| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.341.104.2014 Site U14181J.M. Jaeger, S.P.S. Gulick, L.J. LeVay, H. Asahi, H. Bahlburg, C.L. Belanger, G.B.B. Berbel, L.B. Childress, E.A. Cowan, L. Drab, M. Forwick, A. Fukumura, S. Ge, S.M. Gupta, A. Kioka, S. Konno, C.E. März, K.M. Matsuzaki, E.L. McClymont, A.C. Mix, C.M. Moy, J. Müller, A. Nakamura, T. Ojima, K.D. Ridgway, F. Rodrigues Ribeiro, O.E. Romero, A.L. Slagle, J.S. Stoner, G. St-Onge, I. Suto, M.H. Walczak, and L.L. Worthington2Background and objectivesSite U1418 is located at 3703 m water depth on a slightly elevated region of the proximal Surveyor Fan. It is located between the Aleutian Trench and an abandoned channel, named here the Bering Channel, which also terminated into the trench when active. These channels appear to originate at the base of the slope seaward of the Bering Trough (Fig. F1). This site lies below the westward-flowing Alaska Current, a boundary current within the Alaska Gyre that commonly contains eddies and meanders (Stabeno et al., 2004). Seasonally high productivity is often associated with these eddies (Ladd et al., 2007). The site has likely been supplied with sediment from gravity flows through these adjacent channels, creating a thick (~1 km), seismically stratified deposit (Fig. F2). A large deposit of chaotic seismic facies interpreted as a mass transport deposit (MTD) is found at ~5.8 s two-way traveltime (TWT) (Reece et al., submitted). The top of the MTD forms a reflector that can be mapped to Site U1417, which is dated at ~1 Ma at that site (see “Background and objectives” in the “Site U1417” chapter [Jaeger et al., 2014b]), suggesting that this depocenter may contain an expanded Middle–Late Pleistocene sedimentary sequence. Drilling objectives at Site U1418 were to develop a high–temporal resolution, proximal sedimentary record of Late Pleistocene glacial–interglacial dynamics, fan sedimentation and development, and paleoceanography. A primary objective of drilling at Site U1418 was to constrain the timing of glacial events of the Pacific side of the northwestern Cordilleran ice sheet to test its relation to dynamics of global ice sheets. This drilling allows for the examination of the timing of sediment gravity flow processes and submarine fan deposition relative to glacial advance–retreat cycles, including the age of the imaged MTD. The observation of an abandoned and buried Bering Channel above the MTD (Fig. F3) suggests that it formed subsequent to the MTD and during the Middle–Late Pleistocene, perhaps as a consequence of changes in glacigenic sediment delivery. An expanded Pleistocene record of glacigenic sediment deposition at Site U1418 also allows for the documentation of the spatial and temporal behavior of the geomagnetic field (paleointensity and paleosecular variation) during this time period in an undersampled region of the globe. Seismic reflection data crossing Site U1418 and the two alternate sites include high-resolution Profiles GOA3201 and GOA3202 from the 2004 R/V Maurice Ewing cruise and two crustal-scale St. Elias Erosion and Tectonics Project (STEEP) profiles (Fig. F4; Gulick et al., 2007). A 1989 seismic reflection line (F-6-89-GA-26; Fig. F1) with a vertical resolution in between STEEP07 and GOA3201 and collected by the US Geological Survey ~10 km northeast of Site U1418 reveals the detailed seismic stratigraphy of the sediment dispersal pathways in this region (Fig. F3). Site U1418 lies at crossing Lines GOA3202 and STEEP07, which reveal a seismically stratified depocenter (Figs. F2, F5). Subhorizontal reflectors of varying intensity are mappable over tens of kilometers, implying deposition from suspension, likely overbank deposition from turbidity currents combined with hemipelagic sedimentation from surface waters. Higher amplitude reflectors may indicate periods of enhanced deposition of ice-rafted sediment and/or coarser sediment gravity flows that are associated with the Bering Glacier. Seismic units were identified for Expedition 341 to guide core-log-seismic integration (see “Core-log-seismic integration”) (Fig. F5). All seismic units shallower than the MTD are expected to be correlative subunits of Sequence III at Site U1417 (Reece et al., 2011). Seismic Subunit IIIA is reflective in the higher resolution generator-injector gun data on Line GOA3201. Based on interpretation of Line F-6-89-GA-26, these reflective facies appear to correlate with levee deposition from the Bering Channel (Figs. F3, F5). Seismic Subunit IIIB is interpreted to represent the cessation of sediment transport through the Bering Channel, possibly corresponding with sediment dispersal into easternmost Aleutian Trench (Fig. F6). In seismic Line F-6-89-GA-26, seismic Subunit IIIA appears to be related to hemipelagic and/or channel overbank deposition from turbidity currents in the Aleutian Trench (Fig. F6). These seismic unit boundaries were therefore primary targets while drilling to the base of Sequence III and the top of the MTD. |
|||
