May 18, 2010

Just when I thought the Sr was done...

It seems my Sr mixing model equation for Rome was wrong. I didn't take into account the Sr concentration of the two sources (aqueduct water from the Monti Simbruini in the east and aqueduct water from the Colli Albani), just the relative abundance of water in the mixture. So I hung out with an isotope geochemist for the better part of an hour today and got a better equation. What it showed was that, even though two-thirds of the aqueduct water was coming from outside of Rome, because it has a very low strontium concentration, its contribution was being dwarfed by the local water, which is strontium-rich. Basically, if a person was drinking more than 10% of their water from sources in the Colli Albani, their strontium isotope ratio will be indistinguishable from someone who drank 100% of their water from the Colli Albani.

What this means to me is that, in all likelihood, the strontium range of Rome is much narrower than I thought, perhaps .7090 to .7100 as suggested by my other mixing model. However, I cannot rule out the possibility that there were people living in Rome or the suburbium who got all of their water from the eastern aqueducts. It seems that aqueduct water was not generally mixed at Rome; that is, public fountains, castella, baths, and private houses were fed by one aqueduct. We know a lot about which aqueduct fed which part of the city or which fountain/bath/castellum. As the strontium ratios I measured are from first molars, they reflect an average over the first 3 years of a person's life - the time when a Roman was least likely to be independently mobile. So, if a person grew up in Rome near a castellum (a reservoir that his/her parents drew water from) or lived in a villa that was supplied by the eastern aqueducts (Anio Vetus, Anio Novus, Claudia, and Marcia), that person could easily have gotten over 90% of his water from the lower-Sr sources. Similarly, if a person grew up in the suburbium in a villa that illegally tapped the aqueduct (which was quite common), that person might have a lower Sr signature.

Since I can't rule out the possibility that people living in Rome could have obtained significant amounts of lower-Sr water, this doesn't change my identification of immigrants or my dissertation conclusions. I still need to posit a very conservative Sr range for Rome. Should I ever get more Sr ppm data, I might be able to narrow down the local range at Rome, thereby identifying people who probably came from Campania, which has slightly lower Sr isotope ratios than Rome but similar oxygen ratios.

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