Site U1525

Abstract

International Ocean Discovery Program (IODP) Site U1525 is located on the continental slope at 75°0.0603 ′S, 173°55.2028 ′W in 1776 m of water on the southwestern levee of the Hillary Canyon in a depositional setting similar to that of Site U1524 (~87 km to the northeast). Site U1525 is closer to the continental shelf edge, which is ~60 km southwest of the site, and the mouths of the Pennell and Glomar Challenger Basins. The levee shows asymmetric flanks, with a steep erosional flank (~500 m relief) facing the canyon to the northwest and a less-steep southeastern flank (30 m relief). The northeast flank sharply truncates parallel, high-amplitude reflectors outcropping at the seafloor, and in the seismic-reflection profile chaotic seismic facies suggest that slope failure features affected the middle to lower units). The Hillary Canyon is one of the largest conduits for newly formed Ross Sea Bottom Water (RSBW; a type of Antarctic Bottom Water [AABW]), which is focused in this channel by cascading dense water formed on the Ross Sea continental shelf passing down the continental rise. The site also lies beneath the modern-day westward-flowing Antarctic Slope Current (ASC), which is thought to be stronger here than at the deeper water Site U1524. Seismic-reflection profiles indicate that the targeted sediments at Site U1525 are dominated by channel overspill and drift deposits characterized by stratified, parallel, high- to medium-amplitude seismic reflectors. Low-resolution multichannel seismic-reflection Profile IT94-127 shows that the overall unit is stratified down to the acoustic basement at ~1 km below seafloor (3.5 s two-way traveltime [TWT]). Internal reflectors are spaced ~100 ms apart and are subparallel, laterally continuous, and subhorizontal. In the upper 50 m (0.7 s TWT), the reflectors onlap a highly angular unconformity that deepens and is likely older toward the southeast. These strata are believed to be glacial debris flows interlayered with interglacial hemipelagic units of Pliocene to Pleistocene age that formed a prograding shelf margin fan at the mouth of the Glomar Challenger Basin coeval to or postdating Ross Sea Unconformity (RSU) 2. The continental slope location of Site U1525 should allow for a more continuous assessment of oceanic forcings for (and response to) Antarctic Ice Sheet (AIS) variability than is possible from continental shelf records. The record of deposition at Site U1525 was anticipated to be primarily influenced by RSBW flowing down the Hillary Canyon but modified by an along-slope component associated with the westward-flowing ASC. Consequently, this site will enable assessment of the ocean drivers of ice sheet variability and bottom water production (Objective 2). The direct record of RSBW flow down the Hillary Canyon obtained at Site U1525 could potentially be extrapolated to high-fidelity paleoceanographic records farther afield in the abyssal Pacific Ocean to better constrain the Antarctic influences on the global oceanic deep circulation. The Hillary Canyon is also a main route for glacial sediments being eroded and transported by ice streams from the innermost continental shelf to its edge, and provenance studies of the terrigenous sediment may allow for identification of changing ice sheet drainage pathways through the Neogene and Quaternary. Ice-rafted debris (IRD) could be sourced by icebergs from the Ross Sea or transported from the east by the ASC, providing a proxy for dynamic ice discharge from the Pacific Ocean coastline sector of the WAIS. Pelagic sediments would have been deposited during periods of high productivity or a lull in turbidity current–overspill deposition and will provide proxy records of surface water properties, including sea ice cover, sea-surface temperature, stratification, and salinity. These proxy records will enable an assessment of the magnitude of polar amplification during past warm climates (Objective 2) and the role of oceanic forcing on driving ice sheet fluctuations at those times (Objective 3). In addition, continuous deposition, anticipated at this site through much of the upper Neogene and Quaternary, will allow assessment of the orbital response of the WAIS and adjacent oceanic/biological system over a range of past climatic conditions (Objective 4). The upper Quaternary record in the uppermost 50 m of glacial strata can also shed light on the most recent ice sheet advances.