RESOLVING A TAPHONOMIC ROLE FOR KAOLINITE-ORGANIC INTERACTIONS IN THE BURGESS SHALE

A taphonomic role for clay-organic interactions in Burgess Shale fossilization has been proposed. Experimental evidence indicates that the aluminum-rich mineral kaolinite can foster the polymerization of decaying organics, increasing their resistance to decay, and microbiological data highlights the antibacterial properties of kaolinite. However, key evidence of fossil-kaolinite associations is controversial. Enrichments of aluminum on anatomical features of Burgess Shale arthropods have been attributed to kaolinite-organic bonding during decay, or to kaolinite formation during retrograde metamorphism. Resolving the timing of kaolinite emplacement/formation is key to understanding whether kaolinite played a role in decay suppression. We use in situ selected-area X-ray diffraction to constrain the mineralogy of Burgess Shale fossils and compare it to that of the matrix that surrounds them. Fossils are distinguished from matrices by the presence of kaolinite, pyrite, and to a lesser extent dolomite. Chlorite may be more abundant in the matrix. The preferential survival of kaolinite in association with fossils evidences early diagenetic clay-organic interactions that protected kaolinite from metamorphic transformation. Thus, kaolinite was emplaced/formed during decay and likely played an important taphonomic role. Confirmation of a role for kaolinite points to potential bias in the early animal fossil record to environs/locales where kaolinite is common, e.g. the tropics.