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doi:10.2204/iodp.proc.323.105.2011 Site U13411Expedition 323 Scientists2Background and objectivesThe primary objective of drilling at Integrated Ocean Drilling Program (IODP) Site U1341 (proposed Site BOW-14B; Takahashi et al., 2009) was to study high-resolution Pliocene–Pleistocene paleoceanography at a western flank location of Bowers Ridge (Fig. F1) in the southern part of the Bering Sea. Previous Deep Sea Drilling Project (DSDP) coring (Site 188) and other piston core studies in the region documented relatively high sedimentation rates of 100 and 138 m/m.y., respectively, and the presence of appropriate microfossils for paleoceanographic studies. Bowers Ridge is well situated for study of 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 changes in the warm Alaskan Stream water mass that flows into this region and presumably impacted the distribution of past sea ice coverage. Although productivity in the Bering Sea in general is very high with respect to other parts of the global oceans (Takahashi et al., 2000), the expected productivity at this site is lower than at Site U1339, which is substantially more influenced by the adjacent Bering shelf, which was subaerially exposed during glacial lowstands. Drilling at this site, which is located at a relatively deep water depth of 2177 m, allows for the study of past intermediate water conditions, including changes in the chemical composition of seawater. For example, this site is located just below the modern dissolved oxygen minimum zone (OMZ), which causes the formation of laminated sediments. Slight fluctuations in the intensity or depth of the OMZ should be captured by proxy records of past oxygenation measured at this site and compared to other, shallower sites. This site and the shallower drill sites on Bowers Ridge (IODP Site U1340, water depth = 1295 m, and IODP Site U1342, water depth = 819 m) will be used to compare the vertical extent of water mass conditions. Site U1341 can also be used to study the impact of subseafloor microbes on biogeochemical fluxes in a region of high surface-ocean productivity in the Bering Sea. 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 Bowers Ridge were used to determine subseafloor cell abundances and were then subjected to intensive geochemical analysis to investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean. Site U1341 is located on the western flank of the central part of Bowers Ridge (Fig. F1) in a depression (Fig. F2) ~40 km west of the ridge crest. Close-ups of seismic images in the basin (Figs. F2, F3, F4, F5) indicate that strata dip gently to the west (Figs. F2, 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 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. They encountered mudstone in the interval between 580 and 638 meters below seafloor (mbsf). They reported sedimentation rates of ~100 m/m.y. A piston core study from the same general region provided a sedimentation rate of 138 m/m.y. (Takahashi, 2005), which allows the study of high-resolution paleoceanography. Thus, before drilling we expected to recover Pleistocene to Pliocene sections. |
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