ICELANDIC CHIRONOMIDS: A HOLOCENE ENVIRONMENTAL RECONSTRUCTION FROM NW ICELAND
LANGDON, PETE G. University of Exeter.
Caseldine, Chris J. University of Exeter.
Geirsdottir, Aslaug . University of Iceland.
Chironomids have been shown to respond rapidly to past changes in temperature (e.g. Walker et al. 1991; Lotter et al. 1999; Brooks and Birks 2000; 2001) which make them an attractive palaeoclimatological proxy, particularly as they can be found in abundance in climatically marginal or severe environments. The construction of transfer functions, based on the development of a modern day training set from which individual taxon optima can be deduced, has allowed the quantitative reconstruction of temperatures using chironomids. This is particularly attractive for researchers working on Icelandic terrestrial sediments as the vast majority of palaeoclimatic reconstructions from these sites around the Nearctic relies on evidence from pollen spectra (Miller et al., 2001) which remains difficult to quantify in terms of thermal limits.
Studies on Icelandic chironomids have been largely restricted to ecological studies in freshwater ecosystems (e.g. Jónasson 1979, 1992; Lindegaard, 1992; Garðarsson et al. 1995) with little work have been undertaken on subfossil chironomids. One profile is known to have been examined from Mývatn (Einarsson and Haflidason 1988), which is a eutrophic lake fed mainly by spring water (occasionally with temperatures of up to 30°C), although only a few chironomid taxa were documented in any detail. Mývatn may therefore not be the best place to look for a thermal response from chironomid assemblages - a more suitable location was needed where other proxy climate indicators could be developed to facilitate a comparison with a chironomid reconstruction.
High resolution marine cores have recently been obtained from nearshore fjord deposits in Ísafjarðardjúp (NW Iceland) which span Holocene timescales (e.g. Andrew et al. 2000; 2001). Lakes in the NW region have also been sampled recently with sedimentological and palynological analyses suggesting that the region has been sensitive to climatic changes since deglaciation. NW Iceland was therefore selected as a key area from which to analyse Holocene chironomid faunas due to the presence of these marine and terrestrial data as well as the lack of geothermal activity within the region.
Initial examination of Holocene chironomid assemblages from a site in NW Iceland at Efstadalsvatn (65°55'N 21°40W) has provided an abundance of chironomid remains and application of the Norwegian chironomid-inferred temperature transfer function to the data (S.Brooks and J.Birks, pers. comm.) has produced the first terrestrially based temperature reconstruction for Iceland. The chironomid-inferred temperature reconstruction suggests a rise from ca. 40C at 11,000 cal. BP to ca. 100C by 9500 cal. BP although this is not without a number of oscillations, and the basic pattern is comparable with Greenland ice core reconstructions. The chironomid based reconstruction throughout the rest of the Holocene (the record terminates around 4000 cal. BP) fluctuates between 8.50C and 110C. The taxonomy of species is in general comparable to NW Europe and has therefore been based on the European fauna and the Norwegian transfer function could thus be used for the Icelandic data. There are, however, some important differences in the taxonomy and until an Icelandic training set is derived key issues relating to chironomid temperature reconstructions from Icelandic lake sediments cannot be resolved.
Recent research suggests that at least 33 chironomid genera can be found in Iceland, containing 75 species, with the fauna tending to resemble the European fauna (Hrafnsdóttir et al. 2000). The study of Holocene sediments from a variety of Icelandic non-thermal oligotrophic lakes would thus allow important comparisons, not only in terms of the Icelandic Holocene fauna, but also by comparing the results with the Norwegian and North American faunas. Current investigations have revealed subfossil chironomids which have not been recorded in the modern fauna to date, hence the need to conduct extensive surveys of the lake surface sediments from the region as well as subfossil analyses. Collaborative research is currently underway with Icelandic chironomid experts to resolve some of the issues between the modern and subfossil assemblages.
What is clearly now needed is the creation of a modern day Icelandic chironomid training set in order to develop a chironomid-inferred temperature transfer function for the region. The NW has been selected as a key region due to its lack of geothermal activity and abundance of accessible lakes. Once a model has been developed in the NW, chironomid faunas from geothermal lakes can be sampled and then compared with the model output in order to ascertain the effect of water temperature on the communities.
Over 30 lakes have been surveyed and found to be suitable for developing a modern day chironomid training set for NW Iceland (Figure 1). The littoral region of some of the lakes have been sampled and some initial analysis undertaken. The lakes surveyed were mainly in the NW peninsula, but also extended west towards the Snæfellsnes peninsula, including the region around Stykkishólmur, and covered an altitudinal range from 0-500 metres (Figure 1). Plenty of other lakes were also observed within the region, which would provide over 100 lakes suitable for the development of a modern day chironomid training set.
A number of lakes were surveyed from the NW peninsula in order to provide a detailed sedimentary record and ultimately a high resolution palaeoclimatic reconstruction. Gjögurvatn, in the Strandir area provides the best opportunity to correlate land and sea studies, and Laugarbólsvatn/Efstadalsvatn, two connected lakes, offer the best possibility of a quantitative reconstruction of July temperatures over the past 10 ka from changes in chironomid assemblages. Sedimentological and palynological analyses will also be undertaken on these lakes in collaboration with The University of Iceland and INSTAAR. The combined results from these lakes will provide a unique palaeoclimatic record for the entire Holocene, with the possibility of extracting the anthropogenic effect from the natural forces on climate in the late Holocene.
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Figure 1. Detailed map of NW Iceland showing the location of recently sampled marine cores, surveyed lakes, sites with sediment cores currently being analysed, and proposed coring sites.
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