Monday, September 12, 2011, 12:00PM - 1:00PM
G. Robert Brakenridge
ARC room 620
Early predictions that some supernovae release large quantities of prompt high energy photons are now corroborated by optical identification of core-collapse supernovae associated with extragalactic gamma bursts and x-ray flashes. Given in-galaxy supernova frequency and recurrence statistics, significant Earth-incident events during the past several million years very likely occurred. Such nearby events should have affected terrestrial surface solar UV, the Earth’s climate, and its ecology. The Younger Dryas Stadial, commencing at ~12,900 calendar yr BP, began with sharply cooler temperatures in the Earth’s northern hemisphere, regional drought, paleoecological evidence compatible with increased UV, and abrupt increases in cosmogenic 14C and 10Be in ice and marine cores. In North America,stratigraphic and faunal sequences indicate that a major pulse of mammalian extinctions began very close to 12,830 calendar yr BP and was sudden. A 10 s beamed gamma burst within 2 kpc of the Earth causes spallation and catalytic reactions depleting 35–50% O3, and producing excess NOx species (which favor cooling and drought), 14C and 10Be. An un-beamed, 10^50 erg hard photon impulse at ~250 pc distance produces similar terrestrial atmospheric effects, with recovery times measured in several 10^2 yr. A well-characterized massive star supernova, the unusually close Vela event(d = 250 ± 30 pc; total energy of 1–2 x 10^51 erg; age constrained from remnant nebula shock velocities considerations at ~13,000–16,000 yr) may have initiated the Younger Dryas climate change, and caused the "Rancholabrean" faunal extinctions.