Podcast: Using X-rays to look inside ancient leaves

X-ray of leaf fossil

Optical plus X-ray false colour composite image montage of a 50-million-year-old leaf fossil

22 July 2014 by Richard Hollingham

This week in the Planet Earth podcast, Roy Wogelius and Nick Edwards of the University of Manchester explain how they used extremely bright light from particle accelerators to delve into the chemistry of exceptionally well preserved fossil leaves from the Green River Formation in the US.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Richard Hollingham: This time in the Planet Earth podcast, revealing the chemistry of fifty million year old leaves. I'm Richard Hollingham and I am at the University of Manchester to meet members of an international team studying the lives of ancient plants. I'm in the palaeontology laboratory and with me are Roy Wogelius a GO Chemist and palaeontologist, Nick Edwards. We're looking at one of the benches here and on the bench lit by two lights, it looks like you have brought a cake in covered in tin foil, actually it's not a cake is it?

Nick Edwards: No, actually, what's inside is probably one of the most beautiful fossil leaves anybody is ever going to be lucky enough to lay their eyes on. It is from the Green River formation in the Western United States and it is about fifty million years old. Honestly, it is absolutely stunning.

Richard Hollingham: Let's have a look and peel back the foil here. That is incredible. It looks like someone has, I don't know, pressed a sycamore leaf into some stone.

Nick Edwards: Yeah, precisely, and I mean again that is one of the reasons it is so amazing. It looks like it was laid down yesterday. It is about 80cm-tall but it is absolutely beautifully intact, but one of the really other really cool things about is, and what is quite rare in palaeontology is to see an interaction between two different types of organisms. Normally we get isolated dinosaurs or animal like this but here we are actually seeing an interaction between two completely different species.

Richard Hollingham: How do you mean?

Nick Edwards: So if we look closely on the fossil we can actually see that this leaf has been predated by some sort of insect. You can see that the flesh of the leaf has been eaten away leaving, even after fifty million years the really detailed tiny venation in the leaves.

Richard Hollingham: So they look like little... almost like caterpillars.

Nick Edwards: Yes, even after fifty million years this exact same interaction is happening today where moth larvae are eating the leaf and munching away at the nice soft tissues and rather disgustingly also creating cocoons for themselves out of faeces and silk.

Richard Hollingham: So, Roy, this is a remarkably well persevered leaf. You can see that the cast from caterpillars from larvae on it, but actually you're interested in the chemical processes going on in that. How on earth do you take a fossil and figure out the chemical processes.

Roy Wogelius: That's absolutely right, Richard, and I think if I can just link back to what you said about it looking like a cake. Because it was wrapped in aluminium foil and the reason why we do that is to keep it absolutely pristine, keep it clean. We treat these things the same way that you would treat a forensic sample. This was collected for us by a very good friend of ours at the Black Hills Institute in South Dakota, Pete Larson, so we know the providence of the fossil and we know that it doesn't have any preservatives, there's been nothing added to it, it's a completely pristine fossil. It is a great opportunity for us because it means that we can get in and start to analyse the original chemistry that might go along with the fossil. Now the thinking has been that something like this would be completely carbonised - the logic was that, well, okay, there's nothing left and you can see a beautiful cast but there's nothing there. Now we've been challenging that assumption by what we've been doing here at the University of Manchester and in collaboration with our friends at Stanford Synchrotron Radiation Lightsource-

Richard Hollingham: Hold on. You've talked about the chemistry here, how on earth do you figure it out? You've used particle accelerators to do this - what have you actually done?

Roy Wogelius: We use a technology that is based on synchrotron. It's an amazing piece of equipment. I will never go up on the star ship 'Enterprise' but this is about as close as you're gong to get. Essentially what you do is you take electrons, fire them at very, very high energy, they move around in a circle, in a ring, near the speed of light and when they do that they emit X-rays tangent to the direction of travel. Very, very bright X-rays. It's like the old joke about where do you look for your keys, you look underneath the street light because it is the only place you can see. Well, what we do is we bring our keys, the fossils, we bring them to the street light, which is the synchrotron and we can see things that you can't see anywhere else. Tremendously bright synchrotron X-rays illuminate the material and we can see that not only is there this visible remains, but there is also trace metals and other elements that are only in the leaf and not in the imbedding sediment. By doing that we can work out the biochemistry.

Richard Hollingham: So you know you've got these particular elements there, and you can say that they are definitely from the leaf. How do you then go to figuring out the biochemical pathways, the chemistry of the leaf if you like?

Roy Wogelius: Once we are able to show that the patination actually matches the fossil structure we can then go in and look and see how the metals are bound. In this case they are organically bound and particularly the element that is very, very interesting here is copper. The copper is not a copper sulphide or some kind of mineral precipitate, it is organically bound copper and again we can do that using X-rays. Effectively we can use the X-ray beam like a bit of a sonar and tell the distances to the nearest neighbour of atoms. And by doing that we are able to show that it is organically bound copper, so it is still in its configuration that it was in when the fossil was laid down.

Richard Hollingham: So there is evidence for these chemical pathways and what do they tell you? Was this more or less like, for example, a sycamore leaf?

Roy Wogelius: There is recent work in biology that shows that an awful lot of the copper is bound to specific functional groups in leaf tissue and we get exactly the same thing in the fossil. Here is a great example of it. Nick is standing here and this is something he discovered, so maybe I should let him talk about serration tips.

Nick Edwards: We may remember we all did a little bit of plant biology in primary school and one of the things we look at is the serrations, the saw tooth edges to the leaves, and those often have different shapes depending on the species. In one fossil leaf the serration tips, these tiny little details, may be only half a millimetre across showed high intensity concentrations of copper and all palaeontology research is based on comparing to modern counterparts, that's how we interpret these things. We looked at modern leaves in exactly the same way and indeed in the modern leaves we saw exactly the same thing - copper was concentrated at this tiny scale on the serration tips and that's why this imaging solution is so important because we might be able to say, 'oh look it is very interesting, there's copper there' but these patterns are showing that similar biosynthetic processes are happening even over fifty million years.

Richard Hollingham: So what does that tell you about evolution? Does that tell you that the plant chemistry was almost locked at that point and hasn't changed since?

Nick Edwards: Essentially, yes. Certainly with regard to copper, it is certain dealing with copper and potentially the other trace metals in a very similar way and that's really interesting because previously we may have interpreted biochemical processes in the same way we interpret the evolution of the shape of dinosaurs or any other organisms, but actually we're starting to pick out the landmarks in the evolution of metal evolution in these biochemical pathways.

Roy Wogelius: That's also one of the things that really fascinated us about this fossil. You can see the copper patterns in the leaf. We know that it was relatively high copper, we know it's organically bound copper but what's really fascinating about this is that this leaf was eaten by a hungry caterpillar and there is high copper concentrated in the frass tubes, in the little feeding tubes. So it's a snapshot and it tells us unequivocally that what the chemistry of the leaf was and what the chemistry of the residue was from the feeding, they match.

Richard Hollingham: So what's the ultimate aim of this? Just keep building on this and building up a picture of evolution between then and now?

Roy Wogelius: By looking at the metals, we're looking at things that survived much, much longer than proteins and things that survived much, much longer than DNA. So even though we don't have bones in the soft tissue we do have a hard part which are the metals, and that's part of the reason why we focused on this because looking at the metal chemistry gives us an opportunity to look at the pathways through very, very deep geological time even when we don't have hard parts preserved in the organism.

There are a couple more things that really we want to highlight about this research and one of the key things is the role of copper. It is fundamentally important. We evolve using copper and enzymes very, very early on in the history of life. Right now copper gets concentrated, for example, in your hair as I'm looking at you and in Nick's hair. Birds put a fair amount of copper into their feathers, why do they do that? One of the reasons for that is because it is a biocide and that's exactly the same reason that a lot of your readers sometime during this year are going to be putting Cuprinol on their fence because it acts as a biocide, bacteria don't like it, hence it kills them and it preserves the organic material that is left behind. Well, that's probably exactly why this leaf has relatively high copper in its soft tissue, it is acting as a biocide and that's also acted to help preserve it over geological time and this is a phenomenon that we see over and over and over again.

Richard Hollingham: So, Nick, what's the plan now? You've worked out some of the biochemistry of this leaf. What do you want to do next?

Nick Edwards: I do feel very lucky in this sense because a lot of this research was very curiosity driven. We were very much let's see what we can see. We did the work on fossil animals previously and we decided to just look out these beautiful leaf specimens and this will now open up a wide range of research avenues looking at the potential evolution of the biochemistry of life on earth and that's a really exciting opportunity.

Richard Hollingham: Nick and Roy, thank you very much. You can see pictures of the leaf at Planet Earth online and you can see pictures of our recording today on Facebook and Twitter. That's the Planet Earth Podcast from the Natural Environment Research Council. I'm Richard Hollingham, thanks for listening.