Monday Mourning Part 30

"Guess we are."

"The kid with the broken wrist?"

I nodded. "The skeleton wrapped in the leather shroud."

"Son of a b.i.t.c.h." Charbonneau pushed to his feet. "I'll get her stats into the system right away."

Charbonneau had barely cleared the door when the phone rang. It was Art Holliday, calling from Florida.

"You got the Carbon 14 report?"

"Yes, thank you. I appreciate your turning it around so quickly."

"We aim to please. Listen, I may have something else for you."

I'd forgotten Holliday's offer to perform additional testing.

"For prosecutorial purposes, strontium isotope a.n.a.lysis is still experimental. But we have applied the technique to forensic questions. In one case, we nailed the point of origin of six white-tail deer. Used the antlers. Course, we knew the animals had to have come from one of two states, so we had isotopically distinct geographic localities from which to measure control groups. That made the job easier."

Over the years I've learned that it is impossible to hurry Art Holliday. You go with the flow, half listen to the buildup, and focus on the conclusions.

"We're getting good results looking at immigration and settlement patterns with ancient populations."

That rang an archaeological bell.

"Yours is the group a.n.a.lyzing the pueblo materials from Arizona?'

"Thirteenth- and fourteenth-century burials. Construction and occupation of some of the grander pueblos spanned many generations. Hundreds of people occupied them, probably a mixture of longtime residents and immigrants from outside. We're trying to sort that out."

"Strontium isotope a.n.a.lysis can separate newcomers from lifelong inhabitants of a place?"

"Yep."

The hummingbird revved up again.

"The technique can tell you where someone lived?"

"If you have reference samples. In some circ.u.mstances, if a subject moved from one geographic region to another, Sr a.n.a.lysis can tell where they were born, and where they spent the last six to ten years of their life."

The hummingbird gunned it to red-line.

"Drop back and start from the beginning." I grabbed pen and paper. "Using no word having more than three syllables."

"There are four stable isotopes of strontium, and one isotope, 87 87Sr is produced by the radioactive decay of 87 87Rb. The half-life's forty-eight point eight billion years."

"Much slower than Carbon 14."

"Much slower than my old dog Spud."

Spud?

"The geology of North America shows tremendous age variation," Art sailed on, oblivious to my confusion over the dog reference. "For example, the age of the crust varies from less than a million years in Hawaii, to just over four billion years in parts of the Northwest Territories of Canada."

"Resulting in differences in Sr values in the soil and rock of different regions."

"Yes. But such differences are also due to variations in bedrock composition."

"When you use the term value, do you mean the ratio of the unstable strontium to its stable counterpart?"

"Exactly. It's the ratio of the strontium 87 isotope to the strontium 86 isotope that's important, not the absolute level of each."

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 90 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, a.n.a.lyzed the purified Sr using thermal ionization ma.s.s spectrometry, and collected the Sr ratios by multicollector dynamic a.n.a.lys-"

"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?"

"That's where it gets interesting. Last year we had a case of jumbled remains from a barrel found in some hophead's bas.e.m.e.nt in Detroit. Police knew the victims were business a.s.sociates of the drug dealer who owned the house, but wanted the bones sorted into individuals. None had dental work, all were black, in their mid-twenties, and about the same size. One of the three was born in north-central California, one was from Kansas, and the other was local Michigan talent.

"We didn't have control groups from the three areas in question, so we had to infer the isotope composition of the dietary Sr from the bedrock geology in each region, then work back to the various bones in the barrel. You still there?"

"I'm here."

"Someone who spent their childhood in north-central California should have Sr values in the range of plus thirty to plus sixty." Rustle. "That's exactly where 38428 falls."

For a moment I was taken aback.

"Meaning my girl's from California?"

"Meaning she could be. If you have no other ideas, it's as good a starting point as any. Of course, she could be from another region with similar bedrock geology."

"And my other unknowns?"

"A couple of years back we had a case involving commingled remains recovered from a common grave in Vietnam. The army had IDs for the two soldiers, but wanted the bones separated into individuals. One soldier had grown up in northeastern Vermont. The other was from Utah."

Art gave me no chance to interrupt.

"A study of the Sr isotope composition of the groundwater near St. Johnsbury in Vermont suggested values in the range of plus eighty-four to plus ninety-four. The teeth from one of the soldiers produced Sr values smack-dab in that range."

"The Vermonter."

"Yes. The teeth of 38426 and 38427 produced identical values."

"Meaning these girls were from Vermont?"

"Not so fast. The same rock formations extend across the border into Quebec. What I'm suggesting is that the Sr values of your other two girls are consistent with what I'd expect from people born in the region where the remains were found."

"The Montreal area."

"Yes. Now let's talk bones. For 38426 and 38427, the Sr values in their teeth are similar to the Sr values in their bones."

"Suggesting they didn't stray too far from home."

"Right. But 38428 is a different story."

I waited.

"Her skeletal Sr values are higher than her dental Sr values. What's more, her skeletal Sr values are very similar to the skeletal Sr values for 38426 and 38427."

"The Quebec stay-at-homes."

"Yes."

I took several moments to digest that.

"You're suggesting 38428 was raised in one place, but spent the last few years of her life in another."