As carbon dioxide levels in the atmosphere creep upward, researchers looking to forecast climate change face a problem—we are now in unexplored territory. Carbon dioxide concentrations are at their highest point in at least 800,000 years, so to understand today’s climate system researchers must supplement modern measurements with data from proxies—ways of measuring the state of the planet in prehistoric times.

A new study published in Earth and Planetary Sciences Letters showcases a way to look back into the ocean’s deep history. By examining fossilized zooplankton, scientists at the University of Cambridge discovered the organisms’ shells hold a climate record that goes back around 200 million years. “The only thing that compares are ice cores, which only go back about 400,000 to 800,000 years,” says study co-author Oscar Branson, a PhD student studying biomineralization and palaeoproxies.

The shells preserve information about the conditions in the oceans by continuously picking up sediments on the seafloor and trapping the material within their mineral makeups. The researchers measured their mineral composition using scanning transmission x-ray microscopy, a method originally developed for looking at manufactured products such as microstructures in hard drives.

The x-rays revealed that the shells provide nanoscale data on the environment where and when the individual plankton lived because they trap chemical impurities from the local waters. Additionally, the mineral calcite in the shells reveals day-to-day climate variability through time via their magnesium concentration. Temperature, rather than other environmental factors such as pH level, seems to control how the organism incorporates magnesium, the researchers say. “You can grow [marine microorganisms that have a calcite shell] in different pH waters and show that the effect on magnesium is insignificant compared to the effect of growing them in waters at different temperature,” says study co-author and mineralogist Simon Redfern, a professor at Cambridge’s Department of Earth Sciences.

To Redfern and Branson’s surprise another chemical element, boron, did prove sensitive to pH. The researchers’ next experiments will look at the nanoscale incorporation of boron into marine microorganisms’ calcite shells. Potentially, Redfern says, “the elements within the shells could give us information about the acidity and salinity in the ocean.”

In the meantime the researchers hope their findings will prompt others to compare the environmental data encoded in the plankton shells with known historical events, particularly changes in species and mass extinctions.

- Julianne Chiaet