Sedimentology of the grounding zone of the Kamb Ice Stream, Siple Coast, West Antarctica
Authors:   , Calkin Theo 
Published: November 2021

<div>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. <br></div><div><br></div><div>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.</div><div><br></div><div>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.</div><div><br></div><div>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.</div>