Old Sea Shells Carry Records of Ancient Climate Change

A finding published in the journal Earth and Planetary Sciences Letters suggests that climate changes from millions of years ago are recorded at a daily rate in ancient plankton shells. With the help of a huge X-ray microscope the researchers identified the growth bands in plankton shells that reveal how the shells' chemistry records data of the sea temperature. With this technique the scientists could list the timescale alterations in the ocean temperatures that took place millions of years ago.

The plankton shells carry crucial features like the tree rings that record the details of climate. In order to understand the present times rapid climate change, it is important to know how climate changed in the geological past. One way scientists did this was by analyzing the ice from the poles. The temperature and atmosphere of the planet is recorded by the bubbles of ancient air that is embedded in the polar ice cores. One of the oldest Antarctic ice core records date back to some 800,000 years ago.

When the microbial planktons grow trace amounts of chemical impurities get trapped in the shells that are made of mineral calcite. The scientists have observed that the planktons growing in warmer waters have more chemical impurities, but how this proxy for temperature works, continues to remain a mystery.

 The level of chemical impurity offers clue about the past ocean temperature dating to more than 100 million years ago.

With the help of X-ray microscope the researchers have measured the level of magnesium in the plankton shells. This technique made it possible for the researchers to identify narrow nanoscale bands in the plankton shells where there are high levels of magnesium. These are the growth bands, but in planktons the bands occur daily rather than yearly.

"These growth bands in plankton show the day by day variations in magnesium in the shell at a 30 nanometre length scale. 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," Professor Simon Redfern, one of the experimenters on the project, said in a statement. "Our X-ray data show that the trace magnesium sits inside the crystalline mineral structure of the plankton shell. That's important because it validates previous assumptions about using magnesium contents as a measure of past ocean temperature."

The new measurements highlight that the magnesium settles in the calcite crystals replacing the calcium rather than the microbial membrane, this explains why the temperature affects the chemistry of the plankton shells. This study showed how warmer waters boost high magnesium in calcite.