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Observations of rapid ongoing grounding line retreat, ice shelf thinning and accelerated ice flow from the West Antarctic Ice Sheet (WAIS) may forebode a possible collapse if global temperatures continue to increase. Understanding and reconstructing West Antarctic Ice Sheet dynamics in past warmer-than-present times will inform about its behavior as an analogue for future climate scenarios. International Ocean Discovery Program (IODP) Expedition 379 visited the Amundsen Sea sector of Antarctica to obtain geological records suitable for this purpose. During the expedition, cores from two drill sites at the Resolution Drift on the continental rise returned sediments whose deposition was possibly influenced by West Antarctic Ice Sheet dynamics from late Miocene to Holocene times. To examine the West Antarctic Ice Sheet dynamics, shipboard physical properties and sedimentological data are correlated with seismic data and extrapolated across the Resolution Drift via core-log-seismic integration. An interval with strongly variable physical properties, high diatom abundance and ice-rafted debris occurrence, correlating with partially high amplitude seismic reflection characteristics was identified between 4.2 and 3.2 Ma. Sedimentation during this interval is interpreted as having occurred during an extended warm period with a dynamic West Antarctic Ice Sheet in the Amundsen Sea sector. These records compare to those of other drill sites in the Ross Sea and the Bellingshausen Sea, and thus suggest an almost simultaneous occurrence of extended warm periods in all three locations.more » « less
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In this study, sediments from the Pliocene warm period (3-5 million years ago) from a sediment drift deposit in the Amundsen Sea in Antarctica (drillsite U1533) were treated and analyzed for particle size distribution and sortable silt percentage. These variables help explain how fast ocean currents were moving under warmer climate scenarios. The sediments from beneath the seafloor were treated using hydrogen peroxide to break down any organic matter, followed by boiling with hydrochloric acid to break down carbonate. To remove chemicals, the sediment samples were placed into a centrifuge with 50 mL of DI water for 30 minutes, decanted, then another 30 minutes with 50 mL of DI water. After the samples were treated, they were placed into the Malvern Mastersizer 2000 to measure particle size distributions by using a laser and the resulting scatter patterns of the sediment particles. The results were graphed with depth beneath the seafloor to show the differences in sortable silt percentage over time. The results showed that the sortable silt percentage was low around the onset of the warm period, concluding that the movement of ocean currents during this time period dropped. These results were not typical, as models had predicted that warm periods would have faster ocean currents, and opens up the possibility for future research.more » « less
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Pliocene sediments were recovered during IODP Expedition 379 within the Resolution Drift offshore the Amundsen Sea. Site U1533 was drilled on the margin of a submarine channel extending landward to the continental margin, and Site U1532 was drilled in a more distal position on the thicker portion of the drift. We present new data collected on both sites. Facies assemblages consist of greenish gray clast-bearing mud with a biosiliceous component, interbedded with dark brownish gray laminated silty clay. Due to the close proximity of Sites U1532 and U1533 and the continuous sedimentation in the early Pliocene, individual beds of each facies can be correlated between sites. The red-green channel (a*) in shipboard reflectance spectroscopy and colorimetry data for Site U1533 covaries with the facies descriptions, Ba/Rb and Br in XRF data, ICP-MS bulk elemental ratios such as Sm/Zr, and clay mineralogy. This suggests that a more greenish color of the facies is partially attributed to a larger biogenic component in the sediment relative to the terrigenous supply, and a different provenance from the gray facies. Terrigenous particle size distributions (0-2000 mu) of Site U1533 show that the gray facies are relatively uniform silty clay, whereas greenish gray units show more variability, and a sand component. Sand-rich beds are present in both facies between the top of the greenish units and the bottom of the overlying gray units, and these have a uniform fine-sand mode. Greenish gray units are tentatively interpreted as deposition during ice retreat, with reduced terrigenous supply and higher primary productivity. Although these greenish grey facies can be interpreted as interglacial units, beds with this character do not occur evenly spaced throughout the stratigraphy. Greenish grey facies coincide with low Al/Ti ratios in XRF data for Site U1533. However, Al/Ti ratios change over evenly spaced intervals with orbital frequency and likely record a more complete record of glacial-interglacial cyclicity in sediment delivery than the irregular occurrence of greenish grey facies. This would suggest that some early Pliocene interglacials did not yield suitable conditions for the deposition of the greenish gray facies, and highlights the complex interactions between the ice sheet and the ocean embedded within these paleoarchives. PLAIN LANGUAGE SUMMARY Layers of sediment extracted via deep-sea drilling from beneath the seafloor off the Amundsen Sea, Antarctica, were stacked up over millions of years. The layers were built by pulses of sediment supplied from land ice and biogenic blooms, with distribution of material by ocean currents. The changing color and composition of the layers is an indication of the dominant imprint of ice-related processes versus ocean processes on the sediments that were raining down on the seafloor at any given time. Sedimentation related to the ice and the ocean follows different rhythms related to distribution of heat over time at different latitudes on Earth. The climate archive studied here records how the interference of these rhythms produces ice ages in Antarctica in a previous warm period about 3 to 5 million years ago with atmospheric greenhouse conditions that were like those of today. Investigations of these polar geological climate archives help provide context for the current ice mass loss observed in this same area of Antarctica and its potential sea-level effects.more » « less