The “Paleocene-Eocene Thermal Maximum” (PETM), to give this event its proper title, has now been studied in many places across the world, providing abundant confirmation of the massive and rapid release of fossil carbon, over a few thousand years - followed by a period of recovery extending over at least 100,000 years. What triggered the carbon release? This we do not yet know for certain.
An early (and still favoured) explanation is that the PETM was triggered by destabilisation of subsea methane hydrate deposits at quite shallow depths within the sediments draping the continental slopes (Dickens, 1999). But what could cause such destabilisation? One possible process is uplift of the sea floor – reducing the weight of water bearing down on the unstable hydrates (Maclennan and Jones, 2006). The key to their idea lies in modern-day Iceland, with its volcanoes, and the hot springs in which field geologists can relax happily in the worst of the weather (Figure 6). The Iceland hotspot already existed 55 million years ago (Figure 7).
Could that hotspot have been responsible? The geological record indicates that from time to time the Iceland hotspot gets even hotter. More molten rock reaches the surface when the temperature of the hotspot is relatively high, and study of past volumes of magma suggests that activity pulses at irregular intervals, a few million years apart. Maclennan and Jones appeal to one of these episodic heating events as the trigger for the PETM. Their notion is that this pulse caused uplift of the sea floor of the Paleocene North Atlantic Ocean 55 million years ago. This uplift destabilised methane hydrates and thereby rapidly added large volumes of fossil carbon to the atmosphere. The resulting increase in greenhouse gases caused more trapping at Earth’s surface of heat from another, much larger external source – the Sun.
We are led back to the discussion in Aberdeen in May 1995 that I had with Nicky White, which led within months to our identifying pulses of uplift and sand deposition. As a result of pulses of heat in the early Iceland hotspot, pulses of sand were shed from the uplifted early Scottish landmass. One major pulse occurred 55 million years ago. To the east of Scotland it created the body of sand that later became the reservoir for the four billion barrels of oil trapped in the Forties oilfield. To the west of Scotland, that same pulse may have destabilised methane hydrates on the flanks of the developing North Atlantic Ocean, triggering the warming event.
On the one hand we have a large volume of oil, a significant and famous part of one of the world's notable oil provinces. On the other we find a possible trigger of dramatic climate change, a cautionary tale from geological history. It seems to be telling us: “here is what happens when you release large volumes of fossil carbon”.
At this stage my hero, Socrates, might ask: “Is this not a natural process? What is so special about this element carbon that you make such a fuss?”
End of Part Two. The concluding part of Bryan Lovell’s essay will appear next month.