No description available.

No description available.

No description available.

Climate exerts a profound control on the processes that shape landscapes and produce the sedimentary deposits with which we can interpret the Earth's history. However, we lack a complete understanding of how sensitive tectonically-active, eroding landscapes are to climate and climate change. How does a simple sediment routing system react to a change in rainfall rate? Can mountainous landscapes respond quickly enough to preserve a record of high-frequency climate changes, e.g., glacial-interglacial cycles? What effect does headwater glaciation have on downstream sediment characteristics? Can we quantify past climate changes using the sedimentological properties of terrestrial stratigraphy? Geologists lack complete answers to these questions, among many others. Theoretical work, using physical first principles and numerical models, has produced a range of hypotheses about landscape sensitivity to climate, but we now need empirical data to test and make sense of these ideas. This thesis therefore explores empirically how geomorphological and sedimentological records have responded to climatic gradients across time and space. In the first part of this thesis, the extent to which spatial climate gradients are recorded by the longitudinal geometry of river channels is investigated. I use a simple stream power erosion law to predict an inverse relationship between channel steepness and average precipitation rate, and then test this theory using data from a variety of study areas and two complementary analytical approaches. Climate is found to be an important control on river longitudinal geometry across a range of climatic and tectonic conditions, in a way that conforms to existing theoretical knowledge and also allows the climatic signal to be discriminated from tectonics. This work therefore demonstrates that a widely-used geomorphological measurement - the channel steepness index - is quantifiably sensitive to climate in tectonically-active areas, and these findings offer a new explanation for geographic variations in channel steepness that cannot be explained by tectonics alone. The second part of this thesis focuses on the sensitivity of simple mountain catchment-alluvial fan systems to climate changes associated with the last glacial-interglacial cycle, as expressed in the south-western United States. First, eight debris flow-dominated systems located in the south-eastern Sierra Nevada, California are examined. I establish a detailed chronostratigraphic model for these fan systems by building upon and integrating existing exposure age constraints reported by others, and additionally developing a new technique for estimating the ages of these fan deposits. This technique is based on calibrating the rate of enlargement of common weathering fractures observed in exposed surface boulders, which are shown to widen at a steady and predictable rate post-deposition, and can be used as reliable age indicators for > 100 ka at this location. Using the detailed temporal record of deposition established for these fan systems, a large ( > 30,000 particle) grain size data set that spans the last full glacial-interglacial cycle is examined. I demonstrate that debris flow grain size is a highly sensitive recorder of past climate changes, capturing the glacial-interglacial cycle as a sustained and high-amplitude time series with a rapid response timescale of < 10 ka. These debris flow deposits become significantly coarser-grained with warming and overall drying of the climate, and this thesis outlines quantitative reasons why this signal can be attributed to increasing storm intensity with warming. Finally, these debris flow-dominated systems are contrasted with two carefully-selected stream flow-dominated fan systems in Death Valley, California. Using measures of down-system grain size fining and a self-similarity model of sediment calibre, sediment flux estimates during arid interglacial and wetter glacial climate conditions are derived and compared. This study shows that a decrease in average rainfall rate of ~ 30 % produced a corresponding decrease in sediment flux of ~ 20 %. However, I also demonstrate the circumstances in which signal buffering due to incision and sediment recycling destroys this climate signal. Consequently, this thesis demonstrates both the causes and results of complexity in the relationship between climate change, geomorphology, and well-dated terrestrial sedimentary records. Ultimately, this is an expression of how sediment transport processes, tectonics, the magnitude-frequency distribution of rainfall, and other factors interact to generate different climate responses in different systems. Nevertheless, for both geomorphic and sedimentological records examined here, I demonstrate that the effects of climate can be quantified clearly: channel steepness can be quantified as a function of rainfall rate; debris flow sedimentology can be quantified as a function of storm intensity; and alluvial fan sedimentology in Death Valley can be quantified as a function of glacial-interglacial climate changes. Essentially, this thesis finds that terrestrial landscapes are sensitive to known climate changes in the recent geological past, and this result is profoundly important for improving our ability to decode geomorphic and stratigraphic archives effectively. The data and ideas within this Ph.D. research provide useful opportunities for (i) testing and updating our models of how sediment routing systems respond to climate, (ii) extracting quantitative information about past climates from the sedimentary record, and (iii) predicting the effects of future climate changes on the landscape.

Despite the increasingly frequent use of oxygen isotopes in diatom silica as a proxy for past environmental change, the processes which control these isotope ratios remain poorly understood. Various factors affect the oxygen isotope composition of sedimentary diatom silica, beginning with the atmospheric changes that determine the isotope composition of precipitation through the controls over assimilation of silica within the diatom cell to the processes affecting silica sedimentation and diagenesis. An understanding of these contemporary processes is an important prerequisite for the use of lake sediment records as archives of past climate change. The aim of this thesis was to investigate the cascade of factors which determine the oxygen isotope composition of sedimentary diatom silica, using a case study of Lochnagar, a remote upland lake in Scotland. The climate-lake interface at Lochnagar was studied using bi-weekly monitoring of the oxygen isotope composition of precipitation (6180p) and lake water (6180L) (between December 1999 and September 2005) and monthly monitoring of the cell density and oxygen isotope composition of living diatoms (5180Siiica) (between May 2004 and November 2005, excluding periods of lake ice cover). Three supplementary datasets of monthly 5180p from sites in Great Britain and Eire (Wallingford, Oxfordshire Keyworth, Nottinghamshire and Valentia, Kerry) were also analysed to obtain a wider geographic perspective. 5180Siiica was analysed from the surface sediments of a series of lakes across Europe in order to assess the relationship between climate and 5180Sinca over a large continental gradient. The research demonstrates considerable complexity in the climate-lake-sediment interface at Lochnagar. 6180p is a sensitive climate tracer, reflecting changes in air temperature and precipitation amount, however the response varies with site location. Changes in S18Ol at Lochnagar reflected changes in 6180p, but the response was mediated by limnological processes, primarily the water throughflow rate, which is a function of precipitation amount. Diatom abundance varied markedly over the annual cycle, with peak abundance during Spring, leading to a probable seasonal bias in the sediment 6180Siiica record. 6180SiiiCa values also fluctuated markedly throughout the seasonal cycle, although no systematic relationship with potential physical controls was observed. However, 5180SiiiCa values in the surface sediment dataset do correlate with the continental climate gradient, suggesting that diagenetic effects play an important role in fixing the climate signal. The thesis concludes that potential does exist for the use of 5180SitiCa in palaeoclimate research, despite considerable complexity within the sediment-climate interface. The processes which combine to determine the sedimentary 5180Siiica signal are combined in a basic holistic model to enable a more rigorous approach to interpreting the 5180Siiica record from Lochnagar. However, further detailed research into controls over isotope fractionation during silica assimilation by diatoms, and subsequent diagenesis, is required if 5180Stiica records are to be confidently used to reconstruct past climate.

This thesis provides a basin-level analysis of climate shocks and conflict links, utilising livelihoods and vulnerability toolboxes, including a newly assembled conflict dataset that captures communal, rebel and water conflicts in four Lake Chad Basin (LCB) zones. The thesis draws on multi-method approaches to assess: (i) the manner in which lake drying shapes livelihood drawbacks and opportunities, (ii) the directionality of occupation-based vulnerability to double exposures, (iii) climate conflict interactions in the context of contextual vulnerability and lake drying, and (iv) adaptation-water-conflict integration need for the LCB. Key findings reveal that: (i) asset holdings from unstable water-based activities are a medium through which drying influences livelihoods, (ii) pastoralists are more vulnerable to double exposures because they have limited social networks and income strategies, (iii) rainfall anomalies have dampening effects on conflict and lake drying does not represent a sufficient mediator for climate conflict links. Effects of rainfall anomalies on conflict are more pronounced in the presence of political exclusion in the Chad and Nigeria zones which occupy large areas of the LCB, (iv) policy initiatives increasingly acknowledge the need to preserve the Lake waters, yet initiatives that explicitly integrate adaptation, water and conflict concerns are only beginning to emerge. Two new documents indicating integration have been developed between 2015 and 2016. This thesis is the first to develop a new set of integrated vulnerability tools for use in framing climate conflict vulnerabilities in water scarce environments. It provides a piece of empirically-rich understanding that suggests that climate conflict studies that fail to account for vulnerability forces risk a critical misrepresentation and misunderstanding. The results offer an empirical case to buttress the theoretical critiques already available in the literature. The thesis concludes by outlining recommendations and ways forward that better integrate LCB-related adaptation, water governance and conflict management goals.

No description available.

The East Asian Monsoon (EAM) is an active component of the global climate system and has a profound social and economic impact in East Asia and its surrounding countries. Its impact on regional hydrological processes may influence society through industrial water supplies, food productivity and energy use. In order to predict future rates of climate change, reliable and accurate reconstructions of regional temperature and rainfall are required from all over the world to test climate models and better predict future climate variability. Hokkaido is a region which has limited palaeo-climate data and is sensitive to climate change. Instrumental data show that the climate in Hokkaido is influenced by the East Asian Monsoon (EAM), however, instrumental data is limited to the past ~150 years. Therefore down-core climate reconstructions, prior to instrumental records, are required to provide a better understanding of the long-term behaviour of the climate drivers (e.g. the EAM, Westerlies, and teleconnections) in this region. The present study develops multi-proxy reconstructions to determine past climatic and hydrologic variability in Japan over the past 1000 years and aid in understanding the effects of the EAM and the Westerlies independently and interactively. A 250-cm long sediment core from Lake Toyoni, Hokkaido was retrieved to investigate terrestrial and aquatic input, lake temperature and hydrological changes over the past 1000-years within Lake Toyoni and its catchment using X-Ray Fluorescence (XRF) data, alkenone palaeothermometry, the molecular and hydrogen isotopic composition of higher plant waxes (δD(HPW)). Here, we conducted the first survey for alkenone biomarkers in eight lakes in the Hokkaido, Japan. We detected the occurrence of alkenones within the sediments of Lake Toyoni. We present the first lacustrine alkenone record from Japan, including genetic analysis of the alkenone producer. C37 alkenone concentrations in surface sediments are 18µg C37 g−1 of dry sediment and the dominant alkenone is C37:4. 18S rDNA analysis revealed the presence of a single alkenone producer in Lake Toyoni and thus a single calibration is used for reconstructing lake temperature based on alkenone unsaturation patterns. Temperature reconstructions over the past 1000 years suggest that lake water temperatures varies between 8 and 19°C which is in line with water temperature changes observed in the modern Lake Toyoni. The alkenone-based temperature reconstruction provides evidence for the variability of the EAM over the past 1000 years. The δD(HPW) suggest that the large fluctuations (∼40‰) represent changes in temperature and source precipitation in this region, which is ultimately controlled by the EAM system and therefore a proxy for the EAM system. In order to complement the biomarker reconstructions, the XRF data strengthen the lake temperature and hydrological reconstructions by providing information on past productivity, which is controlled by the East Asian Summer monsoon (EASM) and wind input into Lake Toyoni, which is controlled by the East Asian Winter Monsoon (EAWM) and the Westerlies. By combining the data generated from XRF, alkenone palaeothermometry and the δD(HPW) reconstructions, we provide valuable information on the EAM and the Westerlies, including; the timing of intensification and weakening, the teleconnections influencing them and the relationship between them. During the Medieval Warm Period (MWP), we find that the EASM dominated and the EAWM was suppressed, whereas, during the Little Ice Age (LIA), the influence of the EAWM dominated with time periods of increased EASM and Westerlies intensification. The El Niño Southern Oscillation (ENSO) significantly influenced the EAM; a strong EASM occurred during El Niño conditions and a strong EAWM occurred during La Niña. The North Atlantic Oscillation, on the other hand, was a key driver of the Westerlies intensification; strengthening of the Westerlies during a positive NAO phase and weakening of the Westerlies during a negative NAO phase. A key finding from this study is that our data support an anti-phase relationship between the EASM and the EAWM (e.g. the intensification of the EASM and weakening of the EAWM and vice versa) and that the EAWM and the Westerlies vary independently from each other, rather than coincide as previously suggested in other studies.

No description available.

This study addresses the decadal variability and predictability of the Atlantic Meridional Overturning Circulation (AMOC), and associated key variables, in two IPCC-class climate models. The AMOC variability is analyzed in a new climate model CHIME, which features a novel (largely isopycnic) ocean component. Power Spectral analysis reveals enhanced variability for periods in the range 15-30 years. The primary mode of variability is associated with decadal changes in the Labrador and the Greenland-Iceland-Norwegian (GIN) seas, in both cases linked to the tropical activity about 15 years earlier. These decadal changes are controlled by the low-frequency North Atlantic Oscillation (NAO), associated with a tropical-extratropical teleconnection. Poleward advection of salinity anomalies in the mixed layer also leads to AMOC changes that are linked to convective processes in the Labrador Sea. A secondary mode of variability is associated with interannual changes in the Labrador and GIN Seas, through the impact of the NAO on local surface density. The decadal potential predictability of the AMOC and climate as represented in the non-isopycnic IPSL-CM5A model and CHIME is explored using prognostic and diagnostic approaches. The modelled AMOC has an average predictive skill of 8 and 6 years, respectively. Over the ocean, surface temperature has the highest skill up to 2 decades in the far north of the North Atlantic, in both models. Additional oceanic areas of predictability are identified in IPSL-CM5A in the tropics and subtropics. The spatio-temporal predictability of both surface temperature over land and precipitation differs somewhat between the two models, but is of limited extent compared to that of ocean variables. Predictability of climate arises from the mechanisms controlling the decadal AMOC fluctuations. Predictive skills of AMOC and climate are favoured by extreme AMOC events but the role of minimum versus maximum states remains to be clarified. The expected better predictive skills of CHIME over non-isopycnic models (due to its better preservation of water masses and more coherent internal structure to the anomalies) are not borne out.