A sulfidic driver for the end-Ordovician mass extinction

• Published in: Earth and Planetary Science Letters
• Main Authors: Hammarlund, Emma U., Dahl, Tais Wittchen, Harper, David A.T., Bond, David P.G., Nielsen, Arne T., Bjerrum, Christian J., Schovsbo, Niels H., Schoenlaub, Hans P., Zalasiewicz, Jan A., Canfield, Donald Eugene
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2012
• Subjects: South pole
• Online Access: https://portal.findresearcher.sdu.dk/da/publications/d20d0325-f914-4ad3-bab9-e6a81f7e1422; https://doi.org/10.1016/j.epsl.2012.02.024

The end-Ordovician extinction consisted of two discrete pulses, both linked, in various ways, to glaciation at the South Pole. The first phase, starting just below the Normalograptus extraordinarius Zone, particularly affected nektonic and planktonic species, while the second pulse, associated with the Normalograptus persculptus Zone, was less selective. Glacially induced cooling and oxygenation are two of many suggested kill mechanisms for the end-Ordovician extinction, but a general consensus is lacking. We have used geochemical redox indicators, such as iron speciation, molybdenum concentrations, pyrite framboid size distribution and sulfur isotopes to analyze the geochemistry in three key Hirnantian sections. These indicators reveal that reducing conditions were occasionally present at all three sites before the first pulse of the end-Ordovician extinction, and that these conditions expanded towards the second pulse. Even though the N. extraordinarius Zone appears to have been a time of oxygenated deposition, pyrite is significantly enriched in S-34 in our sections as well as in sections reported from South China. This suggests a widespread reduction in marine sulfate concentrations, which we attribute to an increase in pyrite burial during the early Hirnantian. The S-isotope excursion coincides with a major positive carbon isotope excursion indicating elevated rates of organic carbon burial as well. We argue that euxinic conditions prevailed and intensified in the early Hirnantian oceans, and that a concomitant global sea level lowering pushed the chemocline deeper than the depositional setting of our sites. In the N. persculptus Zone, an interval associated with a major sea level rise, our redox indicators suggests that euxinic conditions, and ferruginous in some places, encroached onto the continental shelves. In our model, the expansion of euxinic conditions during the N. extraordinarius Zone was generated by a reorganization of nutrient cycling during sea level fall, and we argue, overall, that these ...