Primary silica granules-A new mode of Paleoarchean sedimentation

In the modern silica cycle, dissolved silica is removed from seawater by the synthesis and sedimentation of silica biominerals, with additional sinks as authigenic phyllosilicates and silica cements. Fundamental questions remain, however, about the nature of the ancient silica cycle prior to the appearance of biologically mediated silica removal in Neoproterozoic time. The abundance of siliceous sedimentary rocks in Archean sequences, mainly in the form of chert, strongly indicates that abiotic silica precipitation played a significant role during Archean time. It was previously hypothesized that these cherts formed as primary marine precipitates, but substantive evidence supporting a specific mode of sedimentation was not provided. We present sedimentologic, petrographic, and geochemical evidence that some and perhaps many Archean cherts were deposited predominately as primary silica grains, here termed silica granules, that precipitated within marine waters. This mode of silica deposition appears to be unique to Archean time and provides evidence that primary silica precipitation was an important process in Archean oceans. Understanding this mechanism promises new insights into the Archean silica cycle, including chert petrogenesis, microfossil preservation potential, and Archean alkalinity budgets and silicate weathering feedback processes.