Diagenetic alteration in low-Mg calcite from macrofossils: a review

Abstract

The quality of palaeoenvironmental reconstructions based on macrofossil car bon ate critically de pends on preservation of the shell material because post-depositional pro cesses can modify its structural, chemical and isotopic composition, potentially over printing or completely erasing palaeoenvironmental information. A suite of methods can be employed to evaluate the impact of diagenetic processes on the primary geochemical signatures of samples. Here we review the benefits and short comings of the most commonly employed optical (optical microscopy, cathodoluminescence microscopy, scanning electron microscopy) and chemical (trace element abundances, isotopic ratios) screening techniques used to assess the alteration degree of low-Mg calcite macrofossils and summarize the findings on diagenetic trends observed for elemental and isotopic systems in such materials. For a robust evaluation of the preservation state of biogenic calcite, it is advisable to combine a set of complementary techniques. Absolute limiting values of element and isotope ratios for discarding diagenetically altered materials cannot be universally applied, but should rather be evaluated on a case to case basis. The evaluation can be improved by analyses of diagenetic carbonate and altered fossil materials, which help constraining the diagenetic trajectories in the sampled specimens. Quantification of post-depositional alteration is possible, but in most cases the complexity of diagenetic systems hampers the possibility of retrieving original proxy values for palaeoenvironmental reconstructions from partially altered materials.