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Saturday, January 21, 2012

Amak Radiocarbon Dates




Last week I finally got back the radiocarbon dates for last summer's excavation at the Amak Site. Now we know the age of everything we excavated. By the end of the summer we already had a pretty good understanding of the site's stratigraphy, and I had some pretty educated guesses about the age of the various layers we had excavated. See 'Amak Stratigraphy' post -

http://saltonstall.blogspot.com/2011/08/amak-stratigraphy.html

Still they were only 'educated' guesses and I have been horribly wrong before. Once I thought a house we excavated would be around 6000 years old and it ended up 3500 years old instead. That bad guess forced me to re evaluate what I thought I knew about that particular site, and I ended up with a more refined understanding of the site's stratigraphy. I knew why I had made a bad guess. Nonetheless, I don't like being wrong. And so I was very happy when all the dates from last summer's dig came back pretty much as expected.

Radiocarbon dating is a method whereby we send off a piece of charcoal from the layer we want to date to a lab in Miami, Florida and they put it into a fancy machine. The machine determines the ratio in the sample between the radioactive carbon 14 and the stable carbon 12. Carbon 14 is unstable and slowly but surely reverts to the stable carbon 12 with time after an organism like a tree dies. Half of the carbon 14 will have reverted carbon 12 after 5600 years. They calibrated this curve by dating really old bristlecone pine trees where they knew the age of the wood exactly. So if you know the ratio then you know how long the carbon has been dead. Anyway it is complicated and I have probably confused you with my ramblings.

All that really matters is that I sent off charcoal samples to Florida and a couple of weeks later they told me the age of the various samples. They told me that the site was abandoned shortly after 3800 years ago and that the earliest occupation at the site was around 7100 years ago. The main occupation at the site, the time period when they were making all the pretty slate hunting lances, dates to about 5500 years ago (which means that that charcoal sample had about half of its radioactive carbon remaining).

As I said above, the dates came back pretty much as I expected. The only real surprise is that I think the youngest date pretty much dates a massive volcanic ash fall. The ash was churned at the site and the structure we dated looks like it might have been standing when the ash fell (on far left in the profiles). I had hopes that the oldest date would be older - I really want to get a date older than 7500 years (the oldest date so far on Kodiak) - but 7100 is about what I expected for the date of the thin black occupation layer at the bottom of the site. Finally, the 5500 year old layer contains tools typical of the 'Ocean Bay II' era on Kodiak, but it is an early Ocean Bay II occupation. Good dates all! Patrick

Photos: Top photo is an aerial photo taken by a friend of ours Keller W and it shows the site landform and our excavation pretty well. When people lived at the site the bottom third of the photo would have been water while there would have been a lagoon and stream behind the site. Today the ocean is almost a mile away - I actually wonder if that 3800 year old ash fall might have filled up the bay and created a new beach much further down valley. This would have forced the abandonment of the site.

The other 2 images show the same profile from our excavation - the north wall of the main unit. The top white layer represent the latest volcanic ash fall from the 1912 Katmai eruption. The largest volcanic eruption of the 20th century deposited many feet of ash onto the city of Kodiak almost 100 miles away. It dwarfs the Mount St Helens eruption.

1 comment:

Molly Odell said...

Cool - glad all the dates turned out reasonably!

14C doesn't actually revert to 12C - it reverts to 15N! The radioactive isotope 14C forms when atoms of 15N collide with those of 12C while entering the Earth's atmosphere. So, when 14C decays, it reverts to 15N.

I actually just learned this stuff WELL when I had to TA "Fundamentals of Archaeological Method and Theory" last quarter. The funny thing is, I bet most high schoolers have a better grasp of this stuff than most college students (or even me!) because chemistry is so fresh in their minds.