Planktic foraminifera form their shells via metastable carbonate phases

D. E. Jacob,O. B. A. Agbaje,S. M. Eggins,O. Branson,R. Wirth

doi:10.1038/s41467-017-00955-0

Abstract

The calcium carbonate shells of planktic foraminifera provide our most valuable geochemical archive of ocean surface conditions and climate spanning the last 100 million years, and play an important role in the ocean carbon cycle. These shells are preserved in marine sediments as calcite, the stable polymorph of calcium carbonate. Here, we show that shells of living planktic foraminifers Orbulina universa and Neogloboquadrina dutertrei originally form from the unstable calcium carbonate polymorph vaterite, implying a non-classical crystallisation pathway involving metastable phases that transform ultimately to calcite. The current understanding of how planktic foraminifer shells record climate, and how they will fare in a future high-CO2 world is underpinned by analogy to the precipitation and dissolution of inorganic calcite. Our findings require a re-evaluation of this paradigm to consider the formation and transformation of metastable phases, which could exert an influence on the geochemistry and solubility of the biomineral calcite.

Highlights

The calcium carbonate shells of planktic foraminifera provide our most valuable geochemical archive of ocean surface conditions and climate spanning the last 100 million years, and play an important role in the ocean carbon cycle
We present an in situ study of the ultrastructure and mineralogy of O. universa and N. dutertrei, using focussed ion beam (FIB) supported transmission electron microscopy (TEM) imaging and diffraction, and Fourier transform infrared (FTIR) spectrometry
Shell ultrastructure was examined by highresolution TEM (HR-TEM) during our first two analytical sessions (Table 1)

Summary

Introduction

The calcium carbonate shells of planktic foraminifera provide our most valuable geochemical archive of ocean surface conditions and climate spanning the last 100 million years, and play an important role in the ocean carbon cycle. These shells are preserved in marine sediments as calcite, the stable polymorph of calcium carbonate. Many biologically precipitated carbonates are known to form via complex mineralisation pathways involving metastable intermediate phases that transform stepwise into the final shell mineral[11] This provides an energetically and kinetically favourable pathway to calcite formation, by employing metastable particles with high surface energy, which have a lower free energy barrier to nucleation[12]. This would alleviate the requirement for complex, energy intensive ion control mechanisms to explain the geochemistry of foraminifera

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Results

Conclusion