This is a scanned image of a cut and polished slab of the cap dolostone from South China that contains highly carbon-13-depleted carbonate. The view shown is 3 inches wide. Credit: Thomas Bristow |
There’s
a theory about how the Marinoan ice age—also known as the “Snowball
Earth” ice age because of its extreme low temperatures—came to an abrupt
end some 600 million years ago. It has to do with large amounts of
methane, a strong greenhouse gas, bubbling up through ocean sediments
and from beneath the permafrost and heating the atmosphere.
The
main physical evidence behind this theory has been samples of cap
dolostone from south China, which were known to have a lot less of the
carbon-13 isotope than is normally found in these types of carbonate
rocks. (Dolostone is a type of sedimentary rock composed of the
carbonate mineral, dolomite; it’s called cap dolostone when it overlies a
glacial deposit.) The idea was that these rocks formed when
Earth-warming methane bubbled up from below and was oxidized—”eaten”—by
microbes, with its carbon wastes being incorporated into the dolostone,
thereby leaving a signal of what had happened to end the ice age. The
idea made sense, because methane also tends to be low in carbon-13; if
carbon-13-depeleted methane had been made into rock, that rock would
indeed also be low in carbon-13. But the idea was controversial, too,
since there had been no previous isotopic evidence in carbonate rock of
methane-munching microbes that early in Earth’s history.
Unusual textures exposed in the cap dolostone from the field. Hand lens for scale is about 1 inch long. Credit: Thomas Bristow |
And,
as a team of scientists led by researchers from the California
Institute of Technology (Caltech) report in this week’s issue of the
journal Nature, it was also wrong—at least as far as the geologic
evidence they looked at goes.
Their
testing shows that the rocks on which much of that ice-age-ending
theory was based were formed millions of years after the ice age ended,
and were formed at temperatures so high there could have been no living
creatures associated with them.
“Our
findings show that what happened in these rocks happened at very high
temperatures, and abiologically,” says John Eiler, the Robert P. Sharp
Professor of Geology and professor of geochemistry at Caltech, and one
of the paper’s authors. “There is no evidence here that microbes ate
methane as food. The story you see in this rock is not a story about ice
ages.”
To
tell the rocks’ story, the team used a technique Eiler developed at
Caltech that looks at the way in which rare isotopes (like the carbon-13
in the dolostone) group, or “clump,” together in crystalline structures
like bone or rock. This clumping, it turns out, is highly dependent
upon the temperature of the immediate environment in which the crystals
form. Hot temperatures mean less clumping; low temperatures mean more.
Crystals of highly carbon-13-depleted carbonate observed using a light microscope. Credit: Thomas Bristow |
“The
rocks that we analyzed for this study have been worked on before,” says
Thomas Bristow, the paper’s first author and a former postdoc at
Caltech who is now at NASA Ames Research Center, “but the unique advance
available and developed at Caltech is the technique of using carbonate
clumped-isotopic thermometry to study the temperature of crystallization
of the samples. It was primarily this technique that brought new
insights regarding the geological history of the rocks.”
What
the team’s thermometer made very clear, says Eiler, is that “the carbon
source was not oxidized and turned into carbonate at Earth’s surface.
This was happening in a very hot hydrothermal environment, underground.”
In
addition, he says, “We know it happened at least millions of years
after the ice age ended, and probably tens of millions. Which means that
whatever the source of carbon was, it wasn’t related to the end of the
ice age.”
Since
this rock had been the only carbon-isotopic evidence of a Precambrian
methane seep, these findings bring up a number of questions—questions
not just about how the Marinoan ice age ended, but about Earth’s budget
of methane and the biogeochemistry of the ocean.
“The
next stage of the research is to delve deeper into the question of why
carbon-13-depleted carbonate rocks that formed at methane seeps seem to
only be found during the later 400 million years of Earth history,” says
John Grotzinger, the Fletcher Jones Professor of Geology at Caltech and
the principal investigator on the work described.
View from one of the cap dolostone collection sites in south China, looking along the cliffs of the Yangtze Gorges. Credit: Thomas Bristow |
“It
is an interesting fact of the geologic record that, despite a
well-preserved record of carbonates beginning 3.5 billion years ago, the
first 3 billion years of Earth history does not record evidence of
methane oxidation. This is a curious absence. We think it might be
linked to changes in ocean chemistry through time, but more work needs
to be done to explore that.”
In
addition to Bristow, Eiler, and Grotzinger, the other authors on the
Nature paper, “A hydrothermal origin for isotopically anomalous cap
dolostone cements from south China,” are Magali Bonifacie, a former
Caltech postdoc now at the Institut de Physique du Globe de Paris, and
Arkadiusz Derkowski from the Polish Academy of Sciences in Krakow.
The
work was supported by an O. K. Earl Postdoctoral Fellowship, by the
National Science Foundation’s Division of Earth Sciences and its
Geobiology and Environmental Geochemistry program, and by CNRS-INSU
(French research agency).
