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Difficulty: HardAligning Data and Predictions with Viewpoints

Two scientists discuss how Earth's surface remained warm during the Archean Eon (3.83.8 to 2.52.5 billion years ago), when the Sun's energy output was only 70% to 75% of its current value.

Scientist 1
Atmospheric warming was driven primarily by high levels of carbon dioxide (CO2CO_2) and water vapor. CO2CO_2 levels were at least 100 to 1,000 times modern pre-industrial levels due to high volcanic outgassing and slow continental silicate weathering. Methane (CH4CH_4) played a negligible role because atmospheric oxygen (O2O_2), although low, was sufficient to generate hydroxyl radicals that rapidly oxidized methane, keeping its concentration below 1 ppm1\text{ ppm}.

Scientist 2
CO2CO_2 alone cannot resolve the warming because geochemical analysis of Archean paleosols (fossil soils) indicates that CO2CO_2 levels were constrained to less than 100 times modern pre-industrial levels. Instead, warming was sustained by a mixture of CO2CO_2 and CH4CH_4. Methanogenic bacteria in the Archean oceans produced massive quantities of CH4CH_4, which accumulated to levels above 1,000 ppm1,000\text{ ppm}. This was possible because the atmosphere was virtually anoxic (O2O_2 levels less than 10−510^{-5} of modern levels), preventing methane oxidation.

Suppose a geochemical study of 3.0-billion-year-old3.0\text{-billion-year-old} marine sediments reveals that the atmospheric O2O_2 level was 10−610^{-6} of the modern level and the atmospheric CO2CO_2 level was 40 times the modern pre-industrial level. This finding would most directly support the viewpoint of which scientist, if either?

  1. A
    Scientist 1, because the low O2O_2 level would prevent CH4CH_4 from accumulating, confirming that CO2CO_2 was the primary greenhouse gas.
  2. B
    Scientist 1, because a CO2CO_2 level of 40 times the modern level falls within their predicted range of 100 to 1,000 times the modern level.
  3. Scientist 2, because the low O2O_2 level is consistent with anoxic conditions that allow CH4CH_4 accumulation, and the CO2CO_2 level is within their predicted range.Answer
  4. D
    Scientist 2, because the high CO2CO_2 level demonstrates that methane was not required to maintain liquid water.

Answer

Scientist 2, because the low O2O_2 level is consistent with anoxic conditions that allow CH4CH_4 accumulation, and the CO2CO_2 level is within their predicted range.
The correct option accurately maps the findings to Scientist 2's model. Scientist 2 asserts that carbon dioxide (CO2CO_2) was less than 100 times the modern pre-industrial level, and the measured value of 40 times fits this constraint. Furthermore, Scientist 2 claims that methane (CH4CH_4) could accumulate if oxygen (O2O_2) was less than 10−510^{-5} of modern levels. The measured value of 10−610^{-6} is lower than 10−510^{-5}, satisfying this condition for methane accumulation.

Step-by-Step Solution

1
Analyze the new finding's carbon dioxide value and compare it to both scientists' predictions.
The finding indicates CO2CO_2 was 40 times the modern level. This value is less than 100, which contradicts Scientist 1's claim (CO2CO_2 at least 100 to 1,000 times) and aligns with Scientist 2's claim (CO2CO_2 less than 100 times).
This establishes which scientist's CO2CO_2 threshold is satisfied by the new data.
2
Analyze the new finding's oxygen value and compare it to both scientists' conditions for methane stability.
The finding indicates O2O_2 was 10−610^{-6} of the modern level. Since 10−6<10−510^{-6} < 10^{-5}, the atmosphere was indeed virtually anoxic as defined by Scientist 2, allowing methane to accumulate and prevent oxidation.
This determines if the atmospheric conditions support methane accumulation according to the viewpoints.
3
Synthesize the results to determine the overall alignment.
Both the CO2CO_2 level (40 times modern) and the O2O_2 level (10−610^{-6} of modern) align perfectly with Scientist 2's model, supporting Scientist 2.
This leads to the correct matching of the data points to the competing viewpoints.

Key Concept

Aligning Data and Predictions with Viewpoints
Estimated Time:2m 0s
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Aligning Data and Predictions with Viewpoints

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