Monday, 10 December 2012

Core sampling in Windermere - meccano, cheese wires and liquid nitrogen

On Wednesday Carol explained why the work on Windermere is so important. Read all about that here.

But I wanted to know more about the nitty gritty and the hard graft. How do you collect sediment from the middle of a lake, how do you sample it and what's liquid nitrogen got to do with it? Plus why has it taken until now to finish the sampling?! Well here's Carol and her camera to illustrate.....

The journey begins in April

To collect core from the bed of a lake you first have to get out on it. For this you need a coring platform, and this can be a bit like building a giant meccano project - and no we didn’t have any parts left over!!  
The fully built Uwitec coring platform on a serene Windermere
We used a 'piston coring' system with a 'seabed re-entry cone'. What this means is that as each 2m section was pulled out on deck we could guide the next core barrel down the guide wires and through the cone into the exact same hole. 
Deploying the re-entry cone prior to coring
The cores were collected inside plastic liners that were immediately “curated” on deck ..... assigned a unique identifier that included the Latitude and Longitude of the core, the type of coring (piston), the core number and what section number that 2m liner represented.

Daniel Niederietter (Uwitec) and Helen Miller (BUFI student) pose beside a 2m core section in its plastic liner

Me labelling the recovered core section

But what happens to the plastic liners full of mud?!

We transported all the cores back to Edinburgh, and in October Helen Miller, a BUFI PhD student, and myself got together and spent a week splitting and logging the cores at our Marine Operations building in Loanhead. We started off by cutting the plastic liner and end cap with a router and hacksaw, and then using a cheesewire to run down the core.

Me splitting the core using a cheesewire

This divides the core into a working half and an archive half. The working half is cleaned up and then described in terms of grain size, colour, any organic material in it, the type of boundaries between different sediment units such as erosional or graduated, the size and frequency of glacial varves.....and anything else that Helen and I spot!

Helen Miller cleaning off the core surface prior to logging

The archive halves are left pristine as a legacy record for other scientists to use. Once described, the cores were then photographed for a visual record, both in the splitting workshop and later using a 32 megapixel camera for detailed images in our photographic suite.

Now for the science bit!

We recovered one core that appeared to show, condensed into just a 10m section, most of the sedimentary processes and units observed in all the others cores! We decided to concentrate on this core to start with, and so the working halves were transported to Southampton to the National Oceanographic Centre (NOCS). It was here in mid-November that a group of scientists with different specialties met to discuss how we would start sampling this unique core.

The first step was to take a U-channel sample from the centre of the core. This sampled pristine sediment that was hopefully least affected by any coring disturbance or contamination due to contact with the air or water at the edges of the core.

Overview of the U-channel (labelled plastic tube) and slabbing instrument in one of the sections

This is right now in the process of being run through an ITrax system to determine the chemical, density and optical composition of the core. It will then be run through a paleomagnetometer to determine paleointensity and secular variation as recorded by the magnetic minerals within the sediments. If you're interested in this we'll post more as it become available.

The cores were then “slabbed” – extracting a 2cm deep and 1cm wide sample for detailed analysis of any cyclical laminations and glacial varves. The slabs are run through an X-ray initially, and then later a sub-sample is flash frozen in liquid nitrogen and then embedded with resin to bind all the grains together. 

Flash freezing a sediment sample in liquid nitrogen
This process allows thin sections to be taken which can reveal the micro structure of the sediments, including glacial varves and any lithological boundaries.

Scanning Electron Microscope (SEM) image of a lithological boundary

Scanning Electron Microscope (SEM) image of glacial varves
So what's next?

The next stage is to begin to collate the results as they come in. This will probably take a number of months as the various scientists involved work on research as varied as dating organic matter to identifying microscopic diatoms and pollen grains to counting sediment laminations and varves! We will then gather together again and look at how the various results answer the questions that we have, and indeed what other questions they raise!

Thanks. Dr Carol Cotterill

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