Microspheroids from the Clegg Creek Member of the New Albany Shale: New evidence supporting a Late Devonian Impact

摘要

Scientists have identified five great mass extinction events in the past 500 million years of the earth's history. Although the extinction events are agreed upon, only the Cretaceous (K-T) event has produced enough positive evidence that suggests a bolide impact may have been the cause. Although impacts have been suggested as mechanisms for other extinction events, volcanism, global climate change, changes in CO_2 concentrations, and evolutionary diversity changes have also remained viable triggers for consideration. The lack of evidence remains the primary reason for the dismissal of an impact causation triggering the other mass extinction events, including the one that occurred in the Late Devonian. Glassy iron-bearing micro-spheroids have been identified in the Late Devonian Clegg Creek Member of the New Albany Shale. These microspheroids are: (1) too large and dense to have been transported at the time of shale deposition; (2) of a composition and morphology that does not represent material that would have formed in situ by groundwa-ter; and (3) in areas free of soil or overburden that might have contaminated the shale. Therefore, it must be considered that the source of these microspheroids originated from a position above the shale, falling into the sediments before lith-ification, and that an extraterrestrial source is the most viable. Scanning Electron Microscopy/Energy Dispersive X-ray (SEM/EDX) analyses of the microspheroids indicates a roughly spherical shape with a glassy composition that is magnetic and iron-rich in a vitrified to semivitrified matrix. These data strongly suggest that these microspheroids are in fact microtektites (impact ejecta) and most likely originated from a large Late Devonian impact event that may have been the trigger for the Devonian extinction.