Earth’s oceans are teeming with life thanks to the oxygenated waters which allow that life to grow and thrive: just as animals on land need oxygen to survive, so too do the fish and mammals beneath the sea surface. However around 499 million years ago the oceans suffered an event which turned them into a giant dead zone, meaning that they were anoxic, or “oxygen-poor”.
It had only been approximately a hundred million years earlier that the oceans had evolved from a relatively oxygen-poor environment to the oxygen filled environment that we are more familiar with today, during what was called Earth’s Late Ediacaran Period.
However new research shows that around 499 million years ago this changed, for a period of about two to four million years, just as the first animals began appearing on our planet.
“This work is important at many levels, from the steady growth of atmospheric oxygen in the last 600 million years, to the potential impact of oxygen level fluctuations on early evolution and diversification of life,” said Enriqueta Barrera, program director in the National Science Foundation (NSF)’s Division of Earth Sciences, which funded the research.
Reported in this week’s edition of the journal Nature, the authors argue that the transition from oxygen-poor to oxygen-rich was not a simple flick of the switch, as many had previous assumed.
“Our research shows that the ocean fluctuated between oxygenation states 499 million years ago,” said paper co-author Timothy Lyons, a UCR biogeochemist and co-author of the paper.
“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,” specifically the massive explosion of life which took place during the Cambrian Period, some 540 to 488 million years ago.
However, while we now know that the ocean did grow anoxic, we aren’t sure how it happened. “We have the ‘effect,’ but not the ’cause,'” said Benjamin Gill, the first author of the paper, a biogeochemist at UCR, and currently a postdoctoral researcher at Harvard University.
“The oxygen-poor state persisted 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.”
As with so many of the discoveries made about Earth’s past, the results hold information and hints for Earth’s present and future.
“Today, some sections of the world’s oceans are becoming oxygen-poor–the Chesapeake Bay (surrounded by Maryland and Virginia) 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 consequences can provide essential clues to what the future has in store for our oceans.”
Source: National Science Foundation
Image Source: Ben Gill, UC-Riverside and Harvard University