People need mementoes. They are small things (normally) with little intrinsic value (often), but which carries a memory which is far more valuable to the owner than the item is in itself. It can be a bit of jewelry, a cup, or a chair, or even a pebble. Or it can be one of many other things. Somewhere in a museum is Neil Armstrong’s space suit. It is utterly useless on Earth, but it brings back memories of a time half a century ago, when we could still go places. Now all we do is a quick billionaire dip in the sea of space. The will to explore remains in our mementoes. Some time ago I came across my old playpen. I have no memories of being in it (of course) but it brought back memories of being told (in great detail) how I escaped from it before I could even walk. We keep the first shoes of our children, not because they will ever be used again but for the precious memories they bring back. A smell can do the same thing. It is the way our minds work: they need something to hold on to, in order to guide us back to our past. And who knows: memories have power, and maybe one day that space suit will incite our children to restart the journey we abandoned.
Volcanoes also leave us with mementoes. They can be the real deal, the calderas formed by the explosions or the thick layer of ignimbrite which deeply buried an even more distant past. They can also be more fleeting and volatile, such as the smell of smoke and sulphur in the air. The ash on the ground will continue to carry that smell for centuries. Or longer: one of the things we learned on the Moon is that moon dust (a bit like ancient abrasive snow) smells like burnt gun powder. (One of the astronauts had an allergic reaction to it – maybe that is why we couldn’t go back.) We do not know what produces the smell. One suggestion is that it is caused by a reaction with oxygen in the astronaut’s air. The moon dust brought back to Earth does not smell: the smell may be old, but it did not last.
This is also true for volcanoes: once exposed, the smell of old eruptions does not last. Volatile mementoes should be stored safely. And so they have, buried in ancient icecaps where they were deposited on a glacier and covered by the snows of later years. In cold storage, they will last as long as the ice they are in. Dig into the glacier and you can find the layer with the dirt and the smell of an old eruption. These are the Earth’s mementoes.
Ice has a long memory. At least, if it is given a chance. In temperate places ice is just an annual, here today, gone tomorrow, and leaving us no trace or past. It wasn’t always like that. During the ice age, not that long ago, the winter snows would fail to melt in summer. Layers and layers of ice and snow build up to enormous glaciers, huge dumps of frozen water. So much ice was stored that water levels in the oceans fell by 100 meters. And in some places the ice age never ended. The icecaps of Europe and North America are gone, leaving us only their scars on the landscape, the scraped-down bed rock covered in deep scratches. But in Greenland and Antarctica the ice-age icecaps are still there, unmelted and forbidding. The icecap of Antarctica goes back hundreds of thousands of years. Greenland’s ice isn’t quite as old but it too carries a long memory. Even the ash of Toba is still here.
But how do you get these ancient memories out? That is an adventure. You need to find a place where the ice has been undisturbed since before the last ice age. That often means picking the highest place on the glacier, and this can be 3 kilometers or more above sea level. The ice should have stayed in place, and not flowed towards the distant sea: we are looking for the high centre of the icecap, not the more accessible area near the coast. Just getting there is already an experience. The scientists come here for summer, and work in group isolation as if in a remote oil camp. They live in a small community where it can feel at times that the outside world does not exist.
Now the ice is cored: a drill is used to remove a long core from the glacier. The drill itself is also similar to an oil drill. It brings up an ice core which can be kilometers long. Of course, technology never fails to fail under such extreme conditions – the most important people in the camp, after the cook, are the engineers who keep it all running. The engineers know their instruments inside out and know how to keep things working in ways they never learned at school. The scientists can only look up in admiration at these magicians.
The core is brought up in pieces of manageable size. Initial analysis may be done on site, but eventually it all needs to be transported to civilization. The transport has to be fast, as any melt would be a disaster. C130 Hercules planes may be used for this as they can land where nothing else can. I don’t know how they refrigerate the ice during the journey – perhaps they just keep the windows open while flying! After landing, refrigerated trucks are waiting to speed the cores to the laboratory, and specifically to the ice-cold storage space below the laboratory.
If you are interested to see what it is like to live on the ice in a scientific research station, please look at this video of
Ice camp viewing.
After a few months, the summer camp is abandoned as the winter approaches, and the scientists return to the laboratory where the harvest of ice is waiting. Now the laborious laboratory work begins: counting the annual layers, creating a table of what depth in the core corresponds to what age, and after that discovering what is hiding in the ice – often involving melting a bit of it and see what comes out. Science can be a slog – but an exciting slog like no other.
It is important to locate the very best place for the drilling. Once drilling is in progress, you can’t change your mind and decide that 15 km northwest would be so much better. It would be nice to have some existing infrastructure, so first you look whether any existing bases are usable. They may not be. There are some requirements. The location should be deep-frozen year-round: snow that falls should not melt during the summer. So it should be an exceptionally cold location. This favours very high places: temperature drops with altitude, and at 3 km altitude this can make 30C difference. That helps.
In fact, this is the main reason why the huge glaciers of Antarctica and Greenland survived the end of the ice age. They are so thick that the temperatures at the top are much lower than they were when the ice first started growing. Height is a saviour for the ice. But it is also a danger, since when melting finally begins it goes very fast. After all, this mountain is nothing but ice, and ice is prone to change liquid. It works like a negative volcano, forming water rather than lava. As the ice melts, the surface drops in altitude and the temperature rises – so the melt goes faster. A glacier takes ten thousand years or more to grow, but can disappear in a tenth of that time. Just compare Greenland to Sweden: during the ice age they were very similar, but they are so no longer. One is a paradise for the ice hunters, and the other is full of Swedes. Greenland is a scientific treasure because it kept its ice age memories safely in the freezer, something Sweden failed to do – or managed to avoid, depending on your preference.
The second requirement is for enough snowfall each year to give a measurable layer of ice. It takes decades for the buried snow to become ice, and in that time it compresses to perhaps as little as 10% of its snow depth. A meter of snow can give 10 cm of ice. This is enough to give an identifiable and measurable layer. Icecaps tend to be deserts: the air is too cold to contain much moisture. The Western Antarctica WAIS core is an example of relatively fast growth: it averages to about 5 cm of ice per annual layer.
The third requirement is for a thick enough (and stationary) icecap, so that it gives a record that is long enough for the purpose. At 5 cm thick layers, a 3-km ice core (as in the WAIS ice core) provides a 60,000 year record. But the WAIS location is only 1700 meters high. How does that work? Ice comes to the rescue: the weight of the ice has depressed the crust so much that the bottom of the ice is now far below sea level. This is typical: the Hudson Bay is under water because this is where the Canadian icecap was centred. It is still trying to recover from that, and is slowly rising. Every year the Hudson Bay is a little smaller. In general, the thicker the annual layer, the shorter the record. The oldest ice cores (as the 800,000 year one from the centre of Antarctica) are from places where the annual snowfall is very limited. There is a trade-off: it can make the annual layers indistinguishable, giving both lower time resolution and lower accuracy of the dates. You can either have detail, or you can have a very long record. Our icecaps are not thick enough to give both.
What science can we do with all that effort? There are many different ice cores in existence, and they were retrieved for different purposes. For instance, the Greenland TUNU core was retrieved specifically to study atmospheric pollution from Europe over the past 2 millennia. That is a recent-time project which required good resolution but not a long time line. Other projects measure past CO2 levels, using air bubbles captured by the ice. These require extremely long lasting records. The longest ice core record goes back almost a million years. Only in Eastern Antarctica has the icecap has been stable for that long. Imagine breathing in air from a million years ago! When that air was captured in the ice, humanity was still a distant dream. The Greenland ice, on the other hand, goes back no more than 100,000 years: its icecap largely formed during the most recent ice age and there is little point looking for older ice. Over those 800,000 years, CO2 levels in the atmosphere never exceeded 300 pm. We are now above 400 ppm, in uncharted territory. For an account of the quest for ancient air, see https://cdiac.ess-dive.lbl.gov/images/air_bubbles_historical.jpg.
Some science-on-ice projects look for past temperatures. That is done using oxygen. There are two common isotopes of oxygen: O-16 (the usual one) and O-18 (heavy oxygen). Both are equally healthy, by the way. Your body does not distinguish between those, so don’t be tempted to buy ‘heavy oxygen’ for any advertised health purposes. (Luckily no one seems to have caught on to this one yet, so I hope I haven’t given health scammers an idea!) They are of course both present in water, which after all is H2O. Now water with O-18 needs slightly more energy to evaporate. As temperatures drop, this heavy oxygens tends to stay behind in the oceans and slightly less of it enters the atmosphere. Snow forms from atmospheric water, and so it contains the depressed level of O-18. The amount of O-18 can be measured in the ice core. When you see a past temperature record, this is how it was obtained. (At the same time the oceans become enriched in O-18, and this shows up in fossil marine shells. So in the ice, lower temperatures means less O-18, but in marine records it means more O-18. See for a description of the complexities https://earthobservatory.nasa.gov/features/Paleoclimatology_OxygenBalance. Who said that science was easy?) The O-18 and CO2 are measured in the same ice, so it gives an excellent comparison. And CO2 and temperatures have tracked each other very well over the past million years.
Volcanic pollution in the ice is another project. It is identified in several ways: looking for volcanic ash (pretty rare), measuring sulphur, or measuring the conductivity of the ice (a measure of acidity). The latter has the advantage that the ice does not need to be melted. An ash particle can help to identify the guilty volcano, as these can act as a finger print. Sulphate, in contrast, only tells you that something erupted somewhere. Now the scientist needs to turn detective. But often there is insufficient data for the Miss Marple of science to solve the case of the mystery volcano.
Volcanic ice cores have been retrieved from both Greenland and Antarctica. They don’t always show the same eruptions. Eruptions in the tropics can spread their sulphate to both hemispheres. But eruptions at higher latitudes will find their emissions confined to their own hemisphere. Stratospheric sulphate does not easily penetrate the tropics. This is partly because of the dominant winds in the stratosphere, and partly because the lower edge of the stratosphere is much higher in the tropics (20 km) than at higher latitudes (10-7 km): the stratospheric dust runs into the wall of the troposphere as it spreads towards the tropics. Tropical volcanoes have the advantage. Once they penetrate the stratosphere, the sulphate will easily spread in both directions and within months can reach the poles. Therefore, an Indonesian volcano may show up in ice cores at both poles, but an eruption in New Zealand or Alaska may be seen at Antarctica or at Greenland, but not both. This pattern is one of the clues that Miss scientist-detective Marple will be looking for. The stratospheric sulphate spreads a haze across the sky and dims the sun. In the case of a large eruption, it may lower the temperatures. But only tropical volcanoes will do this in both hemispheres. New Zealand’s volcanoes will have little effect on America’s weather, but can make a real impact on Australia.
In an earlier post we discussed the demise of the Greenland Vikings, and saw that for both of their settlements, there is a finger of suspicion pointing at a volcano. But the finger is not pointing at any specific volcano. The evidence for volcanic actors comes from the ice and from circumstantial evidence from elsewhere in the world which suggest a volcanic ‘winter’. (Actually, volcanoes affect the summer more than the winter, but ‘volcanic winter’ sounds more threatening.) The Western Settlement suddenly disappeared during or shortly after the unidentified eruption which caused the weather that let the rats, the fleas and the plague spread across Europe. The date of the demise of the Eastern Settlement is less well determined and it seems to have suffered a slow decline – but it ended in the same general period as the volcanic upheaval in the mid 15th century.
This is about the volcanic eruptions of the Viking era. It is not actually about the Vikings, and I doubt that the Greenland Vikings ever went up the icecap other than to make some blueberry-juice-on-the-rocks. But it gives me a convenient time scale to use. I will look at the volcanic ice core record for the Viking period 900-1500, a time when much of the volcanic world was still undocumented and volcanoes could erupt in secret. Iceland will play a prominent role, both because its volcanoes are over-represented in the ice, and because we know a bit more about which volcano erupted when. But the rest of the world will get a cameo role.
Sailing the seven sulphatic seas
Let’s first look at the Greenland ice cores. There are several available, but the one most sensitive to volcanic eruptions is NEEM. NEEM stands for North Greenland Eemian Ice Drilling: it aimed at getting a particular long core for past-climate research. The ice core extraction was completed in 2012.
I am using the sulphur tables published by Sigl et al in 2015 (https://www.nature.com/articles/nature14565) for the NEEM core from northern Greenland, and the WDC (Wais Ice Divide Core) from the highest point on the icecap of Western Antarctica. As mentioned, WDC only goes back 60,000 years (this area was ice free between the ice ages) but it has high time resolution. This time resolution is essential when looking for volcanic signals. In order to compare different ice cores for all but the very strongest eruptions, the layers need to be dated to better than a few years accuracy.
The plot below shows the sulphur content of each annual layer in the NEEM core, in red. In most years it is very low, but on occasion there is a significant peak: a volcano went off, spreading its considerable load of sulphate to the pole.
The blue line shows the WDC core from Antarctica. Clearly, some peaks are mirrored between red and blue, and some are not. The scientist-detective can now begin her work.
Location, location, location
There are two ways that sulphate can reach the ice core. The first one is if a volcano erupts in the vicinity, so that the ice core sees local pollution, carried directly by the ever-changing wind. The second way is if the sulphate reaches the stratosphere, where it stays up long enough (months to years) to spread to the polar regions. Here, also, location is important. A sulphate spike seen in both hemispheres is likely to have originated from a tropical volcano. One seen only in the north or south will have come from that hemisphere.
Let’s first look at the tropical eruptions. To identify them, I took both the NEEM and WDC cores. I converted both to a common grid (same time steps, by interpolating between data points) and subtracted the mean background so that both averaged around zero with positive peaks where a volcano jumped in. I now set all data points less than 5 ppb to zero. This gets rid of most of the variable background but leaves the volcanic signals in place. Finally, I multiplied the two curves with each other. Any peak seen in only one core will be multiplied by a zero in the other core, and disappear. Only peaks seen in both cores will give anything else then zero. The result is shown in the next plot.
The procedure left me with 10 tropical eruptions. However, one of these appears to be problematic. An eruption is seen in 1230 in Antarctica and in 1231/32 in Greenland. The curves overlap just enough that the overlap shows up as a tropical eruption. This is the only eruption which shows such an offset. It is possible that these are in fact two different eruptions, a southerly one in 1230 and a northerly one in 1231/32. Another possibility is that one of the two has a small error in the layer counting, and that in fact the two eruption signals coincide, and that the eruption was twice as large as it appears. An error of one or two years is not impossible.
To check, I went back to the climate records. No peculiar weather was noted around 1230. (All weather is peculiar, but that of 1230 was no more peculiar than normal.) 1232 was very dry and warm summer. But 1233 was a strange year. It started in March in England with thunder and heavy rain. The wetness lasted the entire summer, with so much rain that fish were swimming among the harvest. All of Western Europe was affected, with flooding in Paris, Cologne and Utrecht. It was called a year of misery. The winter of 1234 was extreme: even the Laguna in Venice froze solid, for the only time in 250 years. In the summer of 1234 people complained about an ‘unhealthy atmosphere’ and ‘unnatural weather’ (Roger of Wendover). Only in 1235 did the weather improve. Was this a volcanic winter? It has the hallmarks of one, and this would agree with a significant eruption around 1232. Perhaps the 1230 and 1232 eruption were in fact the same, large enough to affect the weather.
A recent paper attempted to differentiate between stratospheric and tropospheric eruptions through their isotopic content: Gautier et al 2019. Sulphur comes in two isotopes: a ‘light’ one (S-33) and a ‘heavy’ one (S-34). As in oxygen, the heavier isotope is more reluctant to travel. And the stratosphere is a long way. The result is that stratospheric sulphate has a bit more S-33 than usual. Using this, they find that the 1230 sulphate in the Antarctic ice was stratospheric. That makes a tropical eruption more likely. A volcanic winter (possibly seen in Europe) also fits better with a stratospheric eruption. It seems plausible that the two stratospheric eruptions were the same one: there was indeed a significant tropical eruption, around 1232.
The other 9 tropical eruptions occurred in 1110, 1172, (1192), 1258/59, 1287, 1331, 1345, 1453, 1459. The dates are accurate to 1-2 years. It is possible for a layer to be lost in counting, or to be double counted, and comparison with other information on eruption dates is necessary. The 1110 and 1453 eruptions are considerably stronger in the Greenland core and these may be a bit north of the equator. The 1459/60 eruptions is much stronger in the Antarctica core and may have been in the southernmost tropics. The 1192 eruption is strong in Greenland but is barely above our lower limit in Antarctica: it needs confirmation.
The number of strong tropical eruptions amount to about once per century, with a few more weaker eruptions. Strong eruptions probably should be interpreted as VEI 6 and (rarely) 7.
We can also extract the southern eruptions. I have done this by keeping only the peaks in the Antarctic plots which are well above the background, and using the plot of tropical eruptions to mask out the tropical events. This leaves me with 6 clear eruptions and 2 uncertain ones.
The southern eruptions are in 918, 961, 1040, (1230), 1241, 1270, 1277, (1286). The last one is uncertain as it is also classified as a tropical eruption. It is seen in Antarctica one year before Greenland but it strongly overlaps and is most likely (but not certainly) the same eruption. The 1230 eruption was discussed above: there is a possibility that this also is a single tropical eruption.
The southerly eruptions therefore occur at a similar frequency to tropical eruptions, about once per century. Interestingly, they were mostly in the 13th century, but both the suspect eruptions are in this cluster and this may reduce the significance of the clustering.
The southern eruptions are for the most part weaker affairs than the tropical ones. Only the 1277 eruption shows a strong sulphate peak. Most of these eruptions will be from Chile or New Zealand, regions which can do large eruptions but have too few volcanoes to do this regularly. (Although, one should be aware that Taupo should never ever be taken for granted. Its eruptions are infrequent but ewe-some.)
We can do the same thing for the north, and this gives the plot below. Now there are 17 eruptions visible which are exclusive to the northern hemisphere.
The northern eruptions occur at 939, 1003, 1011, 1021, 1029, 1112, 1138, 1183, 1201, 1210, 1212, 1215, 1220, 1233, 1330, 1341, 1390, 1470, 1478, 1480, and 1496. The 939 and 1183 events are strong; some of the weaker eruptions cluster in time and may be related or may be part of a single longer-lasting eruption.
It is notable that the north shows much more volcanic activity than the south. This is understandable, since outside of the tropics, there is more volcanic activity in the northern hemisphere (Iceland and the northern Pacific ring) than in the southern hemisphere (New Zealand, Chile). Also, the volcanoes are at higher latitudes and therefore closer to the icecap. Iceland is a prime contender for many of the eruptions; others will be from Alaska, Kamchatka and below, and perhaps some from the Azores.
to be continued