During the Paleocene-Eocene Thermal Maximum (PETM), global temperatures rose rapidly, and … — ACT

During the Paleocene-Eocene Thermal Maximum (PETM), global temperatures rose rapidly, and marine sediment records show a sharp decrease in the ratio of carbon-13 to carbon-12 (represented as $\delta^{13}\text{C}$ , measured in parts per thousand, $‰$ ). Two scientists discuss the source of this carbon isotope excursion.

Scientist 1
The PETM warming was triggered by the sudden dissociation of marine methane hydrates (solid lattices of ice trapping methane gas) on the seafloor. Methane hydrates contain biogenic methane, which is highly depleted in $^{13}\text{C}$ (having a $\delta^{13}\text{C}$ value of approximately $-60\text{‰}$ ). Due to this extremely negative signature, releasing approximately $2{}000\text{ Gt}$ (gigatons) of carbon from hydrates is sufficient to cause the observed global decrease in marine $\delta^{13}\text{C}$ .

Scientist 2
The carbon release was caused by volcanic intrusions (sills) from the North Atlantic Igneous Province heating organic-rich sedimentary basins. This heated organic matter released thermogenic methane and carbon dioxide gas. Thermogenic carbon is moderately depleted in $^{13}\text{C}$ , with an average $\delta^{13}\text{C}$ value of approximately $-35\text{‰}$ . To produce the observed global decrease in marine $\delta^{13}\text{C}$ using this less-depleted carbon source, a much larger release of carbon—approximately $4{}500\text{ Gt}$ —was required.

Suppose a new geochemical reconstruction determines that the total mass of carbon released during the PETM was approximately $2{}100\text{ Gt}$ . How does this finding align with the viewpoints of the two scientists?

It supports Scientist 1's viewpoint but contradicts Scientist 2's viewpoint.Answer
B
It supports Scientist 2's viewpoint but contradicts Scientist 1's viewpoint.
C
It supports both Scientist 1's and Scientist 2's viewpoints.
D
It contradicts both Scientist 1's and Scientist 2's viewpoints.

Answer

The finding supports Scientist 1's viewpoint but contradicts Scientist 2's viewpoint, as 2,100 Gt is close to Scientist 1's prediction of 2,000 Gt and far below Scientist 2's prediction of 4,500 Gt.

The correct option is supported because the new finding of 2,100 Gt is consistent with the approximately 2,000 Gt predicted by Scientist 1, while it directly contradicts the 4,500 Gt predicted by Scientist 2.

Step-by-Step Solution

Identify the carbon release prediction made by Scientist 1.

Scientist 1 predicts that approximately

2{}000\text{ Gt}

of carbon was released.

This establishes the predicted quantity of carbon required to explain the isotope excursion according to the first hypothesis.

Identify the carbon release prediction made by Scientist 2.

Scientist 2 predicts that approximately

4{}500\text{ Gt}

of carbon was released.

This establishes the predicted quantity of carbon required to explain the isotope excursion according to the second hypothesis.

Compare the new finding of

2{}100\text{ Gt}

to both scientists' predictions.

The value of

2{}100\text{ Gt}

is close to the

2{}000\text{ Gt}

prediction of Scientist 1, supporting it, but is significantly lower than the

4{}500\text{ Gt}

prediction of Scientist 2, contradicting it.

Determining how the data points align allows us to evaluate which viewpoint is supported or contradicted.

Key Concept

Aligning Data and Predictions with Viewpoints

Question

Answer

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