Mesozoic/Cenozoic Climate
Chapters 14 (pp. 413-416; 448), 15 (pp. 482-488; 504), & 16 (508-527; 538-543)

MESOZOIC CLIMATE

  • GENERALLY WARM (see figures 14.34 & 14.36)
  • 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

  • CHARACTERIED BY AN OVERALL COOLING THAT CULMINATED WITH QUATERNARY ICE AGE (see figures 15.37 & 15.38)
  • Temperature of bottom waters got colder (15°C to -1°C)
  • Temperature of tropical surface waters remains warm (25°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 (see figure 15.39)
  • 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 Antarctic & 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°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 & retreats 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
  • AXIAL TILT - TILT OF EARTH'S ROTATIONAL AXIS VARIES BETWEEN 22° & 24° 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°), precession has the northern hemisphere summer occurring when the Earth is farthest from the Sun, & eccentricity is maximum
  • THE CO2 CONTENT OF THEEARTH'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 (360 ppm & rising) resulting from human activities (burning of fossil fuels, deforestation) will have on the Earth's climate?

    • Study Questions

    1. What are the general characteristics of Triassic-Jurassic & Cretaceous climate & the causes for any differences between these Mesozoic periods?

    2. What are the general characteristics of Cenozoic climate & the differences between the Mesozoic & Cenozoic?

    3. Summarize the tectonic events that had a major effect on Cenozoic climate.

    4. Summarize the general characteristics of the Cenozoic cooling.

    5. What were the times that Antarctica & North America first became sporadically, & then permanently, ice covered?

    6. Summarize the general characteristics of Quaternary glacial advances & retreats.

    7. What are the differences between the terrestrial & marine records of Quaternary climate?

    8. Summarize the effect of variations in the Earth's orbital parameters, their periods, & the ideal situation for glacial advances.

    9. What has been the role of Earth's atmospheric CO2 content in the Quaternary ice age & what is the evidence for this role?

     

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