AUTHIGENIC MINERALIZATION AND EXCEPTIONAL PRESERVATION

Exceptional preservation of non-biomineralizing tissues is, in essence, a race between degradation and mineralization. These two processes are not independent from each other and, in the proper settings, can form negative feedback loops: degradation can create localized microenvironments facilitating authigenic mineralization which in turn limits degradation and promotes preservation. Such feedback loops are critical for exceptional preservation. The role of authigenic mineralization can be fulfilled by a variety of minerals (e.g. silica, phosphate, pyrite, and clay). A comparative taphonomic study of silicification, phosphatization, pyritization, and aluminosilicification shows that the nature of degradation, the timing and duration of mineralization, the size of authigenic minerals, and their diagenetic overgrowth and alteration, are factors that control the quality and taphonomic resolution of exceptional preservation. Fundamentally, regardless of the minerals involved, precipitation of authigenic minerals on degrading organic tissues, which serve as a physical template or a chemical substrate, is a key constructive process driving exceptional preservation. In this light, different styles of Burgess Shale-type carbonaceous compression (e.g., Burgess Shale vs. Chengjiang and Fezouata) can be viewed as examples of a singular taphonomic pathway fulfilled by different authigenic minerals (aluminosilicate vs. pyrite), and exceptionally preserved biotas that were previously regarded as representing distinct taphonomic modes (e.g., 2-D compression in Chengjiang and 3-D pyritization in Gaojiashan) can instead be viewed as end-members of a unifying mineralization model related by the nature and degree of authigenic mineralization. The unifying taphonomic model is supported by the intimate association of mineralization through silica, phosphate, pyrite, and clay in some exceptionally preserved biotas.