Greenhouse Earth

The Greenhouse Era 100 Myr AgoThe Greenhouse Era 100 Myr AgoThe Cretaceous Period of the Mesozoic EraThe Cretaceous Period of the Mesozoic Era

• Global Sea Level – 200 m higher than today

• Shallow seas flooded continental interiors

• Cretaceous is from the Latin word creta which means chalk

Abundant Limestone DepositsAbundant Limestone Deposits

Cretaceous Chalk Outcrop in Denmark

Cretaceous Limestone Outcropin Virginia, USA

Cretaceous GeographyCretaceous Geography

• Continental flooding reduced the amount of surface land.• No evidence of any permanent ice on Earth, even at the

poles.

Fossils of Warm Adapted Animals in the Arctic:Fossils of Warm Adapted Animals in the Arctic:

ReptilesReptiles Champsosaur Champsosaur

Turtles

Fossils of Warm Adapted Animals in the Arctic:Fossils of Warm Adapted Animals in the Arctic:DinosaursDinosaurs

Fossils of Tropical Vegetation Fossils of Tropical Vegetation in the Arctic:in the Arctic:

• Fossil leaf of a tropical breadfruit tree found in the Arctic

Antarctic DinosaursAntarctic Dinosaurs

Small theropod (6 – 8 ft. tall) related tothe raptors and tyrannosaurids

Antarctic DinosaursAntarctic Dinosaurs

Fossil pelvis of a small (6 – 8 ft talland 30 ft. long) sauropod compared to its

giant relative shown below

Brachiosaurus

Fossil EvidenceFossil Evidence

• Fossils of tropical plants and animals– North of the Arctic Circle– South of the Antarctic Circle– Indicate a much warmer Earth

• Tropical corals extended up to 10 degrees higher in latitude than they do at the present.

Cretaceous Polar RegionsCretaceous Polar Regions

• 20o C to 40o C warmer than present-day• The Antarctic today is very cold because

– High elevation: Lapse-rate cooling with increased elevation– High albedo of the ice which reflects insolation

Why Were the Poles So Warm?Why Were the Poles So Warm?

• The simplest explanation is CO2 levels that were up to 10 times present-day levels.

Ocean Heat TransportOcean Heat Transport HypothesisHypothesis

• Today– Deep ocean is filled

with cold dense water• Sinks in polar regions

– A small amount of intermediate depth Atlantic water

• Comes from sinking of salty Mediterranean Sea water that flows into the Atlantic

• Warmer water is denser due to high salinity it gains by strong evaporation

• Changes in the amount of heat transported toward polar regions by the ocean• Cause changes in climate

Warm Saline Deep Water Filled Ocean Warm Saline Deep Water Filled Ocean Basins 100 Myr AgoBasins 100 Myr Ago

• Formed in tropics or subtropics

• Dense due to evaporation• Sank deeper than any

water near warmer poles• Configuration of continents

could have caused this– Large seaway near

northern branch of the Hadley Cell’s descending air

– Dry conditions and evaporation

• Warm salty bottom water could have- Contributed to poleward heat flux to warm the poles- Reduced the large temperature and density contrast between the surface and bottom ocean which would caused faster overturning and more poleward heat flow.

Sea Level Changes and ClimateSea Level Changes and Climate

• Over tectonic time scales, average sea level has risen and fallen by several hundred meters– Transgressions: Rise in sea level– Regressions: Fall in sea level

• Small in comparison to the >4000 m ocean depth– Can have significant effects on climate

Continental MarginsContinental Margins

• Relatively flat– Sea Level Changes of 1:1000 (vertical to

horizontal)• A sea level increase of 1 meter could extend as far

inland as 1 km (1000 m)

– The coastline can move 100’s of km

Low Sea Level (Present-Day)Low Sea Level (Present-Day)

• Coastline is near the break between the continental shelf and the steeper continental slope

• Erosion is dominant on continental margins• Sediment is carried out to the slope and into the deep

ocean

High Sea LevelHigh Sea Level

• Ocean floods the continental shelf to a depth of 200 m or more

• Sediment is deposited on the submerged shelf

Eustatic Sea Level ChangesEustatic Sea Level Changes

• Eustatic refers to global or world-wide

• Evidence is marine sediments of the same age– Deposited on several

continents– At levels well above

present-day sea level.

Tectonic Causes of Eustatic Tectonic Causes of Eustatic Sea Level ChangesSea Level Changes

• Changes in the volume of ocean ridges

• Continental Collisions

• Construction of Volcanic Submarine Plateaus

1. Changes in the Volume of 1. Changes in the Volume of Mid-Ocean RidgesMid-Ocean Ridges

• High elevation from heating from below.

• Submarine lava flows• Expansion of rocks

due to extreme heat• Causes surface of the

ocean to rise

Ridges Subside with TimeRidges Subside with Time

• Initially anomalously high heat flow causes ridges to stand high above the seafloor

• As rock moves away from the crest– Rapid subsidence due to heat loss

• As rock continues to move along the ridge flanks– Rate of heat loss is more gradual

• After 60 Myr– All excess heat is lost– Ridge elevations are a stable depth of 5500 m (average value)

Ridge Depths are Constant with AgeRidge Depths are Constant with Age

• Profiles (and volumes) of past ocean ridges can be reconstructed by using a relationship observed in the modern ocean basins.

– The average depth of all ridges today is 2500 m below the surface

• This is used as the initial depth (0 age)– Age of the crust is obtained from paleomagnetic data

• The ridge crest starts at 2500 depth and gradually deepens with age and increasing distance from the ridge– Rock cools and contracts

Ridge Depth = 2500 m + 350 (crustal age)1/2

(in meters) (at 0 age) (in Myr)

Fast Spreading RatesFast Spreading Rates

• Ridges stand higher– Less time to cool and contract

• Produce a wider, high elevation (“fat”) profile• Reduces the volume of ocean basins displacing water

onto the continents • Transgressions occur resulting in eustatic sea level

changes.

Slow Spreading RatesSlow Spreading Rates

• More time for crustal rocks to cool and contract• A narrower (“thin”) profile results

– Ocean basins are deeper– Less water is displaced

• Regressions occur resulting in eustatic seal level change

Sea Level During Sea Level During the Cretaceous Periodthe Cretaceous Period

• Average spreading rates were at least 50% faster than today

• Evidence of marine transgressions

• Ocean basins had a lower capacity

• Sea level was 200 to 300 m higher than today

2. Collisions of Continents2. Collisions of Continents

• Continents “float” on the denser asthenosphere– Lower density granitic bedrock

Collision Thickens the CrustCollision Thickens the Crust

• A high plateau is formed– Faulting shears off slivers of rock and stacks them on top of

each other• A subsurface low-density root forms down to 60 or 70 km• The crust is double its normal 30 km thickness

– Net area of continental crust is lost– Increases the area of the ocean basins

• Sea level falls

India is Colliding with southern AsiaIndia is Colliding with southern Asia

• The only continental collision within the last 100 Myr• First made contact 55 Myr ago• Still in progress

– Increased area of the ocean by 2 million km2

– Sea level is about 40m lower than 100 Myr ago when no collisions were occurring

3. Construction of Volcanic 3. Construction of Volcanic Plateaus in the OceanPlateaus in the Ocean

• A large region of 110 to 80 Myr old plateaus is buried beneath a thin layer of marine sediment in the tropical Pacific Ocean

• Initially high above the nearby seafloor– Heat from volcanic origin

• Today they are cooler and at lower levels• 80 Myr ago they likely displaced more sea water

– Sea level was probably 40 m high than today

Climate Factors and Sea LevelClimate Factors and Sea Level

• Water Stored in Ice Sheets

• Thermal Contraction of Seawater

Glacial IceGlacial Ice

• Huge amounts of water stored on land– Sea Level is lower during

ice ages

• Water returns to the sea during interglacial periods– During the last ice age sea

level was about 300 m lower than today

Sea Level Today . . .Sea Level Today . . .

• Antarctica’s Ice– Holds enough water to raise sea level 66 meters

• Greenland’s Ice– Holds enough water to raise sea level 6 m

• If all the ice on these continents melted, there would be a eustatic sea level increase of 72 meters

Thermal Contraction of SeawaterThermal Contraction of Seawater

• Warmer water expands– Oceans 80 to 100 Myr ago would have

occupied more space than the cooler ocean water today

• Cooling of the oceans– Low latitude ocean has cooled slightly in the

last 100 Myr– High Latitude Ocean surface and the deep

ocean have cooled by 10o C to 15o C– Sea level has dropped 7 meters

Summary of Sea Level Change Summary of Sea Level Change FactorsFactors

• When all factors are combined, a sea level decrease of 300 to 440 m is estimated

• Rates of sediment deposition indicates sea level should only be 100 to 300 m lower

• This discrepancy (“mismatch”) can be explained by uncertainties in– Past spreading rates– Volcanic plateaus– Sedimentation rates– Other factors

Sea Level Change and Sea Level Change and Climate ChangesClimate Changes

Higher Sea LevelsHigher Sea Levels

• Flooding of– Continental margins– Interior seaways

• Continental extremes of climate would be moderated (water heats up and cools slower than land)– Milder, more maritime winters– Cooler summers

Falling Sea LevelFalling Sea Level

• More land is exposed

• Seasonally climate variations are more extreme

GlaciationGlaciation• Summer melting is a major control on the

amount of glacial ice on Earth– Cooler summers result in less melting

• Causes greater climate cooling and glaciation– (a higher albedo)

• As a result, with higher sea level the moderating effect of water on climate should cause cooler summers with more glaciation– There should be less glaciation with today’s

lower sea level

Glaciation MismatchGlaciation Mismatch• The record of the last 100 Myr shows just

the opposite trend.– High sea levels of 100 Myr ago should have

aided glaciation, but none occurred– Low sea levels of today should oppose

glaciation, but we have ice sheets

• This mismatch indicates that we should assume that sea level change does NOT control long-term climate change.

Asteroid ImpactsAsteroid Impacts

Frequency of Earth ImpactsFrequency of Earth Impacts

• Inverse relationship between size and frequency of impacts– Largest (>10 km): 50 to 100 Myr frequency

• Great environmental impacts (e.g., “extinction events”)

– Smaller objects impact much more frequently

10 km Diameter Meteorite10 km Diameter MeteoriteImpacted 65 Myr AgoImpacted 65 Myr Ago

• Impacted at 20 km per second (72,000 km/hr or 44,712 mi/hr)

• Created an immense shock wave that traveled outward at the same speed.

• Seismic waves equivalent to an earthquake 100 to 1000 times stronger than the strongest recorded earthquakes.

Chixulub ImpactChixulub Impact1 second before impact 5 seconds after impact

60 seconds after impact 1000 years after impact

A Global Extinction EventA Global Extinction Event

• Many paleontologists believe that this was a major cause of the non-avian dinosaur extinction.• But, not only the dinosaurs suffered extinction.• 70% of the living species and 40% of genera became extinct

Planktonic ForaminiferaPlanktonic Foraminifera

• Extinction of all but one of 25 species

Ammonite CephalopdsAmmonite Cephalopds

All became extinct

Evidence of the Impact: Evidence of the Impact: Iridium AnomalyIridium Anomaly

• World-wide distribution of clay containing the element iridium– Ir is rare in crustal

rocks– It is found in much

higher concentrations in some meteorites

Ocean sediments with a layer of enriched Ir

• Closeup view of the boundary clay in the Raton Basin, New Mexico

Boundary ClayBoundary Clay

• Centered on the Yucatán Peninsula of Mexico

• 180-km diameter structure

• Lies beneath layers of sedimentary rock

• Appears to be the right age

Chixulub Meteorite Impact CraterChixulub Meteorite Impact Crater

Evidence for an Impact Evidence for an Impact CraterCrater

Impact Crater StructureImpact Crater Structure

Barringer (“Meteor”) Crater, AZ Lunar Impact Crater

Density Measurements of RocksDensity Measurements of Rocks

• Patterns of low density pulverized rock

• High density rock• Consistent with an

impact crater

Shocked QuartzShocked Quartz

• No, shocked quartz isn't a pyschologically-distressed rock. • It is actually a structurally-altered form of quartz that • Created by a sudden application of extremely high

pressure.

TektitesTektites

• Small pieces of rock that were melted during the proposed impact and hurled into the atmosphere

• Many are found in Antarctica

Deposits of Huge WavesDeposits of Huge Waves

Catastrophic ResultsCatastrophic Results

• Sunlight greatly diminished• Earth's surface temperatures

were drastically reduced. • Sulfuric acid (H2SO4) and nitric

acid (HNO3) resulted from vaporized rock and atmospheric gases– Both would have contributed

to strongly acid rain that might have had devastating effects on vegetation and marine organisms

Climatic and Environmental EffectsClimatic and Environmental Effects

• Over longer periods (decades to centuries) the abrupt injection of carbon biomass into the atmosphere by burning:– Increased CO2 levels– Lead to global warming

• Large impacts are likely not long-term climate changers