At the bottom of a frigid Antarctic lake, a thin layer of
green slime is generating a little oasis of oxygen, a team including UC Davis
researchers has found. It's the first modern replica discovered of conditions
on Earth two and a half billion years ago, before oxygen became common in the
atmosphere. The discovery is reported in a paper in the journal Geology.
The switch from a planet with very little available oxygen
to one with an atmosphere much like today's was one of the major events in
Earth's history, and it was all because some bacteria evolved the ability to
photosynthesize. By about 2.4 billion years ago, geochemical records show that
oxygen was present all the way to the upper atmosphere, as ozone.
What is not clear is what happened in between, or how long
the transition - called the Great Oxidation Event - lasted, said Dawn Sumner,
professor and chair of earth and planetary sciences at UC Davis and an author
on the paper. Scientists have speculated that here may have been "oxygen
oases," local areas where was abundant before it became widespread around
the planet.
The new discovery in Lake Fryxell in the McMurdo Dry Valleys
could be a modern example of such an ancient oxygen oasis, and help geochemists
figure out what to look for in ancient rocks, Sumner said.
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| Lake Fryxell. Credit: Tyler Mackey, UC Davis |
Sumner and collaborators including Ian Hawes of the
University of Canterbury, New Zealand have been studying life in these
ice-covered lakes for several years. The microbes that survive in these remote
and harsh environments are likely similar to the first forms of life to appear
on Earth, and perhaps on other planets.
The discovery occurred "a little by accident,"
Sumner said. Hawes and Tyler Mackey, a UC Davis graduate student working with
Sumner, were helping out another research team by diving in Lake Fryxell. The
lakes of the Dry Valleys typically contain oxygen in their upper layers, but
are usually anoxic further down, Sumner said. Lake Fryxell is unusual because
it becomes anoxic at a depth where light can still penetrate.
During their dives below the oxygen zone, Hawes and Mackey
noticed some bright green bacteria that looked like they could be
photosynthesizing. They took measurements and found a thin layer of oxygen,
just one or two millimeters thick, being generated by the bacteria.
Something similar could have been happening billions of
years ago, Sumner said.
"The thought is, that the lakes and rivers were anoxic,
but there was light available, and little bits of oxygen could accumulate in
the mats," she said.
The researchers now want to know more about the chemical
reactions between the "oxygen oasis" and the anoxic water immediately
above it and sediments below. Is the oxygen absorbed? What reactions occur with
minerals in the water?
Understanding how this oxygen oasis reacts with the environment
around it could help identify chemical signatures preserved in rocks.
Researchers could then go looking for similar signatures in rocks from ancient
lake beds to find "whiffs of oxygen" prior to the Great Oxidation
Event.
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