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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 CO2.

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 on stoneMoss 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 CO2 level had dropped to about eight times its present value.

Figure captionOne 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 CO2.[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 CO2 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:

  1. Sid Perkins, "Did Plants Freeze the Planet?" Science Now, February 1, 2012.
  2. 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.
  3. First plants caused ice ages, University of Exeter Press Release, February 1, 2012.
  4. First land plants may have plunged the Earth into a series of ice ages, Guardian (UK), February 1, 2012.
  5. 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.
  6. Editorial - One and only Earth, Nature Geoscience, vol. 5, no. 2 (February 1, 2012), p. 81.
  7. Mark Fischetti, "Thanks to Plants, We Will Never Find a Planet Like Earth," Scientific American, February 1, 2012.