Newly found evidence of the Late Devonian extinction by microtektites in black shales: Sellersburg, Indiana, USA

Abstract

Our planet has experienced at least five major mass extinction events, which have occasionally brought the flora and fauna to almost complete collapse , yet they have been pivotal in the evolution of life as we know it. This thesis explores potential causes of mass extinctions, focusing particularly on the Late Devonian mass extinction event, which is the least understood of the major five. Over the span of the early Paleozoic and the Devonian, the planet experienced extensive plate tectonic reorganization which resulted in the formation major orogenic belts, extensive rifting and seafloor spreading, as well as associated volcanism related to the emplacement of large igneous provinces (LIPs). Collectively, these tectonic changes drove the development of various sedimentary basins in the Devonian, including a series of foreland and intracratonic basins to form on Laurentia (i.e., present day North America). The Illinois Basin is one such intracratonic basin, which host a thick sedimentary succession, including Devonian strata. The mudstone-dominated Devonian part of basin fill, which was deposited in a shallow and partly restricted marine basin, holds the potential of harboring evidence of one of many drivers, which over the period of some few million years, triggered the Late Devonian mass extinction. By combining conventional petrographic analysis of some black shale samples (from the Late Devonian-aged New Albany Shale group) with X-Ray Fluorescence (XRF), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) analyses, this study investigate microscopic spherules, known as microtektites, which is thought to be dust-sized impact ejecta debris that settled from the atmosphere into this vast basin following a potential lethal meteor impact somewhere else on the planet. . Across the planet, multiple Devonian-aged impacts are known, especially from near or at the Frasnian – Famennian boundary (c. 372.2 Ma), which thus could be the source of these microtektites. The investigated, finely laminated black shales from the Illinois Basin also contain abundant pyrite framboids, recording the anoxic conditions that prevailed in the basin at the time of deposition. Similar black shales of Late Devonian age occur elsewhere on the planet, suggesting that widespread anoxia could be an important factor leading to the mass extinction. Indeed, the scientific community have hitherto, not fully agreed on the key causal factor that drove the Late Devonian extinction event, although volcanic activity related to LIPs, widespread oceanic anoxia, onset of severe glaciations, as well as vascular plant migration on land has variably been proposed as the main drivers. Due to limited number of locations having preserved Late Devonian-aged microtektites, the impact hypothesis is rarely mentioned as a potential cause. Therefore, this thesis aims to rejuvenate the bolide impact-theory, which may have influenced and substantially changed the course of the Late Devonian biotic evolution, or the very least contributed to the environmental crisis that was already gripping the Late Devonian World.