Nature Podcast：维京人登陆北美时间

又到了每周一次的Nature Podcast时间了！欢迎收听本周由Geoff Marsh带来的一周科学故事，本期播客片段里讨论了科学家们如何更加精确地估测维京人登陆北美大陆的时间。欢迎前往iTunes或你喜欢的其他播客平台下载完整版，随时随地收听一周科研新鲜事。

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First up on the show, reporter Geoff Marsh has b

een finding out how the Sun has helped precisely date Viking settlement in North America.

In the 1200s AD, in a couple of texts called the Vinland sagas, epic stories were told of how the Vikings, also known as the Norse, had made the boat journey west from Iceland via Greenland to the Americas. But it wasn’t until the twentieth century that these potentially fictional accounts earned some credibility, as archaeologists found hard evidence of Vikings in Canada at a site called L’Anse aux Meadows in Newfoundland.

The first presence of Scandinavians or Norse or Vikings in North America was really discovered in the 1960s, and that’s very exciting because it’s the furthest west that anybody has ever discovered a European settlement at that point in time.

This is Cat Jarman, an archaeologist specialising in the Viking Age, currently at the Museum of the Viking Age at the University of Oslo.

But there’s been quite a lot of questions surrounding it, and so although we’ve been excited, people have tried to tie it very closely to the saga literature, still questions have remained, especially about the timing and the duration of that settlement.

Unfortunately, the Vikings didn’t write down dates in the way we do today. Instead, they gave timings relevant to important events, like ‘this many winters after that famous battle’. Plus, the sagas were written down several hundred years later, after the Viking Age, and they’re thought to be at least partly fictional, meaning we can’t really take any of the dates at face value unless they’re independently verified. So, researchers have often looked for other clues as to the timing of historical events. In the twentieth century, this pursuit was revolutionised by the introduction of radiocarbon dating.

Radiocarbon is a radioactive form of carbon, and it decays away with a given half-life from the moment it’s formed.

This is Mike Dee from the Centre for Isotope Research at the University of Groningen in the Netherlands.

But luckily, it’s formed continuously in the atmosphere, and it’s formed because the Earth is constantly being bombarded by particles from space, from all different directions actually, and it would basically decay away and perhaps disappear in the atmosphere if it weren’t for the fact that it becomes part of carbon dioxide and is taken up by plants, just like ordinary carbon dioxide, through photosynthesis. And so therefore it gets built into the structure of plants. So, what you need to do is find some organic tissue from the past and measure the amount of radiocarbon in it, and you can trace back up the sort of decay curve and work out approximately how long ago it was when that organism was living.

Which is all well and good, but it’s just not that accurate. But today in Nature, we’re publishing a paper which details a new technique which manages to give radiocarbon dating a sort of turbo boost in terms of its resolution, allowing scientists to resolve dates to the single year.

And this has actually only arisen because in the last nine years officially we in the community have realised that there are moments in time over the last thousands of years when there was a particular signal, particularly actually a little spike in production in the atmosphere, and we think that that was the result of major storms on the Sun, the likes of which nobody has witnessed in modern times. Massive storms like huge solar flares that would throw out all these particles and bombard the Earth and caused a sudden rise in production.

What’s your sort of reference scale? How do we know when these big events took place?

It just turns out that tree rings, when they build in that carbon signal, it doesn’t move across the different tree rings. It stays locked into the year in which that particular tree ring was laid down. And because, as you know from when you’re a kid or whatever, you can count the number of rings in the cross section of a tree and you can work out how old it was when it died, there is a whole science called dendrochronology, and you have long, long, therefore, records, of these tree rings in these dendrochronological archives, in which you know exactly what year each of those tree rings grew. For example, 993 AD, which is the sort of spike that our work concerns, there is this jump in production, which we think is caused by one of these events, and if you go to these archives, all around the world, and there’s like 30 or 40 all around the world, in that exact same growth ring, in that exact same year, you will see this jump.

I would like to know whether your technique or the question about Vikings – what came first? Presumably, the technique came first and then you realised, well, that could be quite interesting for Vikings?

So, these events, these sudden upsurges were really more of interest to solar physicists and astronomers and so on, in the beginning, and we knew that there was one in the eighth century AD and we knew that there was one in the tenth century AD, towards the end of it. And at the time, I was thinking what chronological puzzles or challenges can we examine that we might be able to really resolve with this method, and I started to think about the Viking voyages. The Vikings also used a lot of wood, which is very handy for our work. And I thought, well, here’s a real chance because they might have some wood left over. We might be able to get this really pinned down to the exact year.

Once Mike and his colleagues had decided a target for their new technique, they needed some organic material that could be tied to the Vikings. Luckily, a retired archaeologist, Birgitta Wallace, who’d been active since the 70s at the famous Viking site L’Anse aux Meadows in Newfoundland, Canada, had had the foresight to keep a load of materials in a freezer for future research. So, Mike’s colleague, Margot Kuitems, gave her a visit.

And I went there and she was very helpful, and I opened the freezer and I remember I said, ‘It’s a gold mine.’

So, what were they? Were they bits of Viking longboat or house? What were the bits of timber?

Yeah, so, in that sense, it doesn’t sound like a gold mine at all to many people, probably, because they are actually just bits of wood. So, there were wood chips, which just came off during maybe boat repairs or building activities, and actually there was kind of good news because for this method, it was really important that we had the bark edge preserved, so the outer layers. And if you have, for instance, a statue or another artefact, then often the outer bits are chopped off, and also they are more precious and people won’t allow you to take samples.

So, you opened this fridge and whereas probably anyone else in the world will have opened this fridge and seen these frozen sort of random blocks of wood and probably closed it again, for you it was a gold mine.

Yes.

Can you confidently link this wood to the Norse people and not the Indigenous North Americans?

Because they all show signs actually of distinctive marks that are made by metal tools, and metal tools were not used at the time by the Indigenous American people living in the area.

How lucky that the Viking carpenters didn’t use their offcuts for a nice warm fire.

Exactly, yes.

And as you say, you want bits of wood that have enough growth rings but also that extend right the way to the edge of the tree so that you can work out exactly how old it was.

Yes, so I brought them with me to the Netherlands, to the lab, and over there I investigated the bits of wood under a microscope.

So then you separate out the rings and then you’re looking for these upsurges of carbon-14, aren’t you, these indicators of a solar particle event? How do you spot those? Presumably, you can’t see those under the microscope?

That’s really done by mass spectrometry. Then you get the radiocarbon concentration of the past for each year. And then in one of the samples, finally we found the upsurge in the 29th growth ring counted from the bark edge.

So, you told us that we know that this big cosmic burst of particles happened in the year 993, and you counted the tree rings, so what year can you pinpoint Scandinavians being there in Newfoundland?

Exactly in the year 1021. And even we could work out sometimes what the cutting season was.

Wow, how did you do that?

Well, you can differentiate between early wood and late wood. So, the wood that was growing in the spring or autumn, for instance, you can see that in, for instance, the discolouration of the wood, the wider cells. Yeah, it has to do with the thickness of the cells.

What is it like holding these bits of wood and knowing that there was a Viking chopping this tree down in the spring of the year 1021?

Exactly, 1,000 years ago this year.

Oh, wow, I didn’t think about that. It’s exactly a millennium ago, isn’t it?

It is, yeah. The 1,000 year anniversary this year. Yes.

How apt that the paper is being published in Nature exactly 1,000 years after this ancient woodcutter.

Yes, it’s like we waited with publication, for instance, for this year, but it was…

A very happy accident?

Exactly, yes.

So, for an archaeologist like you, Cat, what does it mean then to have this kind of very specific year in which the Norse people were definitely in this place in Newfoundland?

I think it’s extremely exciting actually because with the presence in North America of these Viking settlers, the dating really has been the big issue because the dates that we’ve had so far actually span pretty much the entire Viking Age, so a period of over 300 years, and that’s not useful to anyone. And that dating really is important to us because it means that we can then relate it to settlement elsewhere. So, actually, to have something that can pin it down to a single year, which is something that we never really get from any site normally, is really quite spectacular. So, you go from this completely vague idea that it happens at some point in the Viking Age to something so specific.

So, of course, this specific year that we got from these results tells us that they definitely were there at that year, but I suppose it doesn’t tell us when they got there. They could have been there for a while before they cut down that particular tree.

Yeah, absolutely, so that is the other question that’s unanswered: how long did they stay for? And certainly the saga literature suggests it could be up to I think something like seven years. But we need to think of that really more as sort of historical fiction than anything, than real written evidence. So, we don’t know that yet, and the fact that they’re so close together could just suggest that this was literally just a seasonal thing. But I think it does open up for dating more artefacts. This is only a very small sample, but if the method works that well on these objects then maybe we could find more things and we can understand more about that duration. So, it’s both exciting because of what it’s telling us right now but I think also the promise it has for the future.

Finally, I wanted to hear from Mike what he saw ahead for this new dating technique. So, this paper has been a really nice illustration, like a sort of proof of concept of this technique. Of course, as you mentioned, these big solar events are vanishingly rare, aren’t they? Is there any hope that the technique could get more sensitive and that smaller fluctuations that there are perhaps more of would work?

So, we would normally measure our references as averages of ten rings because from this ten to the next then to the next ten, it’s not going to change very much, let’s get an average. All of a sudden, there was a rush for measuring every single year, and that’s what we’ve been doing in the last seven or eight years, is measuring every single tree ring, and now we do see little ups and downs that are reproducible all over the world. You do see, perhaps not as dramatic as these great, big jumps, but other sort of imprints on the record that come from maybe smaller solar events, maybe other things that are also unique time markers. So, either maybe we’ll be trying to find more of these big enough events that we can use as hooks to hook parts of floating chronologies and individual events on every few hundred years or every millennium, or we’ll start to be able to really recognise very small variations in the record, and perhaps we’ll be able to date a lot of stuff to the year, particularly if we have a sequence of dates, which is what you get with tree rings. We might be able to date lots and lots of artefacts to the exact year, so who knows, actually, where we’ll be in 10 or 15 years from now.

That podcast piece was produced by Geoff Marsh. In it, he spoke to Mike Dee and Margot Kuitems from the University of Groningen in the Netherlands. He also spoke to Cat Jarman from the Museum of the Viking Age at the University of Oslo. To find out more about when Vikings crossed the Atlantic, check out the paper in the show notes.‍

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