Testing the Supernova Hypothesis Using 3He and 60Fe in Marine Sediments

点击翻译按钮获取中文摘要

Extraterrestrial events can dramatically affect the evolution of life on Earth. The best known example of our Earth's vulnerability to its surrounding neighborhood in space is the meteorite impact 65 million years ago that led to extinction of the dinosaurs. The recent discovery of a radioactive isotope that decays over time (iron-60) in samples from marine sediments on Earth has led to the idea that one or more nearby supernova explosions showered the Earth with particles (atomic nuclei) in the last few million years. In fact, there is the possibility that this event triggered a major environmental change about 3 million years ago. In this project, the researchers will perform an independent test of the idea that one or more supernova explosions influenced the Earth during the last 5 million years. The focus is on Helium-3 in marine sediments which is dominated by the presence of interplanetary dust particles and is not supplied in any significant amount from supernova explosions. Thus, the new results will provide further support for the supernova hypothesis if there is no correlation between helium-3 and iron-60. Astronomical models predict that supernova events should be recorded in marine sediments over a duration of 100,000 to 400,000 years. However, work to date has not been able to resolve the duration of such events and thus the new (helium-3) measurements will provide a significantly improved sense of the duration of such events, and help to answer the question of whether a single event, or multiple supernova events, occurred during the last 5 million years. The timing of the most recent supernova event recorded by iron-60 in the sediment cores lies between 1.7 and 3.2 million years ago (based on dating with cosmogenic beryllium-10). This supernova event may be responsible for major environmental changes that occurred during the Pliocene-to-Pleistocene transition. The objective of this project is to further evaluate the record and timing of this supernova event during the last 5 million years by addressing the following hypotheses:Hypothesis 1. Terrestrial iron-60 is derived from supernova input while helium-3 is derived from interstellar dust particles. Following this hypothesis, we expect to see no covariation in the measured levels of helium-3 and iron-60. The absence of such a covariation would firmly establish a supernova source for the iron-60 origin. Hypothesis 2. Mass accumulation rates determined from the new helium-3 analyses reveal that more than a single SN event is discernible. These events should have a timescale of 100-400 kyr, as predicted from transport time modeling from a supernova within a distance of 100 parsecs from the solar system. Hypothesis 3. The extraterrestrial helium-3 flux from interstellar dust particles determined in the sediment cores increases from Pliocene to Quaternary time. This would be consistent with earlier studies of Pacific sediments, but the increase has not been verified at sites in other ocean basins.The investigative approach provides a critical test of the idea that a nearby supernova significantly affected Earth's surface processes and its biota in the recent geologic past.The project uses sediment core samples recovered by the R/V Eltanin from the South Australian Bight that are housed in the Antarctic core collection at Oregon State University. The project supports a post-doctoral investigator to carry out work at the OSU Marine and Geology Repository and in the Noble Gas Geochemistry Lab. Results from the project will be incorporated into graduate level earth science classes taught at OSU.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.