30 October 2012

Seeking the sea in Cambridgeshire

Metres and metres of sea level change. Timescales of hundreds of thousands of years. Our project is really spectacular! Unfortunately, our field sites aren’t. So far we had visited Norfolk. It just looks like the Netherlands. The sediments were great, though! And this time we would look for very elusive sediments in nearby Cambridgeshire. Since the 19th Century scholars had been writing about the Woodston Beds; deposits, about 400.000 years old, from environments such as rivers, ponds, marshes and the sea. In other words: deposits from near shore environments, reflecting fluctuations in sea level. Exactly what we want! The problem was that these beds had never been very extensive,  and a lot of them had already been eroded away in the meantime. And what Nature had left, Man had had no mercy with; the region is famous for its brick production. One third of UK bricks seems to originate from the Peterborough area. And these bricks are made of Jurassic clays; in order to get to these, you have to dig away whatever lies on top. Bye bye Woodston beds.  And if there was anything left, it had a good chance of having been built on, as Peterborough is a rapidly expanding town. This would not be an easy fieldwork!

Our field area - one can see some difficulties here. From Google Maps

It was going to be the usual team of four: Antony, Tasha, Roland and me. Then Antony and Tasha found a local expert at a conference, and he was interested in getting involved. And then Roland fell ill. So in the end it was just me taking the train from Plymouth, and being picked up from the railway station by the Durham crew, and Harry the local expert. We first would visit the headquarters of the company on whose land we were allowed to core: O&H Hampton. They were most welcoming, brought out all kinds of aerial photographs, and talked us through what the region had been through. The changes were spectacular: from arable land to clay pits to lakes to fly ash dumps to residential areas. Not easy to find an undisturbed site in all this. But if it's there, they know where. Then we went to the site we had picked: just some empty field. Not much to see! We did find clay with oyster beds in it; Woodston beds? It looked promising!  But the next day we would have a look inside, and find out more.

Oyster beds!

The next morning we wanted to start coring. For these sediments you need a percussive drill; by hand you never get through the gravels. In Norfolk we had quite a nice one with us, but in the meantime this one had been helped to its grave by a PhD student. So we had a slightly dodgier model. When we had a look if we could make it work we saw the pull cord was about to snap. So we started the day with impromptu mechanical repair! Luckily, there was a spare cord, and we managed to put it into place, and screw the whole thing back together without being left with some enigmatic bits of which we couldn’t remember where they belonged. We could start coring!

Fixing the pull cord

Over the next four days we turned that field into Swiss cheese. The first core we took struck Woodston beds! They still exist! Even though they turned out to have nothing to do with the oyster beds of the first day - these were Jurassic. But then the next cores only struck gravel. We got the impression we had struck a channel where the beds had been deposited. Core outside the channel, and you only find gravel. We tried to trace the channel so we could find the place with the best sediments. This would have been easy with ground penetrating radar; the plan had been to bring that, but only Roland had ever used it before, and we figured it wasn’t worth me coming by car with that thing in the boot, if we would be stuck at the first moment the machine gave us problems.

And coring!

Preparing the sediments for being taken out of the barrel

When we cored, we did take samples from time to time, to check them for microfossils back at the cottage. That started out promising; we found forams galore. But something was odd. The forams we found high up, which would be the most recent ones, looked pretty and pristine. The ones further down, so older, looked more battered and etched. So far so good. But then the very oldest ones, from the Jurassic Oxford Clay, turned out to be the prettiest, glassy, pristine forams of all. What was going on there? The Oxford Clay is more than two orders of magnitude older than anything that is lying on top. And what was strange too: the species I found in there weren’t what I expected. They were unlike anything that had been described as belonging to Oxford Clay before. Had my sample been contaminated? That would have been one hell of a contamination, as we are talking Very Many forams. Maybe this wasn’t Oxford Clay? But if we got that wrong, what else would we have interpreted wrongly? Back in the lab I’ll have to have a good look at how I can sort out how all of that fits together. No scarcity of puzzles!

If we understand the sediments that these forams came from correctly, then the ones on the left are ~100.000 years old, and the ones on the right ~160 million years old... if that's really true I'll sell their secret to l'Oreal and get rich!

We had started out using the so-called “window corer”; it works well, but the only way of getting the sediments out is by cutting them out, in pieces, through the windows in the barrel (hence the name). Not only does that give you disturbed cores; it also is a hell of a lot of work. Stiff clays are especially hard to get out. So this time we had also brought another system: a closed barrel, with plastic liner. The idea is that you pull the liner out of the barrel, and have a beautiful, pre-wrapped, undisturbed sediment core. But it doesn’t work on all sediments. That’s why we had brought the tried window corer, so we knew we were guaranteed sediments.

A sneaky picture from the previous fieldwork, showing the sampling with the window corer.

It was time to try it out. We cored, with the widow corer, to such a depth the next drive would catch the Woodston beds. We lowered the closed barrel, already renamed “Roland’s condom machine”, into the hole. And hammered it all the way in. So far so good. But now came the harder bit; first we had to jack it out of the hole. And the condom machine came with its own rods, which were connected by sleeves. And these sleeves didn’t fit through our jacking system. We did manage, but we had to move the jacking system from its normal position to resting on the ground, and back, for every 1m rod that came up. Without dropping the whole core into the hole. A lot of hassle.

The new corer! Notice the big thread on the rods, that needs a sleeve around it to connect them. And notice the cool big spanners; there was nothing they fit on, strangely...

When we had it on the surface we were faced by the challenge of getting the sediments out. Tasha knew how it should work; you remove a metal bit, then take out another metal bit that has the sleeving attached, and hop! Out comes the core. Or not. When the metal bit came out without any sign of sleeving we knew we had a problem. We then tried to screw the barrel open on the other end. But to no avail. The technicians will have to solve this one, probably with a pressure cleaner! An unglamorous end for the exclusive Woodston beds.

Plop! The metal thingy Tasha has just pulled out should be attached to the sleeving... and pulling it out should  pull the sediments out too. Well, it clearly didn't work that way!

When we had all the sediments we wanted, we surveyed in the core locations. And then it was a wrap. But it will take quite some effort to find out what it really is we found! Stay tuned...

Goodbye to autumnal Peterborough

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