GeoLog

isotopes

Imaggeo on Mondays: Electron cloud gone wrong

Imaggeo on Mondays: Electron cloud gone wrong

Deciphering the past history of rocks and what they might reveal about the Earth’s future is a key part of geology, and tools such as Ion Probes can be used by Earth Scientists to extract valuable information about a rock’s past.

Today’s Imaggeo on Monday’s image was acquired by Sarah Glynn, a researcher at the University of the Witwatersrand, in South Africa, who was analysing a potential calcite reference material (a mineral homogeneous enough to be considered a benchmark for future analyses on others minerals of the same composition) using Secondary Ion Mass Spectrometry. By assessing its suitability as a reference material for oxygen isotope measurements in calcite (a carbonate mineral), one can more accurately determine the past temperature conditions in which certain rocks formed by looking at the differences in the relative abundances of the isotopes of oxygen (16O & 18O).

When doing analyses with an Ion Probe, it is important to coat the sample (in this case a round epoxy disc, in which some calcite fragments are embedded) with a thin layer of gold to make the surface conductive. As an additional measure to compensate for charge building up, a flood of electrons is directed to the surface of the sample.Occasionally however, if the sample is not sufficiently coated, or the coating itself is damaged – perhaps scratched – a kind of ‘lightning strike’ can occur, which can be quite damaging to both the sample and instrument alike, but can also be strangely beautiful when they create striking fractal patterns, such as was the case in this photograph.

Imaggeo is the EGU’s online open access geosciences image repository. All geoscientists (and others) can submit their photographs and videos to this repository and, since it is open access, these images can be used for free by scientists for their presentations or publications, by educators and the general public, and some images can even be used freely for commercial purposes. Photographers also retain full rights of use, as Imaggeo images are licensed and distributed by the EGU under a Creative Commons licence. Submit your photos at http://imaggeo.egu.eu/upload/.

GeoTalk: What can you learn from a human hair?

Jim Ehleringer is Director of the Stable Isotope Ratio Facility at the University of Utah in Salt Lake City in the US. In the past few years he has been developing a method for deducing people’s diets and travel history from a sample of their hair. Here, he talks to the EGU press office about his research.

Thanks for agreeing to chat, Jim. Can you tell me a bit about how your project started?

This project began by looking at animals and we were interested in animal diet and migration. My colleague Thure Cerling was studying game animals in Africa and we were interested in wandering elephants. The elephants are supposed to spend their time in the park, which has C3 vegetation, but often they leave the park at night and visit the farmlands, which have C4 crops. We could see differences between what they were supposed to be eating and what they were actually eating [by looking at their hair]!

And now you’ve progressed to humans?

It wasn’t long before we started thinking about changes in human diet and in human movements. The first examples were me and my colleagues. As we travelled to new places we would shave and we would record the food we ate and we would look to see if there was a change in the isotopes within our beards as we travelled to China, to Mongolia, to South America, and back to the US.

Could you explain a bit about how your method works?

We use scalp hair as a linear recorder of an individual’s travels and diet. The carbon, nitrogen, sulphur and oxygen in the hair is only produced in the hair follicle and once it is extruded there is no further change. We have now verified a stable isotope model that says something about the regions of origin and travel histories of humans and we’re applying this to previously unidentified murder victims.

So how do you distinguish between the effects of travel and diet?

We can use carbon and sulphur to tell us something about food sources; oxygen tells us something about drinking water. We know that there are variations in the isotopic composition of water across the continents. We then link this with a mathematical model and use the term ‘isoscape’ to describe the maps that we make.

The four regions visited by ‘Saltair Sally’ prior to her death. The blue dot marks Salt Lake City. Oxygen isotopes in her hair were used to track her movements.

The four regions visited by ‘Saltair Sally’ prior to her death. The blue dot marks Salt Lake City. Oxygen isotopes in her hair were used to track her movements. (Credit: Jim Ehleringer)

How big are the variations in oxygen isotope ratios across a country?

It depends on the size of the country. There’s not much variation across the Netherlands, but if I were to take a map of Europe I would find that Vienna is quite a bit different from Paris, which is quite a bit different from London. There’s probably a change of 16 parts per thousand across Europe and our resolution is about 0.2 parts per thousand. So we get maybe eighty resolution units.

Does the type of hair matter?

It all depends on the length of your hair. Hair grows at the same rate – about one centimetre per month – but men tend to keep their hair shorter and women tend to keep their hair longer. The resolution is a function of mass. Typically what we do is to take ten to twelve hairs and bundle them. Then we can get a time resolution of about seven to ten days.

And you’ve applied these techniques to murder cases?

Yes, we’ve now had enough time to develop and test the model and apply it and see some results. The first case was of a murder victim known as ‘Saltair Sally’. Law enforcement discovered hair, bones, and a necklace in a shallow grave near the Great Salt Lake. [From our analysis] we found that she was a resident of nearby Salt Lake City, but that she had travelled frequently. We used our model to predict where she had travelled and found that she had visited four regions. Approximately a year before her death she visited an area which is consistent with Portland, Seattle and the northwest. In fact, she had visited her mother in Seattle and it was by identifying these potential places that law enforcement was able to make the link. This [method] is now being applied on a routine basis in the US.

Jim presented his work at the EGU General Assembly 2014.

By Tim Middleton, University of Oxford

 Reference:

Ehleringer et al., 2014: Evaluating O, C, and N isotopes in human hair as a forensic tool to reconstruct travel. Geophysical Research Abstracts, Vol. 16, EGU2014-1436-1.

GeoTalk: Connecting geoscience and archaeology with Sophie Gangl

This week in GeoTalk, we’re talking to Sophie Gangl, a masters student from the University of Natural Resources and Life Sciences (BOKU), Vienna. Sophie tells us about the intersecting fields of geoscience and archaeology, and shares the benefits of presenting postgraduate projects at an international conference.

Hi Sophie, can you tell us a little about yourself and what you’ve been investigating as part of your MSc course?

I have always been very interested in chemistry, so I decided to do my masters project in the Department of Chemistry at BOKU. Luckily, my current advisor (Thomas Prohaska) had an open position in his working group, the Vienna Isotope Research Investigation and Survey (VIRIS). I was entrusted with a very interesting project: using strontium (Sr) isotope ratios to reconstruct migration and mobility patterns. The aim of the project is to find out if the Early Medieval population of Thunau am Kamp in Lower Austria was local, or if the inhabitants had moved to this site during the course of their lifetimes.

Say hello to Sophie! (Credit: Sophie Gangl)

Say hello to Sophie! (Credit: Sophie Gangl)

How can we use geoscientific methods to answer questions in other subjects, like archaeology?

In nature, the abundance of the isotope 87Sr varies because 87Rb decays to 87Sr over geological time periods. But the absolute amount of the other natural stable Sr isotopes (84Sr, 86Sr and 88Sr) remains constant. This is why 87Sr/86Sr ratios are characteristic for a specific region – it all depends on the age of the geology and the initial amount on 87Rb.

Humans and animals primarily take up strontium through drinking water and their diet, and since strontium has similar chemical properties to calcium, it’s mainly stored in teeth and bones. Tooth enamel is formed during early childhood and doesn’t alter much in later life. Therefore, the enamel-enclosed strontium has the same isotopic composition as the environment an individual lived in during their early years (provided that their food came from the close proximity).

Stontium isotope ratios can be measured using mass spectrometry. For the project I’m working on, Sr isotope ratios were measured using the mouthful that is multi collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS)! To determine residential mobility, 87Sr/86Sr ratios in enamel samples are compared to the local strontium signature, which is specific to that area. If they match, it indicates that the individual was a local (autochthonous).

Studying strontium isotopes in the lab. (Credit: Sophie Gangl)

Studying strontium isotopes in the lab. (Credit: Sophie Gangl)

You were awarded the Outstanding Student Poster Award for your presentation at the EGU General Assembly in 2013, what inspired you to present your work at an international conference?

My advisor encouraged me to attend the EGU General Assembly 2013 with a poster presentation. At first I was overwhelmed by the idea, as I had never attended a conference before. But it turned out to be a great experience and I am grateful for the possibility to participate. I had the opportunity to talk to other scientists and discuss both my research and theirs. Furthermore, I joined several oral and poster sessions where I learned more about research fields outside my own. Participating in an international conference has reaffirmed my plan to embark upon an academic career and it has definitely expanded my horizons.

The award-winning poster presented at EGU 2013: “

Connecting geoscience and archaeology: “Autochthony in the early medieval settlement of Thunau/Kamp, Austria? A question explored by 87Sr/86Sr isotope ratios using MC-ICPMS” (see here for a larger image). (Credit: Gangl, S.; Irrgeher, J.; Teschler-Nicola, M.; Prohaska, T.)

Do you have any tips for fellow postgrads hoping to present projects at a national or international conference?

My number one tip is: just do it! I think a lot of students have concerns about whether their project is of suitable importance or scope to be presented. But everybody can attend, even if their project is just a small one. The feeling you have when other scientists are getting interested in your work is definitely worth the effort.

Finally, with a few months of your masters course to go, what’s next?

After I have finished my studies, I will continue with a PhD. I would like to do research in the field of ultratrace analysis, maybe in forensics, marine or polar research. I hope that I will find an interesting PhD position and am looking forward to starting this exciting and challenging phase of my life.

If you would like to suggest a scientist for a Geotalk interview, please contact Sara Mynott.

Geosciences Column: Dating a bivalve

Just as the rings on a tree can be used to determine its age, the bands on a bivalve’s shell can tell us the how long it’s been around for. Warm, food-filled waters lead to greater growth in the summer and low plankton abundance (the principle food source for filter-feeding molluscs) leads to limited growth during the winter months – hence the banding. But pinning down the age of a bivalve may not be a simple matter of counting bands as sudden changes in temperature and food availability throughout the year could cause multiple bands form within a single year.

This is where a little isotope geochemisty comes in. Oxygen naturally occurs as three sable isotopes: 16O, 17O and 18O and the ratio of these isotopes, particularly that of 18O/16O (known as δ 18O) reflects the temperature of the environment. Since 18O is heavier than 16O (by two neutrons), more energy is required to vaporise water containing 18O than 16O. So in the summer, when there’s more energy for evaporation, more water made up of 18O is evaporated and this leaves less 18O in the water (low δ 18O). The converse is true during winter, when there is little energy available for vaporisation and more water containing 16O is evaporated. In short, high δ 18O indicates low temperatures, and low δ 18O indicates high temperatures.

Since bivalves obtain the materials needed for their shells from the surrounding water, their oxygen isotope ratio reflects that of the water column. The seasonal cycling of δ 18O makes this a great way of verifying the bivalve’s age and checking whether we can trust the information gained from band-counting. This is what a team of geochemists led by Joana Cardoso set out to find: by counting the lines on a shell, can you get a reliable estimate of age?

This is a razor clam (Ensis directus).The white arrows point out the annual growth lines. (Credit: Cardoso et al., 2013)

This is a razor clam (Ensis directus). The white arrows point out the annual growth lines (click for larger). (Credit: Cardoso et al., 2013)

To start the investigation, you need to picture a line along the shell from the edge to the umbo (the most primitive part of the shell, where the two valves join). You can then take sections along this line and analyse the isotope ratios within them to see how δ 18O varies over time. The downside: as you approach the younger end of the shell, there is less and less carbonate available for sampling, to the point where there’s not enough material to do isotope analysis. Not to worry, you can combat this by pooling the carbonate sampled from neighbouring sections. Okay, so your data has slightly less resolution for the early stages of bivalve growth, but you can sample all parts of the shell – problem solved!

Cardoso’s results, published in Biogeosciences, found that δ 18O is an especially good measure of age and not only that, it matches the results you get for growth lines on both the inside and the outside of the shell!

Measured and predicted values for δ 18O in the shell. Carbonate is deposited in the summer and as the temperature rises, the ratio of 18O to 16O decreases, hence the troughs in measured 18O in the graph (white circles). (Credit: Cardoso et al., 2013)

Measured and predicted values for delta (δ) oxygen-18 in the shell. Carbonate is deposited in the summer and as the temperature rises, the ratio of oxygen-18 to oxygen-16 decreases, hence the troughs in measured delta oxygen-18 in the graph (white circles). (Credit: Cardoso et al., 2013)

The annual growth lines appear at the start of each growing season, in early summer. These coincide with the drop in δ 18O (remember this corresponds to a temperature rise). Carbonate that’s deposited from June to September (when there is most growth) provides the most detailed δ 18O data. The absence of data during the winter is because there is little or no growth – if no carbonate is deposited then there are no isotopes to analyse!

So, if you’re looking to age bivalves – count the rings – it’s still a sturdy method, and if you’re looking for more detailed data about both age and environmental conditions, oxygen isotope analysis is what you need.

By Sara Mynott, EGU Communications Officer

Reference:

Cardoso, J. F. M. F., Nieuwland, G., Witbaard, R., van der Veer, H. W., and Machado, J. P.: Growth increment periodicity in the shell of the razor clam Ensis directus using stable isotopes as a method to validate age, Biogeosciences, 10, 2013.