High productivity of early Silurian sea evidenced by post-bloom macroaggregates

Sedimentary structures interpreted as early diagenetically silicified post-bloom macroaggregates are described from early Silurian black radiolarian cherts of Poland. They occur as whitish lenses and/or sheet-like layers of various thickness and lateral extension distributed in certain horizons of the studied radiolarian chert sections. The macroaggregates originated from decomposing plankton bloom biomass comprised mainly of large acanthomorph acritarchs with an admixture of radiolarians and graptolites, detrital quartz, and amorphous organic matter. After accumulation on the sea floor the macroaggregates were overgrown by mats of benthic cyanobacteria and became a source of phosphate and barium diffusing to the surrounding sediment, causing phosphatization of the large bloom-forming acanthomorph acritarchs buried in their proximity. Modern macroaggregates show extremely low fossilization potential and the finding of comparable early Silurian material represents a unique preservation explainable by their rapid burial and very early silicification. The presence of fossil macroaggregates in black radiolarian cherts is testimony of periodic high productivity within early Silurian epicontinental seas.