A geologist is studying the 'Snowball Earth' hypothesis, which suggests that Earth was com… — ACT

A geologist is studying the 'Snowball Earth' hypothesis, which suggests that Earth was completely covered in ice during the Cryogenian period. Two scientists propose different mechanisms for how the planet deglaciated (melted).

Scientist 1
Deglaciation was triggered solely by the slow, continuous accumulation of volcanic carbon dioxide ( $CO_2$ ) in the atmosphere over millions of years. Because the ice cover prevented chemical weathering (which removes $CO_2$ from the air), atmospheric $CO_2$ levels rose to approximately $120,000\text{ ppm}$ ( $350$ times modern levels). This extreme greenhouse effect eventually provided enough warming to melt the equatorial ice. The subsequent rapid weathering of silicate rocks precipitated this massive atmospheric reservoir of $CO_2$ directly into the oceans, forming the thick layers of 'cap carbonates' observed globally today.

Scientist 2
Deglaciation was initiated by a sudden release of methane ( $CH_4$ ) from gas hydrates trapped in marine sediments beneath the ice. A minor geothermal warming event destabilized these hydrates, releasing large volumes of methane into the atmosphere through fractures in the ice sheet. Because methane is a far more potent greenhouse gas than $CO_2$ , it triggered rapid global melting within a few thousand years. The released methane was rapidly oxidized in the atmosphere and oceans to form bicarbonate ions, which precipitated as cap carbonates.

New Evidence
Geochemists analyzed the carbon isotope ratio ( $\delta^{13}C$ , expressed in parts per thousand, $\text{‰}$ ) of the cap carbonates. Volcanic emissions typically have a $\delta^{13}C$ value of approximately $-6\text{‰}$ , whereas biogenic methane from gas hydrates has a $\delta^{13}C$ value of approximately $-60\text{‰}$ . The researchers found that the bottommost (oldest) layers of the cap carbonates had $\delta^{13}C$ values of $-55\text{‰}$ to $-60\text{‰}$ , while the upper (younger) layers gradually shifted to values of $-6\text{‰}$ .

Based on the information provided, how does the new evidence impact the scientists' hypotheses?

It supports Scientist 2's hypothesis because the very low $\delta^{13}C$ values in the oldest carbonate layers indicate that the carbon was initially derived from methane rather than volcanic emissions.Answer
B
It supports Scientist 1's hypothesis because the transition to $\delta^{13}C$ values of $-6\text{‰}$ in the younger layers shows that volcanic $CO_2$ was the primary driver of the initial deglaciation.
C
It weakens both hypotheses because the shifting isotope values indicate that the carbon source changed over time, whereas both scientists claimed the carbon source remained constant.
D
It has no effect on either hypothesis because cap carbonates are formed by chemical precipitation, which does not alter the isotopic composition of carbon regardless of its source.

Answer

The new evidence supports Scientist 2's hypothesis because the very low

\delta^{13}C

values in the oldest carbonate layers indicate that the carbon was initially derived from methane rather than volcanic emissions.

The new evidence shows that the oldest (bottommost) layers of the cap carbonates, which represent the initial phase of deglaciation, have

\delta^{13}C

values of

-55\text{‰}

-60\text{‰}

. This matches the signature of biogenic methane from gas hydrates (

-60\text{‰}

) rather than volcanic emissions (

-6\text{‰}

). This directly supports the hypothesis that the initial melting was triggered by methane release, as proposed by the second scientist.

Step-by-Step Solution

Identify the carbon isotope signatures (

\delta^{13}C

) associated with each source described in the new evidence.

Volcanic emissions are associated with a

\delta^{13}C

value of approximately

-6\text{‰}

, while biogenic methane is associated with approximately

-60\text{‰}

This establishes the baseline signatures needed to interpret the data.

Analyze the carbon isotope profile of the cap carbonates over time.

The oldest, bottommost layers show values of

-55\text{‰}

-60\text{‰}

, and the younger, upper layers show a shift toward

-6\text{‰}

Understanding the chronological sequence of deposition helps identify which source was present at the initiation of the melting event.

Evaluate the impact of this profile on the two hypotheses.

The presence of a methane-like signature in the oldest layers supports the model where methane release initiated deglaciation (Scientist 2), while the absence of a volcanic signature in these initial layers weakens the model where volcanic

CO_2

initiated it (Scientist 1).

This allows us to select the option that correctly describes the evidence supporting Scientist 2 based on the timing of the isotopic signatures.

Key Concept

Evaluating how new physical evidence matches predictions made by competing scientific hypotheses based on isotopic signatures and timing.

Estimated Time:3m 0s

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