The Pliensbachian–Toarcian (Early Jurassic) extinction, a global multi-phased event

    The Pliensbachian–Toarcian (Early Jurassic) extinction, a global multi-phased event Andrew H. Caruthers, Paul L. Smith, Darren R. Gr¨ocke PII: DOI: Reference:

S0031-0182(13)00234-4 doi: 10.1016/j.palaeo.2013.05.010 PALAEO 6499

To appear in:

Palaeogeography, Palaeoclimatology, Palaeoecology

Received date: Revised date: Accepted date:

1 January 2013 27 April 2013 2 May 2013

Please cite this article as: Caruthers, Andrew H., Smith, Paul L., Gr¨ ocke, Darren R., The Pliensbachian–Toarcian (Early Jurassic) extinction, a global multi-phased event, Palaeogeography, Palaeoclimatology, Palaeoecology (2013), doi: 10.1016/j.palaeo.2013.05.010

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ACCEPTED MANUSCRIPT The Pliensbachian–Toarcian (Early Jurassic) extinction, a global multi-phased event

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Andrew H. Caruthersa*, Paul L. Smith a, and Darren R. Gröcke b

Department of Earth Ocean and Atmospheric Sciences, University of British Columbia,

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Room 2020 Earth Sciences Building, 2207 Main Mall, Vancouver British Columbia V6T

b

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1Z4, Canada. E-mail addresses: [email protected], [email protected]. Department of Earth Sciences, Durham University, South Road, Durham DH1 3LE, UK.

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E-mail address: [email protected].

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Tel.: +1 604 224 5669.

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* Corresponding author.

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E-mail address: [email protected]

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ACCEPTED MANUSCRIPT ABSTRACT During the Pliensbachian–Toarcian of Early Jurassic, there is a well-known

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second order marine extinction that is observable at the species and genus levels.

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Ammonite diversity data from successions throughout Europe and parts of the Arctic suggest that this extinction may have been multi-phased with diversity declining over six

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separate intervals. The main-phase of decline begins at the Pliensbachian–Toarcian

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boundary and extends into the Lower Toarcian, to a level that is correlative with the Tenuicostatum / Serpentinum Zone boundary. To date, only the main-phase of extinction

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has been demonstrated as being global in extent and affecting multiple taxonomic groups. This multi-phased extinction has been attributed to regional and global controlling

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mechanisms that are associated with the Volcanic Greenhouse Scenario which links the eruption of the Karoo–Ferrar large igneous province (LIP) to global warming and mass

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extinction.

We compare stratigraphic ranges of ammonite and foraminiferal species in Pliensbachian–Toarcian successions of western North America to the record in Europe and parts of the Arctic in order to test the geographic extent of the multiple phases of extinction. Our results show six intervals of species level decline that correlate with those recognized in Europe: 1) middle of the Lower Pliensbachian (middle Whiteavesi–middle Freboldi Zones), 2) middle of the Upper Pliensbachian (upper Kunae–lower Carlottense Zone), 3) Pliensbachian / Toarcian boundary into the Lower Toarcian (upper Carlottense– middle Kanense Zones), 4) Middle Toarcian (upper Planulata–lower Crassicosta Zones), 5) upper Middle–lower Upper Toarcian (middle Crassicosta–Hillebrandti Zones) and 6) Upper Toarcian (lower Yakounensis Zone).

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ACCEPTED MANUSCRIPT Recognition of this multi-phased event in three separate ocean basins (paleo Pacific, paleo Arctic, and Tethys Oceans), in at least two taxonomic groups, greatly

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expands the known geographic extent of this multi-phased event and argues for a

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controlling mechanism that is global in its reach. In relation to the Volcanic Greenhouse Scenario, our study shows that four of the six pulses of extinction occur within the main-

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phase of Karoo magmatism. The decline in the Early Pliensbachian, previously thought to

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be separate from this event, occurs within error range of the onset of Karoo magmatism and the decline in the Late Toarcian coincides with the later stages of magmatism. These

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observations extend the known duration of this multi-phased extinction event to the Early Pliensbachian and support the Volcanic Greenhouse Scenario, specifically the eruption of

Keywords

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Pliensbachian–Toarcian.

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the Karoo–Ferrar LIP, as a preeminent factor driving the multi-phased extinction of the

Pliensbachian; Toarcian; extinction; western North America; ammonite; foraminifera

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ACCEPTED MANUSCRIPT 1. Introduction Mass extinctions involve the global disruption of a broad suite of environments

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causing a significant proportion of the world’s biota to disappear over a narrow interval

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of geological time (Hallam and Wignall, 1997). It has recently been suggested that greenhouse gas emissions are having a major impact on the marine ecosystem, causing

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extinction and possibly leading to ocean anoxia (Harnik et al., 2012). Understanding

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extinction is therefore a central question for science and society, a question upon which the geological record can shed valuable light.

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One mechanism-chain has recently emerged that links global warming to both first and second order extinction events. It is referred to as the ‘Volcanic Greenhouse

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Scenario’ by Wignall (2005) and it is explored in the current study (Figure 1). This model is evidenced in the sedimentary record by elevated concentrations of organic carbon and

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major perturbations in the geochemical record. The geochemical systems involved include: carbon, nitrogen, molybdenum, manganese, sulfur, strontium and osmium isotopes (Jenkyns, 1988; 2003; 2010; Holser et al., 1989; Magaritz et al., 1992; Gröcke et al., 1999; Hesselbo et al., 2000; 2002; 2004; Jenkyns et al., 2001; McArthur et al., 2000; 2008; Pálfy et al., 2001; Ward et al., 2001; Wignall, 2001; Cohen and Coe, 2002; 2007; Cohen et al., 2004; Payne et al., 2004; Williford et al., 2007; Whiteside et al., 2007; 2010; Pearce et al., 2008; Suan et al., 2008; 2010; Kuroda et al., 2010; Sabatino et al., 2011; Gill et al., 2011; Sanei et al., 2012). The Volcanic Greenhouse Scenario proposes that during the eruptions that form large igneous provinces (LIPs), volcanogenic outgassing of CO2 and other greenhouse gasses results in prolonged global warming (Pálfy and Smith, 2000; Wignall, 2005).

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ACCEPTED MANUSCRIPT Warmer water temperatures then initiate a series of events which produces large-scale environmental change and mass extinction (Pálfy and Smith, 2000; Hesselbo et al., 2000;

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Wignall, 2001). These events are summarized in Figure 1 and include: 1) the sudden

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release of methane hydrate from the continental shelf sediment reservoir, 2) a disruption in ocean water dynamics involving the thermohaline circulation pattern, 3) increased

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weathering and erosion on the continents and 4) ocean stagnation and marine anoxia.

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To date the Volcanic Greenhouse Scenario has been invoked for a number of extinction events occurring over the past 300 million years (Courtillot, 1999; Pálfy and

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Smith, 2000; Wignall, 2001; 2005; Courtillot and Renne, 2003; Suan et al., 2008; Caswell et al., 2009). Four of these occur within the Upper Paleozoic and Mesozoic and

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include, in stratigraphic order: 1) the Emeishan flood basalts and the Middle Permian extinction (~263 Ma in Sun et al., 2010); 2) the Siberian Traps and the end Permian

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extinction (~250 Ma in Wignall, 2001); 3) the Central Atlantic Magmatic Province (CAMP) flood basalt and the end Triassic extinction (~200 Ma in Wignall, 2001); and 4) the Karoo–Ferrar LIP and the Pliensbachian–Toarcian extinction (~183 Ma in Pálfy and Smith, 2000). There is also a connection between the Deccan Traps and the end Cretaceous extinction at ~65 Ma although there is strong evidence that this mass extinction was more heavily influenced by a major bolide impact (Wignall, 2001 and references therein; Keller, 2008). The correlation of the Karoo–Ferrar LIP and the Pliensbachian–Toarcian mass extinction is the focus of this study.

2. The Pliensbachian–Toarcian extinction

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ACCEPTED MANUSCRIPT During the Early Toarcian there was a major extinction among many marine genera and species at a level correlative with the Tenuicostatum / Serpentinum

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(Falciferum) Zone boundary (Hallam 1961; Hallam, 1987; Benton, 1993; Little and

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Benton, 1995; Harries and Little, 1999; Aberhan and Baumiller, 2003; Cecca and Macchioni, 2004; Zakharov et al., 2006; Caswell et al., 2009; Dera et al., 2010; Bilotta et

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al., 2010). One calculation suggests a species extinction intensity that exceeds 90%

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among ammonites, belemnites, bivalves and gastropods (Harries and Little, 1999). However, a more recent calculation of ammonite species extinction in the NW Tethyan

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and Arctic domains suggests measurable losses of 40–65% at the subzone level and 70– 90% at the zone level (Dera et al., 2010).

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In Europe, this Early Toarcian extinction event affected a variety of benthic and pelagic groups in addition to the molluscs including: radiolarians, foraminifera,

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brachiopods, ostracods, dinocysts, crinoids, asteroids, crustaceans, marine reptiles and fish (Hallam, 1986; 1987; Little and Benton, 1995; Hallam and Wignall, 1997; Harries and Little, 1999; Vörös, 2002; Cecca and Macchioni, 2004; Zakharov et al., 2006; Caswell et al., 2009; García Joral et al., 2011). The Tenuicostatum / Serpentinum Zone boundary is considered to be the extinction acme where many benthic and pelagic marine groups were affected seemingly instantaneously (Figure 4 in Harries and Little, 1999). Because it has been observed in many Tethyan and Boreal stratigraphic sections, many have postulated that it was global in extent (Zakharov et al., 2006; Figure 4 in Caswell et al., 2009; Dera et al., 2010). It is this interval, some argue, that coincides with the series of events relating to environmental change within the Volcanic Greenhouse Scenario. Specifically a large negative carbon-isotope excursion (CIE) that is thought to indicate a

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ACCEPTED MANUSCRIPT release from the methane hydrate reservoir (Hesselbo et al., 2000) and the deposition of organic rich sediments co-occurring with a positive excursion in carbon-isotope values

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which is attributed to anoxic marine conditions known as the Toarcian oceanic anoxic

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event or T–OAE (Hallam, 1987; Jenkyns, 1988; 2003; 2010; Little, 1995; Little and Benton, 1995; Jenkyns and Clayton, 1997; Harries and Little, 1999; Jenkyns, 2010).

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A recent study examining Pliensbachian–Toarcian ammonite biodiversity in the

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NW Tethyan and Arctic domains has greatly expanded the known temporal extent of this extinction (Dera et al., 2010). A multi-phased event is indicated where species and

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generic biodiversity declined over six time intervals during the Pliensbachian–Toarcian (Figure 2). These biodiversity declines include the: 1) Ibex–Davoei zone, 2) Gibbosus

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subzone, 3) Pliensbachian / Toarcian boundary, 4) Semicelatum subzone, 5) Bifrons– Variabilis Zones and 6) Dispansum Zone, with the main-phase of the extinction occurring

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at a level that is correlative with the Tenuicostatum / Serpentinum Zone boundary. Events 2–6 are interpreted as contributing to the Pliensbachian–Toarcian extinction while decline number 1, occurring in the Lower Pliensbachian at the Ibex–Davoei Zone boundary, is interpreted by as a regional decline in species diversity restricted to the Mediterranean and northwest European parts of the Tethys Ocean (Dommergues et al., 2009; Dera et al., 2010). To date, this multi-phased extinction has not been established in taxonomic groups other than the ammonites, except for at the Pliensbachian / Toarcian boundary and the Tenuicostatum / Serpentinum Zone boundary (within the Lower Toarcian) where a variety of organisms were affected globally (Harries and Little, 1999; Caswell et al., 2009; Dera et al., 2010). Neither has the multi-phased extinction been shown to be global

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ACCEPTED MANUSCRIPT in extent. Except for decline number 1 across the Ibex / Davoei Zone boundary in the Lower Pliensbachian, Dera et al. (2010) speculate that the multi-pulsed volcanic activity

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of the Karoo–Ferrar LIP could have been a trigger for the extinction phases in the

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Pliensbachian–Toarcian time. If correct, there should be evidence of coeval declines in species diversity that occur globally in tandem with phases of volcanic activity within the

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Karoo–Ferrar LIP.

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2.1 Controlling mechanisms

Currently there are two leading hypotheses as to the cause of the Pliensbachian–

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Toarcian extinction event invoking global controls in one case and regional controls in the other. The global hypothesis is homologous to the Volcanic Greenhouse Scenario

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(Figure 1) and it argues that in the Lower Toarcian, at a time correlative with the Tenuicostatum / Serpentinum Zone boundary, rift-related tectonic activity during the

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break-up of Gondwana initiated the eruption of the Karoo–Ferrar LIP. Outgassing of volcanogenic CO2 from this eruption started a prolonged period of global warming (Pálfy and Smith, 2000) that subsequently destabilized and released ~5000 Gt of methane hydrate from continental shelf sediments (Hesselbo et al., 2000; Beerling et al., 2002). The rapid injection and oxidation of methane reduced oceanic O2 levels and increased organic carbon burial, creating the globally extensive euxinic conditions of the T–OAE. This severe environmental crisis is further thought to have: 1) increased seawater temperatures ~3–5°C in the subtropical seas of the Tethys and 8–10° in the polar domains (Dera et al., 2011; Dera and Donnadieu, 2012); 2) caused a significant increase in continental weathering rates (Jones and Jenkyns, 2001; McArthur et al., 2000; Cohen et al., 2004); 3) caused a major reduction in biocalcification (Mattioli et al., 2004;

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ACCEPTED MANUSCRIPT Tremolada et al., 2005; Suan et al., 2010); and 4) escalated the extinction of marine organisms in the Early Toarcian (see Caswell et al., 2009 and references therein; Bilotta

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et al., 2010; Dera et al., 2010).

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Recently, the timing and the geographic extent of the events contributing to the large extinction in the Early Toarcian have been questioned, thus prompting an

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alternative hypothesis promoting regional controls (Wignall et al., 2005; van de

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Schootbrugge et al., 2005; McArthur et al., 2008; Gómez et al., 2008; Gómez and Goy, 2011; García Joral et al., 2011). Specifically these works have suggested that: 1) the

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negative CIE interval is not a global phenomenon, 2) the main level of extinction in some European sections occurs below the negative CIE interval and 3) anoxic (black shale)

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facies are not globally distributed and, where present, are not coeval. The resulting

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‘regional’ hypothesis therefore invokes a restricted basin model (Küspert, 1982) whereby

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the European epicontinental seaway was the site of several silled basins, in which black shales and 12C-enriched bottom waters formed in response to euxinic conditions brought on by a salinity-driven pycnocline that developed as a result of increased fresh water input (Küspert, 1982; Jenkyns, 1988; Sælen et al., 1996; Schouten et al., 2000; McArthur et al., 2008). In this model, the negative CIE occurred by diachronous upwelling of 12Crich bottom water and was therefore restricted to marine environments in the Tethys Ocean area (van de Schootbrugge et al., 2005; Wignall et al., 2005; McArthur et al., 2008). Furthermore due to the various dynamics within each basin, black shale deposition and co-occurring extinction are also thought to have been diachronous throughout Europe (Wignall et al., 2005; McArthur et al., 2008). These discrepancies therefore raise concern

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ACCEPTED MANUSCRIPT as to the validity of the Volcanic Greenhouse Scenario and therefore necessitate further study.

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The work presented herein is a critical investigation of the Pliensbachian–

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Toarcian extinction. A primary objective is to explore the magnitude and geographic extent of this event by measuring species-level ammonite and foraminiferal diversity

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throughout the Pliensbachian–Toarcian interval in western North America and then

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compare it with correlative ammonite species data previously reported from the northwest European and Arctic domains. This comparison will test previous claims that the

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Pliensbachian–Toarcian extinction is a multi-phased event (e.g. Dera et al., 2010) and also shed light on the apparent discrepancy between the extinction interval and negative

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CIE that is the focus of study in Wignall et al. (2005). Lastly, our work addresses the hypothesized primary controlling mechanism for this extinction by assessing its overall

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timing with respect to previously established eruption ages for the Karoo magmatic province (Jourdan et al., 2008).

3. Methods

3.1 A calibrated geological time scale In order to address these questions it is important to have a well calibrated geologic time scale for the Pliensbachian and Toarcian that can be applied globally. In western North America, Lower Jurassic interbedded fossiliferous marine sedimentary and volcaniclastic successions are extremely useful for calibrating biochronologic and geochronologic time scales. Previous studies have established an ammonite zonal scheme for Pliensbachian and Toarcian successions (Smith et al., 1988; Jakobs et al., 1994; Smith

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ACCEPTED MANUSCRIPT and Tipper, 1996; Jakobs, 1997) that is calibrated with U–Pb and Ar–Ar dates from interbedded zircon-bearing ash beds and lava flows (Pálfy et al., 1997; 2000). This

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calibrated time scale has been further correlated with the ammonite zonal schemes of NW

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Europe, parts of the Mediterranean, and South America (Figure 3). In our study the geologic time scale of Pálfy et al. (2000) and Gradstein et al. (2012) is utilized to

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compare geochemical results. Recent age determinations indicate that the base of the

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Toarcian is at 182.7 (+/- 0.7) Ma (Gradstein et al., 2012) rather than the previous age of 183.6 (+1.7/-1.1) Ma (Pálfy et al., 2000). Herein we use the chronostratigraphic concepts

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and zonal terminology advocated and developed by Callomon (1985). 3.2 Stratigraphic range charts

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Pliensbachian–Toarcian ammonite and foraminiferal species-level diversities in western North America were measured from compiled stratigraphic range charts derived from

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many previous accounts (Imlay, 1955; 1981; Tappan, 1955; Smith et al., 1988; Jakobs, 1992; 1995; 1997; Thomson and Smith, 1992; Jakobs et al., 1994; Smith and Tipper, 1996; Jakobs and Smith, 1996; Smith et al., 2001; Kottachchi et al., 2002; 2003; Caruthers and Smith, 2012). The compiled stratigraphic range charts are provided herein as Supplemental Data. Species ranges were analyzed using metrics established by Foote (2000) as well as Hammer and Harper (2006) including: Total diversity, total diversity minus singletons, estimated mean standing diversity, per-taxon rate, Van Valen rate, and estimated per-capita rate; all of which are defined in Figure 4. The resulting values are a measure of diversity and extinction/origination rates for these two taxonomic groups during the Pliensbachian–Toarcian interval.

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ACCEPTED MANUSCRIPT Throughout western North America, Pliensbachian and Toarcian ammonite faunas are common and are widely distributed in many stratigraphic successions (red circles in

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Figure 5; Figure 6). In comparison, coeval foraminiferal faunas are much less well

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understood. They have only been studied in two areas namely Haida Gwaii and northern Alaska (green and orange circles in Figure 5; Figure 6). The ammonite-bearing

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successions are thought to have been located in the northeast part of the paleo Pacific

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Ocean during the Early Jurassic, whereas the two areas yielding foraminiferal successions are thought to have been located in the paleo Pacific and paleo Arctic Oceans (Miller et

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al., 2002; Smith, 2006). Therefore, the compiled ammonite species ranges were analyzed as one dataset and foraminiferal species were first analyzed according to region (i.e.

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Haida Gwaii and northern Alaska), and then re-analyzed as a unified dataset. In our study, occurrences of foraminifera were placed in context of the

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Pliensbachian and Toarcian ammonite zonal schemes in order to develop comparable stratigraphic range charts. Sections utilized in this study are represented by the green circles in Figure 5 and listed in Figure 6. In northern Alaska, foraminifera are known primarily from well cores extracted from the National Petroleum Reserve on the North Slope (Tappan, 1955; Bergquist, 1966). Co-occurring Pliensbachian–Toarcian ammonite and foraminifera faunas are exceedingly rare in these well cores except for a single core known as the United States Navy South Barrow #3 Core (identified herein as SB #3 core). The SB #3 core contains a diverse foraminiferal fauna that can be dated to the Upper Pliensbachian–Lower Toarcian using the ammonite zone scheme for the high Arctic developed by Zakharov et al. (2006), Nikitenko and Mickey (2004), and Nikitenko

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ACCEPTED MANUSCRIPT et al. (2008) which in turn has been correlated with the North American ammonite zonal scheme as shown in Figure 3.

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The data include confidently and provisionally identified species (see

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Supplemental Data). Systematic descriptions of foraminiferal species from Haida Gwaii were not provided in Kottachchi (2001) or Kottachchi et al. (2002; 2003). Consequently,

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the taxonomic relationships of this fauna to the coeval fauna from northern Alaska (e.g.

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Tappan, 1955) are not certain. However, taxonomic revisions to the northern Alaskan genera (Tappan, 1955) by Nagy and Johansen (1991) and Nikitenko and Shurygin (1992)

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are also applied to the Haida Gwaii fauna. Species re-assigned to new genera include: Kutsevella barrowensis, Laevidentalina pseudocommunis, Grigelis apheilolocula,

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Reussoolina aphela, Ammovertellina irregularis, Ammoglobigerina canningensis, Ammodiscus asper, Reophax metensis, Ammobaculites lobus and Ammodiscus siliceus.

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3.3 Diversity measurements

Ammonite and foraminiferal species diversity is measured using methodology presented in Foote (2000) as well as Hammer and Harper (2006). Diversity is measured from the compiled species level stratigraphic range charts for the Pliensbachian and Toarcian stages (Supplemental Data), first at the zone level and then re-measured with an informally divided scheme where zones were divided into three intervals (lower, middle, and upper; Figure 4A) informally referred to as subzones. Using these intervals, the stratigraphic range of each species was divided into one of four mutually exclusive categories (Figure 4B) following Barry et al. (1995) and Foote (2000). These include: 1) taxa whose first and last appearance are confined to the interval (Nfl), 2) taxa that cross the bottom boundary and disappear within the interval (Nbl), 3) taxa that appear within

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ACCEPTED MANUSCRIPT the interval and cross the upper boundary (Nft), and 4) taxa that range through the entire interval and cross both stratigraphic boundaries (Nbt). Taxa were then assigned a unitary

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weight depending on their stratigraphic range within the interval (Sepkoski, 1975; Foote,

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2000; Hammer, 2003; Hammer and Harper, 2006). Taxa that range through the interval (Nbt) were counted as one unit each, taxa that disappeared (Nbl) or appeared (Nft) within

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the interval were only counted as a half of a unit, and taxa that were confined to the

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interval (Nfl) were counted as a third of a unit (Hammer, 2003). Hammer and Harper (2006) justify these unitary weights by the rationale that the first appearance (Nft) and last

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appearance (Nbl) datum are uniformly distributed throughout the measured interval and that the average portion of the interval length that is occupied by both of these types of

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single-ended taxa is approximately 0.5. Similarly, taxa that are confined to the interval (Nfl) are thought to occupy approximately one-third of the interval length and therefore

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should be counted as such (Hammer, 2003; Hammer and Harper, 2006). Categories were then totaled with respect to each stratigraphic interval (zone or informal subzonal unit) and analyzed using various metrics to calculate taxonomic diversity and extinction/origination rates, as defined in Figure 4C. Lastly, the term singleton has been used to denote either: 1) species that are represented by a single specimen (Buzas and Culver, 1994; 1998), or 2) taxa that are confined to a single stratigraphic interval, essentially any species that is in the Nfl category (Foote, 2000). We use the definition of singleton adopted by Foote (2000) indicating a taxon that is confined to a particular zonal or subzonal interval.

4. Results

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ACCEPTED MANUSCRIPT 4.1 Diversity data Stratigraphic ranges of 206 ammonite and 242 foraminifera species were

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compiled and analyzed at the informal subzone level for Pliensbachian–Toarcian strata in

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western North America. Ammonite species diversity (Figure 7A) is measured as a total sum (Ntot), without singletons, and as an estimated mean. Foraminiferal species diversity

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data (Figure 7B) is only presented as Ntot values, whereas diversity data pertaining to the

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singleton-free and estimated mean metrics for the combined foraminiferal species is presented in Figure 8. In both cases, diversity levels in all three metrics generally follow

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the same overall pattern (within each group independently) and show very similar values, which indicates that taxonomic singletons do not have a strong influence in this analysis.

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Results are therefore presented and discussed herein in terms of the total diversity (Ntot) with occasional reference to singleton-free data.

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Results for the ammonite species diversity analysis (Figure 7A) show a steady increase throughout the Imlayi–middle Whiteavesi Zones of the Lower Pliensbachian. Diversity reaches a high point in the middle Whiteavesi Zone (19 species) and then declines steadily throughout the remaining part of the Lower Pliensbachian, reaching a low point in the middle Freboldi Zone (12 species). From the upper Freboldi Zone to the middle Kunae Zone, there is a sharp increase and diversity reaches its maximum value of 30 species. Also, over this time frame during the lower Kunae Zone, there is an observed offset between the total diversity and singleton-free curves of ~5 species which may have resulted from increased origination at this time (Figure 9). Throughout the remaining part of the Upper Pliensbachian and into the Lower Toarcian, ammonite species diversity undergoes a major decline that consists of two

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ACCEPTED MANUSCRIPT distinct steps (Figure 7A). The initial step is an abrupt decline from the middle Kunae to the lower Carlottense Zones when diversity drops from 30 to 14 species. The second step

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is a decline that begins after the middle Carlottense Zone, crosses the Pliensbachian /

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Toarcian boundary, and reaches a minimum diversity of 6 species in the middle Kanense Zone. From the upper Kanense Zone and into the Middle Toarcian there is a gradual

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increase in species diversity that reaches a high point of 15 species in the middle

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Planulata Zone (Figure 7A). Following this peak in diversity in the Middle Toarcian, there is a gradual decline from the upper Middle Toarcian into the lower Upper Toarcian.

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During this interval, diversity falls to 8 species in the lower Crassicosta Zone and then gradually declines again, reaching low levels (of ~5 species) throughout the Hillebrandti

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Zone of the Upper Toarcian. In the Yakounensis Zone there is an abrupt rise in diversity that reaches 14 species, with a subsequent decline in the upper part of the zone.

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Regional foraminiferal species diversity data (Figure 7B), analyzed independently according to geographic region, shows a much different pattern throughout the Pliensbachian–Toarcian time. The record from Haida Gwaii indicates an initial increase in diversity from ~10 species in the Imlayi Zone to ~40 in the Whiteavesi Zone and then increases, very gradually, to ~47 species throughout the remaining part of the Pliensbachian. Throughout the Lower–Middle Toarcian there is a steady rise in diversity that reaches ~72 species in the upper Planulata–middle Crassicosta Zones. At this point, there is a steady decline to the Upper Toarcian where species diversity in the upper Yakounensis Zone drops to ~33. Data from the Alaska core (Figure 7B) shows consistent foraminiferal species diversity of ~27 species throughout the Imlayi–upper Freboldi Zones of the Lower

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ACCEPTED MANUSCRIPT Pliensbachian. Throughout the Kunae Zone there is a distinct rise in diversity that reaches a maximum of 51 species in the upper Kunae Zone. In the upper part of the Pliensbachian

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extending across the Pliensbachian / Toarcian boundary and into the lower Middle

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Toarcian, there is a major decline in species diversity that is evident as two distinct steps. The first decline is into the Carlottense Zone, where species diversity maintains fairly

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steady levels, decreasing only slightly from 43 to 39 species. The second decline is much

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more significant dropping from 39 in the upper Carlottense Zone to 5 in the lower Planulata Zone. It should be noted that above this interval in the SB #3 Alaska core there

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is a gap in core-recovery and therefore it is not possible to obtain foraminiferal species diversity data from the middle Planulata Zone onward.

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The combined foraminiferal species diversity analysis (Figure 8A) constitutes an analysis of the entire stratigraphic range chart from both British Columbia and Alaska.

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This analysis shows that foraminiferal species diversity maintains a steady increase throughout the Lower Pliensbachian, reaching ~70 species in the upper Freboldi Zone. Following this there is a small, sharp, decline in the lower Kunae Zone to 65 species with a subsequent abrupt increase in diversity throughout the remaining part of the zone where diversity reaches a maximum of 88 species. In the later part of the Pliensbachian–Lower Toarcian there is a gradual decline to 73 species in the middle part of the Kanense Zone. From the middle part of the Lower Toarcian to end of the Middle Toarcian, species diversity fluctuates slightly but remains at this level. In the Upper Toarcian diversity declines, gradually, and reaches ~30 species in the Upper Toarcian. An important point to consider in this diversity analysis is the proportion of longranging taxa compared with the single ended taxa (taxa that contain stratigraphic ranges

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ACCEPTED MANUSCRIPT that end or begin within the measured time interval). As previously mentioned, taxa that cross both measured boundaries (i.e. Nbt value in Figure 4B) are counted as a whole unit,

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single ended taxa counted as a half, and singletons counted as a third. If there is an over

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abundance of Nbt taxa, then changes in diversity within a particular interval may be diminished. In the combined foraminiferal species diversity analysis, there is an

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abundance of foraminifera whose stratigraphic ranges are exceedingly long in

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comparison with those of ammonite species. Therefore the combined foraminiferal species stratigraphic range chart was re-analyzed without the long-ranging species

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(omitting species whose stratigraphic ranges are greater than six ammonite zones within the Pliensbachian–Toarcian interval, or are greater than half of the studied interval). The

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results are presented in Figure 8B.

The combined foraminiferal species analysis without long-ranging species (Figure

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8B) shows an increase in diversity from the Imlayi to Whiteavesi Zones, a very slight decrease across the transition between the Whiteavesi and Freboldi Zones, with a subsequent gradual increase in diversity from the Freboldi Zone to the lower Kunae Zone. Throughout the Kunae Zone, there is a sharp rise in foraminiferal diversity, reaching a maximum of ~40 species in the upper part of the zone. Following this apparent peak in foraminiferal diversity there is a large decline into the Lower Toarcian that is divided into three phases, an initial drop in the lower Carlottense Zone, a subsequent gradual decline throughout the remaining part of the zone and a sharp decline across the Pliensbachian / Toarcian boundary where diversity reaches a low point of 24 species in the middle part of the Kanense Zone. From the upper Lower Toarcian, there is a steady rise in diversity into the Middle Toarcian with one very tenuous decline across the

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ACCEPTED MANUSCRIPT Planulata / Crassicosta zone boundary (number 4? in Figure 8B). Diversity levels reach a Toarcian peak in the Crassicosta Zone at 37 species. Foraminiferal diversity then declines

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gradually throughout the Upper Toarcian.

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4.2 Extinction and origination patterns

Rate metrics established by Foote (2000) were used to assess patterns of

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extinction and origination in ammonite and foraminiferal species throughout the

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Pliensbachian and Toarcian. For this analysis, long-ranging foraminiferal species were excluded for reasons previously discussed. Results show that, in general, the derived rates

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for all four metrics used (i.e. Van Valen with singletons, Van Valen without singletons, per-taxon rate, and estimated per capita rate) had similar values, except for the lower

(Figure 9).

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Kanense Zone. Consequently only the Van Valen metric with singletons is discussed

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Ammonite and foraminiferal species had similar patterns of extinction and origination although the scales were quite different in that extinction rate among ammonites is much higher than in the foraminifera. Throughout much of the Pliensbachian, ammonites showed similar extinction and origination rates that were fairly low (Figure 9A). There is an increase in origination in the lower Kunae Zone as well as a slight elevation in both rates that is offset in the upper Kunae–lower Carlottense Zonal interval. Across the Pliensbachian / Toarcian boundary, covering an interval from the middle of the Carlottense Zone to the middle of the Kanense Zone, there is a very dramatic increase in the extinction rate. There is also a rise in origination at this time, but it is notably smaller than the observed increase in extinction rate. Throughout the

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ACCEPTED MANUSCRIPT Middle–Upper Toarcian the extinction and origination rates do fluctuate, but generally maintain lower levels.

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Foraminiferal species data show low extinction and origination rates throughout

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much of the Pliensbachian with offset increases in both metrics throughout the Kunae Zone interval (Figure 9B). Across the Pliensbachian / Toarcian boundary, covering an

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interval from the middle of the Carlottense Zone to the middle of the Kanense Zone, there

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is a sharp and marked increase in the extinction rate. Within this interval, the origination rate is somewhat elevated but it lags behind the extinction rate considerably. Throughout

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the Middle Toarcian, rates are of similar magnitude as those experienced in the Upper Pliensbachian. In the Upper Toarcian, extinction rates are predominantly higher than

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origination and reach maximum values in the upper Hillebrandti and middle Yakounensis

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zones.

5. Discussion

5.1 Multi-phased extinction

It is currently suggested that the Pliensbachian–Toarcian extinction event in Europe and parts of the Arctic is multi-phased with the two most significant diversity declines occurring at the Pliensbachian / Toarcian boundary and at the Tenuicostatum / Serpentinum zonal boundary in the Lower Toarcian (Dera et al., 2010). Ammonite and foraminiferal species diversity in western North America suggests a similar multi-phased event (Figure 10A). Species diversity in both taxonomic groups declined and reached low points in six separate intervals that correspond to the: 1) middle Whiteavesi–middle Freboldi Zones, 2) upper Kunae–lower Carlottense Zones, 3) upper Carlottense–middle

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ACCEPTED MANUSCRIPT Kanense Zones, 4) upper Planulata–lower Crassicosta Zones, 5) middle Crassicosta– Hillebrandti Zones and 6) lower–middle Yakounensis Zone. These episodes of decreasing

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species diversity correlate well with the multi-phased event recorded in combined

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ammonite data from the northwest European and Arctic domains as documented by Dera et al. (2010) (Figure 10B); and, interestingly, also seem to correlate with declines at the

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generic level in the Neuquén Basin of South America (Riccardi, 2008).

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The new dataset from western North America shows a modest decline in ammonite diversity and a correlative, but smaller, decline in foraminiferal diversity in the

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Lower Pliensbachian that begins in the middle of the Whiteavesi Zone and continues to the middle of the Freboldi Zone (number 1 in Figure 10A). Throughout this interval, the

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extinction rates in both taxonomic groups are slightly elevated in comparison with origination. This modest decline in species diversity occurs over a similar time frame, and

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is of similar magnitude, to the decline in ammonite species diversity across the Ibex– Davoei Zone boundary in the dataset from northwest Europe and the Arctic (number 1 in Figure 10B). This could therefore suggest a controlling mechanism that is global in extent.

Of the five episodes of diversity decline that constitute the multi-phased Pliensbachian–Toarcian extinction described by Dera et al. (2010), all are distinguishable in western North America (numbers 2 to 6 in Figure 10). In both datasets, the main phase of extinction consists of two successive phases of decline (number 3 in Figure 10): one at the Pliensbachian / Toarcian boundary and the other within the middle part of the Kanense Zone (at a correlative interval with the Tenuicostatum / Serpentinum Zone boundary of northwest Europe; Figure 3). At the onset of the main-phase of extinction in

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ACCEPTED MANUSCRIPT western North America, ammonite and foraminiferal diversity reached maximum values of ~30 and 40 species respectively in the Kunae Zone and then began to decline gradually

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into the Lower Toarcian (numbers 2 and 3 in Figure 10A). In the combined northwest

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European and Arctic data (Figure 10B), species diversity rebounds in the uppermost Pliensbachian following the decline in the Margaritatus–Spinatum Zones and then

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subsequently collapses across the Pliensbachian–Toarcian boundary into the Lower

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Toarcian where ammonite species diversity reaches a minimum value. During this mainphase of extinction in western North America, ammonite and foraminiferal diversity

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reach minimum values of 5 and 25 species respectively in the middle part of the Kanense Zone (Figure 10A). These low levels of Lower Toarcian diversity probably resulted from

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extraordinarily high extinction rates observed in both taxonomic groups in the lowest part of the Kanense Zone (Figure 9).

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As mentioned earlier, one of the major discrepancies related to a global control mechanism for this event is the suggestion that the negative CIE only occurred in the Tethys Ocean area at an interval that is above the main extinction horizon (Wignall et al., 2005). However, more recent work has shown that the negative CIE is recognizable in many correlative lower Toarcian successions that are far-removed from the Tethys Ocean (Al-Suwaidi et al., 2010; Caruthers et al., 2011; Suan et al., 2011; Gröcke et al., 2011; Izumi et al., 2012). This suggests that this phenomenon was most likely global in extent and strongly supports influence by the methane hydrate reservoir. Furthermore, our study shows that the main phase of extinction is a progressive decline in species diversity that begins just before the Pliensbachian / Toarcian boundary and extends into the Lower Toarcian where diversity reaches minimum values in three ocean basins (namely the

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ACCEPTED MANUSCRIPT paleo Pacific, Arctic and Tethys oceans) at an interval that is precisely correlative with the negative CIE (Figure 10). This apparent synchronicity between diversity minimums

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during the main-phase of extinction and the negative CIE is a good indication that

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methane release played an important role in this extinction event, working to escalate its effects.

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Following the main phase of extinction in the Kanense Zone, ammonite species

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diversity in both datasets shows a rise into the Planulata Zone of the Middle Toarcian and its European correlative with a subsequent decline into the Hillebrandti Zone of the

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Upper Toarcian. This decline in ammonite species is composed of two separate phases (numbers 4 and 5 in Figure 10). Although the extinction rate during phase number 4 is

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somewhat higher than origination, the observed values are considerably lower than those in the Kanense Zone. Within the later phase (number 5 in Figure 10), ammonite species

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diversity maintains lower levels throughout the Hillebrandti Zone of the Upper Toarcian with low levels also observed in the correlative zone of Europe. During this phase, extinction and origination rates were nearly identical which resulted in no observable change in diversity.

Lastly in the Upper Toarcian, the ammonite dataset from northwest Europe and the Arctic shows an abrupt rise in diversity in the lower part of the Dispansum Zone with a final decline in the upper part of the zone; maintaining lower levels throughout the Pseudoradiosa and Aalensis zones (number 6 in Figure 10). In western North America, this final event is recognizable and is constrained based on the appearance of two ammonite species, Hammatoceras insigne ranging from the upper Hillebrandti–lower Yakounensis zones and Dumortieria cf. levesquei in the upper part of the Yakounensis

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ACCEPTED MANUSCRIPT Zone (Supplemental Data). In the zone scheme of Northwest Europe (Page, 2003), these two species are noted as the index for the Insigne and Levesquei Subzones encompassing

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the lower parts of the Dispansum and Pseudoradiosa zones respectively (Figure 3).

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Therefore, the diversity peak occurring in the Yakounensis Zone (Figure 10A) is potentially correlative with the peak in the lower part of the Dispansum Zone (Figure

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10B). The subsequent decline in diversity in the Yakounensis Zone is correlative with the

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decline in the lower part of the Pseudoradiosa Zone. This thereby constitutes a sixth potential correlative phase of diversity decline evident in North America, Europe and

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parts of the Arctic in the Upper Toarcian.

Foraminiferal species diversity, following the main-phase of extinction, show a

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similar steady increase from the upper Kanense to middle Crassicosta zones with a very small (negligible?) decline across the Planulata–Crassicosta Zone boundary that is

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correlative with phase number 4 in Figure 10. Diversity then declines gradually throughout the Upper Toarcian. During the small decline in diversity at the Planulata / Crassicosta Zone boundary there is no observable trend in extinction rate (Figure 9) and therefore this does not appear to be a recognizable event (in the foraminiferal data). However, in the upper part of the Toarcian, extinction rates are generally higher than origination which could account for the observed decline in diversity. 5.2 Karoo magmatism Currently this multi-phased extinction event is attributed to a variety of paleoenvironmental changes that could be related to the Volcanic Greenhouse Scenario but, except for the main-phase of extinction (number 3 in Figure 10), are believed to be restricted to the Tethys Ocean area (Dera et al., 2010). These include: sea-level

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ACCEPTED MANUSCRIPT fluctuation affecting restricted basins, saline stratification, warming (or cooling) of seawater, changing water current dynamics and variation in geochemical cycles

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(McArthur et al., 2000; 2008; Bailey et al., 2003; Suan et al., 2008; 2010; Dera et al.,

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2010; Dera and Donnadieu, 2012). However, if the six extinction phases are global in extent, it is plausible that the eruption of the Karoo–Ferrar LIP is the underlying

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controlling factor.

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When comparing the approximate timing of this multi-phased event to the previously established, and calibrated, time scale for the Pliensbachian and Toarcian

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Stages (Figure 3), the middle Whiteavesi–middle Freboldi Zone event occurred at 186 My, the upper Kunae–lower Carlottense Zone event occurred at 184 My, the upper

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Carlottense–middle Kanense Zone event occurred at ~183 My, the upper Planulata–lower Crassicosta Zone event occurred between 182 and 181 My, the middle Crassicosta–

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Hillebrandti Zone event occurred between 181 and 180 My, and the Yakounensis Zone event occurred at 179–178 My. A recent study by Jourdan et al. (2008) shows eruption ages for magmatism in the Karoo Basin that occur over ~10 Ma, between 186 and 176 Ma, with the main pulses of magmatism occurring over a window of ~3 Ma from 184 to 181 Ma (Figure 11). A comparison of eruption ages from the Karoo Basin and the multiphased extinction event reveals a correlation, in that: 1) four of the six pulses of extinction occur within the main-phase of magmatism, 2) the initial Lower Pliensbachian decline in species diversity occurs within error range of the onset of Karoo magmatism and 3) the uppermost Toarcian decline correlates with the later stages of Karoo magmatism (Figure 11). This suggests that the eruption of the Karoo–Ferrar LIP was a

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ACCEPTED MANUSCRIPT major underlying factor in the long-term environmental change that resulted in the multiphased extinction.

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In relation to the hypothesis of prolonged environmental change during Karoo–

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Ferrar magmatism, oxygen-isotope data indicate that there were several warming events within the Tethys Ocean area occurring in the Ibex-Davoei Zones, Serpentinum Zone,

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Bifrons Zone, and possibly in the Dispansum Zone (Rosales et al. 2005, Suan et al. 2008,

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Dera et al. 2009, Gomez et al. 2008, Silva et al. 2011, Korte and Hesselbo 2012, Harazim et al. 2012). However, there seems to be evidence of colder intervals that occur in the

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Late Pliensbachian and at the Pliensbachian–Toarcian transition (Suan et al., 2008) which could have resulted from volcanogenic injections of SO2 into the atmosphere during

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Karoo–Ferrar magmatism (Self et al., 2006). During the Early Toarcian, a number of compounding factors could have simultaneously created a significant amount of

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environmental stress which induced episodes of methane release and marine anoxia, and escalated this (main) phase of extinction (as indicated in Hesselbo et al., 2000; 2007; Beerling et al., 2002; Jenkyns et al., 2001; Jenkyns, 2003; 2010; Jourdan et al., 2008; Dera et al., 2010 and references therein).

6. Conclusions This study investigates the Pliensbachian–Toarcian extinction of marine organisms during the Lower Jurassic. A primary objective is to compare new data from western North America with previously established records in European (and other) successions in order to test hypotheses that relate to its duration, magnitude and controlling mechanisms. Analysis of the stratigraphic ranges of 206 ammonite and 242

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ACCEPTED MANUSCRIPT foraminiferal species in North America indicates declines in diversity during six separate intervals throughout Pliensbachian–Toarcian time. These intervals of extinction can be

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further correlated with declines in ammonite species and generic data from a combined

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dataset of the northwest European and Arctic domains (Dera et al., 2010). This suggests that the multi-phased species level extinction during the Pliensbachian–Toarcian interval

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was potentially global. Phases of extinction correspond with the: 1) middle of the Lower

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Pliensbachian (middle Whiteavesi–middle Freboldi Zones), 2) middle of the Upper Pliensbachian (upper Kunae–lower Carlottense Zone), 3) Pliensbachian / Toarcian

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boundary into the Lower Toarcian (upper Carlottense–middle Kanense Zones), 4) Middle Toarcian (upper Planulata–lower Crassicosta Zones) and 5) upper Middle–lower Upper

Yakounensis Zone).

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Toarcian (middle Crassicosta–Hillebrandti Zones), and 6) Upper Toarcian (lower–middle

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Species level data from western North America also support previous conclusions suggesting that the main-phase of extinction is a global event during which diversity declined in a gradual fashion beginning at the Pliensbachian / Toarcian boundary and extending into the Lower Toarcian (Harries and Little, 1999; Caswell et al., 2009). In western North America, species diversity in both ammonite and foraminifera reach minimum values in the middle part of the Lower Toarcian Kanense Zone which coincides with the negative CIE interval described by Caruthers et al. (2011). Recognition of a multi-phased extinction across the paleo Pacific, Arctic and Tethys Ocean basins greatly expands the known geographic extent of this event. Previously, it was suggested that only the main-phase of extinction might be global but it

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ACCEPTED MANUSCRIPT is now apparent that other phases of decline could potentially be global requiring a controlling mechanism that is also global in its reach.

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Previously it was argued that volcanogenic outgassing of CO2 during the eruption

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of the Karoo–Ferrar LIP initiated greenhouse conditions and caused the extinction of marine organisms (Pálfy and Smith, 2000). Our data show a good correlation in timing

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between the eruption ages in the Karoo Basin (Jourdan et al., 2008) and calibrated ages of

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declines in species diversity that constitute this multi-phased event. Four of the six pulses of extinction occur within the main-phase of magmatism, the Lower Pliensbachian

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decline occurs within error range of the onset of Karoo magmatism and the Upper Toarcian decline coincides with the later stages of magmatism. These observations in

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species diversity decline in western North America support the Volcanic Greenhouse Scenario as the most important factor during the Pliensbachian–Toarcian multi-phased

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extinction.

Acknowledgements

Funding for this work was provided by NSERC research grants to Paul Smith (#8493) and Darren Gröcke (#288321), a NERC grant to Darren Gröcke (NE/H021868/1), and a grant from the Alaska Geological Society to Andrew Caruthers. We thank Guillaume Dera, Mike Foote, Jim Haggart, Melissa Grey and two anonymous reviewers for their constructive comments and suggestions.

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Figure captions

Fig. 1. The ‘Volcanic Greenhouse Scenario’ a model showing the potential chain of events leading to environmental change and mass extinction (modified from Wignall, 2001, 2005). Note, figure is composite from many individual events, not every isotopic system is necessarily affected.

Fig. 2. Species and generic ammonite diversity in the Pliensbachian and Toarcian of Europe (modified from Dera et al., 2010). Six major declines in biodiversity are recognized with the largest being in the Lower Toarcian at the Tenuicostatum /

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‘geographic singletons’ refer to taxa that are confined to a specific geographic area;

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‘single-interval taxa’ are confined to a specific subzone; and ‘boundary crossers’ refer to taxa that occur in two separate time intervals. Marg. = Margaritatus, Sp. = Spinatum, Ten.

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= Tenuicostatum, Ser. = Serpentinum, Bif. = Bifrons, Variab. = Variabilis, Thouar. =

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Thouarsense, Dis. = Dispansum, Ps. = Pseudoradiosa, Aal. = Aalensis.

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Fig. 3. Correlative Pliensbachian–Toarcian time scales for Northwest Europe and the Mediterranean (Dean et al., 1961; Schlatter, 1980; Braga et al., 1982; Howarth, 1992;

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Page, 2003); High-Arctic (Zakharov et al., 1997; Nikitenko et al., 2008); Western North America (Smith et al., 1988; Jakobs et al., 1994) and South America (von Hillebrandt,

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2006). Absolute U–Pb and Ar–Ar age data from Pálfy et al. (1997; 2000) and Gradstein et al. (2012). Absolute age dates in bold-face font have an error range that is less than 5 Ma and are interpreted as good quality. Note that the Pliensbachian zone level biostratigraphy in the Mediterranean Province refers to areas of Spain (Page, 2003).

Fig. 4. (A) Example of stratigraphic ranges of hypothetical species, divided at the zoneand informal subzone-levels, used in this study for measuring taxonomic diversity and extinction/origination rates. Taxa from within each informal subzone are identified as being from the ‘lower’, ‘middle’, or ‘upper’ part of the zone. (B) Four fundamental classes of taxa (after Foote, 2000) used in this study to quantify stratigraphic ranges of

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this study (after Foote, 2000). O = Origination rate, E = Extinction rate.

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Fig. 5(A–C). Maps showing the location of previously published Pliensbachian and Toarcian stratigraphic sections in western North America. Sections contain occurrences

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of ammonites (red circles) and foraminifera (green circles) that are used in this study.

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Locality numbers 1–22 refer to the section correlation chart (Figure 6). YK = Yukon, NWT = Northwest Territories, BC = British Columbia, AK = Alaska, OR = Oregon, NV

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Fig. 6. Pliensbachian and Toarcian stratigraphic sections in western North America. Sections contain occurrences of ammonites and foraminifera that are used in this study to

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compile stratigraphic range charts. SB#3 = South Barrow #3 well core, HC = Hicks Creek, CC = Camp Creek, YR Sec. = Yakoun River Section.

Fig. 7. Pliensbachian–Toarcian ammonite (A) and foraminiferal (B) species level diversity in western North America. Data is derived from compiled stratigraphic range charts (Supplemental Data). Ammonite diversity data is compiled from many localities throughout western North America, whereas foraminiferal diversity data is from Haida Gwaii (BC) and the South Barrow #3 core (Alaska). Major declines in ammonite species diversity (A) are evident at six distinct intervals, whereas foraminiferal diversity data (B) only shows two major declines (Kanense Zone and in the upper part of the Toarcian).

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Planulata, Crass. = Crassicosta, Hill. = Hillebrandti, Yak. = Yakounensis.

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Fig. 8. Combined Pliensbachian–Toarcian foraminiferal species diversity from Haida Gwaii and the South Barrow #3 core in Arctic Alaska, western North America. Data in

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(A) is the total combined species from the compiled stratigraphic range chart

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(Supplemental Data). Data in (B) does not include species whose stratigraphic ranges are greater than 6 ammonite zones within the Pliensbachian–Toarcian interval. In (A) only

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small declines in biodiversity are noted in the Kunae, Kanense, and Planulata Zones and a gradual decline in species diversity that occurs throughout the upper part of the Toarcian.

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In (B) declines in species diversity are evident over five potential intervals throughout the

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Pliensbachian–Toarcian time (red numbers). See Figure 7 for abbreviations.

Fig. 9. Extinction and Origination rates for Pliensbachian–Toarcian ammonite (A) and foraminiferal (B) species in western North America. Rate metrics for foraminifera do not contain species whose stratigraphic ranges are greater than 6 ammonite zones within the Pliensbachian–Toarcian time interval. Data shows accelerated extinction rates in the Kanense Zone for both taxonomic groups that correlates with major declines in species diversity at interval 3 (Figures 7, 8). See Figure 7 for abbreviations.

Fig. 10. Ammonite and foraminiferal species level biodiversity in (A) Western North America (this study) and (B) NW Tethys and Arctic domains (Dera et al., 2010). Figure shows a multi-phased event with major declines occurring over six correlative intervals.

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Toarcian where diversity reaches its lowest levels at an interval coeval with the negative

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CIE. Note: the Middle Toarcian event in Dera et al. (2010) is illustrated here as two separate events (#4, #5). Intervals i & ii = approximate extinction intervals previously

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identified by Harries and Little (1999) and Caswell et al. (2009), Wh. = Whiteavesi, Fre.

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= Freboldi, Carl. = Carlottense, Kan., = Kanense, Plan. = Planulata, Crass. = Crassicosta, Hill. = Hillebrandti, Yak. = Yakounensis, Jam. = Jamesoni, Marg. = Margaritatus, Spin. =

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Spinatum, T. = Tenuicostatum, S. = Serpentinum, Var. = Variabilis, Th. = Thouarsense, D. = Dispansum, P. = Pseudoradiosa, A. = Aalensis, negative CIE = negative carbon-

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isotope excursion interval in Caruthers et al. (2011).

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Fig. 11. Diagram comparing the timing of the multi-phased Pliensbachian–Toarcian extinction with emplacement of the Karoo volcanic province. The 40Ar/39Ar age probability density distribution diagram and frequency histogram of volcanic rocks in the Karoo Basin are modified from Jourdan et al., (2008) to show its correlation in timing with the known phases of extinction. Figure suggests that all six phases of diversity decline occur within the duration of Karoo magmatism. Baj. = Bajocian.

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ACCEPTED MANUSCRIPT Highlights

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Duration of Pliensbachian–Toarcian extinction is analyzed in western North America. Species-level diversity data includes ammonoid and foraminiferal occurrences.

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Data shows six correlative phases of diversity decline in three ocean basins.

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Multiple correlative declines argue for global controls for all phases.

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Correlation in timing between multi-phased event and Karoo magmatism.

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