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

Bora Bora Dining and Food at Sunset

Knowing about food will increase the suc­cess of your dating

ResearchBlogging.orgLooking 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.

View Petchey et al. 2011 in a lar­ger map

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.


When he's not tired, fixing his car or caught in train delays, Alun Salt works part-time for the Annals of Botany weblog. His PhD was in ancient science at the University of Leicester, but he doesn't know Richard III.

5 Responses

  1. Geoff Carter says:

    Very inter­est­ing; essen­tially, you are con­tend­ing that the pres­ence of large quant­it­ies of mar­ine fish in a diet effects radiocar­bon dat­ing of bones;
    how much of this because of coral in this food chain?

    • Alun says:

      Short answer is I don’t know. The longer answer is that I think is the bar­rier between atmo­sphere and sea that will delay fresh C-14 enter­ing the mar­ine envir­on­ment. That would mean I’ve prob­ably over-fixated on the coral rather than simply fish in this post.

  2. Arise says:


    I really couldn’t digest the con­vert­ing of car­bon 14 to Nitrogen after an aver­age of 5730 years (this is OK) and then the Nitrogen waft­ing away into the atmo­sphere (totally not cool).

    The carbon-14 iso­tope atom would be bon­ded to other Carbon 12 atoms, form­ing a big biomolecule.…

    So say if there is one car­bon 14 atom out of the 12 car­bon atoms (11 being car­bon 12) of sucrose, you are imply­ing that once this car­bon 14 con­verts to Nitrogen it’ll just van­ish… What will hap­pen to the entire sucrose entity…?

    • Alun says:

      Yes you’re right. The pro­cess for get­ting rid of the Nitrogen is over-simplified. What hap­pens when the Carbon decays into Nitrogen is is the car­bon chain breaks. I’m not a bio­lo­gist, so I’ve had a search, and this seems the most plaus­ible solu­tion. It’s for amino acids, but the prin­ciple should be sim­ilar for most car­bon in a body. The car­bon con­verts to Nitrogen, this breaks the chain of car­bon molecules, effect­ively form­ing free rad­ic­als that need to bond with some­thing else. If the body’s dead then it’ll be broken down by bio­lo­gical pro­cess. Ammonia seems the most obvi­ous product for the Nitrogen, and then enter­ing the Nitrogen cycle, but I couldn’t tell you what the products of the car­bon remains would be.

      I’ll make an edit to tidy that up.

  1. January 5, 2011

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