Peat and pollen analysis

David Beeson, January 2022

For us, northern hemisphere people, the year is edging towards longer days and shorter nights … and about time too! And there are signs that life is at least starting to think about spring. We have snowdrops just coming into flower, the Tulipa sylvestris have popped up above soil level and our thousands of wild daffodils are poking their leaves skywards. Yet the minus seven degrees Celsius last night will have cooled their ardour. But I digress, peat.

Peat is mighty useful. You can burn it, make paper out of it, have therapeutic baths in it and, in the past, spread it liberally around the garden. Although we all try to avoid peat composts these days.

In the soils article, I intimated on the origin of peat – from partially decayed organic plant material. Peat is produced in an anaerobic decomposition, usually in waterlogged soils. If oxygen is later admitted, for example when peat is used in a planting mixture, it will, of course, break down and liberate the carbon dioxide from which it was made. Peat is a carbon store, and a small barrier to enhanced carbon dioxide levels in the atmosphere, and hence Global Warming.  Belatedly, the UK is restoring some of our peat bogs by rewetting them.

Imagine a small lake in an upland area. Leaves fall into it over many autumns and, especially if the water is slow moving, will accumulate and slowly release humic acids. Acid-loving plants colonise and soon one would be able to spot the start of zonation around the lake, with sphagnum a first major component. The sphagnums (bog mosses) semi-decay into peat, and reeds and sedges germinate in it to generate the second vegetative zone. With time, in the UK, willow scrub then alder woodland and eventually a mixed oak woodland would establish.  A true succession as the lake slowly fills, but with the roots of everything probing into the underlying peat.

It is the march of a glorious vegetative succession. And, given time, the lake vanishes completely.

In Shropshire, on the Welsh borderlands, some glacial lakes have been almost lost, because the vegetation has grown over the lake’s surface … yet beware! It is only a surface layer and there is open water beneath! [Smaller situations occur elsewhere, of course. Once in the New Forest a group of students, including me, were told not to run across an area of bog. One did (not me) and sunk up to his armpits.] By gently jumping the ground will shake and tremble beneath you.

As the peat is anaerobic, and certain plant components decay VERY slowly in it, it represents a time machine with the ancient vegetation stored there for us to rediscover. And the component that one seeks is pollen, for they are unique in their designs. Seek out the pollen and you know the historic vegetation.

[It is also possible to sample the vegetation type via DNA analysis, and that avoids a lengthy time spent at the microscope.]

Quiz question: Are these grains the male gametes of the plant? Answer at the end.

Peat borers sample the soil vertically, and acids can remove the bulk of the peat and leave behind the pollen grains. The deeper the sample, the older the peat. The question then arises, how old is the sample? And the answer arrives easily in a laboratory, for the atmosphere contains minute amounts of C¹⁴ which, like all radio-isotopes, gradually decays whether it has remained unfixed or gabbed by photosynthesis into a plant. By measuring the C¹⁴ level remaining, and knowing the decay rate gives the age of the sample. [The C¹⁴ decays to 50% in around 5730 years.]

It is said that 250 million acres (100 million hectares) of land is covered in peat, with a weight of 223 billion tonnes. Quite a carbon store.

Peat is great! A carbon store and it gives us a window into the past. Real Time Travel.

Experiment: view your own pollen grains under the microscope.

Answer: no! They are the microspores, containing two nuclei, and they germinate on the stigma to grow to produce the pollen tube. That contains several nuclei, one of which is the male gamete. Other nuclei join to form the nucellus. But you knew that because you’ve read Plants are Clever 2.

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