Drilling in the Amundsen Sea and results on West Antarctic paleoclimate and ice sheet history

The West Antarctic Ice Sheet (WAIS) has been subject to a very dynamic history as most of its base is grounded below present sea level and, thus, sensitive to climatic changes. Its collapse would result in a global sea-level rise of 3.4-5 m. The reconstruction and quantification of WAIS collapses in warm periods in the geological past will provide constraints required for ice sheet models predicting its future behaviour and resulting sea-level rise. The Amundsen Sea ice sheet drainage sector in particular has shown unusual rapid retreat and dramatic changes over the last decades, which has been suggested to be a precursor to the behaviour of the entire WAIS. The main objective during the RV Polarstern expedition PS104 in early 2017 was the application of the MARUM-MeBo70 seabed drilling system to recover a series of sediment cores at sites from the oldest to the youngest sedimentary sequences of the Amundsen Sea Embayment shelf. The expectation was that these cores provide sample material for proxy analyses required to reconstruct the development and past dynamics of the WAIS in the Amundsen Sea sector. A multi-barrel seabed drill rig was used for the first time to drill unconsolidated sediments and consolidated sedimentary rocks from an Antarctic shelf with core recoveries between 7 and 76%. We deployed the MARUM-MeBo70 drill device at nine drill sites in the Amundsen Sea Embayment. Three sites were located on the inner shelf of Pine Island Bay from which soft sediments, presumably deposited at high sedimentation rates in isolated small basins, were recovered from drill depths of up to 36 m below seafloor. Six sites were located on the middle shelf of the eastern and western embayment. Drilling at five of these sites recovered consolidated sediments and sedimentary rocks from dipping strata spanning ages from Late Cretaceous to Miocene. With IODP Expedition 379 in early 2019, we aimed to drill long cores on the shelf and rise of the Amundsen Sea to target in particular past warm periods of the Pliocene, Miocene and older. Severe sea-ice prevented any shelf drilling. However, continuous late Miocene to Holocene sediments were recovered from a sediment drift on the continental rise, allowing assessment of sedimentation processes in response to climate cycles and trends since the late Miocene. Via seismic correlation to the shelf, we interpret massive prograding sequences that extended the outer shelf by 80 km during the Pliocene through frequent advances of grounded ice. Buried grounding zone wedges indicate prolonged periods of ice-sheet retreat, or even collapse, during an extended mid-Pliocene warm period from 4.2 to 3.2 Ma inferred from IODP Expedition 379 drill records. These results indicate that the WAIS was highly dynamic during the Pliocene and major retreat events may have occurred along the Amundsen Sea margin. More detailed core analysis are still being undertaken. This presentation describes some results of both drilling expeditions, the challenges posed by drifting icebergs and sea ice, and technical issues related to deployment of the MeBo70. It includes also recommendations for similar future drilling campaigns on polar continental margins