Ancient Ocean Temperatures Recorded in Mother-of-Pearl

To understand what the climate will be like in the future, it’s important to look at what the climate was like in the past—not just on land, but in the oceans as well. Without a time machine, however, our estimates of “paleotemperatures” must be indirect, through analysis of proxies such as sediments, ice cores, and tree rings. For ocean temperatures, this has mostly involved decoding the climate signals stored chemically in the shells of zooplankton. However, imperfect preservation causes chemical changes (diagenesis) that could skew temperature readings.

Nacre, or mother-of-pearl, is an iridescent material that lines the insides of mollusk shells. Its iridescence and strength are due to the stacking of thin, aragonite (CaCO₃) plates (or “tablets”) with thicknesses close to the wavelengths of visible light. Researchers have now shown that nacre tablet thickness is correlated with the temperature of the environment in which the nacre formed. This temperature proxy, being physical rather than chemical, is less sensitive to diagenesis.

To establish the method’s feasibility, the researchers used the photoemission electron microscope (PEEM-3) at ALS Beamline 11.0.1.1 to develop polarization-dependent imaging contrast (PIC) maps of mollusk shells from the genera Pinna and Atrina, from modern to the Jurassic (0–200 million years ago). X-ray diffraction studies were also performed at Beamline 12.3.2. The results demonstrate that the crystal structure and mineral composition of the nacre tablets can remain sufficiently intact over geologic time intervals. Nacre tablet thickness can thus serve as a viable paleothermometer, supplementing existing geochemical methods, especially in the shallow marine environments that preserve most of the invertebrate fossil record.