Past climate change - Climate Change: Biological and Human Aspects - page 96

temperatures of intermediate sea waters would increase the chance (and hence the
pacing) of methane hydrate dissociation.
Carbon dioxide, be it from part of the volcanic event, which itself might have been
methane-dominated, or as a result of the oxidation of released methane, would easily
dissolve in the oceans, so increasing their acidity. This in turn would increase the
dissolution of calcite shells of microplankton which are the dominant component
of sea-floor sediments, leaving behind only non-soluble clays; hence the foram-
extinction event. Other species with calcareous shells are also likely to have been
affected (see the ocean acidity subsection in Chapter 6). It is well documented that
a change in the colours of sediments, from bright white carbonate to deep red clays,
marks the Palaeocene-Eocene event (see Figure 3.3). Normal deposition of micro-
scopic carbonate foram shells on the deeper reaches of the sea floor did not resume
for at least 50 000 years, and the total recovery time to a so-called normal state took as
long as 100 000 years. Finally, ocean sediments also reveal a boom in the number of
planktic Foraminifera species a few million years after the IETM/PETM (Emilliani,
1992). This might be expected if the ocean contained an abundance of material from
which to make carbonate. However, whereas this recovery begins immediately after
the IETM event, a more gradual rise in foram biodiversity (as opposed to population
numbers, which, as said, declined during the IETM) began prior to the IETM, so
making it difficult to claim with certainty that the foram biodiversity peak and IETM
were connected, even if this is arguably likely.
It is worth noting that Svensen's team estimate that the rate of carbon released
as methane during this Norwegian sea event - over a period of between 30 and 360
years - is comparable to an average year's worth of carbon release from the burning
of fossil fuels as carbon dioxide in the 1990s. This rate is low (as 1000-2000 Gt of
12 C in total is needed for the CIE), nonetheless the suggestion is that our current
burning of fossil fuels will eventually warm the planet and hence, as in the Eocene,
eventually the oceans, possibly enough to disrupt methane hydrates and so trigger
further greenhouse warming above and beyond that. The relevance to today is that
we are releasing 12 C into the atmosphere by burning fossil fuels and clearing forests;
could we be about to trigger an event analogous to the IETM/PETM?
In addition to the Svensen team's estimate of the rate of carbon release from
these vents, they also estimate that the total carbon release over the period of the
CIE might be 1500-15 000 GtC. Given that the CIE is thought to involve around
2000 GtC or more, if these estimates are accurate the vents themselves may have
been responsible for the CIE on purely carbon-volume terms (Svensen et al., 2004).
However, this alone is unlikely. We know the CIE happened and it resulted in global
warming that affected ocean deep-water temperatures, and so thermal dissociation of
methane hydrates would have been virtually inevitable. In addition, with warming,
carbon was also likely to have been released from soils. The question then becomes
one of whether the North Atlantic volcanic province was the trigger for a separate,
subsequent release of methane from marine clathrates and also, in all likelihood, other
terrestrial carbon pools.
As for comparisons with current carbon releases driving present-day global warm-
ing, it is worth highlighting a few figures. The release of at least some 1000-2000 GtC
was needed to generate the Eocene CIE. This compares with some 6.1 GtC released