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doi:10.2204/iodp.proc.323.103.2011 Site U13391Expedition 323 Scientists2Background and objectivesThe primary objective of drilling at Integrated Ocean Drilling Program (IODP) Site U1339 (proposed Site UMK-4D; Takahashi et al., 2009) was to study high-resolution paleoceanography in the easternmost part of the Bering Sea, a marginal sea expected to exhibit large variations during times of global climate change. Umnak Plateau is located off Bristol Bay (Figs. F1, F2, F3) and is well situated to study past changes in surface water conditions (Fig. F4), sea ice extent, and associated biological productivity. Today, parts of the relatively warm Alaskan Stream surface water flow into the Bering Sea through Umnak and Amukta passes, and hence sea ice is not formed in this region. However, substantial sea ice coverage during the Last Glacial Maximum (LGM), when sea level was ~100 m lower than today, has been noted. This indicates that the influence of relatively warm water from the distal end of the Alaskan Stream was reduced, perhaps because water entering the Bering Sea from the Pacific Ocean through Umnak and Amukta passes was at least partially restricted when sea level dropped; warm Pacific water could have more easily passed through deeper passes, such as Amchitka Strait, in the central and western Aleutians (Katsuki and Takahashi, 2005). Thus, examining past environmental conditions at Umnak Plateau can shed light on the impact of changes in water exchange between the Pacific Ocean and the Bering Sea. Because these eastern passes are fairly shallow (~50 and 430 m), little intermediate or deep water flows out to the Pacific in this region. As such, Umnak Plateau should provide different information than the western sites, which are closer to deep passes where surface water flow may not have been strongly inhibited by sea level changes and where dense water exchange with the Pacific Ocean is more likely to occur (Tanaka and Takahashi, 2005). To make this west–east comparison, records from Site U1339 (water depth = 1870 m) on Umnak Plateau can be compared to those of IODP Site U1341 (water depth = 2177 m). Site U1339 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. Organic-fueled subseafloor respiration and its impact on biogeochemistry in such a highly productive region have not previously been quantified. To do this, sediments drilled at Umnak Plateau were used to determine subseafloor cell abundances and investigate the link between the mass and characteristics of subseafloor microbes and the extent of export productivity from the surface ocean. Scholl and Creager (1973) found Pleistocene diatomaceous sediments with ash layers in the uppermost 120 m, followed by Pliocene diatomaceous sediments below, at Deep Sea Drilling Project (DSDP) Sites 184 and 185, both drilled at Umnak Plateau. They reported sedimentation rates of ~67 m/m.y. and indicated that the diatomaceous sediments include neritic components, which are probably an influence from the Bristol Bay region. A piston core study from the same general region provided a sedimentation rate of 262 m/m.y. (Takahashi, 2005). Thus, before drilling, the predicted ages for the bottom of Site U1339 at ~200 meters below seafloor (mbsf) ranged from mid-Pleistocene to Pliocene. |
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