How to core a peat bog

Peatlands are nature’s history books, storing up to 10,000 years of information on past climate change in their slowly accumulated layers. This is what Bogology is all about – extracting this information, learning from it and applying those lessons to study past, present and ultimately future climate change. But how do we extract that information? How do we go from being researchers stood on a peatland with a heap of equipment to researchers stood on a peatland with a heap of muddy equipment and metres of ancient peat to analyse?!

Over the past week or so, I have been on fieldwork in Sweden making just that transition many times over. It has been an intense and busy trip, but so far we have taken five peat cores, averaging around six metres each. We will have to wait and see exactly how old these cores are, but on the basis of other research in this region, it is likely they stretch back between six and ten thousand years! I have cored many peatlands over the past ten years, but this fact never ceases to amaze me when I pull up a deep core section that looks as well preserved as if the moss had grown only yesterday.

The first thing to think about before starting to take your peat core is where to take it from. This generally involves finding the deepest (and hence oldest) part of the peatland and then selecting a location that is likely to have been sensitive to change over time. To explain what I mean by this, it is necessary to remind you that the surface of peatlands tend to be covered by different features known as hummocks, lawns and pools, which we describe in more detail here. Hummocks are higher, drier areas at one end of the spectrum, which may even have stunted pine or spruce trees growing on them, whereas pools are much wetter, as the name suggests! Pools can have extensive open water, or just be very wet patches filled with extremely soggy Sphagnum moss. Since these two features fall at the wet or dry extremes of a peatland’s surface, they tend to persist over time and taking a core from them might not show much variability, even if climate is causing the bog surface wetness of the peatland to change. Lawns on the other hand are areas of intermediate wetness that respond more sensitively to increases or decreases in rainfall and/or temperature and so are broadly considered the best locations on a peatland from which to take a core, if your aim is to study past climate change.

So, once you have found the deepest part of your peatland (by depth probing along transects with the coring poles until you hit the bottom) and located a lawn from which to take your core, you must set about the coring process. This involves two main pieces of kit – a monolith tin and a Russian corer. The Russian corer will do most of the work, but often the top 50 cm or so of peat can be very wet and full of fresh, rooty vegetation, making it difficult to core well with a Russian corer. This is where the monolith tin comes in. This is a very basic piece of equipment; a tin measuring about 50 cm long by 10 cm wide, with an open front and bottom. The tin is carefully pressed down into the peat until its top is level with the surface. Here, a good old bread knife, the peat fieldworkers invaluable friend, is very useful to cut through the vegetation to ensure the tin slips downwards without distorting the layers of peat. It is then a case of reaching down into the bog to pull up the tin from beneath, plunging ones arm into the frigid, mucky waters of the peatland. This is hardly a glamorous process, but once completed, we have the first section of our core in the bag!

Next comes the coring proper. Russian corers are usually 30 or 50 cm long so our cores must be taken in shorter sections rather than one long, continuous length. Russian corers have a semi-cylindrical chamber and a blade that twists around to cut the peat. They work by inserting them vertically down into the bog with the chamber empty, then rotating the corer through 180 degrees, which fills the chamber with peat. Starting at a depth of 50 cm or so (depending on the length of our monolith) we simply keep going downwards, adding poles as we go, collecting short peat sections, until the base of the peatland is reached. The poles are marked off at specific depths, so we push the corer down to a specific depth, twist to collect the peat and pull it back up, repeating this process many times over. Sometimes the bog might be only three or four metres deep, sometimes, as in Sweden recently, it might be about six metres and sometimes, if you are really lucky (or not depending on your perspective!) it might be even deeper – the deepest core I have ever taken was from a site in Ireland which was just over 12 metres deep! You can see a video below of my colleagues Gunnar Mallon and Stefan Wastegård pulling up the corer at Kortlandamossen in Sweden just a few days ago.

Every core section that is brought up from the deep might cover 500 years or so of history, during which time major global events were doubtless occurring while the peat just continued its quiet, steady and relentless accumulation. It’s quite amazing to imagine, when we pull up our 10,000 year (or more) old lower core sections, that when that peat was vegetation growing at the surface, humans were still living hunter gatherer lifestyles. In individual core sections we can often see changes in the vegetation type of the peat, for example between the dominance of mosses, grasses and shrubs, that indicate that the peatland became wetter or drier at that time. The preservation of the vegetation in the acidic and low-oxygen environment of the peat, even at great depth, is mind-blowing.

Gradually, as the core gets longer and longer, the peat will get more and more compacted. Towards the base of the peat core will be a section of ‘fen’ peat, representing the time before the bog became ombrotrophic. If you are feeling strong, then you may even be able to core down into clay sediments, to the time when the peatland was formally a lake, before it began its slow infilling and long transition to the peatland we and stand upon today with all our coring equipment.

Once all the core sections are extracted, neatly packaged and very carefully labelled (the trouble caused by labelling even a single core section the wrong way around doesn’t even bear thinking about!) the hardest part, after the exhausting process of coring, is getting all that heavy peat back off the bog to the car, to be eventually transported back to the lab, where the real work begins…

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