Research in this area is aimed at understanding the formation and differentiation of the Earth and other planets.
Current research interests include:
Constraints on the timing and nature of accretion and core formation on Earth and the other planets. The timing of volatile loss on early planets planetisimals. The nature and origin of stable isotope variations between planets and planetisimals. This has involved the development of new techniques for analysing very small samples to high-precision for both conventional long-lived radiogenic isotopes, linked to short-lived extinct nuclides to provide key information on early solar system processes, such as volatile behaviour of elements or their oxides.
A major focus of research within the Durham Geochemistry Group is the use of novel stable isotope systems to understand the formation and evolution of the Earth and the other terrestrial planets. Key topics that we are interested in include planetary differentiation and core crystallisation, core-mantle interaction, formation of the Earth’s continental crust and crustal recycling into the deep mantle, and the evolution of the oxidation state of the Earth’s mantle. Novel stable isotope systems can place constraints on these processes as stable isotope effects (fractionation) are driven by changes in bonding environment when isotopic equilibrium is reached. We have exploited the redox-dependence of Fe isotope fractionation within the Earth’s mantle to place constraints on secular variations in mantle redox state and we have are also exploring the possibility of Fe, Zn, Cr and Cu isotope fractionation between the mantle and core using both iron meteorites and high-pressure experiments as analogues of core formation.
Understanding of element behaviour during mantle melting and the relationships between the timescales of mantle depletion and the relationships to the formation of the continental crust is fundamental to constraining the chemical evolution of the Earth. A major part of this work is the assessment of the nature and role of source heterogeneities in the mantle and the influence of magmatic processes on the elemental and isotopic evolution of oceanic basalts using isotope systems such as Pb, Os and Fe.
Subduction drives the recycling of surface material into the Earth’s interior. During subduction, fluids and melts released from the subducting slab rise and induce partial melting in the overlying mantle wedge while the residual slab continues into the deeper mantle and may contribute to the long-term geochemical heterogeneity observed in oceanic basalts. The influence of subduction on the chemical evolution of the Earth is still poorly understood and we know little about the nature of the material transferred from the subducting slab to the mantle wedge or the residual material that is recycled into the deep mantle and what the consequences are for mantle compositional heterogeneity and redox state. This can be investigated using a combination of radiogenic and stable isotope tracers (e.g. stable isotopes of Fe, a redox-sensitive element) in rocks sampling the sub-arc mantle (usually brought to the surface as xenoliths) as well as arc volcanic rocks, the subducting slab (typically preserved in obducted oceanic massifs) and the products of melting in the deep mantle (oceanic basalts).
Magmas erupted at volcanoes serve as chemical (and to some extent physical) probes of the lithosphere and subjacent mantle. At Durham we use volcanic rocks to investigate the nature of the lithosphere and mantle, and the effects of such volcanism on climate.
We are interested in how magma systems behave - how magma differentiates and on what timescales. Such processes will not only control the compositions of erupted magmas, but will also influence the hazards posed by a given volcano. In short, whether a volcano passively erupts basalt at frequent intervals in low volumes per eruption (such as Hawaii) or whether it violently erupts large volumes of rhyolite less frequently (such as in New Zealand) is largely controlled on what happens as the magma makes its way through the lithosphere. The objective of this research is to use the rock components, crystals, glasses, melt inclusions, volatiles) to monitor how magmas have evolved - including the roles of crystal fractionation, magma mixing and contamination - where they have evolved in terms of the architecture of storage systems in the crust, such as size and depth of magma chambers, and how fast they evolved.
Basaltic fissure eruptions are sometimes characterised by the repeated eruption of huge batches of magma, over relatively brief intervals of time, and delivering large masses of volcanic gas to the atmosphere. The release of gases and aerosols may have a significant impact on the atmosphere, ocean chemistry and climate – and many have linked such eruptions with mass extinction events that punctuate the history of life on Earth. However, the precise impact of such volcanism on the atmosphere and biosphere remains highly debated because of uncertainties as to whether the atmospheric effects are sufficient to trigger an environmental response that results in climate change and/or a biotic crisis. Research in this area aims to use a combination of isotope and petrological techniques to constrain: (1) The source of the volatiles, using the exceptional sensitivity of key isotope systems to the presence of crustal material in the melt, linking that information with trace element and volatile measurements on coexisting phases (and melt inclusions). (2) The relative duration of volatile release during volcanism using the 187Re-187Os and Pb isotope systems to monitor variations in melt chemistry and volatile release. (3) The mechanism of volatile release, in particular, which gaseous species are present and the mechanism of S transport, either as gases or crystalline sulfide particles and/or sulfates, traced using highly siderophile element (HSE) abundances and Zn, Cu and S isotopes.
Continental weathering and erosion are amongst the primary processes responsible for the evolution of the landscape and exert a major control on the transport of material from the continents to the oceans, and on the cycles of many elements at the Earth’s surface. Over geological (million year) timescales weathering is the principal feedback regulating atmospheric levels of the greenhouse gas carbon dioxide (CO2), thereby controlling the Earth’s climate. This operates in two ways: First, the chemical weathering of silicate minerals converts atmospheric CO2 to hydrogen carbonate ions, which, along with metal ions, are transported to the oceans where the CO2 is fixed by formation of carbonate minerals by organisms. Second, the production of organic carbon during photosynthesis on land, and its burial in oceanic and floodplain sediments.
Research in this area is focused on understanding the processes of weathering and erosion, preserved in the chemistry of soils, rivers and groundwaters. Much of the silicate weathering and organic carbon production occurs in soils, while rivers (and groundwaters) play a central role, exporting metal ions, and the solid and dissolved carbon mobilised by weathering and erosion. Together, these processes act to negate CO2 input to the atmosphere by volcanic and metamorphic degassing and by the weathering and oxidation of fossil organic carbon from sedimentary rocks.
Soil ecosystem development is regulated by the supply of nutrients from rock weathering and the dissolution of silicate minerals, the decomposition of organic matter, and atmospheric deposition. Over the past twenty years the radiogenic isotopes systems (Sr, Nd and Pb) has been used with considerable success to trace these sources, their variation in time, and across different climate regimes and geographical rock types. The stable isotopes of Li, Mg, Si, Sr and Fe also offer a means of tracing the sources, and mechanisms of transfer and release of elements in soils. Our recent research has been using both radiogenic and stable isotopes to trace the mechanisms of soils formation and the inorganic and biological controls on element behaviour.
Rivers represent an ‘average’ composition of the products of weathering (both waters and suspended sediment), and their isotope and elemental composition yields information on the net effects of weathering on the chemistry of material delivered to the oceans. Recent work has included the study of rivers draining young basaltic (Iceland, Azores and Costa Rica) and ancient shield terrains (Greenland. This research has also included the study of groundwaters (Great Artesian Basin, Iceland) and Lakes, which reflect a combination of local input and biogeochemical cycling within the lake itself. The effects of glacial weathering and transport, and the influence of aeolian transport and deposition are also active areas of our research.
The chemical signal from continental weathering may be significantly modified during the mixing of freshwater and seawater in estuaries. Recent work in this area has focused on the role of weathering of riverine particulates in estuarine-shelf environments, which occurs through dissolution prior to sedimentary burial, by divalent metal exchange for Na on clay mineral surfaces and through secondary mineral formation. This work indicates that the elemental flux from particulate dissolution may be significant and that the processes controlling weathering in seawater are very different to those dictating the composition of riverine dissolved species on land.
The chemistry of the modern oceans largely reflects a balance of inputs from continental weathering (via rivers, groundwaters and aeolian input) and hydrothermal exchange at mid-ocean ridges. Marine sedimentary archives potentially preserve a record of changes in the balance of those inputs over geological time, and such isotope and elemental records may serve as proxies for changes in ocean temperature and salinity, ocean circulation the flux of elements and alkalinity from continental weathering, ocean anoxia and ocean and atmospheric CO2. Research in this area is aimed at quantifying the balance of inputs (and outputs) to the oceans for key isotope systems at the present–day, and the retrieval of past seawater compositions on both short (millennial) and long (million year) timescales to elucidate the relationship between ocean chemistry and the biological response of the ocean, to climatic and tectonic change.
Measurement of both conventional radiogenic and non-traditional stable isotopes, and elements, in the modern oceans, to constrain the sources of input, and elemental and isotope behaviour in seawater.
Retrieval of marine sedimentary records to determine changes in erosional input (from continental weathering) accompanying climate change and subsequent dispersal through ocean circulation, using tracers with a range of residence times to determine both the local and global response. Retrieval of records of ocean circulation and their relationship to key tectonic events and the opening and closing of oceanic gateways.
Retrieval of isotope records from organic-rich sediments and carbonates to provide information on the extent of ocean anoxia (regional or global) accompanying oceanic anoxic events (OAEs). Linking records of oceanic redox, continental weathering and productivity to determine the controls on OAEs. Using the same redox and paleoproductivity proxies to trace the presence of hydrocarbon source rocks in sedimentary basins.
Tracing variations in carbonate sedimentation using stable isotopes. Combining records of deep-ocean carbonate ion [CO32-] with weathering proxies to determine the controls on oceanic [CO32-], and atmospheric CO2, and the relationship to climate change – on both millennial and Cenozoic timescales.
Involving the culturing of biogenic carbonate and silicate to quantify the temperature, pH and growth rate dependence of elemental incorporation and isotope fractionation in these microorganisms, and the role that these factors play in the retrieval of proxy records.
We are also interested in using redox and paleoproductivity proxies to trace the presence of hydrocarbon source rocks in sedimentary basins.
We have collaborative research programmes in the chemical and isotopic tracing of Archaeological artefacts and population dynamics of former communities.
The principle focus of our biomedical research is using plasma spectroscopy to study the effects of Pt-bearing anti-cancer drugs. Currently we are working on cell-lines and clinical patients treated with a variety of drugs including Cis-platin, oxaliplatin, HCT116 and BBR3464. The DNA bound Pt is typically extracted via Qiagen DNA extraction kits or in specific studies investigating DNA-bound Pt adduncts are extracted from cultured and clinical samples using HPLC based separation techniques. Depending on the level of analyte, Pt is measured by either quadrupole ICP-MS, Magnetic Sector ICP-MS or MC-ICP-MS. Instrumental detection limits for MC-ICP-MS are in the femto gramme per mL range (1 fg = 1 x 10-15 g), i.e., 1 x 10-18 moles per mL. This work is has been undertaken in collaboration with Cancer Research Group at the Northern Institute of Cancer Research (NICR), and the CRC Cancer Research Unit, Newcastle University Medical School. School of Biomedical and Health Sciences, University of Western Sydney and Cancer Biomarkers & Prevention Group, University of Leicester, Leicester Royal Infirm.
In addition, we are a participant in the Biomedical Engineering artificial joint design programme run by Durham's School of Engineering, by analysing material wear products.
A variety of biological tracing projects are being undertaken using radiogenic isotopes as tracers of migration habits. This work relies on the uptake of Sr and Pb into organisms. The isotopic composition of these elements reflects that of their habitat/ nutrient source. If physiological elements are selected such as otoliths (fish earbones) and teeth, that grow in increments over time, then a time-resolved record of the migratory habits of the animal can be constrained. This approach can be extended to other organisms such as corals, to monitor temporal changes in water chemistry at particular sites.
Work with the Geological Survey of Canada has been undertaken, investigating the migratory habits of Arctic walrus populations using the Pb isotopic composition of their teeth, determined by laser-ablation MC-ICP-MS. This is the only possible means of constraining migratory habits over timescales of 10's of years. While with Biological Sciences at Durham we are using Sr isotopes in otoliths to trace the breeding and migratory behaviour of fresh water and ocean-dwelling fish. This data is also obtained using laser ablation MC-ICP-MS.
We are interested in pursuing other collaborative studies using these techniques.
The isotopic analysis of plants has been applied to many scientific research areas, and in particular botany, cereal science, archaeology, soil science and agriculture. Its application to the ancient record has not been developed as much, except in the use of carbon isotope stratigraphy, specifically in the Mesozoic. Although a few studies have been incorporated to investigate the ecology and environment the palaeo-flora resided in, a lack of understanding variability in the modern system has been often overlooked. In order to rectify this, detailed isotopic analysis of leaves, fruits and nuts has been initiated. This has involved the intra-leaf isotopic variation of different leaves, and in particular the living fossil, ‘Ginkgo’, as well as during leaf senescence and decay.
The development of steam equilibration techniques for analysing material which exchanges with atmospheric hydrogen has made development of greater resolution and regional data of isotopes in hydrology extremely important. Small-scale regional studies are being conducted on river and lake water, and precipitation has been collected during almost every rain shower in Durham since 2010. With the acquisition of the Los Gatos Liquid Isotope Water Analyser (LWIA) for SIBL the analysis of water has become routine, fast and cheap. An ideal instrument for undergraduate projects and training the LWIA is now being incorporated in archaeological, biological and ecological studies regionally and nationally.
Stalagmites are amongst the most important terrestrial palaeoclimate archives: they are amenable to accurate radiometric dating, provide multiple geochemical proxies, and are generally free of diagenetic alteration. Stalagmite stable isotope ratios and trace element concentrations have received particular attention as palaeoclimate proxies more recently. This research theme is focused on expanding this approach to UK and European speleothems, and further afield to tropical records from the Belize region.
Terrestrial plants can be divided into three groups on the basis of distinctions in photosynthesis and anatomy, and each of these groups have distinctive carbon isotope ratios. Knowing when types of photosynthesis evolved enables understanding of the presence of different plant types across geologic time.
As a result of this and the carbon reservoir size of the ocean compared to the terrestrial realm, isotopic analysis of fossil plants was defined by the isotopic composition of CO2. Carbon isotopic curves generated from oceanic sedimentary records reflect global changes in the carbon cycle, so a stratigraphic analysis of fossil plants through geologic time would reflect similar carbon cycle changes. By wiggle matching terrestrial curves (with some biostratigraphic information) with those generated from time-synchronous oceanic records, the curves can be coupled, and with that the environmental changes in the ocean and terrestrial realms. Hence, providing a clearer understanding of combined biologic, climatic and environmental changes during plant evolution.
The majority of research undertaken in SIBL with respect to ocean science involves the unravelling of past oceanic chemistry, temperatures, circulation patterns and sedimentation. Primarily the research focuses on the Mesozoic time period, and developing a greater understanding of the cause, formation and consequences of ‘oceanic anoxic events’. The approach of the laboratory is to employ as many isotopic systems as possible at high-resolution sampling. In addition, the research group are now undertaking modern shoreline and shelf research to develop a greater understanding of the modern system and how dynamic it is. It is clear that our understanding of the geochemistry and sedimentation of ancient marine sediments has been over-simplified.
This is a major NERC-funded project, specifically focused on the carbon cycle through the Late Cretaceous from various oceanic basins spread around the globe. Stable isotope analysis has been undertaken on samples taken at the sub-Milankovitch level; these are used to compare that record to the sequence stratigraphic framework developed for the basin. It is anticipated that by comparing these two records, a link between the carbon cycle and global sea level change can be unravelled for the Late Cretaceous.
Over the past decade, there has been an increased effort to unravel the cause(s) and consequences of Mesozoic ‘oceanic anoxic events’ (OAEs). This has involved a myriad of approaches ranging from geochemistry, cyclostratigraphy, sedimentology and stratigraphy. However, we are no clearer to understanding OAEs as global phenomena. Black shale deposition and the architecture of black shale deposits in these environments are controlled by climate, oceanography and basin evolution. This last factor controls development of sub-environments in which black shales may accumulate, as well as (partly) controlling the former two. The research group investigating OAEs are involved with combining high-resolution sedimentology and isotope geochemistry in order to develop a greater understanding of the environment in which black shales are deposited, and how this may affect the isotopic ratios preserved in such sedimentary systems.
This research area looks at the environmental conditions, mechanisms, and processes that affect the deposition of sedimentation over geologic time. Oceanic anoxia, and orbital and ocean tide variations are studied to increase our understanding of the causes, and outcomes, of sedimentation types.
This research theme is primarily involved at present with the analysis of beetle chitin and megafauna (e.g. bear, reindeer) collagen.
This research theme is involved with generating modern experimental conditions on plant cereals by incorporating different levels of nitrogen fertilization. Isotopic variations are generated from these lab-grown cereals to generate a modern database from which to compare with archaeological cereals found in the Neolithic Age in the UK.
Research associated with Economic geology focuses primarily on two aspects, the absolute age of sulfide mineralization using sulfide minerals such as molybdenite, pyrite, chalcopyrite, bornite and arsenopyrite, and using the Osmium (Os) isotope composition of sulfide minerals to assess the origin of Os in an ore system and, by inference, the source of metals, e.g., Copper, Gold and Molybdenum.
Renium (Re) and Osmium (Os), in addition to being siderophilic and chalcophilic, are also organophilic and thus can be utilized to determine the depositional age of organic-rich sedimentary rocks, such as oil source rocks. Further to this the application of Re-Os geochronology can be applied to petroleum to yield the timing of oil generated from a source. The Os isotope composition also can provide a valuable fingerprint of an oil to its source.
The Osmium (Os) isotope composition recorded by an organic-rich sedimentary rock on formation inherits that of the water column at the time of deposition. As a result the Os isotope composition reflects the ocean chemistry and can be used to evaluate the controls on palaeo Os ocean chemistry through time and evaluate the cause of major events, such as global ocean anoxia, snowball earth, and mass extinction.