupying depths >= 4000m South Pacific Deep Water). These results indicate the absence of a Circumpolar Deep Water (CDW) connection, like the one found today within the ACC, across the Tasmanian Gateway before 30 Ma. The first evidence of proto-CDW in the Southwest Pacific was previously reported at 30 Ma. Our study however shows the absence of a homogenous deep-reaching proto-CDW in the eastern and western side of the Tasmanian Gateway between 30 Ma and 19 Ma, which indicates a proto-ACC shallower and weaker than the present-day ACC. Between 19 and 4 Ma, we find evidence of a long-term intensification of bottom current flow speeds, coinciding with increasing influence of North Atlantic Deep Water (NADW) in the deep Southwest Pacific. We suggest that the modern deep-reaching ACC flow established at 4 Ma as indicated by a prominent shift to (i) intensified ACC frontal system resulting in enhanced biogenic productivity, (ii) stronger bottom current flow speeds, and (iii) establishment of a homogenous CDW along the polar front in the Southern Ocean Moreover, our results show that the proto-ACC frontal system offshore the Wilkes Land margin was weaker compared to that of the present-day during the late Oligocene-earliest Miocene (24-23 Ma), allowing warm subtropical waters to reach close to this Antarctic margin. In addition, proto-CDW was circulating closer to the Wilkes Land margin especially during interglacial times (e.g., 23.23 Ma), likely due to reduced Antarctic Bottom Water (AABW) export and ice sheets where retreated in the continent. The Antarctic Circumpolar Current (ACC) connects all major ocean basins, links the deep and shallow layers of the oceans and has a strong influence on global ocean circulation, biogeochemical cycles, the stability of the Antarctic ice sheet and thereby Earth´s climate system. However, the timing of the onset of the ACC and the establishment of a vigorous, deep circumpolar flow, similar to presentday remain controversial. Moreover, the links between the ACC and the Antarctic ice sheet in past warmer than today climates are poorly known. This knowledge is essential for improving our understanding on ACC-Antarctic ice sheet interactions in the ongoing climate warming that can inform coupled ocean-ice sheet global climate models used to forecast future changes. In this context, this PhD thesis aims to advance our understanding on the evolution of the ACC since its initiation (proto-ACC) to the time when the modern deep ACC is established over the last 34 million years (Ma). In addition, we aim to relate proto-ACC dynamics offshore the eastern Wilkes Land margin to Antarctic ice sheet behaviour during the warm late Oligocene and the earliest Miocene (24-23 Ma), including the second major Antarctic glaciation (23.03 Ma). To achieve these objectives, we conducted sedimentological, geochemical, and isotopic analyses on sedimentary sequences recovered by the Deep Sea Drilling Project Leg 28 (Sites 269 and 274) and Leg 29 (Site 278) across both sides of the Tasmanian Gateway. In addition, we conducted a study in the glaciated margins of Lake Baikal (Russia). There the tectonic and sea level histories are well known allowing us to test, using bathymetric and seismic reflection data, the climate vs. sea-level changes and tectonic controls on deep-water deposition in glaciated margins. This PhD Thesis shows that between 34-30 Ma, deep waters from the South Atlantic and Indian Ocean did not flow into the Southwest Pacific via the Tasmanian Gateway. Instead, the Southwest Pacific deep water circulation was characterised by the presence of two deep water masses, one occupying depths between ~2500-4000 m (Equatorial-like Deep Water) and another one, a bottom water mass, occupying depths >= 4000m South Pacific Deep Water). These results indicate the absence of a Circumpolar Deep Water (CDW) connection, like the one found today within the ACC, across the Tasmanian Gateway before 30 Ma. The first evidence of proto-CDW in the Southwest Pacific was previously reported at 30 Ma. Our study however shows the absence of a homogenous deep-reaching proto-CDW in the eastern and western side of the Tasmanian Gateway between 30 Ma and 19 Ma, which indicates a proto-ACC shallower and weaker than the present-day ACC. Between 19 and 4 Ma, we find evidence of a long-term intensification of bottom current flow speeds, coinciding with increasing influence of North Atlantic Deep Water (NADW) in the deep Southwest Pacific. We suggest that the modern deep-reaching ACC flow established at 4 Ma as indicated by a prominent shift to (i) intensified ACC frontal system resulting in enhanced biogenic productivity, (ii) stronger bottom current flow speeds, and (iii) establishment of a homogenous CDW along the polar front in the Southern Ocean Moreover, our results show that the proto-ACC frontal system offshore the Wilkes Land margin was weaker compared to that of the present-day during the late Oligocene-earliest Miocene (24-23 Ma), allowing warm subtropical waters to reach close to this Antarctic margin. In addition, proto-CDW was circulating closer to the Wilkes Land margin especially during interglacial times (e.g., 23.23 Ma), likely due to reduced Antarctic Bottom Water (AABW) export and ice sheets where retreated in the continent. Lastly, our results from Lake Baikal highlight that climate is the main control on the development of turbidite system in this glaciated margin. We provide evidence that show that despite the nearly constant lake levels, the late Pleistocene to Holocene changes in lake Baikal turbidite system evolution are the same as marine turbidite systems with ~120 m of sea-level lowering. These results are relevant, when interpreting deep-water deposits in the glaciated margins of Antarctica, which are governed by a complex interplay between ice sheet dynamics, sea level changes and tectonic control.