Project Description

For the first four billion years of earth history, the only life on land was microbial, and soils were simply thin horizons of weathered rock. This changed with the advent of land plants around 430 million years ago, which revolutionised Earth surface environments. For example our recent studies have shown that early plants drove a change from dominantly braided to meandering rivers; trapped mud on land; and even changed the composition of the deep Earth. They also brought about thick soils that are rich in plant-derived organic matter, which are now so crucial to life on Earth. In some cases, oxidation of this incorporated soil zone organic matter contributed to growth of calcium carbonate (‘calcrete’) in the soil zone. Calcrete rocks are themselves useful as they can preserve geochemical archives of environmental change, including for example soil temperatures and atmospheric CO2 concentrations through time.

However calcrete rock characteristics – including both their physical appearance and their geochemical signatures – likely evolved at the same times as bigger land plants with deeper rooting systems developed, although this intriguing hypothesis has not yet been thoroughly tested. For example the first land plants were small and stubby, with poorly developed root systems, so early calcretes should look different to the modern calcretes we find that commonly have fabrics that are directly related to plant roots.

A further step that likely revolutionised soil development from the Mesozoic or latest Palaeozoic onwards was the evolutionary radiation of earthworms. Earthworms are known to improve soil drainage, and raise soil temperatures whilst encouraging bacterial rather than fungal growth. Recent studies have also shown that several earthworm species produce distinctly recognisable calcite crystals, and these should leave a trace in the rock record of fossil soils.

It is likely that both of these big steps in biological evolution drove changes to earth surface environments that are recorded in calcareous fossil soils (palaeosols with calcretes) deposited from the Silurian to Triassic. Here we expect to find big changes in the physical appearances of calcretes from different geological times and places, both in the field and under the microscope. This project will include a deliberate search for the oldest evidence of plant roots in calcrete development, as well as for the oldest annelid-associated calcite crystals. We also anticipate that there will be differences in carbon isotope chemistry of calcretes, in part reflecting changes to factors like atmospheric pCO2 and soil drainage as the co-evolution of biology and land environments progressed.

Essential & desirable candidate skills

Essential: BSc in a Geoscience-related discipline 

Desirable: Microscopy (petrography); Geochemistry; Palaeobiology 


Alex Brasier

Primary Supervisor:

Profile: Alex Brasier
Institution: University of Aberdeen
Department/School: School of Geosciences

Neil Ogle

Secondary Supervisor:

Profile: Neil Ogle
Institution: Queen's University, Belfast
Department/School: School of Natural and Built Environment

Additional Supervisor:

Dr Stephen Bowden 

University of Aberdeen, School of Geosciences



Additional Supervisor:

Dr Neil Davies

University of Cambridge, Department of Earth Sciences



Additional Supervisor:

Dr Jakob Vinther

University of Bristol, School of Earth Sciences




Brasier, A.T., 2011. Searching for travertines, calcretes and speleothems in deep time: Processes, appearances, predictions and the impact of plants. Earth-Science Reviews, 104(4), pp.213-239.

Brasier, A.T., Morris, J.L. and Hillier, R.D., 2014. Carbon isotopic evidence for organic matter oxidation in soils of the Old Red Sandstone (Silurian to Devonian, South Wales, UK). Journal of the Geological Society, 171(5), pp.621-634.

Davies, N.S., Gibling, M.R., 2010. Cambrian to Devonian evolution of alluvial systems: the sedimentological impact of the earliest land plants. Earth-Science Reviews 98, 171-200.

Gibling, M.R, Davies, N.S. 2012. Palaeozoic landscapes shaped by plant evolution. Nature Geoscience 5, 99-105.

McMahon, W.J., Davies, N.S. 2018. Evolution of alluvial mudrock forced by early land plants. Science 359, 1022-1024.

Spencer, C.J. et al. (inc. Brasier & Davies), 2022. Composition of continental crust altered by the

emergence of land plants. Nature Geoscience

Parry, L., Tanner, A, Vinther, J. 2014. The origin of annelids. Frontiers in Palaeontology 57, 1091-1103.

Prud’Homme, C., Lécuyer, C., Antoine, P., Hatté, C., Moine, O., Fourel, F., Amiot, R., Martineau, F. and Rousseau, D.D., 2018. δ13C signal of earthworm calcite granules: A new proxy for palaeoprecipitation reconstructions during the last glacial in Western Europe. Quaternary Science Reviews, 179, pp.158-166.

Spencer, C.J. et al. (inc. Brasier & Davies), 2022. Composition of continental crust altered by the emergence of land plants. Nature Geoscience

Research Methods

This project involves fieldwork in Scotland, England, Wales and the Mediterranean, exploring calcareous soils deposited from 420 million years ago up to the present day. The student will also employ and receive training in laboratory work, including use of optical and advanced electron microscope techniques, as well as stable isotope geochemistry. These are transferrable skills that can be widely applied in the environmental sciences. The student would also be trained in writing of reports and scientific articles, and in presentation skills. The student would benefit from joining an active team of collaborative researchers working on related topics, including for example an existing QUADRAT PhD student who is working on the potential impact of early plants on ancient coral reef health.


This project will help show how calcareous soils developed in response to key biological and geological revolutions. Studies have previously suggested that ‘early land plants’ were likely important for soil development in deepening the critical zone of weathering; trapping mud on land; and mixing carbon into the sediment, from where it can be trapped as either organic matter or carbonate carbon (calcrete limestone), or returned to the atmosphere (as CO2). However the role of early invertebrates in breaking up organic matter and soil zone minerals, whilst also mixing primitive soils, and improving their drainage, is under-explored. We need to know whether their effects are significant because this has implications for development of thick, impervious calcrete horizons in, for example, arid vs semi-arid environments of an increasingly arid modern world. These calcareous soils are an important part of the global C cycle, and their calcrete limestone products can archive physical (microscopic) and chemical (stable carbon isotopic) archives of changes to Earth’s  geosphere, hydrosphere, atmosphere and biosphere. Data generated during this PhD could therefore help to improve global C-cycle models that rely on, for example, soil carbonate carbon isotope values as a proxy for changing atmospheric CO2 concentrations through time. Further, the origin and dispersal of earthworms is currently poorly understood partly due to the limited fossil evidence they produce such as trace fossils, which may be overlooked or obliterated. Therefore, the search for unique earthworm granules offer a unique avenue for tracing earthworms back in time. Currently, such granules have only been found in Pliocene and Miocene aged deposits. 

Proposed Timetable

Months 1-18: This project will begin with fieldwork, exploring i) calcretes of the Lower Old Red Sandstone in Pembrokeshire (Wales) and Moray (Scotland); ii) Upper Old Red Sandstone calcretes in northern England and the midland valley of Scotland; and iii) Permo-Triassic calcretes of Moray (Scotland) and southwest England. Modern examples of Spain will also be visited. Repeat field visits to UK examples may be required in the second year of the project.

Months 6 – 30: Samples collected in the field will be examined using optical and electron microscopy techniques at the University of Aberdeen, and sub-samples will be analysed for carbon and oxygen isotopes at Queen’s University Belfast. This work will begin following the first fieldtrip, and will conclude by the middle of the second year.

Months 9 – 36: Results will be compared with and augmented by data extracted from the literature, resulting in likely articles on topics including i) evolution of soil carbonate petrography from the Silurian to Triassic; ii) influence of invertebrates on Triassic calcareous soil development; iii) implications for the global carbon cycle of changes in calcareous soil development from the Silurian to Triassic. These will be written up in the 3rd year of the project.


  • earth-systems


Not applicable at this time.

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