Award: OCE-1924215

Intellectual Merit The ocean is an important part of Earth's carbon cycle with implications for atmospheric carbon dioxide and climate. Carbon-13 isotopes measured in ocean sediments samples are used by paleoceanographers to reconstruct changes in carbon cycling and circulation in past oceans. One important, yet still unresolved issue in paleoclimate science is the variations in atmospheric carbon dioxide and climate during the ice age cycles of the last million years. Recent work has made progress in understanding this issue by applying climate and ocean models that include paleoclimate tracers such as carbon-13 to simulate the climate and oceans during the height of the last ice age, the Last Glacial Maximum (LGM). The principal investigators of this project have created a model of the LGM ocean and climate that agrees with a suite of paleoclimate tracers, including carbon-13, carbon-14 (radiocarbon), nitrogen-15, oxygen, and temperature and thus can be regarded as a viable state of the climate system for that period. In this project we have developed and applied a new method to decompose oxygen, carbon, carbon-13 cycling in ocean and climate models. This is useful to better understand the complex set of processes that cause changes is these variables. Biology redistributes carbon in the ocean by removing inorganic carbon from the surface during photosynthesis by phytoplankton. The organic carbon is subsequently cycled in the oceans food web and eventually part of it sinks down to deeper levels where it remineralizes back to inorganic carbon and remains sequestered for hundreds of years. This process is often called the biological pump. Upwelling of biologically sequestered carbon leads to outgassing to the atmosphere, but since air-sea gas exchange is slow, outgassing is incomplete, which creates a disequilibrium that propagates back into the ocean interior, increasing carbon storage there. Our results have shown that this biologically mediated air-sea disequilibrium is the dominating component for changes in oxygen, carbon, and carbon-13 in the LGM. This project has thus contributed to a better understanding of the cycling of these gases and isotopes in the ocean. Another important result of this project is that carbon isotopes constrain only the depth of the Atlantic Meridional Overturning Circulation (AMOC) well, whereas its strength is less well constrained by these data and thus much more uncertain in the LGM. We have also shown that, in contrast to previous suggestions, the carbon isotope data do not support the idea that the AMOC was collapsed during Heinrich Standial 1, a cold period in the North Atlantic following the LGM. Other publications have compiled palaeoceanographic data of carbon and oxygen isotopes in a new community-driven international database. Overall, this project has led to the publication of 14 articles in the peer-reviewed literature. Broader Impacts This project has supported the international effort of the Ocean Circulation and Carbon Cycling (OC3) working group, which is part of the Past Ocean Changes (PAGES) project. This collaborative effort has brought together scientists from around the world to synthesize carbon and oxygen isotope data. The resulting databases are now publicly available (https://doi.org/10.5281/zenodo.7234738 and https://doi.org/10.5194/essd-14-2553-2022). During the early Covid years we have organized an online series of 13 talks of users of the University of Victoria climate model. Those talks were useful to bring the international climate modeling community together remotely, without the dangers of transmitting the disease. Thus, the project has contributed to fostering international collaboration in science. The climate model code, including the improvements added in this project, has been made available online, thus contributing to advancing computational infrastructure for research. We collaborated with Oregon State University's Science & Math Investigative Learning Experiences (SMILE) faculty to deliver lessons and professional development opportunities to a statewide network of teachers in rural Oregon school districts. Over 50 teachers participated in two professional development workshops focused on climate change and its connection to the global carbon cycle. As part of this effort, we developed a hands-on modeling lesson that enables students to explore the importance of the global carbon cycle in mitigating anthropogenic carbon and its inherent limitations. The lesson also provided a direct link to PI Schmittners research, enriching students understanding of the importance of climate modeling. Teachers who attended the workshops piloted the lesson in afterschool STEM clubs and, where applicable, incorporated it into their classrooms. According to online logs submitted by teachers, over 350 middle and high school students participated in this lesson. Additionally, teachers provided feedback to refine the lesson for high school classrooms. The revised lesson was subsequently published on the SMILE curriculum website and is available at:Modeling Human Impact on Earths Carbon Cycle. This project has supported two graduate students (US veteran Nathaniel Fillman and UK student Ellen Cliff) and two undergraduate students (Matthew Boling and Elijah Stahr, both from underserved institutions with limited access to Earth sciences). Last Modified: 01/03/2025 Submitted by: AndreasSchmittner