Nature Podcast 1 June 2006

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Derek Thorne: Hello and welcome to this week's podcast from 1st June edition of Nature. I'm Derek Thorne, standing in for Chris Smith. Coming up in the podcast this week, new insight into why New Orleans levees failed during Hurricane Katrina, how Saturn has rolled over one of its moons, and the debate over how laboratory animals should meet their maker. But let's begin this week with news that the weather at the North Pole wasn't always as chilly as it is now. In fact 55 million years ago, the Arctic Ocean was warm enough for a swim. This finding, and many others, have come out of the recent Arctic coring expedition in which a large team of researchers drilled down beneath the Arctic ice to try and uncover the history of the North Pole's climate. Speaking to Chris Smith, here's Rhode Island University's Kate Moran. Nature 441, 601–605 (1 June 2006) ; Nature 441, 606–609 (1 June 2006) ; Nature 441, 610–613 (1 June 2006) ; Nature 441, 579–581 (1 June 2006)

Kate Moran: We've recovered for the very first time a very long climate record from the Arctic Ocean. Previous to this there is only about half a million year's worth of climate record, so we were able to basically extend the information about the climate at the top of the planet from, essentially, the present day to 57 million years ago.

Chris Smith: How did you do that because 57 million years is a long time?

Kate Moran: This is actually a technique we've used in other oceans. It turns out that sediments in the deep parts of the world's oceans get deposited at very slow rates, about a centimetre to two per thousand years, so that means a pile of sediment can represent a very long record, basically like a textbook going back in time, turning the pages back in time. And so what we did was we went back and we cored, we recovered a continuous core section of about 420 metres of sediment below the central Arctic Ocean in about 1,100 to 1,300 metres of water. It was a tough thing to do but we were able to do it.

Chris Smith: Once you got that back to the lab, what fruit has it borne, if you like?

Kate Moran: First, on board we actually had a few scientists on board and we were looking at little bits of each piece of core that came up. We carefully protected most of the core when we were off-shore and while we were out there we found that we had recovered a very important time interval called the Palaeo-Eocene thermal maximum, which was about 55 million years ago. And that was a time where the planet warmed significantly from greenhouse gases. And it seemingly warmed very rapidly, much like today. So this particular time period is important for us to study because then we can see how the Earth responded to these warmings. Now, previous to this, climate modellers had suggested that the difference between the climate at the Equator and the Poles would be quite large during warming periods like today. What we found is that the Arctic Ocean at that time was quite warm as well, surprisingly warm.

Chris Smith: So how warm was it? One of the researchers involved in trying to pinpoint the temperature at that time was Appy Sluijs of the University of Utrecht. He told me what he found.

Appy Sluijs: The key finding is that about 55 million years ago when we know there was an episode of very high greenhouse gas concentrations in the atmosphere, temperatures at the North Pole actually reached 24 degrees and there were also tropical algae swimming in the Arctic Ocean.

Chris Smith: 24 degrees, that actually sounds warm enough for a swim, which seems quite remarkable compared to the temperature, what is the temperature of the Arctic Ocean now?

Appy Sluijs: Well, the temperature of the Arctic Ocean at present is basically zero or below zero, we have an icecap, we have sea ice drifting on the Arctic Ocean at the moment, so lots of difference.

Chris Smith: OK, and I presume, does that mean that there was just no ice there at that time then?

Appy Sluijs: Exactly, yes, there was no ice and there was, as you said, it was actually nice for a swim, it was warmer than the present North Sea, for example.

Chris Smith: Indeed, so how did you make this finding?

Appy Sluijs: We know also for this time interval what kind of species were living in the Tropics and what kind of species were living on the higher latitudes and now it appears that this specific tropical dinoflagellate, this algae, migrated all the way up to the North Pole and to the Arctic Ocean during this time interval. So also on the basis of algae we could say there is actually tropical algae swimming in the Arctic Ocean 55 million years ago. And the other method we use is an organic palaeo thermometer, you could say, basically a way to measure temperature in the Arctic Ocean 55 million years ago, and it's based on fossil molecules made by Archaea and basically on the ratio of various of these molecules there is a way to actually quantify temperature so that way we can actually say it's 24 degrees.

Chris Smith: Appy Sluijs speaking there. So that was one of the major findings, that the Arctic Sea was once very warm. But the expedition also found that this warm period, sometimes called the greenhouse world, may have come to a rather abrupt end. And this might mean our understanding of how greenhouse gases like carbon dioxide affect the world isn't as good as we thought. Here's Kate Moran again.

Kate Moran: If we go younger in our sediment history, to approximately 45 million years ago, we found something that we visually saw in the core and it was a pebble, quite a large pebble, about a centimetre, and we're in the middle of the Arctic Ocean, in very deep water, and in order to move something like that from land to that part of the ocean you need to transport it with something that can float and move, and the only way that we think it could have got there was from ice. And so what we're suggesting, we think that the ocean basin started to freeze at that time, forming sea ice, perhaps icebergs, and that is much, much earlier than anyone ever thought that the northern hemisphere actually began to cool.

Chris Smith: And we know this is a greenhouse-mediated effect?

Kate Moran: If you look at the Earth in that time period, zero to 57 million years ago, we've got two big periods of time, one is the greenhouse world, one is the icehouse world. So we hadn't thought that we would have any evidence of ice during the so-called greenhouse world in the northern hemisphere. Now, at about 42 million years there are suggestions from other studies that that's when cooling began in antarctica. The fact that we have seen cooling now means that it could be that the planet is cooling at the same time at the North and the South Pole. And this again is important because climate changes, either from some kind of atmospheric change, CO2, or from some major change in ocean circulation due to tectonics, like closing of some kind of seaway, and it's those seaway closings that cause slow response, so one part of the planet could cool first and then ocean circulation changes and the other part of the planet could cool later. That's what we thought had happened, but this study suggests that that may not be the case. So I think of us will have to rethink how we moved from the greenhouse world to the icehouse world now.

Derek Thorne: Kate Moran from the University of Rhode Island on how the world went from greenhouse to icehouse.

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Derek Thorne: Still to come in this Nature podcast, what's the best way for lab rats and mice to pass away, and researchers dare to dream that they might have found out how rapid eye movement, or REM, sleep switches on and off. Now, to the city of New Orleans. We've known for some time that New Orleans is gradually sinking, and this subsidence makes it vulnerable to flooding, as occurred last August when the city was struck by Hurricane Katrina. But exactly where and how is this subsidence taking place. Well, a team led by Tim Dixon from the University of Miami has some answers. They've been using satellite data to work out what parts of the city are sinking the fastest. Nature 441, 587–588 (1 June 2006)

Tim Dixon: What we've been investigating is the subsidence of New Orleans and one of the key findings is that we have a pretty good estimate of the average rate of subsidence in New Orleans, it's around six millimetres a year, but perhaps more importantly we have a measure of subsidence of some of the levees and one of the surprising things is that some of those levees are subsiding at much faster rates, up as high as about 25 millimetres a year. And that may be giving us clues as to why those levees failed. Either they subsided over the last 40 years since they were built, so they were simply too low, or the high subsidence rates indicate that the substrate of the soil on which the levees are built is anomalously weak and subject to subsidence when you put a large load on it, like a levee.

Derek Thorne: How is it that you made this finding?

Tim Dixon: We used a technique called permanent scatterer interferometry, and it's based on the use of radar sat data, which is a type of synthetic aperture radar data. And radar has one neat property, that it allows us to measure distance changes between the antenna of the radar and the ground surface.

Derek Thorne: Do you think these findings might help to protect New Orleans in the future from flooding like we saw with Hurricane Katrina?

Tim Dixon: Definitely, I think you could use the findings to say which levees are most prone to subsidence and with that knowledge you could perhaps change some of the substrates, although that might be quite expensive. More simply you could just raise the height of the sea wall based on the knowledge that the height that you build it to is going to be lower in the future, and so you raise it by a metre or two and you've accounted for the subsidence that will likely happen in the future.

Derek Thorne: But looking further into the future, does this really mean that New Orleans is in an unsustainable position, I mean, what does it say about the future of that place?

Tim Dixon: Well, here it gets a little more speculative and our data don't directly address that. My own personal opinion is that New Orleans is OK for the next 20 or 30 or 50 years, but longer term, on the order of one or two hundred years, New Orleans has three strikes against it. One is that a large part of the city is built on these marshy sediments that are subject to the subsidence. The second strike it has is that the larger Mississippi Delta is basically slowly sliding into the Gulf of Mexico. We can measure the rate of sliding now, but we don't know if that's constant or if it could accelerate or slow down in the future. And then of course the third strike against New Orleans is global warming. There's good evidence now that both hurricane frequency and hurricane intensity are going to increase and so I think over the next few decades New Orleans could be fine, longer term I think we have to think about some other options.

Derek Thorne: Tim Dixon from the University of Miami on the uncertain future of New Orleans. Now we've got a round up of some of the other major science news stories of the week with Nature's Jo Marchant, and I gather, Jo, that you've got more news this week on New Orleans and Hurricane Katrina.

Jo Marchant: Yes, that's right Derek. We've got an interview with a member of a team that's just put out an independent report on what happened with Katrina, basically what went wrong, who was to blame. But what we've got is an interview with David Rogers who's a geologist at the University of Missouri who has travelled to New Orleans 11 times as part of his work for the team and we just asked him a little bit about what his role was and what he thinks went wrong and how he feels about the whole process now. Nature 441, 556–557 (1 June 2006)

Derek Thorne: And what does he have to say about all that?

Jo Marchant: He said that he was really surprised by what they found, he was drilling down into the swamp deposits where the levees were to look at the situation there and said he was amazed at how permeable the swamp deposits were. He said he's drilled all over the world and never seen anything as permeable as the swamp deposits underneath New Orleans.

Derek Thorne: And what does that permeability mean, or what could it have meant?

Jo Marchant: What the problem is that there are sheets of metal that go down beneath the levees which are supposed to stop water from seeping underneath them, and because the material underneath the levees was so permeable they just didn't make those sheets anything like deep enough. An early conclusion was that everything was due to over-topping, which is water going over the top of the levee, but what this report says is that a lot of it was seepage related, it was more water being forced underneath the structures, eroding their bases and ultimately knocking them down.

Derek Thorne: So what are the prospects for the future of New Orleans because I gather the hurricane season is due to start again?

Jo Marchant: Well, there are two levels to this. One is the specific fixes for the damaged levees caused by Hurricane Katrina and Hurricane Rita and David Rogers has said to us that even though his band-aids won't be completed until 30th June, which is a little later than they would have liked seeing as the hurricane season starts on 1st June, the other level is the longer terms fixes and changes that need to happen to keep the city safe on a longer time scale and there he's pretty pessimistic actually. He said that they've got the reports, they've come out with a very well thought out plan of attack and he said that the elected officials just don't, in his opinion at least, seem to really care about it. He said that as far as he knows no elected officials showed up to the press conference when the report was released, so he's pretty pessimistic about the chances of much changing in the future, to be honest.

Derek Thorne: OK, thanks for that. So I gather there's another story to do with the Flores Hobbit, so what's been happening there?

Jo Marchant: Well, this is an ongoing story, basically I'm sure everybody will remember these were the remains of tiny people just one metre tall that were discovered on the Indonesian island of Flores and the original researchers that published this in Nature back in October 2004 concluded that these were a new species of hominids, probably descended from Homo erectus. But there is a continuing group of dissenters who don't believe that this is a separate hominid species. They think that these were just deformed modern humans basically. There's a condition called microcephaly which gives sufferers a very, very abnormally small brain and it can cause dwarfism as well, so the argument is that these could just be sort of abnormal individuals as part of a modern Homo sapiens community. Nature 441, 559–559 (1 June 2006) ;

Derek Thorne: What kind of noises are coming from the original group that found these remains?

Jo Marchant: Well this is very interesting because they're now looking at the theory that perhaps after all of this debate, Homo floresiensis didn't descend from the Homo erectus that was found that was around in Indonesia at all. This was the assumption because Homo erectus, Java Man, is the particular individual that everyone would have heard of, is the only hominid species that would have been around in that area at the time that the Homo floresiensis ancestors would have been there. But Java Man is very big, so we've heard that the latest idea from Michael Morwood who is one of the original discoverers of the Hobbits, he's been saying that maybe that is actually quite unlikely that you'd get such a big species evolving into such a small one, and perhaps there are some more hominids that were around in Asia at that time, that we just haven't found yet, with smaller bodies, and that they may have been the ancestors of Homo floresiensis. Nature 441, 624–628 (1 June 2006)

Derek Thorne: OK then, as so finally this week I gather that the Nature news team has been looking into the risk associated with working in a chemistry lab. Nature 441, 560–561 (1 June 2006)

Jo Marchant: Yes, that's right. There was a very serious accident related to chemistry a few months ago at the National Institution of Higher Learning in Chemistry which is in Mulhouse in France. It actually killed a 41-year-old photochemist. And in general that kind of reminded everyone, I think, what the dangers of research can be. So we just wanted to have a look and see, is chemistry really that dangerous? Is it more risky than other fields and what are the main risks in chemistry?

Derek Thorne: And so what have you found there?

Jo Marchant: The main thing we found is that things have certainly changed a lot in the last 20 years. We spoke to a lot of safety officers and a lot of chemists who all said that compared to up to the 1960s, where you had some quite startling practices, mouth pipetting is one, washing hands with benzene, which is now known to be a carcinogen, is another. After a lot of occupational health legislation that came in in the 1970s, things are much safer. But still we were told that there are particular areas where chemistry labs could be doing a lot better. Labs are much too crowded. And waste disposal was the other big issue, most chemistry labs have open bottles where solvents are dumped and it's virtually impossible to say what could happen in those mixtures. When we spoke to the industry it sounds as though the accident rate in academia is much, much higher than in industry. In academia it's more people working alone late at night and being slightly macho even about safety glasses or following particular rules and regulations.

Derek Thorne: That was Nature's Jo Marchant. And if you'd like to discuss these or any of the other science stories this week you can go to the Nature news blog at You're listening to Nature's podcast from the 1st June edition of the journal and I'm Derek Thorne. If you'd like to learn more about the stories featured in this week's programme please go to our website at, and if you have any comments on this or one of our previous podcasts, you can send an email to Next, far, far out in the Solar System, a particular moon orbiting Saturn has been doing some very slow gymnastics. This small icy moon is called Enceladus and for some time scientists have been trying to explain an anomaly. Why is it that a relatively warm part of the moon is at one of the poles? The answer seems to be that this hot spot is less dense than the rest of the moon and so the gravitational force from Saturn has pulled the heavier part of the moon round. Francis Nimmo from the University of California, Santa Cruz, told me more. Nature 441, 614–616 (1 June 2006)

Francis Nimmo: Enceladus, which is one of these moons of Saturn, has an area which is hot and seems to be spewing out water at the surface, but the really funny thing about this hot spot is that it seems to be located exactly at the moon's south pole. And so what we've been trying to explain is why that hot spot should be located at the south pole rather than anywhere else.

Derek Thorne: So what is your theory for that, then?

Francis Nimmo: Our theory is that the reason there's a hot spot there at all is that there's a blob of warm material somewhere inside the moon, either in the ice shell or below in the rocky core, and so this warm blob creates the hot spot you see at the surface, but also because it's warm it's less dense than the surroundings and if you have a spherical object like a moon with a low density blob in one place, the whole moon will roll over until that low density blob ends up at one of the poles.

Derek Thorne: And over what sort of time period did the rolling over happen?

Francis Nimmo: Well, that's a very good question but it was probably rather fast, less than a million years. So geologically speaking it was very rapid, although obviously from our point of view we wouldn't see it happening, it would be too slow.

Derek Thorne: Let's talk about this blob in a little bit more detail. What exactly is it?

Francis Nimmo: We're not really sure, but essentially if you take something like an ice shell and you heat it up then you get warm blobs rising from the bottom, just like in a lava lamp, as the lava lamp heats up you get warm blobs rising. And the reason that they rise is because they're lower density than the surrounding material and so we think that Enceladus it must have generated at least one warm, rising blob, and it was that blob that caused the whole satellite to roll over. And so what that means is that the interior of Enceladus is being heated somehow.

Derek Thorne: Is there any way that you can confirm this finding?

Francis Nimmo: There are a couple of things that we can think about. The most obvious one is to look at the record of the craters on the surface of the moon, because as it travels through space it runs into debris which causes craters at the surface and so the front side of the satellite ends up getting more craters than the back side. But once it has accumulated those craters, then if it rolled over you'd see craters in the wrong place, and so by looking at the distribution of craters on the surface, that might allow us to tell whether or not it really has rolled over.

Derek Thorne: Has this kind of rolling over been seen before?

Francis Nimmo: There's some evidence that even the Earth may actually have done it, and that's based on the fact that you can tell where the magnetic pole of the Earth was in past times. But there's also evidence from one of the other icy moons in the outer solar system called Miranda. Miranda also has things that look a bit like the hot spot of Enceladus and it looks like Miranda actually rolled over several times because it has three of these blobs, only one of which is at the south pole now, and so Miranda rolled over, we think, in response as each of these blobs appeared.

Derek Thorne: Francis Nimmo from the University of California, Santa Cruz, on how Saturn's moon Enceladus rolled into its current position. Next, if you're having a vivid dream then there's a fair chance that you are in a period of rapid eye movement, or REM, sleep. This type of sleep is important for our brains, but many questions still surround it, such as what is it for and how does it work? A team of researchers based in Boston has been trying to work out how it switches on and off by studying particular nerve cells, and also the chemicals these cells use to communicate with one another, called neuro transmitters. Chris Smith got the details from Jun Lu, of the Beth Israel Deaconess Medical Centre. Nature 441, 589–594 (1 June 2006)

Jun Lu: This was a study that's trying to find the neurocircuitry that contains REM sleep.

Chris Smith: So in other words the on-centre for dreaming.

Jun Lu: Right, because REM sleep is a specific stage during which your brain actually becomes highly activated and at the same time you lose your muscle tone, you become paralysed. So also people call it paradoxical sleep. And for a long time people know, somewhere in the brainstem there's a neuron controlling this behaviour.

Chris Smith: So how did you manage to home in on it and track down where those neurons were?

Jun Lu: One of the ways we do that is we put an animal in a condition where they have a lot of REM sleep. The way we do that is basically you give a dark pause [?] in the morning. In that condition an animal, we don't know exactly the reason why, but they have a lot of REM sleep. And then we can look at the particular early gene, called a Fos, see where those Fos are produced, so that way we find in the region of brainstem has high fars expression following REM sleep. And from there we will find a particular cell then we can trace where this cell projects and what is the neuro transmitters those cells contain.

Chris Smith: And where do those cells go and what is the neurochemistry they contain?

Jun Lu: It's very interesting. So what we found is that three groups of neurons are activated in REM sleep. One group projects to the forebrain and those cells contain excitory neurotransmitters. So you can imagine, when these cells become activated they project to the forebrain and your brain becomes highly activated.

Chris Smith: So what's that, glutamate or something?

Jun Lu: It contains glutamate. And also there's another group of cells projects spinal cord and they also contain glutamate. But these cells don't project to the motor neuron directly. And they project to the interneuron. Those interneurons contain an inhibitory neurotransmitter, glycine.

Chris Smith: So that's the correlate of how you become paralysed during sleep.

Jun Lu: Exactly, so when these cells become activated you stimulate a glycine inhibitor in the neuron and that neuron shuts down the motor neuron, that's how you become paralysed.

Chris Smith: And what does the third group do?

Jun Lu: The third group is very interesting because, think about it, when you are in REM sleep you want to maintain REM sleep, so you don't want to just switch to the REM sleep and then you switch back to other states, you want to maintain the time and that's the key of our third group. The third groups contains gaba in the neuron which is an inhibitory neurotransmitter. What these neurons do is they project to another group which is REM of cell group. These cells are inactive during REM sleep but they inhibit REM sleep cell group, all these three groups inhibit it.

Derek Thorne: Jun Lu, from the Beth Israel Deaconess Medical Centre in Boston who's shown that drifting into REM sleep is nothing to do with the sandman. And finally, in the world this year more than 10 million mice and rats will be used in experiments. The vast majority of those will be killed in the laboratory. So it may seem like a morbid question but it has to be asked, what is the best way to kill a rodent? Nature's Emma Marris has been investigating this issue and she found that not everyone agrees on how to administer a peaceful death. Nature 441, 570–571 (1 June 2006)

Emma Marris: Well lots and lots of rodents are killed every year in the course of research and it actually just occurred to me through talking with friends of mine who are involved in animal research that there's more than one method here and there is some active debate about which method is most humane.

Derek Thorne: So what is the current method, as it were, for killing animals?

Emma Marris: Well it does vary a bit, but carbon dioxide seems to have become the overriding default method, at least in the United States and Europe. This is done just by putting the rodent in a tank and slowly adding CO2 or occasionally in the US by sort of chucking the rodent into a tank that's already been filled with CO2.

Derek Thorne: And is there any evidence now to suggest that that's not as painless as people once thought?

Emma Marris: The interesting thing here isn't the pain, it's the distress. It's true that carbon dioxide can be extremely painful at higher concentration. It kind of melts on the mucose membranes of the animal and becomes carbonic acid which is very painful. But at lower concentrations it's much more likely to cause a sort of a vaguer concept which is distress. And this is where the animal may feel like they can't breathe.

Derek Thorne: Are there any alternatives out there then, to CO2 killing?

Emma Marris: One of them, the simplest is actually a procedure called cervical dislocation which is actually manually done. It's just expertly snapping the rodent's neck very quickly with your hands. A lot of people believe that this is in fact more humane than CO2 because it's practically instantaneous as long as you are skilled at it.

Derek Thorne: But I suppose the question here is that although it might be better for the rodent, is it more unpleasant for the person doing it?

Emma Marris: Yes, very much so, and it's interesting that these sort of human behavioural considerations or these aesthetic considerations seem to really have come into the decision making process and a lot of researchers like animals and the idea of holding a live, wriggling animal in their hands and killing it with their hands can be quite emotionally traumatic. So putting it in a box, where it seems to sort of drift off, can seem much more pleasant and the researchers themselves like it better in general.

Derek Thorne: Do you think this issue might be resolved in the near future then?

Emma Marris: Well, it's interesting a lot of people are looking at this right now. There was a conference last February about the possible drawbacks of CO2. The European Commission is looking at its own standards. I think a lot of countries are looking at this and there may be some changes in the future.

Derek Thorne: And of course in comparison to how rodents are killed in the home. I suppose, is it actually a rather better deal in the lab?

Emma Marris: Right, I mean in a way the disagreement here is between varying levels of humaneness. Of course, you're never going to have a 100% humane death because you're killing the rodent. But all of these methods, including CO2 are much more humane than your average kind of domestic encounter with a rodent, which can often be quite messy and extremely traumatic for both sides.

Derek Thorne: Nature's Emma Marris on the debate over the best way to kill laboratory rodents. That's all for this week, so thank you for listening, but do join us next time when we'll be winding back to the formation of the solar system and shaking up the study of earthquakes. And in the meantime, don't forget to check out the Naked Scientist podcast this week. The Naked Scientist team will be answering questions from any and every field of science, and also finding out what makes us happy. That's the Naked Scientist podcast which is freely available from The Nature podcast is produced in a division of Virology at Cambridge University by Anna Lacey, myself Derek Thorne, and Chris Smith, who returns next week.Advertisement:The Nature podcast is sponsored by Bio-Rad, and the centre of scientific discovery for over 50 years. And on the web at