CAREER: Back to the Beginning - Refining marine radiocarbon as a tracer of ocean circulation and time in the North Pacific

About This Grant

Radiocarbon, a naturally-occurring but unstable isotope of carbon that decays away over time, is a powerful tracer supporting insights into the global carbon cycle. Radiocarbon is also an essential tool for independent dating of carbon-bearing biological and geological materials over the past 50,000 years. This isotope is produced by cosmic rays interacting with nitrogen in the atmosphere, and dissolves slowly into the ocean as carbon dioxide. As a result, the ocean is always depleted in radiocarbon relative to the contemporaneous atmosphere, meaning that organisms that grow in the ocean (such as corals, bivalves, zooplankton and phytoplankton) have a lower starting ratio of radio to stable carbon in their bodies. This makes organisms that create their bodies or shells from sea water appear 'old' relative to the ratio of radio to stable carbon expected for an organism exchanging carbon dioxide directly with the atmosphere (for example, a land animal or plant), limiting the precision with which we can interpret marine radiocarbon dates of environmental events. The correction would be simple if the offset of the radiocarbon content or 'reservoir age' of the ocean relative to the atmosphere was constant, however it varies through space and time in complex ways, largely associated with changes in ocean circulation. While researchers know these changes occur, they are constrained by limited data in only a few locations, and there is little consensus on the radiocarbon ventilation history of the Pacific Ocean, which comprises the largest reservoir of carbon actively exchanging with the atmosphere on the planet. To address this knowledge gap, investigators will reconstruct ocean ventilation variability from marine sediment cores in the Northeast Pacific that contain both marine microfossils and terrestrial organic debris during the past 23,000 years. This reconstruction will advance the understanding of the marine carbon cycle, as well as provide an improved tool for dating marine sedimentary records of environmental and geohazard variability. Broader impacts activities include training undergraduate students, a graduate student, and a postdoctoral researcher, redesigning an undergraduate and a graduate class around research themes using new, project-derived cores/samples, and producing training products (Navigating the Academic Research Fleet) for the University-National Oceanographic Laboratory System (UNOLS) to improve participation in seagoing research. This study will leverage sites on the Pacific Northwest margin with confirmed undisturbed sedimentary sequences and demonstrated preservation of coeval benthic and planktic marine foraminifera as well as terrestrial plant debris. Investigators will apply genomic sample identification techniques to confirm genus-level identification of fragmented vascular plant material that are well established in the field of molecular ecology – yet novel in the field of marine radiocarbon dating. The resulting reconstruction, pinned to the atmospheric radiocarbon calibration curve, will offer insights into reorganization of Pacific Ocean circulation and ventilation from the surface to a depth of 2,500 m (modern Pacific Deep Water) during several events of abrupt climate change through the Pleistocene-Holocene transition, allowing for the evaluation of hypotheses surrounding the phase relationship between climate variability in the Pacific and Atlantic. Research outcomes will increase understanding of the marine reservoir age effect in the Northeast Pacific, refining the utility of radiocarbon as a chronostratigraphic tool for applications ranging from understanding changes in regional paleohydrology during past warm periods to the recurrence intervals and rupture lengths of great earthquakes, supporting research themes identified as priorities in the National Academy of Sciences Engineering and Medicine 2015-2025 Decadal Survey of Ocean Sciences. Outcomes will form an essential component of cohort-based undergraduate research experiences, and data will be incorporated into the adaptation of an undergraduate and development of a graduate level course to meet inquiry-based learning objectives. Investigators will also develop an annual webinar series, “Navigating the Academic Research Fleet” to help lower barriers to participation in seagoing research on vessels within the Academic Research Fleet and effectively utilizing NSF/UNOLS Facilities. The webinar series will also address cruise planning tools, logistical best practices, and how to best access these resources. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.