Publication Date:
2016-08-23
Description:
Large igneous provinces (LIPs) are proposed to have caused a number of episodes of abrupt environmental change by increasing atmospheric CO 2 levels, which were subsequently alleviated by drawdown of CO 2 via enhanced continental weathering and burial of organic matter. Here the sedimentary records of two such episodes of environmental change, the Toarcian oceanic anoxic event (T-OAE) and preceding Pliensbachian–Toarcian (Pl-To) event (both possibly linked to the Karoo-Ferrar LIP), are investigated using a new suite of geochemical proxies that have not been previously compared. Stratigraphic variations in osmium isotope ( 187 Os/ 188 Os) records are compared with those of mercury (Hg) and carbon isotopes ( 13 C) in samples from the Mochras core, Llanbedr Farm, Cardigan Bay Basin, Wales. These sedimentary rocks are confirmed as recording an open-marine setting by analysis of molybdenum/uranium enrichment trends, indicating that the Os isotope record in these samples reflects the isotopic composition of the global ocean. The Os isotope data include the first results across the Pl-To boundary, when seawater 187 Os/ 188 Os increased from ~0.40 to ~0.53, in addition to new data that show elevated 187 Os/ 188 Os (from ~0.42 to ~0.68) during the T-OAE. Both increases in 187 Os/ 188 Os correlate with negative carbon isotope excursions and increased mercury concentrations, supporting an interplay between terrestrial volcanism, weathering, and climate that was instrumental in driving these distinct episodes of global environmental change. These observations also indicate that the environmental impact of the Karoo-Ferrar LIP was not limited solely to the T-OAE.
Print ISSN:
0091-7613
Electronic ISSN:
1943-2682
Topics:
Geosciences
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