Sedimentology and biomarker geochemistry of a Kamb Ice Stream subglacial channel, West Antarctica

Abstract

<p>The stability of the Antarctic Ice Sheet (AIS) in response to climate change is of particular interest because of its influence on global climate and global mean sea level. Increasing the understanding of subglacial hydrology and knowing how the AIS behaves can help inform models that predict future AIS behaviour. During the 2021/2022 field season, the NZ Antarctic Science Platform collected five gravity cores from a Kamb Ice Stream (KIS) subglacial channel (Long. -152.292; Lat. -82.471), approximately 5 km landwards from the grounding zone in the southeast sector of the Ross Embayment. The longest (0.53m) core contains five units that differ markedly from what has previously been observed in sediment cores from the Ross Sea. Through grain size, diatom, palynological, XRF elemental, and Nd isotope analysis the depositional mechanisms and sediment source for these units were investigated. Three graded deposits (Units B-D) are inferred to be transported to the core site as concentrated density flows originating from lake drainage events. These events have been hypothesised to occur underneath the Siple Coast ice streams, and the KIS core provides the first sedimentological evidence of this occurring in Antarctica. Surface elevation anomalies of the KIS ice surface suggest a recurrence interval of ~5 years. Units B and C have a composition consistent with derivation from the KIS catchment, but Unit D has a composition indicative of neighbouring Whillans Ice Stream, suggesting multiple active drainage paths upstream of the KIS channel. The lowermost unit comprises a diatom ooze assigned to the Thalassiosira praefraga Zone of the early Miocene age (18.7-18.0Ma). As a potentially in situ deposit, this interval was studied to try and reconstruct environmental conditions of that time. Key diatom taxa include Fragilariopsis truncata, Cavitatus miocenicus, Creania lacyae, Synedropsis chethamii, Thalassiosira nansenii, and T. praefraga. Dominant palynomorphs are Nothofagus lachlaniae complexes, Podocarpidites spp. and Brigantedinium spp., supported by the presence of high-molecular weight n-alkanes (>C25) and n-fatty acids (>C26:0) . The diatoms and pollen are very similar to those reported from similar age material from RISP site J-9 (~600km WNW from KIS-2). Together, the data suggest mean annual temperatures of 5-8°C, mean annual summer temperatures of 4-12°C, an extensively vegetated West Antarctica and widespread phytoplankton deposition in the Ross Sea during the early Miocene.</p>