Modelling the Cambrian Explosion

Summary
We propose to model the depositional and diagenetic contributions to poorly understood geochemical proxies that are of importance to understanding the ‘Cambrian Explosion’ and modernisation of Earth’s biogeochemical cycles.

Project background
The ‘Cambrian Explosion’ records the rapid increase in animal diversity and abundance, as manifest in the fossil record, between ~ 550 and 520 million years ago (Ma). This evolutionary event also coincides with geochemical evidence for the modernisation of Earth’s biogeochemical cycles, as shown by the behaviour of various isotopic proxies.
Two major phenomena of this revolution remain poorly understood and constrained. One is the impact of the rise of the skeletal carbonate factory. Prior to ~550 Ma, all calcium carbonate on the seafloor was precipitated either inorganically or by microbes. But a major outcome of the Cambrian Explosion was that diverse animals rapidly acquired carbonate skeletons, such that this carbonate factory became under increasing biological control. The impact of the appearance of large and diverse biological grains on the dynamics of sedimentation is poorly understood. Equally poorly known, is how this impacted geochemical proxies. Many measurements of isotopes are undertaken using powdered bulk carbonate rock. But these measurements are simply an averaging of both depositional, sedimentary grain and diagenetic components such as carbonate cements precipitated from pore fluids after deposition and during burial. The variable contribution of these two components, how they may have contributed to our bulk signals, and how their values may have changed during the Cambrian explosion, has not been quantified.

Research questions
The overall aim of this project is to quantify, via modelling, the impact of the rise of the skeletal carbonate factory on two key phenomena: sedimentary dynamics and isotopic proxies. This involves the development of two sets of models:
Dynamic models of depositional sedimentation that quantify the impact of increased sedimentation rates due to the rise of skeletal grains.
Porescale models of grains and cement growth to test both the impact of biological grains on isotopic proxies, and the impact of different styles of diagenesis.
We hypothesize that the rise of the biological carbonate factory during the Cambrian Explosion has a significant, and quantifiable, impact, upon both sedimentation style and isotopic proxies.

Methodology
Dynamic models of depositional sedimentation will be coded in Matlab using data derived from the field of relative rates of sedimentation of microbial reefs (stromatolites) and skeletal reefs across the Cambrian explosion. These will produce 4D models, that show the local impact of the skeletal carbonate factory of sedimentary dynamics through time.
Porescale models of grains and cement growth have been developed in-house using a 3D process-based Matlab model, Caclite3D (Hosa and Wood, 2017). This model can create grains of varying size and shape to form a depositional framework. Both early marine and late, burial cements can then be ‘grown’ in remaining pore space. Lattice Boltzmann simulation allows a realistic, flow-based control on the distribution of cement growth. This then allows a quantification of the volumetric contribution of all depositional and diagenetic components. So in turn this can then be calibrated to the measured isotopic compositions of each separate component in real rocks.

Training
A comprehensive training programme will be provided comprising both specialist scientific training and generic transferable and professional skills. The candidate will join two vibrant research groups – one specialising in the carbonate sedimentology, petrology, and the Cambrian Explosion (Wood), and the other in mathematical modelling and inversion techniques (Curtis).

Year 1: Research training, Setting up Matlab models; initial isotopic sampling of components;
Year 2: Fieldwork in Namibia; Testing variable models of sedimentation; continued initial isotopic sampling of components; collation of isotopic proxy data;
Year 3: Further development of porescale models and calibration with isotopic proxy data.

Requirements
A very good first degree in Mathematics, Physics, Geophysics, numerate GeoSciences, or another closely-related subject is required. A Master’s degree with an independent research component is desirable.