Mollusc produces pearl of climate wisdom

Historic evidence of temperature change is revealed by mother-of-pearl layers in the shells of mollusc species that inhabit the ocean shallows.

Scientists in the US believe they have found a new source of hard evidence about bygone climates − very hard and very bygone − in the nacreous layers in the mollusc mineral known as mother-of-pearl.

These layers provide a measure of the temperature at which they were laid down in the tropical seas, and the technique has been tested in a bivalve sample first fossilised 200 million years ago.

The research delivers another independent check on the climate record. Mercury thermometer measurements are only 300 years old – the modern thermometer was first fashioned by Daniel Gabriel Fahrenheit in 1714 – and the earliest systematic temperature records date only from 1659, and were limited to the English Midlands.

Proxy evidence

Global records date only from early last century, so climatologists rely on what they call proxy evidence to compose a climate record for historic and prehistoric times.

Researchers have measured bygone climate change from cores taken deep in the ice caps of Greenland and Antarctica, and from subtle evidence in the planetary bedrock.

They have inferred climate history from the chemistry of foramifera – little calcium carbonate shells deposited on the sea floor – and read the pattern of dramatic change in the annual growth rings of trees.

Old lake beds have been searched by researchers for preserved pollen to read the changing pattern of vegetation – itself a guide to climate shifts – and they have checked their mute evidence against direct or indirect evidence from human witness, by combing through the logbooks of 18th-century whaling ships.

“The only thing you can do to understand climate
in the future is to look at climate in the past”

They have also deciphered the pattern of hurricanes in Caribbean shipwreck records, and gained weather data even from casual references in the texts of plays performed in Athens 25 centuries ago.

And now a biomineral fashioned by a mollusc that inhabits the shallow ocean provides another silent witness to a procession of good years and bad.

Nacre is lustrous, pearly and iridescent, and is deposited on the inside of a mollusc shell one layer at a time.

Pupa Gilbert, a physicist at the University of Wisconsin, and colleagues report in Earth and Planetary Science Letters journal that they used a technique called scanning electron microscopy to measure the thickness of the plates or polygon tablets of nacre laid down in both modern sea shells and in fossilised specimens of Pinnidae, a fast-growing saltwater clam that lives in shallow ocean reefs.

Species of mollusc

“We can very accurately correlate nacre tablet thickness with temperature,” Professor Gilbert says.

“If what you are measuring is a physical structure, you see it directly. 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.”

The mother-of-pearl evidence has the advantage that even if the chemistry of the fossil shell is altered by the circumstances of its preservation, as long as the physical structure is preserved, the nacre structures will testify to the temperature at which they grew.

And since species of mollusc have been settled in the world’s oceans for at least 400 million years, climatologists have yet another ready-reckoner of temperature readings deep in the planet’s history.

“The only thing you can do to understand climate in the future is to look at climate in the past,” Prof Gilbert says.