by Alison J. Smith, Emi Ito, David W. McGee, Natalie Burls, Peter Molnar, Tim K. Lowenstein, Alexander A. Prokopenko, Stephen C. Kuehn

The Pliocene Epoch is the focus of scientific interest as a period of sustained global warmth, with reconstructed CO2 concentrations and a continental configuration similar to modern. Studies suggest that the Pliocene was warmer and largely wetter, at least in the subtropics, than today, which contrasts with the long term hydroclimatic response of drying conditions seen in most climate model simulations. Two features of Pliocene warmth identified from sea-surface temperature (SST) reconstructions have the potential to bring about atmospheric circulation changes that could have supported wetter conditions in western North America (WNA) and other Mediterranean-like climates of subtropical drylands across the globe: 1) a weaker than present-day Pliocene zonal gradient in SST between the western and eastern equatorial Pacific, resembling El Niño-like conditions; and 2) polar-amplified Pliocene warmth supporting a weaker equator-to-pole temperature gradient between the tropics and the mid-latitudes, as this controls Hadley strength. The distribution and seasonality of wet conditions in WNA and the timing of late Pliocene-Quaternary aridification offer the potential to evaluate these two mechanisms. We hypothesize that the hydroclimate of western NA was, in addition to global warmth, dictated by changes in the Pacific surface temperature gradient. During the Pliocene in WNA, large perennial lakes existed for 10^5 to 10^6 years in basins to the east of the Cascades, Klamath Mountains and the Sierra Nevada, but dried out during the Quaternary. The basins, (Butte Valley, Tule Lake, Summer Lake, Lake Idaho, Searles Lake and Blythe Basin) span seven degrees of latitude. Data from these sites will compliment the SST reconstructions from global sites spanning the last 5Ma and investigate the large-scale hydrological cycle controls associated with both global warming and cooling. We propose to acquire new drilled cores from 4 of these basins in order to test our hypotheses. Although individual Pliocene records occur in many areas of the world, the WNA basins are unique as deep perennial freshwater lakes latitudinally arrayed in a subtropics-Mediterranean-like climate region, able to capture a response to Pacific forcing. We request funds to organize an ICDP workshop in Minneapolis, MN, USA in 2020, to extend the research objectives, refine the drilling and site survey plan, and broaden the science team working in PLIOWEST. This proposal is a resubmission. We provide improved global context for this project, an increase in the number of international ICDP scientists, a reduced budget, and clarification of the project details following previous review.

• We expect that the cores we propose to acquire will enable us to understand the processes that governed subtropical precipitation in Late Cenozoic time in Mediterranean-like climates. Specifically,we aim to reconstruct the time history of changes across the Pliocene-Quaternary time in lake status (e.g. lake levels and solute chemistry), stable isotopic composition of precipitation from d18O of carbonates and diatoms, moisture balance from pollen and d13C of leaf wax preserved in lake sediments. The time series enables us to correlate western North American continental records with Pacific marine records, which in turn allows us to examine how regional drying on the continent correlates with changes in zonal and meridional temperature gradients in the Pacific that drive the hydroclimate of western North America. We plan to use established data management systems such as those used by the ICDP, CSDCO/LacCore, Open Core Data, and various community databases for the PLIOWEST project. We will use ICDP and LacCore/CSDCO data management systems to seamlessly share data among project PIs, and at the end of the proprietary use period, deposit data in open community-curated data resources such as Neotoma Paleoecology Database (https://www.neotomadb.org/ ) and IEDA (https://www.iedadata.org/ ). We will adopt digital object identifiers (DOI) for each project dataset, and International Geo Sample Numbers (IGSNs) for all boreholes, cores, sections, halves, and subsamples. Adoption of DOI and ISGN will bring added value to PLIOWEST by inviting additional analyses of samples and reuse of project data by others not directly involved with the project.

Hydroclimate, Northern Hemisphere, Paleoclimate, Paleolakes, Pliocene

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