February 12, 2009

Not types of cheese: tufo, pozzolane, peperino

My task before meeting with the geochemist today was to find out the age of the limestones in and around Rome. Most of Rome is composed of volcanic rock, of course, but there is some travertine in the area as well. Turns out, both the limestones and the lavas are middle Pleistocene in origin. A very useful article on the travertines around Rome even told me that the Sr ratios all cluster in the low .708s. Even though I wasn't tasked with looking into the volcanic rock, I found that the stuff around Rome (Alban Hills, Monti Sabatini, Monti Volsini) is all in the .710s. I should, then, find that any "local" Romans would fall into the range between .708 and .710; but most of them should be more in the .710 area, since the volcanic area around Rome is huge in comparison to the small outcroppings of travertine.

Of course, the vast majority of my samples fall within this range. Interestingly, the pig samples I took from each site are .709-.710. Neither one is so low that I would suspect a travertine signature. (But if anyone wants to send me some rodent teeth from Rome, that would help greatly!) So there are still some outliers in the people. Unfortunately, only one is crystal clear: a guy at around .714. There is also a .712 in the high range and three in the .707-.708 lower range. What does this mean? Who knows. Well, I haven't figured it out yet. It's still entirely possible that these people with Roman signatures were from elsewhere: much of the west coast of Italy is made up of volcanic material. Two published Sr ratios from about 60km north of Rome show volcanic rocks that are .713-.715. So perhaps the .714 guy didn't come from very far. At this point, I'm looking into other possibilities, other places that geologically look like Rome, as well as a possible location for the lower .707-.708 values. The geochemist noted that lower values could indicate younger volcanic rocks, namely basalts, so I combed a map of Italy for an hour today looking for all the places that could have basalts or were similar to Rome. If I can find published Sr ratios for these areas (places like the Dolomites in NE Italy, and volcanoes like Etna and Vesuvius), perhaps that'll help.

The one major drawback of my project is that I don't have archaeological evidence to suggest that people came from anywhere else. Pretty much no one was buried with grave goods, and certainly no one was buried in a way that seemed truly anomalous. It's easier to test a few individuals from two groups of people you think are different than to test people with the simple hypothesis that surely some of them are immigrants. I had hoped that my sampling strategy - namely, to test every single person - would combat this problem, but one clear immigrant is far less than the 20% I had expected based on historical demography. Perhaps the people who were immigrating were the middle classes and the slaves who were bought by the upper classes. It's entirely possible that the lower classes were just Roman poor - or Italian poor who made their way a few dozen kilometers to the city to seek their fortunes. Hopefully, the oxygen analysis will find additional immigrants, and I might get to do some lead isotopes as well. Until then, I have to figure out what to say in my AAPA presentation about the Sr data but, even worse, in my SAA podium presentation about Roman immigrants. I think I left my abstracts just vague enough to get by, though.

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