The Geological Society

The Eocene greenhouse

Prof. Paul Pearson (Cardiff University) brought the BA a steamy tale of rainforests in London and asks - could they be coming back? Ted Nield reports from Liverpool.

Geoscientist Online 8 September 2008

Geologists have long known that the Earth’s past climate has been much warmer than it is today. The Eocene epoch (c. 55 to 35 million years ago) was such a time. The rocks laid down in this period include the London Clay, the soft rock through which many of London's tube lines are cut. This has yielded 'tropical' type floras, including fossil relatives of our modern cinnamon, figs, citrus, mangroves, paw-paw and vines – all of which indicate local mean annual temperatures that were as much as 15°C warmer than today and similar to the hottest tropical jungles on the modern Earth.

But nagging questions remain. If it was so hot in middle latitudes (which was where Britain was back then) what was it like in the tropics? How much hotter was the world overall? Was there any ice at the poles and, if not, when did it appear? How was heat transported around the planet? Perhaps most importantly for us today - were greenhouse gases responsible for the global warmth, and can we reconstruct the atmospheric carbon dioxide level?

To answer questions like these, the palaeoclimate research group at Cardiff University and their collaborators have been investigating Eocene deposits from various places, most notably Tanzania, but also Mozambique, Java, New Zealand, Croatia, Alabama, the northern North Atlantic and the Isle of Wight. All these places have one thing in common - they have clay-the rich marine sediments of Eocene age that contain exceptionally well-preserved microscopic fossils called foraminifera (“forams” to their friends). These lived as plankton in the sunlit upper ocean or on the sea floor, and the researchers look for chemical fingerprints in their shells to determine the temperature and chemistry of the water in which they lived.

A benthic foram from the Eocene - Image courtesy, UCL

Says Pearson: “Each place provides a snapshot or - if we are lucky - a continuous series of snapshots of local conditions. Taken together, and used in conjunction with state-of-the-art climate model simulations, we can start to reconstruct the global climate state, and how it changed through time. We can also assess how well the climate models cope with reconstructing a world that was very different from the modern one, for which they were initially designed.”

Foram shells are made of calcium carbonate, and the researchers are fussy about its quality. “The reason we need exceptionally well preserved microfossils is that subtle alteration of the shells by percolating fluids during fossilisation can affect their chemistry. Clays preserve the fossils very well because they are relatively impermeable and retard the process of recrystallisation that alters the shells and can give misleading data. The Cardiff group helped highlight this problem, showing that much previous data from less well-preserved sediments gave wholly unreliable (and too cold) temperature signals. This produced the 'race for clay' which is now going on.”

Isotopes

“Most of our recent work has been in reconstructing temperatures using a technique first developed in the 1950s; analysing the ratio of oxygen isotopes (¹⁶O and ¹⁸O) in the foram shell. Theory and experiments show that this isotope ratio can be used to reconstruct temperature very accurately (within a fraction of a degree) if we know the isotopic ratio of the water in which the foram lived. Uncertainties in this latter value and various other factors that can affect the oxygen isotope ratio mean that realistic uncertainties are about two to three degrees. However we can refine this using other techniques, such as the Mg:Ca ratio of the shell, which is also affected by temperature and more recently, a completely different technique that analyses the ratio of various organic compounds derived from the membrane lipids of marine archaea (simple organisms similar to bacteria that also live as plankton). This technique, develop by Dutch researchers is called TEX-86 and we are beginning to apply it to our samples also.”

No samples of ancient air remain from such a remote period, so the researchers must use indirect methods to determine atmospheric carbon dioxide levels. “We use the chemistry of foram shells to do this also.” Says Pearson. “The theory is that CO₂ in the atmosphere is generally well equilibrated with the upper ocean where the forams live. Carbon dioxide is an acidic gas, and the ancient acidity would have affected the foram shell, specifically the ratio of the incorporated boron isotopes, ¹⁰B to ¹¹B. By measuring this value in samples spanning 60 million years and making other necessary assumptions we have reconstructed how CO₂ levels varied up to the present day. Other groups have used other indirect techniques and produced similar values, suggesting that the estimates may be broadly correct.”

Chunks

“The samples we need come from rare, clay-rich deposits. Obtaining samples can be a simple case of hammering off chunks from a cliff, as in New Zealand, but is sometimes more of a challenge. We have uncovered very detailed and valuable tropical climate records from Tanzania, where sediments once laid down under the ocean have now been uplifted onto the continental margin. Because the rocks are very weathered near the surface, it is necessary to use mobile truck mounted rigs to drill down up to 150m below the land surface. We have drilled nearly 30 holes, sometimes in remote areas, often camping in the bush and making our own geological maps as we go along.“

The results suggest an Eocene world that was much warmer than the present, and that this warmth was indeed driven by much higher greenhouse gas concentrations than currently exist. "Tropical" climate conditions extended into the polar regions, while the tropics themselves were considerably hotter than present. Pearson believes that these regions may at times have become too hot for many forms of life to survive: “including many plants and in some instances the very plankton that we use to reconstruct climate!”

“Recent results from New Zealand have been use to follow natural climate cycles on timescales of tens of thousands of years. We can deduce from the foram shells and other evidence that there was little ice on the planet and therefore no glacial-interglacial cycle like there is now, but the world did warm and cool cyclically as the orbits changed, even in a wholly greenhouse climate state.”

Researchers think that the high CO₂ levels were caused by long term geological variations in the carbon cycle. In the Eocene large amounts of previously deposited limestone were being subducted into the Earth's mantle and consequently volcanic emissions above the subduction zones were high in CO₂. The world was much less mountainous; the great Himalaya & Alpine and Andean chains were only beginning to rise. The rise of these mountains promoted the weathering of silicate rocks by carbonic acid rain, eventually pulling carbon out of the atmosphere - which was deposited as limestone. These shifts in the carbon cycle took tens of millions of years and caused long-term global cooling, eventually bringing on the current ice age (albeit in interglacial mode just now).

Although the Cardiff group does not conduct global climate model simulations, they collaborate with climate modellers who do. These (for example Matthew Huber at Purdue University) use the latest computer models, the same as those being developed to predict the future. The process of data gathering and model simulation has been a two-way conversation as we try to understand how the ancient planet worked - including factors like the ocean current systems, atmospheric circulation, and solar and greenhouse forcing.

Pearson told the BA: “It has been difficult to 'fit' the geological evidence from the Eocene to the output of the climate models. One problem is that to warm the poles and mid latitudes sufficiently, the modelled tropics tend to get very hot. Alternatively, it is difficult to make the poles as warm as they appear to have been. However our recent data from Tanzania and Java show much warmer temperatures than previously reconstructed (thanks to the clay and our un-recrystallised forams), narrowing the gap between data and models. The most recent models suggest that very extreme greenhouse forcing can fit most of the available evidence.”

And now the future

“We are often asked what our work tells us about the future and the problems of human-induced global warming and ocean acidification. One reason for doing the work is to provide stringent tests for the climate models that historically have been designed to fit the very different situation of the present day. All such models use many short-cuts and approximations, especially of small-scale processes, that may or may not be correct. No model could ever reconstruct the evaporation of the ocean, molecule by molecule, or the drifting of clouds and formation of rain, worldwide. Such a 'model' would end up being as complicated as the Earth itself.”

Another reason for doing the work is that it provides the natural context for what is happening now. Pearson says: “The extreme levels of CO₂ that we reconstruct (over one thousand ppm) may conceivably be reached sometime in the next century if nothing is done to restrict emissions. Such conditions existed in the past - when London was a tropical swamp and monitor lizards, not polar bears, lived near the north pole! “

“We can also use the past to study natural processes of change, for example how the plankton coped with acidic oceans and what happened when climates shifted rapidly in the past (sometimes resulting in mass extinction, as at the end of the Eocene when large ice caps appeared on Antarctica). But the primary motivation of our work is curiosity - to reconstruct large blanks in the history of the planet and how life evolved long before humans existed.”

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