Breaking the Wall of Timing Our History. How Trace Elements Analysis Will Help to Understand the Past of Our Planet
By Detlef Günther
On November 9th I watched TV; I guess football.
Good morning ladies and gentlemen, it is a great pleasure for me to be here today. I can tell you exactly what I did twenty years ago on the ninth of November behind the wall. There is some truth in the sentence: never predict the future, because I can ensure you- even in my wildest dreams- I would not have dreamed to be here with you today celebrating the twentieth anniversary of the falling wall. The falling wall opened up some opportunities for me to develop, and some of them I would like to share today with you: about trace elements and the understanding of the past. For the next fifteen minutes, I would like to give you some snapshots as introduction. I will focus on three applications where we believe actually that some breakthroughs can be achieved, which is related to some ore formation, climate studies, and of course fingerprinting of some old gold objects.
Now analytical chemistry and the field I am doing research in is related to the detection of elements to its traces. This cannot be a single research area. It is embedded in all the other research areas. We have the smallest link, actually, in terms of elements and trace elements, because we all know that those elements play a major role in synthesising new materials and discovering processes from the earth.
Our main interest is related to instrumentation and method developments, which we can apply to reveal stories from the past. Then, of course, we generate some new understanding, which is forcing us to develop further techniques. This circle gets smaller and smaller. Our challenge today is based on physical techniques, which allow us to look from the micro-scale further down into the nano-scale. The challenge for us is actually that the sample mass we can analyse is getting smaller and smaller, and we are related somewhere close to ten to minus fifteen gram. That is related to the spatial resolution, because we don’t want to look into micrometre any more; we would like to look into nanometre.
The technique we have been working on over the last couple of years, which we pushed in the direction where we can gain more information, is shown schematically here. We use an intense laser source to ablate tiny amounts of material. You can follow that here, down here, and then we transport material; we excite it under hot temperatures: 7000K. These tiny particles tell us some stories. But I don’t want to take you back to a chemistry lesson, but if you look into this, you all know this is our periodic table, and our future break will be that all the elements should shine in yellow, because that indicates the detection limits that we would need to reveal more stories.
So far we have driven it in the direction that we can reconstruct some environmental pollution and ore formation processes. We can follow the production of new materials, and we can look into some archives from the past. That is what we have been interested in. I could extend this list to further topics, however I want to concentrate on three distinct little stories.
Ore exploration from the past: lets make a little scenario, and lets assume nature would not do us a favour in enriching some of the elements we are most interested in. Lets assume we would have to dig out our copper and gold and all the precious metals from normal soil. That would cause us some trouble, because our landscape would look like this.
I can ensure that ETH and the University of Zürich would not be influenced, because we survive down here. However, nature has done us a favour in doing it, and we want to study how much do we get out of the earth for industrial purposes.
What you see here is copper ore deposit in Argentina. Of course, it is very important to study how much of the ore is located in here. This is pretty visible. You can see that gold is enriched from magmatic fluid, which was pressed through the quartz. However, nature also left us some remains, micro-inclusions, in these quartz materials, which we can study in detail if we can drill them out properly and if we can analyse these little inclusions. They contain the original composition of the magmatic fluid, which formed this ore body.
Now we developed these kinds of techniques. You can see here these signals, which can be generated in tiny amounts, which is parts per billion of gold we have to detect to make a mass balance calculation. These little holes indicate that we drill these inclusions out successfully. What we found is that a twenty-micron inclusion, which consists of picolitre, contains the entire information of the mass balance of an ore body. We could figure out that ten thousand kilogram of copper are related to one kilogram of gold. The two locations we analysed were from Indonesia and Australia. They all have the same elemental ratio in the magmatic fluid.
Coming to the second story, climate reconstruction. If you look at this schematic, everything that goes into the ocean floor is compacted there and remains there. It is the perfect time scale; because what you see is compacted over time, everything gets deposited here. In case of rivers, they transport a lot of sediments; if there is a high water flow or low sediment flows and indicate if there is a dry or wet period. If we can analyse from the ocean floor some of these sediments with high spatial resolution, we might trace back climate information over thousands of years.
Now, our studies, being in the Caribbean, and we have been participating with a colleague Prof. Haug in the analysis of samples from an Ocean Drilling Program. So, we got some ocean sediments, which we tried to analyse. That is the process you can see: drilling out the sediments. That is finally the sample we end up with. Our aim was to use these kinds of sediments to look into the Mayan society development. We particularly had some indication that they suffered from not having rain consistently, and they had a very highly developed water system.
The sediment you have seen here have been analysed for some tracers and titanium is one of these elements: low- there is a dry season, and wet is indicated by high titanium concentrations. These being the resolution, which has been roughly ten years ago possible to achieve. We wanted to look a little bit deeper into these sediments, and used another technique: micro florescence, which allows us actually to gain forty-micron resolution. So, it means even to a month’s record we could come up with over the last 2000 years.
Of course, you don’t get these instrumentations right from a shelf. Parts of them have been developed in our group, and we started to analyse. I just want to summarise it briefly. That is what I showed you from ten years ago: we can reveal the structure down to micrometre scale and even further. What we found is exactly the collapse of the Mayan society, which you see here the three stages; we can find some negative titanium concentration, which is an indicator that this society suffered from periods where they had not enough rain. Of course, if you come with an analytical result based on trace element that is not direct evidence, then we have to accept that it is only one possibility. But I can show you there are some other possibilities involved in the collapse of the Mayans.
In my last example I would like to show you, and that can be not only used for provenance studies, but I can show you what we are doing with modern materials in the same way as with old materials. We tried to get fingerprints of our samples to determine the origin, to determine if it is real object or a fake, or old or new. In one of the studies, we will look into the Inka gold objects. You can see here the map of Peru, and these are all little ore mines where gold has been taken out of the earth and then processed into objects, which you can see here. Of course, the question is: where are these materials coming from, and are these objects all belonging to this area? That is part of our studies.
I will just show you one case study from an Inca gold collection from the Ebnöter collection in Switzerland. If you analyze this ornament consisting of three pieces, you can see that the first part is a ring; it looks slightly different. It is also in the major element composition- slightly different- however, if we studied the traces and looked into some indications, which elements have been used in the production of these materials and we were able to distinguish them very well. That is what we would like to do in the future: to really set up an origin database; can we trace back the materials to its original source?
That brings me towards the future: where are we going to? I told you that we were developing techniques according to the needs towards smaller and smaller materials. However, if you have to place or put your samples always under vacuum or in certain cells, then you put damage on these samples. We don’t get it, and therefore we have been looking for almost five to ten years into a possibility where we can do this sampling under atmosphere without using any prerequisites.
We could not envision that one day it would be possible and that we have just something like a very little vacuum cleaner, which allows to suck in this material in form of nanoparticles. We analyse it, and we get this information within a very short period of time- something like 30 seconds. That will help us, and I hope will help us to develop further techniques for trace elements in the future. One of the archives already waiting for it: you can see here it is a stalagmite. We don’t have to cut them in the future anymore; we can really analyse them as one piece as they come- sometimes one and a half or two metres long. That will be work in the future.
Let me summarise the presentation a little bit in the way of what we are looking for. In the future, we hope that we get a little bit more sensitivity and improved limits of detection. We would like to have better high spatial resolution and understanding, generating understanding of the role of trace elements. We are not a single research area and single research groups; we are working on a highly interdisciplinary field, and I simply think that there are some walls in between that we have to break down to collaborate even further. Of course, the best way, ladies and gentlemen, is never build a wall.
If I am allowed to come here, I would like to thank my research group, sharing my enthusiasm for this field, and of course as research also needs a lot of sponsors ETH and particularly thanks to my colleagues at ETH. We are already in the approach of setting up a very interdisciplinary research project. With that I would like to thank you very much for your attention.