Continuous late Miocene to present records on West Antarctic Ice Sheet dynamics: Summary of IODP Expedition 379 to the Amundsen Sea

The Amundsen Sea sector of Antarctica has long been considered the most vulnerable part of the West Antarctic Ice Sheet (WAIS) because of the great water depth at the grounding line, incursion of warm Circumpolar Deep Water, and the lack of substantial buttressing ice shelves. Ice flowing into the Amundsen Sea embayment is currently undergoing the most rapid changes of any sector of the Antarctic ice sheets, including substantial grounding line retreat over recent decades as observed from satellite data. Recent models suggest that a threshold leading to collapse of WAIS in this sector may have been passed already and that much of the WAIS could be lost even under relatively moderate greenhouse gas emission scenarios. Drill cores from the Amundsen Sea provide tests of several key questions about controls on ice sheet stability. Since the Amundsen Sea drainage basin currently has the largest negative mass balance of ice of anywhere in Antarctica, geological tests of ice-sheet stability in this region are thus of prime interest to future predictions. IODP Expedition 379 successfully drilled two sites on the continental rise of the Amundsen Sea in January-March 2019, despite operational difficulties. Site U1532 is located on a large sediment drift and penetrated to a depth of 794 mbsf with 90% core recovery. Nearly continuous cores were collected from the Pleistocene down through an expanded Pliocene–uppermost Miocene sequence. Site U1533 was drilled to a depth of 383 mbsf (70% core recovery) into a more condensed sequence down to the upper Miocene on the lowermost flank of the same sediment drift, recovering a complete Pleistocene–uppermost Pliocene composite section and a correlative, but more condensed, Pliocene section to that recovered at Site U1532. The cores of both sites contain unique records to study the cyclicity of ice sheet advance and retreat processes as well as ocean-bottom circulation and water mass changes. In particular, Site U1532 revealed distinct cyclic Pliocene lithofacies alternations with an excellent paleomagnetic record, which will be suitable for high-resolution, sub-orbital scale climate change studies of the previously sparsely sampled Pacific sector of the West Antarctic margin. Coarse-grained sediments, interpreted as ice-rafted debris (IRD), were identified throughout all time periods recovered. Cyclicity interpreted to represent relatively warmer periods, variably characterized by higher microfossil abundance and higher counts of IRD, alternating with colder periods, characterized by dominantly gray laminated terrigenous muds, is a dominant feature of the cores. Initial comparison of these cycles to published records from the region suggests that those units interpreted as recording warmer time intervals in the core relate to interglacial periods and those units interpreted as being deposited during colder periods tie to glacial periods. The association of lithological facies at both sites predominantly reflects the interplay of downslope and contouritic sediment transport with phases of relatively more pelagic sediment input. Despite the lack of drill cores from the shelf, our records from the continental rise reveal the timing of glacial advances onto the shelf and, thus, the expansion of a continent-wide ice sheet in West Antarctica at least back to the late Miocene