Paleoventilation Studies of the North Pacific Ocean

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Under this award the PIs will use the radiocarbon content of paired samples of benthic and planktonic foraminifera to determine the ventilation state of the North Pacific Ocean at key intervals during the most recent deglaciation. These estimates will help to understand the timing of and the connections between some of the most dramatic changes in North Pacific sedimentation in the past 100,000 years. Climate-linked changes in the composition of sediments from the eastern Pacific continental margin (organic content, lamination) are well documented, but it has been difficult to determine the cause of these changes. Some studies have implicated changes in upper ocean productivity, while others assign a dominant role to changes in ventilation of the intermediate-depth bottom waters in their northwest Pacific outcrop regions. It is important to determine the contributions from each of these two mechanisms because they each imply different climate forcing. Productivity-based explanations for organic rich / laminated sediments imply changes in the physical and biological conditions of the surface ocean (wind field, stratification, nutrients) all along the eastern margin of the Pacific. In contrast, ventilation-based explanations imply changes in the factors that control dense water formation in the high-latitude North Pacific (winds, stratification, sea ice formation). The proponents will determine benthic-planktonic 14C age differences on key deglacial intervals from a set of sediment cores from water depths ranging between 1 and 4 km in the northwest Pacific and the Okhotsk Sea and from a new set of Bering Sea cores over a similar depth range. To complement these radiocarbon-based ventilation estimates, they will use the d13C difference between pairs of co-existing benthic foraminifera to estimate bottom water oxygen concentrations over the same depth range and time intervals, and they will use the accumulation rate of benthic foraminifera to estimate the flux of organic matter to the sea floor. These three sets of data (B-P .14C, d 13C, and BFAR) will document the relationships between ventilation rate, bottom water chemistry, and carbon flux from the LGM through the deglacial. The data may help resolve the long-standing debate about the role of productivity versus the role of bottom water ventilation in controlling the accumulation of organic-rich sediments and laminated sediments at intermediate depths throughout the eastern Pacific. The project will provide half-time postdoctoral support for two years.