Browsing by Subject "Toarcian"

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Carbonate platform demise and recovery at the Toarcian Oceanic Anoxic Event : high-resolution data from the Adriatic Carbonate Platform
(2017-06-28) Ettinger, Nicholas Patrick; Martindale, Rowan C; Kerans, C. (Charles)
The sedimentological, stratigraphic, and geochemical signature of the Toarcian Oceanic Anoxic Event (OAE) on the Bahamian-style, Adriatic Carbonate Platform of Southern Europe is examined here. New stratigraphic sections correlated to previously published data show that a healthy and diverse metazoan carbonate factory characterized the flat-topped platform prior to the onset of the OAE. Carbon isotopes of marine organic matter and bulk carbonate, as well as trace element data (V, Ni, Mo, Mn, Hg) were used to identify the stratigraphic location of the OAE on the outer platform. A negative organic carbon isotope excursion of -2.2‰ coincident with enrichments in Mo and Fe was used to correlate to time equivalent strata in both the inner platform and adjacent basin. The onset of the OAE heralded a carbonate factory collapse on the outer platform that was driven by deoxygenation, eutrophication, and possibly ocean acidification. At the same time, a shallow intrashelf basin developed, while the adjacent deep-water basin accumulated black shales. The cessation of carbonate production throughout the OAE yielded a significant drowning unconformity on the outer platform as sea-level continued to rise, while restricted, organic-rich mudstones accumulated on the inner platform under a euxinic regime. Recovery of carbonate production is characterized mainly by calcitic fauna and ooids. By the Middle Jurassic, ooids generated on the outer margin spilled into the adjacent basin forming a thick deep-water oolitic fan, and they prograded to the inner platform, reestablishing a flat-topped carbonate platform.
Impact of Increased Terrigenous Sediment in Near-Shore Reef Systems
(2020-12) Logie, Tyler
Changes in the flux of terrestrial sediment into the ocean can cause marine ecosystems to suffer severe environmental stress. The physical effects of increased terrestrial erosion towards the shallow ocean can decrease light quality, increases the energy that reef builders must expend, and in extreme cases can cause burial of near-shore reefs. The influx of new terrestrial sediment can also have a notable effect on the chemistry of the ocean by increasing the flux of nutrients. This can alter the trophic state of the area, favor the expansion of algae over metazoan reef builders such as corals, and potentially cause a decrease in free oxygen available in the water column. The effects of increased terrestrial sediment flux can be seen most clearly during time intervals during which a spike in atmospheric CO2 concentration causes the global silicate weathering rate to increase. These events are commonly associated with global marine extinctions, as is the case at the Permian/Triassic boundary and during the Toarcian Ocean Anoxic Event. Isotope and trace element geochemistry can be used to reconstruct changes in the global silicate weathering rate, marine nutrient content, and fraction of terrestrial sediment in a marine setting. By combining these geochemical data with modern studies of reefs under sediment stress, the mechanisms driving reef collapses in the geologic record are better understood. This information can be applied to research attempting to predict the impact of increased sediment flux associated with climate change.