Global climate has undergone large changes during the last deglaciation (21,000 to 11,000 years ago), with a warming of ~3.5°C globally, a sea level rise of ~80 m, and the disappearance of the large continental ice sheets. The large magnitude of the signals and the availability of extensive paleoclimate data make this period an excellent target for investigating the mechanisms underlying climate change and for the validation of state-of-the-art Earth System Models (ESMs). However, up until now model simulations could not be directly compared with the paleoclimate records, as the models did not simulate the geochemical tracers ("proxies") on which the reconstructions are based. To address this issue and provide insights into several long-standing scientific questions about the evolution of the ocean's overturning circulation during the deglaciation, this project will perform the first transient ocean simulation of the deglaciation with the newly developed isotope modules in the ocean model of the Community Earth System Model (CESM1), allowing for direct model-data comparison of multiple marine paleo proxies. This project will contribute to STEM workforce development by supporting the training of two graduate students in climate modeling and model-data comparisons and by providing project leadership experience for an early career female scientist, who will work in close collaboration with U.S. and international colleagues. Results from this project will be used in undergraduate and graduate classes at the University of Colorado at Boulder and the University of Wisconsin-Madison and will be incorporated into public outreach through a project website and through general-audience talks. The output of the simulation (called C-iTRACE-O) will be made freely available though the Earth System Grid gateway, allowing the entire paleoclimate community to use it for their research. The simulation will also be contributed to the Paleoclimate Model Intercomparison Project transient deglacial working group, allowing for an evaluation of the CESM C-iTRACE compared to the deglacial simulations from other ESMs.
By directly simulating oceanic carbon-isotopes (d13C and D14C), Neodymium (eNd), and Protactinium/Thorium (231Pa/230Th) for the deglaciation and by carrying out sensitivity experiments to investigate the physical mechanisms causing the changing isotopic and geotracer distributions in the transient simulation, it will be possible to address some long-standing scientific questions through direct comparisons to observations and by evaluating tracer-circulation relationships in the model itself, such as:
- What was the source and routing of the low-radiocarbon CO2 released during the Mystery Interval? - What is the implication of the d13C evolution in the Atlantic for the deep ocean circulation during the last deglaciation and how much of the d13C change was caused by nutrient- and air-sea gas exchange effects versus circulation changes? - Why is the deglacial evolution of Antarctic Intermediate Water controversial among proxies, notably in eNd and carbon isotopes? - How does 231Pa/ 230Th represent the deglacial evolution of North Atlantic deep circulation and Southern Ocean upwelling?
