1. Gamma Ray Burst: Initiator of the Late Ordovician Mass Extinction? Tanya Harrison

4. Late Ordovician Mass Extinction ~440 Million Years Ago ~440 Million Years Ago –Two extinction events ~0.5-2 Myr apart 85% of marine species became extinct 85% of marine species became extinct Coincided with global ice age, 70-100 m drop in sea level, and draw-down of half of the atmospheric CO 2 content Coincided with global ice age, 70-100 m drop in sea level, and draw-down of half of the atmospheric CO 2 content

5. Ice Age Onset Atmospheric O 2 levels about the same as today Atmospheric O 2 levels about the same as today CO 2 levels 8-20 times higher than today CO 2 levels 8-20 times higher than today Began during a time of relative warmth & high climate stability Began during a time of relative warmth & high climate stability

6. Possible External Forcing Mechanism—Gamma Ray Burst High-energy astronomical events High-energy astronomical events –Particle energies ~10 17 —10 20 eV –Avg power output 5*10 44 W –Galactic event rate ~1.5*10 -7 per year –Nearly impulsive energy input on order of ~10 seconds

7. GRB Atmospheric Effects Dissociation & Ionization: Dissociation & Ionization: N+O2  NO+O N+O2  NO+O N+NO  N2+O N+NO  N2+O Ozone Depletion: Ozone Depletion: NO+O3  NO2+O2 NO+O3  NO2+O2 NO2+O  NO+O2 NO2+O  NO+O2 O3+O  O2+O2 O3+O  O2+O2

8. Equatorial Burst Orientation Global atmospheric effects in ~2 weeks Global atmospheric effects in ~2 weeks Max O 3 depletion at equator initially, but greatest at poles within a year Max O 3 depletion at equator initially, but greatest at poles within a year Short-lived O 3 production at pole ~5-6 months after burst Short-lived O 3 production at pole ~5-6 months after burst Immediate max DNA damage @ equator, max at mid to low latitudes within a few months Immediate max DNA damage @ equator, max at mid to low latitudes within a few months

9. Near-Polar Burst Orientation Never fully affects opposite hemisphere Never fully affects opposite hemisphere Max DNA damage at ±30° initially; becomes greatest at mid to low latitudes after a few months Max DNA damage at ±30° initially; becomes greatest at mid to low latitudes after a few months

10. Ordovician GRB Orientation? Recovering fauna were preferentially derived from high-latitude survivors  implies near-equatorial burst Recovering fauna were preferentially derived from high-latitude survivors  implies near-equatorial burst Ocean surface-dwelling trilobites showed greater extinction rates than their deep sea cousins Ocean surface-dwelling trilobites showed greater extinction rates than their deep sea cousins –Explained by GRB, not by ice age alone

11. Nitrate Rainout Avg GRB could produce 0.5 g/m 2 mean global deposition of nitrates Avg GRB could produce 0.5 g/m 2 mean global deposition of nitrates –Would have been beneficial for life’s transition to land, seen after the Ordovician Combination of increased UVB, nitrates, and low pH in shallow water environments could contribute to marine life extinction Combination of increased UVB, nitrates, and low pH in shallow water environments could contribute to marine life extinction

12. Chemical Signatures Radioisotope generation via spallation Radioisotope generation via spallation Nitrate deposition Nitrate deposition High δ 13 C coinciding w/ high δ 18 O High δ 13 C coinciding w/ high δ 18 O Ir or 244 Pu to look for GRB alternatives Ir or 244 Pu to look for GRB alternatives

14. Conclusions GRB explains: GRB explains: –Onset of ice age and extinctions that occurred before the ice age began –Preferential recovery of high-latitude survivors –Preferential extinction of sea surface and shelf-dwelling organisms –Explosion of life onto land after the Ordovician

15. Bibliography Berry, William B.N. Late Ordovician mass extinctions: Causes and consequences—A view from Laurentia. Catastrophic Events Conference, July 2000. Berry, William B.N. Late Ordovician mass extinctions: Causes and consequences—A view from Laurentia. Catastrophic Events Conference, July 2000. Derner, Charles D. et al. Cosmic Rays from Gamma Ray Bursts in the Galaxy. The Astrophysical Journal, Vol. 628, Issue 1, L21-L24, Jul 2005. Derner, Charles D. et al. Cosmic Rays from Gamma Ray Bursts in the Galaxy. The Astrophysical Journal, Vol. 628, Issue 1, L21-L24, Jul 2005. Hecht, Jeff. Gamma rays may have devastated life on Earth. New Scientist, 24 September 2003. Hecht, Jeff. Gamma rays may have devastated life on Earth. New Scientist, 24 September 2003. Kazlev, M. Alan. The Ordovician. Palaeos (παλαιος): www.palaeos.com/Paleozoic/Ordovician/Ordovician.htm Kazlev, M. Alan. The Ordovician. Palaeos (παλαιος): www.palaeos.com/Paleozoic/Ordovician/Ordovician.htm Marshall, Jim. Stable Isotopes: What Can They Tell Us About Oceans, Atmospheres, and Mass Extinction? Asociación Cultural Paleontológica Murciana: www.educarm.es/paleontologia/html/marshall.htm Marshall, Jim. Stable Isotopes: What Can They Tell Us About Oceans, Atmospheres, and Mass Extinction? Asociación Cultural Paleontológica Murciana: www.educarm.es/paleontologia/html/marshall.htm www.educarm.es/paleontologia/html/marshall.htm Melott, A.L. et al. Climatic and biogeochemical effects of a galactic gamma ray burst. Geophysical Research Letters, Vol. 32, 21 July 2005. Melott, A.L. et al. Climatic and biogeochemical effects of a galactic gamma ray burst. Geophysical Research Letters, Vol. 32, 21 July 2005. Melott, A.L. Did a gamma-ray burst initiate the late Ordovician mass extinction? International Journal of Astrobiology, Vol. 3, Issue 1, 55-61, Jan 2004. Melott, A.L. Did a gamma-ray burst initiate the late Ordovician mass extinction? International Journal of Astrobiology, Vol. 3, Issue 1, 55-61, Jan 2004. Sheehan, Peter M. The Late Ordovician Mass Extinction. Annual Review of Earth and Planetary Sciences, Vol. 29, 331-354, May 2001. Sheehan, Peter M. The Late Ordovician Mass Extinction. Annual Review of Earth and Planetary Sciences, Vol. 29, 331-354, May 2001. Thomas, Brian C. et al. Terrestrial Ozone Depletion Due to a Milky Way Gamma-Ray Burst. The Astrophysical Journal, Vol. 622, Issue 2, L153-L156, Apr 2005. Thomas, Brian C. et al. Terrestrial Ozone Depletion Due to a Milky Way Gamma-Ray Burst. The Astrophysical Journal, Vol. 622, Issue 2, L153-L156, Apr 2005. Thomas, Brian C. et al. Gamma-Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects. The Astrophysical Journal, Vol. 634, Issue 1, 509-533, Nov 2005. Thomas, Brian C. et al. Gamma-Ray Bursts and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects. The Astrophysical Journal, Vol. 634, Issue 1, 509-533, Nov 2005.