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Early Oceans May Have Been Devoid Of Oxygen


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
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 she had 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: those that lie beneath
waters, 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.
Figuring out just how much less oxygen was in the oceans in the ancient
past is the next step.  [...]