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Multi-proxy sediment records from four lakes in the Venezuelan Andes document changes in tropical climate over timescales of decades to millennia. The results are grouped into three topics: the Little Ice Age, the Holocene climate history, and atmospheric moisture balance of South America from oxygen isotopes. A 1500-year reconstruction of climate history and glaciation indicates four glacial advances occurred between 1250 and 1810 A.D. These advances are coincident with solar activity minima. Temperature declines of 2.3 to 3.4° C and precipitation increases of 25 to 70% are required to produce the observed glacial responses. These results highlight the sensitivity of high-altitude tropical regions to relatively small changes in radiative forcing, implying even greater responses to future anthropogenic forcing. On longer timescales, the Venezuelan Andes were generally wetter during the early Holocene. The middle Holocene was a time of low lake levels and reduced moisture balance whereas the late Holocene was wetter, with the wettest period occurring during the Little Ice Age. The pattern of millennial climate variability in Venezuela appears to be either a wet-dry-wet (Andes) or dry-wet-dry (lowlands) sequence. Comparison with climate records from North, Central and South America suggests this pattern is widespread near the northern and southern edges of the tropical monsoon climate regime and along the Andes near the equatorial Pacific. The isotopic composition of Andean precipitation reflects evaporation conditions over the Atlantic Ocean, moisture recycling over the South American lowlands and uplift to the Andes. The isotopic composition of precipitation in the Venezuelan Andes, reconstructed from lake sediment diatom oxygen isotope records, show a 2.4 ‰ decrease during the Holocene. This decrease reflects a reduction in the moisture entering South America which reaches the Andes. Ice cores from Peru and Bolivia exhibit similar isotopic trends. Direct orbital changes in solar insolation cannot explain the synchronous trends in both climatic and isotopic histories throughout the neotropics. However, sea surface temperature variation in the tropical Pacific may explain these trends because modern interannual variability in this region has similar effects in both hemispheres.

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The Ediacaran Period (635 – 541 Ma) is a critical transition in the history of the Earth and life. It is marked by the appearance of enigmatic Ediacara-like fossils and macroalgae, which had existed since the early Proterozoic, but started to diversify morphologically and ecologically during this geological period. Nevertheless, paleontologists have been studying Ediacaran fossils for more than a century, the key questions about these fossils remain unanswered, including their phylogenetic affinities, taphonomic history, ecology, and paleoenvironmental distribution. Thus, new ways of investigation need to be employed to unveil enigmas of Ediacaran organisms. As well as, scientists need to engage other representatives of Ediacaran assemblages, such as macroalgae, to fully understand how those communities operated in the past. The chapters of this original research in this dissertation demonstrate innovative approaches and methods for studying the paleoecology and taphonomy of Ediacaran macrofossils. The second chapter presents the results of a geochemical analysis to resolve taphonomic and paleoecological questions about the Ediacara-like discoidal holdfast fossil Aspidella. Stable isotope data of organic carbon, carbonate carbon/oxygen, and pyrite sulfur were integrated with iron speciation data to reconstruct the taphonomy, paleoecology, and paleoenvironments of Aspidella fossils. The third chapter presents results from a comprehensive analysis of macroalgae from Proterozoic to early Paleozoic. In this study, a database of macroalgal fossil was updated and analyzed in order to gain insights into the big-picture evolutionary patterns of macroalgal morphology and ecology through time. These methods and approaches offer new opportunities to test major paleobiologic and geobiologic hypotheses, thus improving our understanding of the history of the biosphere and the Earth system. / Ph. D.

Ph.D., Faculty of Science, University of the Witwatersrand, 2011 / There is ongoing speculation about how an increasingly arid environment contributed to the extinction of Paranthropus robustus, given that a mosaic landscape with a major part of the area consisting of predominantly open grassland environment accompanied by an escalating cooler drier climate remains the persistent palaeoecological reconstruction for this species. It has been suggested that P. robustus, a dietary specialist, was not able to adapt to an increasingly xeric habitat. This notion has been challenged by recent multi-disciplinary research on P. robustus remains, including stable light isotope and dental microwear analyses, which portray a more complex diet. Paranthropus robustus is present in a number of key fossil assemblages spanning the period ca. 1.8 to 1.0 Ma. Analysis of the stable carbon isotope composition of bioapatites and dental microwear texture analysis of different bovid taxa, associated with P. robustus remains from five discrete deposits, were used to reconstruct dietary behaviour and by inference availability of local resources. The overall pattern emerging from the bovid data indicates a more mixed and varied diet than previously thought, suggesting a heterogeneous environment, and hence a less static ecological profile for Paranthropus. The significant occurrence of mixed diets and relatively few obligate C4 grazers suggest that although C4 grasses were available in a mosaic environment, a C4-dominated ecosystem was not present. Swartkrans Member 2 (ca. 1.6 Ma) contains substantially more C3 feeders than other P. robustus deposits, signifying a vegetation community structure that was more C3-dominated than the other deposits. There is an apparent indication of shifting vegetation structure between P. robustus deposits. Thus, despite its derived craniodental morphology, P. robustus seems to have thrived through a range of climatic and ecological shifts by selecting from a variety of available foods present on the landscape.

Thesis (Ph. D.)--University of Washington, 1994. / Vita. Includes bibliographical references (leaves).

Thesis (Ph. D.)--University of Chicago, Dept. of the Geophysical Sciences, June 2003. / Includes bibliographical references. Also available on the Internet.

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