Description: 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.

Description: The Amundsen Sea sector of Antarctica has long been considered the most vulnerable part of the West Antarctic Ice Sheet because of the great water depth and retrograde slope at the grounding line, the observed incursion of warm Circumpolar Deep Water onto the shelf, and the lack of substantial buttressing ice shelves. Notably, ice flowing into the Amundsen Sea embayment has been undergoing rapid changes over recent decades. International Ocean Discovery Program (IODP) Expedition 379 accomplished two successful drill sites on the continental rise of the Amundsen Sea in January-March 2019, the first from this sector, despite significant logistical limitations, including persistent sea ice that prevented access to all proposed continental shelf sites and abundant mobile icebergs that forced loss of ~50% of drilling time. Site U1532 is located on a large sediment drift and penetrated to a depth of 794 m below seafloor with 90% recovery. Nearly continuous cores were collected from the Pleistocene into the upper Miocene. Site U1533 reached 383 m below seafloor (70% core recovery) in a more condensed sequence down to the upper Miocene at the lowermost flank of the same sediment drift. The cores from 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 a sequence of Pliocene lithofacies, with an excellent paleomagnetic record for very high-resolution, sub-orbital scale climate change studies of the previously sparsely sampled region. Coarse-grained sediments, interpreted as ice-rafted debris, were identified throughout all time periods recovered. Proximal sources in West Antarctica are confirmed for crystalline rock detritus in some intervals. Cyclicity interpreted to represent relatively warmer periods, variably characterized by higher microfossil abundance and higher counts of ice-rafted debris, alternating with transitional and colder periods, characterized by dominantly gray laminated terrigenous muds, is a dominant feature of the cores. Despite the lack of sites on the shelf, the 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.

Description: Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Quaternary Science Reviews 177 (2017):265-275, doi:10.1016/j.quascirev.2017.10.029.

Description: Most outlet glaciers of the Cordillera Darwin Icefield (CDI; Patagonia, 54⁰S) are currently transitioning from calving to land-based conditions. Whether this situation is unique to the modern climate or also occurred during the Holocene is entirely unknown. Here, we investigate the Holocene fluctuations of outlet glaciers from the northern flank of the CDI using a multi-proxy sedimentological and geochemical analysis of a 13.5 m long sediment core from Almirantazgo fjord. Our results demonstrate that sedimentation in Almirantazgo fjord started prior to 14,300 cal yr BP, with glacier-proximal deposits occurring until 13,500 cal yr BP. After 12,300 cal yr BP, most glaciers had retreated to land-locked locations and by 9800 cal yr BP, Almirantazgo fjord was a predominantly marine fjord environment with oceanographic conditions resembling the present-day setting. Our sediment record shows that during the first part of the Holocene, CDI glaciers were almost entirely land-based, with a possible re-advance at 7300–5700 cal yr BP. This is in clear contrast with the Neoglaciation, during which CDI glaciers rapidly re-advanced and shrank back several times, mostly in phase with the outlet glaciers of the Southern Patagonian Icefield (SPI). Two significant meltwater events, indicative of rapid glacier retreat, were identified at 3250–2700 and 2000–1200 cal yr BP, based on an increase in grain-size mode and related inorganic geochemical parameters. This interpretation is additionally supported by concomitant decreases in organic carbon of marine origin and in Cl counts (salinity), reflecting higher terrestrial input to the fjord and freshening of the fjord waters. Overall, our record suggests that CDI outlet glaciers advanced in phase with SPI glaciers during the Neoglaciation, and retreated far enough into their valleys twice to form large outwash plains. Our results also highlight the potential of fjord sediments to reconstruct glacier variability at high resolution on multi-millennial timescales.

Description: This research was supported by an EU Marie Curie FP6 postdoctoral fellowship to S.B., by National Geographic Grant 8379-07 (to S.B.), by COPAS Center FONDAP Grant 150100007 and COPAS Sur-Austral CONICYT PIA PFB31 (to C.L and S.P), and by IDEAL Center FONDAP Grant 15150003 (to C.L.).