This research provides the first Pliocene paleo-oceanographic observations from the Bering Sea and supports the objectives of IODP expedition 323 for understanding the associated factors influencing climate change during the Pliocene and Pleistocene, including a warm period of the early Pliocene, when pCO2 levels were similar to today¡¦s levels. The flow of water masses between the Bering Sea and the Arctic and Pacific oceans, changes in Arctic ice volume, and whether dense intermediate waters formed in the Pacific during this time will be investigated as some of the possible triggers of past climate change. The high resolution sediment records obtained from this cruise, with relatively large-amplitude signals, provide a unique opportunity to understand how insulation changes in the upper atmosphere propagate through the climate system and to further test if the orbital cycles of climate change primarily operate on 23ky versus 40 ky timescales. This study utilizes K % logging data from the cruise with paleomagnetic, sedimentological (eg., grain size analysis) and paleontological (eg., analysis of microfossil assemblages; d18O and d13C measurements of benthic and planktonic forams) measurements from the drill cores. The information expected from this study is critical for understanding the fundamental drivers and feedbacks between ice sheets, oceanic and atmospheric circulation and the global carbon cycle.
Broader Impacts: This project significantly advances large investments previously made in an important IODP expedition. The data derived from this investigation can provide important information for paleoclimate/ocean models, and is relevant towards projections of future climate under IPCC AR4 scenarios. The project includes a team of early and mid-career scientists, and includes international collaborations with other scientists of the IODP community. Support is also provided for undergraduate and graduate education, and for public outreach and education.
The Bering Sea is a dynamic region with the highest rates of biological productivity in the world’s oceans, seasonal sea ice distribution that responds readily to climate change, and the potential for intermediate water formation which is tied to heat flux and climate conditions of the North Pacific. We conducted three main studies that focused on different timescales of change. In the first study, we generated long records of changes over the last five million years to examine Bering Sea conditions during the early Pliocene, a period of global warmth. We found the Bering Sea was 6 degrees warmer in the early Pliocene warm period compared to today, indicating high latitude amplification of the global signal which was 2-3 degrees warmer than today. And, we found that there was a continuous and plentiful supply of nitrate, and diatom assemblages that imply very high levels of paleoproductivity in the Pliocene warm period. In the second study, we explored the role of intermediate water formation, within the Bering Sea, in glacial-interglacial climate change of the North Pacific. This involved generating geochemical records of intermediate water conditions over the last million years. We found that during glacial periods, intermediate water formed in the Bering Sea by brine rejection during sea ice formation; this salty water sank to intermediate depths and was exported out of the Bering Sea thereby participating in large scale overturning circulation and climate change in the North Pacific region. During interglacials, brine formation was negligible. Furthermore, we found that the production of intermediate water in the Bering Sea occurred only during major glaciation when sea level dropped below the 50 meter sill depth of the Bering Strait. With sea level drop, flow through the Bering Strait stopped, affecting the temperature and salinity of the North Atlantic and North Pacific. Our results show that North Atlantic and North Pacific intermediate water formations were both enhanced when flow through the Bering Strait was cut off. Thus, the Bering Strait serves as an important gateway that moderates climate in both the Atlantic and Pacific regions. In the last study, we examined millennial scale climate changes in the Bering Sea over the last 65 kyrs. We found pronounced and large amplitude variations at sub-orbital to millennial scales. During warm events, diatom and isotopic analyses show high productivity while the surface ocean was stratified possibly related to sea ice formation/melting. The millennial scale events seem to be correlated to similar events recorded in the Greenland ice core, suggesting that these millennial scale events propagate throughout the Northern Hemisphere, at least, and probably via the changes in the ocean. Overall, our three studies indicate that the Bering Sea oceanographic and climatic changes occurred on a large range of timescales, and that these changes are not occurring in isolation in the Bering Sea. Rather, climate variations across timescales are linked to North Pacific, North Atlantic, and global climate change.
