The transition from the warm, “greenhouse” conditions of the Pliocene to the cold, “icehouse” conditions of the Pleistocene marks a significant development in climate history. The deep-ocean is the largest dynamic reservoir of heat and carbon dioxide in the climate system that is accessible on timescales of Plio-Pleistocene climate change. Therefore, changes in the state of the deep-ocean may have played an important role in large scale Plio-Pleistocene climate change via variability in the meridional overturning circulation of the Atlantic (AMOC). In this thesis, paleoceanographic reconstructions of Plio-Pleistocene Atlantic deep-water circulation are presented from the perspective of Ocean Drilling Program Sites 1264 (2505m depth) & 1267 (4350m depth), situated at ~30oS in the Southeast Atlantic. Reconstructions are based on high-resolution (~<5,000 year time-step), down-core measurements of oxygen (δ18O) and carbon (δ13C) stable isotope ratios in benthic foraminifera. During the Pliocene, widespread high δ13C values in the Atlantic and at Sites 1264 & 1267 indicate low nutrient conditions and active deep-water renewal. The early Pliocene closure of the Central American Seaway (CAS) (~4.7-4.2 Ma) is considered to have been influential in establishing strong deep-water formation in the North Atlantic. Evaluation of δ13O and δ13C records from Site 1264 and throughout the North Atlantic, however, indicate that the CAS closure event had only a limited impact outside of the Caribbean Basin. Meanwhile, during the interval ~3.6-2.7 Ma, δ13C-gradients between Sites 1264-1267 are near zero and suggest strong North Atlantic Deep Water (NADW) prevalence in the Southeast Atlantic, similar to or stronger than the modern situation. The transition into Pleistocene style glacial-interglacial cycles at ~2.7 Ma is associated with a reduction of NADW prevalence in the Atlantic, particularly during glacials and at depth. At ~2.4 Ma, δ18O and δ13C records from Sites 1264 & 1267 reveal marked changes in deep-water circulation. Large (>0.5‰) δ18O-gradients emerge, with heaviest values seen at Sites 1264 & 1267 compared to records from the North Atlantic. At the same time, δ13C values increase at Sites 1264 & 1267. The combination of high δ18O and δ13C values at Sites 1264 & 1267 is consistent with enhanced export of a dense component of NADW that enters the Atlantic from the Nordic Seas by spilling over the Iceland-Scotland Ridge. Comparisons with other North Atlantic records suggest that the pathway of Iceland-Scotland Overflow Water (ISOW) was restricted, flowing along the abyssal East Atlantic and piling up at Walvis Ridge. Between ~2.0-1.5 Ma, maximum δ13C values and minimum δ13Cgradients within the North Atlantic and between the North Atlantic and Sites 1264 & 1267 indicate that the overall export of NADW was strongest for the Pleistocene. After ~1.5 Ma, Atlantic δ18O-gradients begin to reduce, along with δ13C values, although δ13C-gradients still imply strong NADW export. Starting at ~1.3 Ma and across the Mid Pleistocene Transition (MPT), Atlantic δ18O-gradients reduced markedly, as did North Atlantic-Pacific δ18O-gradients but to a lesser degree. After ~0.9 Ma, glacial reductions in NADW presence at depth are the most severe of the entire Plio-Pleistocene, while interglacial export of NADW into the Atlantic remained almost as high as pre MPT conditions. Changes in the strength of AMOC during the Plio-Pleistocene are inferred through comparisons of Atlantic deep-water history with records of sea surface temperature from the high latitude North Atlantic, South Atlantic and North Pacific. I propose that AMOC played an important role in the evolution of Pleistocene climate. Enhanced northward heat transport, due to an increase in the strength of AMOC at ~2.4 Ma, limited the growth of continental ice sheets and sea ice within the North Atlantic region. This may have been caused by increased equator-to-pole thermal gradients and decreased atmospheric moisture transport, increasing salinity, as the global climate cooled. A strengthening of AMOC at ~2.4 Ma is paralleled by significant deep-water changes recorded at Sites 1264 & 1267, implicating enhanced ISOW export into the Southeast Atlantic as an important component of AMOC at this time. A maximum in AMOC occurred between ~2.0-1.5 Ma, along with warming in the mid latitude North Atlantic. Scavenged heat from the South Atlantic promoted enhanced cooling of the Southern Hemisphere and the expansion of sea ice at ~1.5 Ma. Feedbacks originating in the Southern Ocean then acted to cool the globe and eventually pre-condition the climate for the MPT.

In order to better understand the interactions between climate change, landscape erosion and sedimentation, a detailed study was conducted on Plio-Pleistocene non-marine deposits of the Palm Spring Group in the Fish Creek-Vallecito basin, California, USA. Three inter-related studies focused on (1) local response to global climate change in late Pliocene-early Pleistocene time, (2) large-scale evolution of lithofacies architecture, and (3) climate modulation of late Pliocene sediment flux on Milankovitch time scales. Stable isotopes and paleosol classification reveal that between ~4.0 and 0.75 Ma, aridity increased in the study area concurrent with a shift towards a less intense and more winter-dominated precipitation regime. These changes are interpreted to reflect the long-term waning of summer monsoon precipitation in southern California. A dramatic and enigmatic reorganization of basin strata occurred at 2.9 Ma. Detailed basin analysis shows that locally-derived sediment was supplied by the predecessors of two modern drainages, Vallecito and Carrizo creeks. Initial progradation of alluvial deposits from these two sources across the Colorado River delta plain began between 4.0-3.4 Ma. At 2.9 Ma, rapid progradation of these two deposystems was coeval with emplacement of a megabreccia and transgression of Borrego Lake. My data indicate that tectonic realignments at both local and regional scales drove this reorganization. Time series analysis of rock magnetic data from a densely-sampled stratigraphic section of the lacustrine Tapiado Formation reveals that between 2.9 and ~2.75 Ma landscape denudation in the Carrizo Creek catchment was partly modulated by orbital obliquity. Peaks in landscape denudation implied by my data correspond to obliquity highs. More frequent high intensity precipitation events (i.e. monsoons and tropical storms) probably drove increased erosion during these time periods relative to obliquity lows. The breakdown of this relationship at around 2.75 Ma corresponds to a dramatic increase in northern hemisphere glaciation and may reveal a reduction in monsoonal influence in southern California. A geologic map of the Fish Creek-Vallecito basin is included as a supplemental file to this dissertation. This dissertation contains previously published and unpublished coauthored material.

Thesis (M.Sc.)--Memorial University of Newfoundland, 2004. / Bibliography: leaves 311-318. Also available online.

Thesis (Honors)--College of William and Mary, 2009. / Includes bibliographical references (leaves 70-72). Also available via the World Wide Web.

Mesoamerica is considered a biodiversity hot spot with levels of endemism and species diversity likely underestimated. For mammals, the patterns of diversification of Mesoamerican taxa still are controversial. Reasons for this include the region's complex geologic history, and the relatively recent timing of such geological events. Previous studies, however, support the view that substantial migration between North (NA) and South America (SA) occurred prior or/and during the Great American Biotic Interchange (GABI) ~3.5 Ma. This was followed by repeated periods of isolation during Pleistocene climatic oscillations, which produced most of the diversification in the region. From a North American origin, the subfamily Sigmodontinae migrated to SA, where most of its present day diversity exists. The taxonomic history of this subfamily, and of Oryzomynii, its largest tribe, has been exceptionally complex. Recently, extensive studies have helped to clarify genealogical relationships among major clades, but have left the evolutionary histories of several groups unresolved. Such is the case for the genus Handleyomys that includes nine species; seven of which are endemic to Mesoamerica; and of its phylogenetic position among closely related genera Euryoryzomys, Hylaeamys, Oecomys, Nephelomys and Transandinomys. The results supported the monophyly of Handleyomys, and four clades with inter-generic levels of divergence within the genus, three of these clades restricted to Mesoamerica (the alfaroi, chapmani and melanotis species groups). Furthermore, the estimated time for the split of the Mesoamerican Handleyomys is on average, 2.0 Myr older than the proposed migrations to NA during the GABI. In addition, the position of Handleyomys as the sister clade to Euryoryzomys, Hylaeamys, Oecomys, Nephelomys and Transandinomys was well supported, as it was a biogeographic hypotheses that depicted a polyphyletic origin for these genera and Handleyomys 5.5-6.0 Ma. The integrative approach implemented in this dissertation allowed the development of more biologically realistic hypothesis than has previously been conducted in Mesoamerica, where half of the endemic mammals are listed under the IUCN Red list; and where mammals with small ranges, which are the most vulnerable to extinction, are found largely outside reserves. The continued decline of the ecosystems health in this region calls for a more precise account of its biodiversity for its proper conservation; and for rigorous biogeographic studies for its management, since the region also serves as a biological corridor for intercontinental connectivity.

Thesis (Ph. D.)--State University of New York at Binghamton, Department of Anthropology, 2006. / Includes bibliographical references (leaves 263-281).

No description available.

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Les plantes et les insectes forment l’un des principaux niveaux trophiques des écosystèmes au cours des 325 derniers millions d’années. Aujourd’hui, l’augmentation rapide et continuelle de la température principalement causée par l’activité humaine depuis les derniers siècles, perturbe la balance des écosystèmes sur Terre. En conséquence, comprendre le rôle des interactions entre les plantes et les insectes, à travers le temps mais aussi les réseaux trophiques, est essentiel. Le registre fossile est une opportunité exceptionnelle d’examiner les réponses aux interactions plante-insecte lors de longues variations climatiques et à travers des traces de réaction de la plante sachant que la Terre a déjà été soumise à de nombreux changements climatiques. Durant les derniers 3 millions d’années, des oscillations entre de longues périodes froides et de courtes périodes chaudes ont eu lieu. Les écosystèmes Européens ont particulièrement été impactés par ces oscillations. Le Langerstätte de Willershausen (Allemagne) a été particulièrement étudié. C’est un gisement contenant plus de 8000 feuilles fossiles. Ces feuilles relatent d’une paléoforêt ayant existé il y a 3- 2,6 Ma dans un climat plus chaud qu’aujourd’hui (ca.+5°C). Dans ces conditions climatiques, de nombreuses espèces d’écosystèmes Méditerranéens étaient présentes, telles que l’Érable de Montpellier ou l’Olivier. En comparaison, d’autres paléoforêts ont été prise en compte : Berga (du même âge et proche de Willershausen) et Bernasso (plus jeune que Willershausen (2,16 — 1,96 Ma) localisée dans le sud de la France près de la Méditerranée. Ces forêts sont comparables notamment du fait des nombreux taxons communs qu’elles partagent. En outre, certaines de ces espèces sont aujourd’hui endémiques de la région du Caucase, telles que le Parrotie de Perse ou encore l’orme du Caucase. Le but de cette étude a été de déterminer en quoi les différences climatiques peuvent être impliquées dans les changements des interactions plante-insecte au sein des paléoforêts Européennes de la fin du Pliocène - début du Pléistocène. Les résultats obtenus ont permis de mettre en évidence les impacts de la saisonnalité des températures et précipitations facteurs impactants les interactions plante-insecte des paléoforêts Européennes. Il est apparu que les écosystèmes sujets à d’intenses saisonnalités hydriques ont pu engendrer une plus grande spécialisation des interactions plante-insecte déduite d’un fort taux d’interactions spécialistes observées. En parallèle, les températures les plus froides durant l’année semble être un facteur important dans la faible diversité de dégâts, probablement dû à un faible métabolisme de la majorité des insectes. L’absence de corrélation convergente entre la richesse des plantes et la richesse des interactions pourrait suggérer que l’influence des facteurs climatiques surpasse l’impact potentiel des interactions biotiques locales. Pour l’ensemble de ces paramètres qui ont pu avoir un impact sur les interactions plante-insecte, nos connaissances actuelles sont encore insuffisantes. Il serait intéressant de focaliser davantage d’études sur les forêts modernes avec les méthodes appliquées dans le fossile. C’est dans cette intention qu’une partie de cette étude a étudié une espèce de plante (Parrotia persica) actuellement endémique de la forêt Hyrcanienne (Iran). Cette forêt est supposée être une forêt analogue des paléoforêts Européennes étudiées dans cette thèse. Pour le moment, les observations qui ont été faites en Iran semblent corroborer notre interprétation. Au final, les études sur les interactions plante-insecte des forêts anciennes et actuelles, combinés avec les études de changements climatiques, pourraient nous permettre de mieux caractériser les relations entre les insectes et les plantes au sein d’une forêt. / Insects are the most diverse animals on Earth, and neatly associated with plants they represent two of the major groups of organisms both in species diversity and biomass quantity. The majority of their interactions involves insect feeding and insect parasitism mostly on leaves. Plant and insect compose one of the main trophic levels in ecosystems over the 325 million years. Today, the continuous and fast rising of temperature mostly due to human activities since the last century is disturbing the balance of ecosystems on Earth. Consequently, to understand the role of plant and insect interactions, through time but also trophic networks, becomes crucial. The fossil record is an exceptional opportunity to survey responses of plant-insect interaction to climate variations over long time interval through traces of plants reactions caused by interaction with insects, as Earth has already experienced many climate changes. For the last 3 million years, oscillations between long cold periods and short warm periods have occurred. Europe ecosystems has been particularly impacted. The Lagerstätte of Willershausen (Germany) was specifically study. It is an exceptional fossil outcrop that contains ca. 8000 collected fossil leaves. These leaves testify a paleoforest developed there around 3—2.6 Ma ago in a climate warmer than today (ca. +5°C). Under these conditions, many plant species typical of the Mediterranean ecosystems were settled there, such as Montpellier maple or Olive tree. For comparison, other paleoforests were studied: Berga (similar in age and geographically close to Willershausen) and Bernasso (younger than Willershausen (2.16—1.96 Ma) and located in southern France close to Mediterranean. These forests were compared as many common plant taxa were similar between each other. Furthermore, some species today endemic to the Caucasian region, such as Persian ironwood or Caucasian elm, were also present in these outcrops. The aim of this study is to determine how far the climate differences could be involved in the changes of plant-insect interactions in European paleoforests of the late Pliocene – early Pleistocene. Results highlighted the impacts of both hydric and temperature seasonality, hitherto underestimated in the fossil record, on the patterns of plant-insect interaction in the European paleoforests. It appeared that ecosystems subject to intense hydric seasonality could led to higher specialization of plant-insect interaction inferred by higher rate of observed damages due to ‘specialists insects’. In parallel, the coolest temperature during the year seems to be a major factor in the low diversity of damage in paleoforest, presumably due to lower insect metabolism. Absence of convergent correlations between plant richness and damage richness could suggested that influence of climatic factors override impact of these local biotic factors. In order to understand the whole parameters that could have an impact on plant-insect interactions, our current knowledges are still insufficient. It would be wise to make more investigations on modern forests with the methods as applied in fossil record community structure studies. These investigations could help to understand the factors potentially involved in the establishment of a pattern of plant-insect interactions. It is in this perspective that a part of this study was precisely focused on one plant species (Parrotia persica) currently endemic to the Hyrcanian forest (Iran). This forest is supposed to be an analogue forest of the European paleoforests as those studied in this thesis. For now, observations made in Iran tend to corroborate our interpretation. Finally, the studies on plant-insect interactions in past and extant ecosystems, combined with the study of climatic changes, should permit us to better characterize the relations between plants and insects in forests through time.