Collaborative Research: Did the Formation of the Great Unconformity Trigger Oxygenation and the Cambrian Explosion?

大不整合(GU)代表了岩石记录中最大的时间间隔之一。因为紧随其后的是寒武纪辐射，这是生命在生物多样性和复杂性方面最显著的增加之一，长期以来，人们一直认为这些现象是有联系的。地质学家提出，这一古迹是由地壳的大规模侵蚀造成的。随着岩石的风化和溶解，一种对生命至关重要的元素磷被释放出来，并被河流带到海洋中。磷的增加被认为刺激了蓝藻之间的光合作用，从而增加了海洋和大气中的氧气含量。这种氧合作用可能是导致寒武纪辐射的导火索，因为复杂的生命需要氧气。尽管谷在生命中最重要的转变之一中扮演着潜在的角色，但人们对它发生的时间和地点知之甚少。确定GU形成的时间和导致其形成的最后一次侵蚀事件的大小，对于确定在地球历史的这一关键时期生命和环境变化的可行机制至关重要。本研究项目是利用锆石和钛铁矿(U-Th)/He方法的进展来破译Gu的发育。结果直接检验了将大陆侵蚀与氧气和生命爆发联系起来的假说。该项目还通过培训本科生和研究生以及K-12教师来增进对科学技术的理解。有人提出，大不整合面下的侵蚀是超级大陆解体、雪球地球冰川或全球隆起的结果，可能向海洋输送了生物限制性的营养物质，从而助长了寒武纪的辐射。然而，大不整合面的时间、全球同步性和大小以前没有经过测试或限制。(U-Th)/He热年代学的最新进展使人们能够获得解开这一标志性特征的历史所需的热历史。该项目将获取横跨两个不同克拉通边缘的两个横断面上的锆石和钛铁矿(U-Th)/He数据。研究地点将以充分开发新元古代和寒武纪地质约束为目标，这将是缩小可行热史范围以区分古层模型的关键。这一结果将极大地改善对每个边缘的新元古代冷却历史的限制，这些限制将被用于测试GU开发和意义的竞争模型。这一奖项反映了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。

The Great Unconformity (GU) represents one of the largest gaps of time in the rock record. Since it is followed by the Cambrian Radiation, one of life's most significant increases in biodiversity and complexity, it has long been thought that these phenomena are linked. Geoscientists have proposed that the GU was caused by large-scale erosion of Earth's crust. As rocks weathered and dissolved, an element critical to life, phosphorous, was released and carried to the ocean by rivers. The increase in phosphorous is proposed to have spurred photosynthesis among cyanobacteria, which then increased the oxygen content of the ocean and atmosphere. This oxygenation was perhaps the fuse that led to the Cambrian Radiation since complex life requires oxygen. Despite the GU's potential role in one of life's most important transformations, very little is known about when and where it occurred. Determining when the GU developed and the size of the last erosion event that lead to its formation are critical for identifying viable mechanisms for life and environmental change during this pivotal interval of Earth history. This research project is making use of advances in zircon and titanite (U-Th)/He methods to decipher GU development. Results are directly testing hypotheses that relate continental erosion to oxygenation and the explosion of life. The project is also advancing scientific and technological understanding through the training of undergraduate and graduate students and K-12 teachers. It has been proposed that the erosion below the Great Unconformity occurred as the result of supercontinental breakup, Snowball Earth glaciation, or global eustasy, and may have delivered bio-limiting nutrients to the ocean that fueled the Cambrian Radiation. However, the timing, proposed global synchroneity, and magnitude of the Great Unconformity have not been previously tested or constrained. Recent advances in (U-Th)/He thermochronology allow access to the thermal histories required to unravel the history of this iconic feature. This project will acquire zircon and titanite (U-Th)/He data for samples along two transects across two different cratonic margins. Study sites will be targeted to fully exploit Neoproterozoic and Cambrian geologic constraints, which will be vital for narrowing the range of viable thermal histories to enable discrimination between GU formation models. The results will dramatically improve constraints on the Neoproterozoic cooling history of each margin, which will be used to test competing models for GU development and significance.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.