Archaeologists prove the secret to a successful date is knowing what is on the menu
Looking from the outside, one of the most underrated areas of archaeological research at the moment is the Archaeology of the Pacific. It’s possible to make exciting discoveries anywhere in the world. In Polynesia though, it’s hard not to. The reason is that Polynesian archaeology has an odd contradiction. There’s been some excellent research done in the Pacific, yet it’s likely to be wrong. The problem is in the dating.
Take Easter Island. The big story there is the ecological collapse of the island. We know there was an ecological collapse because settlers arrived AD 800, their settlement patterns changed around AD 1200 and when they were discovered by Europeans there were relatively few people on the island. We know they were on the island in AD 800 because that’s been radiocarbon dated. If those dates were wrong, like if they were too old and settlers arrived later, then it’s not just a matter of tweaking dates on the timeline in textbooks. Suddenly there’s no native-caused population crash to explain.
Across the Pacific it turns out that many radiocarbon dates are too old. Testing the human factor: radiocarbon dating the first peoples of the South Pacific by Petchey et al. (2011) is a paper that helps explain why, but also shows which dates are accurate. First here’s a brief reminder on how radiocarbon dating works.
Your body mass is about 18% Carbon. Most of the mass is Oxygen in water molecules, but Carbon comes second. That Carbon has to come from things you eat. If you’re a vegetarian that will all come from plants. If you eat meat, then the animal you’re eating is likely to have eaten plants — or else it’s lunch will. Ultimately all the Carbon in your body comes from plants. They get it from Carbon Dioxide in the atmosphere.
The Carbon in the atmosphere is usually Carbon-12. That’s Carbon with six neutrons and six protons, but some is different. Cosmic rays sometime zap Nitrogen, with seven neutrons and seven protons. That zap converts a proton into a neutron giving it eight neutrons and six protons. Now it only has six protons it’s become Carbon, but it’s heavier than normal Carbon so we call it Carbon-14 because of its atomic weight. Carbon-14 is not stable. Sooner or later a Carbon-14 nucleus will spit out a beta-particle through radioactive decay. The energy the particle takes away converts a neutron back into a proton, making the Carbon-14 back into Nitrogen.
You can’t predict how long it will take for any single Carbon-14 atom to do this, but on average half of them will turn back to Nitrogen after 5730 years and
waft away into the atmosphere re-enter the Nitrogen cycle (edit due to comment by Arise). So if you’ve got a big enough sample, and you know how much Carbon-14 is missing you can date anything derived from plant matter like bones, books made from paper, charcoal from trees cut for burning etc. The trick is to know how much Carbon-14 is missing.
When you get a sample to analyse you don’t get a neat pile of Carbon-14 atoms and a big gap with missing atoms. You just have the Carbon in the sample, so how do you know how much is missing? You look at the Carbon-12.
To make the maths easy we’ll say that 1% of Carbon is Carbon-14. So a plant will also be 1% Carbon-14 as long as it keeps breathing in Carbon Dioxide. When it dies it stops breathing and the radiocarbon clock starts ticking. If we want to date a dead plant with 99g of Carbon-12 in it, we’d expect 1g of Carbon-14 in it if it died yesterday because there hasn’t really been much time for it to start decaying. If it only had 0.94g of Carbon-14 we’d know there was 0.06g missing and with a half-life of 5730 years, that works out at around 500 years.
That’s fine for plants, but what about human bones? Generally if you’re eating fresh plants you’re taking fresh Carbon-14 into your body during your life and your radiocarbon clock starts ticking when you die. The same goes for animals, so if there’s only a short delay between killing an animal and you eating it then that’s not going to make a big difference to archaeological dating. So eating cows or pigs is no big problem. However, the geography of Polynesia is different to somewhere like Europe or America. Here’s a map of the sites sample by Petchey et al that shows it.
The Pacific Ocean has a lot of water. Not good for pasture, but very useful if you want to eat fish. The thing about fish is their food chains are massive. Big fish eat smaller fish, who eat even smaller fish, who eat tiny fish who peck at coral and so on. Algae can bring in Carbon-14 but they have to get eaten by an awful lot of things before they make their way into humans. The radiocarbon clock starts ticking when the algae stop breathing and if their molecules get recycled through the coral reefs and the fish that live on them it can be decades or centuries before their Carbon ends up in a human. That means you can’t calibrate the amount of Carbon-14 a human bone should have if the person had a marine diet the same way you can if they ate what they farmed.
The difficulty is in working out what a person ate. You can’t even say that people at one specific site ate a 65% terrestrial and 35% marine diet. Fashions in foods and availability via trade mean that these proportions could change over time. Petchey et al have been able to match up work on chemical isotopes that reveal the source of food and then apply this information to recalibrate the radiocarbon dates. In some cases, like Watom Island (Yellow) they’ve been able to push the dates of sites back slightly. The chemical analysis matches well with archaeological evidence of pig bones in middens. The terrestrial diet means that an old date is more likely to be closer to the real date than a marine diet. Across in the east on Tonga they have a more secure date showing the site at Pea Village (Violet) is around a 1000 years later. The large dependence on shellfish and reef fishes means that the dates appeared older than they would have if they’d been eating pigs.
They’ve also been able to show that some burials are later deposits in earlier sites. That’s means that they can be eliminated from discussions about the colonisation of the Pacific. On the other hand it might also be evidence for the re-use of sites and help people discover what made an important funerary site in Polynesia.
The classic tale of a major scientific finding is that you simply find a Big New Thing, and The Textbooks Must Be Rewritten. It’s not so often that people talk about how previous research could be wrong. The satisfying thing about this paper isn’t simply the new dates, it shows how intelligent people doing their best could have come to the wrong conclusion and explains how they got their dates. Being able to integrate new findings with previous work rather than ignore them shows you know what you’re talking about. It is possible that new evidence at some of the sites will overturn a few of the dates in this paper. Perhaps a new midden will be found changing what we think about the ancient diet of people at a specific site. Still, because you can see the working you’ll still be able to use it as a source for future research even if parts are out-of-date.
Any time I see dates quoted for Polynesian sites now, the first thing I’ll be looking for is whether or not the archaeologists have taken into account the effects of diet. The cliché is true: you are what you eat.
Petchey, F., Spriggs, M., Leach, F., Seed, M., Sand, C., Pietrusewsky, M., & Anderson, K. (2011). Testing the human factor: radiocarbon dating the first peoples of the South Pacific Journal of Archaeological Science, 38 (1), 29–44 DOI: 10.1016/j.jas.2010.07.029
Photo: Bora Bora Dining and Food at Sunset by Duncan Rawlinson / Online Photography Course. Licenced under a Creative Commons BY licence.
Map: The map is based on information from Petchey et al. 2011, but is not their map and might have some inaccuracies at close scale. This is because I was too cheap to pay Elsevier £18:49 to re-use the map from the article.