Transcript Week4_orbital

The trigger for the initiation of the PETM was (probably) a period of intense
flood basalt magmatism (surface and sub-surface volcanism) associated with
the opening of the North Atlantic,
by generating metamorphic methane from sill intrusion into basin-filling carbonrich sedimentary rocks
SEQUENCE OF EVENTS AT PETM (Eocene climate maximum).
1. Hypothesis – initial cause? The eruption of the North Atlantic Igneous Province (the
head of the Iceland mantle plume). 60 to 55 My.
2. Climate gets (generally) warm.
3. Ocean circulation changes – conveys surface warmth to deep ocean. Bottom
water starts warming up.
4. Methane hydrates stored in the sediments now become unstable. Some continental
margin slopes are de-stabilized, and slump, exposing more hydrates – which
decompose to methane. Strong positive feed-back!
5. Large (1,500 gigatons) amounts of organic carbon are vented into the ocean and
atmosphere.
6. The methane in the atmosphere and the oxidation of CH4 to CO2 (both greenhouse
gases), cause a strong temperature spike (the PETM).
7. This increased global temperature causes more water evaporation, more coastal
run-off, more nutrients into the ocean.
8. This increases biological productivity which removes the CO2 from the atmosphere
(the biological pump), and temperatures ‘cool off’.
9. Heat spike (few 1000 years). Cooling off period – 70K to 100K years.
The methane hydrate
‘spike’.
OC 450: Orbital Controls on Climate
(Chaps 8 and 10)
Main Points:
• Small cyclic variations in the earth’s orbital
characteristics affect the distribution of solar
radiation on earth and, in turn, the growth and
retreat of ice sheets over the last 1M years.
• Evidence for these cyclic variations in climate
are clearly demonstrated in the deep sea
carbonate d18O record.
• Reconstructions of sea level indicate the
history of ice sheet growth and retreat.
THE CAUSE OF GLACIAL / INTERGLACIAL CYCLES
•Based on climate proxy records of the last 0.5 Ma, a general scientific consensus
has emerged that variations in summer insolation at high northern latitudes are the
dominant influence on climate over tens of thousands of years.
•The logic behind this is that - times of reduced summer insolation could allow
some snow and ice to persist from year to year, lasting through the ‘‘meltback’’
season.
•A slight increase in accumulation from year to year, enhanced by a positive snowalbedo feedback, would eventually lead to full glacial conditions.
•At the same time, the cool summers are proposed to be accompanied by mild
winters which, through the temperature-moisture feedback, would lead to
enhanced winter accumulation of snow.
•Both effects, reduced spring-to-fall snowmelt and greater winter accumulation,
seem to provide a logical and physically sound explanation for the waxing and
waning of the ice sheets as high-latitude insolation changes.
Orbital Effects on Solar Insolation
1. Variations in the tilt of the earth’s axis.
2. Variations in the shape of the earth’s
elliptical orbit.
3. Variations in the position of the earth’s tilt in
its elliptical orbit.
•
All three of these orbital variations have
affected the distribution of solar insolation on
earth over the last 4.5B years.
Variations in Tilt
Angle of tilt varies from 22º to 24º
Higher tilt causes stronger
seasonality.
No tilt, no seasons.
Periodicity of
Tilt
Elliptical Orbit
Shape of earth’s elliptical orbit varies from more
circular to less circular (eccentricity).
Periodicity of
Eccentricity
Variations in
Axial Wobble
(Precession)
Variations in
Precession
Periodicity of Precession
Periodicity of
Precession
Superimposed
on Eccentricity
Periodicity
Combined
Periodicity of
Tilt, Precession
and
Eccentricity
(as sine waves)
Spectral Analysis of Climate Records
Effect of Orbital Changes of Solar
Insolation on Climate
• Milankovitch (1920) hypothesized that
the orbital induced change in solar
insolation was a primary driver of climate
change on earth.
• At the time his theory was not taken too
seriously, but as climate records
improved, there was clear evidence that
orbital variations in solar insolation are an
important component of climate change.
High
Latitude
Orbital
Insolation
Change
Current Solar Insolation Distribution
H
Ice Sheet
Mass Balance:
Temperature
Dependence
Effect of
Changes in
Summer
Insolation
Ice Sheet Distribution during the Last
Glacial Maximum (LGM) ~20K yrs ago
Solar Insolation Changes
Red line marks 20K yrs BP
d18O of CaCO3 in Ocean Sediments
• A proxy for Ocean Temperature and Ice Sheet Volume
that extends back millions of years.
• Ocean Temperature vs d18O Relationship
Δtemp/Δd18O = -4.2 ºC/1 ‰
• Ice Sheet Volume Relationship
-an increase in d18O corresponds to an increase in
Ice Sheet Volume (quantify later)
• Higher d18O means colder ocean and greater ice sheet
volume
Correlation between d18O record deep
sea CaCO3 sediments and Orbitally
forced Solar Insolation Changes
Strength of Tilt and Precession
Periodicities in a Climate Record
Dashed = tilt period
Solid= Spectral analysis of
d18O in deep-sea carbonates
Dashed = precession period
Solid= Spectral analysis of
d18O in deep-sea carbonates
Slow Cooling
and Change
in Dominant
Periodicity in
d18O-CaCO3
Record
Spectral
Analysis of
Insolation
and d18OCaCO3
Records
Changes in Glacial Threshold
Reconstructing
Sea Level
Changes
Present Elevation (m) of Shorelines
from 124K years ago
Benchmark: Mean Sea Level at 124,000 yrs BP = +6m (Ruddiman)
Reconstructing Paleo Sea Level
Sea Level Change and its impact on the
d18O of Seawater (and CaCO3)
As ice sheets grow, the d18O of seawater increases (and vice versa)
The d18O of CaCO3 precipitated by forams depends on the d18O of
seawater. Thus the d18O-CaCO3 sediment record reflects both ice
sheet volume change and ocean temperature change.
Sea Level Effects on d18O-CaCO3 Record
• At LGM (20,000 yrs BP), sea level was 120m lower
than today based on Barbados coral reef record.
• Calculate the d18O change in the ocean due the transfer
of 120m of ocean to glacial ice sheets.
(3800m)*(0 ‰) – 120m (-35 ‰) = 3680m (d18Ogl oc)
d18O glacial ocean (at LGM) = 1.1 ‰
• Thus the transfer of water from ocean to ice sheets at
the LGM left the ocean with a d18O which was 1.1 ‰
higher than today’s ocean.
Ice Volume Correction on d18O-CaCO3 record
Ice volume
change
is 1.1 ‰
d18O change due to ocean temperature decrease is 0.65 ‰ after the 1.1
‰ ice volume correction has been applied to the observed 1.76 ‰
change. This equals a 2.7 ºC decrease in ocean temperature.
Conclusions
• Earth’s orbital changes affect the distribution of
solar insolation (especially important at high
latitudes).
• Ice sheet growth is likely impacted mostly by
changes in summertime insolation which affects
ablation rates.
• Whether or not orbital changes in solar insolation
are sufficient to cause the growth or retreat of ice
sheets depends on the ‘glacial threshold’ at the time,
which in turn depends on other climate factors (e.g.,
atmospheric CO2 levels, position of continents,
ocean and atmospheric circulation rates, etc.).
Conclusions
• There is a strong correlation between the
periodicity of the d18O-CaCO3 record preserved in
deep sea sediments and orbital insolation change
(at 23K, 41K and 100K years). Support for
Milankovich’s theory.
• Reconstruction of paleo sea levels indicate that
changes in ice sheet volume had the major
(dominant) impact on the d18O-CaCO3 record.
• Ocean temperature decreased by ~2.7 ºC
during the Last Glacial Maximum (LGM) and sea
level was 120m lower than today.