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

Abstract

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.

La Corriente Circumpolar Antártica (CCA) conecta todas cuencas oceánicas, las aguas superficiales y profundas de los océanos e influye en la circulación oceánica global, los ciclos biogeoquímicos, la estabilidad del casquete de hielo Antártico, y por ende en el sistema climático terrestre. El origen y la evolución de la CCA hasta la alcanzar la configuración actual de una corriente circumpolar, fuerte y profunda, sigue creando controversia. También, es poco el conocimiento sobre la relación entre la evolución de la CCA y la del casquete de hielos Antártico en épocas cálidas del pasado. Sin embargo, en el contexto actual de calentamiento global, este conocimiento es importante para poder informar los modelos acoplados del sistema climático océano-criósfera, utilizados en las predicciones de cambios futuros. En este contexto, esta tesis doctoral tiene como objetivos el avanzar nuestro conocimiento sobre la evolución de la CCA desde su inicio (proto-CCA) hasta el establecimiento de la CCA actual, abarcando los últimos 34 millones de años (Ma). Además, pretende relacionar la dinámica de la proto-CCA con la del casquete de hielos en el margen continental de la Tierra de Wilkes durante el Oligoceno cálido y el Mioceno inferior (24-23 Ma), periodo que incluye la segunda mayor glaciación continental en la Antártida (23.03 Ma). Para lograr estos objetivos, hemos realizado análisis sedimentológicos, geoquímicos e isotópicos en sedimentos recuperados por el Deep Sea Drilling Project (DSDP) Leg 28 (Sites 269 y 274) y Leg 29 (Site 278) a cada lado del Paso de Tasmania. Además, se ha realizado un estudio en el Lago Baikal (Rusia). La historia tectónica y de los cambios del nivel del Lago Baikal son bien conocidas permitiéndonos testar, usando datos batimétricos y de sísmica de reflexión, el control climático vs. nivel del mar y tectónica en la formación de depósitos profundos en márgenes continentales influenciados por la actividad glaciar. Los resultados de esta tesis revelan que entre 34-30 Ma, no había flujo de aguas profundas del Atlántico Sur y del Océano Indico a través del Paso de Tasmania. La circulación profunda en el Pacifico Sur estaba caracterizada por la presencia de dos masas de agua, una ocupando profundidades entre ~2500-4000 m (tipo Agua Profunda del Pacifico Ecuatorial), y otra ocupando profundidades >= 4000m (Aguas Profundas del Pacífico Sur). Estos resultados indican la ausencia de la Corriente Profunda circumpolar (CDW), componente principal de la CCA, atravesando el Paso de Tasmania antes de 30 Ma. La primera evidencia de una proto-CDW en el Pacifico suroccidental había sido informada a los 30 Ma. Sin embargo, nuestro estudio muestra la ausencia de una proto-CDW homogénea en la parte occidental del Paso de Tasmania entre 30 y 19 Ma, lo que indica la existencia de una CCA más somera y débil que la actual. Entre 19 y 4 Ma, los sedimentos registran un aumento en la velocidad del flujo de corriente la profunda, coincidiendo con un incremento de la influencia de las Aguas Profundas del Atlántico Norte (NADW) en el Pacífico suroccidental. Nuestros datos sugieren que la formación de una CCA similar a la actual tuvo lugar a los 4 Ma indicado por un marcado cambio en: (i) la intensificación del sistema frontal y como resultado en la productividad biogénica, (ii) intensificación en la velocidad de flujo de la corriente, y (iii) el establecimiento de una CDW de composición homogénea. Además, nuestros resultados indican que la existencia de una proto-CDW más débil que la actual durante el Oligoceno superior-Mioceno inferior (24-23 Ma) permitía la llegada de aguas cálidas subtropicales cerca de la Antártida. Nuestros datos muestran que la proto-CDW circulaba más próxima al margen de la Tierra de Wilkes durante periodos interglaciares (e.g., 23.23) probablemente debido a la reducción en la producción de Aguas Profundas Antárcticas (AABW) y cuando los casquetes estaban retirados en el continente. Por último, nuestros resultados en el Lago Baikal muestran el clima como el factor que controla el desarrollo de sistemas turbidíticos en el margen del lago influido por procesos glaciares. Pese a que los niveles del lago permanecen casi constantes durante el Pleistoceno superior y el Holoceno, la evolución de los sistemas turbidíticos es similar a la observada en sistemas turbidíticos marinos con ~120 m de descenso del nivel del mar. Estos resultados son importantes a la hora de interpretar depósitos profundos en los márgenes glaciares de la Antártida, en los que gobierna una compleja interacción entre factores de control que incluyen la dinámica glaciar, los cambios del nivel del mar y la actividad tectónica.