Development of Technique: Systematically measuring the cosmogenic 26Al/10Be production ratio at a variety of altitudes and latitudes

点击翻译按钮获取中文摘要

Geologists use many rare isotopes of different elements to date events in the past, both natural and those related to humans. One particularly widely applied system compares the ratio of 26Al to 10Be in samples that have been exposed at the surface of the earth and then buried. To create ages from a measured ratio of 26Al/10Be, researchers need to know the ratio of 26Al/10Be after exposure but before burial. While this ratio has been estimated before, its value is not well known and appears to vary around the world. This project will be the first to measure the 26Al/10Be ratio in over 150 different samples that have already been collected and will thus provide the information needed for many researchers in a variety of fields (geology, glaciology, archeology) to generate more accurate 26Al/10Be burial ages. The new data will also allow much better understanding of the uncertainty of these ages. This work will train a graduate student in science and will improve dates of ancient human remains as well as past changes in Earth's climate, including the coming and going of glaciers.Knowing the surface production ratio of 26Al/10Be is critical for the calculation of two-isotope burial ages because a measured 26Al/10Be ratio less than the production ratio can be interpreted quantitatively as the duration of burial after surface exposure of a sample. Although 26Al/10Be dating has been widely applied (and many of the applications have been supported by NSF), the 26Al/10Be ratio at production is not well constrained by data. Recent measurements suggest that the ratio is at least 8% higher in Greenland than the canonical value of 6.75, which is derived primarily from one series of early 1990s measurements made at high elevation in the mid latitudes. Physics-based models and some other new data suggest that the 26Al/10Be surface production ratio should vary systematically with latitude and altitude. Here, this project will test for spatial variance and determine the 26Al/10Be surface production ratio empirically as a function of latitude (5-45° in both hemispheres) and altitude (0 to 5000 meters above sea level.). The team will do this by making 150 high-precision 26Al analyses of archived samples already prepared and previously analyzed for 10Be. The team will compare this data-driven determination of the 26Al/10Be surface production ratio with predictions made using numerical models of nuclide production. The project will increase the broader impact of this work by disseminating the results both at meetings and in at least one peer-reviewed publication. This will ensure that the community is aware of and can easily use the data that are generated to improve the accuracy of 26Al/10Be two-isotope dating. Additional broader impacts include training a student who will do much of the research and writing.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.