by Melvyn Bragg
They also found a really dramatic change in the chemical signature in these rocks at this particular point.
Keen to place these prehistoric events in the right time period, Melvyn asked Mark Maslin for more context. We heard that geologists split the whole of the 4.5 billion years of the earth’s existence into periods, to help identify and understand when massive changes in the earth have occurred, both environmental and biological. The end of the non-avian dinosaurs marked the start of an eon – the Cenozoic, the life period – and of the Palaeocene. That lasted for 10 million years with little change in the environment, before the very warm spike, which marked the boundary with the next period, the Eocene. This warm spike between these eons, the Palaeocene–Eocene Thermal Maximum or PETM, was abrupt.
The PETM was crucial for mammals, which had been expanding and changing since the end of the dinosaurs 10 million years before, and new species evolved. Melvyn suggested that the mammals used to be crushed by dinosaurs and started to peep out of their holes again after the dinosaurs were crushed.
MARK MASLIN: Yesss, in some ways. Mammals evolved originally 225 million years ago and were oppressed by the dinosaurs for about 120 million years …
MELVYN BRAGG: Slaves to the dinosaurs.
MARK MASLIN: Well, not quite. I mean there is a wonderful fossil from China of what can only be described as a killer badger and it has, in its stomach, baby dinosaurs and eggs, so we did actually occasionally get one back on the dinosaurs.
During the PETM, there appeared animals like even-toed ungulates, which include camels, cattle, goats, giraffes and even whales, porpoises and dolphins. Significantly for us, this is also when primates first appeared and social monkeys evolved. This, then, was when human evolution started off. Monkeys became social, primates evolved from them and we evolved from these in east Africa later on. A lot of people have speculated about why monkeys became social, but one factor may have been that the warmer temperatures, with forests at the poles, meant there was suddenly a massive expansion of the environments that suited mammals.
MARK MASLIN: You have warmth, subtropical temperatures up to Antarctica and the Arctic, you suddenly expand the range. Then you get groups of mammals moving into new environments that have no other competition, and therefore changes and different things have been tried out. And working as a tribe, as a group, as a social group seemed to be very, very successful, and that suddenly took over.
If someone had been around then to look down on the planet from the sky and take a photograph, as Tracy Aze explained, the world would not have looked all that different from the way it does today. There would have been a recognisable Africa and North and South America. Some things, though, would have been quite different and these were significant to global temperatures.
TRACY AZE: We don’t have a Himalayan mountain chain because India is yet to crash into that tectonic plate that causes the Himalayan mountain chain. We also have South America still attached to the Antarctic continent. And the Arctic basin was much more restricted than we see in the modern day. There was a gap between North America and South America, so the Panama isthmus was open and that allowed for exchange between the Atlantic Ocean and the Pacific Ocean.
As Antarctica was still joined to South America, there was no circumpolar current as there is today, a current that seems to aid the thermal isolation of Antarctica, keeping it cool by stopping the warmer waters from reaching it. The difference in the Arctic basin was significant, as this restricted the phenomenon we see today where the cold salty water in the high latitudes, north and south, moves down to the sea floor and drives ocean circulation. Until the PETM, the deep-water currents were probably only being generated in the Southern Ocean and this would have affected circulation. This is important as the oceans are regulators of heat distribution.
TRACY AZE: One of the things we think happened in the Palaeocene–Eocene Thermal Maximum is this background state, where we think we were generating deep waters in the Southern Ocean, suddenly flipped and we started generating deep waters in the North Atlantic, which was a change in the state of ocean circulation and would have happened very rapidly. That would have had an impact on how we spread and distribute heat.
Turning to the evidence of temperatures really rising, allowing crocodiles and rainforests at the poles, Jane Francis said there are many places across the world where we can find support for this. There is a really good sequence of plants in Wyoming, in the Bighorn Basin, with layer upon layer of fossil plants that originated before the PETM, during the PETM and then afterwards. Before the temperature rise, this Basin had plants like the Everglades, and then they became more like those of Mexico. The changes in the leaf shape edges and size also showed evidence of the warming.
MELVYN BRAGG: Why the edges of leaf … ?
TRACY AZE: If you have a more jagged edge on a plant leaf, that is associated with cooler temperatures than leaves that have smooth edges, and we know that from modern [times]. When we look at the past and see assemblages of plants dominated by jagged leaf edges, we know that temperatures would have been cooler than assemblages dominated by smooth edges. And the size of the leaves can tell us about precipitation. When we see big leaves we know that there were higher levels of precipitation at that time as well.
In some cores that have been drilled from sections of the ocean floor originating around the PETM, there is a very dramatic change in the colour of the rocks, from whitish grey to reddy brown. These grey rocks have a lot of lime in them, a lot of carbonate, and then, with the PETM, the rocks changed and the carbonate disappears. To find absolute evidence of the temperature, we can look at the carbon and oxygen isotopes in these cores extracted from the seabed.
JANE FRANCIS: These are chemical signatures that are trapped in the shells of the small animals, the forams that were formed at that time. And we can use them to work out what the carbon was like in the ocean and what the temperature was like in the ocean. So that is direct evidence of what the ocean temperatures were at that time.
The shells tell us which species existed at that time. Scientists have drilled over 400,000km of deep-sea sediments in over 1,400 different sites all over the globe since the 1960s. What those scientists drilling in the 1980s noticed, particularly Ellen Thomas, the micropalaeontologist and palaeoceanographer, was a clear extinction that occurred both at the surface and in the deep ocean, and then new species appeared. Mark Maslin pointed to another discovery in the shells, which is that we can see the different chemicals inside them. Sometimes the shells contain magnesium where calcium should be, and the amount of magnesium that goes into them is related to the water temperature, which can then be calculated.
MELVYN BRAGG: We were talking about this warming spike. What was it? What sort of temperature is outside Broadcasting House now and what was it then?
MARK MASLIN: Global temperatures, on average now, are about 15–16°C for the whole planet. You are looking at the Palaeocene probably being somewhere more like 17–18°C. On top of that, we add another 5°C on ocean temperatures and global temperatures, so you’re looking at an average temperature for the planet of about 22°C, which is 7°C warmer than the average today.
MELVYN BRAGG: It doesn’t seem massive; it’s more like a football score than a rugby score, isn’t it? Can you convince us, Jane?
JANE FRANCIS: Let’s go to the Arctic, [where] we’ve got ice at the poles now and we’ve got polar bears and seals. But if we go and look at the rocks that are 50–55 million years old, in the Arctic we can see crocodiles, we can see lemurs, we can see hippo-like animals, we can see Florida Everglade-type conditions.
All of the environments, the trees and plants, that had lived much nearer the equator where it was warm, were suddenly able to spread to the high latitudes. We can use the fossils to track this migration into much warmer latitudes and the movement is quite distinct at this time.
Melvyn wanted to drill down into the evidence of what caused the sudden spike. There was, we heard, a massive inje
ction of carbon into the atmosphere, thousands of gigatonnes (a gigatonne is 1,000,000,000 metric tonnes).
MARK MASLIN: To put that into context, the amount of carbon that our huge industrial complex in the world puts into the atmosphere every year is about 4 gigatonnes. We’re talking about 2,000 to 7,000 gigatonnes of carbon that were injected in [the PETM in] a very short period of time.
One of the early ideas for the source of the carbon was coal, as there are seams from that period, and it was thought that this may have been burning freely. Another idea was that there may have been permafrost in the regions towards the poles, which melted as the temperatures increased, releasing trapped carbon. Jane Francis also suggested there may have been volcanic eruptions at that time, as we know that, in certain parts of the globe, there were plate tectonic activities, especially in the Atlantic region, which may have put more carbon in the atmosphere. One of the big ideas is that the warming released methane, which was trapped on the sea floor, frozen as nodules, and is thirty times more potent than carbon dioxide in terms of its greenhouse potential.
JANE FRANCIS: A lot of it is stored, it comes from rotting animals and plants on the land. A lot of it gets stored in the oceans, it’s frozen as nodules on the sea floor today. And if you have a little bit of warming or you have an event that disturbs the sea floor, these frozen nodules are then released into the water and they melt and the methane is released. At the moment, the jury’s out on whether there’s one single cause, and I think that we are probably looking at a mixture of all of those ideas.
As more information comes to light, that jury has to reassess its views. Until ten years ago, Mark Maslin suggested, the consensus would have been that methane was definitely the cause of the temperature spike. Now it is more likely to be one cause of several. The methane hydrates on the seabed arise when bacteria break down the organic matter for food, but because, without oxygen, they cannot oxidise the matter fully, methane is produced. The methane bubbles up to the surface of the sediment only to be chilled by the ocean, so water freezes around it, enclosing it in icy cages or clathrates. If you raise one of these clathrates onto a ship’s deck, you can light it and a blue flame appears from the ice as it melts. In the Palaeocene, over several million years, huge reservoirs of organic matter had rotted down and created these stores of methane. It is thought that the warming at the end of the Palaeocene may have just tipped the temperature of the seabed over and suddenly all this methane erupted, creating what are sometimes called ‘burps of death’.
MARK MASLIN: The reason we call it burps of death is because, if you do it slowly, what happens is the methane goes into the water column and dissolves in the ocean. We know that, strangely enough, from Deepwater Horizon, the BP disaster, [where] methane was released but it just disappeared into the ocean. You have to do this explosively, you have to really release all of it, and [then] it gets through the water column and bursts out into the atmosphere. That allows you to get thousands of gigatonnes of carbon into the atmosphere and get this incredible piece of warming.
The consequence of the warming was colossal, Melvyn observed, even though the numbers involved were not spectacular. The impact was worldwide, too, Jane Francis added, while other events in the geological past were regional or in one half of the earth and not the other. Moreover, while all were affected, there were some areas affected even more than others.
TRACY AZE: If you look at some of the local changes in the tropics, for example, [with] some of the work we did in Tanzania, we were looking at sea surface temperatures getting towards 40°C, which is probably about as hot as your bath.
Mark Maslin argued that the PETM helps us understand current climate change, as the PETM suddenly became an event where nature was injecting huge amounts of carbon, just as we have been injecting huge amounts of carbon into the atmosphere over the past 100 years. One of the first things that interested researchers in the PETM was that it appears as though, left to nature, it took about 100,000 years for all that extra carbon to be taken out. We have already put about 1,000 gigatonnes of carbon into our atmosphere since the industrialisation and we are looking at putting in another 1,000 gigatonnes by the end of this century. The PETM is so interesting when looking at climate change, he said, as the scale of carbon emissions that we could put in by the end of the century is at the bottom end of the PETM.
Leaf fossils from the Palaeocene–Eocene Thermal Maximum.
MARK MASLIN: We’re actually doing our own natural experiment at the moment, which we can look back at [in] 55 million years and go, ‘Well, what does that tell us?’ The first thing it tells us is, if we don’t actually take out CO2 physically, the natural system will take 100,000 years.
The cooling at the end of the PETM followed the reduction in the levels of carbon in the atmosphere. On short timescales, the carbon dioxide was absorbed into the oceans, which became more acidic, Tracy Aze said. Over hundreds of thousands of years, the carbon dioxide in a warm atmosphere would have increased the weathering rate of rocks, and then the weathered rocks would be flushed into the ocean via rain and rivers. Once there, organisms would have made their shells from carbon and that would have ended as sediment, locked on the sea floor.
Other periods of warmer temperatures were to follow the PETM, towards the end of the Eocene, but these built up slowly over millions of years, rather than in the sudden, dramatic way that had such an effect on evolution. Then, at the end of the Eocene, about 34 million years ago, there were the first signs of major ice caps on earth. We are now in a very cold period of time in earth history, with ice at both poles and a lot of increased sea level potential locked up in that ice, even if it seems relatively warm after the ice ages. Jane Francis thought that we are not going to get back into a glacial age for a very long time, as the carbon dioxide levels are high now and it is going to take a while for the earth to work out what to do with all this carbon in the atmosphere.
MARK MASLIN: If ever.
In the studio afterwards, Melvyn wondered how the modern age can be compared with the PETM when we are in a cold phase now, with ice caps. Tracy Aze suggested that the PETM is not a direct analogue to the modern age. The increase in carbon in the atmosphere now has happened over years measured in hundreds, while the PETM was over thousands. While it is not directly analogous, it is one of the only things we can learn from, because this kind of event has not happened that many times in the past.
There was evidence in 2016 of methane hydrates releasing methane from the bed of the North Atlantic, but Jane Francis understood that this was being trapped by seawater at present, even though that may change if ocean temperatures increase.
JANE FRANCIS : There are so many cycles in the earth system and they’re like cogs in a wheel, and one interacts with the other, and so trying to understand a very complicated cogged wheel system is what we’re trying to do.
MARK MASLIN: About ten years ago, people were very worried about the clathrates in the bottom of the ocean. We now think that, in the next couple of hundred years, they will actually remain quite stable. But this has shifted the focus on to all of the methane hydrates in the Arctic underneath the tundra, because the tundra is melting, and all the permafrost. We’re really worried about that methane. And we have no way of really estimating how much methane is stored there.
PHOTOSYNTHESIS
Three and a half billion years ago, this planet was a hostile and barren place. The atmosphere was toxic and contained no oxygen, and life on earth was restricted to a variety of unsophisticated single-celled organisms that lived in the sea. But then a new type of organism emerged, one with an amazing new capability: it could harvest energy from sunlight and use it to fuel its own activities. This phenomenon is known as photosynthesis and is almost certainly the most important chemical process on earth. Plants and some other organisms depend on it for their energy, and almost all life is ultimately reliant on it for its survival. It is responsible for the food we eat and the air we breathe and, without it, the earth
would still be sterile rather than as it is, teeming with life.
With Melvyn to discuss photosynthesis were: Sandy Knapp, botanist at the Natural History Museum; Nick Lane, honorary professor of evolutionary biochemistry at the Department of Genetics, Evolution and Environment, University College London; and John F. Allen, honorary professor at University College London.
Photosynthesis, Sandy Knapp told us, is a very simple, elegant chemical reaction that involves an organism taking water and carbon dioxide and, with the help of light, turning those into glucose and oxygen. Without photosynthesis, that blue and green ball that we see from space would look like Mars. When organisms were able to make their own food by creating something from light, that, in turn, allowed other organisms to feed on them. Plants are autotrophs, organisms that make their own food, and (subject to the caveats raised by John Allen after the programme) anything that does photosynthesis is an autotroph.
SANDY KNAPP: We are completely hopeless, we’re heterotrophs, we depend on other organisms for our food. So, without these photosynthetic organisms, we would have nothing to eat and nothing to breathe and so all of life really depends upon autotrophs.
The essential ingredients are water, carbon dioxide and light, but photosynthesis needs a few other things as well.
SANDY KNAPP: Plants need nitrogen and phosphorous to make the enzymes that drive the reactions of photosynthesis and they also need an element, magnesium, which sits like a spider at the centre of the chlorophyll molecule, which is one of the light-harvesting pigments in leaves.
Melvyn suggested that photosynthesis was like an engine room, and had nods back from all three guests. Nick Lane then took us to the structures within the plant that make photosynthesis possible. This happens in structures inside the cell called the chloroplasts, which were once bacteria in their own right and are now known as cyanobacteria. They became captured by more complex cells, probably 1–1.5 billion years ago, and they continued doing what they did before.