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Puddingstone - second slice


In the second of three instalments, Bryan Lovell* looks back over a life in sandstones and oil, ponders the significance for Homo sapiens of events that happened 55 million years ago…

Read Part One

Read Part Three

Geoscientist 18.7 July 2008

Part Two – Hot blobs rising

We already knew from BP's detailed study of the North Sea that the Forties sandstones formed one of a series of sand bodies shed from ancient Scotland (Figure 1) (Stewart, 1987). These separate sheets of sandstone were laid down on the floor of the ancient North Sea in a number of episodes, at intervals of a million years or more. In between, muds were deposited - in time forming the seals that now prevent oil leaking to the surface. What we in the oil gang did not know in 1995 is just how Scotland had been pushed up out of the chalk sea. This was what Nicky White had presented to us.
Fig. 2 Stratigraphical columns showing uplift of area of Scotland at 55 million years, first (and more) to west and then a little later (and less) to the east. (From Rudge & others, 2008.) Nicky and I published our ideas (White & Lovell, 1997) and then began to seek a fuller understanding of how these pulses of sand originated and their fundamental cause. We came to realise that control of regional uplift by pulsed magmatic underplating, which we had favoured as an hypothesis in the years following our 1995 encounter, was only one particular example of a more general phenomenon. A decade later, during doctoral research funded at Cambridge by BP, our colleague Max Shaw Champion recognised evidence in his work and that of John Underhill (ex-Shell) and Friobjorg Biskopsto at Edinburgh University, for successive uplift of the sea floor in two different areas: first to the west of Scotland and then, 0.3-1.6 million years later, to the east of Scotland (Figure 2) (Shaw Champion & others, 2008; Rudge & others, 2008). This transient uplift took place around 55 million years ago. Peak uplift was at least 490 metres in the west, and some 300 metres in the east.

Dan McKenzie listened to Max’s explanation of this diachronous uplift at one of the Bullard Laboratories’ regular Friday afternoon research seminars. Max showed how he could generate a set of numbers for the timing and extent of the uplift, and identify a probable cause - a hot blob, travelling from west to east deep below Scotland as a result of convection in Earth's interior. At Dan’s suggestion, John Rudge took Max’s numbers and generated a quantitative model of the asthenospheric flow involved. Geology had guided geophysics, and in return geophysics repaid geology handsomely, with an understanding of a fundamental control of high-frequency changes in the elevation of Earth’s surface. Could we now explain those numerous marine transgressions and regressions for which a cause (in non-glacial times) had so long been elusive?

The “Hot Blob” caused transient uplift of Paleogene Scotland, leading to first regression and then transgression on an impressive scale. As uplift took place and more land emerged from the Paleogene ocean, rivers carried quantities of sand east to the shores of the early North Sea. Those sands were then carried further offshore to form the Forties sandstones. The volume is impressive, estimated at over 3000 cubic kilometers for the Balmoral-Forties submarine fan of sediment alone (Reynolds, 1994). Once again, this took place during that special time in Earth history - 55 million years ago, the probable age of the Hertfordshire Puddingstone.

Fig 3 Paleogeography of Britain and Ireland around 55 million years ago (right), showing a coastline hinting strongly at the present-day configuration of land and sea. Outcrop of Paleogene rocks shown (left) for reference. Coincidence? The Scottish uplift was also felt in England. With our new information we can sketch with more understanding the geography of the London and Hampshire basins of 55 million years ago (Figure 3). We see a recognisable outline of present-day Britain and Ireland emerging from the chalk sea as a result of both transient thermal uplift and permanent magmatic underplating. And Hertfordshire lay right on that Paleogene coastline – just where you might expect to find white sand and rounded pebbles on warm beaches (Figure 4).

Figure 4 - the Puddingstone

Fig 11. Illustration from Gerald Dickens (1999), showing rapid release of fossil carbon at 55 million years. Its subsequent removal from the atmosphere and oceans took over 100,000 years.

Meanwhile, on the ocean floor away to the west, in the developing North Atlantic Ocean, an episode of dramatic global climate change was being recorded in deep-sea sediments. Thanks to the international programme of deep-sea drilling, we can now read that record 55 million years later. We can do this using the thousand-year definition of the astronomical timescale, created by Nick Shackleton and his colleagues. This brings the story onto a human timescale: we are thereby led to some uncomfortable conclusions about the effects of burning all that North Sea oil.

It is commonly said that our present-day release of carbon into the atmosphere of Earth is an uncontrolled experiment with an unknown outcome. That is not really true. There has already been a release of fossil carbon comparable in rate and volume to that which we are now causing (Figure 5) – and it happened 55 million years ago (Norris & Rohl, 1999). This was long before we were around to light so much as a camp fire - so we didn’t do it, but now we know about it. Although we cannot predict the outcome of our own experiment, the main effects of the 55 million year release of fossil carbon provide hefty clues about what is likely to happen. Admittedly starting conditions were different. The Late Paleocene was already warmer before the large release of fossil carbon than Earth is now. A world map of Paleocene land and sea does not look wholly familiar to us, but the story we can read from rocks formed at the time is clear enough.
Earth became a lot warmer (Cohen & others, 2007). Even on the deep ocean floor, temperatures increased by several degrees centigrade. The boundary between the Paleocene and Eocene epochs is defined by the resulting extinctions in the fossil record. Oceans became notably more acid, and then received large volumes of carbon as recycling of the released gases took place. It was over 100,000 years before the planet returned to something approaching its previous state. The whole episode may plausibly be regarded as an earlier, and complete, version of the experiment on which we have ourselves embarked.

Figure 6 - Advantages of a handly plume. The 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.
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.