Past climate change - Climate Change: Biological and Human Aspects - page 78

The above account is the standard story. More recently the notion has arisen that
the early terrestrial colonising vegetation, such as mosses (Bryophyta), could have
caused silicate drawdown of carbon dioxide, much like the later silicate drawdown
that occurred with the rise of vascular plants (see section 3.3.3). The idea is that,
even though mosses do not have the root system of vascular plants, they could have
enhanced the erosion of rock, albeit far more modestly that vascular plants: after all,
mosses have to access mineral nutrients too. Not only would this (as we shall shortly
see) result in carbon dioxide drawdown, causing cooling, but this erosion of rock (the
surfaces of which were previously devoid of such life) released nutrients. This would
have released phosphorous, which then got washed into the oceans, so enhancing
marine biological productivity that further drew carbon dioxide to the point where
glaciation took place. In 2012 British researchers Tim Lenton, Michael Crouch and
colleagues supported this idea with experiments determining the effect of mosses on
rock and then applied the results to a model of the carbon cycle to see the likely effect
on atmospheric carbon dioxide. This idea certainly fits the 13 C evidence of a change
in global carbon cycling; however, further corroborating work is needed.
Microbialites (including stromatolites, thrombolites and dendrolites) were formed
by microbial mats that were common in the late anaerobic Earth. They made a
resurgence following the late-Ordovician extinction and in one sense this is analogous
to the opportunistic fungi at the end-Permian and Cretaceous-Tertiary extinctions.
However, in this case the opportunity came both in the form of a ready food supply (the
seas containing dead marine life would have provided nutrients for algae, which would
release phosphorous in the way that Lenton et al. explored) and reduced pressure from
grazing animals. The microbialite resurgence lasted some 5 million years (Sheehan
and Harris, 2004). There is also some evidence for a similar stromatolite resurgence
associated with the later end-Permian extinction (see section 3.3.5).
3.3.2 Late-Devonianextinction(365-363.5mya)
The late-Devonian extinction appears to have been a succession of extinction pulses.
Many cephalopods were decimated, as were the armoured fishes that are associated
with much of the Devonian. Many coral reef species and crinoids (sea lilies) were
also lost. Both terrestrial and marine systems were affected, the tropics more so than
the poles. There is much debate as to the cause of this cooling but one of the preferred
theories is multiple asteroid strikes (possibly five significant ones; there is evidence
from Morocco). Another cause is likely to have been the spread (through changes
in ocean circulation) of anoxic (oxygen-deficient) deep water. Of course, there could
have been more than one factor operating.
3.3.3 Vascularplantsandtheatmosphericdepletionofcarbondioxide
(350-275mya)
Vascular plants (the pteridophytes and seed plants) have a vascular system of xylem
and phloem that enables them to have a free-standing structure, and hence they
would have gained height on the land compared to the more primitive plants. Equally
importantly they have extensive roots that helps to break up bedrock. Vascular plants