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doi:10.2204/iodp.proc.323.107.2011 Site U13431Expedition 323 Scientists2Background and objectivesThe primary objective of drilling at Integrated Ocean Drilling Program (IODP) Site U1343 (proposed Site GAT-4C; Takahashi et al., 2009) was to study high-resolution Pliocene–Pleistocene paleoceanography at a location proximal to the gateway to the Arctic Ocean. Additionally, this site is closer to the current seasonal sea ice limit, and its ~2 km water depth provides information regarding the history of mid-depth water mass characteristics in the Aleutian Basin. This site is located at a depth of 1953 m on a topographic high clearly separated from the Bering shelf (Figs. F1, F2, F3, F4, F5). Hence, it was anticipated to have received a lower supply of reworked terrigenous sediment from the shelf during the interglacials or from subaerially exposed land during the glacials than a location directly downslope of the Bering shelf. Site U1343 is 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 encounters the Bering shelf. The BSC is at ~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, 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 (Taniguchi, 1984; Springer et al., 1996). Because of the expected high organic carbon supply to the seafloor, especially during the interglacial sea level highstands, it is possible that the oxygen minimum zone (OMZ) previously extended to the depth of this site. In order to compare the vertical extent of water mass conditions across the basin and relate the OMZ to paleoproductivity, records from shallower drill sites on Bowers Ridge (IODP Site U1340, water depth = 1295 m, and IODP Site U1342, water depth = 818 m) and other slope sites will be used. This site is also located close to the maximum extent of present-day seasonal sea ice cover. Thus, this site is expected to have been covered by seasonal or perennial sea ice during the glacial sea level lowstands. This 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 Bering Sea drill sites. Organic-fueled subseafloor respiration and its impact on biogeochemistry 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 (Takahashi et al., 2000). Sedimentation rates at this site were not previously known because piston cores were unavailable. However, rates of ~180 m/m.y. were observed in an earlier site survey piston core study (Takahashi, 2005) at IODP Site U1344 (Fig. F1). Thus, we expected to recover Pleistocene to Pliocene sections. |
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