New evidence suggests early oceans bereft of oxygen for eons

Early life may have lived very differently than life today

Arlington, Va.-As two rovers scour Mars for signs of water and

the precursors of life, geochemists have uncovered evidence that

Earth’s ancient oceans were much different from today’s. The

research, published in this week’s issue of the journal Science,

cites new data that shows that Earth’s life-giving oceans

contained less oxygen than today’s and could have been nearly

devoid of oxygen for a billion years longer than previously

thought. These findings may help explain why complex life barely

evolved for billions of years after it arose.

The scientists, funded by the National Science Foundation (NSF)

and affiliated with the University of Rochester, have pioneered a

new method that reveals how ocean oxygen might have changed

globally. Most geologists agree there was virtually no oxygen

dissolved in the oceans until about 2 billion years ago, and that

they were oxygen-rich during most of the last half-billion years.

But there has always been a mystery about the period in between.

Geochemists developed ways to detect signs of ancient oxygen in

particular areas, but not in the Earth’s oceans as a whole. The

team’s method, however, can be extrapolated to grasp the nature

of all oceans around the world.

“This is the best direct evidence that the global oceans had less

oxygen during that time,” says Gail Arnold, a doctoral student of

earth and environmental sciences at the University of Rochester

and lead author of the research paper.

Adds Enriqueta Barrera, program director in NSF’s division of

earth sciences, “This study is based on a new approach, the

application of molybdenum isotopes, which allows scientists to

ascertain global perturbations in ocean environments. These

isotopes open a new door to exploring anoxic ocean conditions at

times across the geologic record.”

Arnold examined rocks from northern Australia that were at the

floor of the ocean over a billion years ago, using the new method

developed by her and co-authors, Jane Barling and Ariel Anbar.

Previous researchers had drilled down several meters into the

rock and tested its chemical composition, confirming it had kept

original information about the oceans safely preserved. The team

members brought those rocks back to their labs where they used

newly developed technology -called a Multiple Collector

Inductively Coupled Plasma Mass Spectrometer-to examine the

molybdenum isotopes within the rocks.

The element molybdenum enters the oceans through river runoff,

dissolves in seawater, and can stay dissolved for hundreds of

thousands of years. By staying in solution so long, molybdenum

mixes well throughout the oceans, making it an excellent global

indicator. It is then removed from the oceans into two kinds of

sediments on the seafloor: oxygen-rich and those that are oxygen-

poor.

Working with coauthor Timothy Lyons of the University of

Missouri, the Rochester team examined samples from the modern

seafloor, including the rare locations that are oxygen-poor

today. They learned that the chemical behavior of molybdenum’s

isotopes in sediments is different depending on the amount of

oxygen in the overlying waters. As a result, the chemistry of

molybdenum isotopes in the global oceans depends on how much

seawater is oxygen-poor. They also found that the molybdenum in

certain kinds of rocks records this information about ancient

oceans. Compared to modern samples, measurements of the

molybdenum chemistry in the rocks from Australia point to oceans

with much less oxygen.

How much less oxygen is the question. A world full of anoxic

oceans could have serious consequences for evolution. Eukaryotes,

the kind of cells that make up all organisms except bacteria,

appear in the geologic record as early as 2.7 billion years ago.

But eukaryotes with many cells-the ancestors of plants and

animals- did not appear until a half billion years ago, about the

time the oceans became rich in oxygen. With paleontologist

Andrew Knoll of Harvard University, Anbar previously advanced the

hypothesis that an extended period of anoxic oceans may be the

key to why the more complex eukaryotes barely eked out a living

while their prolific bacterial cousins thrived. Arnold’s study

is an important step in testing this hypothesis.

“It’s remarkable that we know so little about the history of our

own planet’s oceans,” says Anbar. “Whether or not there was

oxygen in the oceans is a straightforward chemical question that

you’d think would be easy to answer. It shows just how hard it

is to tease information from the rock record and how much more

there is for us to learn about our origins.”

Figuring out just how much less oxygen was in the oceans in the

ancient past is the next step. The scientists plan to continue

studying molybdenum chemistry to answer that question, with

continuing support from NSF and NASA, the agencies that supported

the initial work. The information will not only shed light on

our own evolution, but may help us understand the conditions we

should look for as we search for life beyond Earth.

NSF Program Contact: Enriqueta Barrera, ebarrera@nsf.gov, (703)

292-8550

National Science Foundation: Cheryl Dybas, cdybas@nsf.gov ,

(703)292-7734

University of Rochester: Jonathan Sherwood,

jonathan.sherwood@rochester.edu , (585)273-4726