Past climate change - Climate Change: Biological and Human Aspects - page 98

The answer to this is that matters are more complex than dealing with a simple
carbon pulse into the atmosphere. In 2004 an Anglo-American research team, led
by Gabriel Bowen and David Beerling, suggested that terrestrial and, importantly
separately, oceanic carbon cycling may have changed significantly as a result of the
Eocene carbon release and that this in turn would have a consequential regional
climate change effect (as opposed to the average global increase in temperature
expected). Turnover of soil organic matter therefore probably increased dramatically,
suggesting vigorous plant growth in a number of terrestrial ecosystems (due to the
extra warmth and precipitation, and increased carbon dioxide from the slow oxidation
of methane), so doubling terrestrial carbon cycling. The picture they construct is that
there would need to be an increase in soil moisture and humidity of at least 20%. Water
vapour is itself a greenhouse gas and this would have helped further the IETM, during
which the Earth appears to have switched into a new climatic mode. However, water
vapour concentration is a response to warming and would not itself maintain it, as we
perceive from the geological record, but reinforce something else that was going on.
The Bowen-Beerling team could not state with confidence what this factor (or factors)
might have been but did make a number of suggestions. These included changes in
atmospheric and/or ocean circulation and heat transport, higher atmospheric carbon
dioxide due to circulation and ocean chemistry changes, or higher methane due to
increased wetlands arising from the higher precipitation, humidity and temperature
as well as some of the carbon release from soils once global temperatures had started
to rise (Bowen et al., 2004).
All the above notions are likely factors but in themselves do not fully explain the
duration of the Eocene CIE. Yet, pulling together the evidence published over the past
decade, the following scenario is one well worth considering.
This is what may have happened. An event such as magma intrusion into biogenic
carbon rich sediments, as reported by Svenson et al. (2004), could (if the conclusions
of the analysis are to be believed) have resulted easily in the release of more than
enough 12 C to generate the Eocene CIE. (Not surprisingly, you will recall, major
volcanic activity also took place at the time of the Toarcian CIE.) Yet, even if this
were so, the raising of deep-ocean water temperature by 4-5 ◦ C was very likely to
have destabilised those marine methane hydrate deposits that were already close to
dissociation. After all, the degree of methane hydrate stability is a function of both
temperature and pressure so that those deposits that were closer to the surface would
be more likely to dissociate before than those deeper. These latter deposits would
only dissociate with further abyssal warming and that would take time. Furthermore,
there is evidence suggesting that there was an abrupt change in ocean circulation
during the Eocene warming event, from a more temperature-driven thermohaline
circulation (see Figure 4.1 and associated text) to a more salinity-driven halothermal
circulation. Foram benthic (deep-water) carbon-isotope evidence from four ocean
basins dating from the time of the IETM/PETM event also suggests a switching
of ocean circulation during the climate event itself (Nunes and Norris, 2006). This
would have exposed other methane hydrate strata to thermal destabilisation that had
previously been protected by cooler currents. In short, methane hydrate deposits
dissociated not simultaneously but gradually, as the oceans slowly warmed and as
the ocean circulation changed. Further, once the planet began to warm up, other