| Summary: | This thesis examines the spatial and temporal expression of Holocene glacier fluctuations in southeast Iceland. The study uses geomorphological evidence to reconstruct the former extent of Lambatungnajökull -a non-surging, valley glacier flowing from the eastern flank of the Vatnajökull ice-cap. Lichenometry is used to date recent glacial landforms and decode the pattern of glacier fluctuations over the last 300 years. Tephrochronology is used to date older features (<10,000 yr). The results show a pattern of fluctuations driven by climatic change. During the Lateglacial-Early Holocene Period the glacier terminus was situated at the present-day coastline. The ice-margin has retreated c. 20 km during the last 10,000 years. At least four periods of glacier re-advance have been identified, at c. 5000,3000,1600 and 170 years BP. Overall, the cumulative ice recession since c. 10 ka BP represents an ELA rise of c. 400 m which equates to an increase in mean air temperature of at least 2°C, assuming constant precipitation levels. Since the late 181" century, Lambatungnajökull has been in overall retreat. Moraines dated using two different lichenometric techniques indicate that the most extensive period of glacier expansion during historical time culminated shortly before c. AD 1795, probably in the 1780s. Recession over the last 200 years has been interrupted by re-advances in the 1850s, 1870s, and c. AD 1890. In the 20`h century, most notably in the 1930s and 1940s, Lambatungnajökull receded more rapidly than at any time during the previous 150 years. However, brief cold spells (-5 yrs), centred around the years AD 1918 and AD 1964, temporarily halted glacier recession. Lambatungnajökull has only retreated slightly over the last 20 years. The degree and nature of glacier retreat since 1930 compares well with similar-sized glaciers in southern Iceland. Furthermore, the pattern of glacier fluctuations over the last 150 years reflects the temperature oscillations recorded at nearby meteorological stations. Much of the climatic variation experienced in southern Iceland, and the glacier fluctuations that result, can be explained by secular changes in the North Atlantic Oscillation. A shift to more zonal atmospheric circulation and a weaker Icelandic Low - resulting in a greater frequency of negative NAO anomalies - may have been responsible for the cooling and associated glacier advances of the 18`h and 190' centuries. One implication of this work relates to the exact timing of the Little Ice Age in the Northeast Atlantic. The advanced position of glaciers during the late 18`" century suggests that this period represented the culmination of the Little Ice Age in Iceland. This contrasts with the current consensus that the Little Ice Age 'glacier maximum' in southern Iceland was during the late 19`h century. Other implications concern lichen-dating and its wider applications. Firstly, this research shows that the 'growth' curve of yellow-green Rhizocarpon lichens over the last 270 years in southeast Iceland is not linear. Although growth rates appear constant for periods of several decades, the growth 'curve' is exponential overall, with larger (older) lichens apparently growing more slowly than smaller lichens. Secondly, growth rates of Rhizocarpon Section Rhizocarpon in Iceland vary as a function of climate, with growth in the moist, maritime, southeast being c. 40% faster than in the cooler and drier northwest. Thirdly, this growth rate relationship - across the Northeast Atlantic region as a whole - can best be expressed in terms of climatic 'oceanicity' (r2 = 0.95). This latter relationship could be used to estimate lichen growth rates in areas where dating curves cannot be constructed. Finally, these findings suggest that lichen growth rates are likely to have varied in response to climatic change. In Iceland, slow-growing lichens, such as Rhizocarpon, probably grow more rapidly now - since the climatic amelioration of the 1920s and 30s - than they did in the cooler and drier periods of the 18's and 19'" centuries
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