Archaeologists prove the secret to a successful date is knowing what is on the menu

Jan 5, 2011 Alun Science, The Past 5 Comments Archaeology, Colonisation, Dating, Oceania, Pacific, Polynesia, Prehistory, Radiocarbon, Research Blogging

Know­ing about food will increase the suc­cess of your dating

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

Cred­its:
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.