Transcript Greenhouse Earth

Greenhouse Earth
The Greenhouse Era 100 Myr Ago
The 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 Deposits
Cretaceous Chalk Outcrop
in Denmark
Cretaceous Limestone Outcrop
in Virginia, USA
Cretaceous 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:
Reptiles
Champsosaur
Champsosaur
Turtles
Fossils of Warm Adapted Animals in the Arctic:
Dinosaurs
Fossils of Tropical Vegetation
in the Arctic:
• Fossil leaf of a tropical
breadfruit tree found in
the Arctic
Antarctic Dinosaurs
Small theropod (6 – 8 ft. tall) related to
the raptors and tyrannosaurids
Antarctic Dinosaurs
Fossil pelvis of a small (6 – 8 ft tall
and 30 ft. long) sauropod compared to
its giant relative shown below
Brachiosaurus
Fossil 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 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?
• The simplest explanation is CO2 levels
that were up to 10 times present-day
levels.
Ocean Heat Transport Hypothesis
• Changes
in the amount of heat transported toward polar
regions by the ocean
• Cause changes in climate
• 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
Warm Saline Deep Water Filled Ocean
Basins 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 Climate
• Over tectonic time scales, average sea
level has risen and fallen by several
hundred meters
– Transgressions:
– Regressions:
Rise in sea level
Fall in sea level
• Small in comparison to the >4000 m ocean
depth
– Can have significant effects on climate
Continental 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)
• 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 Level
• Ocean floods the continental shelf to a depth of
200 m or more
• Sediment is deposited on the submerged shelf
Eustatic 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
Sea Level Changes
• Changes in the volume of ocean ridges
• Continental Collisions
• Construction of Volcanic Submarine
Plateaus
1. Changes in the Volume of
Mid-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 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 Age
• Profiles (and volumes) of past ocean ridges can be
reconstructed by using a relationship observed in the
modern ocean basins.
Ridge Depth = 2500 m + 350 (crustal age)1/2
(in meters)
(at 0 age)
(in Myr)
– 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
Fast 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 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
the 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 Continents
• Continents “float” on the denser asthenosphere
– Lower density granitic bedrock
Collision 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 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
Plateaus 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 Level
• Water Stored in Ice Sheets
• Thermal Contraction of Seawater
Glacial 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 . . .
• 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 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
Factors
• 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
Climate Changes
Higher 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 Level
• More land is exposed
• Seasonally climate variations are more
extreme
Glaciation
• 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 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 Impacts
Frequency 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 Meteorite
Impacted 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 Impact
1 second before impact
60 seconds after impact
5 seconds after impact
1000 years after impact
A 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 Foraminifera
• Extinction of all but one of 25 species
Ammonite Cephalopds
All became extinct
Evidence of the Impact:
Iridium 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
Boundary Clay
• Closeup view of the boundary clay in the
Raton Basin, New Mexico
Chixulub Meteorite Impact Crater
• Centered on the
Yucatán Peninsula of
Mexico
• 180-km diameter
structure
• Lies beneath layers
of sedimentary rock
• Appears to be the
right age
Evidence for an Impact
Crater
Impact Crater Structure
Barringer (“Meteor”) Crater, AZ
Lunar Impact Crater
Density Measurements of Rocks
• Patterns of low
density pulverized
rock
• High density rock
• Consistent with an
impact crater
Shocked 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.
Tektites
• Small pieces of rock that were melted during the
proposed impact and hurled into the atmosphere
• Many are found in Antarctica
Deposits of Huge Waves
Catastrophic 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 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