Dr R.M. Coggon, Prof D Teagle, Prof Blair Thornton
No more applications being accepted
Competition Funded PhD Project (European/UK Students Only)
About the Project
Project Rationale:
Ocean chemistry reflects a dynamic balance between riverine inputs, biological processing, sediment burial, and hydrothermal exchanges with ocean crust. This project aims to quantify hydrothermal exchange, providing a baseline to investigate vital Earth processes from records of past ocean chemistry. Current models of seawater geochemical evolution generally only consider high temperature hydrothermal venting along ridge axes, ignoring the lower temperature reactions across the vast ridge flanks. Investigating ridge flank exchange requires scientific ocean drilling through thick sediments into the oceanic crust, but our current sampling is biased towards young and very old crust. Dr Coggon is therefore leading an International Ocean Discovery Program experiment to drill an age-transect (7-63 Ma) across the western flank of the Southern Mid-Atlantic ridge (Expeditions 390/393; winter 2020/spring 2021). Core recovery in upper oceanic crust is typically <30%, but careful integration of core observations with continuous downhole logging data allows us to compensate for any resultant sampling bias. Established qualitative approaches that visually match ‘unrolled’ digital images of core exteriors with downhole logging images are labour intensive, and prone to error. This PhD will develop machine-learning approaches to compensate for incomplete core recovery, enabling global extrapolation of observations of hydrothermal exchange in the recovered cores.
Methodology:
Chemical exchange within the ocean crust is determined from geochemical analyses of representative samples of the different alteration features and their fresh counterparts. To quantify the net hydrothermal fluxes through a drilled section of ocean crust the distribution of the rock types and alteration features downhole must be determined; hence an accurate stratigraphy is required. Wireline logging tools yield continuous downhole records of in-situ physical properties within boreholes, which can be integrated with core observations to compensate for incomplete recovery. To quantify ridge flank hydrothermal exchange, the PhD student will:
1. Use state of the art machine learning methods to develop algorithms to automatically construct quantitative stratigraphies from wireline data and borehole images. The student will develop a semi-supervised Convolutional Neural Network (CNN), that is supplied with partially labelled input data (borehole images) so that it can ‘learn’ to identify different rock-types, automatically generate volcanic stratigraphies, and flag intervals it can’t confidently classify for closer human inspection.
2. Determine trace element, major element and isotopic compositions of representative samples from ridge flank cores.
3. Combine these geochemical analyses with the reconstructed stratigraphies to quantify the hydrothermal exchange through drilled sections of crust and extrapolate them to global fluxes.
Training:
All doctoral candidates will enrol in the Graduate School of NOCS (GSNOCS), where they will receive specialist training in oral and written presentation skills, have the opportunity to participate in teaching activities, and have access to a full range of research and generic training opportunities. GSNOCS attracts students from all over the world and from all science and engineering backgrounds. There are currently around 200 full- and part-time PhD students enrolled (~60% UK and 40% EU & overseas). Specific training will include:
• The student will be able to attend postgraduate modules in specialist topics relevant to the project, both within the School of Ocean and Earth Science as well as the Schools of Mathematical Sciences, and Electronics and Computer Science (ECS).
• The student will also be encouraged to attend at least one international summer school, and to present their research at national and international conferences.
• The student will be encouraged to apply to sail on a scientific ocean drilling expedition. Participation on a two-month IODP expedition with a world-class science team provides a unique training experience and is a career-building opportunity leading to enduring long-term research relationships.
• Training in all necessary geochemical techniques, including sample preparation, trace element analysis by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and isotope analyses via Thermal Ionization Mass Spectrometry (TIMS).
• The student will attend the European Petrophysics Consortium/UK-IODP Petrophysics Summer School (University of Leicester), receiving 36 hours of Continuing Professional Development-accredited training and a Techlog Fundamentals training certificate.
References
Coggon & Teagle 2011, Hydrothermal calcium-carbonate veins reveal past ocean chemistry. Trends in Analytical Chemistry, vol 30, 1252-1268, doi:10.1016/j.trac.2011.02.011
Tominaga et al, 2009, Determination of the volcanostratigraphy of oceanic crust formed at superfast spreading ridge: electrofacies analyses of ODP/IODP Hole 1256D. Geochemistry, Geophysics, Geosystems, vol 10, Q01003, doi.org/10.1029/2008GC002143.