[M-IS14] Biogeochemistry

[MIS14-P07] Geochemical changes occur during fossilization learnt from well-preserved deep sea fish fossils

*Haruka Katada¹, Hidekazu Yoshida², Tatsuo Oji², Yoshihiro Asahara¹, Masayo Minami³, Yui Kouketsu¹ (1.Graduate School of Environmental Studies, Nagoya University, 2.Nagoya University Museum, 3.Division for Chronological Research, Institute for Space–Earth Environmental Research, Nagoya University)

Keywords:Taphonomy, Fossilization, Pyrite, Miocene

Several biological and chemical changes occur during fossilization of organisms. However, the nature and mechanism of fossilization are not yet completely known. We investigated the changes in chemical content during fossilization using the fossils from the Yamami Formation of the Morozaki Group distributed on the Chita Peninsula, southwest Japan. The recent large excavation of the Yamami Formation enabled the study of deep-sea fish fossils, which preserve detailed organ textures as fossils and are excellent materials for fossilization studies. To study the various paleontological issues based on fossils, it is crucial to understand the chemical and mineralogical processes that play an important role in the fossilization process. In this study, we performed chemical mapping using X-ray microscopy, mineral identification using Raman spectroscopy, and chemical analysis using inductively coupled plasma mass spectrometry and elemental analysis. Further, the chemical compositions of living and fossil fish were compared to define geochemical changes during fossilization. Carbon and nitrogen were lost significantly during fossilization, whereas slight changes were observed in phosphorus and calcium concentrations, which are major constituent elements of hard tissues. Meanwhile, iron and sulfur were highly concentrated during fossilization in fossil fish than in living fish due to pyrite formation. We concluded that in-situ framboidal pyrite formation and sulfur-reducing bacteria played important roles in the preservation of soft tissue textures in the fossilization of the Morozaki Group. Oxidation of pyrite is also important for fossil preservation because of the low solubility of iron oxides. The geochemical transformation from organ decay to mineralization, such as pyrite during fossilization, is a crucial process for preserving the detailed textures of organs as fossils.