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doi:10.2204/iodp.proc.323.109.2011 Site U13451Expedition 323 Scientists2Background and objectivesThe primary objective of drilling at Integrated Ocean Drilling Program (IODP) Site U1345 (proposed Site NAV-1B; Takahashi et al., 2009) was to study high-resolution Holocene–late Pleistocene paleoceanography at a location proximal to the gateway to the Arctic Ocean at a water depth of ~1008 m. Site U1345 is located on an interfluve ridge near the large, broad head of the Navarin submarine channel off the Bering Sea shelf (Figs. F1, F2, F3, F4, F5) (Normark and Carlson, 2003). This site was anticipated to have had high fluxes of terrigenous sediment from the shelf during glacials and deglacials. This site is also in an area of high biological productivity known as the "Green Belt," which is formed by nutrients brought to the photic zone by tidal mixing and transverse circulation at the shelf edge and eddies in the Bering Slope Current (BSC). The BSC originates in the incoming Alaskan Stream water that flows through the western Aleutians into the Bering Sea (Taniguchi, 1984; Springer et al., 1996). Site U1345 is located at the center of the modern oxygen minimum zone (OMZ) and is expected to record past changes in OMZ intensity. Given high biogenic and terrigenous flux, sedimentation rates at this site are expected to be high, with intermittent millimeter to submillimeter laminations (see below), and future work should allow for the reconstruction of detailed climate change on submillennial timescales. This record can then be compared with high-resolution records from Ocean Drilling Program (ODP) Sites 893 (Santa Barbara Basin) and 1002 (Cariaco Basin) and the Greenland Ice Sheet Project 2 (GISP2). Site U1345 provides the northernmost constraint on the vertical structure of watermasses in the Aleutian Basin in relation to the sites at Bowers Ridge and Umnak Plateau. Site U1345 is located close to the maximum extent of present-day seasonal sea ice cover. Thus, this site is sensitive to changes in seasonal and perennial sea ice cover during glacial–interglacial cycles. Because of its proximity to the location of sea ice formation and brine production, this site, as well as IODP Site U1344, may provide crucial information regarding the formation of intermediate or deep watermasses in the Bering Sea in the past. Site U1345 has high surface-ocean productivity and can also be used to study the impact of subseafloor microbes on biogeochemical fluxes. Organic carbon–fueled respiration and its impact on sediment geochemistry in such a highly productive region have not previously been quantified. Sediments from this site can also be used to investigate the link between subseafloor microbial communities and export production from the surface ocean (Takahashi et al., 2000). Reports of sedimentation rates at this site vary significantly, ranging from 14 cm/k.y. during the Holocene and 91 cm/k.y. during the Last Glacial Maximum (LGM) to as much as 242 cm/k.y. during the last deglaciation (Cook et al., 2005). Thus, before drilling, we expected to recover sections from the Holocene to late Pleistocene. |
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