Mother of pearl holds data about ancient ocean temperatures

Monday, December 19, 2016, 11:45 AM - Earth scientists at the University of Wisconsin-Madison have found an unusual key to determining ancient ocean temperatures.

Mother-of-pearl, also called nacre (pronounced nay-ker) is a "lustrous, tough-as-nails biomineral" that makes up the opalescent exterior coating of pearls. It's also sometimes found in the inner lining of mollusk shells, such as nautilus, pinctada (saltwater oysters), and abalones.

Published in the journal Earth and Planetary Science Letters, a team led by University of Wisconsin–Madison physics Professor Pupa Gilbert found that the layered composition of nacre within a mollusk reveals an accurate account of temperatures as the material is formed.

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Mother-of-pearl is created as mollusks place polygon-shaped tablets (layers) of the mineral aragonite together like brickwork, forming the layers of nacre. These tablets are microscopic.

The image below shows a micrograph (a microscopic photograph) of a fracture surface in nacre, revealing the aragonite tablets.

“We can very accurately correlate nacre tablet thickness with temperature,” Gilbert said in a statement.

Using a scanning electron microscope, Gilbert and her team examined a cross and measured the thickness of the layered tablets that make up the nacre in a shell.

Based on the ocean temperatures measured in modern shells (where the ocean temperatures were already known and verified at the time the shells were forming), researchers were able to confirm that the thickness of the tablets in older shells adds up accurately.

"You just measure nacre tablet thickness, the spacing of the lines, and it corresponds to temperature," Gilbert says. "When the temperature is warmer, the layers get thicker."

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The study examined fossil samples of nacre dating back 200 million years.

Its findings offer scientists a new method of measuring ancient ocean temperatures, building on previous measures currently in effect that use biominerals to determine the environmental conditions during which materials formed in the past.

In Photos: Enormous pearl could be world's largest. See it below.