CDP: Site survey investigation for ICDP drilling in Pliocene lake at Butte Valley, CA

Southern Methodist University, Dallas TX

Investigators

Abstract

The Pliocene Epoch (5.3 to 2.6 million years ago) shares many geological similarities to our current time, as a period of sustained global warmth characterized by CO2 concentrations and a continental configuration similar to modern. For these reasons, the Pliocene is considered an appropriate analogue for testing the capabilities of models used to predict climate and ecological changes over the coming century. Modern long-term hydroclimatic simulations, for example, forecast subtropical regions to become drier in response to increasing CO2 levels. These regions, however, show wetter and warmer conditions with global warming during the Pliocene, suggesting the presence of sources of precipitation that are not included into current model projections. To account for these sources and to understand the changes in atmospheric circulation that cause them, we need to reconstruct the distribution and seasonality of wet conditions across a wide range of latitudes and hydroclimatic changes during the Pliocene. This reconstruction is possible across the western North American continental margin, where a set of deep, permanent lake basins preserves the record of Pliocene paleoclimate conditions locked in the sedimentary fill. Unlocking this record requires drilling into these paleolacustrine lake basins, retrieving and analyzing sediment cores. The PLIOWEST project (proposed to the International Continental Scientific Drilling Program) plans to drill these lake basins and to capture the changing hydroclimate gradients over time. To plan for drilling, this award will fund a geophysical investigation necessary to understand the characteristics of Butte Valley, California, one of the sites selected for retrieving of lacustrine cores. The site is a dry lake basin just east of the Cascade Range that persisted for 105 to 106 years during the Pliocene, but subsequently dried during the Pleistocene. The basin has been sculpted by the tectonic and volcanic processes that shaped the Pacific northwest, and contains one of the longest and most complete archives of Earth climate through the Pliocene and Pleistocene. The geophysical survey will image the characteristics and geometry of the sedimentary sequences and guide the selection of the drilling site. The lacustrine deposits and the valley infill are not only relevant for past and current climate studies; they are also critical aquifer and groundwater resources, particularly in a time of drought, as the one California has been experiencing. The data acquired and the results of this project will be relevant to watershed managers, agricultural planners and stakeholders in western North America, including Native American tribal communities. The project will carry out a high-resolution geophysical site investigation through collection and interpretation of seismic reflection data. Twenty kilometers of 2D multichannel seismic reflection data will be acquired using a 100 kg trailer-mounted accelerated impact seismic source firing at 10 m intervals on a 5 m receiver interval spread. These parameters have the advantage of achieving both the high spatial and high temporal resolutions required for defining the drilling site. The goals of the project are to image the architecture of the basin and the geometry of the lacustrine deposits down to depths greater than 500 m, to resolve unconformities, and to identify presence of faults that may result in hiatuses in sedimentation and/or truncations of sedimentary record. The geophysical investigation will also ensure the identification of potential hazards to drilling and obstacles (e.g., basaltic sills), that may interfere or prevent the retrieval of complete stratigraphic succession. In addition, by illuminating the lacustrine basin infill, the survey will provide the context for the interpretation of lithological and stratigraphic information in cores retrieved during the drilling project. 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.