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Coring on a larger scale

Hammering in the core barrel with a road drill

Hammering in the core barrel with a road drill

The older the sediments, the more macho the equipment. Do you want to core a thousand years of salt marsh sediments? Just bring your hand auger and a gouge. And some muscle power, and a willingness to get really close to the muddy side of science. Do you want to core through interglacial sediments? That might not be so easy. As the name “interglacial” already suggests; these sediments are quite likely to be wedged between glacial sediments. And these may very well consist of gravelly stuff that’s awful to core through. If you need to drill through these to get to the sediments of your choice, bring at least a road drill. The percussive force of the drill will break through all the pesky flints you might encounter below. And a core barrel that’s rammed down with such force won’t be easily pulled out, so a big sturdy jack system is needed to pull the barrel up through the sucking mud and obstructing gravel.

Jacking out the core barrel

Jacking out the core barrel

A set-up like that can take you through many metres of unforgiving sediment, but there are limits to it. We know that because we tried it. If the core barrel is stuck, you have to pull the levers of the jack with all the power you can muster, but you might find what that results in is that you push the jack into the ground, rather than pull the barrel up. You might add some solid wooden beams to place under the jack to stop it from sinking into the ground; that will just break them. We tried that too! So there is a moment when you have to upgrade from the road drill. And the next step up is quite a step. The next step up is a drill rig. We had pushed the limits of the road drill set-up in the hunt for our iGlass interglacial sediments. So we had to take the next step. Fortunately, we had expected that, and there was budget for it.

The drill rig erected at Horse Fen

The drill rig erected at Horse Fen

The difference between a road drill and a drill rig is mainly the size. Another difference is that if you rent it, you get people to work it with that. So from struggling through brambles and barbed wire to get to a muddy field where we had to do everything physically possible to not lose the core barrel underground, we were suddenly upgraded to overseers who watch other people get muddy and achy and tired. And these muddy, achy and tired people would give us what we couldn’t possibly get ourselves: wide, intact sediment cores, all the way through the interglacial sediments to whatever lies below. And they delivered!

It was in rural Norfolk we got to meet our drillers. They arrived in a Land Rover with the rig as a trailer behind it. We asked them to set it up at Horse Fen, one of our critical field sites, where we had not been able to core the lower contact of the interglacial sediments. We had to set up at some distance from the old drill holes, in order no not disturb our badger neighbours. So the men erected the quadripod, hammered metres of casing into the ground, and started drilling through the glacial sands on top of our interglacial clays. These we don’t need for our research, so we just chucked these away. That saves time. They drilled down to where we expected the clay to start. And they drilled deeper. And deeper. We started to get worried. By the time we were wondering if they were drilling into an entirely glacial succession we hit the clay. The stratigraphy is very laterally variable in this area! We cored and retrieved the clay, which was a lot thicker than we expected, and finally hit the freshwater peats. And then the rig did what we had hired it for: it drilled through these. We knew we had a beautiful, undisturbed contact in our core liner. What would we find below? That was something we would not know until we got to the lab!

Extracting the core from its liner

Extracting the core from its liner

Sealing the core sections with warm wax

Sealing the core sections with warm wax

Coring in the dark

Coring in the dark

Removing a section of casing

Removing a section of casing

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Big, unicellular science

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A shallow water foraminifer, Ammonia sp. (it’s about 0.4 mm big)

Metres of sea-level change. Hundreds of thousands of years. The entire globe. The scale of the iGlass research project is rather large. But some of the research is based on the humblest of creatures: benthic foraminifera.
If you have a really big one it might be an entire millimetre. Benthic foraminifera are unicellular protists who live on top of, or in the top layer of, marine sediment. Every species has its own niche; for instance, some like coarse sediments, some like fresh food, some like warm water, etcetera. Some of the more adventurous species even live in salt marshes. These are, if you will, the scuba divers among foraminifera; all foraminifera need salt water to live, but these brave souls have become accustomed to do without for substantial amounts of time.

Three specimens of Aubignyna perlucida; two of these have their shells filled with pyrite nodules

The reason why foraminifera, in spite of being such modest creatures, get so much attention from the palaeoclimate community is that they build some sort of skeleton, and aretherefore often very well preserved in sediments. They can be tens of millions of years old and still look like they died last week. They take the information they contain into their graves, and in this case that’s a good thing. If you know which species prefers what environment, you can “read” sediments they are found in.

Drilling a core in sediments that are several hundreds of thousands of years old

The York and Durham teams drilled cores through interglacial sediments.

We drilled in the middle of Norfolk, but we know that area has been inundated in previous interglacials, and indeed, we found both freshwater and marine sediments in our cores. And using the foraminifera in them, we are reading them. We are looking for changes reflected in the various species encountered; will we get a gradual change from the shallowest foraminifera, which are the ones that actually live above sea level but still within reach of high tide, through the ones that live in shallow water, to those that prefer deeper waters? Or will we see several of such sequences? Or an instantaneous shift from no foraminifera to relatively deep-water species? Watch this space for results…

The foraminifera sorted by species
(each numbered rectangle is about 2.3 mm wide)

We found some 10, 000 foraminifera (and counting). They were picked out of the samples by two different scholars, so we keep them all in order to be able to check we have comparable ideas of which species is which (since you ask; no, identifying the species is not a straightforward task). Keeping them also allows us to always go back to the source material in case of questions arising. And after the questions iGlass asks have been answered, who knows what other questions such a collection can help tackle!

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