Marine bivalve geochemistry and shell ultrastructure from modern low pH environments

Abstract. Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis (from the Mediterranean) and M. edulis (from the Wadden Sea) combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island of Ischia. The shells of transplanted mussels were compared with M. edulis collected at pH ~8.2 from Sylt (German Wadden Sea). Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.</jats:p

Experimental diagenesis: insights into aragonite to calcite transformation of Arctica islandica shells by hydrothermal treatment

Abstract. Biomineralised hard parts form the most important physical fossil record of past environmental conditions. However, living organisms are not in thermodynamic equilibrium with their environment and create local chemical compartments within their bodies where physiologic processes such as biomineralisation take place. In generating their mineralised hard parts, most marine invertebrates produce metastable aragonite rather than the stable polymorph of CaCO3, calcite. After death of the organism the physiological conditions, which were present during biomineralisation, are not sustained any further and the system moves toward inorganic equilibrium with the surrounding inorganic geological system. Thus, during diagenesis the original biogenic structure of aragonitic tissue disappears and is replaced by inorganic structural features

Structure and ferroelasticity of tetramethylammonium-hexafluoroosmate(IV), (TMA)(2)[OsF6]

(TMA)(2)[OsF6] has at room temperature a rhombohedral structure (space group R (3) over bar) which is pseudocubic (K-2[PtCl6]-type). a = 8.0565(6) Angstrom, r = 20.020(3) Angstrom (hexagonal setting). Double refraction decreases linearly with increasing temperature and disappears above 434 K which indicates cubic symmetry of the high temperature phase. The change from the cubic high temperature structure to the rhombohedral structure is characterized by a ferrorotative tilt of the [OsF6] octahedra and the TMA tetrahedra. Evidence of ferroelasticity follows from the switching of the trigonal c-axis by the application of directional pressure. The phase transitions of (TMA)(2)[OsF6] (Fm (3) over barm --> R (3) over bar) and of the recently reported (TMA)(2)[PtF6] and (TMA)(2)[IrF6] (Fm (3) over barm --> I4/m) are described by Landau's theory