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doi:10.2204/iodp.proc.323.104.2011 Site U13401Expedition 323 Scientists2Background and objectivesThe primary objective of drilling at Integrated Ocean Drilling Program (IODP) Site U1340 (proposed Site BOW-12B; Takahashi et al., 2009) was to study high-resolution Pliocene and Pleistocene paleoceanography in the southernmost part of the Bering Sea at a topographic high on Bowers Ridge (Fig. F1), where relatively good calcium carbonate preservation is expected. 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. In particular, its location allows for an examination of the influence of the warm Alaskan Stream water mass entry into this region, which influenced the distribution of past sea ice coverage (Katsuki and Takahashi, 2005). Although productivity in the Bering Sea in general is very high with respect to other parts of the global oceans, it was expected to be lower at this site than at the Bering slope sites, which are substantially more influenced by the nearby Bering shelf, which was partially exposed during glacial lowstands. The relatively shallow water depth of this site should allow us to study physical and chemical changes in upper water mass conditions such as the low dissolved oxygen concentration conditions that caused the formation of laminated sediments at a site on the Bering shelf at a similar water depth (Cook et al., 2005). The vertical structure of past water masses can be determined by comparing results at this site with those at other drilling sites on Bowers Ridge (IODP Site U1341, water depth = 2177 m, and IODP Site U1342, water depth = 837 m) (Fig. F1). Site U1340 on Bowers Ridge 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. Samples used to study organic-fueled subseafloor respiration and its impact on biogeochemistry at the highly productive region of IODP Site U1339 and other Bering slope sites can be compared to analyses at the Bowers Ridge sites, including Site U1340; however, the high-resolution sampling that occurred at the Bering slope sites was not performed at Site U1340. Sediments drilled at Bowers Ridge 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. Site U1340 is located on the eastern flank of the southern part of Bowers Ridge (Fig. F1) at 1295 m water depth in a basin ~10 km east of the ridge crest (Fig. F2). Close-ups of the seismic images in the basin (Figs. F3, F4, F5) indicate that strata dip to the east (Fig. F4). Some shallow sections do not have continuous parallel strata, but most of the rest of the section appears to have continuous features in the seismic images (Figs. F4, F5). The ages of basement and the deeper sediments are unknown, but sediments as old as upper Miocene were found at Deep Sea Drilling Project (DSDP) Site 188 on the western flank of Bowers Ridge (Scholl and Creager, 1973). Specifically, Scholl and Creager (1973) found recent to upper Miocene diatom ooze interbedded with silt-rich diatom ooze and diatomaceous silt. They also found calcareous nannofossils in the uppermost 120 m and planktonic foraminifers in the uppermost 300–400 m. In the interval between 580 and 638 meters below seafloor (mbsf), they encountered mudstone. They reported sedimentation rates of ~100 m/m.y. A piston core study from the same general region provided sedimentation rates of 80 m/m.y. (Takahashi, 2005). Thus, before drilling we expected to recover sections ranging from the entire Pleistocene to the Pliocene and possibly Miocene. |
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