Chapter 17 Outline

GEOLOGIC RECORD OF THE LATEST CRETACEOUS

• TECTONIC ACTIVITY
  • RAPID SEAFLOOR SPREADING & SUBDUCTION IN THE PACIFIC OCEAN
    • Large amounts of granite around the rim of the Pacific
  • EXTENSIVE FLOOD BASALTS IN INDIA - DECCAN TRAPS
  • CONTINENTAL CONFIGURATION SIMILAR TO TODAY
    • Asia & North America connected
    • South America, Antarctica & Australia connected
    • Africa & India isolated continents
• OCEANS
  • SEAFLOOR SPREADING RATES ACTUALLY DECREASED FROM A MAXIMUM ABOUT 85 MILLION YEARS AGO TO THE END OF THE CRETACEOUS
    • Progressive shrinkage of inland seaways & regression, exposing more of the continents
• CLIMATE
  • GENERAL COOLING, ALTHOUGH STILL WARM
  • SEASONAL TEMPERATURE FLUCTUATION IN MID-PALEOLATITUDES WAS ~8^oC
• ASTEROID IMPACT
  • IRIDIUM ANOMALIES AT K/T BOUNDARY - ASTEROIDS CONTAIN 10,000 TIMES MORE IR THEN CRUSTAL ROCKS
    • Iridium anomaliy first found in Gubbio, Italy by Walter Alvarez
    • Well over 100 sites worldwide have been found to have Ir anomalies
    • Occurs on land & in the ocean
  • SHOCKED QUARTZ ALSO FOUND AT K/T BOUNDARY - FROM INTENSE PRESSURES OF IMPACT
  • MICROTEKTITES ALSO FOUND AT K/T BOUNDARY - FROM INTENSE TEMPERATURES OF IMPACT MELTING TARGET & ASTEROID MATERIAL, WHICH ARE THROWN INTO THE ATMOSPHERE, WHERE THEY COOL
  • SOOT-RICH HORIZONS FOUND AT SEVERAL K/T BOUNDARY SITES
    • Resulted from global wildfires ignited by the intense heat of impact
  • IMPACT SITE BURIED BENEATH NW YUCATAN PENINSULA OF MEXICO - CHICXULUB CRATER
    • ~180 km in diameter
    • Evaporite deposits (gypsum = calcium sulfate) underlie the region
    • Caused massive tsunami ("tidal wave") that resulted in chaotic deposits along the Gulf Coast region of Texas & Mexico and in Haiti, which also acquired a 1-meter thick bed of glass formed of melted evaporites & continental crustal material
  • EFFECTS
    • Blockage of sunlight for ~3 months from material blasted into the atmosphere
    • Short-term (~1 month) global warming from the heat energy generated by impact
    • Long-term (several years) global cooling from material blasted into the atmosphere
    • Long-term (several years) global warming from shutdown of organic productivity & release of carbon dioxide into the atmosphere
    • Acid rain by reaction of nitrous oxide produced by the impact with water to form nitric acid & production of sulfuric acid from the evaporites at Chicxulub
      • ALTHOUGH PROBABLY NOT RESPONSIBLE FOR MASS EXTINCTION, COULD EXPLAIN ABSENCE OF DINOSAUR BONES NEAR THE K/T BOUNDARY (THEY WERE DISSOLVED)
• VOLCANISM
  • C.B. OFFICER OF DARTMOUTH HAS CHAMPIONED THE IDEA THAT MANY OF THE DATA CITED AS SUPPORT FOR AN ASTEROID IMPACT COULD ALSO BE CAUSED BY VOLCANISM
    • Ir enrichment was found in Hawaiian lava, from hotspot activity originating deep within the earth, suggesting the Deccan Trap eruptions might be responsible for the Ir anomaly at the K/T boundary
    • Shocked quartz can also originate volcanically
    • Furthermore, no impact crater had been found initially
  • FEW GEOLOGISTS ARE CONVINCED BY OFFICER'S ARGUMENTS; MOST CONCUR THAT AN ASTEROID IMPACT OCCURRED AT THE K/T BOUNDARY
    • The Deccan Traps were flood basalts, not the kind of explosive volcanism necessary to distribute Ir globally
    • Shocked quartz produced by impact is different than that produced by volcanism

BIOLOGIC RECORD OF THE LATEST CRETACEOUS

• STRATIGRAPHIC NOTE
  • IR ANOMALY IS A GREAT STRATIGRAPHIC MARKER
• OCEANS
  • EPEIRIC SEAS & CONTINENTAL SHELVES
    • Teleost marine fish (most modern fish are teleost) apparently suffered fewer extinctions than other groups
    • Plesiosaurs & ichthyosaurs declined & went extinct at or before the K/T boundary
    • Mosasaurs thrived throughout the Late Cretaceous & went extinct at the K/T boundary
    • Ammonite mollusks went extinct at the K/T boundary
    • Bivalved mollusks suffered many species extinctions within the last 10 million years of the Cretaceous
  • PLANKTONIC FORAMINIFERA SEEM TO HAVE GONE EXTINCT ABRUPTLY & OCEAN PRODUCTIVITY SUFFERED A RAPID AND COMPLETE BREAKDOWN AT THE K/T BOUNDARY
• TERRESTRIAL RECORD
  • PLANTS
    • Pollen record of Montana, North Dakota & Wyoming shows Cretaceous species go extinct precisely at the Ir anomaly, followed by a spike of fern pollen and then a return of flowering seed plants characteristic of the Paleocene; that of southern & central Canada indicates a series of extinction events before, during & after the K/T boundary
    • Megafloral record in the U.S. indicates a major & significant extinction precisely at the K/T boundary, while that of New Zealand indicates little change
  • VERTEBRATES
    • Differential survivorship - ~90% of aquatic species but only ~10% of terrestrial species survived in Montana
    • Non-avian dinosaurs appear to have gone extinct abruptly at the K/T boundary based on limited data from the Western Interior of the U.S.

EXTINCTION HYPOTHESES

• MUST BE TESTABLE & MUST EXPLAIN MORE THAN DINOSAUR EXTINCTION
  • FASTOVSKY AND WEISHAMPEL FEEL THAT ONLY THE ASTEROID IMPACT MEETS THESE CRITERIA
  • EXTINCTION OCCURRED BECAUSE OF COLLAPSE OF PRIMARY PRODUCTIVITY IN THE OCEANS & ON LAND - ORGANISMS DEPENDENT DIRECTLY ON PRIMARY PRODUCERS SUFFERED GREATER EXTINCTION RATES THAN THOSE ORGANISMS THAT ALSO RELIED ON DETRITUS
    

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