Frazer proves that the Mo does not stop work in the palaeo lab..
Enjoying the Palaeo-Mo’s? If so for more picture and and the opportunity to donate visit the OU Roc Soc team Movember pages.
For more Open University Mos’
CHIRONOMIDAE AS A PALAEO-ECOLOGICAL TOOL
Chironomidae is a family of two-winged flies more commonly referred to as non biting midges. This diverse group of insects have been known for a long time to be sensitive environmental indicators. Early research in the field showed that the trophic status of lakes could be classified according to the characteristic chironomid assemblages found within them (Thienemann, 1922). Furthermore the head capsules of the larvae are well preserved within the sedimentary record. As a result palaeolimnological researchers became increasingly interested in the potential for using Chironomids to track the trophic development of a lake through time by examining the changing assemblages within the accumulated sediments. With geographically close lakes displaying significantly different midge faunas the potential for the insects being used as climatic indicators was dismissed and the following hypothesis became established: Chironomid assemblage composition reflects in-lake variables, e.g. lake depth, pH, dissolved oxygen, trophic status and substrate. However work by Walker and Matthews (1989) demonstrated that temperature was by far the most significant variable in controlling the broad scale distribution and abundance of midge fauna.
Walker and Matthews realised the potential for the non biting midge to be used as a palaeoclimatic indicator from two initial observations. Firstly within the fossil records, as climate began warming following the deglaciation of the northern hemisphere, the relative abundance of taxa associated with cold oligotrophic lakes (Heterotrissocladius) abruptly declined. Secondly they noticed the best analogues for late glacial assemblages were found in modern day arctic and alpine settings. Overall Walker and Matthews concluded that the northern limit of temperate taxon was controlled by cold summer air and/or water temperatures. The southern limit of Arctic species was instead driven by cold oxygenated refugia in the profundal zone of deep, temperate lakes. These temperatures were significant with respect to the insect’s life cycles as many species require critical temperature thresholds to complete pupation and emergence stages.
Since the pioneering work of Walker and Matthews (1989) and others the debate linking Chironomids to temperature has raged. Debate has centred upon what controls chironomid distribution and how suitable, if at all, the insects are in the context of palaeoecological studies. Recently Velle et al. (2010) discussed some key factors which must be considered when working on chironomid based temperature resonstructions.
Below I present some of the debate around the midge-environment-temperature debate; focusing on both midge distribution and identification and the potential of this proxy as a indicator of past environmental and climatic change.
COMMENT ON DISCUSSION OF PROXIES IN HUNTLEY (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:
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.