Monday Mourning

I nodded. “The skeleton wrapped in the leather shroud.”

“Son of a bitch.” 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 analysis 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 analyzing 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 analysis 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 circumstances, if a subject moved from one geographic region to another, Sr analysis 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, ⁸⁷Sr is produced by the radioactive decay of 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 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.”