The carbonate shells of tiny marine plankton, foraminifers, are important archives of geochemical records of past climates. Understanding how these plankton make their shells is essential to correctly interpret the geochemical climate signals recorded in them. Using electron microscopy and infrared spectrometry on ultra-thin slices cut from these shells, Dorrit Jacob from Macquarie University in Australia, together with her colleagues from the Australian National University and the GFZ German Research Centre for Geosciences in Potsdam, Germany, has resolved puzzling issues surrounding foraminifer shells.

The researchers have discovered that, contrary to long-standing textbook knowledge, these shells do not form as calcite, but instead, are originally formed as the metastable carbonate vaterite and only later transform into calcite. "These findings are important for understanding how chemical elements are incorporated into the shells and how to read these climate archives correctly," explains Dorrit Jacob. "This promises to resolve hotly debated discrepancies between observations on natural shells and those seen in chemical laboratory experiments."

The presence of vaterite instead of calcite in these abundant organisms also means that foraminifer shells are much more susceptible to ocean acidification than has been previously thought, which carries drastic ramifications for their survival in the future oceans.