The Mi-1 event marks an episode in Earth history where the Antarctic ice sheet experienced a significant growth in ice volume. At its largest extent, at 23.03 Ma, coincident with the Oligocene Miocene boundary, the Antarctic ice sheet was similar in size to the present Antarctic ice sheet. The Mi-1 event was only short lived and soon after its maximum extent, Antarctic ice volume returned to pre-Miocene levels. During the Early Miocene, however, the Antarctic ice shield was more dynamic and characterised by periods of advancing and retreating ice sheets. The triggers initiating the rapid deglaciation after the Mi-1 maximum, and the parameters controlling Early Miocene Antarctic ice sheet variability remain under debate. To date, most research on the Mi-1event and the Early Miocene rely on deep sea sediments rather than on samples from the Antarctic shelf itself, where the influence of a dynamic ice sheet would have been greatest. The choice as study site fell on the Cape Melville Formation, because it is known to be the only outcrop in Antarctica which hosts Early Miocene sediments and shows signs of sediments which were deposited as a result of the Mi-1 event. Previous work and the findings from two field seasons suggests that the base of the Cape Melville Formation is a diamictite which represents an episode of maximum ice sheet extent, associated with the Mi-1 glaciation. The diamictite is overlain by a succession of siltstone and sandstone layers which represent episodes of ice sheet retreats and advances. Three ash layers were deposited as a result of volcanic eruptions in the upper Cape Melville Formation. This study suggests, using a combination of radiometric dating of an ash layer(20.79 ± 0.22 Ma), a high resolution magnetic susceptibility record which uncovered the presence of pronounced eccentricity cycles and the results from magnetostratigraphy, that the sediments of Cape Melville were deposited from 22.7 until 21.3 Ma, encompassing the Early Miocene. In a multi-proxy approach, using techniques from environmental magnetism (e.g., FORC-PCA, variations in magnetic susceptibility and its temperature dependence), geochemistry (elemental composition and elemental ratios) and sedimentology, I derived a model which describes the palaeoenvironment at the time of sediment deposition. According to the environmental model, sediments were deposited under an ice sheet from 22.7 until 22.1 Ma. From 22.1 to 21.4 Ma, sediments were deposited in a more dynamic environment where the ice sheet retreated further and sediments where deposited proximal to the calving line of the ice sheet and the open ocean. While an increase of CO2 between 22.7 and 22.5 Ma is thought to have promoted a decrease in ice volume, between 22.1 until 21.4 Ma, changes in orbital parameters induced several episodes of ice sheet retreats and advances. This study shows that mainly changes in orbital parameters, especially changes in eccentricity, rather than changes in CO2, controlled ice volume variability at the time when the sediments of the Cape Melville Formation were deposited.
| Date of Award | 11 Sept 2025 |
|---|
| Original language | English |
|---|
| Supervisor | Bethany Fox (Main Supervisor) & Thomas Smyth (Co-Supervisor) |
|---|