Entrapment and transformation of post-bloom radiolarians in cyanobacterial mats as a factor enhancing the formation of black cherts in the Early Silurian sea

Abstract

ABSTRACTBlack radiolarian cherts are widespread in the lithological records of the Silurian; however, the processes governing their formation remain unclear. Analyses of Early Silurian cherts of Poland have revealed that they are composed of degraded remains of cyanobacterial mats enclosing variable numbers of dissolved radiolarian tests. In optical and scanning electron microscope (SEM) images, this cyanobacterial necromass, containing entrapped radiolarians and forming massive or laminated cherty deposits, exhibits a complex taphonomic and diagenetic history. These cyanobacterial mats underwent syndepositional to early diagenetic silicification in a peculiar environment which was highly enriched with dissolved silica, due mainly to a massive deposition of opaline tests of post-bloom radiolarians, which settled on mats covering large areas of the sea bottom. It is assumed that precipitation of the chert precursor silica occurred in the highly porous mats and, to a great extent, was governed by the activity of sulfate-reducing bacteria (SRB), which resulted in changes in pH in the mat profile. This process was a key factor governing the dissolution of the radiolarian tests and subsequent reprecipitation of dissolved radiolarian silica in the mats' microenvironment. Photosynthesis carried out by cyanobacteria and the decompositional activity of sulfate-reducing bacteria resulted in: i) a shift of the oxygen gradient below the mat, and ii) strong pH fluctuation within the mat profile. During the first stage of bacterial degradation of the cyanobacterial necromass (SRB I stage), an increase in pH caused dissolution of the radiolarian tests. During the second stage of bacterial degradation (SRB II stage), as the decomposition of organic matter proceeded, a significant amount of organic acid was produced, causing a reduction in pH in the mat interior. Presumably this triggered rapid silica precipitation from the pore waters, which were highly saturated with silica. The availability of radiolarian tests and the rate of their dissolution was therefore a key factor governing the rate of silicification in Early Silurian cyanobacterial mats.