MESOZOIC CLIMATE

• GENERALLY WARM
  • Continents strattle the equator & none are at the poles
  • Equatorial currents nearly circle the world (flow west into Tethys)
    • Complete circum-equatorial ocean circulation through Tethys & North Atlantic (after separation of Pangea into Laurasia & Gondwana)
• VERY DRY IN THE TRIASSIC & THE JURASSIC; WETTER IN THE CRETACEOUS
  • Pangea is so big that moisture cannot reach the interior
  • Mountain ranges resulting from formation of Pangea create numerous rain shadows
  • By Cretaceous time, the continents have dispersed significantly & sealevel is high from fast spreading along the MOR

CENOZOIC CLIMATE

• CHARACTERIZED BY AN OVERALL COOLING THAT CULMINATED WITH QUATERNARY ICE AGE
  • Temperature of bottom waters got colder (15 degrees C to -1 degrees C)
  • Temperature of tropical surface waters remains warm (25 degrees C)
  • No continental glacial ice sheets at the end of the Cretaceous & the beginning of the Cenozoic
  • Antarctica & Greenland permanently ice-covered in the Quaternary
    • Continental glacial ice sheets periodically cover large parts of North America & Eurasia
• RELATED TO CONTINENTAL DRIFT & DECREASING SEALEVEL FROM DECREASING SEAFLOOR SPREADING RATES ALONG THE MOR
  • Early Eocene (55 my ago) - opening of the northernmost Atlantic allows cold deep water from the Arctic Basin to flow south down the Atlantic
  • Late Eocene (45 my ago) - separation of Antarctica & Australia results in thermal isolation of Antarctica by Circumpolar Current
  • Late Eocene & Early Oligocene (~40-35 my ago) - significant glaciation in Antarctica begins
  • Oligocene & Early Miocene (35-15 my ago) - glaciers sporadically present in Antarctica
  • Late Miocene (15 my ago) - permanent ice in Antarctica & sporadically present in northern hemisphere
    • Documented by tills drilled beneath ice & continental shelf of Antarctica & by tills in Alaska, ice rafted sediment in deep sea off Antarctica, & oxygen isotope record of benthic foraminifers
    • Extremely cold (-1 degrees C) bottom water formed beneath glacial ice shelves adjacent to Antarctica
  • Late Pliocene (2.4 my ago) - permanent ice in Greenland
    • Latest Miocene (5 my ago) - uplift of Isthmus of Panama cut off circum-equatorial ocean circulation & epeirogenic uplift of Rockies deflects moisture-laden Gulf air northward
      • Uplift of tibet plateau probably had impact as well
    • Documented by ice rafted sediments in north Atlantic & oxygen isotope record of benthic foraminifera
• CHRONOLOGY OF QUATERNARY GLACIAL ADVANCES & RETREATS
  • Terrestrial glacial deposits record 4 advances & retreats of ice in North America & Eurasia during the last 1 my
  • Marine oxygen isotope record of benthonic foraminifera, temperature reconstructions based on planktonic foraminifera, radiolaria, diatioms & coccoliths, & coiling directions of planktonic foraminifera record more than 30 advances & retre ats of ice in North America & Eurasia in the last 2.4 my (15 advances & retreats in the last 1 my)
    • Deep sea record is more complete & is not eroded
    • Terrestrial record is partly erased with each glacial advance
  • Glacial to interglacial alternations are apparently caused by variations in earth's orbital parameters that vary amount of sunlight received at critical locations on earth's surface
    • Axial tilt - tilt of earth's rotational axis varies between 22 degrees & 24 degrees over a period of 41 ky
    • Precession - rotational axis precesses with respect to stars over a period of 26 ky (north pole now points to Polaris, but in 13 ky will point to Vega)
    • Eccentricity - small changes in shape of earth's slightly elliptical orbit occur over a period of 100 ky
    • Glacials occur when northern hemisphere summers are cool (although winters are relatively warm) because snow has to last through the summer
      • Results when tilt is small (22 degrees), precession has the northern hemisphere summer occurring when the earth is farthest from the sun, & eccentricity is maximum
• THE CO2 CONTENT OF THE EARTH'S ATMOSPHERE ALSO PLAYS A ROLE
  • CO2 is a greenhouse gas & traps heat in the atmosphere
  • The CO2 content of air trapped in bubbles in Greenland & Antarctica ice cores is low (200 ppm) during glacials & high (290 ppm) during interglacials
  • Air trapped in bubbles in 80 my old amber (fossil pine pitch) has high (700 ppm) CO2 content
    • The Cretaceous was very warm
    • High Cretaceous CO2 content probably resulted from 1) volcanic eruptions associated with fast spreading rates along the MOR & with vast outporings of basalt in India (Deccan Traps), & 2) less CO2 dissolved in warmer Cretaceous ocean water
  • What do you suppose the effect of increased CO2 content of the present atmosphere (350 ppm & rising) resulting from human activities (burning of fossil fuels, deforestation) will have on the earth's climate?