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

地外事件可以极大地影响地球上生命的进化。我们地球在太空中对周围邻居的脆弱性的最著名的例子是6500万年前的陨石撞击，导致恐龙灭绝。最近在地球海洋沉积物样品中发现了一种随时间衰减的放射性同位素（铁-60），这导致了一种想法，即在过去的几百万年里，一次或多次附近的超新星爆炸使地球充满了粒子（原子核）。事实上，这一事件有可能在大约300万年前引发了一次重大的环境变化。在这个项目中，研究人员将对一个或多个超新星爆炸在过去500万年中影响地球的想法进行独立测试。重点是海洋沉积物中的氦-3，它主要存在于行星际尘埃颗粒中，并且没有从超新星爆炸中提供任何显着的量。因此，如果氦-3和铁-60之间没有相关性，新的结果将为超新星假说提供进一步的支持。天文学模型预测，超新星事件应该在10万至40万年的时间内在海洋沉积物中记录下来。然而，迄今为止的工作还没有能够解决这些事件的持续时间，因此新的（氦-3）测量将提供对这些事件持续时间的显着改进的感觉，并有助于回答在过去500万年中发生的单个事件或多个超新星事件的问题。沉积物芯中铁-60记录的最近一次超新星事件发生的时间是在170万至320万年前（基于宇宙成因铍-10的年代测定）。这一超新星事件可能是上新世到更新世过渡期间发生的主要环境变化的原因。该项目的目标是通过解决以下假设，进一步评估过去500万年来这一超新星事件的记录和时间：假设1。地球上的铁-60来自超新星的输入，而氦-3来自星际尘埃颗粒。根据这一假设，我们预计氦-3和铁-60的测量水平不会出现协变。如果没有这种协变，就可以确定铁-60是一个超新星源。假设2.从新的氦-3分析确定的质量积累率表明，不止一个SN事件是可辨别的。这些事件应该有一个100-400 kyr的时间尺度，正如从距离太阳系100秒差距内的超新星的传输时间模型预测的那样。假设3.从沉积物芯中测定的星际尘埃颗粒的地外氦-3通量从上新世到第四纪增加。这与早期对太平洋沉积物的研究是一致的，但这种增加还没有在其他海洋盆地的地点得到证实。调查方法提供了一个关键的测试，证明附近的超新星在最近的地质历史中对地球表面过程和生物群产生了重大影响。该项目使用了R/V Eltanin从南澳大利亚湾，被安置在南极核心收集在俄勒冈州州立大学。该项目支持一名博士后研究员在俄勒冈州立大学海洋和地质储存库和惰性气体地球化学实验室开展工作。该项目的成果将被纳入俄勒冈州立大学教授的研究生水平的地球科学课程。该奖项反映了NSF的法定使命，并被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.