Reconstructing palaeoclimates using beetle remains

This is a light hearted extract from of the newsletter The Black Country Geological Society. This was part of the regular section entitled ‘The Dudley Bug’ written by members Alison Roberts and Chris Broughton.

An effective way of reconstructing Quaternary palaeoenvironments is to investigate the range and distribution of beetle populations. The most commonly used species to reconstruct Pleistocene climates are the Coleoptera beetles. There are 300,000 known species of this beetle. They are suited to this method of dating because they are a varied group with distinct tolerance ranges. Beetles are extremely abundant as they comprise over a third of insect species. The high preservation rates allow them to be identified as belonging to a living species. Beetles are very well preserved due to their hard chitinous teguments. When found within anoxic soft sediments they can be easily identified to species level. The male genitalia are often used to correlate beetle remains as far back as the early Quaternary compared to modern equivalents. This indicates that there has been highly stable morphology of beetles for the past 70 million years. There has been little evolutionary change of insect species on the continental masses.

The beetles provide an accurate indication of temperatures, and therefore reflect the thermal climate of each hemisphere. The temperature ranges between summer and winter can be deduced by studying the fossil species assemblages. Beetles are extremely sensitive to temperature change, for example in the Netherlands beetles have responded to climate change since 1890, ie within decades.

Beetle populations can indicate palaeoclimate using the mutual climatic range (MCR) method. The theory for this technique is: assuming that present day climatic tolerances for a beetle species are known, then fossils of that same species indicate a palaeoclimate within the same tolerance range.

Data provided by beetles include the warmth of the summer and temperature range between summer and winter. Where a majority of the coexisting species lie is the mutual climatic range. The accuracy can be tested using living communities of Coleoptera; in most cases there is excellent agreement between them. An example of this is seen in major rapid warming periods around 13,000 years BP, and also around10,000 years BP with cooling in-between. The warming is associated with a northward migration of Southern Mediterranean beetle species. In this example the climate is estimated to have average temperatures up to 18ºC, annually ranging between 9ºC and 30ºC. Precision varies for the radiocarbon dating of beetle remains from ±45 to ±325 years with an average of ±120 years.

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