Collaborative Research: Using Multisystem Deep-Time Thermochronology to Decipher Neoproterozoic Exhumation Patterns in Time and Space

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Earth’s history as recorded in rocks is frequently incomplete in any one place, with gaps of missing time known as unconformities. While common, such gaps typically occur at different times in different places. A major exception is the unusual abundance of such gaps across multiple continents shortly before the start of the current geological Eon, and shortly before the diversification of shelly fossils in the famous Cambrian Explosion. The origin of this global gap in the rock record, known as the Great Unconformity, has been a subject of debate for some time. One of the recently proposed hypotheses links the unconformity to glacial erosion during the global “snowball Earth” ice ages that occurred about 715-660 and 740-735 million years ago. Because erosion decreases the depth (and thus temperature) of rocks beneath the surface of the crust, one test of this hypothesis involves minerals known as thermochronometers. These minerals record the past temperatures that they have experienced over time, due to (for instance) the rate of diffusion of isotopes produced in these minerals at a known rate by radioactive decay. While time alone provides some constraints on the cause of erosion, the time resolution of thermochronometers is limited in rocks this old. Consequently, in order to distinguish glacial erosion from erosion associated with the normal operation of plate tectonics, we propose to study not only the timing but also the spatial pattern of erosion over this time period, using thermochronometers collected from both stable continental interiors and less stable continental margins. Since the glacial erosion hypothesis predicts substantial erosion of stable crust in the interior of the continents, while the tectonic hypothesis predicts erosion only near tectonically active regions, the spatial distribution of erosion will allow us to determine whether erosion associated with the Great Unconformity was the result of glacial processes, tectonic processes, or both. The results will allow us to formulate and test new questions about the environmental consequences of the Great Unconformity, and the relationship between the Great Unconformity and the Cambrian Explosion. In addition to producing peer-reviewed publications and open-source software, our results will be incorporated into professional video content designed to be accessible to the broader public and for use in undergraduate courses. The Neoproterozoic Era encompassed a number of significant changes in Earth’s systems, including major diversification and complexification of the biosphere, episodes of extreme glaciation, and breakup of the supercontinent Rodinia. Preliminary data suggest that this time interval also saw a period of surprisingly robust erosional exhumation on the order of several km. Such exhumation could be a key link in connecting Earth-system processes, if it were widespread enough in extent, significant enough in magnitude, and had the correct timing. Recently, Keller et al. (2019) proposed a link between widespread glaciation and cratonic exhumation, specifically linking Neoproterozoic “Snowball Earth” glaciations to the phenomenon of widespread unconformity spanning the late Neoproterozoic. However, this proposal has subsequently been contested by Flowers et al. (2020), who instead attribute late Neoproterozoic exhumation and the Great Unconformity to normal tectonic processes associated with the breakup of Rodinia, and propose that Neoproterozoic glaciation had little if any erosive impact. Unfortunately, many previous thermochronologic studies, including that of Flowers et al., have focused on regions that were cut by Neoproterozoic faults, not truly tectonically stable — requiring the two hypotheses to be differentiated by timing alone, a tricky proposition given the large time uncertainty of thermochronologic time-temperature (t-T) inversions. Here we will propose a new thermochronologic test, based instead on the contrasting spatial patterns of exhumation predicted by tectonic and glacial mechanisms between stable cratonic interiors and less stable, tectonically active regions. This project will provide integrated research experience and professional-development training for a first-generation- postdoctoral fellow, who will collaborate across three institutions (Lehigh, Dartmouth, and Illinois), and will support two early-career PIs. Undergraduates at all three collaborating institutions will also be engaged in the project as summer interns and receive exposure to the research process, including experimental planning and communication of results. Given the level of public interest in the Great Unconformity, we will collaborate with Kindea Labs to produce content that can be integrated both in undergraduate class lessons and in popular science media.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.