The scientists at the University of Wisconsin-Madison have discovered a new way of measuring ocean temperatures in the past, by analysing the mother-of-pearl (or nacre) biomineral that lines seashells.
Based on their research into the physical attributes of the nacre in fossil and present-day shells, the biomineral appeared to provide an accurate record of temperature as the material is formed in a mollusk.
“We can very accurately correlate nacre tablet thickness with temperature,” said Professor Pupa Gilbert, explaining that the nacre is formed as mollusks lay down microscopic polygonal tablets of the mineral aragonite like brickwork to build layers of the shiny biomineral.
Studying fossil samples as old as 200 million years from a mollusk in the family Pinnidae (large, saltwater clams), the scientists used a scanning electron microscope and a cross-section of the shell to measure the thickness of the layered microscopic tablets that make up the nacre in the shell. The thickness of the tablets would correlate with ocean temperatures as measured in modern shells when ocean temperatures were known at the time the shells were formed.
“If what you are measuring is a physical structure, you see it directly,” said Professor Gilbert. “You just measure nacre tablet thickness, the spacing of the lines, and it corresponds to temperature. When the temperature is warmer, the layers get thicker.”
According to her, the new method was more accurate than studying the shell's chemistry as the chemistry can be altered by diagenesis. Diagenesis occurs over geologic time, during or after sediments rain down on ocean beds to form sedimentary rock. Fossil shells may partially dissolve and re-precipitate as calcite, which fills cracks in aragonite nacre, thus skewing the shell's chemical analysis, if analysed as a bulk sample.
She added that “if the physical structure is altered by diagenesis, you will notice immediately that nacre is no longer layered, and so you will know that it’s not worth analysing that area. If just a few nacre tablets are preserved, their thickness can easily be measured.” This means that the new technique can augment current geochemical methods used to assess past temperatures, and help to reconstruct ancient climates, especially the shallow marine environments that preserve most of the world’s invertebrate fossil record.
“The only thing you can do to understand climate in the future is to look at climate in the past,” Professor Gilbert concluded.