[Eoas-seminar] Reminder: Geology Thesis Defense - Andrew Kleinberg - June 29, 1:15pm - 104CAR

eoas-seminar at lists.fsu.edu eoas-seminar at lists.fsu.edu
Wed Jun 28 12:00:06 EDT 2017

Title: Investigation of Paleoredox Conditions Across the Llandovery-Wenlock (Silurian) Boundary: Implications for the Ireviken Extinction Event and Carbon Isotope Excursion
Major Professor: Seth Young

            The Ireviken extinction event occurred within the early-middle Silurian, spanning the Llandovery-Wenlock boundary, ~431 million years ago and has been proposed to have been initiated at least partly through the transition from late Ordovician- early Silurian icehouse conditions to middle Silurian greenhouse conditions. This extinction, like many of the biotic crises throughout the Paleozoic, coincides with the rising limb of a subsequent carbon isotope excursion and the magnitude of the Ireviken carbon isotope excursion (CIE) has recorded δ13C values of ≥+4‰ worldwide. Epicontinental seaways have long been thought to have been the most likely place for high enough organic carbon burial to have the capability of producing this positive isotope excursion. However, the Ireviken extinction event and its subsequent carbon isotope excursion had both occurred during times of increased carbonate production, rather than organic carbon burial, in these shallow epeiric seas. This led to the development of a two steady-state ocean climate model, known as primo (P) and secondo (S) states, that could best explain these lithostratigraphic and biostratigraphic trends. This model demonstrates how alternating between climate states can affect ocean circulation via changing the site of deep water formation and ultimately induce anoxia throughout portions of the deep ocean. It assumes of a globally deoxygenated deep ocean with δ13C alone and lacks any direct evidence for pervasive deep ocean anoxia.  It is for that reason that this study has conducted the first paired δ34S-carbonate associated sulfate (CAS) and δ13C study along with a carbonate paleoredox proxy, I/(Ca+Mg) ratio analysis, to bring new insights on the paleoredox conditions of the late Llandovery-early Wenlock oceans during the Ireviken extinction and CIE. Covarying positive shifts in δ13CCarb and δ34SCAS are consistent with the onset of sea-level rise following the end of Late Ordovician-Early Silurian icehouse conditions and represents an overall increase in the fraction of anoxic waters in the global ocean. Trends in δ13COrg show overall low oxygen trends and downwelling of nutrient poor water masses resulting from expansion of epeiric seas. I/(Ca+Mg) ratios during this time also indicate local and pervasive low oxygen conditions. Shallow oxic surface waters were shown to be in direct exchange and within proximity to anoxic water masses suggesting a shallow chemocline prior to the Ireviken CIE and an expanded one during. The net effect of organic carbon and pyrite burial results in an overall increase in atmospheric O2, eventually cooling climate, reestablishing thermohaline circulation, and, therefore, ending the Ireviken CIE.

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