How low can you go? Adventures in the fine slicing of peat.

I’ve written elsewhere about the sheer volume of work that is required to produce results in our field of research. As if there wasn’t already enough to do, I’m currently involved in a line of research that has the potential to increase that volume of work tenfold!

The time resolution of any given study using peat cores to reconstruct past environmental or climate change is primarily dependent on two things – the rate at which the peat accumulated and the spacing and depth of the samples taken.

It’s standard practice to take samples that span 1 cm of vertical depth. Depending on what methods you are using, how long your core is and what your research questions are, these 1 cm samples could be contiguous (i.e. 0-1, 1-2, 2-3, 3-4 cm etc – this is quite rare) or spaced at regular intervals such as every 2, 4 or 8 cm (this is more common due to time constraints).

The average rate at which ombrotrophic peat bogs have accumulated over the Holocene is about 1 cm every decade. So this means that if you’ve taken your 1 cm deep samples and analysed them contiguously, you may end up with a ‘decadal resolution’ record – i.e. each of your samples represents the average conditions over a decade and you have one sample per decade. If your samples are more widely spaced, every 4 cm for example, you might have one sample every 50 or so years. This is what we mean when we talk about the time resolution of a peat record. It’s very dependent on how well your core is dated (read more about dating here), but dating is a story for a whole other blog.

All different archives of past climate information have different time resolutions, depending on how they formed. We’ve seen already that peat bogs might be thought of as having decadal resolution at best. However other archives (such as tree rings, ice cores, corals or even varved lake sediments) can provide annual resolution records. Think of the way a tree grows with its yearly rings – a distinct record of annual changes. Peat bogs, clearly, do not provide such an explicit record of their shorter-term changes.

However, that’s not necessarily to say that if you were to slice your peat samples very finely, less than 1 cm thick, you couldn’t resolve a record of past changes with a less than decadal time resolution. It was exactly this question that I investigated during my PhD and subsequently published some journal papers on (see here and here for example).

Of course, it’s a far more complex question than I have laid out here. During my PhD, I used a custom-built fine peat slicer to take 1 and 2 mm thick samples from very rapidly accumulating peat bogs. I then analysed these and tried to understand if the results could be interpreted as a sub-decadal resolution record of past climate changes, or whether other processes local to the bog itself masked that climatic information at this fine a scale.

Just imagine – analysing samples every 1 mm! My contemporaries at the time were using traditional approaches with widely spaced samples and were analysing metres of core and thousands of years of stratigraphy. On the other hand, I was progressing my research at snails pace – if I got through 1 cm worth of samples, perhaps ten to twenty years worth in a day, I would be over the moon; it certainly required an adjustment in expectations!

Perhaps you won’t be surprised to learn that I didn’t come up with any firm answers (that’s science for you!). There were some tantalising findings – very rapid changes that seemed to suggest that it was possible to produce these fine-resolution records – but these results need more replication and a deeper understanding. It’s also interesting to note that different methods are not all equally suited to this approach – for example testate amoeba reproduce rapidly and are therefore sensitive to even seasonal changes on a bog whereas plants tend to respond more slowly and therefore wouldn’t necessarily show the same type of response in our core samples. The images below, for example, show why it’s problematic to look at plant remains at such fine scales – they show where roots and grasses have been sliced into 1 mm thick sections. These could then be found in consecutive samples – clearly, this won’t be telling us about climate.

The issues of dating (using radiocarbon and tephra techniques) and the roles of internal (i.e. ecological processes happening in the bog itself, which, in the context of this research, we can think of this as ‘noise’) and external (i.e. climate driven changes which we can think of as the ‘signal’) factors driving changes in the peat stratigraphy are especially important in this line of research.

In the end, I probably came up with more questions than answers. However, that’s not necessarily a bad thing and this summer I’ll be heading out on fieldwork to take cores with which I’ll try to answer some of these questions. More on that when it happens!

References:

(Links to these articles can be found in the blog. Please contact me if you don’t have access to either of my papers but would like to read them).

Amesbury, M. J., Barber, K. E. and Hughes, P. D. 2011. The methodological basis for fine-resolution, multi-proxy reconstructions of ombrotrophic peat bog surface wetness. Boreas 40, 161-174.

Amesbury, M. J., Barber, K. E. and Hughes, P. D. 2012. Can rapidly accumulating Holocene peat profiles provide sub-decadal resolution proxy climate data? Journal of Quaternary Science 27, 757-770.

Sullivan, M. E. and Booth, R. K. 2011. The potential influence of short-term environmental variability on the composition of testate amoeba communities in Sphagnum peatlands. Microbial Ecology 62, 80-93.

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