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.

Cricetidae, which includes a wide range of mice, rats, and voles, is the second-most diverse and abundant family of mammals. Distributed across the Americas and Eurasia, this rodent family has been extensively studied for its success and adaptability in the face of climate and habitat change, and its utility for ecological reconstruction and the biochronology. While cricetids are known from many fossil sites in western North America, few have been found and studied from the terrestrial sites in the east prior to the Pleistocene. The Gray Fossil Site (GFS) in eastern Tennessee is well-known for its diverse flora and fauna, particularly large mammals. Small mammals from the site have remained understudied, and although a few studies have mentioned cricetids, a comprehensive investigation is needed to describe the large number of cricetid specimens found at GFS. This project uses qualitative and quantitative methods to examine the morphology of fossil cricetids from GFS, focused on molars and jaw elements because of their abundance and the rich taxonomic and ecological information they provide. Materials have been studied using a stereomicroscope and photographed using a digital microscope camera. Occlusal morphology is described and compared to modern cricetids and published records from across North America. Specimens from GFS include at least 5 taxa, and the two most abundant taxa at the site are the woodrat Neotoma, which has high-crowned molars and still lives in the region today, and the deer mouse-like Postcopemys, which has low-crowned molars. While many GFS specimens can be categorized into these two genera, distinctive morphological variations seen in some individuals suggest the presence of new species. The array of cricetids present is distinct from those at contemporaneous sites and reinforces the spatial and temporal uniqueness of the Gray Fossil Site. Situated in the biodiversity hotspot in the Appalachian region today, GFS provided habitats for cricetid taxa with diverse body sizes and dietary preferences in the Pliocene.

The high sensitivity of high latitudes to global climate changes is the stimulus for the study of ancient Arctic ecosystems under greenhouse conditions. With an increasing number of studies, including the most recent Intergovernmental Panel on Climate Change (IPCC) report highlighting Pliocene climate as key example for the study of Earth system sensitivity to higher levels of atmospheric CO₂, the need for accurate proxy records for this period is crucial. In order to investigate Pliocene climate, I used stable isotopic studies of fossil molluscs, moss and wood from two fossil forest deposits in the Canadian High Arctic. Temperatures were determined for an Early Pliocene (4-5 Ma) fossil forest site located on Ellesmere Island using 'clumped' and stable isotopic analysis of mollusc shells and stable isotope values of fossil wood. Mollusc inferred growing season (May-Sept) temperatures derived using two independent techniques were estimated to be 11-16° C warmer than present (1950-1990) Ellesmere Island temperatures. Tree ring inferred growing season (June-July) temperatures (JJ) were 10-16° C and mean annual temperatures (MAT) were 18-20° C warmer than present (1950-1990). Mean annual and growing season (JJ) temperatures were also determined using fossil wood from a younger (2.4-2.8 Ma) late Pliocene-early Pleistocene site on Bylot Island. This deposit represents the remains of a flora that grew during an interglacial warm period during the transition to large-scale Northern Hemisphere glaciation that occurred between 2.5 to 3 million years ago. Mean annual temperatures were ~12° C and growing season temperatures were ~13° C warmer than present (1923-2010). The interglacial setting of the Bylot Island site and the warm temperatures suggests that prior to using such sites as true analogues of future conditions we may need to consider how close the feedbacks operating then were to the feedbacks we might expect in the future. However, that temperatures so much warmer than present existed in the high Arctic during a period when levels of atmospheric CO₂ were at nearpresent levels indicates that we may be moving beyond our ability to use the Pliocene as an example of the future.

Abstract The primary objective for this study is to determine the variations of germanium (Ge) to silicon (Si) ratios in diatom shells from down core sediments off the southwestern coast of the Africa. Because Ge behaves like Si in the ocean and the major source of these two elements is rivers, Ge/Si ratios of the diatom are thus expected to record the intensity of weathering on continents. Two sediment cores (Site 1081 and 1084) were collected during the ODP Leg 175 in August, October 1997 and were analyzed for intervals between 3.1 and 1.9 Ma. Generally, the (Ge/Si)diatom ratios in this study show the similar trend with those reported from the Antarctic Ocean. The decline of Ge/Si ratios after 2.5 Ma is caused by the high opal mass accumulation rate, indicating the increasing rivers input of silica to the ocean. Relatively high Ge/Si ratios of diatom shells during warm periods shown by the depleted benthic foraminiferal £_18O values from the North and Equatorial Atlantic, suggesting a scenario of intensified chemical weathering on continent. Cold periods signified by £_18O-enriched values are associated with lower Ge/Si ratios. Very likely the reduced Ge/Si values reflect the prevailing weathering on continents was less chemical-dissolving due to the dry weather in cold stages.

Various methods exist for measuring and analyzing dental wear patterns in mam-mals, and these patterns have been extensively studied in ungulates. Mesowear has proven useful as a method to compare large numbers of individuals, particularly fossil individuals, observe trends through time or between groups, and estimate paleoenvi-ronmental conditions. Levels of attrition (tooth-on-tooth wear) and abrasion (tooth-on-food wear) can be readily compared by observing the shape of the cusp and relative crown height of the tooth. This study uses a modified method of mesowear analysis, examining actual cusp angles of the population of Tapirus polkensis from the Gray Fossil Site, a densely canopied, hickory and oak dominated forest located in Gray, Tennes-see. Crown height and cusp angle were measured for 38 specimens arranged into eruption series from young juveniles to old adults. Results found a strong correlation between eruption series and cusp angle with a steady increase in mean angle as the individuals increase in age. A strong correlation between cusp angle and crown height was also found. Overall, the population showed relatively low wear rates, as would be expected of a forest-dwelling browser. As a mesowear analysis across all age groups for a population has not been conducted before, this study could be useful for measuring relative wear rates at different life stages and could be applied across other com-munities.

Yunnan, southwestern China, represents a modern biodiversity center for Rubus (Rosaceae). The history for this high modern diversity remains poorly known due to the lack of fossil evidence. In this report, fossil pyrenes of Rubus are taxonomically studied from the late Pliocene (Piacenzian) of Lanping County, northwestern Yunnan. These pyrenes show a greater morphological variation than that of extant Rubus pyrenes within the same species, indicating that they belong to different taxa of Rubus. Based on comparisons with both modern and other fossil species, our fossil pyrenes are assigned to five taxa, including a newly established one, Rubus lanpingensis nov. sp. These fossils suggest a somewhat high species diversity of Rubus in Lanping, a small area in northwestern Yunnan, during the late Pliocene. This provides the first fossil perspective for an understanding of the historical background of the modern Rubus diversity in a limited geographic area of Yunnan. The inferred palaeobiodiversity is probably associated with a large environmental heterogeneity in a limited area of Yunnan at that time.

Cricetidae, which includes a wide range of mice and rats, is the second-most diverse and abundant family of mammals. Though presently distributed across the Americas and Eurasia and also known from many fossil sites in western North America, few records have been studied from the east prior to the Pleistocene. This study has identified seven species of cricetids from the Gray Fossil Site (GFS), an early Pliocene site located in the northeast corner of Tennessee well-known for preserving fossils of many large mammals. While a few studies had previously noted the occurrence of cricetids at GFS, this study provides a detailed description of the cricetid assemblage and helps expand our understanding of small mammals from this site. Specimens were examined under stereomicroscope and DinoLite digital microscope camera, allowing qualitative assessment of occlusal morphology and quantitative comparison to modern cricetids and published fossil records from across North America. The most common taxa at the site included the deer mouse Peromyscus, which is currently the most abundant and widely distributed mammal in North America, deer mouse-like Postcopemys, and the woodrat Neotoma, which is larger rat with high-crowned molars that still lives in the region today. Other less common ones, like Sigmodon, Repomys, and Neotomodon, are also present. While most of the GFS specimens can be categorized into known genera, distinctive morphological features suggest the presence of new species. This assemblage of cricetids differs from those found at contemporaneous sites, emphasizing the spatial and temporal uniqueness of GFS. The Pliocene cricetid taxa at GFS exhibit diverse body sizes and dietary preferences, providing insights into the paleoecology of the region. As a biodiversity hotspot in the Appalachian region today, GFS was likely an important habitat for cricetid evolution during the Pliocene.

The Pliocene world c.5.3 Ma to c.2.58 Ma exhibited a relatively stable climate with a warmer global mean surface temperature than present-day by ~2 °C to 3 °C, and palaeoclimate analysis from this interval is used to understand climate drivers in ‘warmer world’. Previous oxygen isotope thermometry investigations of Pliocene southern North Sea Basin (SNSB) Aequipecten opercularis from the Coralline Crag Formation in Suffolk, UK repeatedly reveal evidence of a cold-temperate climate regime. Contrastingly, other biological proxies record a warm-temperate/sub-tropical regime. This investigation concentrated on oxygen, carbon and microgrowth increment widths (MIWS) of fossil shell material from Pliocene SNSB spanning an interval of~4.4 Ma to ~2.5 Ma. The study sites included shallow marine Pliocene formations from the western and eastern SNSB, the Ramsholt Member of the Coralline Crag Formation, Suffolk UK, and the Luchtbal Sands and Oorderen Sands Members of the Lillo Formation, Belgium, and the Oosterhout Formation in the Netherlands. Oxygen isotopic palaeotemperature results showed cooler summer temperatures than presently in the SNSB, which were reflective of a cool-temperate regime. There was no evidence of warm-temperate or sub-tropical summer palaeotemperatures in the Pliocene SNSB as suggested by other planktonic proxies. This investigation discussed the possible causal factors for the cooler – than- expected winter and summer palaeotemperatures in the ‘warmer’ Pliocene world as recorded by this proxy. Discrepancies between the cool summer benthic palaeotemperatures from the bivalves and the warmer sub-tropical or warm-temperate summer palaeotemperature estimations from planktonic biological proxies was rectified by the application of a theoretical summer stratification factor (SSF). However, rectifying the discrepancies between cooler (cold-temperate) benthic winter palaeotemperatures and the warmer winter palaeotemperatures from other proxies was difficult because stratification does not occur during the winter. Dormancy behaviours in the warm- temperate –sub-tropical organisms was proposed as a suitable mechanism to allow their coexistence with the cool-tolerant bivalves, which were able to grow and feed underneath the thermocline during the summer months. Therefore, the investigation showed how the Pliocene SNSB exhibited a greater seasonality than occurs presently in the SNSB. The driver for the cooler winter temperatures in the Pliocene SNSB was not identified. Localised explanations including continental wind effects, interannual variations in MOC strength, and increased storm activity in the winter bringing cooler water into the SNSB were all suggested as potential drivers. Global features of climate including interglacial/glacial cycles and orbital forcing effects were factors also proposed for the overall mixed palaeotemperature signal in the Pliocene SNSB.

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.

The Gray Fossil Site (GFS), a Mio-Pliocene (4.5–7 Ma) locality in the southern Appalachians, boasts the most diverse pre-Pleistocene salamander fauna in North America: Desmognathus sp., Plethodon sp., Notophthalmus sp., a Spelerpinae-type plethodontid, and Ambystoma sp. Because greater taxonomic resolution can result in more precise paleobiological interpretations, additional specimens were studied here. ETMNH 8045, a nearly complete articulated ambystomatid, appears most like Ambystoma maculatum in dentition and vertebral proportions. ETMNH 18219, an isolated vomer, is consistent with modern Pseudotriton and Gyrinophilus in possessing a postdentigerous process and a similar dentigerous row morphology. If these taxa, or species of similar ecological preferences, occurred around the GFS, it seems unlikely they co-inhabited the sinkhole lake. Aquatic stages of Pseudotriton and terrestrial Gyrinophilus last multiple years; their presence could further support a perennial lake interpretation. Modern A. maculatum preferentially breed in vernal pools; confirmed identification could suggest local seasonal wetlands.