https://doi.org/10.1130/G47781.1 Manuscript received 20 April 2020 Revised manuscript received 29 June 2020 Manuscript accepted 6 July 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 13 August 2020 Ocean acidification during the early Toarcian extinction event: Evidence from boron isotopes in brachiopods Tamás Müller1*, Hana Jurikova2,3, Marcus Gutjahr2, Adam Tomašových1, Jan Schlögl4, Volker Liebetrau2, Luís v. Duarte5, Rastislav Milovský1, Guillaume Suan6, Emanuela Mattioli6,7, Bernard Pittet6 and Anton Eisenhauer2 1 Earth Science Institute, Slovak Academy of Sciences, Dˇ umbierska 1, 974 01 Banská Bystrica, Slovakia 2 GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany 3 GFZ Deutsches GeoForschungsZentrum–Helmholtz-Zentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany 4 Department of Geology and Paleontology, Faculty of Natural Sciences, Comenius University in Bratislava, Mlynská dolina, Ilkovicˇova 6, SK-842 15 Bratislava, Slovakia 5 University of Coimbra, MARE–Marine and Environmental Sciences Centre–Department of Earth Sciences, Polo II, Rua Sílvio Lima, 3030-790 Coimbra, Portugal 6 Univ Lyon, Univ Lyon 1, ENSL, CNRS, LGL-TPE, F-69622, Villeurbanne, France 7 Institut Universitaire de France (IUF), 75231 Paris Cedex 05, France ABSTRACT methane, have also been postulated (Hesselbo The loss of carbonate production during the Toarcian Oceanic Anoxic Event (T-OAE, ca. et al., 2000; Them et al., 2017). 183 Ma) is hypothesized to have been at least partly triggered by ocean acidification linked The changes in the carbon cycle are globally to magmatism from the Karoo-Ferrar large igneous province (southern Africa and Antarc- expressed as a short negative shift in the car- tica). However, the dynamics of acidification have never been directly quantified across the bon-isotope record at the Pl-To boundary (Lit- T-OAE. Here, we present the first record of temporal evolution of seawater pH spanning the tler et al., 2010), followed by a broad positive late Pliensbachian and early Toarcian from the Lusitanian Basin (Portugal) reconstructed excursion that is interrupted by a major negative on the basis of boron isotopic composition (δ11B) of brachiopod shells. δ11B declines by ∼1‰ (∼6‰) carbon isotope excursion (CIE) during across the Pliensbachian-Toarcian boundary (Pl-To) and attains the lowest values (∼12.5‰) the T-OAE (Hesselbo et al., 2007; Müller et al., just prior to and within the T-OAE, followed by fluctuations and a moderately increasing 2017). Marine carbonate factories dominated by 11 trend afterwards. The decline in δ B coincides with decreasing bulk CaCO3 content, in bivalves, corals, and algae disappeared after the parallel with the two-phase decline in carbonate production observed at global scales and onset of the negative CIE (Trecalli et al., 2012; with changes in pCO2 derived from stomatal indices. Seawater pH had declined signifi- Brame et al., 2019), and nannoplankton fluxes cantly already prior to the T-OAE, probably due to the repeated emissions of volcanogenic declined in epicontinental basins (Mattioli et al., CO2. During the earliest phase of the T-OAE, pH increased for a short period, likely due 2009). The coincidence between the timing of to intensified continental weathering and organic carbon burial, resulting in atmospheric the CIE, indicating a major increase in CO2 CO2 drawdown. Subsequently, pH dropped again, reaching the minimum in the middle of emissions, and the collapse in carbonate pro- the T-OAE. The early Toarcian marine extinction and carbonate collapse were thus driven, duction indicate ocean acidification as one of the in part, by ocean acidification, similar to other Phanerozoic events caused by major CO2 potential drivers of these changes (Trecalli et al., emissions and warming. 2012). However, a direct quantification of pH is lacking. To fill this gap, we measured the boron INTRODUCTION Suan et al., 2010; Trecalli et al., 2012). These isotope composition (δ11B) of brachiopod shells The Pliensbachian-Toarcian (Pl-To) bound- changes and the associated ecosystem crisis in conjunction with their δ13C and δ18O from the ary and the Toarcian Oceanic Anoxic Event have been linked to the emplacement of the Peniche section (Global Boundary Stratotype (T-OAE, ca. 183 Ma) constituted a transient Karoo-Ferrar large igneous province (southern Section and Point of the Toarcian Stage) in the interval of global warming, development of Africa and Antarctica) and consequent green- Lusitanian Basin (Portugal; Comas-Rengifo widespread anoxia, enhanced organic carbon house gas release (Caruthers et al., 2013). Dur- et al., 2015; Duarte, 2007). This section com- burial, and acceleration of the hydrological ing the T-OAE, volcanogenic greenhouse gas bines exceptional stratigraphic resolution across cycle, resulting in a mass extinction and a col- emissions induced by thermal metamorphosis the Pl-To boundary and the T-OAE with reliable lapse of carbonate production (e.g., Jenkyns, of coal deposits in the Karoo basin most likely preservation of geochemical signals in calcitic 1988; Bailey et al., 2003; Cohen et al., 2004; triggered carbon-cycle perturbations (McElwain shells (Suan et al., 2008; Rocha et al., 2016). et al., 2005; Percival et al., 2015), although Here, we evaluate the timing and intensity of other sources, such as dissociation of methane ocean acidification by reconstructing temporal *E-mail: [email protected] hydrates from marine sediments or terrestrial evolution of seawater pH. CITATION: Müller, T., et al., 2020, Ocean acidification during the early Toarcian extinction event: Evidence from boron isotopes in brachiopods: Geology, v. 48, p. 1184–1188, https://doi.org/10.1130/G47781.1 1184 www.gsapubs.org | Volume 48 | Number 12 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/48/12/1184/5186144/1184.pdf by guest on 26 September 2021 Figure 1. (Upper panel) Paleogeographic loca- tion of the Lusitanian Basin (Portugal; red star) during the Early Jurassic Legend: (adapted from Ikeda and Peniche 13 Hori, 2014). LIP—large bulk rock ˡ C Hesselbo et al. (2007) igneous province. (Lower 13 Tethys brachiopod C this study Panthalassa ˡ13 panel) Stratigraphic log brachiopod C Suan et al. (2008) and scale (after Hesselbo ˡ18 brachiopod ˡ O this study et al., 2007; modified fol- brachiopod 18O Suan et al. (2008) lowing Duarte et al., 2018) ˡ11 brachiopod B this study Karoo-Ferrar plotted against bulk δ13C ˡ11 LIP brachiopod ˡ B this study excluded from pH calibration (Hesselbo et al., 2007) and Early Jurassic published (Suan et al., ca.183 Ma 2008) and new (this study) ) n 13 18 m o ( i ¹³C (‰VPDB) ¹¹B (‰NIST951) brachiopod δ C, δ O, and t ˡ ˡ¹೙O (‰VPDB) ˡ t a 11 e h B (error bars indicate e -3 -2 -1 0213465 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 12 13 14 15 16 δ g m g i n r a e t o o two standard deviations Z S F H on replicated analyses). 35 Yellow line shows the three-point moving aver- e 30 age; horizontal purple n o Z line indicates the exact i n n o i o t 25 s a level of the Pliensbachian- i v m r e o L Toarcian boundary. The F n a i o 20 r Toarcian Oceanic Anoxic c i r e a o o v Event (T-OAE) is defined as T r a y l 15 r C T-OAE the interval between onset a o E b of negative carbon isotope a . C Z 10 excursion and the inflec- m u h tion point in the Levisoni p r o 5 ammonite zone, above m y l o which values tend toward P 0 . Pl-To a more positive direction, Z m and coincides with the u t a m -5 n n F i deposition of organic-rich a i p e h S d c black shales in many Euro- e a . b m Z -10 s e . pean sections (Müller et al., n L g e r i l a P 2017). Pl-To—Pliensba- M -15 chian-Toarcian boundary; -3 -2 -1 0213465 -3 -2.5 -2 -1.5 -1 -0.5 0 0.5 12 13 14 15 16 Z.—Zone; Marg.—Margar- itatus; Fm.—Formation; Lithology and fossils: marlstone limestone blue marlstone sandstone lamination burrows brachiopod belemnite ammonite VPDB—Vienna Peedee belemnite; NIST951—Boric acid isotopic standard. METHODS plemental Material1). Sample preparation and in this model has a mean value of 7.7 and ranges The δ11B composition of marine biogenic elemental, as well as δ11B and 87Sr/86Sr, analy- between 7.4 and 7.9 for the latest Pliensbachian carbonates is presently regarded as the most ses were performed according to the methods (ca. 184 Ma). With this range of pre-event sea- 11 reliable pH proxy (Gutjahr et al., 2017). Articu- of Jurikova et al. (2019) and Krabbenhöft et al. water pH, we computed δ Bseawater and seawater late brachiopods secrete low-Mg calcitic shells (2009) on pre-cleaned dissolved powders, with pH with two different δ11B-pH calibrations: (1) nearly in isotopic equilibrium with seawater major- and trace-element content (Ca, Mg, Al, scenario 1, where biological influence on boron (Brand et al., 2013) and exhibit a pH-depen- Sr, Mn, B) determined on a quadrupole induc- incorporation into brachiopod shells is consid- dent δ11B relationship (Lécuyer et al., 2002; tively coupled plasma–mass spectrometer (ICP- ered (Lécuyer et al., 2002), resulting in a mean 11 11 Penman et al., 2013; Jurikova et al., 2019). We MS) (Agilent 7500x), δ B on a multicollector δ Bseawater of 36.6‰ (range = 34.9‰−37.5‰); present major- and trace-element concentration, ICP-MS (Thermo Scientific Neptune Plus), and and (2) scenario 2, where boron incor- δ11B, δ13C, and δ18O, as well as 87Sr/86Sr com- 87Sr/86Sr via thermal ionization mass spectrom- poration follows inorganic fractionation 13 11 11 position of brachiopod shells collected from the etry (TIMS) (ThermoFisher TRITON).