Sedimentology of the grounding zone of the Kamb Ice Stream, Siple Coast, West Antarctica

The grounding line of the Siple Coast incorporates six major ice streams, which together drain around a third of the West Antarctic Ice Sheet. Previously, the ~2000 km-long feature had only been sampled and directly observed at Whillans Ice Stream. This thesis examines glaciomarine sediment and processes operating at the presently stagnant Kamb Ice Stream (KIS) grounding zone ~3.3 km seaward of the modern grounding line (Lat. -82.78, Long. -155.16), where the ice is 590 m thick and overlies a 30 m thick water column. KIS-GZ is the planned site for a deep drilling project in 2023. The sea floor was accessed using a hot water drill in the 2019/20 Antarctic field season. A remotely operated submersible (‘Icefin’) was deployed under the ice shelf, which provided 800 m of sea floor video toward the grounding line. A small number of short (~0.6 m) gravity cores were collected from the seafloor, one of which was examined in this study.

The Icefin video imagery was processed using Structure-from-Motion (SfM) software, enabling the identification of two previously unrecognised sea floor sedimentary facies. One is defined by ubiquitous cm-scale ripples in fine-medium sand, where the ripples are aligned with the prevailing tidal currents flowing parallel to the grounding line. Observed current speeds are too low for the ripples to be generated under the modern oceanographic regime. The second facies is defined by abundant dropstones in mediumcoarse sand. A transition zone separates the two facies. Previously unidentified decimetre-scale bedforms are present in the transition zone and near the borehole.

The lithological, geochemical, and microfossil properties of the gravity core were analysed. The core sediment is sandy diamicton with weak stratification defined by decimetre-scale changes in clast abundance. Mineral counts, zircon ages, Nd/Sr isotopes, and an immature composition indicate this sediment is sourced within the Kamb catchment. The core also contains reworked late Oligocene-late Miocene diatoms. Quaternary diatoms are absent. Ramped pyrolysis radiocarbon dating was attempted on 19 carbon fractions obtained from samples at four depths. Two pyrolysis fractions yielded ages of 31.5-33.2 ka, while the rest did not contain measurable radiocarbon. This likely reflects the reworking of radiocarbon-dead material into the sediment and can only be considered a maximum age for deposition.

Together, the sediment and video data suggest deposition of the sea floor sediment at the core site occurred subsequent to the stagnation of KIS ~160 years ago. I assume that sediment concentrations are relatively uniform along the length of the ice stream and calculate that up to ~2.7 m of diamicton was rapidly deposited at the core site as the grounding line retreated and englacial sediment melted out and settled through the water column. Accumulation in recent decades has been comparatively low. During the period of reduced sedimentation, the sea floor diamicton has been reworked to varying degrees to form ripples and winnowed lag deposits, resulting in a textural patchwork at km scale.