Elemental migration and fixation during post‐depositional process of deep‐sea fish buried in turbidites

Sedimentary rocks preserving fossils undergo alterations during burial, compaction, uplift and weathering. During these post‐depositional processes, elements such as iron, sulphur and calcium migrate and are later fixed in and around the organism. However, the relationship between elemental migration/fixation and fossilisation is still unclear. In this study, well‐preserved deep‐sea fish fossils, living deep‐sea fish and host rocks were quantitatively analysed to understand which elements were consumed and/or enriched during fossilisation and subsequent alteration. Mineralogical and geochemical analyses show soft tissues, such as eyes and luminous organs, are composed of framboidal pyrite and carbonaceous material, hard tissues of apatite and the host rocks predominantly of volcanic glass–derived heulandite. This suggests that rapid pyritisation within the tuffaceous sediments led to the preservation of soft tissues of deep‐sea fishes. The occurrence of heulandite indicates burial of a few kilometres. The exposed surface and/or the surface of fracture of the host rocks and some fossil bones are coloured reddish‐brown, as shown by the Fe enrichments of these parts. These enrichments can be interpreted as a result of Fe migration and fixation caused by pH reactions between acid fluid dissolving Fe ions and high pH pore water in heulandite or bone apatite. Rapid early diagenetic armoring by pyritisation protected the shapes and textures of organs. Quantitative analysis of elements demonstrates that rapid early pyritisation resulted in a drastic enrichment of Fe even after sequential diagenetic processes, such as compaction, deep burial, uplift and erosion.