Late Quaternary palaeoclimatic reconstructions in Patagonia using chironomid analysis

This thesis investigates Late Quaternary palaeoclimate in Patagonia to increase our understanding of the nature and timing of climate change in the mid-latitude regions of the Southern Hemisphere. Palaeoenvironmental reconstructions are presented which date back to the Last Glacial Interglacial Transition (> c. 13.5 cal ka BP) from two lakes in Chilean Patagonia: Laguna Boal in the Chonos Archipelago (44° 39'S, 73° 39'W) and Laguna Leta in the Chilean Lake District (41° 33'S, 73° 10'W). The investigation focuses principally on the analysis of sub-fossil Chironomidae (Insecta: Diptera) to infer palaeoenvironmental conditions. Combined 14C dating and tephrochronology provide the chronological framework for the records. As ecological information about the modem Patagonian Chironomidae fauna is scarce, faunal assemblages at 78 lakes were analysed in conjunction with potentially controlling environmental variables in order to aid the ecological interpretation of sub-fossil assemblages. Analysis of the modem data raised taxonomic issues related to identification of sub-fossil larval specimens, highlighting the need for more Neotropical taxonomic studies. Multivariate analysis of relationships between the modern day fauna and environmental variables indicated that the relationships present were not strong enough to produce a transfer function for the region. The environmental factors that varied most with the taxonomic assemblages were the organic content of substrate, lake depth, altitude, latitude and longitude. Water temperature, which has previously been demonstrated to have a strong effect on Chironomidae distribution, was not significantly correlated with the Patagonian faunal assemblages. These results indicate that the fauna encountered in this study may be more eurytopic than that studied in previous Holarctic investigations. Although the strength of environmental taxonomic relationships was insufficient to warrant production of a transfer function, the investigation allowed qualitative interpretations of the down-core record to be made in conjunction with previously published data on Holarctic midge ecology. The records from Laguna Leta and Laguna Boal both indicate that climate oscillated during the transition from glacial to interglacial conditions, as fluctuations in lake depths are Late Quaternary Palneoclituntic Reconstructions in Patagonia using Clrironornid Analysis suggested by changes in both the chironomid assemblages and sediment properties. Results are generally equivocal in terms of determining whether such changes were precipitation or temperature driven. Nonetheless, such inferred changes add valuable evidence to the assertion that climatic instability prevailed throughout this time period in Patagonia an argument that has been refuted by previous palynological work. Climatic interpretation of the Laguna Boal Holocene record is somewhat equivocal, but indicates that local, basin specific changes may have determined the assemblage versus palaeoclimatic controls. However, at a Laguna Leta a drop in lake levels during the early Holocene may have been followed by a notable rise in water level at c. 7.8 cal yr BP. This latter record supports previous palynological work in the region indicative of a concomitant rise in effective precipitation. The results of this pioneering study suggest that the use of chironomid analysis as a palaeoclimatic technique for this area is promising, but that more information on the midges' modern distribution in Patagonia is required to improve its application in future studies. Late Quaternary Palaeoclimatic Reconstructions in Patagonia using Chirooronoid Analysis