Number crunching in palynology

May 10, 2012

Simulated pollen counts

Simulated pollen counts

Our understating of vegetation in the past, and how it has changed through time, comes mainly from the examination of macrofossils (e.g. wood and leaves) and microfossils (e.g. pollen and spores) found in the sedimentary record. The potential for microscopic fossils to provide an insight into past vegetation change on a landscape scale was pioneered by von Post (Von Post, 1916, reprinted 1967) and has been subsequently used to understand changes in regional floras (Godwin, 1956), and address conservation issues (Willis et al., 2007). Analysis of fossil pollen and spores (palynology) is now widely used on late Quaternary timescales to answer ecological questions linking vegetation and wider environmental/climatic change; these include:

  • Has there been a change in major vegetation type (biome)? For example a change between woodlands and grassland vegetation.
  • How have the ecosystem dynamics altered? For example the presence or absence of fire.
  • How has the diversity within the ecosystem changed? For example increase or decrease in sample richness.

Palynological analysis relies on obtaining a sub-sample of the pollen contained within the sediment at a specific depth (time) which allows the vegetation at that time to be reconstructed. This sub-sample is known as a pollen count. To build up a picture of vegetation change through time it is necessary to generate a sequence of pollen counts. The size of the sub-sample (pollen count) required from any particular depth (time period) is dependent on the nature of the vegetation association being investigated and the ecological question being addressed . For example, the amount of pollen analysed to determine if the vegetation was predominantly wooded or grassland is different to that required to provide information on the biological diversity within the vegetation assemblage.

Discussed below are some of the conventions related to choosing an appropriate pollen count size within palynology, with particular reference to the challenges of dealing with diverse tropical floras.

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Nitrogen isotopes in lake sediments

February 28, 2012

As a component of my doctoral research, I am examining nitrogen (N) isotopes within sediments obtained from Lake Bosumtwi (West Africa). Below I review and comment on the key uses and limitations of using N isotopes to interpret past environmental change with particular reference to lake sediments. Discussion is based on the key text by Talbot (2001).

Talbot, M.R. 2001. Nitrogen isotopes in palaeolimnology. Tracking environmental change using lake sediments. Volume 2. Physical and geochemical methods (ed. by W.M. Last and J.P. Smol), pp. 401-439. Kluwer Academic Press, Dordrecht.

NOTE: This text is avaliable to Open University students as an ebook via the library

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Thinking about proxies

January 31, 2012

All areas of research have strengths and limitations which are readily acknowledged by the scientists involved. The reconstruction of past climates (palaeoclimates) from biological indicators contained within the fossil record (proxies) presents some specific challenges; for example key limitations might be gaps in a sedimentary sequence or post-depositional degradation of samples. Understanding and interpreting data sets in the face of these challenges require the researcher to develop a wide range of skills. Huntley (2012) focuses upon the uncertainties within palaeoclimate reconstruction which he considers to be “frequently overlooked” (p. 2) by scientists making climate reconstructions from proxy records. Specifically Huntley urges researchers to consider carefully:

  • What a given proxy is actually capable of reconstructing, i.e. what climate variables controls its distribution?
  • What other variables might be influencing the proxy, i.e. could there be multiple influences, might these vary through time?
  • What is the spatial relevance of the proxy, i.e. macro versus micro scale?
  • Can multiple proxies be compared, either within or between sites?

In other words: which and how many climatic variables can be reconstructed form any one aspect of the fossil record?

Below I review and comment on some key arguments made by Huntley (2012) related to the use of proxies in reconstructing palaeoclimates.

Huntley, B. (2012) Reconstructing palaeoclimates from biological proxies: Some often overlooked sources of uncertainty. Quaternary Science Reviews 31: 1-16.

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