Radiocarbon estimates of the past ocean are valuable because they have the potential to trace the distribution and transport of water masses, which are both important for understanding the role of ocean circulation in climate changes. A well-known problem with radiocarbon reconstructions from paired measurements of benthic and planktonic foraminifera is that most studies assume that the offset between the apparent radiocarbon age of the local surface water and the contemporary atmosphere (the "surface reservoir age") has been constant through time. Modeling simulations show that surface reservoir age probably changed by hundreds of years at centennial to millennial timescales in the past. Unaccounted-for changes in surface reservoir age are problematic, since they can introduce as large a signal into a radiocarbon reconstruction of the ocean interior as the changes associated with ocean circulation. In this collaborative project, the research team will match volcanic ash layers (tephras) from marine sediment cores in the southeast Bering Sea to independently-dated tephras from nearby Sanak Island. This will allow reconstruction of surface reservoir ages from this location and testing of the hypothesis that reservoir age varied by hundreds of years in the past. The team will also be able to re-evaluate the conclusions of studies based on foraminiferal radiocarbon where the authors assumed constant surface reservoir age, including the hypothesis that North Pacific intermediate water ventilation was antiphase with the North Atlantic during the last deglaciation. The results of this study have potential impact across disciplines, including improving the dating of archaeological remains from the earliest humans in the eastern Aleutians, and the understanding of the timing of waves of migration with ecological and climatic changes from that time period. Funding also supports research experiences for undergraduates from Williams College (an RUI institution).
More specifically, the recently published tephrochronology from the last 17 ky from Sanak Island in the eastern Aleutians (Misarti et al. 2012) presents a new and unique opportunity to reconstruct surface reservoir ages and ventilation from the subarctic Pacific, since Sanak Island is near the Umnak Plateau in the southeast Bering Sea, which has been cored for high-resolution paleoceanographic work. The research team will measure major and trace element abundance with electron microprobe and laser-ablation-inductively-coupled-plasma mass spectrometry to match tephras between Sanak Island and three nearby marine sediment cores spanning 1 to 2 km water depth, creating tephra-based age models for the cores. They will then measure a time series of planktonic radiocarbon in the sediment cores to produce a record of local surface reservoir age changes from the SE Bering Sea. They will measure benthic radiocarbon at the same resolution and use the tephra-based age model to calculate the Delta14C of the water column at 1 to 2 km in the Bering Sea. With this reconstruction, the team will be able to evaluate proposed changes in North Pacific circulation and Bering Sea ventilation from the last deglaciation, and constrain the end-member properties of North Pacific water masses from the past.
