Planktic foraminifera form their shells viametastable carbonate phases

Abstract

The calcium carbonate shells of planktic foraminifera provide our most valuable geochemicalarchive of ocean surface conditions and climate spanning the last 100 million years, and playan important role in the ocean carbon cycle. These shells are preserved in marine sedimentsas calcite, the stable polymorph of calcium carbonate. Here, we show that shells of livingplanktic foraminifersOrbulina universaandNeogloboquadrina dutertreioriginally form from theunstable calcium carbonate polymorph vaterite, implying a non-classical crystallisationpathway involving metastable phases that transform ultimately to calcite. The currentunderstanding of how planktic foraminifer shells record climate, and how they will fare in afuture high-CO2world is underpinned by analogy to the precipitation and dissolutionof inorganic calcite. Ourfindings require a re-evaluation of this paradigm to consider theformation and transformation of metastable phases, which could exert an influence on thegeochemistry and solubility of the biomineral calcite.