Australia: The Land Where Time Began

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Oceanic Anoxic Event 2 - Lithium Isotope Evidence of Enhanced Weathering

About 93.5 Ma during the Ocean Anoxic Event 2, that lasted for 440,000 years, atmospheric concentrations of CO2 were high, associated with rapid global warming, marine anoxia and euxinia, resulting in loss of habitat and a mass extinction event. According to the authors1 it is believed enhanced biological productivity was responsible for the marine anoxia, though it is uncertain what the trigger for the upsurge in biological production was and what led to the rapid recovery. The authors used measurements of lithium isotopes from carbonates that spanned the interval that included the OAE2 to assess the role silicate weathering played. They concluded from their results that the lightest values of the lithium isotope ratio δ7Li during the OAE2, which indicates high weathering levels, and therefore the removal of CO2 from the atmosphere, which they attributed to an enhanced hydrological cycle. They simulated the evolution of  δ7Li and the Ca, Sr and Os isotope tracers which suggested a scenario in which there was an eruption of a large igneous province leading to high levels of CO2 with rapid global warming, which in turn initiated the OAE2. A roughly 200,000 year pulse of increased weathering of mafic silicate rocks, that removed CO2 from the atmosphere, accompanied this simulated warming. Primary productivity was stimulated by nutrients being delivered to the oceans as a consequence of the weathering. The authors1 suggest the rapid recovery from the greenhouse state was allowed by this process, as well as burial of organic carbon.

Sources & Further reading

  1. Pogge von Strandmann, Philip A. E., Hugh C. Jenkyns, and Richard G. Woodfine. "Lithium Isotope Evidence for Enhanced Weathering During Oceanic Anoxic Event 2." Nature Geosci 6, no. 8 (08//print 2013): 668-72.
Author: M. H. Monroe
Last updated 06/08/2013
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