Dinosaur-destroying asteroid struck in spring

Host: Nick Petrić Howe

Welcome back to the Nature Podcast. This week, working out the season that the dinosaurs perished.

Host: Benjamin Thompson

And how warmer nights are affecting wildfire intensity. I’m Benjamin Thompson.

Host: Nick Petrić Howe

And I’m Nick Petrić Howe.

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Interviewer: Nick Petrić Howe

One day, 66 million years ago, a meteor around 10 kilometres wide hit the Earth and led to one of the most famous mass extinctions – the event that marked the end of the time of the dinosaurs and the start of what some call the age of mammals. But a question you might not have asked about that dramatic day was what time of year was it? Well, now we may have an answer thanks to a new paper in Nature. I called up one of the authors, Melanie During, to find out more, and started by asking a very important question for any palaeontologist. What’s your favourite dinosaur and why?

Interviewee: Melanie During

Oh my gosh, I don’t have one. I always refuse to have a favourite dinosaur. No, because the thing is I love them all, and I love the forgotten animals that lived alongside them perhaps even more. I can never devote all my love to just one dinosaur.

Interviewer: Nick Petrić Howe

That is fair enough. So, dinosaurs, non-avian dinosaurs I should say, went extinct in a mass extinction event around 66 million years ago, which was coupled with an asteroid impact, and in a new paper in Nature you claim to have pinned down this event to a specific season. So, first of all, I was wondering why is that something you would be interesting finding out, what season it was when this mass extinction event occurred?

Interviewee: Melanie During

Well, there’s one thing that you need to keep in mind when we speak about the season of the impact, because the season in the Northern Hemisphere is always the exact opposite of the season in the Southern Hemisphere. So, it’s actually ‘seasons’, but that’s not such a catchy title so we stuck to one. So, the thing is that of course it matters because the impact was going to happen, and there is a large concatenation of dramatic events that happens when the impact occurs. So, the impact occurs and you get all of these dramatic effects, but the first one in the worst. You get the actual impact. You get the infrared radiation. Literally being on the surface will kill you. So, then the season suddenly matters because if you’re below ground, you at least survive that first blow, and the thing about extinction, as you mention, it kills the non-avian dinosaurs. It is a very specific extinction. If you compare this extinction to the other extinctions and you take the tree of life and then literally it’s like a branch gets cut off suddenly. Whereas, generally speaking, this affects more than one group. This is such a specific extinction that maybe the season mattered. So, spring or summer, when most animals are above ground eating, is a much more risky season than autumn or winter, when animals are below ground and even plants have already shed their leaves so they’re a lot less vulnerable.

Interviewer: Nick Petrić Howe

So, for example, in winter, say, an animal might be hibernating underground to survive, and that might coincide with a meteor impact, so I guess, yes, it does matter. So, to figure this out, you looked at the fish that died on the day of the impact. Now, you’re going to have to walk me through this a little bit. For a start, how do you find fish that died on the specific day of the impact?

Interviewee: Melanie During

So, the fish were found with impact spherules in their gills, and these impact spherules, they have a fall back time to Earth of 15-30 minutes after the impact. And because these fish were found with impact spherules in this deposit, we can tell that they died there and then, and were likely even buried alive.

Interviewer: Nick Petrić Howe

And when you say impact spherules, is that just the sort of debris that was thrown up by the impact?

Interviewee: Melanie During

Yeah, exactly. Because the meteorite struck with such force, Earth rock was flung out into space, it crystallised in low-gravity conditions, and then rained back down to Earth. And we can recognise them very well because they’re often hollow in the centre or they have a glassy core, and that’s because they crystallised in zero gravity. Everything that crystallises on Earth has their lightest elements come to the top. If something crystallises in space, it stays in the centre.

Interviewer: Nick Petrić Howe

So, you’ve got these rocks that have been thrown up by the meteor, they’ve gone up into space, they’ve crystallised, and then they’ve come back down and impacted these poor fish. And then with these fish, you looked at their bones, and so from their bones you were able to tell what season it was when they died. How did you do this?

Interviewee: Melanie During

So, bone growth is very comparable to trees. So, you have to imagine it like this. When bones growth for a new year starts, it actually starts by stopping. That sounds really weird, but growth is so minimalised – up to absent – in the winter months, that when the growth starts again in spring, the first thing it does is it deposits a line of arrested growth. The literal separation between, okay, here’s where growth ended and then starts again. So, that’s what happens in spring. And then in spring, bone cells start to be created, and they are more abundant when there is more food. So, it’s always spring and summer because then they’re eating, and in summer there is, generally speaking, a little bit more, the weather is better. You’ve got the whole food chain working with you towards summer. So, everything is eating more and more and more, and you’ve got a high peak in bone cell size and density around the summer. Then you get to autumn, and in autumn the food availability drops down again. The weather doesn’t become so favourable and then you see that the growth is slowing down, and then you get lamellar bone growth – that’s like a really slow form of bone growth. So, you get a couple of lines of lamellar bone growth until winter. Then nothing happens until spring, and then first you get the line of arrested growth again, and then the bone cell production starts again.

Interviewer: Nick Petrić Howe

So, given all this, you’ve got your fish bones, you’ve got your data, you brought it all together, and you come down on it is spring. Spring in the Northern Hemisphere is when this is happening and I guess autumn in the Southern Hemisphere. What does this all tell us about this mass extinction event?

Interviewee: Melanie During

I think that what it tells us is that we need to start looking into behaviour of latest Cretaceous fauna, and flora of course but particularly fauna, because I think that the groups that were capable of, I would call it, torpor or hibernation or aestivation, so being able to just shelter in place, shut down most of your vital functions to a minimum and just hang in there until the situation has improved. There’s many groups that are capable of that, and I don’t think it’s a coincidence that most of them survived. And I think it’s a good idea to start looking into such behaviour in latest Cretaceous fauna. That is of course something that was not in the scope of this study, but I think that this study certainly helps direct future research into the end-Cretaceous extinction.

Interviewer: Nick Petrić Howe

So, things that were able to be dormant, like hibernating mammals, may have been more likely to have survived. So, what’s next for this research, or where do you think this needs to go next?

Interviewee: Melanie During

I would look for patterns in hibernation. Look at those who go into burrows because I think it’s as simple as this: when the meteorites struck, unless you were underground, you were dead to begin with.

Interviewer: Nick Petrić Howe

That was Melanie During of Vrije Universiteit Amsterdam in the Netherlands and Uppsala University in Sweden. For more on this story, there’s a news article that you can check out. There’ll be a link to that and the paper in the show notes.

Host: Benjamin Thompson

Coming up, we’ll be hearing how warming nights are increasing the duration and intensity of wildfires. Right now, though, it’s the Research Highlights, read by Noah Baker.

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Noah Baker

The gender pay gap tends to shrink when you tell workers what their colleagues earn. Some activists have suggested that implementing pay transparency — when employees know each other’s salaries — might help to close the gender pay gap by putting pressure on employers to provide equal wages. Now, researchers have put the theory to the test. They assembled a data set of roughly 100,000 academics in eight US states, and compared the salaries of men and women that switched to transparent pay. They found that after the policy was introduced, there was an associated rise in the wages of underpaid women relative to men, and the gender pay gap closed by 2-6 percentage points. The researchers also found that the relationship between academic performance, like numbers of papers published, and salary was weakened after the transition to pay transparency. You can read more about these findings in Nature Human Behaviour.

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Noah Baker

Did you know that King Tut had a dagger formed from a meteorite? Well, he did, and researchers have now delved deeper to discover more about how the pharaoh’s knife was forged. Chemical analysis of the iron dagger found in King Tutankhamen’s tomb suggested that an iron nickel meteorite was a probable source of the blade’s metal. But that posed a conundrum, as King Tut reigned in the fourteenth century BC, before iron working was common in Egypt. Now, researchers have used an X-ray fluorescence technique to map the knife in new detail. They found that the nickel was distributed in a cross-hatched pattern typical of a group of meteorites called octahedrites. But for the blade to have retained this pattern, they say it must have been forged at relatively low temperatures. Additionally, the elemental composition of the decorative hilt suggested that stones were glued on with lime plaster, which was also uncommon in Egypt at the time. All this led the authors to speculate that the dagger was foreign, and might even be a knife mentioned in an ancient diplomatic correspondence – a gift to Tutankhamen’s grandfather from an Anatolian king. You can read more in Meteoritics and Planetary Science.

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Interviewer: Benjamin Thompson

Next up on the show, we’re going to be talking about wildfires and how climate change is affecting their intensity and efforts to control them. Now, this is something we’ve looked at on the podcast before, but last week a paper was published in Nature which looked at the subject from a different angle: namely, how changes to the climate at night are affecting wildfires around the world. To find out more, I called up Jennifer Balch from the University of Colorado, Boulder in the US. I started by asking her the role that night time plays in fire progression.

Interviewee: Jennifer Balch

So, the night is really important for fire because as the sun sets, there’s a drop in temperatures and there’s also a reaccumulation of moisture that happens, and that’s really important for determining how dry fuels are on the ground, and so there is a natural slowing and dampening of fires that happens at night.

Interviewer: Benjamin Thompson

And why have you looked at night time specifically, and what was the question you were trying to answer?

Interviewee: Jennifer Balch

So, there has been a lot of work looking at the effects of maximum temperature and, essentially, the gas pedals on fires in many places around the world, but we haven’t looked as much at the brakes on fires, and so simultaneously to warming temperatures changing the maximum temperatures, we’re also seeing changes in the minimum temperatures around the world as a function of global warming. And so, this study was really trying to take a deep dive into those brakes on wildfires and looking at how fires progress over 24-hour periods, and not only a single 24-hour period, but across the entire life of a wildfire event. Because very often, fires will go for many days or weeks at a time, and so what we tracked was essentially how a fire was behaving on an hourly basis from day to night, from day to night, from day to night, until that event ended.

Interviewer: Benjamin Thompson

How did you go about trying to figure this out then? I know that essential to your work is something called vapour pressure deficit (VPD) – how does that fit into the story?

Interviewee: Jennifer Balch

Yeah, so, the vapour pressure deficit, it combines temperature and humidity variables together to essentially say, okay, how much moisture could the atmosphere hold in a given temperature? For example, if you walk into a hot, dry desert, VPD is really high because the atmosphere can hold a lot of moisture. It’s really hot and there’s virtually no moisture in the air already, versus if you go into a swampy, hot system, there’s already a lot of moisture in the atmosphere and so it has a harder time pulling more water and more moisture out of the underlying fuels that are on the ground.

Interviewer: Benjamin Thompson

And how do you fit that into your work then? How have you looked globally at VPD changes over time?

Interviewee: Jennifer Balch

So, the way that we were able to track VPD’s influence on fire at night is essentially we looked at the minimum vapour pressure deficit and said, okay, how is that minimum changing, and that minimum VPD often occurs at night. We start to see the lowest temperatures and the highest humidity values somewhere in the very early morning hours, when it’s still dark and moisture has reaccumulated into fuels. And so, we essentially tracked minimum vapour pressure deficit on a 24-hour period across the last 40 years, and what we found was that it’s increased by 25%. And what that means is it’s reflective that our nights are getting hotter and drier as a function of the increase in temperature globally. And then we also looked at how that was related to individual fire events and how we were seeing fire behaviour change on an hourly basis, and we saw the amount of active fires detected by satellites essentially tracking VPD really closely. And we did this for over 80,000 fires across the world. We then looked at, okay, how is the intensity of fire changing at night, and globally, fires are essentially putting out more heat or they’re getting more intense.

Interviewer: Benjamin Thompson

Jennifer, one of the things that stood out to me in your paper is that you saw that there are some nights where it never really got cool enough or moist enough to stop fires in their tracks. What sorts of numbers did you see there?

Interviewee: Jennifer Balch

Yeah, so we found that more than a fifth of the burnable world has seen a week-long increase in the number of flammable nights. And so, what that means is our fires can burn for longer periods. And we’ve seen that play out in our fire seasons, for example, across the western United States, where in the past couple of years we’ve had very long-burning wildfires. So, for example, the Cameron Peak fire in Colorado started in August and burned all the way through until 2 December when snow finally put that fire out. So, the ability of that fire to move from day to night, to day to night, to day to night, for weeks and months on end to me is an indication that we’re losing the night brakes on wildfires.

Interviewer: Benjamin Thompson

And it seems like in your analysis that places like the western United States were maybe disproportionately affected compared to other places in the world.

Interviewee: Jennifer Balch

So, the effect of increasing VPD is not a uniform blanket across the planet. There are places in the world that are seeing more or less increase in minimum VPD, and there’s a couple of places that are particularly important. One is the western US and the other is in the northern latitude evergreen forest systems. A huge swath of our planet in northern latitudes had seen a 10 percent increase in fire intensity in just the last 18 years in the fires that are burning at night.

Interviewer: Benjamin Thompson

I mean, if we’re seeing this increase in potential nights where fires are more likely to keep burning in different parts of the world, what do you think this means for efforts to contain things like wildfires? Have you spoken to any firefighters about your work?

Interviewee: Jennifer Balch

Yes, in fact, since this paper came out, just in the last couple of days I’ve heard from several folks in the firefighting community, and not just here where I live in the western US but also in Australia, saying that this is completely consistent with their experience. And essentially what the firefighting community has observed and the brutality of this is that they’re exhausted because they are fighting fires 24/7. They don’t get the fires slowing down or cooling off at night, so they’re not getting a break and, frankly, with warming and anthropogenic climate change, we’re just asking too much of our firefighters.

Interviewer: Benjamin Thompson

I mean, what happens now then? You’ve shown this work saying that night times are changing. Where do we go now? What happens next?

Interviewee: Jennifer Balch

So, I do expect that we’re going to see more fires and more extreme fires in the next couple of decades, and we have to rethink where we’re putting homes and where we’re putting people in flammable places. I’ve done work that showed in the US more than a million homes were within wildfire boundaries over the last two decades, and there are another 59 million homes that were within a kilometre. I live in Boulder, Colorado, and we experienced the most devastating wildfire in Colorado’s history on 30 December. The Marshall Fire burned more than 1,000 homes. Literally in my back yard I could see the fire burning from a hill just behind my house. And it’s one thing to study wildfires for 20 years, but it’s a whole other thing to see it in my own back yard and to have colleagues and friends whose homes burned. Essentially, this paper is adding additional evidence to the fact that we need to be worried about our fire risk and we need to rethink how we’re building into flammable places.

Interviewer: Benjamin Thompson

That was Jennifer Balch. Look out for a link to her paper in the show notes.

Host: Nick Petrić Howe

Finally on the show, it’s time for the Briefing chat, where we discuss a couple of articles that have been featured in the Nature Briefing. Ben, what have you found for us to discuss this time?

Host: Benjamin Thompson

Well, I’ve got a story that I read in Science, and it’s describing how the escape of some fluorescent fish in Brazil has got some researchers worried.

Host: Nick Petrić Howe

I take it from this then that these fish are somewhere they’re not really supposed to be.

Host: Benjamin Thompson

Yeah, absolutely right, and in this case we’re talking about zebrafish that have been engineered to glow red and green. And these fish are about as long as a matchstick, and zebrafish are used a lot in research and fluorescent zebrafish were initially made for research. But in the kind of early 2000s, they were sold for commercial purposes, for people to have at home in their aquariums. And apparently they’ve escaped from some fish farms are living and thriving in the creeks in this area called the Atlantic Forest in the east coast of Brazil.

Host: Nick Petrić Howe

Right, okay, but these are obviously engineered fish, they’re not necessarily supposed to be there. So, are they having some impact on the wildlife around there?

Host: Benjamin Thompson

Well, this is a big question, Nick, and I think it’s an important question to ask, and from what I understand, this is a very rare example of where an engineered animal has become established in the wild. And one of these fish was found in a water course in Florida in the US a while ago, but it was only one, and researchers think that predators probably took care of it because they’re pretty easy to spot, let’s be honest. But in Brazil, levels of predatory fish seem to be lower in these areas so they’re not keeping numbers in check, and so the researchers behind this work have been sort of working out what’s been going on. And these fluorescent fish, it turns out, have been breeding year round, and also they’ve been eating a lot of things in the area, so insects and algae and zooplankton. But in terms of what effect that’s actually going to be having, that’s kind of up in the air to be honest with you. As we know, ecosystems and biomes are really kind of in a delicate balance, and so what happens as these fish continue to breed and their population expands is unknown. Now, some researchers are saying, well, if they do go into other areas, the same thing might happen that happened in the US and predators will just eat them. But others are saying, well, I think we need to keep a close eye and see what’s going on here.

Host: Nick Petrić Howe

So, what happens now with these fish?

Host: Benjamin Thompson

Well, again, it’s kind of a question mark to be honest with you, but in the paper they do put forward some management practices to try and stop these escapes happening. And what’s interesting is this article says that there’s a ban on the sale of these fish in Brazil but local fish farms keep breeding them and they are for sale, apparently, in stores all over the country. So, preventing this happening and preventing colonisation of other parts of Brazil seems to be a really, really important thing. But that’s my story this week, Nick. What have you brought to the Briefing chat?

Host: Nick Petrić Howe

Well, we can stick in South America and unfortunately stick with other ecological concerns. I’ve been reading about an oil spill in Peru, off the coast of Peru, and I was reading about that in Nature.

Host: Benjamin Thompson

Right, and that doesn’t sound like good news at all.

Host: Nick Petrić Howe

No, I mean, the tagline for this article is ‘scientists are appalled at the environmental damage’, and the situation is still evolving but it could be anywhere up to 10,000 barrels of oil have been spilled off the coast. And one of the really devastating aspects of this is that the oil has spread into three marine protection areas and so there could be many animals and many fish that are affected by this.

Host: Benjamin Thompson

And what do we know then about when this spill happened and maybe why it happened?

Host: Nick Petrić Howe

So, this happened on 15 January, which you may actually remember is when a volcano erupted near Tonga. Now, this is 10,000 kilometres away from where this occurred, but the oil companies say that the waves, as a result of this explosion, caused a tanker that was pumping oil into a refinery to get thrown about and cause the oil spill. Now, this claim is still under investigation, so the National Maritime Authority of Peru will investigate that claim and see how that pans out, but that’s what we know at the moment.

Host: Benjamin Thompson

And, Nick, you say there that the article says scientists are appalled at the ecological damage. What are we seeing then? What damage has been observed thus far?

Host: Nick Petrić Howe

Well, many birds have been killed already. Many more are covered in oil, and many, many fish will have been affected by this. And what’s really concerning is that many birds fish off the coast of Peru and so even if they’re not affected by the oil directly, they’ll eat contaminated fish and that could again affect them. So, there are estimates saying that hundreds of thousands of birds could be affected as it spreads, and it’s really quite a disaster for Peru as a country as well because it is a fishing country. A lot of people rely on fish for their livelihoods and this could have a significant impact on many people’s livelihoods.

Host: Benjamin Thompson

So, it seems to be that there could be quite a long tale then in terms of ecological damage to the environment and to people’s livelihoods. What happens now?

Host: Nick Petrić Howe

Well, Peru is no stranger to oil spills. There were several a few years ago. And so, with this one, which could well be the largest one that has been seen, many scientists are pushing for an end to the reliance on oil and a push away from fossil fuels because it’s a very high-risk activity and we can produce energy from different sources.

Host: Benjamin Thompson

Well, let’s leave it there then for this week’s Briefing chat. And listeners, to learn more about those stories and how to sign up for the Nature Briefing, check out the links in the show notes.

Host: Nick Petrić Howe

That’s all for this week. But if you can’t wait until we’re back next week, you can reach out to us. We’re on Twitter – @NaturePodcast – or you can send an email to podcast@nature.com I’m Nick Petrić Howe.

Host: Benjamin Thompson

And I’m Benjamin Thompson. Thanks for listening.