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Friday, September 24, 2010

How old are these bones?

Over the last month I have sent in 22 samples of charcoal for radiocarbon dating analysis. This is how archaeologists find out the age of the things they excavate. At 3-600 bucks a sample, it is an expensive process, and this is far and away the most samples the museum has ever sent in for analysis. But we did a lot of digging and the samples are from our work at Alitak, King Salmon River, Cliff point, and a few from our surveys on the south end of Kodiak (the USFWS gave us some money to run a few dates of samples we collected for them in years past). Now comes the exciting part - getting the results back!

This week I found out the age of the midden we excavated at Mitksqaaq Angayuk during Community Archaeology. I had hope it would be 2 to 3 thousand years old and it turned out to 3400 years old (see older posts on dig from August for details). Perfect! This is really exciting because it means Molly can study how Alutiiq peoples' subsistence changed over the course of 3400 years in one place. Finding a midden that reflects 3500 years of hunting and gathering shells, fish, and sea mammals in one spot is really difficult. It is important for Molly because she can now argue that any changes that she sees in the midden composition reflects overall climate/human behavior and NOT differences in the location of the middens (i.e. if she had to study various middens from around the island to look at subsistence changes through time).

So the bones and shells in the above photo are 3400 years old! That is pretty darn old, and I only know of one midden on the entire archipelago that is older. It also means that Miktsqaaq Angayuk is the oldest continuiously occupied midden on the whole archipelago. No other midden, that I know of, on the whole archipelago has such time depth. It also means that the harpoons Leslie found at the bottom of the midden are over 3000 years old (see posts on dig from August).

Photos: Top: Close up of bones (fauna) from the 3400 year old midden. Second photo is of the midden after we 1/4 inch wet screened it. We had to wet screen it to find the charcoal we used for radiocarbon analysis - note the little black blocks of charcoal mixed in with the shells and bones. Finally the bottom photo is of one of the excavation walls and shows the midden stratigraphy. The shell and bones from about the middle of the wall on down are all older than about 3400 years (everything below the top rocky layer). Also note beach shingles at the bottom of the excavation - that is the 4000 year old beach that the people living at the site dumped their trash onto. Patrick


Molly O said...

Yay! Very good news! =)

jenny said...

hello! nice blog! i'm a swedish archaeologist and it is nice to read about what's going on at other sites in the world! a quick question; what type of tree is the charcoal of? and why do you use charcoal to radiocarbon dating analysis and not the bones or other macrofossil?

Zoya, Patrick, Nora and Stuart said...

We have ID'd the wood in the past and it is usually alder, cottonwood, willow, spruce or driftwood. If we can we try and date twigs or seeds. But in general we date what we can get - often it is VERY difficult to find enough of anything to date.

We do tend to shy away from shell or bone because of the 'marine resevoir effect'. Marine mammal and shell incorporate 'ancient' carbon into their carbon and tend to date too old. It can be done but you have to calibrate it etc. So I just tend to date wood charcoal and prefer to date twigs or seeds if I can. I also don't usually have enough funding to run AMS dates - so we need 5 or more grams of pure charcoal to run a date (300 vs 800 bucks!). Patrick