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Strontium isotope dating

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While the identification of the source of shells used as personal ornaments is crucial for determining home range and exchange networks of prehistoric hunter-gatherers, it is often difficult to identify the coastal versus fossil origin of the shells as most genera used as beads were available both at beaches and fossil outcrops.

Cretaceous strata in Alabama and Mississippi (USA) represent one of the most complete records of shallow marine deposition worldwide for the Upper Cretaceous.

Absolute dating of the time of deposition of sedimentary rocks is an important problem, but one that is very difficult to solve.

Accurate dates depend on thorough re-setting of isotopic clocks.

Authigenic minerals are deposited directly from seawater and hence display good initial Sr isotope homogeneity.

However, they are highly susceptible to recrystallisation after burial and do not necessarily remain closed systems.

In practice, the two distinct dating approaches associated with these sediment types have tended to converge.

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In the ideal case, multiple samples from the same lava flow should plot perfectly along a line with slope e Ar give lines whose slopes supposedly reflect the ages of lava flows.

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.

Here we apply this methodology to the whole Upper Cretaceous, using teeth of two fossil shark genera () collected from variable facies.

Shark teeth collected from a bentonite mine in Monroe County, Mississippi, were also analyzed and compared with the radiometric date of the bentonite layer.