The rocks of the future: lithification of anthropogenic geomaterials in the Anthropocene

The Anthropocene is a proposed geological epoch encompassing significant human impact on the Earth (e.g. Waters et al. 2016). In a geological and geomorphological context, humans are agents in sculpting the landscape and creating artificial ground (e.g. Price et al. 2011). Examples of artificial ground deposited by humans on the Earth’s surface range from excavated natural rock, such as spoil from deep mining, to anthropogenically-created materials such as steelmaking process wastes (e.g. Price et al. 2011; Riley et al. 2020). In Great Britain, ~40 km3 of artificial ground (equivalent to six times Ben Nevis) has been deposited over the past 200 years (Price et al. 2011). Given that the worldwide annual anthropogenic shift of sediment is estimated to be ~57000 Mt, greatly exceeding that of transport by rivers to the oceans (22000 Mt) (Price et al. 2011), lithification of artificial ground will play a major role in creating the rocks of the future.
Artificial ground poses a variety of challenges. Many types of artificial ground are unsuitable for building foundations on due to their physical properties while the unconsolidated nature of artificial ground can lead to devastating mass movement (e.g., failure of colliery spoil tips (Siddle et al. 1996)). Artificial ground will often contain toxic chemicals (e.g., metals, (Hobson et al. 2017)), or lead to secondary pollution of surrounding watercourses (e.g., acid mine drainage from colliery spoil (e.g. Simate and Ndlovu 2014)). However, artificial ground can present opportunities too. ‘Brownfield’ land – underlain by artificial ground – is frequently reused for construction and development (e.g. Hammond et al. 2021). Due to its varied chemical and physical properties, artificial ground can enhance biodiversity within an area (Macgregor et al. 2022). Artificial ground with certain chemical properties can also potentially be used as CO2 sinks (Washbourne et al. 2015; Riley et al. 2020). Given these challenges and opportunities, it is important to understand the evolution of the physical and chemical properties of artificial ground. Understanding this is also timely given ever-expanding volumes of artificial ground and political focus on repurposing land underlain by artificial ground for development. One such potential physical and chemical change is lithification.

Project Aim and Research Questions
The broad aim of this project is to investigate the processes involved by which unconsolidated artificial ground becomes rock, and the implications of this lithification. This aim will be addressed by investigating case studies (Fig. 1) focusing on artificial ground dominated by slag (the by-product from iron and steel making) where the artificial ground has already become lithified – current analogues to future anthropogenic rocks. To explore this overall aim, the following research questions will be addressed:
1. How does slag-dominated artificial ground become lithified, both in terms of its primary anthropogenic deposition, and after natural reworking?
2. What is the change in mechanical properties due to lithification and how can this be important?
3. What is the scope for widespread natural lithification of artificial ground in the UK and beyond?

Click on an image to expand

Image Captions

Fig. 1: field photograph of lithified reworked slag (left, 1 m diameter square denoted); polished section of lithified slag-dominated artificial ground (right, horizontal field of view = 40 mm); images taken by John MacDonald.


Fieldwork at case study sites (likely in the UK) will be conducted to collect field data (e.g., clast analysis, drone imagery) and samples for further analysis in the laboratory. Scanning electron microscope imaging and chemical mapping will be used investigate mineralogical and chemical changes with lithification. X-Ray Diffraction will be used to determine anthropogenic rock mineralogy and laser ablation mass spectrometry will fingerprint microchemistry. There is also scope for various other types of geochemical analysis (e.g., carbon isotopes), depending on the outcomes of the other analyses. Samples will be tested for their mechanical properties using unconfined compressive strength analysis to assess the impact of lithification. A combination of historical map analysis and satellite imagery will be analysed using a GIS approach to map and assess the scope for lithification in slag-dominated artificial ground.

Project Timeline

Year 1

Literature review; mapping of artificial ground (RQ3); fieldwork and sample collection (RQ1); sample lab analysis (RQ1); processing field data (RQ1)

Year 2

Additional fieldwork (RQ1); mechanical properties analysis (RQ2); sample lab analysis (RQ1); data processing and writing

Year 3

Developing conceptual models with RQ1&2 data; mapping scope for lithification UK-wide and beyond (RQ3); completion of data analysis; writing

Year 3.5

Completion of thesis and submission of manuscripts for peer review and journal publication

& Skills

This project will equip the student with a range of analytical and transferable skills which are desirable for careers in research or industry.
Research Methods
Fieldwork at the case study sites will be conducted with the supervisory team. Full training will be given in all of the laboratory techniques to be used in the project, mainly at the University of Glasgow but also in collaboration with some external facilities.
Researcher Development
Technical & personal skills development will be undertaken with guidance from doctoral advisors and within the framework of the DTP Researcher Development Statement. Researcher developmental training will be provided by IAPETUS2 and supplemented by the University of Glasgow. The School of Geographical and Earth Sciences at the University of Glasgow (GES) has a large research research student cohort (currently ~60 PhD students) that will provide peer-support throughout the research program. The scholar will participate in GES’s annual progression assessment and post-graduate research conference, providing an opportunity to present their research to postgraduates and staff within the School, and to also learn about the research conducted by their fellow postgraduate peers. Additionally, skills in NERC’s ‘most wanted’ list for PhD student training will be developed, including in multi-disciplinarity, data management, numeracy, and fieldwork, in addition to principles and practice of various other laboratory analytical techniques such as stable isotope geochemistry. Training and experience in national and international conference presentations, and preparation and submission of papers to international peer-reviewed journals will also be provided.

References & further reading

References cited above:
Hammond EB, Coulon F, Hallett SH, Thomas R, Hardy D, Kingdon A & Beriro DJ (2021) A critical review of decision support systems for brownfield redevelopment. Science of the Total Environment, 785,147132.
Hobson AJ, Stewart DI, Bray AW, Mortimer RJG, Mayes WM, Rogerson M & Burke IT (2017) Mechanism of Vanadium Leaching during Surface Weathering of Basic Oxygen Furnace Steel Slag Blocks: A Microfocus X-ray Absorption Spectroscopy and Electron Microscopy Study. Environmental Science & Technology, 51,7823-7830.
Macgregor CJ, Bunting MJ, Deutz P, Bourn NAD, Roy DB & Mayes WM (2022) Brownfield sites promote biodiversity at a landscape scale. Science of the Total Environment, 804,150162.
Price SJ, Ford JR, Cooper AH & Neal C (2011) Humans as major geological and geomorphological agents in the Anthropocene: the significance of artificial ground in Great Britain. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 369,1056-1084.
Riley AL, MacDonald JM, Burke IT, Renforth P, Jarvis AP, Hudson-Edwards KA, McKie J & Mayes WM (2020) Legacy iron and steel wastes in the UK: Extent, resource potential, and management futures. Journal of Geochemical Exploration, 219,106630.
Siddle HJ, Wright MD & Hutchinson JN (1996) Rapid failures of colliery spoil heaps in the South Wales Coalfield. Quarterly Journal of Engineering Geology, 29,103-132.
Simate GS & Ndlovu S (2014) Acid mine drainage: Challenges and opportunities. Journal of Environmental Chemical Engineering, 2,1785-1803.
Washbourne CL, Lopez-Capel E, Renforth P, Ascough PL & Manning DAC (2015) Rapid Removal of Atmospheric CO2 by Urban Soils. Environmental Science & Technology, 49,5434-5440.
Waters CN, Zalasiewicz J, Summerhayes C, Barnosky AD, Poirier C, Gałuszka A, Cearreta A, Edgeworth M, Ellis EC, Ellis M, Jeandel C, Leinfelder R, McNeill JR, Richter Dd, Steffen W, Syvitski J, Vidas D, Wagreich M, Williams M, Zhisheng A, Grinevald J, Odada E, Oreskes N & Wolfe AP (2016) The Anthropocene is functionally and stratigraphically distinct from the Holocene. Science, 351,aad2622.

See also:
MacDonald JM, Brolly CV, Slaymark C, Spruzeniece L, Wilson, C & Hilderman R (accepted) The mechanisms and drivers of lithification in slag-dominated artificial ground. The Depositional Record, TBC, TBC.
MacDonald JM, Khudhur FWK, Carter R, Plomer B, Wilson C, Slaymark C, (2023) The mechanisms and microstructures of passive atmospheric CO2 mineralisation with slag at ambient conditions, Applied Geochemistry, 152, 105649.

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