Abstract. The assessment of diagenetic overprint on microstructural and geochemical data gained from fossil archives is of fundamental importance for understanding palaeoenvironments. The correct reconstruction of past environmental dynamics is only possible when pristine skeletons are unequivocally distinguished from altered skeletal elements. Our previous studies show (i) that replacement of biogenic carbonate by inorganic calcite occurs via an interface-coupled dissolution–reprecipitation mechanism. (ii) A comprehensive understanding of alteration of the biogenic skeleton is only given when structural changes are assessed on both, the micrometre as well as on the nanometre scale.In the present contribution we investigate experimental hydrothermal alteration of six different modern biogenic carbonate materials to (i) assess their potential for withstanding diagenetic overprint and to (ii) find characteristics for the preservation of their microstructure in the fossil record. Experiments were performed at 175 °C with a 100 mM NaCl + 10 mM MgCl2 alteration solution and lasted for up to 35 days. For each type of microstructure we (i) examine the evolution of biogenic carbonate replacement by inorganic calcite, (ii) highlight different stages of inorganic carbonate formation, (iii) explore microstructural changes at different degrees of alteration, and (iv) perform a statistical evaluation of microstructural data to highlight changes in crystallite size between the pristine and the altered skeletons.We find that alteration from biogenic aragonite to inorganic calcite proceeds along pathways where the fluid enters the material. It is fastest in hard tissues with an existing primary porosity and a biopolymer fabric within the skeleton that consists of a network of fibrils. The slowest alteration kinetics occurs when biogenic nacreous aragonite is replaced by inorganic calcite, irrespective of the mode of assembly of nacre tablets. For all investigated biogenic carbonates we distinguish the following intermediate stages of alteration: (i) decomposition of biopolymers and the associated formation of secondary porosity, (ii) homoepitactic overgrowth with preservation of the original phase leading to amalgamation of neighbouring mineral units (i.e. recrystallization by grain growth eliminating grain boundaries), (iii) deletion of the original microstructure, however, at first, under retention of the original mineralogical phase, and (iv) replacement of both, the pristine microstructure and original phase with the newly formed abiogenic product.At the alteration front we find between newly formed calcite and reworked biogenic aragonite the formation of metastable Mg-rich carbonates with a calcite-type structure and compositions ranging from dolomitic to about 80 mol % magnesite. This high-Mg calcite seam shifts with the alteration front when the latter is displaced within the unaltered biogenic aragonite. For all investigated biocarbonate hard tissues we observe the destruction of the microstructure first, and, in a second step, the replacement of the original with the newly formed phase.

Not all dogs and cats with a positive blood culture met the criteria for sepsis. Bacteraemia caused by Gram-positive versus Gram-negative bacteria cannot be distinguished based on clinical or laboratory parameters, and bacterial culture and susceptibility testing have to be performed for the right choice of antibiotic treatment.

We report results of CaCO 3 crystallization experiments by counter diffusion in agar gel with two different solid contents (0.5 and 2 wt %) and two solute concentrations (0.1 M CaCl 2 , 0.1 M Na 2 CO 3 ; 0.5 M CaCl 2 , 0.5 M Na 2 CO 3 ). Solute concentration and hydrogel strength influence the characteristics of the gel−mineral composite formation. High reagent solution concentrations give rise to high supersaturation and high growth rates. When combined with a light gel, single crystal composites form; in a dense gel, the aggregates are mosaic crystal composites. Low reagent solution concentrations result in low supersaturation and low growth rates; when combined with a light gel, single crystal composites form; in a dense gel, the precipitate is a co-oriented polycrystal composite. Gel occlusion within the mineral increases with gel density. Gel distribution inside the mineral is homogeneous for high growth rates. For low growth rates, the gel accumulates locally in the precipitates. Light gels are pushed ahead by the growing crystals, and gel occlusion into the mineral is decreased; at low reagent solution concentrations, slightly more gel gets occluded. In conclusion, agar gel solid content determines the amount of gel occlusion and calcite orientation organization; reagent solution concentration influences the mode of gel distribution inside the mineral/gel composite aggregates.

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