Recovery of siliceous ecosystems promoted Early Silurian climatic cooling

Throughout Earth's history, the delicate balance within the marine silicon cycle—between silicate weathering coupled to the formation of carbonate and chert versus reverse weathering—has been a fundamental driver of long-term climate stability. However, the ecological rise of siliceous organisms reduced the significance of this process in climate regulation during the Phanerozoic. Despite their importance, siliceous organisms are prone to diagenetic alteration, forming authigenic quartz, which makes direct study challenging. This study investigates Late Ordovician to Early Silurian marine mudstones, known for their richness in authigenic quartz, and uses high-precision scanning electron microscopy coupled with geochemical analyses. We find that the proportion of authigenic quartz (predominantly microcrystalline quartz) began to increase in the upper Hirnantian and reached a peak of 73.13 % in the Rhuddanian, exhibiting an inverse trend to previously reconstructed temperature variations. This microcrystalline quartz originates from the biogenic silica produced when siliceous organisms absorb dissolved silicon from seawater. Once deposited into sediment, it undergoes burial and dissolution, followed by nucleation and growth on micro-bleb (silicon-enriched extracellular polymers produced by microorganisms) surfaces or within locally supersaturated zones, eventually aggregating into ring-shaped clusters and aggregates. We speculate that the recovery of siliceous ecosystems in the Early Silurian provided sufficient silica for the formation of microcrystalline quartz, suppressing reverse weathering, which may have exacerbated or prolonged global cooling.