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

  • Full or part time
  • Application Deadline
    Thursday, January 09, 2020
  • Competition Funded PhD Project (European/UK Students Only)
    Competition Funded PhD Project (European/UK Students Only)

Project Description

Summary
What were the triggers that allowed animals to form skeletons in the Cambrian Explosion?

Project 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 predation (1). Abiotic factors proposed include the increased availability of oxygen (2) (Figure 1), or a rise in the concentration of calcium in seawater (3). 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 precipitates (4). 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/Ca(5, 6,7).
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 Cambrian (8). 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.

Research questions
Chemical tracers reveal profound changes in oceanic composition for this time interval, and significant perturbations to major biogeochemical cycles all implies strong environmental controlson 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 transition. 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.


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 spatialand 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.

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 a vibrant research group, and receive specialist training in state of the art analytical skills in world class 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.

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.

Funding Notes

NERC E4 DTP

References

References

1. Knoll 2003, Geobiology; 2. Towe 1970, Jour. Amer. Sci., 3. Brennan et al., 2004, Geology; 4. Zhuravlev and Wood, 2008, Geology; 5. Wood et al., 2016, Geology; 6.Porter, 2002, Science; 7. Zhuravlev and Wood, 2009, Geology; 8. Wood et al., 2015, Precambrian Research. 9. Wang et al. 2018, Nat. Communications

Related Subjects

How good is research at University of Edinburgh in Earth Systems and Environmental Sciences?

FTE Category A staff submitted: 104.98

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