Wednesday, 31 October 2012

How Science Works: seeing through rock

Amendment to the last blog: Marcello Ruta is now at the University of Lincoln.

Last week I was back in Leicester sorting out the samples that we collected in the field. It was not the most exciting week, mostly consisting of re-boxing the 250 rock samples and answering lots of emails. At least I was away from the wet weather for a while… but not for long, as now I am back in Scotland in the field! So a short blog this time about what we do with all the rock collected.

The most simple method you can use to study sedimentary rock samples is to ‘cut and polish’ the rock. What this means is that you cut the sample with a rock saw and then polish up the surface you have cut, giving a shiny, polished rock surface. The finer the rock polish you use the more detail you can see. 

How to see through rock?
The rock below is one we samples in August 2012, to give an example of this method. 

Step 1. Take a sample of the rock. Here you can see the rock in the field and in the lab. It is a grey/green sediment that is well cemented and looks pretty uniform. This type of rock in the Ballagan Formation is called a cementstone.

Step 2. Cut the rock in half using a rock saw. Don’t do this at home! Here is the outside of the rock once it has been cut in half, again not much to see.

Step 3. Polish the cut half. Here you can see the polishing wheels we use in Leicester, along with different size grades of polishing powder. The powder is a synthetic carbon-silicate called carborundum, which is very hard (hardness of 9 on Moh’s scale). It takes at around an hour to get a good polish on a rock sample.

Step 4. Photograph polished rock surface. This is best done on a scanner at high resolution. Now you can see the rock interior.
The result: In the field it is hard to tell what it is made of as it all looks very much the same, and quite boring. But when we have cut into it, you can see a lot of detail. Some of the features revealed in this sample are 1) a grey/green colour change from base to top, the boundary of which is burrowed or rooted (see the close up picture to the right); and 2) a muddy rock at the bottom of the sample, which may contain microfossils. There are also some as yet unidentified crystal filled voids…. And none of this was visible before the cutting and polishing, so a great result! 

What is the next step?
I will be using the cut and polished rock surfaces of complex specimens as a guide to select which parts of the rock are most interesting and will merit further analysis. The next step is to move onto making thin sections (thin slices) of the rock, which give another, more in depth level of detail. You can then study the thin sections using various microscopes, the most powerful of which is a Scanning Electron Microscope. We have some preliminary thin sections of our samples, which have come out really well. I will blog more about the thin sections and how we analyse them in future posts.

Until next time

Monday, 29 October 2012

Week 3 in Scotland – fossils, seaweed and mud

Week 3 of my Scottish Borders fieldwork was the team fieldwork week. We all got together to do some extensive sampling, look for fossils, examine a few different sites and discuss the science together. 

Who was there from team TWeed? And what were they doing?

University of Leicester – me and Sarah Davies: looking for ostracods (just me on that one!), sedimentary logging, describing sedimentary structures in the field and collecting samples for analysis back in the lab.
ambridge team (Cambridge Museum of Zoology and the Sedgwick Museum, Cambridge) – Tim Smithson (vertebrate palaeontologist), Rob Clack (amateur vertebrate palaeontologist), Sarah Finney (vertebrate curator), Ket Smithson (new PhD student on fossil fish). Project supporter Marcello Ruta from the University of Bristol (tetrapod specialist) also visited. The team were examining different sites and fossil collecting.

British Geological Survey – Tim Kearsey: sedimentary logging, sampling, palaeosols expert, integration of BGS maps and records, digital recording using the GPS.

University of Southampton – John Marshall, palynologist and expert in Devonian/Carboniferous climate change: sampling with the Leicester and BGS team, to examine the microfossil (palynomorph and charcoal) content of the samples.

National Museum of Scotland – Palaeozoic fossil experts and curators Andrew Ross, Stig Walsh, Sarah Stewart and Simon Howard: fossil collecting and identification.
It was a very busy week: the Leicester, BGS and Southampton team logged another 50 metres of section, and took around 250 rock samples. To produce a high resolution data set (i.e. lots of data points throughout the Ballagan Formation) we were taking samples approximately every metre throughout the section. For each one we recorded the sample position on the sedimentary log, the GPS co-ordinates of the sample and a photograph of where the sample was taken.

Left: Tim Kearsey (BGS) and Sarah Finney (Sedgwick Museum). Tim is holding a precise GPS locator, so we can pin-point where the samples come from and digitally record that information. Right: Tim Kearsey and John Marshall looking very enthusiastic despite the rain.

What will we use the samples for? We will study the following:
  • Sedimentology of the rocks, the types of sediment they are composed of. This can tell us about how the sediment formed, for example comparing the composition of different sandstone bodies can tell us if they all came from the same source.
  • Microfossil content of the sediments, revealed by processing and sieving the samples (see blog 3). We will look for vertebrate, invertebrate and plant microfossils. The type and number of microfossils can tell us what the environment was like.
  • Geochemistry of the rocks, which can tell us about the source of organic matter in the rocks and the environment in which the rocks formed.
The vertebrate palaeontologists in the team had a busy week, collecting bulk samples from different parts of the section exposed on the foreshore, to be worked on back in Cambridge over the next few months. Already scales of a group of fishes not previously found in the Early Carboniferous of the Scottish Borders have been identified. Three members of the team, Ket, Sarah and Marcello, had not been to Burnmouth before and Tim showed them the different beds where fossils had been collected in the past. Tim also took Ket, who will be studying for a PhD on some of the fishes being collected in the Borders, to the inland sites. These are all in the banks and beds of the local rivers. Unfortunately, because of all the recent rain, these rivers were in flood and collecting was impossible. They'll have to be patient and hope for more clement weather in the spring.

And on Tuesday the weather was quite dreadful, with heavy rain all day. We all got quite soggy, so at high tide we all trudged back to the cottage for a cup of tea to dry out our socks!

Mid-afternoon on ‘dreadful Tuesday’, we’re having a cup of tea and discussions in the cottage, while drying off our socks!

In the next blog will be about cutting and polishing rock samples to reveal their secrets.

Until next time