The contribution of organic-rich mudstones to the Mesoproterozoic silicon cycle: criticality of microbial life

Abstract The secular evolution of marine silicon, carbon and phosphorous cycles influences interpretation of Earth’s life and climate histories. In today’s ocean, planktic siliceous skeletons are deposited in sediments as particulate biogenic opal that transforms to authigenic quartz. Si biomineralizing organisms only radiated in the early Phanerozic and in the absence of biosiliceous skeletal grains, the pathway for Si transfer from seawater into Pre-Cambrian sediments remains enigmatic. Here we determine the abundance and stratigraphic distribution of authigenic quartz in the c. 1.38 Ga Velkerri Formation (Australia), a key archive of Mesoproterozoic ocean chemistry. Petrographic, geochemical and isotopic analyses demonstrate that a major fraction of quartz (up to 45 % of the rock volume) is not of extrabasinal, detrital origin but instead authigenic and concentrated in subunits rich in organic carbon and phosphorus, indicative of high primary productivity. We argue that this authigenic quartz results from recrystallization of amorphous silica sequestered from sea water due to the presence of cyanobacteria. We probabilistically estimate that the Velkerri Formation contains several thousands cubic kilometres of authigenic quartz representing an important and previously unrecognised mineral record of silicification in Mesoproterozoic sedimentary basins, critically linked to bacterial blooms and organic carbon burial.