High-resolution records of atmospheric circulation and past rainfall from soils based on U-series and stable isotope SIMS approaches

点击翻译按钮获取中文摘要

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).There are relatively few records that reflect large-scale changes in atmospheric circulation during the last glacial-interglacial cycle, and very few with sufficient temporal resolution and/or continuity to resolve the timing and sequence of changes. To understand how these changes occurred in the past, terrestrial climate records that are sensitive to changes in precipitation are necessary. Although there are a substantial number of short-term high-resolution terrestrial paleoclimate records, there are relatively few records that span the last few glacial-interglacial cycles. These sparse records have proven insufficient to resolve the mechanisms behind the large-scale shifts in effective moisture that occurred during glacial-interglacial transitions. In addition, it has been challenging to resolve the effects of temperature and changes in moisture source from changes in the amount of precipitation using existing proxy data.Intellectual merit: This project will develop high-spatial resolution analytical approaches using secondary ion mass spectrometry (SIMS) to provide spatially and temporally continuous records of climate and atmospheric circulation over the last few glacial-interglacial transitions (10 to 200 ka) using pedogenic opal. 230Th-U SIMS dating will be complimented by the development of multi-isotope proxies in order to develop high-temporal spatial resolution records that connect to form a regional grid, and are thus uniquely capable of reflecting past changes in atmospheric circulation. Specifically, the investigator will test the hypothesis that uranium isotopic and trace element variations in dated pedogenic opal provide a robust archive of paleoprecipitation. Changes in moisture sources will be detected using δ18O, and changes in weathering intensity will be assessed using δ30Si. The existing geochronological records from soils show variations in initial (234U/238U) activity ratios and trace elements over the last 200 ka. The investigator hypothesizes that initial (234U/238U) variations are recording changes in past rainfall amounts. However, the exact mechanism for these variations must be more closely examined before quantitative data can be developed. Therefore prior to expanding her network of soil sites, she proposes to test the concordance of the initial (234U/238U) values and the validity of the U-series proxy using the modern soil system. Development and validation of the U-series proxy would enable paleoclimate information to be extracted directly from the geochronologic measurements of soils and could potentially be extended to speleothem.Broader impacts: This research will have important implications for our understanding of the rates and mechanisms of climate change in terrestrial environments. By working with the climate modeling community, the data and network of sites will provide unique and much needed calibration data that will greatly improve the ability to predict and understand the current changes in atmospheric circulation. As the samples she plans to analyze are modern analogues for paleosols, the approaches and understanding she develops will also have important implications for these materials and may enable new records to be developed at key intervals back in Earth?s history. To make her records available, she will host a database of published U-series data at Stanford, and work with other scientists to include existing records. Secondly, she will work with the outreach program in the School of Earth Sciences at Stanford. She plans to: 1) accept high school interns to work with her in the field and in her laboratory, 2) work with the K-12 program ?Expanding Your Horizons? to develop a module describing the climate system, how it varied in the past, and how it has influenced the landscape of both the local environment and greater California.