For at least a billion years of the distant past, planet
Earth should have been frozen over but wasn't. Scientists thought they knew
why, but a new modeling study from the Alternative Earths team of the NASA
Astrobiology Institute has fired the lead actor in that long-accepted scenario.
Humans worry about greenhouse gases, but between 1.8 billion
and 800 million years ago, microscopic ocean dwellers really needed them. The
sun was 10 to 15 percent dimmer than it is today - too weak to warm the planet
on its own. Earth required a potent mix of heat-trapping gases to keep the
oceans liquid and livable.
For decades, atmospheric scientists cast methane in the
leading role. The thinking was that methane, with 34 times the heat-trapping
capacity of carbon dioxide, could have reigned supreme for most of the first
3.5 billion years of Earth history, when oxygen was absent initially and little
more than a whiff later on. (Nowadays oxygen is one-fifth of the air we
breathe, and it destroys methane in a matter of years.)
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| Full structural formula of the methane molecule |
"A proper accounting of biogeochemical cycles in the
oceans reveals that methane has a much more powerful foe than oxygen,"
said Stephanie Olson, a graduate student at the University of California,
Riverside, a member of the Alternative Earths team and lead author of the new
study published September 26 in the Proceedings of the National Academy of
Sciences. "You can't get significant methane out of the ocean once there
is sulfate."
Sulfate wasn't a factor until oxygen appeared in the
atmosphere and triggered oxidative weathering of rocks on land. The breakdown
of minerals such as pyrite produces sulfate, which then flows down rivers to
the oceans. Less oxygen means less sulfate, but even 1 percent of the modern
abundance is sufficient to kill methane, Olson said.
Olson and her Alternative Earths coauthors, Chris Reinhard,
an assistant professor of earth and atmospheric sciences at Georgia Tech
University, and Timothy Lyons, a distinguished professor of biogeochemistry at
UC Riverside, assert that during the billion years they assessed, sulfate in
the ocean limited atmospheric methane to only 1 to 10 parts per million - a
tiny fraction of the copious 300 parts per million touted by some previous
models.
The fatal flaw of those past climate models and their
predictions for atmospheric composition, Olson said, is that they ignore what
happens in the oceans, where most methane originates as specialized bacteria
decompose organic matter.
Seawater sulfate is a problem for methane in two ways:
Sulfate destroys methane directly, which limits how much of the gas can escape
the oceans and accumulate in the atmosphere. Sulfate also limits the production
of methane. Life can extract more energy by reducing sulfate than it can by
making methane, so sulfate consumption dominates over methane production in
nearly all marine environments.
The numerical model used in this study calculated sulfate
reduction, methane production, and a broad array of other biogeochemical cycles
in the ocean for the billion years between 1.8 billion and 800 million years
ago. This model, which divides the ocean into nearly 15,000 three-dimensional
regions and calculates the cycles for each region, is by far the highest resolution
model ever applied to the ancient Earth. By comparison, other biogeochemical
models divide the entire ocean into a two-dimensional grid of no more than five
regions.
"Free oxygen [O2] in the atmosphere is required to form
a protective layer of ozone [O3], which can shield methane from photochemical
destruction," Reinhard said. When the researchers ran their model with the
lower oxygen estimates, the ozone shield never formed, leaving the modest puffs
of methane that escaped the oceans at the mercy of destructive photochemistry.
With methane demoted, scientists face a serious new
challenge to determine the greenhouse cocktail that explains our planet's
climate and life story, including a billion years devoid of glaciers, Lyons
said. Knowing the right combination other warming agents, such as water vapor,
nitrous oxide, and carbon dioxide, will also help us assess habitability of the
hundreds of billions of other Earth-like planets estimated to reside in our
galaxy.
"If we detect methane on an exoplanet, it is one of our
best candidates as a biosignature, and methane dominates many conversations in
the search for life on Mars," Lyons said. "Yet methane almost
certainly would not have been detected by an alien civilization looking at our
planet a billion years ago - despite the likelihood of its biological
production over most of Earth history."
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