Late Ordovician geographic patterns of extinction compared with simulations of astrophysical ionizing radiation damage

Abstract

Abstrac.t-Based on the intensity and rates of various kinds of intense ionizing radiation events such as supernovae and gamma-ray bursts, it is likely that the Earth has been subjected to one or more events of potential mass extinction level intensity during the Phanerozoic. These induce changes in atmospheric chemistry so that the level of Solar ultraviolet-B radiation reaching the surface and near-surface waters may be approximately doubled for up to one decade. This UVB level is known from experiment to be more than enough to kill off many kinds of organisms, particularly phytoplankton. It could easily induce a crash of the photosynthetic-based food chain in the oceans. Certain regularities in the latitudinal distribution of damage are apparent in computational simulations of the atmospheric changes. We previously proposed that the late Ordovician extinction is a plausible candidate for a contribution from an ionizing radiation event, based on environmental selectivity in trilobites. In order to test a null hypothesis based on this proposal, we confront latitudinal differential extinction rates predicted from the simulations with data from a published analysis of latitudinal gradients in the Ordovician extinction. The pattern of UVB damage always shows a characteristic strong maximum at some latitude, with substantially lower intensity to the north and south of this maximum. We find that the pattern of damage predicted from our simulations is consistent with the data assuming a burst approximately over the South Pole, and no further north than -75°. We predict that any land mass (such as parts of north China, Laurentia, and New Guinea) which then lay north of the equator should be a refugium from the UVB effects, and show a different pattern of extinction in the "first strike" of the end-Ordovician extinction, if it were induced by such a radiation event. More information on extinction strength versus latitude will help test this hypothesis.Melott 2

Astrobiological Effects of Gamma-Ray Bursts in the Milky Way Galaxy

“…Furthermore, given that GRBs have collimated emissions, which beam radiation on the order of a kpc, they are expected to pose a significant danger to life on planets in general, and Earth in particular. As such, these events may have been responsible for mass extinction events on the Earth (Thorsett 1995;Melott et al 2004;Thomas et al 2005aThomas et al , 2005bThomas et al , 2015Melott & Thomas 2009;Melott & Bambach 2013).…”

Ordovician–Silurian Lilliput crinoids during the end-Ordovician biotic crisis

“…With the buildup of ice on Gondwana, sea level fell, resulting in a catastrophic loss of marine habitats (e.g., Berry and Boucot 1973;McKerrow 1979;Sheehan 2001). Many hypotheses have been advanced to explain the global changes at this time, including nutrient levels in the ocean, uplift on the continents, rates of silicate and carbonate weathering, reduction of poleward heat transfer in the oceans, orbital eccentricity cycles, gamma ray bursts, volcanic eruptions, among others (e.g., Kump et al 1999;Sutcliffe et al 2000;Sheehan 2001;Herrmann et al 2004a;Melott and Thomas 2009;Young et al 2010). Originally thought to be relatively short (Brenchley et al 1994), this glacial epoch is now recognized to have continued through the Llandovery (Grahn and Caputo 1992;Grahn and Caputo 1994;Brenchley et al 2003;Ghienne 2003;Herrmann et al 2004b;Lefebvre et al 2010).…”

Earliest Silurian graptolites from Kalpin, western Tarim, Xinjiang, China

“…Melott & Thomas 2009, Harper et al 2013, Baarli 2014. Because of their use in biostratigraphy, graptolites from strata near the Ordovician-Silurian boundary have been well studied from many localities worldwide.…”