Final answer:
The likely cause of the carbon isotope excursion (CIE) at the close of the Eocene is thought to be a combination of factors including volcanic activity, climate change, and the input of organic carbon from terrestrial sources.
Step-by-step explanation:
The likely cause of the carbon isotope excursion (CIE) at the close of the Eocene (roughly -3.0 ‰) is thought to be a combination of factors including volcanic activity, climate change, and the input of organic carbon from terrestrial sources. During the Eocene, there was an increased input of volcanic carbon dioxide (CO₂) into the atmosphere, leading to a warming climate. This resulted in changes in the carbon cycle, causing a shift in the ratio of carbon isotopes.