### The Paleocene-Eocene Thermal Maximum During the Paleocene-Eocene Thermal Maximum (PETM… — ACT

### The Paleocene-Eocene Thermal Maximum

During the Paleocene-Eocene Thermal Maximum (PETM), a rapid injection of carbon into the atmosphere and oceans caused global warming and a significant negative carbon isotope excursion (CIE)—a sharp decrease in the ratio of carbon-13 ( $^{13}\text{C}$ ) to carbon-12 ( $^{12}\text{C}$ ) in sediment samples. Two hypotheses explain the source of this carbon.

Hypothesis 1 (Methane Clathrate Release)
The carbon was released via the sudden dissociation of methane clathrates (methane ice trapped in deep-sea sediments). Because clathrates contain biogenic methane, they are extremely depleted in $^{13}\text{C}$ ( $\delta^{13}\text{C} \approx -60\text{‰}$ ). Due to this extreme depletion, a relatively small mass of carbon (approximately $1,500\text{ to }2,000\text{ gigatons}$ ) is sufficient to cause the observed CIE.

Hypothesis 2 (Volcanic Outgassing)
The carbon was released by volcanic activity associated with the North Atlantic Igneous Province. Magma intruded into organic-rich basins, venting thermogenic methane and carbon dioxide ( $\text{CO}_2$ ). This volcanic/thermogenic carbon is moderately depleted in $^{13}\text{C}$ ( $\delta^{13}\text{C} \approx -30\text{‰}$ ). Because it is less depleted than biogenic methane, a much larger mass of carbon (approximately $4,000\text{ to }5,000\text{ gigatons}$ ) is required to cause the observed CIE.

New Evidence
Researchers recently calculated the total mass of carbon added to the oceans and atmosphere during the PETM onset to be approximately $4,500\text{ gigatons}$ .

Based on this new evidence, how does the calculated mass of $4,500\text{ gigatons}$ of carbon affect the two hypotheses?

A
It supports Hypothesis 1 and weakens Hypothesis 2, because the calculated mass falls within the predicted range for biogenic methane release.
It supports Hypothesis 2 and weakens Hypothesis 1, because a clathrate release of this mass would have produced a much larger negative CIE than was actually observed.Answer
C
It weakens both Hypothesis 1 and Hypothesis 2, because a carbon release of $4,500\text{ gigatons}$ would exceed the maximum capacity of both seafloor reservoirs and volcanic basins.
D
It has no effect on either hypothesis, because the total mass of carbon released is independent of the isotopic composition of the carbon source.

Answer

The new evidence supports Hypothesis 2 and weakens Hypothesis 1 because the calculated mass of

4,500\text{ gigatons}

falls within the

4,000\text{ to }5,000\text{ gigatons}

range predicted by the volcanic outgassing model, and releasing that amount of highly depleted clathrate methane would have caused a much larger excursion than observed.

The correct option is supported because the new evidence of

4,500\text{ gigatons}

of carbon falls directly within the

4,000\text{ to }5,000\text{ gigatons}

range predicted by Hypothesis 2. A release of

4,500\text{ gigatons}

of biogenic methane from clathrates, which is extremely depleted in

^{13}\text{C}

, would have produced a carbon isotope excursion far more negative than what was recorded in PETM sediments, thus weakening Hypothesis 1.

Step-by-Step Solution

Identify the predicted carbon mass required for Hypothesis 1.

Hypothesis 1 predicts that a mass of

1,500\text{ to }2,000\text{ gigatons}

is sufficient to cause the carbon isotope excursion.

This is due to the extreme

^{13}\text{C}

depletion of biogenic methane (

\delta^{13}\text{C} \approx -60\text{‰}

Identify the predicted carbon mass required for Hypothesis 2.

Hypothesis 2 predicts that a mass of

4,000\text{ to }5,000\text{ gigatons}

is required to cause the excursion.

This is because volcanic/thermogenic carbon is less depleted in

^{13}\text{C}

(

\delta^{13}\text{C} \approx -30\text{‰}

Compare the new evidence to the predictions of both hypotheses.

The calculated mass of

4,500\text{ gigatons}

falls within the range predicted by Hypothesis 2 but exceeds the range predicted by Hypothesis 1.

This supports the volcanic outgassing hypothesis and weakens the clathrate release hypothesis, as a release of

4,500\text{ gigatons}

of clathrate methane would cause a much larger excursion than observed.

Key Concept

Evaluating Competing Hypotheses Using Quantitative Evidence

Estimated Time:2m 0s

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