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 CH 4 to CO 2 (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 CO 2 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 d 18 O 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

 18 O of CaCO 3 in Ocean Sediments A proxy for Ocean Temperature and Ice Sheet Volume that extends back millions of years. Ocean Temperature vs  18 O Relationship Δtemp/Δ  18 O = -4.2 ºC/1 ‰ Ice Sheet Volume Relationship -an increase in  18 O corresponds to an increase in Ice Sheet Volume (quantify later) Higher  18 O means colder ocean and greater ice sheet volume

Correlation between  18 O record deep sea CaCO 3 sediments and Orbitally forced Solar Insolation Changes

Strength of Tilt and Precession Periodicities in a Climate Record Dashed = tilt period Solid= Spectral analysis of  18 O in deep-sea carbonates Dashed = precession period Solid= Spectral analysis of  18 O in deep-sea carbonates

Slow Cooling and Change in Dominant Periodicity in  18 O-CaCO 3 Record

Spectral Analysis of Insolation and  18 O- CaCO 3 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  18 O of Seawater (and CaCO 3 ) As ice sheets grow, the  18 O of seawater increases (and vice versa) The  18 O of CaCO 3 precipitated by forams depends on the  18 O of seawater. Thus the d 18 O-CaCO 3 sediment record reflects both ice sheet volume change and ocean temperature change.

Sea Level Effects on  18 O-CaCO 3 Record At LGM (20,000 yrs BP), sea level was 120m lower than today based on Barbados coral reef record. Calculate the  18 O change in the ocean due the transfer of 120m of ocean to glacial ice sheets. (3800m)*(0 ‰) – 120m (-35 ‰) = 3680m (  18 Ogl oc)  18 O glacial ocean (at LGM) = 1.1 ‰ Thus the transfer of water from ocean to ice sheets at the LGM left the ocean with a  18 O which was 1.1 ‰ higher than today’s ocean.

Ice Volume Correction on  18 O-CaCO 3 record  18 O 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 CO 2 levels, position of continents, ocean and atmospheric circulation rates, etc.).

Conclusions There is a strong correlation between the periodicity of the  18 O-CaCO 3 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  18 O-CaCO 3 record. Ocean temperature decreased by ~2.7 ºC during the Last Glacial Maximum (LGM) and sea level was 120m lower than today.