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

地外事件可能会极大地影响地球上生命的进化。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。最近发现的一种放射性同位素(铁-60)会随着时间的推移在地球上的海洋沉积物样本中衰变，这导致了这样的想法：在过去的几百万年里，附近的一次或多次超新星爆炸给地球带来了大量粒子(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，研究人员将对过去500万年来一次或多次超新星爆炸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，它主要是星际尘埃粒子的存在，超新星爆炸没有提供任何大量的氦-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，超新星事件应该被记录在海洋沉积物中，持续时间为10万到40万年。然而，到目前为止，工作还不能确定这些事件的持续时间，因此新的(氦-3)测量将大大改善对这些事件持续时间的认识，并有助于回答在过去500万年中是否发生了单一事件或多个超新星事件的问题。沉积核中的铁-60记录的最近一次超新星事件发生在170万至320万年前(基于宇宙成因铍-10的测年)。这一超新星事件可能是上新世向更新世过渡期间发生重大环境变化的原因。这个项目的目标是通过解决以下假设来进一步评估过去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来自超新星输入，而氦-3来自星际尘埃粒子。根据这一假设，我们预计测量到的氦-3和铁-60的水平不会出现协变。如果没有这样的协变，将坚定地为铁-60的起源建立一个超新星来源。假设2：从新的氦-3分析中确定的质量积累率显示，不止一次SN事件是可辨别的。这些事件的时间尺度应该是100-400 KYR，从距离太阳系100秒内的超新星的传输时间模型中预测出来。假设3.在沉积岩芯中测得的来自星际尘埃颗粒的地外氦-3通量从上新世到第四纪都是增加的。这与早先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大洋盆地的地点得到验证。调查方法为附近的超新星在最近的地质过去对地球表面过程及其生物群产生了重大影响的想法提供了一个关键的测试。该项目使用的是俄勒冈州立大学南极岩芯收集的沉积物岩心样本，这些样本是由埃尔塔宁号从南澳大利亚湾采集的。该项目支持一名博士后研究员在俄亥俄州立大学海洋和地质库和稀有气体地球化学实验室开展工作。该项目的成果将被纳入俄亥俄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，并通过使用基金会的智力优势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.