Stable isotope facility PhDs awarded

2017

November 2017: Congratulations to Chris Sargeant on successfully defending his PhD thesis: The application of highly resolved tree–ring isotopes for reconstructing the ecohydrological interactions within riparian forests.

Chris Sargeant

The research conducted within this thesis aims to advance our understanding of plant–water interactions, specifically the identification of the source(s) of water available for tree growth within riparian zones. Through highly resolved tree–ring isotopic analyses, it is possible to deconvolve the δ18O signature of water utilised by a tree during the growing season and relate this to dynamic hydroclimatic processes. In doing so, this methodology allows for the reconstruction of the seasonal evolution of tree source water availability for the historical period. The utility of this method is initially demonstrated for the determination of the isotopic signature of water utilised by individuals of co–occuring riparian tree species over ten consecutive years. It is found that species–specific seasonal source water variability is indicative of tree rooting depth and the hydroclimatic partitioning of water to each of the floodplain hydrological reservoirs. The technique is then developed in to a methodological 'toolkit', comprised of tool groups representing differing levels of data availability. The aim is to enable continued progress in this research area, even under conditions of sub–optimal data availability, whilst also making the suite of methods accessible to other researchers and resource managers. Finally, the hydroclimatic expression of seasonal tree source water avaialbility is characterised across a climate gradient utilising over 800 δ18O cellulose measurements. The results show that the seasonal progression of tree water availability responds to near– and far–field hydroclimatic processes, but this is mediated by species' functional type and site lithology. Where near–field processes dominate water availability, there is a potential for increased vulnerability to localised drought conditions and species–specific competition for water resources. Retrodictions of seasonal water source use may prove useful in forest restoration and rehabilitation efforts, assessment of forest vulnerability to future hydroclimatic regimes and for improving agroforestry and sustainable water management practices.

Chris was based at the University of St Andrews and also supervised at the BGS by Prof. Melanie Leng.

November 2017: Congratulations to Leslie Bode for defending her PhD thesis: In search of a local Palaeoenvironmental Record: Combining Archaeobotany and stable carbon isotopes to investigate water stress at the Epipalaeolithic site of Kharaneh IV in the Azraq Basin, Jordan.

Leslie Bode

This thesis employs two approaches to investigate water stress at the early and mid Epipalaeolithic site of Kharaneh IV in the Azraq Basin, Jordan. Firstly, the archaeobotanical analysis explores the local environment by using the ecology of identifiable charred seeds to indicate water availability (autoecology). Included alongside this is a seed catalogue, which presents the unique archaeobotanical assemblage recovered through sampling. Secondly, to further explore the local palaeoenvironment and due to the potential broad hydrological tolerances of some species, stable carbon isotope δ13C analysis of the archaeobotanical remains is used to track changes in water stress during the occupation of the site. These analyses provide a complementary approach to traditional archaeobotanical studies. Combined, these data offer considerable insight into questions about the local environment, particularly water stress, and the potential use of plants during the occupation of Kharaneh IV. The results presented here demonstrate that Kharaneh IV experienced variable water stress throughout its occupation, with a drying out of the site coincident with the end of occupation. This signature of drying is found within both the isotopic and autoecological analyses, providing multiple lines of evidence for this pattern. This thesis serves as a case study for the usefulness and inferential power of multi–method approaches that combine archaeobotanical and isotopic analysis.

Leslie was jointly supervised by staff in the Department of Archaeology, the School of Geography (at University of Nottingham) and the BGS.

2016

June 2016: Congratulations to Will Dickens on successfully defending his PhD thesis: "Reconstructing glacial and environmental change on the South Orkney plateau, sub Antarctica".

Will Dickens

Glaciers and ice shelves in the Antarctic Peninsula (AP) region, including neighbouring sub–Antarctic islands, have undergone sustained retreat over the past two decades resulting in enhanced global sea–level rise. These changes have been linked with rapid regional increases in atmospheric and ocean temperatures far greater than the global mean. To understand the mechanisms of glacier retreat and help predict the future response of Antarctic ice sheets, it is crucial that longer term ice–sheet history is investigated. The glacial histories of sub–Antarctic islands are particularly poorly known despite their potential to provide important insights into the sensitivity of sub–Polar ice caps to atmospheric and ocean forcing. This study uses new marine geophysical and geological data to reconstruct the style, timing and rates of past glacial and environmental change(s) on the South Orkney Plateau (SOP), at the northern tip of the AP. A comprehensive compilation of available bathymetric data reveals several large cross–shelf troughs emanating from the South Orkney Islands. These extend to the shelf edge in the north, whilst a large glacial depocentre (a grounding zone complex) on the southern shelf indicates that grounded ice previously extended to the mid shelf in the south. A seismic line acquired through the complex reveals that shelf stratigraphy was formed by repeated glacial advances which span the Plio–Pleistocene but could extend back to Miocene times. Former ice–cap dynamics during the most recent glaciation have been reconstructed from high–resolution multibeam data collected within and between cross–shelf troughs. Lineations within troughs indicate streaming ice, whilst their absence in inter–trough areas implies slower moving ice. Channels and grooves on the northern shelf suggest meltwater played a key role in former ice–cap dynamics. Detailed analyses of sediment cores combined with geophysical observations show that the grounding line retreated from the outer shelf prior to 16690 cal. yrs B.P., with mid and inner shelf deglaciation between 14600–12950 cal. yrs B.P. Deglacial moraines are absent from the northern shelf, although a suite of moraine ridges and grounding zone wedges from intra– and inner–trough regions on the southern shelf suggests a complex pattern of grounding line retreat. Initial deglaciation of the SOP was probably driven by postglacial sea–level rise. Ice cap history is supplemented with a multi–proxy analysis of a long piston core recovered from an inner shelf basin. δ18Odiatom data provide a detailed record of glacier discharge and melting through the Holocene and highlight the importance of tropical teleconnections in controlling the climate of the SOP and wider Antarctic environment. Crucially, the record shows a rapid and sustained increase in glacial melt several hundred years before the onset of anthropogenic warming, suggesting environmental response to recent forcing is more complex than previously thought.

Will was supervised at BAS by Dr James Smith and at the BGS by Prof Melanie Leng.

May 2016: Congratulations to Shirley Curtis–Summers on successfully defending her thesis entitled "Reconstructing Christian Lifeways: A bioarchaeological study of medieval inhabitants from Portmahomack, Scotland and Norton Priory, England."

Shirley Curtis–Summers

This thesis investigates lifeways of medieval Christian communities from Portmahomack, Northeast Scotland and Norton Priory, Northwest England, to ascertain the extent to which skeletal indicators of diet, disease or trauma reflect religious or social influences. Osteology and palaeopathology methods on human adult and sub-adult skeletons from Portmahomack (6th to 17th century) and Norton Priory (12th to 16th century) was undertaken to provide evidence relating to the four key themes proposed in this study: 'biological or familial affinity', 'the living environment', 'trauma and conflict', and 'diet and nutrition-related stresses'. Stable carbon and nitrogen isotope analysis of bone collagen from adult humans from Portmahomack (including and a sub-sample of sub-adults) and Norton Priory were measured for dietary reconstructions. Faunal bone collagen was also analysed from Portmahomack and Norton Priory (plus a selection of fish bones from Chester Cathedral) to provide isotopic baselines to reconstruct human diets.The results suggest past lifeways of Christian communities from Portmahomack and Norton Priory can indeed be successfully reconstructed through bioarchaeology. The evidence from this study has found that skeletal traits, alongside burial evidence, can elucidate familial affinities, especially from Norton Priory, and that differences in cultural and religious practices are reflected within the living environment of ecclesiastic and lay groups. Evidence of violence, reflecting interpersonal conflict and vulnerability was found from both Portmahomack and Norton Priory, which was inconsistent with the role of ecclesiastic and lay communities that were expected to follow strict Christian doctrines. Stable isotope data revealed a diachronic change in diet at Portmahomack; no fish were consumed during the monastic period, whereas significant amounts were consumed by layfolk in the later periods, suggesting Christian dietary practices changed over time. The isotope data from Norton Priory reflected a more homogeneous diet that did not change greatly over time, suggesting conformity to the same fasting practices. Overall, this study has demonstrated that adopting a multidisciplinary bioarchaeological approach, integrating skeletal, chemical, archaeological, and historical evidence, results in a powerful research tool that enables reconstructions of medieval Christian lifeways and interpretations of religious and social influences therein.

Shirley did her PhD at Liverpool University where she was supervised by Dr Jessica Pearson and at the BGS by Dr Angela Lamb.

February 2016: Congratulations to Jack Lacey for successfully defending his PhD thesis: "Late Quaternary Palaeoenvironmental Reconstruction from Lake Ohrid."

Jack Lacey

Lake Ohrid is a large, deep, ancient lake located on the Balkan Peninsula and has an extraordinary degree of endemic biodiversity. A deep drilling campaign was carried out in 2013 as part of the Scientific Collaboration on Past Speciation Conditions in Lake Ohrid (SCOPSCO) project to better understand the influence of climate and environmental change on evolutionary patterns and endemism. Initial results from a 569 m sediment succession drilled in the centre of the basin indicate continuous lacustrine conditions over the past ca. 1.2 Ma. This thesis presents new stable isotope data from carbonate within the upper 248 m of the composite profile, covering the last ca. 640 ka at a millennial–scale resolution (≈ 0.5 ka). Isotope data are used to provide a long–term palaeoenvironmental reconstruction, which is supported by a quantitative model of modern lake water isotope composition and a detailed multi–proxy investigation of climate evolution since the last glacial period. Water balance calculations confirm Ohrid to be an evaporative system with a complex hydrology. Variations in the isotope composition of modern lake water are suggested to represent long–term fluctuations in water balance. A trend from wetter to drier conditions through the Holocene is consistent with regional and hemispheric processes related to changes in insolation. Over the last ca. 640 ka, endogenic calcite is precipitated in abundance during warm stages, however carbonate is negligible during glacial periods with the exception of discrete horizons comprising early diagenetic authigenic siderite. The oxygen isotope composition of lake water, calculated using calcite and siderite, indicates more evaporated conditions during warm stages and fresher lake water in glacial phases. Low frequency variability shown by calcite isotope data suggests relatively stable conditions before ca. 450 ka, a transition to a wetter climate between ca. 400–250 ka, and a trend to drier climate conditions after ca. 250 ka. Higher frequency millennial–scale oscillations and abrupt climate events observed during warm stages are likely associated with regional climate change as a function of orbital forcing. This study emphasises the potential of Lake Ohrid as a valuable archive of climate change in the central Mediterranean region and demonstrates the efficacy of isotope data for palaeoenvironmental reconstruction at Lake Ohrid.

Jack did his PhD at the University of Nottingham and BGS and was supervised by Melanie Leng and Matt Jones. His examiners were: Internal George Swann and the External Examiner Dr Mick Frogley (University of Sussex).