Friday, February 1, 2013

UK National Tree Week; Palynology


So, in the absence of fossils, how do palaeobotanists know what type of trees were around during different periods of geological time? The science of palynology!

And what is palynology? Well palynology simply means, “the study of dust”, and in the context of looking for evidence of ancient plant and tree life, palynology is the study of palynomorphs such as pollen, spores, and dinocysts, (this list is not exclusive see the links below for more information) as well as unidentified particulated organic matter and kerogen. Whilst palynology does not include the study of microfossils with calcareous or siliceous tests, (this falls under the category of micropalaeontology) stratigraphical palynology is a branch of micropalaeontology concerned with the study of fossil palynomorphs all throughout geological time.

So aside from palynology being a useful tool to help palaeontologists see the “bigger picture” with regards to periods of geological time, what else is palynology used for? The most useful practical application is stratigraphy, and the dating of rocks during petroleum exploration, mining and water exploration, but palynology has many academic uses too. Aside from being used to unravel the history of plant life on Earth, it can be used for palaeoclimate studies, palaeoenvironment studies, taxonomy and phylogeny, archaeology and palaeoecology.

So how are palynological samples prepared?
Well first of the sedimentary host rock needs to be digested to leave the organic matter behind, and this is done first by submerging in hydrochloric acid to remove the carbonates present, and then hydrofluoric acid to remove the silicates. The sample is then oxidised (unless the slides required are kerogen slides, then no oxidisation is required). The organic matter is then separated from the acid mix. There are two ways to do this; decanting the acid off and replacing with distilled water until you have a neutral mix, or by “washing” the sample through a very fine grained sieve- one capable of retaining the organic water, without washing it down the sink (again, if a kerogen slide is required, then no sieving takes place). The samples may then be stained to highlight particular palynomorphs. Once all of the process required have taken place the sample is mounted on a cover slip, left to dry and then studied (often using a high powered microscope, a traversable slide, and the “England Finder” method of study). And if you want to try this as home, please remember that any time you are using acid you should be using a fume cupboard and wearing protective clothing!

So, what evidence do we have that the “tree-scape” of the UK has changed over time? Using palynology, it’s possible to accurately date the agricultural revolution (the period of time when Neolithic hunter-gatherers settled down on farms) in the UK, and where it started and how and when it spread to different areas, as well as telling us what types of trees once existed. For example the now barren area of Dartmoor in the South West of England was once covered in forest, but clearing for agriculture and the subsequent failed farming left the soil depleted of nutrients, and no trees have really grown there since.

If you want to find out more about palynology and how the landscape of the UK has changed throughout geological time then head to the links provided below.


PDF- Introduction to the Mesozoic and Tertiary palaeobotany of Great Britain
http://jncc.defra.gov.uk/pdf/v22chap1.pdf

Links:
http://www.ucl.ac.uk/GeolSci/micropal/spore.html

http://www.sciencedirect.com/science/journal/00346667

http://palynology.geoscienceworld.org/

http://www.palynology.org/

http://www.geo.arizona.edu/palynology/

Image: W.A.S. Sarjeant


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