No description available.

This thesis documents and analyses the extinction and origination patterns of acritarchs and prasinophyte algae at the Llandovery/Wenlock boundary in the Lower Silurian on the island of Gotland, Sweden. Closely spaced samples were collected from two sections: Lusklint 1 and Lickershamn 2, spanning the upper part of the Lower Visby Beds and almost all of the Upper Visby Beds. Errors associated with the palynological processing technique have been assessed and a new photographic technique has been developed. Five new species are described. At least eight extinctions affecting the conodont record have been reported at this level (named the Ireviken Extinction Event (Aldridge et al. 1993, Jeppsson 1997)). The Ireviken Event has been interpreted as an example of the change from a P to an S climate state (Jeppsson 1993). The data show a significant turnover in the phytoplankton at this time, with most of the extinctions at the end of the event (86.3% in the top four metres of the Lusklint 1 section), after many of the conodont extinctions had already taken place. The originations are more numerous than the extinctions (54 species originate at Lusklint 1 as opposed to 44 that became extinct) and they are distributed through the whole of the Ireviken event. There is an uneven distribution across the event with more originations in the Lower Visby Beds forming a convex pattern. The two section analysed were compared using graphic correlation, but palynomorph range end data show considerable scatter. Peaks in the number of palynomorphs per gram of sediment suggest that the two sections completely overlap. Deposition of the thickest bentonite produced a marked drop in the number of palynomorphs per gram of sediment, but a marked rise in numbers in the following few centimetres, probably caused by a fertilization effect. d13C values at Lusklint 1 remain stable in the Lower Visby Beds, but show a marked gradual rise in the Upper Visby Beds. The d18O values for the same interval vary little. The P and S model does not successfully explain all the changes recorded, but is the most comprehensive model available at this time. Additions to the model, incorporating planktonic dimenthyl sulphide production and iron fertilization are presented.

Controversy surrounds the Quaternary palaeoenvironmental history of Amazonia. It is unclear whether moist evergreen forest, savannah or seasonally dry forest dominated the Amazon basin at the last glacial maximum (c. 21,000 years before present). The main source of information on past vegetation change in Neotropics are fossil pollen records collected from within ancient sediments. In part the controversy surrounding the palaeoenvironmental history of Amazonia stems from a poor understanding of these fossil pollen records. In order to improve interpretations of the fossil pollen record it is essential to better understand the nature of the pollen rain produced by modern ecosystems. Once the characterise pollen spectrum from an ecosystem has been established, it can be sought in the fossil pollen record. However, few such modern pollen-vegetation studies exist from the Neotropics and no one covers all the ecosystems that have controversial palaeoenvironmental histories. In this thesis, the modern pollen rain from the three Neotropical ecosystems with controversial palaeoenvironmental histories are characterised. This is achieved by studying modern pollen rain from the Noel Kempff Mercado National Park area, Bolivia. to identify the characteristic pollen from these ecosystems four factors are considered: i) the spatial variation in the pollen rain, ii) the temporal variations in the pollen rain, iii) the statistically significant pollen types, and iv) the pollen-vegetation relationship. These analyses have revealed a small number of taxa that can be used to characterize these ecosystems. Further analyses have shown that the relative proportions of Didymopanax, Alchornea, Anadenanthera, Melastomataceae/Combretaceae, Moraceae/Urticaceae, Myrtaceae, Palmae, Pteropsidia (trilete), Poaceae and Solanum can be used to differentiate between these ecosystems. These findings have implications for the competing theories of Neotropical vegetation change. In the light of this research a comprehensive reassessment of the fossil pollen records from Amazonia is required.

No description available.

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 aresuggested 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

No description available.

No description available.

No description available.

No description available.

No description available.