Stable isotope facility PhDs awarded 2015

October 2015: congratulations to Lyndsey Fox who successfully defended her PhD thesis: "Mid Miocene orbital climate variability and biotic response in the Pacific Ocean".

Lyndsey Fox

During the Miocene, the Earth's climate transitioned from an extended phase of global warmth (Miocene climatic optimum) into a colder mode with the establishment of a permanent and stable East Antarctic Ice Sheet (EAIS). The mechanisms which drove this extreme climate shift are still poorly understood, because continuous, well–dated Miocene sedimentary archives are still scarce. Reliable sea surface temperature estimates are crucial to any reconstruction and modelling of past ocean salinity and density, water column stratification, thermohaline circulation, and ice volume. Despite extensive studies of benthic foraminifera, existing planktonic foraminiferal records for this interval are extremely scarce and of low resolution. Consequently, the impact of global warming and cooling on tropical surface waters and the propagation of orbital cycles in the Earth System are unknown. The overarching aim of this thesis is to investigate the nature and variability of early–middle Miocene climate and the relationship to orbital variations in solar insolation, in order to better understand the extent and magnitude of the global middle Miocene Climate Transition (MMCT) and the subsequent cooling/EAIS events. Furthermore, this study aims to investigate changes in the thermal structure of the Pacific Ocean during the development of MMCT to examine Pacific Ocean circulation across the middle Miocene climatic events.This is achieved through high resolution planktonic foraminiferal stable isotope analysis, spectral analysis and wavelet transform analysis. The first ever high–resolution (3 kyr) astronomically–tuned record of δ18O and δ13C from planktonic foraminifera for the eastern equatorial Pacific Ocean (15.56–13.35 Myr) is presented here. These data provide vital new information on sea surface temperatures and primary productivity changes at the tropics during the middle Miocene, at a resolution not achieved in any previous study, which sheds new light on the extent and magnitude of the MMCT and associated carbon–isotope excursion. In order to assess the reliability of these new records this thesis also goes on to document the taxonomy and palaeobiology of Miocene tropical planktonic foraminifera and their response to times of climatic stress. Finally the data from Site U1338 is compared to Site 1146 in the western equatorial Pacific Ocean, to reconstruct bottom and surface water conditions and changes in ocean dynamics across the equatorial Pacific during this highly complex interval of climate history.

Lyndsey did her PhD at the University of Leeds and the BGS, at the BGS she was supervised by Prof. Melanie Leng.

August 2015: congratulations to Niklas Hausmann on successfully defending his PhD: "The Shellmounds of the Farasan Islands – an isotopic study of seasonality and coastal exploitation".

Niklas Hausmann

The focus of this thesis is to assess the value of the coastal landscape of the southern Red Sea after the aridification of the environment following the early Holocene wet period (11,000–6,000 cal BP). It presents data from the Farasan Islands shell midden complex, encompassing over 3,000 shellmounds accumulated between 6,500 and 4,500 cal BP, and indicating heavy reliance on marine molluscs as food. In the context of the overall aridity, there are crucial questions surrounding such intensive shellfish exploitation: a) were shellfish the main food source on the islands? b) were they a reliable food source and could they have supported a permanent settlement? c) was the exploitation of shellfish linked to the environmental change? Exploitation patterns of shellfish are reconstructed using seasonality data based on 2,100 stable isotope measurements (δ18O and δ13C) of the marine gastropod Conomurex fasciatus (Born 1778). This enables an assessment of the seasonal consumption of this species, and hence whether it could have been exploited all year round, or whether movement to the mainland (with its more temperate mountains) was necessary. Additionally, environmental data based on the same proxy is used to reconstruct climatic conditions. Early and late periods (6,500–4,800 cal BP) are compared to analyse the degree of aridity and the possibility of a longer–lasting early Holocene wet phase. Results indicate that year–round shellfish gathering took place, with more intensive exploitation occurring in the summer dry season, and that 6,500 years ago climate was already extremely dry. This suggests that the intensive coastal exploitation was not due to landscape aridification, it also indicates that seasonal migration to the mainland was not a necessity, as shellfish were available throughout the year.

Nik did his PhD at the University of York and the BGS, at the BGS he was supervised by Prof Melanie Leng and Hilary Sloane.

July 2015: congratulations to Nia Blackwell on successfully defending her PhD: "The biogeochemistry of iron precipitates from abandoned coal mines in the south Wales coalfield: a study using hydrology, stable isotope geochemistry and microbiology."

Nia Blackwell

Waters draining from many abandoned coal and metal mines in the UK often have concentrations of potentially harmful elements that exceed the EU water quality guidelines. In the South Wales coalfield drainages from abandoned coal mines are characterised by circumneutral pH and elevated concentrations of Fe. The precipitation of ochreous sediment, mainly as the Fe(III) mineral goethite, is also widespread. These ferric Fe precipitates have the potential to sustain complex microbial communities as the Fe(III) minerals readily function as terminal electron acceptors for Fe(III)–reducing bacteria. In the natural environment this is usually coupled with the oxidation of organic matter; the sources of which vary seasonally. This study investigated the sources of contaminated mine drainage in the South Wales coalfield using water chemistry and isotopic techniques. The temporal and spatial changes in the microbial community present within an accumulation of Fe precipitates at one mine site was also investigated using a range of traditional and next generation sequencing techniques. The activity of bacteria closely related to known Fe(III)–reducers was assessed by adding a simple organic compound as an electron donor to the sediment. Finally, the potential of microbially–reduced goethite for the remediation of contaminated mine drainage was also investigated...

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Nia did her PhD at the university of Aberystwyth and at the BGS where she was supervised at the BGS by Dr Barbara Palumbo Roe, Dr Jenny Bearcock, and Prof Melanie Leng.

July 2015: congratulations to Sam Broom–Fendley on successfully defending his PhD: "Targeting Heavy Rare Earth Elements in Carbonatite Complexes."

Sam Broom–Fendley

The heavy rare earth elements (HREE) are generally considered to be the most critical of the REE, indispensable for many high-tech applications such as smart-phones and electric vehicles. Currently, carbonatites are the main source of REE due to their high REE grade; most carbonatites, however, are HREE-poor. This thesis presents the findings on HREE mineralisation at the Songwe Hill carbonatite, in the Chilwa Alkaline Province (CAP) of south-eastern Malawi. Across all carbonatite types at Songwe, whole-rock Y and P2O5 concentrations correlate positively, indicating that phosphate minerals have a strong control over the HREE contents. This is confirmed through textural and geochemical analyses (Laser ablation inductively coupled plasma mass spectrometry and electron microprobe [LA ICP-MS and EPMA]) of apatite, which show that it can be subdivided into 5 different types (Ap-0-4), found at different stages of the paragenetic sequence. The chemistry of each of these apatite types becomes progressively more HREE-enriched, up to 3 wt. & Y2O3, and ultimately culminating in xenotime crystallisation.

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The isotope component of Sam's thesis was supervised at BGS by Dr Tim Heaton.

Kimberley Leanne Davies

June 2015: congratulations to Kimberley Leanne Davies on successfully defending her PhD: "Biogeochemical interactions in thermokarst lakes: investigations into methane processes and lake biota."

Processes in the cryosphere can trigger large-scale greenhouse gas flux to the atmosphere, including methane emissions via ebullition (bubbling) from thermokarst lakes. Methane emissions specific to thermokarst lakes have yet to be fully explored or quantified, and lake sediments hold potentially useful information about methane cycling. This study investigated the relationship between surface sediment properties and current patterns of large-scale methane emissions and how these emission patterns could be used to link methane availability to lake food webs. Two main questions were addressed: how do sediment-based properties record the spatial distribution of methane fluxes, and can we better understand how methane is utilised in lakes and lake food webs? Samples were analysed from the surface sediments of two thermokarst lakes in Interior Alaska. The spatial distribution and stable carbon-isotope compositions of lipid biomarkers were used to assess the presence of methane oxidising bacteria (MOB) in the surface sediments. Based on other studies, it was hypothesized that MOB would be highest in the methane ebullition zone of a lake: this was the case in one lake but not in the other, which may reflect differences in source and mode of transportation of methane and affect the availability of methane for oxidation by bacteria. Fossil chironomids were analysed to establish whether (i) chironomid assemblages varied with the strength of methane flux, and (ii) the stable carbon-isotope values of taxon-specific chironomid samples reflected assimilation of MOB that could be linked to areas of highest methane production. Assemblages showed subtle variations in composition. The δ13C values were depleted, which might be linked to the assimilation of MOB; however, high within-lake and within-taxon variability in δ13C values obscured any correlation with patterns of methane production and oxidation. This suggests there may be problems with using down-core δ13C values of fossil chironomids to infer past methane fluxes. Factors underlying the observed heterogeneity may be related to thermokarst dynamics or organic or inorganic sediment properties, and these require further exploration. Kim was supervised at Southampton by Prof Mary Edwards and Prof Pete Langdon, at the BGS by Prof Mel Leng and at Bristol by Prof Rich Pancost.

Tom Bishop

January 2015: congratulations to Tom Bishop on defending his PhD: "A Palaeolimnological Investigation of Central Patagonian Climate During the Holocene".

The southern westerly winds are the dominant control on climate in central Patagonia, and there is ample evidence of changes in the position and strength of the winds throughout the Holocene, but the timing, nature and extent of these changes is unclear. This study addresses the lack of palaeoclimatic data for the region east of the North Patagonian Icecap to address the uncertainties surrounding Holocene climate in the central Patagonian region. By correlating multi-proxy (sediment δ13C & C/N, chironomids and magnetic susceptibility) data from two palaeoclimatic reconstructions derived from lake sediment sequences, this study identifies a unique climate history for central Patagonia, where mid-Holocene shifts observed elsewhere in Patagonia are subdued, but late-Holocene aridity between 1300–2800 years B.P. and cooler conditions from c.1500 years B.P. onwards are present. Chironomid stratigraphies appear to respond primarily to lake level changes, and recently developed chironomid temperature transfer functions fail to provide reasonable estimates of temperature change over the past c.120 years at these study sites. Modern vegetation δ13C, C/N and modern water δ2H and δ18O data are presented, along with age/depth models for the two study sites derived from 14C, 210Pb, 137Cs and tephrochronological markers. Geochemical data for a number of previously known and unknown tephras are also presented.

Tom was supervised at University of Southampton by Dr Peter Langdon and others, and at BGS by Prof Melanie Leng