ICE DYNAMICS AND OCEAN PRODUCTIVITY DURING THE LATE MIOCENE, OFFSHORE WILKES LAND, EAST ANTARCTICA by Rebecca Frances Pretty A thesis submitted to Victoria University of Wellington in partial fulfilment of the requirements for the degree of Master of Science in Geology Antarctic Research Centre Victoria University of Wellington 2019 ii ABSTRACT The middle Miocene Climatic Transition (~14 Ma) is commonly interpreted to represent the significant advance of the East Antarctic Ice Sheet (EAIS), and the transition to a hyper-arid climate and a stable polar-styled ice sheet. However, an increasing number of studies provide evidence for continued instability and abundant meltwater processes influencing the low-lying margins of the EAIS during the Late Miocene (~11.6-5.3 Ma). The history of the EAIS during this period remains ambiguous due to the sparse number of records, and those that do exist have poor age resolution. This thesis investigates Integrated Ocean Drilling Program Site U1361 (64°24.5°S 143°53.1°E), located on the lowermost continental rise of the Wilkes Land margin. It aims to assess the variability of the EAIS and associated changes in palaeoceanography offshore one of the largest marine-based sectors of East Antarctica, the Wilkes Subglacial Basin, and to establish if the EAIS was responding to orbital forcings during the Late Miocene. The study period (~11.7 to 10.8 Ma) contains six intervals of nannofossil-rich mudstones, interbedded with laminated mudstones and diatom-rich mudstones. Nannofossils are absent elsewhere in core U1361A, which covers the past ~14 Ma. To identify the sedimentary and depositional processes which influenced this anomalous interval of calcareous biological productivity, a high-resolution record (using ~450 samples) of Iceberg Rafted Debris (IBRD), grain size analysis and bulk geochemistry XRF analysis have been developed. A lithofacies scheme has been established and used to provide an interpretation of the shifting sedimentary processes through time. Repeating cycles of faintly laminated mudstones were interpreted to represent the influence of bottom current activity on overbank turbidites, that spill onto a channel leeve, during glacial periods. The contouritic nature of the facies is likely associated with the low-relief channel-levee system at this time. Interglacial sedimentation is characterised by an increase in biogenic content, IBRD and bioturbation, with deposition occurring during biologically productive open marine conditions. Two types of biogenic productivity are present over the interval (silica and/or carbonate). The intervals of diatom-rich mud were interpreted to be associated with enhanced upwelling of Circumpolar Deep Water (CDW). While, the nannofossil-rich mudstones suggest a significantly warmer climate, with coccolithophore production iii proposed to represent the influence of meltwater and shifting Southern Ocean frontal systems, acting to restrict nutrient upwelling and increase water temperatures. Nannofossil-rich muds are only present for a short interval (700 kyr), suggesting that the anomalous depositional environment between ~11.7 to 11.0 Ma was potentially related to the significant retreat and surface meltwater processes at the EAIS margin. Interglacial sedimentation at Site U1361 is also accompanied by an increase in grain size (i.e. silt), interpreted to represent oceanic current intensification which acts to restrict the deposition of finer material, relative to glacial intervals. This intensification of ocean current strength may have resulted in increased heat delivery to the EAIS margin triggering a terrestrial based ice sheet, and the delivery of nutrients stimulating marine productivity. Spectral analysis of the mean grain size (MGS) and IBRD Mass Accumulation Rate (MAR) records revealed that during the Late Miocene (~11.7 to 10.8 Ma), the ice sheet was paced by ~100 kyr eccentricity cycles, and a low frequency ~20 kyr processional component. This is consistent with two Late Miocene �18O records that are also paced by eccentricity, suggesting that the Antarctic Ice Sheet was contributing a significant signal to the global �18O record during this interval of the Late Miocene. However, the presence of nannofossil suggests a warmer world, rather than the colder climate state that are often inferred to lead to eccentricity/precession variability. A recent hypothesis proposed by Levy et al., (2019) invokes that a warmer climate may lead to surface melt processes which at high latitudes are dominated by eccentricity/precession. Although this style of climate is commonly thought to have occurred prior to ~14 Ma in East Antarctica, the evidence presented in this study suggests such a state existed at the Wilkes Land margin until at least ~11.0 Ma. iv ACKNOWLEDGMENTS This thesis would not have been possible without the support and guidance of so many people. First and foremost, a massive thank you to my supervisors, Rob McKay and Gavin Dunbar. Rob, thank you for your constant support and guidance through this project. Your encouragement and enthusiasm helped to bring down my stress level and made this project very enjoyable to be part of. Gavin, thank you for your helpful insight into lab work, particularly with XRF lab work, and useful discussions on data interpretations. I would also like to thank many others who contributed to this research. A big thank you to Jane Chewings for your endless help and encouragement in the laboratory. Without your guidance I don’t think my lab work would have been possible. A massive thank you to Bella Duncan for spending hours teaching me the methods of biomarker analysis and data processing. Lastly, thank you to the SGEES and ARC staff and students who made my time as an undergraduate and postgraduate student at VUW a very enjoyable experience. To my family, I could not have done this without you. Mum and Dad, thank you for your love and support over the years, allowing me to strive for excellence and to never give up. To Benjamin and William, the best brothers an older sister could have! Thank you for checking up on me, it has been great having you both in Wellington over the last couple of years. To my friends, each one of you has been amazing, providing endless encouragement, love, laughter and some much-needed proofreading. A special thank you to Damian, for your support and love throughout this project. I would like to acknowledge the Rutherford Discovery Fellowship to Rob McKay for funding this project. Lastly, the shipboard scientists of the International Ocean Drilling Program Leg 318 for successful recovery and previous analysis on such an amazing sedimentary core. v TABLE OF CONTENTS 1 Chapter one: Introduction ......................................................................................... 1 1.1 Context ............................................................................................................... 1 1.2 Study Location ................................................................................................... 4 1.3 Thesis aims and objectives ................................................................................. 6 2 Chapter two: Background ......................................................................................... 7 2.1 Antarctic ice sheet history – proximal and far-field records .............................. 7 Cenozoic climate and initiation of the Antarctic Ice Sheets ....................... 8 Mid-Miocene Climatic Optimum.............................................................. 11 Mid-Miocene Transition ........................................................................... 12 Late Miocene – was the EAIS dynamic or stable? ................................... 12 Pliocene climate and ice sheet style .......................................................... 14 2.2 Global productivity during the Late Miocene .................................................. 15 2.3 Internal drivers of climate change .................................................................... 18 Orbital cycles ............................................................................................ 18 Milankovitch theory of the Ice Ages ......................................................... 19 Milankovitch cycles- the pre-100 kyr world ............................................. 20 Orbital patterns deep sea records during the Late Miocene ...................... 21 Southern oceans role in atmospheric CO2 concentrations ........................ 22 2.4 Sedimentation on glaciated margins................................................................. 24 Turbidity currents ...................................................................................... 25 Bottom currents ......................................................................................... 27 Pelagic and hemipelagic settling ............................................................... 28 Iceberg Rafted Debris ............................................................................... 28 Glacial/interglacial sedimentation ............................................................. 30 2.5 Nannofossil production .................................................................................... 31 vi Significance of nannofossils in Antarctica ................................................ 32 2.6 Biomarkers (focusing on n-Alkanes)................................................................ 32 2.7 Productivity proxies ........................................................................................