Subfossil Fracture-Related Euendolithic Micro-burrows in Marble and Limestone

Marble and limestone in desert environments of Namibia, Oman and Saudi Arabia is found to be affected by micro-burrows formed by inferred endolithic microbiological activity. Up to 10 m long bands of parallel micro-burrows, as wide as 0.5 mm and up to 30 mm long, formed along fractures inside the rock. The structures are exposed by erosion and appear to be subfossil and no longer active. In some cases, the micro-burrows are bordered by calcrete that formed along fractures. In fresh outcrops, the micro-burrows are filled with white calcium carbonate with internal structures and depleted in several elements. The rims of the micro-burrows contain 1 µm wide contours enriched in P and S. Fluorescence microscopy, Raman spectroscopy and δ¹³C values confirm the presence of biological material. The fossils are too old to conserve DNA or protein. It is presently unclear which organism may have been responsible for the formation of these structures, that are described here for the first time.