It is difficult to draw comparisons between the Earth system of the Ordovician and that of our own day, because the distribution of terrestrial mass at the surface of the oceans was dramatically different than the distribution of continental land masses as we know it, with implications for ocean circulatory phenomena; the exchange of gases between the lithosphere, atmosphere, and ocean; the surface albedo (solar reflectance) of the Earth; and so on. Still, it is interesting to ask what could have caused the CO2 drawdown we observe in the climatological record of the late Ordovician. One hypothesis is that a key role was played by the earliest terrestrial plants.
These were avascular plants, similar to the bryophytes—mosses, hornworts, and liverworts—that feature in the understory of damp biomes today, along with lichens, a class of holobionts composed of algal or cyanobacterial colonies scaffolded on fungi. Lacking the endogenous vascular structure of tracheophytes, bryophytes and lichens depend on some mineral or ligneous substrate to provide both a physical scaffold and a growth medium. But in the Ordovician, there were no ligneous vascular plants, nor was there much of a humus. Terrestrial surfaces consisted of bare rock, notably basalts, newly erupted from the Earth’s magma. The minerals in this rock—phosphorus, magnesium, iron, calcium, potassium—provided the nutrition for the growth of bryophytes and lichens. Plant life depends on plants’ evolved strategies for accelerating the weathering of silicate mineral substrates, in the process drawing carbon out of the atmosphere and sequestering it in the ground. Experimental and simulation studies suggest enhanced weathering by avascular plants might have been sufficient to tip the Earth’s climate into a glacial cycle. Recently, enhanced weathering has been suggested as a strategy by which humans might intervene in the carbon cycle to slow climate change.