Transcript Astronomical Control of Solar Radiation

Climate Cooling

Proxy evidence
indicates an erratic
cooling
 Over both poles
and mid latitudes
 Roughly equal
cooling in first and
second half of
interval
Tectonic Scale Cooling Mechanism?
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Lower volcanic CO2 emissions
Increased weathering
Increased ocean heat
transport
Tectonic changes
 Atlantic widened and Pacific
narrowed
 India and Australia
separated from Antarctica
 India and Australia moved to
lower latitudes
 India collided with Eurasia
 Key oceanic gateways open
and closed
BLAG Hypothesis
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Depends of global spreading
rates
55-15 mya general decrease
in spreading
 Produce cooling
15 mya to today spreading
increased
 Produce warming
Consistent with record prior
to 15 mya
Inconsistent with record
from 15 mya to present
Cannot alone explain cooling
Uplift Weathering Hypothesis

To explain cooling, 3 criteria must be met
 High elevation terrain today must be
unusually large
 High terrain must cause unusual amount of
rock fragmentation
 Fragmentation and exposure must enhance
chemical weathering
Elevation on Earth

Most high elevation caused by subduction of
oceanic crust and volcanism
 Mountain ranges associates with subduction
common throughout geologic time
 Deep-seated heating and volcanism
 East African plateau
• Mechanism of uplift not unique to last 55
my
 Existence of uplifted terrains like the Tibetan
Plateau
 Not common through geologic time
 Conclude – amount of high elevation terrain is
unusually large during last 55 my
Physical Weathering High
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Does the amount of
high elevation terrain
result in unusual
physical weathering?
Most likely given 10
fold increase of
sediment to the Indian
Ocean
 Steep terrain along
southern Himalayan
margin
 Presence of
powerful South
Asian monsoon
Chemical Weathering
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Global chemical weathering rates difficult to
determine
 Dissolved ions in rivers clue
 Today concentration modified by human activity
 Difficult to distinguish ions from hydrolysis and
dissolution
 Only hydrolysis important on long term
 Lots of rivers contribute ions to ocean
Chemical weathering rates in past very difficult to
quantify
 Need chemical indicator of hydrolysis
 Isotopes of strontium and osmium
Strontium Isotopes
 87Sr/86Sr
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for carbonate rocks
has been measured throughout
the Phanerozoic
Curve reflects relative
contributions of Sr to the
ocean
 Continental weathering
 Hydrothermal activity along
mid-oceanic ridges
General decrease in Early
Phanerozoic due to increasing
activity along mid-ocean ridges
Late Cenozoic increase in 87Sr
due to increased rates of
continental weathering by
glaciation
87Sr/86Sr
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& Chemical Weathering
Increase in 87Sr/86Sr in
Cenozoic could be
 Increase in chemical
weathering
 Delivers more Sr and
more radiogenic Sr to
ocean
 Rock type being
weathered is more
radiogenic
 No change in rate of
chemical weathering
No unique solution
Osmium Isotopes
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Radiogenic Os formed from Re
Re enriched in certain phases
 Organic-rich shales
 Weathering of organic-rich shales
 Certain minerals in granitic rocks
 Hydrolysis reactions!
Os residence time in ocean short
Rivers draining the Himalayans not particularly rich
in Os nor in radiogenic Os
 Available evidence indicates Himalayans not a
source for strongly radiogenic osmium
Infer Chemical Weathering Rates
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Tibetan-Himalayan complex very large and at high
elevation
Steep slopes receive lots of rainfall
Heavy rains produce high suspended load
 Probably also provide high dissolved load
BLAG or Uplift Weathering?
No “proof” of either hypothesis exists
 BLAG explains well cooling from 55-15 mya
 Uplift weathering supported by conditions
in Tibetan-Himalayan Complex
 Would a combination of the two hypotheses
explain best global cooling over last 55 my?
 Did Himalayan uplift balance increased CO2
from enhanced spreading?
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Ocean Heat Transport
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Although it appears “cool tropics paradox” is
resolved
 Several important tectonic events influenced
oceanic circulation
Opening or closing of critical gateways
 Narrow passages linking major ocean basins
 Change heat and salt balance
Two critical gateways
 Opening of Drake Passage producing the
Antarctic Circumpolar Current
 Appearance of the Isthmus of Panama stopped
equatorial flow between Atlantic and Pacific
Opening of Drake Passage
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Opening the gap
between South America
and Antarctica 25-20
mya allowed start of
ACC
 Prior to opening, flow
from north kept
Antarctica warm
 Onset of ACC
proposed to initiate
glaciations on
Antarctica
Timing of Opening
Drake Passage opened 25-20 mya
 Glaciations on Antarctica began 35 mya
 Most intense glaciation 13 mya
 Ocean GCM models
 Indicate that opening of Drake Passage
had no effect on ocean/atmosphere
temperatures
 Antarctica cold with or without ACC
 Models crude
 Smaller grid
 Affect of ACC on other deep currents
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Isthmus of Panama
Closure within last 10 my
 Complete closure 4 mya
 N. America glaciations 2.7 mya
 Stopped westward flow of warm salty water
 Redirecting flow in Atlantic into Gulf
Stream
 Northward flow of salty water slow sea ice
formation
 Reduced sea-ice cover made more moisture
available on land
 Triggered growth of ice sheets
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Results of Closure
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Ocean GCM model results
 Agree with redirection of west
flowing warm saline water into
Gulf Stream
 Also stops return flow of low
salinity water into Atlantic from
Pacific
 Further increase salinity of
Gulf Stream
GCM model predicts reduction in
sea ice in N. Atlantic
 Did not affect atmospheric
moisture
 However, warmed N. Atlantic
and increased summer
melting of snow and ice
Assessment of Gateway Changes
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Illustrates fundamental disagreement
 Stopping pole-ward flow enhanced glaciations
 Starting pole-ward flow enhanced glaciations
Argument centers about role of latent heat
 Warmer ocean releases more latent heat to
atmosphere
 Supply moisture in atmosphere for ice sheet
growth
Appears that more sensible heat transferred
 Promote melting and ablation of glacial ice
 Ablation of glacial ice important
Importance of Gateways
Not satisfactory explanation for long-term
global temperature changes
 Discrete events that affected circulation
 One-time events cannot explain well
long-term temperature changes
 Clearly affect circulation patterns
 Closure of Isthmus of Panama
 Increased rate of NADW formation
• Redirected dense water to north
– Easier to form bottom water
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Brief Tectonic-Scale Change
Attempt to explain erratic nature of cooling
 Volcanic aerosols
 Formation of sulfuric acid droplets or
particles
 Sulfate aerosols block incoming solar
radiation when in stratosphere
 Burial of organic carbon
 Brief intervals of enhanced burial
 Reduction in atmospheric CO2
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Earth’s Active Volcanoes
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Most volcanoes associated with subduction
andesitic and relatively explosive
Explosive eruptions between 23.5°N and 23.5°S
have most effect on climate
Sulfate Aerosols
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Aerosols that reach
stratosphere
 Attain maximum
concentration within
months of eruption
 Concentrations
decline
exponentially as
particles settle
 Cooling effect follows
concentration
Documentation of Effect
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Effect of sulfate aerosols difficult to
detect in geologic record
 Crater size
 Volume of volcanic ash deposits
 Geographic area of ash fall deposit
 Caveat is that sulfur content  ash
content
 Ice core records show ash deposits and a
record of sulfuric acid
 Limited time resolution
Size of effect
Even massive eruptions that send sulfate
aerosols into stratosphere
 Produce cooling for only a few years
 Multiple eruptions required for significant
long-term cooling
 Multiple large eruptions unlikely
 Global cooling countered by increased CO2
 Volcanic eruption also a source for CO2
 CO2 residence time in atmosphere higher
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Burial of Organic Matter
Changes in the rate of burial of organic
matter affect atmospheric CO2
 Rate of burial of marine organic matter
sensitive to:
 Changes in rates of production
 Nutrient supply
• Change in upwelling
• Change in delivery of nutrients from
land
 Changes in mode of preservation
 Bottom water oxygenation
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Cooling 13 mya
Organic carbon-rich sediments deposited
along California coast 13 mya
 Coincided with global cooling
 Strong winds enhanced upwelling
• Termed the Monterey Hypothesis
 Timing of maximum organic carbon burial
lags maximum cooling rate by 3 my
 Coastal deposition of organic-rich sediments
 May be exposed during sea-level low stand
 Organic matter oxidized, CO2 released
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Future Research Directions
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What is needed to better resolve
mechanisms underlying tectonic-scale
changes?
 Detailed record of atmospheric CO2
 Geochemical tracer for chemical
weathering
 Better understanding of feedbacks in
climate system
 More detailed ocean general circulation
models