Granular iron deposits, seawater chemistry, and earth evolution
LE3 .A278 2018
Pufahl, Peir K.
Bachelor of Science
Earth & Environmental Sciences
Iron formation and ironstone are marine chemical sedimentary rocks enriched in iron. Iron formation is Precambrian in age whereas ironstone is exclusively Phanerozoic. Both have been exploited as economic sources of iron for steel manufacture. Although formed of a variety of contrasting lithofacies types, a striking similarity is the conspicious presence of granular deposits with coated grains. Coated grains record physical, chemical, and biological processes intrinsic to the paleoenvironment of deposition. Coated grains forming iron formation are composed of nuclei of blocky chert enveloped by hematitic cortical layers. These grains are interpreted to have precipitated from suboxic seawater along shallow, high-energy regions of the paleocoast within photosynthetic oxygen oases. Ironstone coated grains are generally nucleated on subangular detrital quartz grains and are characterized by cortices composed of interlaminated chamosite, apatite, and hematite. These redox sensitive coatings imply formation beneath the sediment-seawater interface as pore water Eh fluctuated during erosive exhumation and reburial of grains by shelf currents or changes in surface ocean productivity that moderated the export of sedimentary organic matter to the seafloor. These findings provide further context for the depositional and oceanographic conditions controlling the accumulation of coated iron grains. Their physical and chemical conditions of formation provide a new window into understanding seawater chemistry, the benthic iron cycle, and biological evolution through deep time.
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