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

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

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.