the trigger for the initiation of the petm was (probably) a period of intense flood basalt magmatism...

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 carbon-rich 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 snow-albedo 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

18O of CaCO3 in Ocean Sediments

• A proxy for Ocean Temperature and Ice Sheet Volume that extends back millions of years.

• Ocean Temperature vs 18O Relationship

Δtemp/Δ18O = -4.2 ºC/1 ‰

• Ice Sheet Volume Relationship

-an increase in 18O corresponds to an increase in Ice Sheet Volume (quantify later)

• Higher 18O means colder ocean and greater ice sheet volume

Correlation between 18O 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 18O in deep-sea carbonates

Dashed = precession period

Solid= Spectral analysis of 18O in deep-sea carbonates

Slow Cooling and Change in Dominant Periodicity in 18O-CaCO3

Record

Spectral Analysis of Insolation and 18O-

CaCO3 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 18O of Seawater (and CaCO3)

As ice sheets grow, the 18O of seawater increases (and vice versa)

The 18O of CaCO3 precipitated by forams depends on the 18O of seawater. Thus the d18O-CaCO3 sediment record reflects both ice sheet volume change and ocean temperature change.

Sea Level Effects on 18O-CaCO3 Record

• At LGM (20,000 yrs BP), sea level was 120m lower than today based on Barbados coral reef record.

• Calculate the 18O change in the ocean due the transfer of 120m of ocean to glacial ice sheets.

(3800m)*(0 ‰) – 120m (-35 ‰) = 3680m (18Ogl oc)

18O glacial ocean (at LGM) = 1.1 ‰

• Thus the transfer of water from ocean to ice sheets at the LGM left the ocean with a 18O which was 1.1 ‰ higher than today’s ocean.

Ice Volume Correction on 18O-CaCO3 record

18O 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.

Ice volume change

is 1.1 ‰

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 18O-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 18O-CaCO3 record.

• Ocean temperature decreased by ~2.7 ºC during the Last Glacial Maximum (LGM) and sea level was 120m lower than today.