Ice dynamics and ocean productivity during the Late Miocene, offshore Wilkes Land, East Antarctica

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 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 𝛿¹⁸O records that are also paced by eccentricity, suggesting that the Antarctic Ice Sheet was contributing a significant signal to the global 𝛿¹⁸O 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.