Tag: radiocarbon

Getting a bit cross-eyed…

I’ve been working on dating the various samples from the WALRUS project.  Since walruses are marine mammals, direct C14 dating is problematic.  In fact, that is one of the things we are looking at with the project.  We ran a number of sample pairs of walrus  and terrestrial material and compared the offsets.  The bad news, there is not just one offset.

That meant that we couldn’t just use that to correct walrus dates.  So, we are reliant on construction of chronologies using the caribou dates we have run, as well as other available dates, to figure out how old the walrus samples are.  This is simple when we have a terrestrial sample from the same context.  When we don’t we need to see if we have dates from earlier and/or later levels (both is better) which gives us boundaries for the context we are trying to date.  To do that, we need to understand how the layers were arranged.  I’ve been using

For some sites, we have very little information beyond the mound or midden square from which the samples came, or the depth of the arbitrary level they were excavated from.  For others, we have more detailed stratigraphic information.  I’ve been developing schematic descriptions of stratigraphy for the sites from which we have samples , using the Harris Matrix as a means of representation.  Some of these are relatively easy to do and others are more complex.  For simplicity’s sake I have only been including sampled contexts in the Harris matrices, although someday I can add other excavated units.  The  one on the left below is from a site where we have little information on most mounds (we have samples from six), and those we do have info on were apparently dug in arbitrary levels, as far as can be told from the field notes.  The one on the right shows the two sampled  mounds from a site where I have extremely detailed provenience information.

 

These help me check that the dates we have are consistent with the stratigraphy, and are helpful in construction of more complex dating models in OxCal.   The process helps me think about how the models should be built, and also serves as a bit of a check, since having a crude model with some dates makes it easier to spot cases where a complex model may have been specified incorrectly (and therefore is giving incorrect results).

Walakpa Zooarchaeology

Last week I went to the Alaska Marine Science Symposium 2017 meetings in Anchorage.  I’ve never gone to those meetings before, but we were presenting two posters on finds from Walakpa.

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At the poster session.  Photo courtesy Raphaela Stimmelmayr.

One was on the results of the necropsy of a mummified seal found in a 1944 ice cellar.  I was excited about it because it was pre-bomb with a pretty tight date and therefore helpful for refining radiocarbon correction factors.  It turns out to be the first mummified seal reported from anywhere outside the Dry Valleys of Antarctica!  Who knew?

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The second was presenting some preliminary results of investigations on a polar bear skull which eroded from Walakpa, and was recovered and turned in by Kenneth Brower.  It turned out to be somewhat unusual in shape, as well as being really big (maybe the 4th largest ever measured).  That bear must have been HUGE!

theoldone_amms2016_final

 

Lab work

So far it has been pretty busy with Tony here.  He’d picked out a number of possible samples in advance, and we’ve been finding them.  I’ve also been going through the column samples we took last year at Walakpa, and finding contexts that have 2-3 datable samples of both land and marine animal bone.  That way, there will be multiple terrestrial dates to give us both a date on the level and information to use to develop correction factors for marine mammal bones.  Marine mammal bones tend to give radiocarbon dates that are too old, but sometimes they are the only thing available to date.  People generally don’t run dates in those circumstances, but if there were decent correction factors available it would be possible.

We went over to the Inupiat Heritage Center this morning to look at the material they have from Utkiagvik.  It looks like there is enough for a whole project there, although the location of the 1981 field notes was unknown for the moment.  Some of the staff came out to the lab later and looked at a couple of sewn objects.  The mystery one is made out of gut which has been stitched to form a sort of pointed tube, although it is pretty crumpled up.  If we get a conservator to visit, it would be great if they were able to rehydrate it a bit and straighten it out.

Tomorrow we are going to be going through boxes of faunal material from Pingusugruk to find suitable samples.  There are hundreds of boxes, so Tony is trying to select some contexts to pull, and then I have to ID appropriate bones.  I am really hoping that the intern  shows up for work.

On the Walakpa front, I’ve been talking with the UIC Science logistics folks, working on how to handle temporary housing for those transiting in and out of the field, and starting to work on travel arrangements.

A day in the lab

I spent much of Friday in the lab, selecting items from Walakpa to send off for radiocarbon dating.  We had a reasonable set of samples from 2013 and funds to run the dates, but given that that entire area is gone, I wanted to get some idea of how old some of what was exposed this fall is.  That meant I had to make some choices about what got sent and what didn’t.

We had managed to collect several caribou bones, but most of them were ex situ (not in their original location).  There are also several samples of plant material from known locations which are much more likely to be informative.  Everything had been frozen as soon as it came in from the field due to the aggressive mold we had had to deal with last year.  For carbon dating, the lab needs to have a certain minimum weight to work with (varies by each type of material), which means that the samples had to be thawed enough to allow them to be split, cleaned, and dried enough to make sure that the weights were accurate.

Beta Analytic has got a slick new sample submission interface that I had never used before.  It has a few quirks, which meant that I had to quadruple check the submissions to fix things.  I got better at it, so .  In the end, it prints out a barcoded form that you put in the package with the samples.

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By the time I finished, it was too late to mail the samples on Friday.  The US Post Office in Barrow doesn’t have any counter service on Saturdays, so they’ll get mailed on Monday.

As a result, I didn’t get to see much of the sun that day.  It is almost time for it to go down for the winter, and we’ve had so much cloudy weather this year, it was a pity to miss a rare sunny day.  By the time I had finished, the sun was down, and this was the view from the BARC.

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UnPhotoshopped iPhone picture of the sunset

 

Sunshine & blue skies…

…and a full crew.  We started work on the burial under the plank.  It took quite a while, as the plank was complex to define.  It was all one piece, in some places up to 9 cm thick, and had obviously required a great deal of work and skill to make.  The top surface looked like the outer surface of the tree.  The bottom surface showed evidence of burning, in some areas completely charcoal.  It should be good for C14 dating.

The remains are those of a large male.  Preservation is a bit variable, but it looks like there might be some ribs that could yield aDNA.  In any event, he will be safely out of the trail.

Many on the crew did STPs.  So far nothing has shown up.  It is beginning to look as if there is a gap in the burials (or at least a much thinner distribution).  I’ve begun to wonder if this could be the result of a village move due to erosion, which brought the village close to the cemetery and made them skip a bit of ground to put the new cemetery at a proper distance from their residences.

At least it was a beautiful day.

One turns into three…

It was a great day in the field, with really lovely weather.  Dennis O’Rourke got in last night and joined us, as did Rhett Herman, & his student Jared Palmer with the GPR gear.

Crew members at the start of the day

We started excavation of the burial that had been found in the road.  As with most road burials it has suffered some disturbance.  At first it looked like there was a large man (found part of his pelvis), then a neck vertebra from a small person showed up, then one skull, then another, and then the nasal area from a third (!) person.  We still have more to do tomorrow, so this may change, but at the moment it looks like there may have been two burial side by side, and a third burial was dug across one of them at a later date.

Starting excavation of the burial

The GPR  guys had a good workout.  They had set up the units on carts, which had worked well yesterday on the beach near NARL, but something at Nuvuk must be different, because they described it as being like “pushing a shopping cart in sand.”  Naturally, this meant that they got less done than they had hoped, but they saw some things and will have a plan view in the morning.  I saw them this evening, and they’d already changed the configuration to a dragable sled, which seems like it might make tomorrow better for them.

GPR gear ("Eva") in foreground, with flagged survey grid in rear
Jared pushing the GPR lawnmower in a brief foggy period

The slog continues…

I am still plowing through the literature seeking out information on C14 dates.  Some of it is really hard to come by, with a date attributed to a house but no  information on the sample, either what it was or where it came from.  Then I look at the information on the artifacts from that house, turn to the plates (not naming any names here, but there are multiple offenders) to look at the artifacts, and see that the plates say they are from a house nearby.  Obviously either the text or the plates are wrong (unless they both are, but I’d rather not go there), so now one is left quite unsure of what was really being dated, and what sorts of artifacts were actually associated with that date.  Cross-dating based on artifact assemblages takes another one on the chin.

Another example from today: a date on wood and skin (what kind? caribou, seal, polar bear?) from a burial for which the description seems to indicate that it was only a few baby teeth!  It’s one thing to have typos in a dissertation, but in published books that people are expected to pay money for?  If people don’t read carefully, and compare about three different places in the book at once, it’s all too easy to accept a date at face value and assume that what is said about what it was found with is correct.  Then it gets mentioned elsewhere, and people read it there and pass it on, and so forth.

Such dates do not get high scores for context or association with the event being dated.  Actually they get zeros, since those factors are unknown.

All this in aid of a handbook article (well, two articles, since this C14 stuff should make an article as well).  On the other hand, a number of people say it should be useful.  I’m sure they’re all really glad that I’m doing it and they aren’t.  Can’t say I blame them.

Occam’s Razor is going to be coming in handy

The fun with radiocarbon dates continues.  I did manage to get a proposal off to a client, make some preparations for the summer field season and take care of the usual admin sorts of things.  Otherwise, I was working on the C14 dates.

It was slow going, in part because I read French much more slowly than I do English, and I was working my way through the Blumer compendium of St. Lawrence dates, which requires looking in at least 3 places to figure out how to evaluate the dates.  In some cases, one also has to go to other books to look at what the original excavator recorded (or didn’t).  Thank goodness for the American Museum of Natural History and their very nice downloadable PDFs (although the link seems troubled at the moment) of their Anthropological Papers.  I had a couple of them on my hard drive, which saved me a trip to get the actual books.

Anyway, St. Lawrence is going to be quite a mess.  There are a lot of whale and walrus dates, and Dumond  has calculated a correction for them by paired dating with terrestrial plants.  The only problem is that the whales in St. Lawrence are the same stock as the whales they catch here, and whalebone C14 doesn’t turn over very fast (a couple of decades at least) so they average the ∂C13 over that period.  That means that the correction factor for those whales should be the same anywhere in their range.  We’ve worked on it here for the Nuvuk graves, and the correction factor that works is much smaller.  I’m guessing walrus ingest relatively huge amounts of old carbon and skewed the calculations…

There was a very nice evening sky on the way home.

Sky from NARL. It was actually brighter, but this exposure shows the colors best.

Wrestling with dating

That’s what I’ve been doing lately.  No, not that kind of dating.  I’ve been wrestling with how old sites are, combined with the catching up after travel and working on taxes and other forms, it’s kept me busy enough that blogging kept coming in second to sleep.

I got started on this because I agreed to write a chapter for a forthcoming handbook of arctic archaeology dealing with Western Thule to Late Precontact in Northern & Western AK.  The northern part was fairly easy, since this is where I have been working for years, and I know the literature inside out, and am responsible for most of the C14 dates in that time frame, as it turns out (a lot of the major sites here were excavated before C14 dating was invented) but I needed to brush up a bit on the more southern parts of the area, I felt.  When I started doing that, I realized that terminology was a bit fuzzy (early investigators tended to think the entire sequence had to be present, and to call things, say, “Birnirk” because they thought there must be Birnirk, rather than because there were any diagnostic (types or designs only found in one culture) Birnirk artifacts at the site.

Then it became clear that people were using some artifact types as “index fossils” without being clear that they were really only in use for a limited time period.  So if they found such an artifact in a feature, they assumed the feature was used at the same time as all other features with that artifact in them.  Fairly recent C14 dating of some such artifacts has shown that some are pretty good to use, and others were being made for hundreds of years, so they aren’t really much help.

There is a pretty good dendrochronological (tree ring) series for the southern part of the area, due to Lou Giddings‘ pioneering work.  This has enabled people to date wood, although in some cases the possibility of wood reuse seems not to have been considered thoroughly, and only one log dated in a feature.  Then artifacts and artifacts assemblages (groups of artifacts often found together) found in that feature have been dated from the dendro date, and similar assemblages have been assumed to be about the same age.

Add the fact that there are a number of beach ridge complexes  in the area (Cape Krusenstern, Cape Espenberg), which develop over time.  Once people figured that out, it was a logical (and frequently correct) assumption that maritime-adapted people would choose to live on the ridge closest to the ocean.  From there, it was only a short step to deciding that all features on a particular ridge were fairly close in age.  In fact, as people have started doing more C!4 dating, and understanding how to interpret the dates better, it’s clear that isn’t the case.

As you can see, it’s not a pretty picture.  Add the tendency of earlier researchers to conflate time periods (as they understood they) with archaeological cultures, and things get really confusing.  Since this is a handbook, which one assumes is meant to be around for a while (although come to think of it, the Encyclopedia of Marine Mammals, in which I have a co-authored article, is in a 2nd edition less than seven years after the original came out), one would like to write an article that will not be rendered obsolete immediately.

So….  I’ve been going through the literature actually looking at the details of dates given for sites, dendro, C14 or otherwise, and am evaluating them on 9 different criteria to try to winnow out dates that are really reliable to base the chronology on.  That doesn’t mean other sites/features won’t be mentioned, but it does mean I’ll avoid putting hard dates on them, or using them as a basis to date yet other sites.

This is not fun.  I just hope it’s productive.