Mesozoic/Cenozoic climate
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?