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Mapping amorphous SiO2 in Devonian shales and the possible link to marine productivity during incipient forest diversification

Wed., Oct. 12, 2022 4:00 p.m. - Wed., Oct. 12, 2022 5:00 p.m.

Location: CW 237.3 and Zoom

All are welcome to listen/watch . . .

Speaker: Dr. Hilary Corlett, Department of Earth Sciences, Memorial University, St. John’s, NL

Abstract:The Late Devonian saw an increase in terrestrial organic matter abundance through forest diversification and the development of root networks. This increased weathering and pedogenesis would have likely affected the marine realm; however, interactions between the lithosphere, hydrosphere, and atmosphere are not well understood in the Paleozoic. Several C13 excursions are recognized in Frasnian marine shales and carbonates worldwide and are referred to collectively as the punctata Event, named for the corresponding conodont zone. The cause of these excursions is thought to be linked to the diversification of forests taking place at this time. It is predicted that increased soil weathering likely resulted in nearshore eutrophication, upwelling, and oxygen stratification in the world’s oceans. Productivity (phyto- and zooplankton) associated with upwelling is not able to be directly measured and, at present, geochemical proxies are used to track paleoproductivity in ancient ocean basins. One of the proxies used for this purpose is excess silica, which is silica that does not correlate with elements typically associated with clays (e.g., Al, Ti) and is instead attributed to a biogenic source. In this study, excess silica is detected in the Frasnian-aged Duvernay Formation and corresponds to δC13 (org) excursions associated with the punctata Event. In addition, longwave hyperspectral imagery is used to detect opaline silica that may be sourced from radiolarian tests. This continuous dataset through several Duvernay cores across the basin allow for mapping of and correlation of biogenic silica during the punctata Event, confirming that increased productivity was likely caused by elevated nutrients entering the marine realm from newly developed soil horizons. The results of the study demonstrate the potential for using longwave hyperspectral imagery for mapping ancient upwelling events associated with major climatic shifts.