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UW-Madison professor Pupa Gilbert holding an abalone shell: mother-of-pearl. (Photo: Jeff Miller/UW-Madison)

Mother of pearl can tell ocean temperature, depth

UNITED STATES
Tuesday, May 15, 2012, 01:40 (GMT + 9)

Nacre or mother of pearl is produced by a host of mollusc species. Scientists from the University of Wisconsin-Madison have now shown that it can be used as a thermometer and pressure sensor which reveals both the temperature and ocean depth at which it formed.

"We found a strong correlation between the temperature at which nacre was deposited during the life of the mollusc and water temperature," explained Pupa Gilbert, a UW-Madison professor of physics and chemistry and the senior author of the new report published in the Journal of the American Chemical Society (JACS).

"All other (temperature) proxies are based on chemical analyses and the relative concentration of different elements or isotopes. This could be our first physical proxy, in which the microscopic structure of the material tells us the maximum temperature and maximum pressure at which the mollusc lived," Gilbert added.

If these techniques can be applied to fossil nacre, scientists can start to accurately rebuild a global record of ancient environments and environmental change.

"If the correlation holds, we would have a thermometer that goes back in time, a paleothermometer of how hot or cold water temperatures were when the nacre formed," said Gilbert.

Researchers can also use the thickness of the nacre layers -- which varies for the water depth at which the material was assembled by a mollusc -- to learn more about environmental conditions of the present and past.

"The maximum temperature can be measured by how disordered the nacre crystal orientations are, while the maximum pressure can be taken from the thickness of the nacre layers."

Along with UW-Madison graduate student Ian C Olson, Gilbert subjected nacre from eight mollusc species from different environments to a technique that can map the orientation of nacre crystals. The team observed uneven thicknesses, widths and angles of the crystalline "bricks" that, together with an organic mortar, are set by the mollusc to form mother of pearl.

"We wondered why the shells were so different and concluded that the key parameters to test were the environmental ones, including maximum, minimum and mean annual temperatures as well as maximum and minimum water pressure, which depends on water depth," said Gilbert.

The team found an extremely high correlation between the microscopic structural characteristics of their nacre samples and the temperature and pressure information obtained from the various harvesting sites.