Integrated Ocean Drilling Program, United States

Paleoclimate and paleoceanographic studies present opportunities to study the dynamics of the climate system by examining how it responds to external forcing (e.g., greenhouse gas and solar radiation changes) and how its interacting components generate climate oscillations and abrupt changes. Of note is the amplified recent warming of the high latitudes in the Northern Hemisphere, which is presumably related to sea ice albedo feedback and teleconnections to other regions; both the behavior of sea ice-climate interactions and the role of large-scale atmospheric and oceanic circulation in climate change can be studied with geologic records of past climate change in the Bering Sea. Over the last 5 m.y., global climate has evolved from being warm with only small Northern Hemisphere glaciers to being cold with major Northern Hemisphere glaciations every 100-40 k.y. The ultimate reasons for this major transition are unknown. In addition, climate cycles on orbital and millennial timescales characterize the variability found in most continuous paleoceanographic records. Although regional environmental cycles and trends reflected in the sediment have been documented in some regions, the mechanisms by which they propagate globally are not understood. Possible mechanisms responsible for both the long-term evolution of global climate as well as the generation of high-frequency climate oscillations involve processes such as intermediate water ventilation and sea ice formation in the North Pacific. However, the paucity of data in critical regions of the Pacific such as the Bering Sea has prevented an evaluation of the role of North Pacific processes in global climate change. Because North Pacific Intermediate Water (NPIW) has the potential to be influenced by dense water forming in the Bering Sea and because changes in sea ice distribution have potential far-field impacts, the Bering Sea may be critically involved in major climate changes. Thus, drilling in the Bering Sea may help answer questions not only about the global extent of climate trends and oscillations but also about the mechanisms that produce them. In addition to having important sedimentary records of past climate change, the Bering Sea is a region of relatively high surface productivity, elevated intermediate-water and deepwater nutrient concentrations, and microbially mediated biogeochemical cycling. Thus, Integrated Ocean Drilling Program (IODP) Expedition 323 was also dedicated to examining for the first time subseafloor biomass and microbial processes in high-productivity regions. The major objectives of Expedition 323 in the Bering Sea are 1. To elucidate a detailed evolutionary history of climate and surface-ocean conditions since the earliest Pliocene in the Bering Sea, where amplified high-resolution changes in climatic signals are recorded; 2. To shed light on temporal changes in the origin and intensity of NPIW and possibly deeper water mass formation in the Bering Sea; 3. To characterize the history of continental glaciation, river discharges, and sea ice formation in order to investigate the link between the continental and oceanic conditions of the Bering Sea and adjacent land areas; 4. To investigate, through comparison to pelagic records, linkages between ocean/climate processes that occur in the more sensitive marginal sea environment of the Bering Sea and those that occur in the North Pacific and/or globally. This objective includes an evaluation of how the ocean/climate history of the Bering Strait gateway region may have affected North Pacific and global conditions; and 5. To constrain global models of subseafloor biomass and microbial respiration by quantifying subseafloor cell abundance and pore water chemistry in an extremely high productivity region of the ocean. We also aim to determine how subseafloor community composition is influenced by high productivity in the overlying water column. During Expedition 323 in the Bering Sea, 5741 m of sediment (97.4% recovery) was drilled at seven sites covering three different areas: Umnak Plateau, proximal to where the modern Alaskan Stream enters the Bering Sea; Bowers Ridge, in the open waters of the Bering Sea and also proximal to the glacial Alaskan Stream entry; and the Bering Sea shelf region, proximal to the modern sea ice extent. Four deep holes that range in depth from 600 to 745 m below seafloor (mbsf) and generally span 1.9-5 Ma in age were drilled. Some basement of older age was recovered at one site (U1342). The water depths of the drill sites range from 818 to 3174 m in order to characterize past vertical water mass distribution and circulation. The highlights of our findings include the following: 1. An understanding of the long-term evolution of surface water mass distribution during the past 5 m.y., including the southward expansion of seasonal sea ice to Bowers Ridge between 3.0 and 2.5 Ma and the intensification of seasonal sea ice at both Bowers Ridge and the Bering slope at approximately 1.0 Ma (the mid-Pleistocene Transition); 2. The characterization of intermediate-water and deepwater masses, including evidence from benthic foraminifers and sediment laminations, for episodes of low-oxygen conditions in the Bering Sea in the last 5 m.y.; 3. The terrigenous and biogenic sedimentary history of the Bering Sea, including evidence for strong climatological and sea level control of siliciclastic deposition at all sites. Records of lithostratigraphic variations indicate that Bering Sea environmental conditions were strongly linked to global climate change; this is apparent both in long-term, million year trends and in orbital, millennial, and shorter oscillations within the lithostratigraphic records generated at sea; and 4. A large range of inferred microbial activity with notable site-to-site variations, including significant activity as deep as 700 mbsf at the Bering slope sites, and, in contrast, low rates of microbially mediated sulfate reduction at Bowers Ridge.