My research is based on a coastal archaeological site over karst, next to a river mouth in the north of Puerto Rico (northeastern corner of Caribbean Archipelago). I am excavating a site that was originally excavated in the 70s to a depth of 210cm bs. My own excavations this summer were only to 45cm. The radiocarbon dates previously published are not reliable, and I want to produce new dates. From the old excavations I only have available for dating bone or marine shell. From my own excavations I have also burnt organics, including seed, and I already sent a set of samples that will help me calibrate the local marine reservoir effect.
I want to get a date from the deepest level (200 - 210cm bs) from which I have three molluscs: Chiton, Fissurella nodosa and Astralium phoebium. These represent two different marine ecosystems: intertidal rocky shore and sea-grass. Is there any difference on their process of obtaining 14C? Does it make any difference if the species is is intertidal or submerged? Which specimen would be best to select?
The sample used in the MRE calibration is an infaunal, soft bottom species (Tagelus divisus).
Thanks in advance.
Dear Isabel,
I forward your question to Antoine Zazzo (C14 zooarchaeologist), because he is a specialist.
I would suggest you the Polyplacophore, for the raison that they are local and rocky.
Do you have any rodent (Dasyproctidae or Cricetidae) or fruit bats (Phyllostomidae) in your samples ? I would select these thruly local and granivorous animals for C14.
Best,
Sandrine
Hi,
Depositional transport is not really an issue in your case since it is an archaeological site so these shells were likely colected live and consumed on site by people.
What really matters then is the preservation state of your shells. Make sure to avoid chalky shells, recristalized material...etc. You can check the preservation state with XRD or with SEM observations. Thick shells are safer.
If you are not sure whether these shells have been influenced by freshwater during their life, their d18O and d13C ratios would tell you.
cheers,
Matthieu
Thank you all!
André and Matthieu: The samples are from an archaeological site and in direct association with other archaeological materials so I am not worried about transport beyond human selection and consumption. They are all in good conditions with no evidence of abrasion. The comment regarding crystallization and mineralization is very important. I will make sure we look at that before making final selection.
I do worry about possible old carbon because of the karstic location. I hope that the set of charcoal and marine shell that we sent over (recovered in direct association with each other and within a ceramic vessel) will help us calibrate the local MRE and give us a better grasp on old carbon input to calibrate the current and future 14C dates we produce.
Sandrine, these samples were of the early excavation (Ovidio Dávila's), where no small-sized sieve was used (and I wonder if sieves were used at all!). For the level we want to date (200 - 210cm below surface) there is only fish and marine shell. I wanted to send anything terrestrial, but there is nothing for this level. If you hear back from Antoine, let me know.
I will double check for re-crystallisation, but thanks to your advice I am inclined to send the Chiton.
Again, big thanks to all.
Isabel
Dear Isabel,
As suggested by others, most commentators recommend dating suspension-feeding bivalves rather than gastropods (the latter often ingesting 'old' carbon from the environments they live in). There are some useful guides to marine shell taxa selection for dating, including the Oceania Marine Reservoir Dating Project website at: http://www.radiocarbondating.com/imagesOMR/homepageframe.html. Two papers led by Fiona Petchey comparing various bivalves and gastropods in the Caution Bay sequence at Caution Bay in Papua New Guinea which appeared in Radiocarbon (2012) and Archaeological and Anthropological Sciences (2013) are very useful too.
However, I think your major issue will be the impact of limestone on radiocarbon activity in all of the taxa in your deposit. Fiona Petchey as a very useful summary of some of the impacts of these issues with examples from the Pacific in her paper available for free download at: http://press.anu.edu.au/wp-content/uploads/2011/05/ch117.pdf
Good luck with your research!
Regards,
Sean
Hoolllaaaaaa Isabeel!..tan pronto acabe en Junio nos comunicamos!! Exito!!!
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