Mother of pearl or nacre, such as this from a New Zealand Paua shell, is one of nature’s wonder materials. Made by a host of mollusks, the material has proven to be an accurate barometer of environmental conditions as signatures of both water temperature and water depth reside in the material, according to new research by UW-Madison professor of physics and chemistry Pupa Gilbert. |
Nacre—or mother of pearl, scientists and artisans know—is one of nature’s amazing utilitarian materials.
Produced
by a multitude of mollusk species, nacre is widely used in jewelry and
art. It is inlaid into musical instruments, furniture and decorative
boxes. Fashioned into buttons, beads and a host of functional objects
from pens to flatware, mother of pearl lends a lustrous iridescence to
everyday objects.
In
recent years, subjecting the material to the modern tools of scientific
analysis, scientists have divined the fine points of nacre architecture
and developed models to help explain its astonishing durability: 3,000
times more fracture resistant than the mineral from which it is made,
aragonite.
Now, in a new report (Thursday, Feb. 16) in the Journal of the American Chemical Society
(JACS), scientists from the University of Wisconsin-Madison show that
nacre can also be deployed in the interest of science as a hard-wired
thermometer and pressure sensor, revealing both the temperature and
ocean depth at which the material formed.
“We
found a strong correlation between the temperature at which nacre was
deposited during the life of the mollusk and water temperature,”
explains Pupa Gilbert, a UW-Madison professor of physics and chemistry
and the senior author of the new JACS report. “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 mollusk lived.”
The
new study was conducted using mother of pearl from modern mollusks, but
Gilbert notes that nacre is widespread in the fossil record going back
450 million years. If the techniques used by the Wisconsin group can be
applied to fossil nacre, scientists can begin to accurately reconstruct 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,” says Gilbert.
The
material also holds a distinctive signature—the thickness of the nacre
layers—for the water depth at which the material was assembled by a
mollusk, potentially providing even more insight into environmental
conditions of the present and past.
“These
are two independent parameters, measured by different aspects of nacre
structure,” the Wisconsin physicist explains. “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.”
Working
with UW-Madison graduate student Ian C. Olson, the lead author of the
JACS report, Gilbert subjected nacre from eight mollusk species from
different environments to a technique capable of mapping the orientation
of nacre crystals. From the different shells, they observed uneven
thicknesses, widths and angles of the crystalline “bricks” that,
together with an organic mortar, are laid down by the mollusk 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,” says Gilbert.
Comparing
the structural maps of nacre from the different mollusks and
environmental data from the places where the animals were collected,
Olson, Gilbert and their collaborators found an extremely high
correlation between the microscopic structural characteristics of their
nacre specimens and the temperature and pressure data obtained from the
various environments where they were collected.
Contributing
to the new study were Reinhard Kozdon and John Valley, both of the
UW-Madison department of geoscience. The work was funded by the U.S.
National Science Foundation.
