An overview of our research
The McMahon Group conducts research at the interface of palaeobiology and astrobiology, using both dedicated laboratory space and shared facilities at the UK Centre for Astrobiology, the School of GeoSciences, and the wider University. Our work benefits from numerous local and international collaborations. Specific areas of interest include:
1. The formation of pseudofossils and dubiofossils on early Earth and Mars
Ambiguous structures interpreted as fossil microorganisms provide key evidence in the search for life on the early Earth and may soon be found on Mars. To understand such objects, palaeobiologists and astrobiologists have extensively explored the microbial fossil record and investigated how microbial cells and communities can become fossilized under different conditions. This important work has had an unfortunate unintended consequence: we are now highly trained in detecting microbial fossils, but lack objectivity; we see fossils even where they do not exist. To counter this tendency, it is important to investigate how non-biological processes (e.g., far-from-equilibrium processes of chemical reaction, diffusion, and precipitation) can produce pseudofossils. Our experiments have shown that microscopic filaments made of iron oxide minerals — including hematite tubules interpreted by other workers as the oldest fossils on Earth — may result from naturally occuring, abiotic "chemical gardens" formed by such processes (McMahon, 2019). We are currently investigating other possible occurrences of similar pseudofossils and dubiofossils. Key collaborators in this area include Anna Neubeck and Sigrid Huld in Uppsala, and Oliver Steibock and Pamela Knoll at Florida State University.
2. Taphonomy: Processes of fossil preservation on Earth and Mars
The fossil record is an amazingly rich archive on biological evolution. However, the record presents a distorted view: most environments, organisms, and types of biological information are completely absent, while others are greatly overrepresented. To correct this distortion we must understand the complex chemical and biological processes by which fossils actually form — particularly those rare fossils that are "exceptionally well preserved" and therefore play an outsized role in palaeobiology. Experimental studies of organic decay and preservation ("experimental taphonomy") offer important clues, but there is much still to be done. In particular, the roles of microrganisms in the preservation of animal and plant tissue are still coming into focus (Briggs and McMahon, 2016). We have helped to show that microbes play a key role in preciptating clay minerals and iron oxides that mediate the fossilization of plant tissues (Locatelli et al., 2017). We have also shown that different types of clay mineral can retard the growth of marine bacteria and may thereby slow down the decay of organic tissues, perhaps helping to explain the remarkable preservation of Burgess Shale-type fossil assemblages (McMahon et al., 2016). We have investigated how sea anemones decay in order to guide interpretations of some of Earth's oldest fossil animals, which may represent similar organisms (McMahon et al., 2017). We have helped to discover new evidence that iron sulfide minerals resulting from bacterial sulfate reduction mediated the preservation of some of Earth's oldest fossil animals (Liu et al., 2019; Shore et al., 2021). Finally, we have shown how the science of fossil preservation — taphonomy — provides important clues about where fossils (broadly construed) might be found on Mars (McMahon et al., 2018). Ongoing work is addressing the pathways and processes by which microorganisms like bacteria can become fossilized.
3. Subsurface astrobiology: habitability and biosignature preservation underground on Earth and Mars
On Earth, pores and fractures underground are home to large numbers of microrganisms. We have led studies evaluating the total biomass of this hidden "deep biosphere" at the present day (McMahon and Parnell 2014) and in the geological past (McMahon and Parnell, 2019). On Mars and other cold planets, the deep subsurface may offer a warm refuge for microbial life from the harsh conditions at the surface. We have therefore investigated the habitability of subsurface environments on Mars and other planets by reevaluating the circumstellar habitable zone (McMahon et al., 2013), reviewing the physical and chemical controls on subsurface habitability (Parnell and McMahon, 2016), highlighting the potential for porous basalt to host microorganisms (McMahon et al. 2013), demonstrating that basaltic martian meteorites contain methane (Blamey et al., 2014) and investigating how geological processes can release chemical energy to support life (McMahon et al., 2016). We have also investigated how life in the subsurface might produce lasting traces that might be detectable in samples that have reached the surface (through erosion, for example) on Earth and Mars. For example, we have reviewed the fossil record of Earth's deep biosphere (McMahon and Ivarsson, 2019), investigated uranium isotopes as a potential biosignature of subsurface bacterial life (McMahon et al., 2018), and reported sulphur isotopic evidence of life in English gypsum veins that are similar to gypsum veins on Mars (McMahon et al. 2020). John Parnell at the University of Aberdeen is a key collaborator on much of this work. Work on this topic from 2017–2019 was supported by the European Commission through a Marie Curie Fellowship held by Sean McMahon (D-BIOME).
4. The origin of life and other evolutionary milestones
The origin of life and major turning points in the history of the biosphere are areas of central concern to astrobiology. In collaboration with Anna Neubeck of the Uppsala University, Sean McMahon is currently co-editing a volume titled, "Prebiotic Chemistry and the Origin of Life" for Springer. We have contributed to studies of the origin and earliest evolution of life on Earth (Brasier et al., 2011; 2013), and also to palaeontological studies of early animals and their microbial milieu (Anderson et al., 2017; Liu et al., 2019; Shore et al., 2021), and early terrestrial vertebrates (McCoy et al., 2016). We have shown that some purported fossils from the early Archean may not be what they seem (McMahon, 2019) and addressed the distribution of biomass on Earth over time (McMahon and Parnell, 2019). Work is ongoing to descibe new microbial fossil material from the Ediacaran of Newfoundland, Canada, in collaboration with Jack Matthews (Oxford University Museum of Natural History) and Alex Liu (University of Cambridge).
5. The philosophy of astrobiology and space exploration
Astrobiology and space exploration pose philosophical (e.g., ethical, epistemological, aesthetic) questions as well as scientific problems. We have investigated aesthetic objections to terraforming Mars (McMahon, 2016) and shown how geoconservation approaches can be applied to safeguarding natural and human heritage in the solar system (Matthews and McMahon, 2018). We have also investigated and reviewed the various ways in which astrobiologists should, should not, and do appeal to "Sagan's dictum", i.e., "extraordinary claims require extraordinary evidence" (McMahon, 2020; see also Cockell et al., 2021). Ongoing work in this area is supported by a new collaboration with Peter Vickers (Durham University).