### Deep Methane Origin Scientists debate the source of deep crustal methane (CH_4) deposi… — ACT

### Deep Methane Origin

Scientists debate the source of deep crustal methane ( $CH_4$ ) deposits. Two main hypotheses have been proposed:

Hypothesis 1
Methane is biogenic in origin, produced by anaerobic microbes (methanogens) that consume organic matter buried in sedimentary rock layers. These microbes cannot survive at temperatures above $120^\circ\text{C}$ or in environments lacking organic material.

Hypothesis 2
Methane is abiogenic in origin, formed in the mantle through inorganic chemical reactions between carbon dioxide ( $CO_2$ ) and hydrogen-bearing minerals under high temperatures and pressures. This methane then migrates upward through faults into the crust, carrying trace mantle gases like helium.

A new study analyzes gas samples from a deep borehole drilled into an ancient granitic shield, a region containing no sedimentary rocks or organic matter. The borehole reached a depth where temperatures are constant at $150^\circ\text{C}$ . The gas samples recovered from this depth contain high concentrations of methane and mantle-derived helium isotopes.

Based on this information, how does this new evidence affect the two hypotheses?

It weakens Hypothesis 1 because the borehole environment exceeds the temperature limit for methanogens and lacks organic matter, and it supports Hypothesis 2.Answer
B
It supports Hypothesis 1 because it proves methane exists at deep levels, and it weakens Hypothesis 2 because abiogenic reactions cannot occur in granitic crust.
C
It has no effect on Hypothesis 1 because methanogens can adapt to higher temperatures, and it weakens Hypothesis 2 because helium is not an inorganic mineral.
D
It weakens both Hypothesis 1 and Hypothesis 2 because neither hypothesis accounts for the presence of methane in deep granitic shield regions.

Answer

The new evidence weakens Hypothesis 1 because the borehole environment exceeds the temperature limit for methanogens and lacks organic matter, and it supports Hypothesis 2 because the presence of mantle-derived helium aligns with migration from the mantle.

The correct option correctly evaluates the impact on both hypotheses. The borehole temperature of

150^\circ\text{C}

exceeds the survival limit (

120^\circ\text{C}

) of methanogens in Hypothesis 1, and the lack of sedimentary rocks or organic matter further contradicts the biogenic source requirement, thereby weakening Hypothesis 1. Meanwhile, the presence of mantle-derived helium indicates a deep mantle source and subsequent upward migration into the crust, supporting Hypothesis 2.

Step-by-Step Solution

Analyze the conditions in the borehole provided by the new evidence.

The borehole environment has a temperature of

150^\circ\text{C}

, lacks sedimentary rocks and organic matter, and contains methane along with mantle-derived helium.

This establishes the factual parameters to compare against the claims of each hypothesis.

Evaluate the impact of these conditions on Hypothesis 1.

Hypothesis 1 states methanogens cannot survive above

120^\circ\text{C}

and require organic matter in sedimentary layers. Because the borehole has a temperature of

150^\circ\text{C}

and has no sedimentary rocks or organic matter, the presence of methane here contradicts and weakens Hypothesis 1.

Determines whether the evidence supports or conflicts with the first model.

Evaluate the impact of these conditions on Hypothesis 2.

Hypothesis 2 states methane forms in the mantle and migrates upward into the crust, carrying trace mantle gases like helium. The presence of mantle-derived helium in the granitic crust borehole supports this upward migration, supporting Hypothesis 2.

Determines whether the evidence supports or conflicts with the second model.

Key Concept

Evaluating the Impact of New Evidence

Estimated Time:1m 30s

Question

Answer

Step-by-Step Solution

Key Concept

Common Mistakes

Topics

More questions from this topic