I let him go on.
“For example, basaltic lavas, limestone, and marble all have very low Sr ratios, whereas those of sandstone, shale, and granite are commonly high. Clay minerals have some of the highest.”
“So differences in geologic age and/or bedrock composition produce variations in Sr isotope ratios in different geographic regions.”
“Precisely. But one final thing to keep in mind is that because ratios are so messy to remember, with all those decimals, we usually compare a measured Sr ratio to the average Sr ratio of the whole Earth. If the measured ratio is greater than this, it yields a positive value. If it’s less than this, it gives you a negative value.”
“What does this have to do with establishing where someone was born?”
“Strontium is an alkaline-earth metal, chemically similar to calcium.”
I made the link. “Strontium is absorbed by plants from the soil and water. Herbivores eat the plants, and on up the food chain.”
“You are what you eat.”
“So the Sr isotope composition of an organism’s bones and teeth will reflect the Sr composition of its diet during the period those body parts were forming.”
“You’ve got it.”
“My grandmother used to worry about strontium in her food.”
“Your granny wasn’t alone. The biological processing of strontium was studied extensively in the 1950s because of the potential for radioactive 90Sr ingestion due to aboveground testing of nuclear weapons.”
A light was going on.
“You’re saying strontium is incorporated into a person’s bones and teeth, much like calcium.”
“Right.”
“And calcium in the human skeleton is replaced on roughly a six-year cycle.”
“Yep.”
“So, like skeletal Ca, skeletal Sr reflects an individual’s diet over the last six years of life.”
“Six to ten,” Art said.
“But Ca levels don’t change in tooth enamel as they do in bone. Once laid down, enamel is stable.”
“And the same is true of Sr. So dental enamel continues to reflect the average dietary Sr isotope composition ingested when the tooth was formed.”
“So if someone relocated from the place in which she was living when her teeth were forming, that individual’s dental and skeletal Sr levels would differ. If she stayed put, those levels would remain similar.”
“Precisely. Enamel values suggest place of birth and early childhood. Bone values suggest place of residence during the last years of life.”
A thought stopped me in midscribble.
“Doesn’t our food come through national and international networks these days?”
“We drink local water, at least most of the time.”
“True. Tell me what you did with my specimens.”
“After removing all extraneous materials, we ground them. Then we separated out the Sr by ion-exchange chromatography, analyzed the purified Sr using thermal ionization mass spectrometry, and collected the Sr ratios by multicollector dynamic analys—”
“Art.”
“Yes?”
“What did you find?”
“One of your three saw a bit of the world.”
22
“GO ON.”
“First, let’s talk teeth. Two of your individuals overlap in their dental Sr values.”
“Which two?”
Paper rustling.
“Let’s see…38426 and 38427. For them I’d expect a childhood diet with an average Sr value of plus ninety to plus one hundred five. But 38428 is statistically distinct. The Sr isotope composition of that individual’s dental sample suggests a childhood diet with an average Sr value of plus fifty to plus sixty.”
“Meaning 38428 was not born in the same region as the other two?”
“Correct.”
“Can you tell where she’s from?”
