Palaeogeography, Palaeoclimatology, Palaeoecology 395 (2014) 222–232 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Cause of Upper Triassic climate crisis revealed by Re–Os geochemistry of Boreal black shales Guangping Xu a,b,⁎, Judith L. Hannah a,b,c,HollyJ.Steina,b,c, Atle Mørk d,e, Jorunn Os Vigran d, Bernard Bingen b, Derek L. Schutt f, Bjørn A. Lundschien g a AIRIE Program, Colorado State University, Fort Collins, CO 80523-1482, USA b Geological Survey of Norway, 7491 Trondheim, Norway c CEED, University of Oslo, 0316 Oslo, Norway d SINTEF Petroleum Research, NO-7465 Trondheim, Norway e Norwegian University of Sciences and Technology, NO-7491 Trondheim, Norway f Department of Geosciences, Colorado State University, Fort Collins, CO 80523-1482, USA g Norwegian Petroleum Directorate, NO-4003 Stavanger, Norway article info abstract Article history: The Triassic Period is bracketed by two of the ‘big five’ Phanerozoic mass extinctions. Though long viewed as a Received 2 April 2013 period of climatic stability, emerging data suggest multiple climatic swings and at least one severe ecological cri- Received in revised form 13 December 2013 sis. Linking these climatic instabilities with probable causes is hampered by poor age control within the Triassic Accepted 19 December 2013 time scale. Here we present new Re–Os ages for shale sections straddling Middle–Upper Triassic stage bound- Available online 29 December 2013 aries. Nominal Re–Os isochron ages of 236.6 and 239.3 Ma for the top and base of the Ladinian (upper Middle Triassic) bring absolute time into the contentious Triassic time scale, and place the beginning of the Late Triassic Keywords: 187 188 Triassic about 12 m.y. earlier than previously assigned. A marked decrease in initial Os/ Os in Upper Ladinian shale Ladinian–Carnian boundary records input from Wrangellian flood basalts — an instigator in the Carnian (Late Triassic) Pluvial Event and ac- Kong Karls Land companying radiation of key fossil groups (e.g., dinosaurs and calcareous nanoplankton). An absolute time scale Svalis Dome is proposed for the Anisian–Ladinian–Carnian boundaries based on Re–Os geochronology. Svalbard © 2013 Elsevier B.V. All rights reserved. Re–Os geochemistry 1. Introduction a global climate and oceanographic perturbation (Furin et al., 2006; Dal Corso et al., 2012). Accurate age information is essential to interpret The Carnian Stage (Upper Triassic) is characterized by a major eco- processes underlying these biotic crises. logical crisis (Simms and Ruffell, 1989). The crisis began with a major The radiometric time scale for Late Triassic ties the magnetostratigraphy decrease in carbonate productivity in the early Carnian, followed by a of Tethyan marine sections to the cycle-scaled terrestrial Newark significant episode of very humid conditions known as the Carnian Plu- magnetic polarity chrons (Gradstein et al., 2004, 2012). Subsequent vial Event (CPE). The CPE spurred a marked turnover of faunal and floral intervals are scaled assuming equal duration of ammonoid subzones. assemblages in Late Julian (early Carnian). Subsequently, a major ma- Re–Os geochronology for black shales has been successful for rine extinction at the Julian–Tuvalian (Late Carnian) boundary impacted documenting depositional ages and paleoenvironmental changes many ammonoid and conodont taxa, even as some terrestrial groups in- (Cohen and Coe, 2002; Hannah et al., 2004). New Re–Os isochron ages creased in diversity with increasing humidity (Raup and Sepkoski, for stratigraphically well-constrained sedimentary rocks from Triassic 1982; Simms and Ruffell, 1989; Hornung et al., 2007; Dal Corso et al., sections in the Boreal realm are presented. The Ladinian–Carnian 2012). With the return to more arid conditions in the Late Carnian, stage boundary, as defined by palynology and ammonoid fossils, is many water-dependent terrestrial taxa died off, while other groups bracketed using the upper Ladinian Botneheia and lower Carnian (calcareous nanoplankton, scleractinian reef builders, and dinosaurs) Tschermakfjellet formations from east of Kong Karls Land in the north- expanded rapidly (Furin et al., 2006). The cause of the Carnian crisis is ern Barents Sea. These new ages, along with our recent age constraints not fully understood (Preto et al., 2010), but the prevailing explanation on the Anisian–Ladinian stage boundary (Xu et al., 2009), confirm re- is that volcanism related to the Wrangellian oceanic plateau, now ac- cent revisions proposed for the controversial Middle–Late Triassic creted to the Cordilleran margin of North America, could have triggered time scale (Ogg et al., 2008; Gradstein et al., 2012). In addition, the data define variations in the initial seawater 187Os/188Os ratio, a tracer for global changes in magmatism, continental weathering, and ⁎ Corresponding author at: AIRIE Program, Colorado State University, Fort Collins, CO 80523-1482, USA. Tel.: +1 970 491 3816; fax: +1 970 491 6307. atmospheric chemistry (Cohen and Coe, 2002; Cohen et al., 2004; 187 188 E-mail address: [email protected] (G. Xu). Hannah et al., 2004). A sharp drop in initial Os/ Os ratios in the 0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2013.12.027 G. Xu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 395 (2014) 222–232 223 early–mid Carnian may reflect atmospheric and oceanic input from may be within the upper part of drill core 7831/02-U-01 or lower part of Wrangellian flood basalts, affirming their role as a trigger for the 7831/02-U-01 (Fig. 2, Table S1 and Vigran et al., in press). Carnian Pluvial Event and accompanying Carnian ecological crisis. The upper part of core 7831/02-U-01 (from 19 to 14 m depth) differs lithologically from core 7831/02-U-02; it belongs to Tschermakfjellet Formation which grades into the De Geerdalen Formation (Riis et al., 2. Description of samples and stratigraphy 2008)(Figs. 1b,c and 2). The Tschermakfjellet Formation consists of gray, non-bituminous shales in a coarsening-upwards sequence Shales from two shallow stratigraphic drillholes (7831/02-U-01 reflecting prodeltaic conditions. These shales have only moderate or- and 7831/02-U-02), cored by the Norwegian Petroleum Directorate ganic contents compared to the underlying core (Fig. 3) and are similar east of Kong Karls Land (Riis et al., 2008), were sampled for Re–Os to the Tschermakfjellet Formation as seen on Edgeøya (Lock et al., dating (Fig. 1). The 7831/02-U-02 core represents the Botneheia 1978), with sparse fossils and siderite beds, and abundant pyrite nod- Formation, which comprises organic-rich shale containing cherty hori- ules. The presence of the Aulisporites astigmosus Composite Assemblage zons and phosphate nodules. The units are markedly similar to expo- Zone in this interval indicates an early Carnian age (Table S1) (Vigran sures in eastern Svalbard (Krajewski, 2008; Riis et al., 2008) where the et al., in press). The gray shales in the lower part of the core 7831/02- Botneheia Formation is ca. 90–110 m thick and spans lower Anisian to U-01 (from 28 to 19 m depth) contain sparse phosphate nodules and upper Ladinian deposition. Its upper part, known as the Blanknuten cherty horizons, as observed more abundantly in the dark shales of Member, is a notably organic-rich, calcareous black shale with locally the Botneheia Formation in core 7831/02-U-02 and at Edgeøya abundant phosphate nodules. The formation was deposited in a re- (Krajewski, 2008). The lower part of core 7831/02-U-01 may represent gressive and mostly anoxic restricted shelf environment (Mørk et al., a gradual transition from the Botneheia to phosphate- and chert-free 1999; Krajewski, 2008). The lithology of core 7831/02-U-02 resembles Tschermakfjellet Formation. Palynological evidence at the base of core that of the Blanknuten Member on Edgeøya (Krajewski, 2008). Three 7831/02-U-01 as in the core below is equivocal, permitting either a 14–18 cm intervals of black shale from the Botneheia Formation were late Ladinian or early Carnian age (Table S1). The clear shift in sedimen- sampled from 7831/02-U-02 (Fig. 2). tary regime between the two cores resembles the stratigraphic succes- The recognition of the Echinitosporites iliacoides Composite sion on eastern Svalbard (Lock et al., 1978; Mørk et al., 1999; Krajewski, Assemblage Zone indicates that the lower part of core 7831/02-U-02, 2008). Two sampled intervals, 27 and 31 cm thick, in 7831/02-U-01 from 18.79 to 13.88 m, is Ladinian (Table S1) (Vigran et al., in press). were taken from the best laminated sections between 20.74 and The ammonoid ex gr. Nathorstites sp. Juv. was found in a phosphate 19.13 m (Fig. 2). nodule at 12.32–12.27 m of core 7831/02-U-02 (Fig. S1), which can be Riis et al. (2008) proposed that prodelta shales of the of late Ladinian (Nathorstites mclearni) or early Carnian age (Nathorstites Tschermakfjellet Formation prograded from SE to NW. Lithostratigra- lindstroemi and Nathorstites mcconnelli). Nathorstites specimens from phy and biostratigraphy together (Weitschat and Dagys, 1989) show core 7831/02-U-02 have globular internal whorls which indicate a late that the base of the Tschermakfjellet Formation and its equivalents, Ladinian age (Wolfgang Weitschat, personal communication, 2007). the Skuld and Snadd formations, are time-transgressive (Fig. 1c). The deposits from the upper part of the 7831/02-U-02 core (11.43 to The Global