Sunday 5 February 2012

Did the first land plants cause an Ordovician glaciation?

The Ordovician Period lasted roughly 44 million years, from approximately 488 million years ago to about 444 million years ago. It is thought to have been extremely warm, possibly the warmest geological period since the evolution of large, complex lifeforms, with a CO₂ rich atmosphere, average sea temperatures reaching 45 °C and most of the continents covered by shallow seas. However the end of the period was marked by a glaciation event, the Hirnantian Glaciation, which saw a drop in sea levels, glaciers across large chunks of Africa and South America, and the second most severe mass extinction event in the fossil record (after the End Permian). Scientists have struggled to explain this sudden climatic shift, since the Ordovician atmosphere appears to have been too rich in CO₂ for glaciers to develop (CO₂ is a powerful greenhouse gas so an atmosphere with a lot of it will be a warm one), and consequently some fairly silly explanations have been put forward.

Map showing the distribution of land-masses, and the extent of sea coverage on them, from the Middle Ordovician. From The Paleontology Portal.

This month, in a paper in the journal Nature Geoscience, a team led by Timothy Lenton of the College of Life and Environmental Sciences at the University of Exeter and the Earth and Life Systems Alliance at the School of Environmental Sciences, University of East Anglia, lay out a new theory in which they propose that the first land plants may have caused a drop in CO₂ levels, brining about the Hirnantian Glaciation and indirectly causing an extinction event in the oceans.

Lenton et al. note that the Ordovician was a time of considerable mountain building (orogeny) in the northeast of North America, and that along with mountain building comes the potential for mountain erosion. They also note that the period saw the spread of the first non-vascular land plants. Since mosses are associated with erosion of rocks in the modern world, Lenton et al. reasoned that this might also have been the case in the Ordovician.

To measure the extent to which modern mosses help erode rocks Lenton et al. set up an experiment in which samples of granite and andesite (rocks commonly associated with newly formed mountains) were cultured with micro-organisms known to aid erosion, and with or without macerated (chopped up) mosses. They found that the cultures with the moss were much better at releasing a range of nutrients into the culture medium than those without.

Two cultures from Lenton et al.'s study; granite chips with (left) and without (right) moss. From Lenton et al. (2012), Supplementary Material.

Lenton et al. accept that moss has had a long time to evolve since the Ordovician, but suggest that the ability to release nutrients from rocks would have been just as advantageous to Ordovician mosses as it is to modern ones. They suggest that these early mosses may have released large amounts of nutrients into the Ordovician oceans, causing vast algal blooms, and that these algal blooms may have in turn absorbed much of the CO₂ from the atmosphere, causing the climate to turn abruptly colder, and bringing about the End Ordovician Mass Extinction.