The Late Devonian period hosts a series of global biological extinction events of such magnitude as to be counted among the major mass extinctions of the Phanerozoic. Yet no consensus has yet been reached regarding the causative mechanisms of extinction and associated changes in marine sedimentation, biogeochemical cycling, sea level and climate. Testing of competing hypotheses for Late Devonian extinctions?which include sea-level fluctuations and regression, climatic cooling, ocean anoxia, bolide impact, and/or massive volcanism?is currently hampered by a lack of sufficient temporal resolution in paleobiological, tectonic and proxy climate records. In this study, a multi-disciplinary research team of paleontologists, biostratigraphers and isotope geochemists will combine high-precision U-Pb zircon geochronology on interstratified volcanic ash beds in key stratigraphic successions with quantitative biostratigraphy applied to a global Late Devonian multi-taxa database compiled from the literature and new high-resolution sampling. The U-Pb geochronology will utilize the chemical abrasion and EARTHTIME quadruple spike isotope dilution methods to measure ca 0.1 Ma resolution ages, which will calibrate the construction of the first highly resolved composite standard for the Late Devonian via constrained optimization. With this high resolution age model the research team will interrogate several fundamental questions relating to the abruptness of the late Devonian biotic events, their global synchrony, and process drivers. Specific hypotheses to be addressed include the synchrony and causality between extraterrestrial impacts or massive volcanism and biotic crises, whether the rates of associated eustatic rise and fall and fluctuations in proxy records of carbon cycling and temperature are consistent with Milankovich-band orbital forcing, and if feedbacks between climate, glacioeustasy, ocean anoxia, and the carbon cycle caused the Late Devonian biotic crises. This research will impact STEM human resource development via participation of undergraduate students in a multi-disciplinary international science team; develop international scientific collaborations; support the science mission of the EARTHTIME geochronology initiative, and its associated inreach and outreach programs; provide fundamentally improved chronologies for numerous other Paleozoic climate studies; and improve our understanding of deep time climate state transitions potentially analogous to those leading to our current icehouse. The scientific process and content of the proposed research will be captured by a parallel NSF-funded STEM education initiative developing a series of web-based learning objects to teach the science of geochronology and Earth history.
Marine strata, deposited in the Upper Devonian, approximately 385 to 360 million years ago, contain zircon-bearing tephra beds from volcanic eruptions that enable high precision dating of the strata using U/Pb decay analysis that is accurate to 0.1 million years. Fossils in the strata enable global correlation of these beds and recognition of events that lead to mass extinctions and global changes in this interval marked by the first extensive forests and evolution of terrestrial vertebrates. In addition, geochemical analysis of the strata - stable carbon isotopes and magnetic susceptibility, tied into evident lithologic cycles of deposition (Image 1 - Walnut Creek), allow recognition of globally forced climate change on the scale of 16,000, 20,000, 40,000, and 100,000 year intervals. These high resolution cycles allow recognition of changes at a finer level than possible using biostratigrpahy alone and will lead to recognition of 100 year scale packages(!). Thus, the conditions leading to the mass extinction event at the Frasnian-Famennian Boundary in the Upper Devonian, one of the "big five" mass extinctions in the Phanerozoic characterized by the elimination of extensive reef environments and organisms, can be investigated in regard to timing and spatial changes. The terminal Devonian interval has also been investigated (Image 2 - Exshaw). The resolution of the changes are at a larger scale due to the depositional setting of the study, in a more offshore environment where depositional rates were slower and thus the interval is condensed and higher resolution is not possible. Nonetheless, strata can be correlated globally based on biostratigraphic and geochemical criteria and the Devonian-Carboniferous boundary narrowly constrained (Image 3 - Jura Creek). These globally recognized events are case studies for planetary change and clues to the interpretation of modern changes in Earth climate.
