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Plankton Chemistry Points to Future Carbon Cycle Issues

The chemical composition of the shells of marine plankton has allowed scientists to more precisely estimate past ocean conditions and predict future changes due to global warming.

The shells of foraminifers record information about the environment in which they grow, so an international team of researchers used transmission electron microscopy to examine ultra-thin slices of the shells. The results have been published in Nature Communications.

“Magnesium found in the plankton shells, for example, is used to calculate seawater temperatures going back tens of millions of years,” said lead researcher Prof Dorrit Jacob of Macquarie University’s Department of Earth and Planetary Sciences. “Understanding how the shells develop is key to understanding how magnesium and other elements get into the shells, and therefore how we read the shells’ climate records.”

The researchers from Macquarie University, the GFZ German Research Centre for Geosciences and The Australian National University found that these plankton shells first form as the unstable carbonate vaterite, which eventually transforms into stable calcite. “This was a big surprise,” Jacob said. “Since the 1950s we’ve thought the shells were made directly of calcite, and this is what we have been teaching students to this day.”

“Which type of carbonate forms first, vaterite or calcite, determines how much magnesium is incorporated into the shell. This finding about how foraminifer shells form will now enable us to estimate past ocean temperatures more precisely, and more accurately predict future climate change.”

The presence of unstable vaterite in these abundant organisms also means that foraminifer shells may be far more susceptible to ocean acidification than previously thought. This could have drastic ramifications for carbon dioxide transfer to the deep ocean and sea floor in the marine carbon cycle, as foraminifer shells are dense and assist rapid sinking of organic matter.