Mineralogy and geochemistry of some trace fossils filled or lined with fecal pellets from the Cenozoic of the Northern Apennines and the Tunisian Atlas

Fossil burrows containing fecal pellets, Tubulichnium mediterraneum (Zouza Member, Tunisia) and Alcyonidiopsis isp., along with the hosting silty marl (Pagliaro Formation, Italy), were examined to interpret their mineralization. Various mineralogical techniques were applied. Siderite is the primary mineral phase of the fillings. Their zonation reflects progressive diagenetic alterations moving inward from the surface, with the extent of alteration varying across different samples and locations. A model for the burrow mineralization and its alterations is proposed, outlining for the first time a complete sequence of mineralization events. Initially, burrows were filled with fecal pellets and mineral particles derived from the surrounding sediment. Siderite formed under more reducing conditions in relation to the host rock due to the decomposition of organic matter, i.e., mucus and fecal pellets. The δ13C values in siderite (−10.3 to −8.5 ‰PDB), indicates formation in a suboxic zone through a process analogous to dissimilatory bacterial iron reduction (DIRB) in sediments, influenced by bacterial sulphate reduction (BSR) and possibly calcite transformation. The $δ^{18}O$ data suggest formation from marine pore fluids, possibly with biological influence. Mildly alkaline and oxidizing fluids led to the siderite surface alterations, forming goethite rims. Continued dissolution of the cores of siderite-goethite grains resulted in hollow goethite grains. Cavities formed by siderite leaching, or cracks formed due to physical weathering, were filled with Mn-Fe oxyhydroxides. The latest stage involved formation of a thin goethite layer on the sample's surfaces.