1. Methane Hydrates and the
Paleocene-Eocene Thermal
Maximum
Aaron Munsart
May 9, 2015
GEOS 442-500

2. Introduction
A global warming induced greenhouse gas event occurred at the boundary between the
Paleocene and Eocene epochs about 55 million years ago. This event is highly debated
scientifically because the conditions under which it was initiated share many similarities with
today’s current global climate change argument. The primary mechanism or driver of the
Paleocene-Eocene Thermal Maximum (PETM) are the dissolution of methane hydrates in the
world’s oceans. The natural flux of methane during the Paleocene-Eocene is of a similar value to
today’s anthropogenic sources of methane
Methane (CH4) is a greenhouse gas roughly 21 times more efficient at warming Earth’s
atmosphere than carbon dioxide. It is stored naturally in the world’s oceans as Clathrate Hydrates
which are solids made up of water and methane that are found in the pore spaces between marine
sediments. These clathrate hydrates are extremely volatile and are dependent upon temperature.
The release of methane and other greenhouse gases such as carbon dioxide and water vapor
during the Paleocene-Eocene thermal maximum caused an increase in global temperatures of 4°
Celsius over less than ten thousand years. It is hypothesized that the carbon isotope excursion
was the main cause of the global warming event.
This warming event caused many substantial changes to both living and nonliving things
on the planet. It rapidly changed oceanic circulation, sea surface temperatures, ocean
acidification, and resulted in the extinction of many species of foraminifera in the deep ocean
regime as well as many terrestrial species of plants and animals. This global warming event has
remained misunderstood mainly due to the lack of proxies necessary to uncover the initial
sequence of events.
Materials and Methods
Various different approaches are taken in the literature to further understand this global
warming event. One approach included a model of the planet and its hydrologic cycle assuming
the mass of the present day methane hydrate reservoir, processes that control hydrate formation
and transport, and carbon transport can occur between oceanic methane hydrates and the
inorganic carbon reservoir during an overall increase in bottom water temperatures (Dickens et
al. 1997). In terms of these assumptions and their application towards the carbon isotope
excursion, only the present day concentration of methane hydrate can explain the decrease in 13C

3. and thus the global warming event. Dickens’ model was created by using a deep-ocean organic
foraminiferal record of tooth enamel and carbonate rock in North America, Europe, and New
Zealand and was based on the model developed by Walker and Krasting in 1992. Walker and
Krasting developed this model by characterizing the ocean into different reservoirs, i.e.
cold/warm, thermocline, Indian, Atlantic, etc. The most controversial aspect of Dickens’ model
was the release of methane into the atmosphere rather than into the oceans. The reasoning behind
this was the lack of detail concerning ocean reservoir parameters in the corresponding literature.
To analyze the effect the global warming event had on different species of terrestrial
animals, Koch utilized mammalian dental apatite and soil nodule carbonate in Clarks Fork-
Bighom Basin (Gingerich 2006). Gingerich also analyzed carbonate tests on foraminifera to
estimate 18O concentration and relative trends.
The standard use for 13C used in the literature is Vienna Pee Dee Belemnite (V-PDB). V-
PDB simply calcite from fossils known as belemnites with age dating back to the Cretaceous
period. The standard use for 18O and 16O used in the literature is known as the Vienna Standard
Mean Ocean Water (V-SMOW).
Another approach uncovering the Paleocene-Eocene thermal maximum analyzed ocean
drilling site data (Thomas et al. 2002) (Tripati & Elderfield 2005). In Thomas et al, stable
isotopes of carbon and oxygen were used based on deep-ocean foraminiferal shells in the
Weddell Sea, Southern Ocean. This is the most complete deep-sea Paleocene-Eocene thermal
maximum section to date. However, they did have limited measurements because the organisms
large enough for analysis were very rare. Tripati and Elderfield’s paper utilized Mg/Ca as a
temperature proxy to deep-sea foraminifera to overcome the limit on ocean temperature
constructions based on foraminifera. These ocean temperature constructions are primarily based
on 18O which is dependent on temperature. They used their Mg/Ca proxy to analyze change in
deep sea temperature, upper latitude sea surface temperature, ocean circulation patterns, bottom
water temperatures, and they combined the proxy with the sea surface temperatures to analyze
tropical regions of warming. They also created the first deep-ocean stable isotope record for the
Pacific.
A similar approach to Thomas et al. and Dickens et al. was used to analyze ocean
acidification utilizing foraminiferal shells and the geologic stratigraphy of marine clay layers

4. (Zachos et al. 2005). In theory, an increase in oxidation and oceanic input of carbon should
decrease pH eventually followed by recovery of the ocean back to a stable/neutral pH.
Results
Dickens et al 1997:
During the Paleocene-Eocene thermal maximum, a large amount of carbonate and
organic matter was deposited and resulted in the carbon isotopic excursion of -2.5% over 10,000
years. This excursion eventually rebounded exponentially to near initial values. Dickens et al.
hypothesized that the excursion signified the distribution of 1.4-2.8x1018 grams of methane via
oceanic hydrates to the atmosphere and surrounding inorganic carbon reservoir. The carbon
isotope excursion caused by the distribution of methane throughout the system is the primary
mechanism for the global warming event. The intensity and length of the carbon isotope
excursion depends on mass and isotope concentration of the carbon input into the system, size of
reservoir, and exchange ratios between mediums. Refer to Figure 1 for carbon isotope excursion
data during the Paleocene-Eocene thermal maximum.
The high intensity and speed of the warming event is uncertain but could involve changes
in tectonism, thermohaline circulation, and atmospheric partial pressure of carbon dioxide. As
there was a decrease in 13C, the enrichment of 12C was taking place throughout the Earth’s
system. The conventional hypotheses for this enrichment are inaccurate but include increased
volcanism and the loss of terrestrial biomass to the atmosphere and ocean. Dickens’ main
hypothesis surrounded the idea that a rapid warming of the deep-ocean resulted in a displacement
of sediment geotherms which released methane from oceanic hydrates. To further analyze this
hypothesis, they looked closer into the dynamics of the lysocline. The lysocline is the depth in
the ocean by which the dissolution of calcite reaches a maximum. This property of the ocean is
dependent on the amount of deposited calcite within the system. Dickens et al. concluded that the
increased dissolution of carbonate resulted in the shoaling of the lysocline. The dynamics of this
process correlated with the time sequence of events in the Paleocene-Eocene thermal maximum
as follows. They lysocline would shoal at first and then deepen. This shoaling of the lysocline
correlates with the initial conditions during the Paleocene-Eocene thermal maximum during
which that substantial amount of carbon was being transported into the oceans thus displacing
they lysocline upwards in the water column. Once dissolution begun to take place, the lysocline
then deepened as a result.

5. Atmospheric carbon dioxide also rose 70-80ppm with the inclusion of methane to
atmosphere or ocean respectively during the 10,000 year carbon isotope excursion period. This
increase in carbon dioxide concentration accounts for roughly 2° Celsius in overall temperature
increase. It may also have caused a decrease in longwave radiation being emitted by Earth
resulting in higher global sea surface temperatures. Their model showed that an increase of 4°
Celsius in global temperatures should shift sediment geotherms and thereby dissociating 14% of
oceanic methane hydrates.
Dickens et al also presents an idea in the paper arguing that the carbon isotope excursion
is simply just a coincidence. This argument assumes that all 13C in all reservoirs on Earth
respond simultaneously to any input of carbon. The release and oxidation of methane in the
model performed in this paper supports this argument. On the contrary, the model also suggests
that warming preceded the massive input of carbon because simply the input of carbon cannot
explain all of the warming evident during the Paleocene-Eocene. His model was plausible by
predicting only 2° Celsius of the 4° Celsius. Most of his conclusions are qualitatively consistent
with observations. The only conclusion he proved based on the geologic record was that the
carbon isotope exclusion occurred at -2.5% over 10,000 years and that it exponentially returned
to its initial value.
Figure 1. Graph of 13C vs. Relative
depth in water column as a function
of time. (Dickens et al 1997)

6. Gingerich 2006:
The Paleocene-Eocene epoch is the period in which most species of mammals appear in
time. Several modern species appear suddenly during this time period lacking any necessary
precursors for such an introduction into the world. Most of these modern species were
significantly smaller than their descendants, i.e. alligators. Gingerich utilized primarily carbon
and oxygen isotopes in marine cores at ODP sites to explain mammalian and plant changes
during this time period. She successfully observed and measured a five million year trend of 13C
depletion from 4% to 1% at the end of the time period. Despite the significant decreases in 13C,
18O remained relatively stable during this time. 18O is dependent upon temperature and since the
majority of the Paleocene-Eocene was relatively warm, it is acceptable to observe little to no
change in 18O. The Paleocene-Eocene thermal maximum is easily observed in her data as a brief
100,000 year warming event distinctly different from the five million year high. This observation
is based on the rapid decrease in 18O in carbonate tests in foraminifera during this time period.
Rapid warming of the southern ocean came as a result of the carbon isotope excursion which
translated to an overall increase in global temperatures. This directly correlates with deep-
ocean/benthic foraminiferal extinction as these species of animals are highly sensitive to changes
in temperature. In coordination with Koch, they were able to correlate the carbon isotope
excursion as an indicator of global change as well as accurately correlate marine and continental
geologic strata.
The enrichment of 12C during the carbon isotope excursion mostly derived from biogenic
sources of methane stored as methane hydrate in shallow buried sediments on continental-ocean
margins. This source of carbon is very light with a 13C of about -60%. It is estimated that
approximately 1500Gt of light carbon must have been added to the oceans and atmosphere to
enable a carbon isotope excursion of -2.5%. This translates to approximately 1/6th of the present
day carbon content allocated in oceanic methane hydrate. This process only eventually slowed
down because the methane reservoir became empty.
They establish the idea for a trigger event for the Paleocene-Eocene thermal maximum
known as astronomical pacing. They explain how milankovitch cycles, which normally play a
large role in changes in climate, caused extreme seasonal contrasts with eccentricity maxima in
the 100,000 and 405,000 year orbital cycles as well as an eccentricity minima in the 2.25 million

7. year cycle. This longer cycle at the eccentricity minima favored the buildup of methane hydrate
and thus later inevitably its release, the extinction of up to 50% benthic foraminifer.
Thomas et al. 2002:
Thomas et al suggests that the initial carbon isotope excursion was instantaneous and was
preceded by a brief gradual warming of sea surface temperatures. Methane must have mixed
from the surface ocean downwards because a significant amount of methane hydrate dissociated
into the atmosphere before oxidation could take place. A sequence of events from bottom to top
include: 1. 18O of surface plankton decreased by 1.5%. 2. Initial decrease in 18O in thermocline
plankton and the 4% decrease in 13C surface plankton correspond to initial warming events and
carbon isotope excursion. 3. 13C decrease in thermocline plankton by 2.5%. The onset of the
Figure 2. Extinction of foraminifera relative to carbon isotope excursion and depth in water
column (Gingerich 2006)

8. temperature maximum would have occurred before the decrease in 18O if it had resulted from
erosion or induced methane dissociation. The most plausible explanation is the thermal
dissociation of methane hydrates from top to bottom. The methane once it had reached the
atmosphere was oxidized into carbon dioxide and used towards the synthesis of bicarbonate
causing calcification within the atmosphere and upper-ocean. This is evident by a decrease of
18O gradually downwards.
Tripati & Elderfield 2005:
The Paleocene-Eocene global warming event was short lived and characterized by a
reduction in carbon content in deep-ocean sediments, a large negative excursion in marine and
terrestrial biomes, and a large release of methane as a result via methane hydrates and the
displacement of the carbon content. This global warming event changed many processes that
occur in the world’s oceans. For example, a rapid change to convection occurred in the North
Pacific Ocean as a result of the warming of the deep waters by 3-5° Celsius. This process alone
could have driven the Paleocene-Eocene by maintaining high levels of carbon dioxide and water
vapor in the atmosphere for a prolonged period of time. This idea of circulation induced global
warming could have further exacerbated the problem by enabling the displacement of methane
hydrates in the ocean thus causing them to grow more volatile. Solubility of carbon dioxide in
seawater would have also decreased because of temperature and salinity changes promoting a
higher carbon dioxide concentration in the atmosphere. The warming of tropical oceans
promotes a higher order intensity change in the hydrologic cycle, increased evaporation, and
change in saturation vapor pressures. All of these changes are consistent with the proxy data for
surface water salt content and humidity.
Evaporation drives the release of water vapor into the atmosphere. This increase in water
vapor causes an increase in the radiative absorption properties and sensitivity to rising carbon
dioxide and methane concentrations. Meaning, more warming due to greenhouse gases.
Eventually, sequestration of carbon would take place via the biological pump, terrestrial
productivity, and weathering which would eventually result in global cooling over 100,000 to
200,000 years. Temperature and hydrologic changes also drive a return to deep sinking in the
southern hemisphere and shut down convection in the Pacific while moving closer towards
recover of the Earth system.

9. Figure 3. Mg/Ca proxy data; temperature and 18O as a function of water depth
(Tripati & Elderfield 2005)
Figure 4. Sequence of Events during the Paleocene-Eocene Thermal
Maximum; Pre-Event(Left); High carbon isotope excursion(Middle);
Cooling/Recovery Period(Right). (Tripati & Elderfield 2005)

10. Discussion
In terms of the carbon isotope excursion, all of the authors in the literature agree that it
was the main driver during the Paleocene-Eocene thermal maximum. Dickens 1997 was
discounted by Thomas 2002 through their top to bottom method and not by dissolution solely.
There is a progressional decrease in 18O values in the water column. These same plankton record
pre/peak excursion 13C values. This demonstrates a direct correlation between Thomas’ top to
bottom method and the carbon isotope excursion. However, Thomas agrees with Dickens that
the thermal dissociation of methane is the primary driver of the Paleocene-Eocene thermal
maximum.
Further research and the development of new more detailed proxies needs to be
undertaken to fully be able to understand this and similar events. Some of the papers referred to
how similar this event was to today’s present greenhouse gas situation. I find this to be extremely
relevant and useful to further understanding how our planet changes as a result of a global
warming event induced by greenhouse gases. This process could very well occur again if the
global temperature rose to a level high enough to repeat the patterns and sequence of events
present during the Paleocene-Eocene thermal maximum. The Gingerich paper refers to how
milankovitch cycles can play a major role in climate change on our planet. We see this
throughout history and at the present time. We are already seeing changes to global circulation
and deep water circulation in the upper latitudes. There are a lot of parallels within this problem
to ones we are trying to understand today.
Conclusion
The Paleocene-Eocene thermal maximum event is best explained by Thomas et al in this
sequence of events. Global warming occurs first in surface waters slowly moving downward
through the water column to the thermocline and intermediate depths. Subduction or
downwelling of warmer surface waters leads to thermal dissociation of methane hydrate with a
significant ratio of sediment to hydrate content. Methane from oceanic hydrates then dissociates
after it becomes unstable and reaches the atmosphere prior to oxidation taking place. Oxidation
drives the conversion of methane toward carbon dioxide. This exponential increase in
greenhouse gases results in increased radiative absorption, increased evaporation, and increased
sensitivity to further increases in greenhouse gases. By this time, global warming will grow until

11. the methane reservoir runs dry. Once the reservoir runs dry, Earth then goes into a recover or
cooling phase to allow the sequestration of the buildup of carbon in the atmosphere out of the
system.
References
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