Past climate change - Climate Change: Biological and Human Aspects - page 93

Fig. 3.3 AmarinecorefromtheearlyEocene.Theabruptdepletionofwhitecalcareousforams-thedarkband-is

clearlyseen.PicturecopyrightJimZachosandreproducedwithpermission.

became extinct, this last in no small part due to increased ocean acidity. Wherever in
the world cores have been taken of early Eocene marine sediments this depletion of
calcareous forams is seen (see Figure 3.3).
But what caused this abrupt warming and, indeed, the increased ocean acidity? The
picture of what probably happened has been built up over a number of decades of
research and even now there is considerable debate as to the various factors involved
and how they interacted. Yet, as we shall see, a broad picture is emerging and one
that increasingly appears to be relevant in the 21st century.
Analyses of both marine carbonate fossils and terrestrial fossils from this time
reveal a sharp decline in the normal (low) ratio of 13 C to the common 12 C (see
Figure 3.4). The logical explanation for this CIE is that a reservoir of 12 C flooded
the active biosphere (i.e. its living components). As noted with the Toarcian event,
because photosynthetic enzymes have evolved a stonger affinity for 12 C than 13 C
it is likely that the flooding carbon was ultimately biogenic (generated from life).
However, such was the amount of 12 C flooding the biosphere, it must have come
from a 12 C-dominated reservoir not usually associated with the short-term carbon
cycle, but from the deep carbon cycle. Carbon-12 reservoirs include the living part of
the biosphere itself (short-term or fast carbon cycle), fossil fuels of the deep carbon
cycle (or fossil fuel-type sources, since no one was refining fuel back then), methane
hydrates (clathrates) in submarine continental slopes and some sea floor sediments
(both deep carbon cycle), and volcanic gases (which ultimately owe much to the
tectonic recycling of organic sediments and which are again part of the deep carbon
cycle and so can be 12 C-rich).
With regard to the first of these options, if all the carbon associated with the living
terrestrial component of the biosphere was turned to carbon dioxide (for example,
by some sort of cataclysm causing, say, global forest fires) then this still would not
have been enough to provide the amount of 12 C released. More than three times the
amount found planet-wide in terrestrial plants and animals would have been required.
Soils and detritus globally contain sufficient carbon (see Chapter 1) but releasing all
of this would have necessitated an extinction event of unprecedented severity and