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doi:10.2204/iodp.proc.323.106.2011 Site U13421Expedition 323 Scientists2Background and objectivesThe primary objective of drilling at Integrated Ocean Drilling Program (IODP) Site U1342 (proposed Site BOW-15A; Takahashi et al., 2009) was to study high-resolution Pliocene–Pleistocene paleoceanography at a relatively shallow water depth on Bowers Ridge (Fig. F1), where relatively low sedimentation rates were observed in an earlier site survey piston core study (Takahashi, 2005). Bowers Ridge is well situated to study the past extent of water mass exchange with the Pacific Ocean through adjacent Aleutian passes such as Amukta, Amchitka, and Buldir (Figs. F2, F3, F4, F5). As at other Bowers Ridge sites, the record of changes in the flow of the warm Alaskan Stream water mass into the Bering Sea and its impact on the distribution of past sea ice coverage (Katsuki and Takahashi, 2005) is of particular interest. A previous site survey piston core study found more open water conditions during the Last Glacial Maximum (LGM) at proposed Site BOW-8A (Takahashi et al., 2009), which is located in almost the same spot as Site U1342, than at Site BOW-12A, near the ridge crest where IODP Site U1340 is located (Fig. F1). Although productivity in the Bering Sea in general is very high with respect to other parts of the global oceans, expected productivity at this site, as well as the other Bowers Ridge sites, is lower than at IODP Site U1339 on Umnak Plateau, which is substantially more influenced by the adjacent Bering Sea shelf. With its relatively shallow water depth of 818 m, Site U1342 is the shallowest of the Bowers Ridge sites (Site U1340, water depth = 1295 m, and IODP Site U1341, water depth = 2140 m); therefore, Site U1342 provides an important constraint on the intensity and depth of the water column oxygen minimum zone (OMZ). A previous site survey piston core study reported sedimentation rates of ~32 m/m.y. (Takahashi, 2005), and Pliocene-age sediments at the bottom of the sedimentary section are expected. This drill site at Bowers Ridge can also be used to study the impact of subseafloor microbes on biogeochemical fluxes. Organic-fueled subseafloor respiration and its impact on biogeochemistry in such a highly productive region have not previously been quantified. To do so, sediments drilled at Site U1342 will be 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). Compared to other IODP Expedition 323 drill sites where detailed microbiological studies took place, Site U1342 is expected to have lower (but still high) surface-ocean productivity. As such, because of its more open ocean location farthest away from the high-productivity zone of the shelf, Site U1342 serves as the low-productivity end-member of the expedition's microbiological study. |
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