Polymerized Tubular Silicates in Lower Cambrian Carbonates – Biology or Chemistry?

Sammanfattning: Shallow marine environments from the Lower Cambrian period were very different to shallow marine environments of today. Tubes of what was thought to be silica recovered from a partly dolomitized carbonate sample collected form a site near Indian Springs, Nevada could help further understand the chemistry and characteristics of such environments. The aim of this study is to present a detailed morphological description of the tubular structures found in the rock sample as well as to give a description of the environment in which they were produced. This was achieved by first studying the samples with several different analytical techniques including Scanning Electron Microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDS) and subsequent evaluation of the obtained data. This resulted in the discovery that the tubular structures have a wall mainly consisting of non-detrital silica crystals. The crystals are arranged in string-like structures which overlap to form tube walls of varying thicknesses. The observed tubular structures are typically straight and ca. 0,23 to 0,25 mm long, but slightly curved and rare bifurcating tubes were also seen. The elemental mapping of a thin section of tubes showed that the wall-forming crystals consist mainly of silica (SiO2), however, small concentrations of calcium (Ca) were found inside many of the silica crystals leading to the conclusion that positive calcium ions may have acted as a seed for the silica crystals. A comparison of the tubular structures with other tubular fossils of the same geological time period is presented as well as a discussion of whether the specimens are of biological origin, and consequently a biosignature, as these could be used in future research when studying material from other planetary bodies.