Transcript Plate Tectonics and Climate Change

Plate Tectonics and Climate
Glaciation on Continents –
The Polar Position Hypothesis
• Two Key Testable Predictions
– When continents are near the poles they
should have ice sheets
– If no continents are near the poles no ice
sheets should appear on Earth
• Does not consider world-wide climate
changes
– Only considers positions of the continents
Seafloor Spreading Has Moved Continents
• During the past 500 Myr continents have changed position between
– Warm low latitudes
– Colder higher latitudes
• If latitude alone is the controlling factor, these movements should have
produced predictable glaciations
Three “Icehouse Eras” During the
Last 500 Myrs
South Pole Positions Correlate to
Periods of Glaciation
• Changes in the
position of the pole
– Slow movement of
Gondwana across a
stationary pole
• 430 Myr ago
– S. Pole position
consistent with
glaciation in the
Sahara
South Pole Positions Correlate to
Periods of Glaciation
• From 325 to 240 Myr
ago
– Gondwana continues
to move across the
South Pole
– A huge region on the
southern continent
was glaciated
South Pole Positions Correlate to
Periods of Glaciation
• Gondwana’s
glaciation ended
about 240 Myr ago
– It moved away from
the pole and merged
with northern
continents forming
Pangaea
The Polar Positions Hypothesis:
Some inconsistencies
• The first southern glaciation (430 Myr ago)
– Brief in terms of geologic time
– 1 to 10 Myr in duration
• The slow motion of Gondwana across the
South Pole doesn’t easily explain a brief
period of glaciation
Lack of Ice Sheets on Land over the
South Pole
• Land existed at the South Pole for almost 100
Myr without glaciation
• This argues against the hypothesis being the
only requirement for large-scale glaciations.
Lack of Ice Sheets on Land over the
South Pole
• After the breakup of Pangaea Antarctica, India, and
Australia moved back over the South Pole.
• No ice developed
– Antarctica remained directly over the pole from 125 Myr ago to
almost 35 Myr ago, but free from ice.
• Again, this argues against the hypothesis being the only
requirement for large-scale glaciations.
Pangaea’s Climate
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Extended from high northern latitudes to high southern latitudes
Almost symmetrical about the equator
Wedge-shaped tropical seaway indented the continent from the east
Represented almost 1/3 of Earth’s surface. It spanned:
– 180o of longitude at it’s northern and southern limits, both near 70o latitude
– ¼ of Earth’s circumference at the equator
Climate Models Input . . .
• Sea Level
– Rock evidence indicates S.L. comparable to
today’s
• Topography
– To minimize errors caused by incorrect guess
as to the distribution of mountains
• Interior land represented as a low-elevation
plateau with a uniform height of 1000 m and
gradually sloping towards the sea along the
continental margins
Climate Models Input . . .
• Higher CO2 level than today
– Compensates for a weaker Sun (about 1%)
– This is because geologic evidence indicates a warmer
Earth
• Absence of polar ice
– Fossil vegetation
– Palm-like trees at latitudes as high as 40o were
not killed by hard freezes on Pangaea
• Indicates that the hard freeze limit was at a
higher latitude than today’s limit of 30o to
40o
Precipitation on Pangaea
• Arid low latitudes,
especially in the
continental interior
• Large land area under the
dry, descending portion of
the Hadley Cell
• Large expanse of land in
the tropics
– Trade winds lose moisture
by the time they reach the
continental interior
Supported by Evaporite Deposits
• Mesozoic Rifting
• Opens the Atlantic
• Evaporites in
shallow basins
• Salts precipitated in lakes or in coastal
margin basins
• Limited exchanges of water with the ocean
• Requires an arid climate
• More evaporates precipitated during the later phases of Pangaea
than during any time in the last several hundred million years
Temperatures on Pangaea
Patterns switch back and forth
between hemispheres with
changes in the seasons.
• Continental interior
– Season extremes of heating in summer and cooling in winter
• May explain lack of ice sheets in high latitudes because summers
were so warm that rapid summer melting prevented the build-up of
snow.
• Freezing average daily winter temperatures extended to
40o latitude
Monsoons on Pangaea
• Strong reversal between summer and winter
monsoon circulations
• Winter Hemisphere has high
pressure over the interior of
the continent
- Weak insolation and high
radiative cooling
- Air sinks building high
pressure
- Air flows out towards the
ocean
• Summer Hemisphere has
strong solar heating
- Air rises and a strong low
pressure cell develops.
- Causes a net inflow of humid
air
Monson Circulation and Seasonal
Precipitation
• Eastern margins from 0o to 45o latitude
– Winds reverse directions between seasons
• Extremely wet summers
• Dry winters
Geologic Evidence – Red Beds
Permian – U.K.
Triassic - CA
L. Permian, Triassic
Palo Duro Canyon,TX
• Sedimentary rocks stained red by oxidation
– Wet season provides the moisture
– Rust forms in the dry season or interval
– Red beds are widespread on Pangaea and is consistent with the
model of high seasonal changes in moisture
Effect of Pangaea’s Breakup on Climate
• Northern Hemisphere continents moved farther
northward
– High latitude ocean water displaced
– Steeper global temperature gradient resulted
Change in Oceanic Circulation
• A single ocean
(Panthalassa) with a
single continent
– Simple pattern
• Separate continents
– More complex
circulation
– Affects atmospheric
circulation
– Warm an cold currents
– Conveyer
The BLAG Spreading Rate
Hypothesis
• Also known as the Spreading Rate Hypothesis
• Proposes that climate changes in the last several
hundred million years:
– Caused mostly by changes in the rate of CO2 input to the
atmosphere
– CO2 input driven by plate tectonic processes
• Named using initials of its authors
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Robert Berna
Antonio Lasaga
And . . .
Robert Garrels
CO2 Released into the Atmosphere
by Plate Tectonics
• Most CO2 is released
– At Mid Ocean ridges
– By Subduction Volcanoes
CO2 Released into the Atmosphere
by Plate Tectonics
• A smaller input of CO2 is released at hot spots
– Most are not associated with plate boundaries
Distribution of Hot Spots
• Identified by volcanic activity and structural uplift
within the last few million years
Rate of Seafloor Movement Controls
Delivery of CO2 from Rocks into the Air
• Rates of plate motion presently varies from plate to plate
• South Pacific spreads up to 10X faster than the Mid-Atlantic Ridge
Age of the Seafloor
• Magnetic data shows widely varying rates over millions of years
• Continue to change
Fast Spreading
• Larger releases of CO2 to the ocean
• Results in faster subduction
– Larger volumes of carbon-bearing sediment and rock melt
Increased CO2 Causes an Initial Shift
Towards a Greenhouse Climate
• Activates increased chemical weathering
– combined effect of temperature, precipitation, and vegetation
• CO2 drawn out of atmosphere at a faster rate
• Negative Feedback
Slow Plate Movement
• Slow CO2 input results in cooling
A Colder Icehouse Climate
• Decreased chemical weathering
– Decreased removal of CO2 (greater amount remains in the
atmosphere
– Reduces the rate of cooling
• Negative Feedback
Carbon Cycling in the BLAG Hypothesis
• Carbon cycles continuously between rock
reservoir and the atmosphere
Removal of Carbon from the Atmosphere
• Carbon from chemical weathering
– Ends up in shells of marine life
– Forms sediments when marine organisms die
Return of Carbon to the Atmosphere
• Suduction
– Some sediment is scraped off, eroded and redeposited
– Most is taken into Earth’s interior
• Doesn’t reach the mantle
• Returned to the atmosphere by volcanism
Does Data Support BLAG?
• Data does seem to support the BLAG Hypothesis
The Uplift Weathering
Hypothesis
• Asserts that chemical weathering is:
– The active driver of climate change
– Not just a negative feedback to BLAG
Available Surfaces Affect the
Rate of Chemical Weathering
• BLAG views chemical weathering as responding
to three climate factors:
– Temperature
– Precipitation
– Vegetation
• The Uplift Weathering Hypothesis considers
availability of fresh rock and mineral surfaces to
be weathered
– This exposure can override the combined effects of
BLAG’s three factors
Rock Exposure and the Rate of
Weathering
• As rocks an minerals physically disintegrate, the
total surface area of the particles increases
Increased Surface Area Results in a
Faster Weathering Rate
• The proportional increase of weathering far exceeds the
estimated result from changes in temperature,
precipitation, and vegetation.
Uplift and Weathering
• Tectonics results in the uplifting of Earth’s
crust and the formation of mountains at
many plate boundaries.
• In regions of uplift exposure of freshly
fragmented rock is enhanced.
Factors Increasing Weathering
Rates in Uplifting Areas
• Steep Slopes
– Erosional processes
are unusually active
– Higher frequency of
earthquakes in young
mountain regions
along plate boundaries
• Dislodge debris and
further weaken bedrock
Factors Increasing Weathering
Rates in Uplifting Areas
• Steep Slopes
– Erosional pocesses
called Mass Wasting
are unusually active
• Rock slides and falls
• Landslides
• Flows of water
saturated debris
– Removal of overlying
debris exposes fresh
bedrock
Mass Wasting or
Mass Movement is . . .
• the movement in which
– bedrock,
– rock debris,
– or soil
– moves downslope in bulk, or as a mss,
because of the pull of gravity.
• Examples
Rockfalls
Talus
An apron of fallen rock fragments that accumulates
at the base of a cliff.
Yosemite Valley Rockfall, 1999
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Two 80,000 ton slabs of an overhang broke off
Slid a short distance over steep rock and then flew 500 meters,
launched as if from a ski jump
They shattered upon impact and created a huge dust cloud.
Debris Slide
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A coherent mass of debris moving along a surface
Rotational debris slide (slump) if the movement is
along a curved surface.
Debris in the upper part
remained mostly intact as
it moved in blocks.
Debris in the lower portion
flowed with rotational
sliding.
La Concita, CA
(1995)
Earthflow and
Slumping
Earthflow
Earthflow in Santa Tecia, El Salvador, January 13, 2001
Factors Increasing Weathering
Rates in Uplifting Areas
• Steep Slopes
– Mountain Glaciation
• Pulverizes underlying
bedrock
• Carries sediment to
lower elevations
• Increases regional rates
of chemical weathering
Factors Increasing Weathering
Rates in Uplifting Areas
• Steep Slopes
– Heavy precipitation
generated on
• High but narrow
mountain belts
– Intercept moisture
carried by tropical
easterlies and midlatitude westerlies
• Large plateaus create
their only monsoonal
circulation (e.g., Tibetan
Plateau) by pulling
moisture from adjacent
oceans
Tectonic Uplift
Ocean-continent convergence
• Subduction occurs relatively steadily over time
• Total amount of high mountain terrain on Earth remains constant through time
- Locations and heights of individual ranges may vary
Tectonic Uplift
Continent-continent collision – the Himalayas and Tibetan Plateau
Active Tectonic Uplift Cools
Climate
• Uplift accelerates
chemical weathering
• Draws CO2 out of the
atmosphere
– Cools climate
• Greenhouse
Conditions
– Slower uplift
– Less chemical
weathering
• More CO2 in
atmosphere
Does Data Support the Uplift
Weathering Hypothesis?
• Data does seem to support the hypothesis.