PhD
PhD by Research
Our PhD by research programme is based on independent study, guided by your assigned supervisors and support system. It typically [...]
Sampling site
Research Project Overview
Radiocarbon dating using Carbon 14 (14C) has become an integral part of the archaeological inventory of tools (Aitken, 1990). Unfortunately, one of the problems that can arise in using marine shells for radiocarbon dating is contamination by old carbon (e.g. Bezerra et al., 2000; Busschers et al., 2014; Allen et al., 2019).
Shells may incorporate dissolved carbon dioxide or bicarbonates from “old” and deep ocean waters that are not in equilibrium with the atmosphere (reservoir effect), or freshwater runoff from limestone areas (hard water
effect). This is further compounded by observations reported by Dye (1990) that limpet shells collected from carbonate substrates dated older than they should, probably due to their ingesting ‘old’ carbonate from the limestone rocks they scraped their food from. Whereas the first two issues can be compensated for, the latter one is problematic as it is practically impossible to compensate for without detailed studies on how these species accumulate old carbonate from feeding.
This has led to a devaluation of mollusc shell material for dating marine coastal contexts, despite the fact that they are increasingly considered pivotal habitats in the evolution of our species (Colonese et al., 2011; Fa, 1998; Fa et al., 2015). Being able to account for these differences would make a significant contribution not only to the amount of available material for dating from human settlements, but also to the discipline of radiocarbon dating in marine habitats, and by extension make a significant contribution to the study of our species with regard to its associations with the marine environment.
Project Team
Darren Fa, University of Gibraltar
Kerry Allen, Centre for Climate, the Environment and Chronology (14CHRONO), Queen’s University, Belfast.
Analysis of substrates and species in Sardinia
Significance of the research
The objectives of this study are to establish, what, if any, are the effects of substrate upon grazing molluscs with regard to the values they produce when dated using 14C.
Marine mollusc shells are commonly found in prehistoric sites around the world and it is well established that rocky intertidal zones offered predictable and renewable food sources that could be harvested by all members of a group (Bailey 2004; Fa 2008).
Limpets, periwinkles, and mussels all were consumed by prehistoric humans and their shells have the potential to provide good material for 14C-derived chronological control of prehistoric events (Stiner et al. 2003).
Given the important role that these organisms have played in past societies (food, decoration) the results of this relatively simple exercise may have significant consequences upon both our understanding of the life histories of these marine species and also on their knock-on effects with regard to understanding the past human societies that lived along the coast.
Research methods
In this study we measure the 14C of shells of intertidal limpets from both the limestone rocks of Gibraltar and the varied substrates of Sardinia to determine variations in mollusc 14C content.
Live samples of limpets that typically occupy different shore levels (Patella rustica, Patella ulyssiponensis, Patella ferruginea (under licence) and Cymbula safiana) were collected from intertidal limestone and basalt rocks at sites on the eastern side of Gibraltar, as well as P. rustica from limestone, sandstone, granite and basalt rocks on the western coast of Sardinia at Tharros, Seu, Mal di Ventre and Capo San Marco.
Samples of Phorcus turbinatus, a grazing topshell, were additionally collected from Gibraltar for purposes of comparison, as well as filter-feeding mussels (Mytilus galloprovincialis, Perna perna) which were also collected from all locations as mussels are filter feeders and should be unaffected by substrate type.
All shell samples were radiocarbon dated and were also tested for carbon and oxygen stable isotopes. Details of methods can be found in the published article.
Molluscs from Gibraltar
Location in Rosia bay for sample collection.
Project collaborators
The radiocarbon and stable isotope measurements in this study were partly funded by a Research Grant (2021) from The Prehistoric Society awarded to Kerry Allen. All other project funding was provided by a research grant from the University of Gibraltar awarded to Darren Fa.
Grateful thanks need to go to the various colleagues at various locations who freely gave of their time to collect samples for us:
- Igor Gutiérrez-Zugasti (University of Cantabria, Spain),
- Maite Vasquez Luis (Instituto Español de Oceanografía, Mallorca, Balearic Islands, Spain),
- Marcos Rubal Garcia (University of Porto, Portugal),
- Esteban Álvarez-Fernández (University of Salamanca, Spain).
Special thanks must go to Stefania Coppa (Istituto per lo studio degli Impatti Antropici e Sostenibilità in ambiente marino and Consiglio Nazionale delle Ricerche, Sardinia, Italy) and Stephen Warr (Gibraltar Government Department of the Environment, Sustainability Climate Change and Heritage, and University of Gibraltar, Gibraltar).
Relevance to the wider community
Initial results indicate that apart from Patella rustica, there were no significant F14C offsets.
In short:
- F14C for P. rustica is significantly depleted compared to other species.
- This is particularly evident from locations with a carbonate substrate.
- Lower δ13C values indicate higher metabolic carbon in P. rustica.
What is different about this species?
- Patella rustica inhabits the splash zone of the intertidal.
- P. rustica must withstand long periods of desiccation and starvation.
- They accomplish this by having a low metabolic rate and the ability to store glycogen.
- P. rustica feeds mainly on cyanobacteria, which can take up carbon directly from the substrate.
Our results therefore suggest that shells from upper-shore species will have a very large uncertainty in the reservoir age and, therefore, are not suitable for radiocarbon dating, especially if they have come from an area of carbonate geology.
However, and very importantly, indications are that grazing species from lower-shore habitats, which experience daily immersion, may be suitable for dating from any substrate.
Patella rustica Linnaeus 1758
Pathways to impact
If our findings are corroborated by other studies, significant collections of lower-shore mollusc species such as limpets that have historically been neglected for radiometric dating can now be utilised for this and other purposes, providing additional data to inform existing hypotheses and the generation of new ones.
Of course, the interesting results obtained for upper-shore grazing species such as P. rustica now require us to investigate the question of exactly what processes are taking place in the upper sections of carbonate geology shores to generate the results obtained.
Project-related outputs
Allen, K. and Fa, D. A. 14C offsets in marine limpets: Observations on upper-shore mollusks and implications for their use in radiocarbon dating. Radiocarbon. Published online 2026:1-14. doi:10.1017/RDC.2025.10184
Selected References
- Aitken MJ. 1990. Science-Based Dating in Archaeology. London: Longman.
- Allen, K., Reimer, P., Beilman, D., & Crow, S. (2019). An Investigation into 14C offsets in Modern Mollusk Shell and Flesh from Irish Coasts shows no Significant differences in areas of Carbonate Geology. Radiocarbon, 61: 1913-1922. doi:10.1017/RDC.2019.119
- Bezerra, F.H.R., Vita-Finzi, C. & Pinheiro Lima Filho, F. (2000). The use of marine shells for radiocarbon dating of coastal deposits. Revista Brasileira de Geociencias, 30: 211-213.
- Busschers, F.S. et al. (2014). Radiocarbon dating of late Pleistocene marine shells from the southern North Sea. Radiocarbon, 56: 1151-1166. DOI: 10.2458/56.16505.
- Colonese, A., Mannino, M.A., Bar-Yosef Mayer, D.E., Fa, D.A., Finlayson, J.C., Lubell, D. & Stiner, M.C. (2011). Marine mollusc exploitation in Mediterranean prehistory: an overview. Quaternary International, 239: 86–103.
- Dye, T. (1994). Apparent ages of marine shells: Implications for archaeological dating in Hawai’i. Radiocarbon, 36: 51-57.
- Fa, D.A., Finlayson, J.C., Finlayson, G., Giles Pacheco, F., Rodríguez-Vidal, J. & Gutíerrez López, J.M. (2016). Marine Mollusc exploitation as evidenced by the Gorham’s Cave (Gibraltar) excavations 1998-2005: the Middle-Upper Palaeolithic Transition. Quaternary International, 407: 16–28.
- Fa, D.A. (2008). Effects of tidal amplitude on intertidal resource availability and dispersal pressure in prehistoric human coastal populations: the Mediterranean-Atlantic transition. Quaternary Science Reviews, 27 (23-24): 2194–2209.
- Ferguson, J.E., Henderson, G.M., Fa, D.A., Finlayson, J.C. & Charnley, N.R. (2011). Increased seasonality in the Western Mediterranean during the last glacial from limpet shell geochemistry. Earth & Planetary Science Letters, 308: 325–333.
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