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doi:10.2204/iodp.proc.323.108.2011 Site U13441Expedition 323 Scientists2Background and objectivesThe primary objective of drilling at Integrated Ocean Drilling Program (IODP) Site U1344 (proposed Site GAT-3C; Takahashi et al., 2009) was to study high-resolution Pliocene–Pleistocene paleoceanography at a location proximal to the gateway to the Arctic Ocean at the deepest water depth of IODP Expedition 323. Site U1344 is located at a depth of ~3200 m along the small summit of a canyon interfluve ~10–15 km southeast of Pervenets Canyon, a large submarine canyon that deeply and widely incises the Beringian continental slope (Figs. F1, F2, F3, F4, F5) (Normark and Carlson, 2003). Pervenets Canyon, along with Zhemchug Canyon, adjacent to IODP Site U1343, was discovered in the early 1960s by the Soviet fishing industry and named after one of the trawlers that found it. It is commonly presumed that at times of glacially lowered sea level the head of Pervenets Canyon was one of the outfall locations for the Anadyr River, which presently drains the Russian northeast and enters the Bering Sea at the Gulf of Anadyr. This location was anticipated to have received a supply of terrigenous sediment from the shelf during both interglacials and glacials. Site U1344 is located in an area of high biological productivity called the "Green Belt." The Green Belt is formed by the Bering Slope Current (BSC), which originates from the Alaskan Stream water that flows into the Bering Sea through the western Aleutian Islands. The water that enters the Bering Sea moves eastward along the Aleutian Islands and consequently impinges upon the Bering shelf break. The bottom depth of the BSC is ~300 m, and its flow is forced to turn to the northwest once it meets the slope and shelf; eddies and instabilities in its flow cause upwelling along the shelf break. Moreover, tidal mixing causes further vertical mixing of the water masses along the BSC, enhancing biological productivity within the Green Belt (Taniguchi, 1984; Springer et al., 1996), which is adjacent to the northwest-trending shelf break, where high primary productivity in the surface waters and high organic carbon accumulation at the seafloor take place (Springer et al., 1996). However, because of Site U1344's deep water depth, organic carbon supply to the seafloor was expected to be lower here than at other gateway sites or Site U1339. Thus, expectations for this site recording conditions in the oxygen minimum zone (OMZ) were relatively low. Instead, Site U1344 serves as the deep end-member site, where deepwater conditions of the Bering Sea can be studied and against which data from shallower sites can be compared to obtain a history of changes in the vertical gradients of oceanic conditions. Site U1344 is also located close to the maximum extent of present-day seasonal sea ice cover. Thus, this site was expected to have been covered by seasonal or perennial sea ice during glacial lowstands. Because it is adjacent to the Bering shelf, a high amount of terrigenous sediment supply was expected, especially during glacial lowstands. This relatively deep drill site in the gateway region to the Arctic Ocean can also be used to study the impact of subseafloor microbes on biogeochemical fluxes in the highest surface-ocean productivity areas of the drill sites in the Bering Sea. Organic-fueled subseafloor respiration and its impact on sediment geochemistry in such a highly productive region have not previously been quantified. To do so, drilled sediments in the gateway region were used to determine subseafloor cell abundances and to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean. Sedimentation rates at this site have been estimated at 170–180 m/m.y. based on earlier site survey piston core studies (Takahashi, 2005; T. Sakamoto et al., unpubl. data). None of the piston cores taken in these studies recovered sections from the Holocene, possibly indicating erosion during the recent past. Thus, before drilling we expected to recover Pleistocene to Pliocene sections. |
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