What this tells us is that the heat content of the crust declines to
a tipping point were the natural sinks no longer rapidly sponge up
all available oxygen. One can safely presume life processes were
long since forming oxygen.
Thus we can predict a similar genesis for Venus as its crust cools,
or any other planet for that matter that share our characteristics.
Thus the prospects for living planets is generally excellent. With
Venus we presently have a super dense atmosphere of CO2 and surely
plenty of both oxygen and carbon tied up in the crust itself.
Impacting the planet with water bearing comets would speed the
surface temperature drop and speed things up although plate tectonics
is not yet underway there as far as we can see. We could be really
wrong about that however, and what may be missing is significant
mountain building
Geological record
shows air up there came from below
by Staff Writers
Princeton NJ (SPX) May 25, 2012
The influence of the
ground beneath us on the air around us could be greater than
scientists had previously thought, according to new research that
links the long-ago proliferation of oxygen in Earth's atmosphere to
a sudden change in the inner workings of our planet.
Princeton University
researchers report in the journal Nature that rocks preserved in the
Earth's crust reveal that a steep decline in the intensity of
melting within the planet's mantle - the hot, heat-transferring rock
layer between the crust and molten outer core - brought about ideal
conditions for the period known as the Great Oxygenation Event (GOE)
that occurred roughly 2.5 billion years ago.
During the GOE - which
may have lasted up to 900 million years - oxygen levels in the
atmosphere exploded and eventually gave rise to our present
atmosphere.
Blair Schoene, a
Princeton assistant professor of geosciences, and lead author C.
Brenhin Keller, a Princeton geosciences doctoral student, compiled a
database of more than 70,000 geological samples to construct a
4-billion-year geochemical timeline. Their analysis uncovered a
sharp drop in mantle melting 2.5 billion years ago that coincides
with existing rock evidence of atmospheric changes related to the
GOE.
Based on this
correlation, the researchers suggest in Nature that diminished
melting in the mantle decreased the depth of melting in the Earth's
crust, which in turn reduced the output of reactive, iron oxide-based
volcanic gases into the atmosphere. A lower concentration of
these gases - which react with and remove oxygen from the atmosphere
- allowed free oxygen molecules to proliferate.
The Princeton research
offers the strongest data-driven correlation yet between deep Earth
processes and the GOE, Schoene said. Previous hypotheses are largely
based on qualitative observations of the rock record and
computational models that simulate how this rapid oxygenation might
have occurred. The Princeton research, however, is based on a
statistical analysis of the geologic record and the chemical traces
of deep-Earth activity it has preserved, Schoene said.
"The perspective
behind past efforts to connect geologic processes to
the Great Oxygenation Event has been hypothetical, saying that 'If
the Earth had been X, there would have been reaction Y,'"
Schoene said. "But these ideas cannot be tested experimentally
because they are largely notional. In our paper, we have the evidence
to say, 'The Earth was like this,' and then propose a hypothesis that
can be tested by examining the same rich database of mantle and
deep-crust changes we used in our work."
A change in subsurface
activity around the time of the GOE has been noted before, Keller
explained. But evidence of that shift is geochemically subtle,
especially after billions of years. The database he and Schoene
created allowed them to show more precisely how the geochemical
makeup of the crust changed through time, resulting in a more
detailed hypotheses about how this would affect the atmosphere,
Keller said.
"Research in this
area has been largely qualitative, but with this much data, we can
pick up finer features in the geologic record, particularly a level
of detail related to this sudden change 2.5 billion years ago that
people had not seen with such clarity before," Keller said.
A missing piece of
the GOE puzzle?
Woodward Fischer, an assistant professor of geobiology at the California Institute of Technology who specializes in the GOE, said that the Princeton research could help shed more light on an important factor in Earth's oxygenation that is not well understood. Fischer is familiar with the paper but had no role in it.
The dominant theory of
oxygenation is that an abundance of photosynthetic life emerged some
hundreds of millions of years before the GOE and began producing
oxygen via photosynthesis, Fischer said. The problem is that this
output would not have been enough to overcome "sinks" that
were absorbing more oxygen from the atmosphere than was being put
into it. So, a lingering question is what happened to those sinks to
bring about oxygenation.
Keller and Schoene
show how one of the primary sinks - volcanic gases - might have
suddenly been neutralized, Fischer said. The exact effect this would
have had on atmospheric oxygen levels is difficult to know - even
recent fluctuations are hard to gauge, he said. Nonetheless, the
clear and objective data the researchers use strongly suggests that a
quick reduction in volcanic gases brought about by a drop in
mantle-melt intensity was an important precursor to oxygenation,
Fischer said.
"This paper
offers a really striking assessment of changes occurring in the solid
Earth that greatly helped set the stage for one of the most marked
environmental transitions in Earth history," Fischer
said.
"And their
methodology precludes a strong tendency that researchers, as humans
invested in our work, have to look for anecdotal geological evidence
and conclude based on coincidence that events co-occurring in time
must have been related," Fischer said. "The statistical
approach taken by the authors in this paper really lets the data
shine and reveals that there were important secular changes in the
way the Earth made igneous rocks, and that these changes were
possibly part of an interplay between life and deep-Earth processes."
Keller and Schoene
fashioned their expansive database from previously reported rock and
trace element analyses, which are increasingly available through
online databases. They focused on changes in the chemical composition
of basalt, a byproduct of melting in the Earth's mantle.
When melting in the
mantle is high, Keller said, basalt contains greater concentrations
of "compatible" elements such as chromium and magnesium
that are ordinarily found in the mantle. Less intense melting, on the
other hand, results in basalt with a higher content of incompatible
elements such as sodium and potassium that are found closer to
the Earth's surface.
From their
examination, Keller and Schoene saw that the Earth's mantle has
undergone a gradual cooling since the planet's early history, which
is consistent with scientists' expectations based on heat loss at the
Earth's surface. Around 2.5 billion years ago, however, the levels
of compatible elements in the sampled basalt plummeted, indicating
that the magnitude of melting deep in the mantle dropped off
suddenly.
Keller and Schoene
confirmed their findings by checking them against existing analyses
of crust-level "felsic" rocks such as granite, which form
when hot basalt merges with other minerals. Heightened melt activity
in the mantle leads to deeper melting in the Earth's crust, and
felsic rocks can indicate the intensity of mantle melting, Keller
said.
The researchers
conclude that when melting happens at a great depth in the crust then
the concentration of the iron-oxide gases in magma increases. When
emitted into the air by volcanoes, these gases bond with free oxygen
and essentially remove it from the air. On the other hand, when crust
melting becomes shallower, as they observed, atmospheric levels of
those volcanic gases drop and free oxygen molecules can flourish.
Connecting
the Earth's systems
In a broader sense, said Schoene, his and Keller's research depicts a close interaction between the Earth's geologic and biological systems that is becoming more apparent. "In science, it is becoming increasingly obvious that seemingly different systems act together and the question is how," Schoene said.
"Overall, this
analysis strengthens emerging arguments that interaction between the
solid Earth and biosphere are very intimate and important,"
he said. "This is strong evidence of how biological and
geological systems might work together, and it suggests that
important planetary change is not simply the result of life dragging
the rest of the planet along."
Fischer of Caltech
added that this interplay of systems applies to various events in the
planet's history - such as mass extinctions - that are the result of
multiple factors both above and below the Earth's surface. Decidedly
more difficult is tracing how these events influenced one another and
ultimately led to a greater planetary change, he said.
"Because of the
complicated questions of how solid Earth changes lead
to biological innovations, scientists now have to start thinking
deeply and working across the boundaries of what have traditionally
been pretty rigid subdisciplines in the Earth sciences," Fischer
said.
"It's clear from
research like this," he said, "that there is hay to be made
by interdisciplinary efforts to connect processes and mechanisms from
the solid to the fluid Earth, and to understand that interplay with
an ever-evolving biology."
Related Links
Princeton University
The Air We Breathe at TerraDaily.com
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