Monday Mourning — читать онлайн бесплатно полностью

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.