Perturbation of the marine nitrogen cycle during the Late Ordovician glaciation and mass extinction Genming Luo a,b,c, ⁎, Thomas J. Algeo a,d,e , Renbin Zhan c , Detian Yan f , Junhua Huang c , Jiangsi Liu a , Shucheng Xie a a State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China b Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA c State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China d Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA e State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China f Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China abstract article info Article history: Received 3 May 2015 Received in revised form 14 July 2015 Accepted 16 July 2015 Available online 26 July 2015 Keywords: Hirnantian Mass extinction Glaciation Nitrogen cycle N 2 O South China The Late Ordovician was a critical interval in geologic history, during which both the biosphere and marine envi- ronments underwent severe perturbations, including one of the ‘Big Five’ Phanerozoic mass extinctions and the massive but short-term (~0.5-Myr) Hirnantian glaciation. The onset and termination of the Hirnantian glaciation have been widely accepted as the triggers for the two extinction pulses that comprise the Late Ordovician biocrisis, but the mechanisms that caused the Hirnantian glaciation itself remain poorly known. Here, we analyze the nitrogen isotope composition (δ 15 N) of two sections in South China (Wangjiawan and Nanbazi) in order to better understand nitrogen cycle perturbations in the Late Ordovician ocean and their relationship to contempo- raneous climatic and biogeochemical changes. Low δ 15 N (~1‰) in the upper Katian and lower Rhuddanian of both sections suggests intensive (i.e., near-quantitative) denitrification and, thus, nitrogen fixation as the main source of biologically available nitrogen for primary producers. A positive δ 15 N excursion in both sections during the Hirnantian indicates weaker (i.e., non-quantitative) denitrification, possibly as a result of more vigorous ther- mohaline circulation and improved ocean ventilation. Weaker denitrification would have reduced the flux of N 2 O, an intermediate product of denitrification, to the atmosphere. N 2 O is a potent greenhouse gas, and a major decline in its production would have led to cooler climatic conditions and, ultimately, the Hirnantian glaciation. A global survey of published nitrogen isotope records suggests that similar processes operated broadly within the Late Ordovician global ocean. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The Hirnantian, the terminal stage of the Ordovician, witnessed severe upheavals in the biosphere, climate, and marine environments (e.g. Sheehan, 2001; Chen et al., 2006; Yan et al., 2010; Finnegan et al., 2011; Melchin et al., 2013; Harper et al., 2014). Marine biotas were decimated by the first of the ‘Big Five’ Phanerozoic mass extinctions, in which ~85% of extant marine species died out (see Sheehan, 2001 for review). The extirpation of marine faunas occurred in two pulses, the first at the beginning and the second near the end of the Hirnantian stage (e.g. Harper et al., 2014). In the interval between these extinction pulses, there flourished a distinctive, globally distributed fauna domi- nated by brachiopods and trilobites that is known as the “Hirnantia Fauna” (e.g., Rong et al., 2002). Global cooling during the Middle and Late Ordovician led to a brief (~0.5-Myr-long) glaciation, during which continental ice mass may have two times greater than during Pleistocene glacial stages (Brenchley et al., 1994, 2003; Finnegan et al., 2011). A recent carbonate C \\ O clumped isotope study suggests that tropical sea-surface temper- atures decreased by ~ 5 °C at that time (Finnegan et al., 2011). Severe cli- mate change is inferred to have been the main cause of the latest Ordovician mass extinction (e.g., Brenchley et al., 1995; Saltzman and Young, 2005; Yan et al., 2010; Finnegan et al., 2011). However, the causes of the onset and termination of the Hirnantian glaciation as well as the exact relationships between climate and biotic changes during the Late Ordovician remain poorly known. Accompanying the Hirnantian glaciation and mass extinction were significant perturbations of the global carbon cycle, as indicated by a near-ubiquitous positive shift in the carbon isotopic composition of both carbonates (δ 13 C carb ) and organic matter (δ 13 C org ) (e.g., Wang et al., 1997; Kump et al., 1999; Saltzman and Young, 2005; Chen et al., 2006; Fan et al., 2009; LaPorte et al., 2009; Yan et al., 2009a; Young Palaeogeography, Palaeoclimatology, Palaeoecology 448 (2016) 339–348 ⁎ Corresponding author at: State Key Laboratory of Biogeology and Environmental Geology, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China. E-mail address: logming@gmail.com (G. Luo). http://dx.doi.org/10.1016/j.palaeo.2015.07.018 0031-0182/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo