PhD: The origin of biomineralisation: oxygenation, nutrient cycling, and seawater chemistry

Background: Animals with skeletons or shells dominate our seas today, and play a key role in the long term carbon cycle. But this process - known as biomineralisation - is a relatively recent evolutionary innovation. Diverse animal skeletons appeared and then rapidly diversified in the late Ediacaran to early Cambrian during the so-called ‘Cambrian Explosion’ (550–520 Ma). This suggests the operation of external triggers or a rise in predation1. Abiotic factors proposed include the increased availability of oxygen2, or a rise in the concentration of calcium in seawater3. Uncertainty persists, however, as to both the record of shallow marine oceanic redox during this interval and its relationship to changes in seawater chemistry.
Most early metazoan skeletons were calcium carbonate (CaCO3), forming as aragonite, low-Mg calcite, and high-Mg calcite, which also formed major abiotic precipitates4. This is of note because early metazoan skeletal groups co-opted different carbonate minerals through the Cambrian Explosion in concert with ambient ocean chemistry, inferred to be driven mainly by changing seawater Mg/Ca6,7
The global nature of ocean redox chemistry throughout this period is also complex, with shallow waters being only intermittently oxygenated in the Ediacaran and deeper water anoxia persisting in many areas until the mid Cambrian8. But we still know little as to how redox might relate to changing seawater chemistry such as Mg/Ca ratios, as well as whether there was a rise in the availability of nutrients such as phosphorous, or in the carbonate saturation state of surface waters - all potentially facilitating the radiation of heavily skeletonised biota. Indeed geochemical and fossil data have not been fully integrated to demonstrate cause and effect, and thus the precise environmental context of what triggered the rise of animal skeletons remains unclear.

Key research questions: Chemical tracers reveal profound changes in oceanic composition for this time interval, and significant perturbations to major biogeochemical cycles all imply strong environmental controls on evolution. In particular, nitrogen (N) isotopic variations show a close coupling between the behaviour of the N cycle with major bio-events across the Ediacaran–Cambrian transition9. Initial data suggest that pulses of oxygenation occurred during a series of discrete intervals, which allowed biological innovations in the earliest animal ecosystems. This remains, however, to be tested.

We hypothesize that coupled changes in seawater redox, nutrient, and Mg/Ca dynamics may have, facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy.

Methodology: Fieldwork in Namibia, and possibly Siberia and/or Mongolia will sample well-documented Ediacaran-Cambrian sections that define shelf-to-basin transects within individual sedimentary basins to construct high-resolution profiles, together with palaeontological and lithological distribution data. We will document the spatial and temporal distribution of early carbonate cements together with N isotopes, and integrate these with local ocean redox dynamics via Fe speciation. We will combine the record of macrofossils with these geochemical data within a relative depth framework to interrogate the evolution of seawater chemistry and biotic response through this critical time interval.

Timetable: Year 1: Fieldwork in Namibia; initial redox analysis; biotic distribution assembly;
Year 2: Fieldwork in Namibia/Siberia/Mongolia; continued redox data acquisition; N isotope data acquisition;
Year 3: Petrography; final redox data acquisition; final N isotope data acquisition.

Training: A comprehensive training programme will be provided comprising both specialist scientific training and generic transferable and professional skills. The candidate will join vibrant research group, and receive specialist training in state of the art analytical skills in worldclass micro-analytical facilities. The student will receive training in carbonate sedimentology and a number of relevant geochemical techniques. All these skills are in high demand in both academia and industry, and will thus provide the student with excellent future employment prospects.

Project Summary: Global biogeochemical cycling and the rise of animal ecosystems is a focus of international attention, and our research team has the essential specialist expertise to make a major contribution to this topic.

Requirements: A very good first degree in Geology, or Geochemistry, or GeoSciences, or Biosciences, or other closely-related subject is required. A Master’s degree with an independent research component is desirable.

References: 1Knoll 2003, Geobiology; 2Wood et al., 2019, Nature Evo Ecol; 3Brennan et al., 2004, Geology; 4 Zhuravlev and Wood, 2008, Geology; 5Wood et al., 2016, Geology; 6Porter, 2002, Science; 7Zhuravlev and Wood, 2009, Geology; 8Wood et al., 2015, Precambrian Research. 9Wang et al. 2018, Nat. Communications