PhD: Understanding the Theatre of the Cambrian Explosion: Sedimentary and Geochemical Characteristics of the Earliest Phanerozoic Marine Sediments

Importance of the area of research concerned:
Life has played a major role in constructing the rock record: most obvious in biogenic sediments, this influence on the Earth’s lithosphere is in fact more pervasive, with changing mediation of physical and chemical processes having much influence on the composition of the geological record. Only through recognising and explaining irreversible lithological changes, synchronous with major events in evolution, can we fully understand the base-line conditions of an abiotic planet, and unravel the intrinsic interactions between lithosphere, atmosphere and biosphere that make the planet Earth apparently unique. This research tackles these challenges from the perspective of a crucial chapter of life evolution: the Cambrian explosion. Although well-documented palaeontologically, the physical and chemical changes during this formative event in Earth history need more attention, because prominent unique Cambrian lithologies such as extensive marine red beds or shallow marine sheet-sands, and unusual geochemical properties presently lack the unified explanation that may provide insights into the functioning of hydrosphere and lithosphere on Earth as a living planet.
Project summary :
The Cambrian oceans were unlike any younger or older ocean systems on Earth. Seafloors were extensively sculpted by bioturbating organisms for the first time – fundamentally and irreversibly complicating the physical boundary between hydrosphere and lithosphere. At the same time, continental masses were as yet unstabilized by vegetation, resulting in highly irregular and accentuated sediment flux to shallow marine settings. This project will synthesise evidence to understand the Cambrian seas as a theatre of evolution, by 1) characterizing the geochemical and sedimentological characteristics of some distinctly Cambrian lithologies (marine red beds, sheet sands); 2) exploring the geochemical differences between bioturbated and unbioturbated sedimentary units; 3) explaining these characteristics within the context of ongoing ocean and life evolution and the evolving geochemical records.
What will the student do?:
The student will produce detailed sedimentological logs and develop sedimentary- and ichno-facies models for key Cambrian sections from the ancient continents of Avalonia (field sites in Wales and northern France) and Laurentia (field sites in northern Scotland and Quebec), during four field seasons. These field investigations will contextualize samples that will be collected for geochemical analysis. The primary geochemical analyses that will be done are measuring the sulfur isotopic composition of sedimentary sulfide minerals extracted from a range of rocks, as well as the iron speciation and potentially isotopic composition of various iron fractions within the rock. It has been hypothesised that the onset of bioturbation caused a fundamental shift in the function of sedimentary redox processes and the preserved mineralogy and isotopic composition of various minerals and rocks. This hypothesis will be tested through detailed sedimentological and geochemical characterisation of the chosen field sites.