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Saturday, February 20, 2016

How old is this Site?

The white Katmai Ash fell in AD 1912 the Glaciers melted sometime between 12-18 thousand years ago - all the other layers date to somewhere in between

We've been digging at the Kashevaroff Site for 3 years now, and based on the types of artifacts we've been finding and the stratigraphic evidence I have had a pretty good idea of the age of the various layers we've uncovered. But a few days ago we got back the results of three radiocarbon dates I had run on charcoal from the site.  Now I have official confirmation of my educated guesses.  And guess what?  My estimates matched the radiocarbon results pretty much exactly! 

This does not always happen.  Once I thought we had been excavating a 6000 year old house and then after radiocarbon analysis it ended up as 3000 years old.  Radiocarbon dates are a good check, and this time they confirmed rather than refuted my interpretations of the site's stratigraphy.

The ancient Alutiiq structure seen in the profile in the picture above - the flat surface in the lower right above the mottled ashes is the floor of the structure - ended up a little older than I thought it would be.  It had ground and chipped tools so I had thought 6300 years.  It ended up dating to around 6700 years old.  One of the oldest dates for ground tools on the Archipelago.

We also believe the inhabitants were catching seals and smoke processing the meat on site (we only found calcined sea mammal bone - no fish).  As suspected this activity was on going 4 to 5000 years ago.  The grooved cobbles pictured below might have been used to hold down nets used to catch seals.  If so these are the oldest net sinkers ever found on Kodiak.  Later in time netsinkers became more standardized and had the notches on the long rather than short axis.

It's nice to finally have some firm dates!

These notched cobbles were created around 4300 years ago

The charcoal at the bottom of this small processing pit dated to around 4700 years ago

Raw data sheet for the radiocarbon dates

Radiocarbon dating works because every living thing incorporates carbon into their bodies (all carbon based life anyway) and a percentage of this carbon is the radioactive isotope carbon 14.  Through time and at a steady, known rate the radioactive carbon breaks down into normal carbon 12.  So by measuring the ratio between carbon 12 and radioactive carbon 14 one can get an idea of how long ago the organism died and stopped incorporating carbon.  The half life of radioactive carbon 14 is about 5700 years ago - so if half the expected carbon 14 is present then the organism died around 5700 years ago.

However,  radiocarbon time is not the same as real time. Scientists figured out the decay rate of carbon 14 by dating the individual tree rings of really old bristlecone pines.  They knew the exact age of the wood they were dating and were able to create a curve showing the age of the wood and the amount of radioactive carbon left in the wood (see multiple points on curve below).  What they discovered is that the curve wobbles all over the place - it turns out that the cosmic rays that create carbon 14 vary slightly in intensity through time.  Hence radiocarbon time, unlike real time, speeds up and slows down.

And this is why all radiocarbon dates should be calibrated into real time.  At times (see below) a radiocarbon date can intercept the curve in a number of places, and there is also always a degree of possible error.  For this reason all radiocarbon dates end up as an actual range of time and not a single point in time.  But depending on the age that range can be small or large.  For instance, the calibration curve shown below shows a lot of wobbling between 4500 and 5000 years ago - radiocarbon time was slow, the curve flat, and the range of the date is large.  Another of the dates shown above has a smaller age range because between 6500 and 6800 years ago radiocarbon time was moving fast and there was a steeper curve.

Anyway, the take home message is that radiocarbon dates are NOT all that precise.  I'm happy with plus or minus 100 years or so, and fully realize that they do not represent a point in time.  Patrick

How raw radiocarbon dates are calibrated against a curve

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