Both papers I discussed before go no further than 20.000 years, and there’s more to life. In the first outreach blog post I already describe the use of the fossil record; if you only know what things are like in the last 20.000 years your knowledge is too narrow. Climate depends on boundary conditions, and there’s more possible variation in these than there is in the last glacial-interglacial cycle. And now the time has come to go have a look at the previous one! But before I get to disclosing my monsoon reconstruction, and its implications, I have to elaborate on some background again. And not only for that reason; the article with the monsoon reconstruction in it is not yet published... it’s not even submitted. I regret that; I’m impatient! But I have a very overworked co-author, and it just takes long.
So what did I do before I got underway with the monsoon? I ended up on the side track of proxy evaluation. Proxies being the things we measure to indirectly find out what we want to know. We measure all kinds of things on the sediments and the microfossils we sieve out of them, but someone somewhere must have found out how these sediments relate to the environmental parameters we want to reconstruct. And generally, it turns out to be quite complicated. So there’s still lots of work being done on fine-tuning the proxies we have, and finding new ones. And I added to that body of work, by taking two ways of reconstructing sea water temperature, performing them on the same sediments, and seeing if they would give the same answers. And if not, try to find out why.
A random picture of a random machine used in proxy studies, in this case a spectrographer of Cambridge university
Something like “temperature” seems so straightforward. But is it? If someone writes “the temperature 128.000 years ago was 25°C”; what do they mean? Global annual average? At one meter above sea level, as modern temperatures are given, or somewhere else? And if they are more specific, and say things you are more likely to read in literature, such as “sea surface temperature in the South China Sea 135.000 years ago was 17°C”, it still is a bit ambiguous. I wonder if a surface technically can have a temperature. And would it be, again, an annual average?
Generally speaking, if someone writes about sea water temperatures they would mean the annual average temperature of the upper part (typically 50m) of the water column. If it is something else it would be specified. But if we read the information as documented in, for instance, the shell of a foraminifer, would that information indeed give just that? Not very likely.
A lady about to measure temperature over a depth interval directly
All reconstructions have their own peculiarities. And they tend to not give you something as regular as the annual average over a spatial interval of nicely round figures. And that can be a problem, but it can also be an advantage. I already explained in the monsoon blogpost that we for instance use the different seasonal cycles of the various foraminifera species for monsoon reconstruction. If one lives only in summer and another one all the time you can reconstruct the difference between summer and annual average from measuring the species separately. A similar thing can be done with ocean stratification. Measure one species that lives in the upper 50m and one that lives 100m deep. If they contain the same signal, the whole water column down to 100m must have been mixed. And it may not be evident to everybody why water stratification matters, but I’ll get to that in some later blog post.
So what was it I did for that third article? I compared two methods. One is the already described method of Mg/Ca on planktonic foraminifera. The other one is new to this blog: it is alkenone unsaturation. Alkenones are organic molecules that a specific type of plankton, viz. coccolithophorids, seem to use to keep the contents of their cells at the right viscosity, or something like that. Coccolithophores, by the way, are algae with a calcite skeleton, and they are small but abundant; they produce about half the oxygen in our atmosphere (so maybe you should say “thank you, coccolithophorids!” next time you breathe in), they are what school board chalk is made of, and what the white cliffs of Dover are made of too. And their alkenones have 37 C atoms, and some of the bonds between them are double and some are not, and just how many of them are double depends on temperature. And the good thing is, that these alkenones are amazingly robust, and you can just leave them lying around on the sea floor for millions of years, and then still measure their (unaltered) number of double bonds.
These cliffs, on the Isle of Wight, are the same stuff as these near Dover.
So what did we expect to see, using both these methods? To be honest, we should not have had any noteworthy difference. The coccolithophorids (or cocco’s for short) responsible for the alkenones have, at least in modern days, a quite similar distribution in both time and space to our foraminifera. So they should be recording the same thing! And if they don’t, it might be due to the plankton behaving differently then compared to what they do now, and we might be able to find out how. That would then be excellent information for the interpretation of all records of these kinds.
So what did we see? Remember we were reconstructing the previous interglacial, and quite some time around it. So we expected to see the low temperatures of the end of that ice age, then the warming up to the interglacial, and then the stepwise descent into the next ice age. And both temperature records thus constructed indeed showed that. But not in the same way! The alkenones showed much higher temperatures over the whole period, and the glacial-interglacial differences were much smaller.
Our Mg/Ca temperature record showed us temperatures in the range that we expected. Glacial temperatures like these in the last glacial, and then the rise into the interglacial, which at its peak was a bit warmer than the Holocene, and then intermediate temperatures. So if the little critters had not been changing their preferences concerning when and where they live in the last 150.000 years, the last interglacial was what we largely already knew from literature.
This is what human skulls looked like at the time. Picture taken from: www.talkorigins.org/faqs/homs/arago.jpg
Then the alkenones. High temperatures all the way! And to a certain extent that was expected; nobody knows why but they always give you higher temperatures than you can really explain. They magically seem to give annual average temperatures, even though they do not live in representative parts of the year! In our own core this offset is also seen in the last 20.000 years. But the difference in the previous interglacial is much bigger; 3.5°C on average. That’s much!
We first tried to explain this by assuming the alkenones were right, and the forams had just had a different preference in these times, but we didn’t manage; you would see a change in environment also in their isotopic composition, and there was nothing there.
Then we had to try the other way around. The forams were right and the cocco’s had changed! But that went wrong too. They could not reasonably have moved up in the water column, for they already live so shallow. It would be difficult to imagine them living in the warm intermonsoon season, as the whole reason it’s so warm is there is no water mixing, and that means no food.
We also considered disruptive mechanisms. Maybe the forams had been dissolving! Maybe the alkenones, which tend to be found in much finer sediments than the bulky forams, were just flushed in form elsewhere by some passing current. Maybe squirmy wormy things had been overturning the sediment, and misplaced interglacial alkenones in layers of glacial sediments! But there always was some argument against.
Arty picture of a brittle star; one of these creatures that is likely to disturb our beloved sediments
So in the end we had to give up. And since we can explain the foram-based record, but not the cocco-based record, we tend to believe the forams. The fact that cocco’s already had a shady image only strengthens our suspicions. But what would have been great would have been if we would have detected some change in environmental preference of one of our small creatures in the difference in temperature signal, and we haven’t. Too bad! Sometimes it just works out that way. At least now I have a better idea of how to evaluate temperature records of these kinds...
This article, by the way, was a difficult one to write. For most of the blog-readers the other names on the author list probably are just names, but of course there’s people behind them. Harry Elderfield happens to be a British Mg/Ca god, while Edouard Bard is a French alkenones god. One of them is a soft spoken, amiable man, while the other one is a genius who cannot stoop to the level of mortals, and often seems to die of impatience and indignation when confronted with the inferior utterings of those. And I was just a shy little PhD student. The article clearly sings the praise of forams, so I had to be very very careful in my wordings to get this article past the cocco man. And that would surely be the soft spoken, amiably man? Unfortunately not...
Edouard gave me a hard time on this one. If I had sent around a draft version, and I would see I had a response from him, I would always first breathe out slowly, gather all my courage, and then open the mail. And then let the shouting poor over me. But he was never unjust, and by whipping me so hard he managed to get everything out that I had in me. And it paid off; the article did get attention. It was top download of the journal for a while. And even though it did not deliver what we had hoped, I am actually above average proud of this one!