Radiocarbon dating a hearth from 2019 changed my whole view on fieldwork

I sent a charcoal sample from what I thought was a Bronze Age pit in Kent to a lab last month. It came back dated to 2019, which is when I remember a group of students having a campfire near that exact spot. The kicker is the layers above it looked undisturbed, so I never questioned the context. My mentor just laughed and said 'garbage in, garbage out' when I told him. Has anyone else had a modern contamination ruin a sample? How do you screen for that before sending stuff off?

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matthew_baker18d ago

Yeah that exact thing happened to me back in 2017 with a hearth in Somerset. I stopped sending charcoal from any context less than 30cm deep unless I can see the full profile with my own eyes and there's a clear cut feature. What works for me now is I always grab a soil sample from right next to the charcoal and send that for a basic pH and organic content check first. If the soil looks like it's been disturbed or has modern roots running through it, I just don't bother with the expensive dating. Also I started asking the lab to do a quick carbon isotope check before the full run, it's like 20 quid extra and catches bomb carbon from the 60s and later. The layers above your pit might have looked fine but roots can wiggle down and pull modern carbon with them, so now I dig out the whole pit bottom and look for root channels before I even bag anything. It sucks to lose a sample like that but honestly it's a good reminder that field notes lie more than we want to admit.