Key takeaways:

1. Tropical waters near the equator once had relatively more oxygen in the ocean—the exact reverse of today, where the tropics host large marine dead zones.
2. A planetary tipping point probably between 570 and 500 million years ago flipped ocean oxygen patterns completely.
3. The oxygen pattern reversal coincided with an episode of animal life diversification.

The tropical oceans that once served as oxygen-rich havens for Earth's earliest complex life have become the planet's largest marine dead zones. The dramatic reversal occurred hundreds of millions of years ago and researchers are now beginning to better understand its timing.

A new study led by former Syracuse University doctoral student Ruliang He and co-authored by his advisor, Earth and environmental sciences Professor Zunli Lu, reveals that Earth's ancient tropical oceans were fundamentally different from today's. The research, recently published in Nature Geoscience, shows marine oxygen levels were higher near the equator than at the middle latitudes during the Proterozoic Eon, which began 2.5 billion years ago and ended 539 million years ago. Oxygen-rich tropical oceans are the exact opposite of the modern pattern and this discovery provides insights into how rising atmospheric oxygen transformed ocean chemistry and set the stage for animal evolution.

“The ocean is vast and the content of dissolved oxygen varies greatly at different locations, just like temperatures in Syracuse and Miami,” He says. “The dissolved oxygen content in geologic past was usually established from rock records of individual sites. In this study, we tried to understand its distribution pattern on a global scale.”

The authors compiled and analyzed a massive dataset of geochemical measurements from marine sedimentary rocks spanning the past 2 billion years. Using iodine-to-calcium ratios preserved in ancient carbonates, the team reconstructed oxygen levels across different latitudes and time periods.

"Oxygen is like a light switch that decides the chemical form of iodine in seawater,” Lu says. “The oxidized form of iodine gets preserved in carbonate rocks deposited in global oceans over time, slowly writing a history book of oxygen for us to flip through page by page.”

A Reversed Ocean

The research reveals that during the Proterozoic Eon, when atmospheric oxygen was very low, tropical waters had the highest oxygen concentrations. These were essentially oxygen oases in an otherwise largely anoxic ocean, sustained by photosynthetic organisms producing oxygen faster than it could be consumed. Today's pattern is reversed as warm tropical waters hold less dissolved oxygen, while cooler mid-latitude waters can hold more. Upwelling in tropical regions further depletes oxygen as organisms consume it while breaking down organic matter.

To understand the mechanisms driving this reversal, the research team partnered with Alexandre Pohl, paleoclimatologist at the National Center for Scientific Research (CNRS) in France. Earth system modeling experiments suggest that the shift occurred as atmospheric oxygen crossed a critical threshold—likely around 1 percent of present levels—fundamentally reorganizing ocean biogeochemistry.

The models demonstrate that at low atmospheric oxygen levels, biology controls oxygen distribution, with photosynthesis creating localized oxygen-rich zones. Once atmospheric oxygen crosses the threshold, physical processes take over, leading to a modern-like distribution of ocean oxygenation.

A Threshold for Life

According to available data, the transition likely occurred between 570 and 500 million years ago, hence immediately before one of the most dramatic episodes in the history of life: the Cambrian explosion, when animal diversity increased exponentially and marine ecosystems were fundamentally restructured. The study suggests that crossing the oxygen threshold increased overall oxygen availability and therefore reorganized where oxygen-dependent life could thrive.

“The understanding of how Earth’s physical/chemical environment evolved with biology is far from complete,” Lu says. “With this study, we are opening a new door to explore this relationship in multiple dimensions, not just through time.”

He's doctoral work built on Lu's expertise in iodine geochemistry and his laboratory's capabilities for analyzing ancient carbonates. The research demonstrates Syracuse University's growing strength in paleoceanographic research, combining analytical tools, theoretical frameworks and international collaborations to address fundamental questions about Earth history.

The research also involved contributions from Virginia Tech, Stanford University, the University of California Riverside and Northwest University in China.

The research was supported by the U.S. National Science Foundation and NASA's Interdisciplinary Consortia for Astrobiology Research program.