Collaborative Research: P2C2--Western United States Hydroclimate during the Last Interglacial: Developing Proxy Records and Using Model Intercomparison to Glimpse the Future

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Freshwater availability is critical to agricultural, energy and urban systems in the arid to semi-arid western U.S. Quantifying hydroclimate change during past warm periods, such as the Last Interglacial (LIG; ~129,000-116,000 years before present), is essential for evaluating climate model simulations of Earth’s past. This project aims to reconstruct variations in the spatial and seasonal distribution of precipitation in the western U.S. during the LIG during which global temperature are comparable to low-end 21st century projections. The project will include the application of novel methods (calcium and triple oxygen isotopes) on stalagmites from Lake Shasta Caverns (northern California) and Titan Cave (northern Wyoming), and will provide new chronological data and novel measurements (carbonate clumped and triple oxygen isotopes) at four lake sites in the northeast Great Basin (Bear Lake, Utah/Idaho and Bonneville Basin, Utah/Nevada), and southwest Great Basin/Mojave Desert (Lake Manix and Owens Lake, California). The anticipated results of this project are past records of changes in temperature, rainfall amount and moisture source and seasonality. These new past climate records will be integrated into a regional proxy-data network used to benchmark climate model simulations of the Last Interglacial period (127 ka).The paleoclimate modeling intercomparison (PMIP4) and coupled model intercomparison project (CMIP6) efforts of the climate modeling community include Tier 1 LIG (127ka) simulations as a benchmarking target for models with similar CO2 levels, different orbital configuration, unchanged topography and higher sea level. The validation of hydroclimate-relevant variables at a regional scale using climate models requires robustly dated and spatially distributed paleoclimate observations in order to place the LIG water cycle in the western U.S. in the context of geologic and future climate change. Yet, few quantitative estimates of paleoclimate exist for this interval, which offers geochronologic challenges too as it is beyond the limit of radiocarbon age control. This project seeks to improve our understanding of LIG climate in the western U.S. by providing: 1) new observations from speleothem records, 2) new U-Th dating of existing lake cores, and 3) the application of novel semi-quantitative proxies for temperature, precipitation and humidity to speleothems and lake sediments. Through the development of new high-resolution proxy time-series from speleothems and lake sediments, we will provide new insight into the behavior of the North American Monsoon during a past warm period and reconstruct hydroclimatic response to abrupt climatic events during the penultimate deglaciation (Termination II) and Marine Isotope Stage 5 (MIS5). The research uses existing lake cores recovered and archived by previous NSF- and USGS-funded continental drilling projects, and recently collected speleothems known to span much of MIS5 and the LIG.The potential Broader Impacts include quantifying regional hydroclimate changes during past warm periods and benchmarking of climate model simulations for the last interglacial. Other Broader Impacts include translating the project research outcomes via a workshop (paleo to policy) convening water resource managers, government agencies (e.g., USGS), climate and weather risk assessment and projection companies and early career paleoclimate researchers from across the U.S. This workshop will emphasize on the information that terrestrial past climate records (lake sediments, tree rings, speleothems) can provide in terms of hydroclimate data beyond the instrumental records needed by policy makers to assess climate variability and extreme events such as drought and floods. The researchers will produce a white paper, policy briefs for water resource managers and a presentation at an education and outreach session at a scientific meeting (American Geophysical Union). The project will provide scientific training, mentorship and professional development for two graduate students and one postdoctoral researcher. Through joint field trips and collaborations, these students and postdoc will have opportunities to develop interdisciplinary expertise in climate archives, novel proxy systems, geochronology and model-proxy comparison. In addition, the researchers will recruit undergraduate students to complete summer internships related to the project. The undergraduate students will be recruited through (1) the Earth Horizons program and partnership between Vanderbilt University and Tennessee State University, a Historically Black University (HBCU); (2) the Leadership Alliance program at Brown University funded through NSF-REU-EPSCOR. The undergraduate students will be encouraged to develop their research into presentations at regional, national and international scientific meetings (e.g., AGU, GSA).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.