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Mediterranean salt: isotopes and ice.

At the end of the Miocene epoch, 6 million years ago, the Mediterranean Sea suffered a "salinity crisis." Its connections to other oceans were severed and, according to some estimates, the amount of seawater that evaporated was 30 times as great as the sea's present volume, leaving more than 1 million cubic kilometers of minerals and salts in the basin.

One idea, proposed last decade, to explain this event is that the Mediterranean became isolated by a global drop in sea level, brought on by the growth of ice sheets. While some lines of evidence support ice sheet expansion in the late Miocene, one of the best indicators of increased ice volume--elevated concentrations of oxygen-18 isotopes in the calcium carbonate shells exreted by ocean organisms--has led to contradictory results; one group has found elevated levels, but other have not. Now, David Hodell, Kristin Elmstrom and James Kennett at the University of Rhode Island in Narragansett report enriched oxygen-18 levels in deep-sea cores from five sites in the Atlantic and Pacific oceans--which supports increased ice in the Miocene. Hodell suspects that other researchers failed to find such enrichments either because they didn't sample their cores at small enough intervals or because the crucial sedimentary layers were lost during coring.

Hodell's group also found that the oxygen-18 levels fluctuated quite rapidly during the late Miocene, echoing similar variations in the Mediterranean salt layers. "We're speculating that these variations reflect ice volume changes and sea level changes of 40 to 50 meters," says Hodell. Such sea level drops by themselves could not account for the extremely thick salt layers, however. Hodell thinks a slow uplift of the region contributed to isolating the Mediterranean. The more rapid rise and fall of sea level, he says, could then have triggered the high-frequency cycles of evaporation and basin refill.
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Title Annotation:Mediterranean Sea suffered 'salinity crisis' in Miocene epoch
Author:Weisburd, Stefi
Publication:Science News
Date:Nov 23, 1985
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