Paleoenvironmental parameters accessible with these tools are not restricted to temperature, but include changes in seawater pH (B 2), freshwater input and ocean upwelling (Ba 3, 4), as well as the paleonutrient distribution in the oceans (Cd, Zn 5). Since the development of the earliest paleo-thermometers, which provided high-resolution proxy records from Sr/Ca ratios in aragonitic coral skeletons 1, the number of trace element proxies multiplied, now comprising elements such as Mg, Cd, U, Zn, B, Ba and Mn. Trace elements in the skeletal hard parts of aquatic animals are sensitive recorders of environmental parameters, thus enabling the reconstruction of environments and climates of the past. This outstanding feature is most likely facilitated through the non-classical crystallization pathway of bivalve shells. This modification at the atomic level enables the bivalve to accommodate many orders of magnitude more manganese in its aragonitic shell than found in any non-biogenic aragonite. Secondly, it indicates that the structure of the aragonitic host is locally altered such that manganese attains an octahedral, calcitic coordination. This, firstly, provides firm experimental evidence for manganese being primarily located in the inorganic carbonate. More importantly, the unique chemical sensitivity of valence-to-core X-ray emission enables us to show that manganese is always coordinated by a CO 3-octahedron. manganese always has the same valence as calcium. We verify that in the shells of all studied species manganese is incorporated as high-spin Mn 2+, i.e. Here we clarify this issue, using state-of-the-art X-ray absorption and X-ray emission spectroscopy in combination with band structure calculations. This approach, however, relies on a thorough understanding of how manganese is incorporated into the shell material –a point that remained controversial so far. Determining the manganese concentration in shells of freshwater bivalves provides a unique way to obtain information about climate and environmental changes during time-intervals that pre-date instrumental data records.
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