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(PP194) Periodic irradiation from the cosmos over geologic history: potential biological impact.
Welsh, James*,1, 1 University of Wisconsin-Madison, Wisconsin Rapids, WI, USA
ABSTRACT- Purpose/Objective: Throughout Earth's history of approximately 4.6 billion years, there have been intermittent astrophysical phenomena that have resulted in significant ionizing radiation exposure. Such sources include supernova explosions, gamma ray bursts, soft-gamma repeaters, stellar collapses, and powerful solar flares. Additionally, Earth's magnetic poles have sporadically reversed, transiently decreasing the protective magnetic field that shields us from charged particulate solar and galactic cosmic rays. In principle, these episodes of increased radiation dose could have had significant impact on the biosphere, such as mass extinctions or accelerated bursts of evolution. Materials/Methods: Data has been gathered from various sources regarding the frequency and types of episodes that generate substantial quantities of ionizing radiation. Calculations are made about the frequency of events that might be of significance to life on Earth based on the proximity to the planet. Factors such as atmospheric attenuation and shielding by the geomagnetic field are also considered. Geophysical events such as polarity reversals and geomagnetic excursions are also analyzed for their potential to affect the biosphere via radiation dose at sea level. Data on the timing of geomagnetic reversals was compared to fossil record over the past 150 million years. Results: Gamma ray bursts from anywhere in the universe are detected by orbiting observatories at a rate of almost once per day but only a small fraction are close enough to deliver dose at sea level. Supernovae occur in our galaxy roughly once or twice per century. Together, these sources are estimated to result in sea level doses of 1 Gy every 5-10 million years depending on the model of atmospheric attenuation used. Very high doses from gamma ray bursts within our own galaxy, potentially capable of producing mass extinctions might occur once in 2 x 107 to 1 x 109 years. Neutrino doses from stellar collapses and supernovae could be biologically significant due to the high LET/RBE recoil products generated by nuclear interactions but the cross-section for interaction is extremely small and uncertain, thus the biological impact remains unclear. Solar and galactic cosmic rays are generally deflected by the geomagnetic field but sporadic polarity reversals may be accompanied by a decrease in field strength by an order of magnitude and persist for hundreds of years, potentially resulting in significant increases in high-LET cosmic ray bombardment at sea level. A review of the fossil record and the 5 major biological extinction episodes does not show an obvious correlation with geomagnetic reversals. Conclusions: Throughout geologic history there have been numerous episodes of increased ionizing radiation dose to the biosphere due to episodic astronomical and geophysical phenomena. Precise estimates of absorbed doses are uncertain due to uncertainty surrounding the attenuation characteristics of the atmosphere. Experiments are being designed to better answer this question. The possible biological consequences of this intermittent exposure such as mass extinctions or increased rates of speciation is a fascinating subject of speculation but the present data do not support an obvious association with geomagnetic reversals.
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