PRIMARY SILICA GRANULES — INSIGHTS INTO SILICA PRECIPITATION MECHANISMS IN EARLY ARCHEAN SEAWATER

The principal mechanisms of silica precipitation and deposition in Precambrian oceans contrast strongly with those dominating the Phanerozoic silica cycle. The first silica biomineralizating organisms evolved during late Proterozoic (sponges) and early Paleozoic (radiolaria) time. Since Ordovician time, biogenic silica has represented the ocean’s largest silica sink, drawing down the concentration of dissolved silica in seawater to a few ppm. In contrast, prior to this biological innovation, the silica cycle was dominated by purely chemical processes. Fundamental questions remain about the mechanics of this abiotic Precambrian silica cycle, particularly with respect to the processes responsible for removing silica from seawater. The observation that sedimentary cherts are especially abundant in Archean sequences has lead previous authors to hypothesize that amorphous silica precipitated as a primary mineral in Archean marine waters.

The recent discovery that many pure chert layers in early Archean rocks formed as sedimentary beds of sand-sized, subspherical silica granules has provided direct evidence for primary silica precipitation within the water column. These silica granules occur in sedimentary cherts from a variety of paleoenvironments; their abundance and widespread distribution in Archean sedimentary rocks suggest that they represented a significant primary silica depositional mode. We look to modern siliceous hot springs for insight into analogous amorphous silica precipitation processes. Our findings suggest that silica granules may have formed via multiple stages of polymerization and aggregation. The environmental conditions promoting rapid silica polymerization are consistent with putative Archean ocean chemistry. Most critically, these include high salinity and/or high concentrations of dissolved silica, paired with near-neutral pH. The occurrence of these favorable conditions was likely episodic, as the silica granules often appear as discrete cm-scale layers within banded cherts.