Ordovician Carbon Sinks
February 28, 2012
Although
Lovelock's Gaia hypothesis might be a little too heavy on
philosophy and light on actual
science, there's no question that the presence of
living organisms has shaped
Earth's environment. It's also true that Earth's
biosphere owes its particular composition to the handy
symbiosis between
plant and
animal metabolism. Plants take in
CO2 from the
atmosphere and create
oxygen. Animals breathe
O2 and exhale CO
2.
Aside from a large dose of
anthropogenic carbon dioxide, the atmosphere seems to regulate itself quite well, perhaps because of control mechanisms like those postulated by the Gaia hypothesis. In the long history of the Earth, things weren't always that way.
There was a time when there were few plants on Earth' surface. Then, about 450 million years ago, plant life began to cover the Earth during the
Ordovician period. These early land plants were primitive,
non-vascular mosses, probably evolved from
green algae.
Moss growing on a dry stone fence. Primitive mosses were the first land plants.
(Photo by P. Smith, via Wikimedia Commons))
The Earth was a very different place about 460 million years ago. The Earth was still subject to frequent impacts from large
meteorites. Atmospheric carbon dioxide was about twenty times the present level, and the
average global temperature was 5°C (9°F) higher than it is now.[1] This is all the more remarkable, because the
Sun emitted 6% less
radiation than it does today. Earth's land masses consisted of two
supercontinents,
Gondwana (a.g.a. Gondwanaland)
Laurasia. The most interesting thing about the the end of this
geological period, the
Late Ordovician, is that there was a tremendous cooling of the Earth and an onset of
glaciation.
This glaciation appeared in two events, and it extended over a period of about ten million years.
Ice covered most of Gondwana, which was situated near the
South Pole. This glaciation may have been a contributing factor to the
mass extinction of
sea creatures than occurred in the Late Ordovician.[1]
Climate models predict that such glaciation during that period could not have occurred unless the atmospheric CO
2 level had dropped to about eight times its present value.
One way to check your browser's fonts - The Icelandic language.
Gígjökull, an outlet glacier extending from Eyjafjallajökull, Iceland.
(Photo by Andreas Tille, via Wikimedia Commons))
A recent paper in
Nature Geoscience, written by a team of
UK geoscientists from the
University of Exeter, the
University of East Anglia (Norwich, UK), the
John Innes Centre (Norwich, UK) and
Oxford University, connects the initial vegetation of the Earth with this glaciation.[1-6]
The cause was not their
fixation of carbon from the atmosphere; rather, it was a
chemical effect induced by the
weathering of
Earth's crust caused by the vegetation. This weathering had two effects. The plants removed
calcium,
magnesium,
phosphorus and
iron from rocks, which made them more chemically active, and it made nutrients available for marine vegetation.[1,3-4] It further exposed a greater surface area of rock material, which allowed formation of
carbonates from the atmospheric CO
2.[1]
The UK scientists tested their
hypothesis in
laboratory experiments. They grew the non-vascular moss,
Physcomitrella patens, on
andesite, a common
silicate rock, in sealed containers in the presence of
water for 130 days. As a control, they also sealed containers with just rock and water.[1,3] The moss increased the weathering of calcium by a factor of 3.6, and magnesium by a factor of 5.4.[1-2] Climate models predict that just 15% coverage of the Earth in such moss would have reduced the atmospheric CO
2 level to a little over eight times what it is today, low enough for the glaciation.[1-2]
In experiments on
granite, the moss increased weathering of iron sixty-fold, and phosphorus, 170-fold. Both of these nutrients would have encouraged marine algae growth.[1] Late in the Ordovician, vascular plants would have populated Earth's surface, bringing on the second observed glaciation.[1-2]
Oxford's
Liam Dolan, one of the study's authors, had this comment on their research.
"For me the most important take-home message is that the invasion of the land by plants – a pivotal time in the history of the planet - brought about huge climate changes. Our discovery emphasizes that plants have a central regulatory role in the control of climate: they did yesterday, they do today and they certainly will in the future."[3]
However, lest we think that planting more trees in the
Amazon rainforest will solve all our
global warming problems, lead author,
Tim Lenton, cautions that "...It would take millions of years for plants to remove current carbon emissions from the atmosphere."[3]
An editorial in the same issue of Nature Geoscience in which this research was published brings us back to the Gaia hypothesis mentioned at the beginning of this article. The editorial remarks that many
exoplanets are being discovered, but it's not just Earth's size and
orbit from its sun that makes it "Earthlike." Plants have modified Earth's atmosphere, land surface and oceans far beyond what the planet looked like early in the Ordovician.[6,7]
References:
- Sid Perkins, "Did Plants Freeze the Planet?" Science Now, February 1, 2012.
- Timothy M. Lenton, Michael Crouch, Martin Johnson, Nuno Pires and Liam Dolan, "First plants cooled the Ordovician," Nature Geoscience, vol. 5, no. 2 (February 1, 2012), pp. 86-89.
- First plants caused ice ages, University of Exeter Press Release, February 1, 2012.
- First land plants may have plunged the Earth into a series of ice ages, Guardian (UK), February 1, 2012.
- Rob Waugh, "The arrival of the first plants cooled the planet so much that it triggered a series of ice ages," Daily Mail (UK), February 1, 2012.
- Editorial - One and only Earth, Nature Geoscience, vol. 5, no. 2 (February 1, 2012), p. 81.
- Mark Fischetti, "Thanks to Plants, We Will Never Find a Planet Like Earth," Scientific American, February 1, 2012.
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