Ironstone of the Ferrona Formation, Nova Scotia, and the biogeochemical cycling of Fe and P
LE3 .A278 2017
Pufahl, Peir K.
Bachelor of Science
Early Ordovician phosphatic ironstones of the Ferrona Formation, Nova Scotia, provide a unique window into the biogeochemical cycling of Fe and P at the onset of the Great Ordovician Biodiversification Event (GOBE). The Great Ordovician Biodiversification Event (ca. 480 to 445 Ma) represents the most important sustained increase in marine biodiversity in Earth history. While the Cambrian Explosion produced skeletonized organisms with a range of body plans, the GOBE records the diversification of these animals. The Ferrona Formation is composed of at least four aggradational parasequences that record high-frequency fluctuations in relative sea-level during an overall marine transgression. Each decameter-scale cycle reflects progradation of ironstone- rich middle shelf deposits over deep-water mudstone, as accommodation filled. Lithofacies associations suggest Fe and P were delivered to this storm-dominated shelf via upwelling of anoxic ferruginous waters. Such upwelling stimulated primary production and deposition of organic matter in distal lithofacies near the locus of upwelling. As ferruginous water was advected away from the upwelling front, and mixed with oxygenated seawater, Fe-(oxyhydr) oxides precipitated in pore water to produce ironstone. In deeper environments with minimal terrigenous clastic input, Fe-redox pumping and bacterial respiration of sedimentary organic matter, concentrated P in pore water to precipitate sedimentary apatite. Seemingly, these processes also operated in shoreface sediments to produce the hematite, Fe-silicate, and sedimentary apatite-rich lithofacies that characterize many ironstones around the world. This model contrasts longstanding ideas that rely on a continental source of Fe. It also highlights the potential connection between seawater redox conditions, nutrient cycling, and biological events such as the GOBE
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