"One of our tasks was to summarize the most important discoveries about carbon dioxide and oxygen in the atmosphere and ocean over the past 500 million years," says Syracuse University geochemistry professor Zunli Lu, lead author on the paper. "We reviewed how those physical changes affected the evolution of life in the ocean. But it's a two-way street. The evolution of life also impacted the chemical environment. It is not a trivial task to understand how to build a habitable Earth over long time scales."

The team from Syracuse University, Oxford University, and Stanford University investigated the complex feedback mechanisms between ancient life forms, such as plants and animals, and the chemical environment during the Phanerozoic Eon, which began around 540 million years ago.

At the beginning of the Phanerozoic, atmospheric carbon dioxide levels were high, and oxygen levels were low, conditions that would challenge many modern organisms. Ocean algae played a critical role by absorbing atmospheric carbon dioxide, locking it into organic matter, and producing oxygen through photosynthesis.

The ability of animals to survive in ocean environments was influenced by oxygen levels. Lu is examining where and when ocean oxygen levels may have varied during the Phanerozoic using geochemical proxies and model simulations. Co-author Jonathan Payne, a professor of Earth and planetary sciences at Stanford University, compares ancient animals' metabolic requirements to their survival or extinction in the fossil record.

As photosynthetic algae reduced atmospheric carbon dioxide and increased oxygen levels, their photosynthetic enzymes became less efficient. Algae adapted by developing internal compartments to manage photosynthesis more effectively under these new conditions.

"For algae, it is changes in the environmental ratio of O2/CO2 that seems to be key to driving improved photosynthetic efficiency," says co-author Rosalind Rickaby, a professor of geology at Oxford. "What is really intriguing is that these improvements in photosynthetic efficiency may have expanded the chemical envelope of habitability for many forms of life."

Ancient photosynthesizers had to adjust to the changes in the environment they had created, Lu notes. "The first part of the history of the Phanerozoic is increasing habitability for life, and then the second part is adaptation."

To further understand the interplay between life and the physical environment, as well as the limits on habitability, the authors suggest that future research should focus on mapping the spatial patterns of ocean oxygen, biomarkers for photosynthesis, and animals' metabolic tolerance as shown in fossil records.

Research Report:Phanerozoic co-evolution of O2-CO2 and ocean habitability

Related Links
Syracuse University
Explore The Early Earth at TerraDaily.com