As your new student representative I thought I would kick things off by telling you a little about some recent fieldwork undertaken as part of my PhD research ‘People, Land-use and Time’ which is investigating prehistoric human-environment interactions in western Ireland. I am carrying out pollen analysis (which is nearly complete!), chironomid analysis and lake sediment geochemistry on two sediment cores extracted from lakes within County Clare. In this short piece I am going to tell you about the process of extracting a 10.59m long core from Lough Inchiquin. The lough, being around 30m at its deepest part, would push the equipment (and us to be quite honest!) to its limits in order to extract the sediment core.
One of the great things about being involved with the Palaeoenvironmental Research Unit at NUI Galway is the access to the Usinger piston corer and associated platform that has the ability to work at depths of up to 40m (thankfully!). This was going to be the first lake core that I had ever extracted and was the deepest lake that any of the researchers involved had attempted. For those of you that don’t know what this Usinger piston corer looks like – it looks like this..
The first day on site at Lough Inchiquin near Corofin, Co. Clare, was spent putting together the equipment which results in essentially a metal raft which has a hole located in the centre through which the sediment core is taken. This requires quite a lot of organisation and consultation with the manual as to which order things have to be assembled. This kind of fieldwork on large lakes is generally only undertaken in the summer and everyone needs a recap as to what is involved.
For the process of extraction all of the equipment and people needed has to be on the raft which is anchored in place on the lake. We wanted to take the sediment core from the deepest part of the lake and so a quick foray with a small boat and depth sounder was required. The deepest part of the lake was actually located quite close to the northern shore of the lake and the ruins of Inchiquin Castle. This was measured at 30m and with a resulting 10.59m of sediment eventually taken resulted in, we think, the deepest core ever extracted in Ireland!
Once the location the raft needed to be in had been identified it was a matter of floating the corer and platform out using the boat and attempting to anchor it in position using both anchor bags and ropes to the shore. The equipment is generally not used in windy or wet conditions and so trying to plan ahead for this fieldwork in the west of Ireland, albeit in June, was quite tricky. Despite a good forecast the conditions were less than ideal with strong winds making the anchoring of the raft highly problematic. It floated around for quite some time before we eventually gave up at 10.30pm and decided to try again tomorrow.
The next day we were more successful and got the platform into the correct position. We then began the process of extraction with a team of seven on the raft and two on the lakeshore. Throughout the day we managed five drives – the first three of which resulted in a full 2m segment of sediment. The final two drives were much harder, however, and we couldn’t seem to get the corer through the sediment. Had we hit glacial clay already?! No, no – that would be far too easy! We instead, swapped to a thinner coring tube and managed to extract a further 2m of sediment.
I should really give a breakdown of how the sediment is actually extracted using this equipment. The first stage of the extraction is to lower down metal casing from the raft to the lake floor which will guide the coring tube when making the drive. The coring tube itself is then filled with water and the piston head inserted into it. What is called a transition rod is then attached to this and lowered down the hole using a rope. Extension rods are then attached in succession to get the coring tube down to the required depth. So, after every 2m drive, a further 2m of extension rods have to be attached. What is called a driving rod is then attached last of all. A drive is made by pushing the driving rod down which in turn pushes the coring tube into the sediment – and the end of each drive the driving rod has been pushed down by 2m and a core of sediment has filled the coring tube.
The next (fun) stage is to bring UP the coring tube which is aided by a lever, some rope and a couple of strong individuals! As the coring tube is being pulled up, the extension rods have to be successively removed at quite a quick pace (my job!). The coring tube will then be the last to come up and has to be laid in a horizontal position to ensure the sediment remains inside. The transition rod has to be detached and the piston head removed which can be quite amusing to watch as the piston head inevitably gets a little stuck and has to wiggled free within the confines of a small metal raft floating on a lake…
The coring tube is then sent over to the lakeside, delivered by boat, and the raft team start to take the next drive while the sediment is removed from the tube at the lakeside. The extruder, which consist of a ratchetting mechanism, pulls the coring tube backwards and deposits the sediment itself into plastic half-tubing. The sediment is cut to produce 1m segments and also has to be split longitudinally using another half-tube and a guitar string. This produces an A segment (containing the most/best sediment) and a B segment which will later both be described in the lab. The sediment is then packaged and labelled.
On the third day, we went back onto the raft for one last drive, again having to use the thinner coring tube, which resulted in a final 2m segment taking the total depth of sediment to 10.59m. We had finally hit glacial clay and we had got all that we needed. The raft then had to be pulled back to the lakeside and the process of dismantling the equipment began.
The three days of fieldwork were tough, physically demanding work but aided by great people, attitudes and good pub grub at the end of the day. The sediment we managed to extract is of great quality and has left me with over 3m of prehistoric material to work with. The pollen analysis for this core is now complete and has been done at 4cm intervals resulting in 90 samples. Periods of reduced and increased human activity can be identified in the data and when combined with the archaeological evidence from this region will provide me with a fascinating insight into the prehistoric landscape and human occupation of this area.
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