### The Late Ordovician Mass Extinction
The Late Ordovician Mass Extinction (LOME), which occurred approximately 445 million years ago, resulted in the loss of about 85% of marine species. Two scientists discuss the potential triggers and environmental mechanisms responsible for this event.
Scientist 1
The LOME was primarily caused by a sudden, intense period of global cooling initiated by the growth of the Gondwanan ice sheet. This glaciation locked up water, causing global sea levels to drop by over , which eliminated shallow epicontinental sea habitats. The subsequent rapid deglaciation released vast amounts of freshwater, creating a stratified ocean. This stratification slowed thermohaline circulation and led to widespread marine anoxia (oxygen depletion) in the warming oceans, driving the second pulse of extinction. Throughout both pulses, atmospheric carbon dioxide () levels decreased significantly due to the rapid silicate weathering of the rising Appalachian Mountains, which drew down and drove the cooling.
Scientist 2
The LOME was triggered by large-scale volcanism in the Altai-Sayan region, which released massive quantities of greenhouse gases, primarily and sulfur dioxide (), into the atmosphere. The immediate result was intense global warming and severe ocean acidification, which devastated marine calcifiers. As volcanic activity subsided, the rapid chemical weathering of the newly exposed volcanic rocks caused a sharp drawdown of atmospheric , leading to a brief, secondary cooling phase and minor glaciation. The primary driver of the marine extinction, however, was widespread ocean anoxia. This anoxia persisted from the initial warming phase through the cooling phase because elevated temperatures and continental runoff fertilized massive algal blooms, whose decomposition depleted marine oxygen.
Match each environmental variable on the left to the statement on the right that best describes the specific point of disagreement between Scientist 1 and Scientist 2.
- Initial global temperature change during the onset of the extinction eventScientist 1 asserts that cooling occurred, whereas Scientist 2 asserts that warming occurred.
- Primary geological trigger of the extinction eventScientist 1 attributes it to glaciation, whereas Scientist 2 attributes it to volcanic eruptions.
- Mechanism driving marine anoxiaScientist 1 attributes it to slowed circulation from melting ice, whereas Scientist 2 attributes it to algal blooms.
- Trend in atmospheric levels at the initiation of the eventScientist 1 claims levels decreased due to continental weathering, whereas Scientist 2 claims levels increased due to outgassing.