It is widely accepted that the oceans became oxygen-rich to the point they are today about 600 million years ago, in the Late Eriacaran Period. Biochemists at the University of California, Riverside have recently found evidence that the ocean went back to being anxic, or oxygen-poor, around 499 million years ago, soon after the first appearance of animals on the planet. Their findings suggest that the ocean stayed relatively anoxic for 2-4 million years. It is possible that these conditions may have been common for a longer period.
The researchers argue that this fluctuation in the ocean’s oxygen state is what drove the evolutionary explosion in the Cambrian Period (540 to 488 million years ago).
In the January 6 issue of Nature, they report that the widely accepted view that the transition from a generally oxygen-rich ocean during the Cambrian period to the fully oxygenated ocean today was not a simple switch.
“Our research shows the ocean fluctuate between oxygenation states 499 million years ago,” said co-author Timothy Lyons, a professor of biogeochemistry, whose lab led the research, “and such fluctuations played a major, perhaps dominant, role in shaping the early evolution of animals on the planet by driving extinction and clearing the way for new organisms to take their place.”
While oxygen is a necessity for many ocean-dwellers, many bacteria thrive and even require anoxic conditions. Understanding how the environment changed can help scientists find out how life evolved and flourished during the early stages of animal evolution.
“Life and the environment in which it lives are intimately linked,” said Benjamin Gill, the first author of the research paper, who worked in Lyon’s lab as a graduate student. Gill explained that when the ocean’s oxygenation states change rapidly, some organisms were not able to cope. Changes in states of oxygenation also effect levels of other elements such as iron, phosphorus and nitrogen.
“Disruption of these cycles is another way to drive biological crises,” he said. “Thus both directly and indirectly a switch to an oxygen-poor state of the ocean can cause major extinction of species.”
The team is now working on finding a reason for why the oceans became anoxic 499 million years ago.
“What we have found so far is evidence that it happened,” Gill said. “We have the ‘effect,’ but not the ’cause.’ The oxygen-poor state persisted for 2-4 million years, likely until the enhanced burial of organic matter, originally derived from oxygen-producing photosynthesis, resulted in the accumulation of more oxygen in the atmosphere and ocean. As a kind of negative feedback, the abundant burial of organic material facilitated by anoxia may have bounced the ocean to a more oxygen-rich state.”
Gill explains that understanding past events can help refine our view of changes happening on the planet today.
“Today, some sections of the world’s ocean are becoming oxygen-poor — the Chesapeake Bay and the so-called ‘dead zone’ in the Gulf of Mexico are just two examples,” he said. “We know the Earth went through similar scenarios in the past. Understanding the ancient causes and consquences can provide essential clues to what the future has in store for our ocean.”
In the study, Lyons, Gill and their team examined the carbon, sulfur and molybdenum contents of rocks they collected from the U.S., Sweden and Australia. Combined, these analyses allowed the team to infer the level of oxygen in the ocean at the time the rocks were deposited. They were able to compile a biogeochemical history of the ocean by looking at successive rock layers.
Lyons and Gill were joined by Seth A. Young of Indiana University, Bloomington; Lee R. Kump of Penn State University; Andrew H. Knoll of Harvard University; and Matthew R. Saltzman of Ohio State University. Gill is a postdctoral researcher at Harvard University. The study was funded by a grant from the U.S. National Science Foundation.
Copyright © 2011 by Marine Science Today, a publication of OceanLines LLC