The chemistry of eolian quartz dust and the origin of chert

Temporal and spatial sources of silica for chert remain poorly constrained. Modern sources to the worlds oceans include silica in rivers > aeolian (dust) deposition > sea floor vents and submarine weathering. However, changes in aridity and dust flux during the Phanerozoic may explain variations in the ocean silica cycle and times and places of chert formation. The chemistry of fine quartz dust (FQD) provides a chemical mechanism for the transformation of FQD to polymorphs of silica in chert; FQD is readily dissolved, then reprecipitated as Opal-A by either biotic or abiotic processes. An unequivocal relation between increases in dust flux and biogenic opal-A in the western Pacific Ocean during the past 200 kyr helps explain why dust was a probable source of silica for Phanerozoic chert in disparate depositional environments ranging from the deep sea to epicontinental seas, and continental settings (lakes, aeolinites, and arid climate silcretes). The data table contains percent of size fraction and settling times.