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PALEOCENE EOCENE THERMAL MAXIMUM (PETM)

By Charles Rhodes, P. Eng., Ph.D.

MASS SPECTROMETER:
There is an instrument known as a mass spectrometer. Mass spectrometers were perfected during WWII as part of the development of the atomic bomb. A subsequent invention known as a quadrupole mass spectrometer further improved mass spectrometry.

A mass spectrometer allows precise separation of ions by mass to charge ratio. Since charge is quantized a mass spectrometer separates the atoms of a chemically pure sample into its isotopes and precisely measures to over five significant figures the relative numbers of atoms of each isotope.
 

STABLE ISOTOPES:
A stable isotope has a nucleus that does not change with time. Examples of stable isotopes of carbon and oxygen are: C-12, C-13, O-16 and O-18. The C-13 atomic nucleus has one more neutron than C-12 atomic nucleus. The O-18 atomic nucleus has 2 more neutrons than the O-16 atomic nucleus. Because these nuclei are stable the total numbers of C-12, C-13, O-16 and O-18 atoms on Earth has remained unchanged during the last 100 million years.

In a typical oxygen sample the ratio of: (O-18 / O-16) ~ 0.00204

In a typical carbon sample the ratio of (C-13 / C-12) ~ 0.01108
 

OXYGEN ISOTOPE RATIO:
However, these isotope ratios vary depending on the location where the sample comes from. For example the ratio of: (O-18 / O-16) is lower in a land borne glacier than in ocean water. The reason for this change in ratios is that land borne glaciers are formed from rain and snow, which occurs as a result of ocean evaporation. Lighter molecules evaporate more easily than heavy molecules. Since the total numbers of O-16 and O-18 atoms are constant, as the amount of land borne ice increases, the ratio of (O-18 / O-16) in the ocean also increases. Thus the ratio of (O-18 / O-16) in the ocean at any time in the geological past indicates the total amount of land borne ice at that time.

Ocean life incorporates into its CaCO3 shells and bones carbon and oxygen that has the same isotopic ratios as the ocean around it. When a marine animal dies its shell and bone matter accumulate on the sea floor. Provided that the sea is not too acidic this matter forms sea floor sediment. Core drilled samples taken from the sea floor at different points around the world show oscillations in the sediment (O-18 / O-16) ratio that are synchronized with oscillations in the atmospheric CO2 concentration gas bubbles in core drilled samples from Greenland and Antarctic glacier ice. These oscillations are due to the periodic land borne glaciations of the Earth that occur during ice ages.

Antarctic ice drill cores allow us to look back in time about 400,000 years. However, sea bed sediment drill cores allow us to look back in time about 100 million years. The time resolution for the most recent 2.5 million years is excellent. Thus we know that for at least the last 2.5 million years there have been oscillating land borne glaciations on Earth.
 

CARBON ISOTOPE RATIO:
Prior to the industrial revolution carbon dioxide in the Earth's atmosphere was in equilibrium with carbon dioxide in the shallow ocean. In the shallow ocean CO2 is primarily stored in water soluble Ca(HCO3)2 which forms (HCO3)- ions. At pre industrial conditions there was about 32.76 times as much easily releasable CO2 in (HCO3)- ions in the bulk oceans as in the atmosphere.

Carbon dioxide (CO2) in the atmosphere behaves in a manner analogous to land borne ice. The (C-13 / C-12) ratio in the atmosphere is less than in the shallow oceans. Hence, as the amount of CO2 in the atmosphere increases, so also does the ratio (C-13 / C-12) in the oceans. Thus isotopic analysis of ocean sediments also reveals the oscillations in the CO2 concentration in the atmosphere looking back millions of years. When the (C-13 / C-12) ratio in the oceans decreases the (C-13 / C-12) ratio in the atmosphere increases and vice versa.

Land borne plants acquire carbon from atmospheric CO2 via photosynthesis. Land borne plants decay to form fossil fuels. Hence land borne biomatter and fossil fuels has a lower (C-13 / C-12) ratio than does marine life.
 

PETM NOTES:
There is a sedimentary layer that extends over the entire Earth that is referred to as the PETM (Paleocene Eocene Thermal Maximum) layer. This layer has a characteristic color and has been dated by multiple methods as having been formed between 56 and 55 million years ago.

Analysis of the PETM sedimentary layer indicates that the initial PETM hot period lasted less than 20,000 years and was followed by a sustained warm period that lasted 200,000 years. The Earth took a further 300,000 years to return to normal conditions.

Analysis of animal fossils found above and below the PETM sedimentary layer indicates that there was a global extinction of large land animals during the PETM. All large land animals that existed prior to the PETM, that due to their size could not rapidly adapt to the higher temperatures and higher sea levels, became extinct during the PETM.

On dry land rocks and fossils formed during the PETM are characterized by a major decrease in the (C-13 / C-12) isotope ratio, indicating that the trapped carbon in these rocks and fossils came from CO2 in the atmosphere, biomass or fossil fuel, not from CO2 in the ocean.

The (O-18 / O-16) isotope ratio in the ocean sediment during the PETM fell to its lowest ever level, indicating sufficient warming to cause complete melting of the Antarctic ice cap.

During the PETM the (C-13 / C12) isotope ratio in the ocean went to its lowest ever level, indicating a large injection of organic carbon from massive combustion of land borne biomass and fossil fuels.

This transient organic carbon injection into the atmosphere caused a high transient atmospheric CO2 concentration and a step decrease in planetary albedo. This combination is known as thermal runaway. The resulting high atmospheric temperature persisted for sufficient time to warm the ocean causing the ocean to emit CO2 and trap the Earth in its "warm" state. This effect is known as "warm" state trapping.

The increase in ocean temperature raised the steady state atmospheric CO2 concentration to over 618.52 ppmv, which raised the Earth's emission temperature to over 273.15 deg K trapping the Earth in its "warm" state. The atmospheric steady state CO2 concentration then slowly decreased to about 618.52 ppmv over about 200,000 years. During this period, while the steady state atmospheric CO2 concentration remained in excess of the critcal threshold of 618.52 ppmv, ice and snow could not form. Hence the planetary albedo remained low and the Earth remained trapped in its "warm" state.

At steady state the warm ocean could not net absorb atmospheric CO2. The steady state atmospheric CO2 concentration and the steady state ocean (HCO3)- ion concentration could only be reduced via a slow photosynthesis process that converts ocean-atmosphere pool CO2 into biomass and fossil fuels and a slow process that reacts CO2 with wet silicate rock to form carbonate rock.

When the atmospheric CO2 concentration eventually decreased enough that the Earth's emission temperature dropped below 273.15 degrees K, the freezing point of water, the planetary albedo rose to its normal "cool" state value. The ocean then gradually cooled which ended the PETM.

All large land animals on Earth, including humans, have evolved since the PETM. Most of these animals function best in the ambient temperature range 5 deg C to 30 deg C. These animals have evolved various means of living through short high or low temperature excursions, but a prolonged temperature excursion will usually lead to species extinction.
 

RECOVERY FROM PETM:
We know that during the PETM the initial transient atmospheric CO2 concentration, which was about 2000 ppmv, decayed with a time constant of 41 to 82 years to a steady state value of about 700 ppmv and then remained above 618.52 ppmv for about 200,000 years. Further decay of the atmospheric CO2 concentration to about 300 ppmv took a further 300,000 years as it required photosynthesis to extract carbon from the ocean-atmosphere pool. Further decay of the atmospheric CO2 concentration to 280 ppmv occurred during the subsequent 50,000,000 years.
 

PETM CONCLUSIONS:
The most important aspect of PETM data is that it conclusively shows that when there is sufficient CO2 in the atmosphere the Earth will spontaneously shift from its normal "cool" state into its "warm" state and if there is sufficient CO2 in the ocean-atmosphere pool the Earth will become trapped in its "warm" state. In the "warm" state the emission temperature Ta is sufficient to completely melt the polar glaciers and to cause a global extinction of large animals.

In both of the "cool" and "warm" states the Earth's infrared radiation emission equals the Earth's solar power absorption. Between these two locally stable states, at Ta = 273.15 K, there is a step decrease in planetary albedo Fr from 0.3 to 0.1 with increasing emission temperature.

The step change in planetary albedo Fr at emission temperature Ta = 273.15 deg K separates the two locally stable states and contributes to positive feedback during the spontaneous transition between the two locally stable states. The physical cause of the positive feedback lies in the highly ocean temperature dependent relationship between the concentrations of CO2, H2O, CaCO3 and Ca(HCO3)2. As the ocean temperature rises so also does the steady state atmospheric CO2 concentration.

The normal locally stable "cool" state corresponds to a dominant cloud temperature of Ta < 273.15 deg K and a planetary albedo Fr = 0.297. The locally stable "warm" state corresponds to a dominant cloud temperature of Ta > 273.15 deg K and a planetary albedo Fr of about:
Fr = 0.1
 

TEMPERATURE CHANGES:
As long as the emission temperature is less than the freezing point of water, at 273.15 K, we can calculate the atmospheric emission temperature increase corresponding to a change in atmospheric CO2 concentration and the corresponding increase in atmospheric water vapor concentration. However, when the emision temperature exceeds the freezing point of water there is a step decrease in planetary albedo (solar reflectivity) and hence a step increase in atmospheric emission temperature. There is also a corresponding increase in ocean evaporation rate.

The corresponding ground level temperature increase during the PETM was sufficient to cause the global extinction of large animals, as indicated by the fossil record before and after the PETM period.

Previous work by this author on mathematical analysis of the Earth's infrared emission spectrum as recorded by the Mars Global Surveryor space craft on November 24, 1996 indicates that the atmospheric emission temperature at an atmospheric CO2 concentration of 360.76 ppmv is 270.70 deg K and the atmospheric emission temperature increase corresponding to doubling of the atmospheric CO2 concentration from 360.76 ppmv to 721.52 ppmv is 3.15 deg C.
 

TEMPERATURE CONFUSION:
Note that when the average emission temperature over Hawaii was measured to be 270.7 K the ground level atmospheric temperature in Hawaii was measured to be 297 K.

There is considerable confusion in the published literature relating to the increase in temperature that occurred during the PETM. The PETM affected the following tabulated temperatures to different extents. In order to properly interpret a calculated change in temperature during the PETM it is necessary to determine both the assumptions that were used to calculate the change in temperature from the observed change in carbon isotope ratio and the assumptions that should have been used. Generally readers must rely on the measured carbon and oxygen isotope ratios and the locations where the PETM sediment or fossil samples were found to make sense of calculated temperature changes. In general the average temperatures in the ocean, in the atmosphere at sea level and the emission temperature at the top of the clouds are not the same.

TEMPERATUREDESCRIPTIONTYPICAL VALUE
Bulk Ocean Temperaturein deep ocean ~ 2 C
Ocean Surface Temperatureat ocean surface ~ 11 C
Ground Level Air Temperaturein the shade~ 15 C
Atmospheric Emission Temperatureat top of cloud layer-2.45 C

 

SWITCHING ACTION:
The PETM was likely triggered by an astro-physical event which enabled widespread combustion of accumulated biomatter and exposed fossil fuels. The resulting transient atmospheric CO2 concentration was about 2000 ppmv. When the switching point steady state atmospheric CO2 concentration (~ 618.52 ppmv) was exceeded the ocean warmed and the Earth became trapped in its "warm" state. The resulting continuous high temperature and increased solar radiation absorption caused the polar ice caps to completely melt and caused extinction of all large land animals.
 

PETM TRIGGER EVENT:
A reasonable question to ask is what event triggered the PETM?

We are certain that a trigger event enabled and/or caused widespread combustion of biomass and exposed fossil fuels. The trigger event likely reduced the planetary albedo for a sufficient period to dry out the majority of biomass on the Earth's surface and hence cause extreme forest and tar pool fires. Thus the trigger event almost certainly raised the emission temperature by at least 4 degrees C for several months. We know that whatever the trigger event was it is something that on a geological time scale happens only rarely. On a geological time scale ordinary forest fires are frequent, not rare events.

So the question becomes: "What event that is rare on the geological time scale triggered the initial sudden prolonged warming 55 million years ago?"
 

TRANSIENT INCREASE IN SOLAR IRRADIANCE:
If a star comparable in size and age to our sun passed within 4 astronomical units (4 Earth orbit radii) of the Earth the temporary increase in received solar irradiance would be sufficient to raise the emission temperature by 4 degrees C which would lower the planetary albedo and hence trigger a PETM like event.
 

P>PETM SEQUENCE OF EVENTS:
A sequence of events that seems to fully account for the observed PETM data is as follows:
1) Prior to the PETM the Earth's atmospheric CO2 concentration was ~ 300 ppmv;
2) At the commencement of the PETM another star comparable to our sun in size and brightness passed within 4 astronomical units (4 Earth orbit radii) of the Earth;
3) The resulting transient increase in solar irradiance for several years raised the Earth's emission temperature by at least 4 degrees C.
4) The increase in emission temperature prevented formation of cloud ice crystals, which sharply reduced the Earth's planetary albedo.
5) As a result of the temporary increase in solar irradiance and the reduced planetary albedo the Earth's surface temperature rose and almost all accumulated biomatter and exposed fossil fuels dried out and burned.
6) Combustion of biomatter and exposed fossil fuels injected a large amount of non-equilibrium CO2 containing C-13 depleted carbon into the Earth's atmosphere. The Earth's atmospheric CO2 concentration immediately rose to about 2000 ppmv.
7) This transient CO2 concentration decayed with an exponential decay time constant of 16 to 32 years;
8) The high solar irradiadiance and high atmospheric CO2 concentration together heated the oceans surface layers which raised the steady state atmospheric CO2 concentration to over 618.52 ppmv;
9) The high steady state atmospheric CO2 concentration kept the emission temperature high and the planetary albedo low and hence trapped the Earth in its warm state;
10) Simultaneously most of the C-13 depleted organic carbon was absorbed by the ocean which caused a geologically rapid decrease in the C-13 / C-12 ratio in ocean sediments;
11) Over about 200,000 years plant photosynthesis and rock weathering gradually converted excess (HCO3)- ions in the oceans and excess CO2 in the atmosphere into fossil fuels and carbonate rock, reducing the amount of carbon in the ocean-atmosphere pool and hence reducing the steady state atmospheric CO2 concentration down to below 618.52 ppmv.
12) At an atmospheric CO2 concentration of about 618.52 ppmv the Earth transitioned from its "warm" state to its normal "cool" state allowing an increase in planetary albedo and hence gradual ocean cooling.
13) Plant photosynthesis and silicate rock weathering continued to extract carbon from the atmosphere and oceans until the normal "cool" state condition with a steady state atmospheric CO2 concentration of about 300 ppmv was reached after a further 300,000 years.
 

This web page last updated January 18, 2017.

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