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。我们的地球在太空中对周围邻居的脆弱性最著名的例子是6500万年前导致恐龙灭绝的陨石撞击。最近在地球海洋沉积物样本中发现了一种随时间衰减的放射性同位素（铁-60），这使人们产生了这样的想法：在过去的几百万年里，一个或多个附近的超新星爆炸使地球上的粒子（原子核）大量出现。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，它主要是由星际尘埃颗粒的存在所主导的，而不是由超新星爆炸提供的大量氦-3。因此，如果氦-3和铁-60之间没有相关性，新的结果将进一步支持超新星假说。天文模型预测，超新星事件应该在10万到40万年的时间里记录在海洋沉积物中。然而，迄今为止的工作还不能解决这些事件的持续时间，因此新的（氦-3）测量将提供这种事件持续时间的显著改进，并有助于回答在过去500万年中发生的是单一事件还是多个超新星事件的问题。沉积物岩心中的铁60记录的最近一次超新星事件的时间在170万到320万年前（基于宇宙起源的铍-10定年）。这个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。这个项目的目标是通过解决以下假设，进一步评估过去500万年中这一超新星事件的记录和时间。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，我们预计在氦-3和铁-60的测量水平中不会看到共变。如果没有这种共变，就可以确定铁60的超新星来源。假设2。从新的氦-3分析中确定的质量积累率表明，不止一个SN事件是可识别的。根据距离太阳系100秒差距的超新星的传输时间模型预测，这些事件应该有100-400光年的时间尺度。假设3。在沉积物岩心中测定的来自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，但是这种增加还没有在其他海洋盆地得到证实。这种调查方法为最近地质历史中附近的超新星显著影响地球表面过程及其生物群的想法提供了一个关键的测试。该项目使用了R/V Eltanin从南澳大利亚湾回收的沉积物岩心样本，这些样本被安置在俄勒冈州立大学的南极岩心收藏中。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以及稀有气体地球化学实验室开展工作。该项目的研究结果将被纳入俄勒冈州立大学的研究生地球科学课程。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.