photos by Daniel Grossman
Many ocean creatures build limestone shells using calcium, an element abundant in seawater. Sometimes, by chance these creatures also incorporate the element strontium into their shells -- strontium is an element in seawater that is chemically similar to calcium but found in much lower concentrations. For every 1000 calcium atoms, seashells contain just a few atoms of strontium. Seawater also contains several isotopes of strontium, two of which researchers use for dating shells: strontium-86 and strontium-87.
Scientists have discovered that the relative amount of strontium-86 and strontium-87 in seawater has changed over time. In the last 40 million years, the sea has steadily become enriched in strontium-87. Thus, at any given time during this long geological interval the ocean contained a unique ratio of the two strontium isotopes. When an animal incorporates strontium into its shell, the ratio of the two isotopes at that moment is preserved, or recorded, like an indelible time stamp. Neither of the two isotopes decay radioactively, so the ratio remains unaltered even after the animal dies and its shell becomes a fossil. Millions of years later, researchers can determine the age of marine fossils by extracting the strontium and comparing the ratio of the two isotopes with ratios known to have occurred in seawater in the past.
The Pliomax team uses this method of dating in laboratories specifically built for measuring strontium isotopes. They begin by cutting off the contaminated outer layers of shells found in the field. They dissolve clean inner material in acid. They extract the strontium from the solution. The researchers work in a clean room with air 1000 times more pure than that on nearby city streets. A tiny speck of dust in dirty air could alter the strontium ratio of samples and invalidate results. The clean room is constructed virtually free of any metal fixtures, which could also contaminate samples. The team takes the solution of pure strontium, contained in a few drops of liquid, and paints it onto a small coil of wire that they bring to an adjoining laboratory. There, a mass spectrometer, an instrument for weighing atoms, measures the relative quantities of the two strontium isotopes.