Researchers at Cambridge University report using powerful X-rays to examine plankton shells that show growth bands similar to tree rings that demonstrate chemical changes revealing ocean temperatures.
Unlike tree rings that show annual growth, in plankton the bands occur daily, they said.
As microbial plankton grow in ocean waters their shells trap trace amounts of chemical impurities, and scientists have noticed plankton growing in warmer waters contain more impurities.
The amount of impurity, measured in fossil plankton shells recovered from ocean floor sediments, provides a record of past ocean temperature dating back more than 100 million years ago, the researchers said.
"For slow-growing plankton it opens the way to seeing seasonal variations in ocean temperatures or plankton growth in samples dating back tens to hundreds of millions of years," Cambridge researcher Simon Redfern said.
It's important to understand current climate change in the light of how climate has varied in the geological past, the researchers said, which makes the kind of data revealed by plankton important.