Manganese Speciation in Bivalve Shell Aragonite

SOLDATI, ANALIA L.; JACOB, DORRIT E.; GLATZEL, PIETER; GRATTAGE, JANINE; GECK, JOCHEN

Praga

Congreso; Goldschmidt2015; 2015

Goldsmidth Conferences

Monitoring the manganese concentration in shells of freshwater bivalves provides a unique way to obtain information about climate and environmental changes during periods that pre-date our instrumental data records. This approach, however, relies on a thorough understanding of how manganese is incorporated into the shell material ? a point that remained controversial so far. Here we clarify this issue using state-of-the-art X-ray absorption and X-ray emission spectroscopy in combination with band structure calculations.We show that manganese is incorporated as high-spin Mn2+ and is coordinated by a CO3-octahedron. This, firstly, provides firm experimental evidence for manganese being located in the inorganic carbonate, thus supporting its use as climate proxy.Secondly, it indicates that the structure of the aragonitic host is locally altered such that manganese attains an octahedral, calcitic coordination. The latter modification at the atomiclevel enables the bivalve to substitute many orders of magnitude more manganese in its aragonitic shell than found in any geological or synthesized aragonite.