Travel Reference
In-Depth Information
Synchronizing our watches
An enormous amount of new understanding about the history of the oceans
came from study of the deep sea cores collected by the Ocean Drilling Pro-
gram. We learned that the ocean basins are young compared to continents
and that they are ephemeral. Yet although we greatly improved our under-
standing of the
history
of ocean basins, the deep sea cores did little to im-
prove our understanding of polarity changes, because the soft muck brought
up by the drill cores disintegrated easily and so yielded little information
about ancient magnetic directions. It was thus virtually impossible to corre-
late between the reversal records found in this undersea lava samples (with
their isotopic ages) and the biostratigraphic time scale made up of land-
based fossil occurrences. Correlation between the deep sea samples (with ra-
diometric and magnetic information) and the European stratotype sections
(with fossil information) was thus problematic at best.
This problem was especially pronounced for the Cretaceous. Nearly all
stages originally defined by d'Orbigny and his followers were identified in
shallow-water limestone, which, it turned out, was without any magnetic
minerals or even ash beds appropriate for radiometric dating. Even more
troubling, the various outcrops were discontinuous; they were isolated in
various villages or seacoasts and could not be assembled into any sort of com-
posite sequence or pile. What was needed was a readily accessible record of
hard,
magnetically
measurable strata in Europe containing both fossils and a
paleomagnetic signal. If a thick sequence of fossiliferous
strata
could be
found with a record of the magnetic reversal history, the reversals could be
used to determine the age of the strata. Such a record was discovered in the
early 1970s, in the Apennine Mountains of Italy.
The discovery of the best region for calibrating reversals and the geo-
logical time scale as defined by fossils came about by accident. In his recently
published book T
rex and the Crater of Doom,
geologist Walter Alvarez noted
that he and co-worker Bill Lowrie undertook the Herculean labor of sampling
thousands of oriented cores from thick sedimentary successions of white lime-
