PATTERN AND TIMING OF THE LATE DEVONIAN BIOTIC CRISIS IN WESTERN CANADA: INSIGHTS FROM CARBON ISOTOPES AND ASTRONOMICAL CALIBRATION OF MAGNETIC SUSCEPTIBILITY DATA

MICHAEL T. WHALEN Department of Geosciences, University of Alaska, Fairbanks, Alaska 99775-5780 USA e-mail: [email protected]

DAVID DE VLEESCHOUWER MARUM–Center for Marine Environmental Sciences, Leobenerstrasse, 28359 Bremen, Germany, and Earth System Sciences, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium

JOSHUA H. PAYNE Shell Exploration and Production Co., 150-A North Dairy Ashford, Houston, Texas 77079 USA

JAMES E. (JED) DAY Department of Geography–Geology, Illinois State University, Normal, Illinois 61790-4400 USA

D. JEFFREY OVER Department of Geological Sciences, State University of New York–Geneseo, Geneseo, New York 14454 USA

AND

PHILIPPE CLAEYS Earth System Sciences, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium

ABSTRACT: Carbon stable isotope data from western Canada, in combination with biostratigraphic control and astrochronologic constraints from magnetic susceptibility data, provide insight into the pace and timing of the Frasnian–Famennian (F–F; Late Devonian) biotic crisis. In much of the world, this event is characterized by two organic-rich shales, which display geochemical anomalies that indicate low-oxygen conditions and carbon burial. These events, commonly referred to as the Lower and Upper Kellwasser events (LKE and UKE), have been linked to the expansion of deeply rooted terrestrial forests and associated changes in soil development, chemical weathering, and Late Devonian climate. The d13C data generated from organic matter record a 3 to 4% positive excursion during each event. These data and other geochemical proxy data reported elsewhere corroborate hypotheses about enhanced biological productivity, driven by terrigenous input under exceptionally warm climatic conditions. In this hypothesis,a boom in primary production leads to successive development of anoxic bottom water conditions, decreased biotic diversity, and net transfer of carbon from the atmosphere to the ocean floor. Despite the importance of the F–F events, a precise chronology for the events is lacking due to limited biostratigraphic resolution. Each of the F–F events falls within one conodont zone, with durations estimated on the order of 0.5 to 1.0 Myr. The LKE occurs very high in Frasnian Zone (FZ) 12, while the UKE begins within FZ 13B, just below the F–F boundary. A previous analysis of high-resolution magnetic susceptibility data from the studied sections in western Canada identified 16.5 eccentricity cycles, each lasting 405 kyr, within the Frasnian strata and one in the earliest Famennian. The present study reports d13C anomalies associated with the LKE and UKE in the same sections. The LKE and UKE intervals comprise 7 to 8 and 13 to 13.5 m of stratigraphic section, respectively. Based on our analysis, this implies that they represent only one 405-kyr eccentricity cycle or less.We estimate that the duration of the LKE was a bit more than half of a long eccentricity cycle (~200–250 kyr), while the UKE was more protracted, lasting a full long eccentricity cycle (~405 kyr). The onset of both events is separated by one-and-a-half 405-kyr eccentricity cycles, indicating that they occurred about 500 to 600 kyr apart. This work demonstrates the utility of magnetic susceptibility, or other long time-series proxy data, used in conjunction with astronomical calibration to provide insight into the pacing of significant events in geologic time.

KEY WORDS: Frasnian–Famennian, biotic crisis, magnetic susceptibility, carbon isotopes, astronomical calibration

INTRODUCTION application of time-series analysis to high-resolution proxy records. Several studies have successfully identified the imprint of astronomical The precise timing and pacing of events in Paleozoic time are two of climate forcing in Paleozoic records and calibrated portions of the the fundamental issues in stratigraphic analyses, and great strides toward geologic timescale (for a review on this topic, see Hinnov 2013). understanding these factors have been made in recent years through the Magnetic susceptibility data are commonly used for this type of analysis

New Advances in Devonian Carbonates: Outcrop Analogs, Reservoirs, and Chronostratigraphy DOI: http://dx.doi.org/10.2110/sepmsp.107.02 SEPM Special Publication No. 107, Copyright Ó 2016 SEPM (Society for Sedimentary Geology), ISBN 978-1-56576-344-9, eISBN 978-1-56576-345-6, p. 185–201.

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FIG. 1.—Global correlation of positive carbon isotope excursions associated with the Lower (LKE) and Upper Kellwasser (UKE) events (modified from Joachimski et al. 2002). in Cenozoic and Mesozoic rocks (Shackelton et al. 1995; Weedon et al. UKE (Table 1). Wang et al. (1996) documented a broad positive d13C 1999; Boulila et al. 2008a, 2008b; Hinnov 2013) and, more recently, excursion near the F–F boundary in western Canada; however, their have been applied to sequences of Paleozoic age (De Vleeschouwer et al. sampling was rather coarse, and the thickness of the interval that 2012a, 2012b, 2014a; Da Silva et al. 2013; Wu et al. 2013). records the excursion (~40 m) strongly suggests that it encompasses Our understanding of Late Devonian sea-level history and both the LKE and UKE. Many records of these Kellwasser events associated geobiologic events is founded on the pioneering work of were documented in shallow epeiric sea depositional settings, and it Johnson et al. (1985, 1996), who built a convincing database remains unclear whether the global ocean experienced widespread supporting the interpretation of eustatic sea-level events during the low-oxygen conditions. Recent proxy data from redox-sensitive trace Devonian. Several Devonian transgressions are associated with metals in Australia and south China (George et al. 2014, Whalen et al. widespread deposition of black shales, carbon isotope anomalies, 2015) imply that the latter may not have been the case. A model to and/or biotic crises (Joachimski and Buggisch 1993, House 2002, drive high productivity, development of low-oxygen conditions, and Racki 2005). Perturbations of the carbon cycle recorded in such events eventually two successive and closely spaced biotic crises involves the demonstrate the complex relationships among terrestrial plant expansion of deeply rooted terrestrial forests and associated changes innovations, sea-level change, paleoceanography, and biotic crises in soil development, chemical weathering, nutrification of epeiric seas, (Algeo et al. 1998, Bond and Wignall 2008, Stigall 2012). The and ultimately major Late Devonian climate perturbations (Algeo and Frasnian–Famennian (F–F) Kellwasser horizons mark the culmination Scheckler 2010, Algeo et al. 2010). The global carbon cycle appears of a series of such Late Devonian transgressive black shale events to have played a central role throughout these events. Therefore, this (House 2002, Racki 2005). paper aims to gain insight into the pattern and timing of the carbon The cause of the F–F biotic crisis remains contentious, and it has cycle perturbations, to better understand the ultimate processes driving alternatively been linked to extraterrestrial impact (Claeys et al. 1992, the Late Devonian events. Racki 1999, McGhee 1996, Wang et al. 1996), climatic cooling (Copper 1986, Stearn 1987, Joachimski and Buggisch 2002), climatic warming (Thompson and Newton 1988), and marine anoxia paired Geologic Setting and Stratigraphy with eutrophication (Joachimski and Buggisch 1993, Algeo et al. 1998, Murphy et al. 2000, Chen et al. 2002, Bond and Wignall 2008, Devonian rocks exposed in western Alberta range from middle Whalen et al. 2015). Multiple factors appear to have been in play Givetian to Famennian in age and consist of isolated and attached during both Kellwasser events, including rapid climate warming carbonate platform complexes and associated slope and basin facies followed by cooling, transgression, and onlap of anoxic waters (Figs. 2, 3; Switzer et al. 1994). The strata were deposited at a low produced by eutrophication driven by plant-induced terrestrial paleolatitude along the western continental margin of Laurasia weathering (Algeo et al. 1998, Joachimski et al. 2009). (Witzke and Heckel 1988, Scotese and McKerrow 1990). A regionally In this paper, we use the astrochronologic framework for the extensive tropical carbonate ramp system first developed in the Frasnian of western Canada (as published in De Vleeschouwer et al. Western Canada Sedimentary Basin during the Middle (Late Givetian) 2012b) to interpret new high-resolution sequence and biostratigraphic Devonian transgression across a widespread subaerial unconformity analyses and stable carbon isotopic data. This approach provides (Figs. 2, 3; Switzer et al. 1994). Fine-grained siliciclastic sediment insight into the pattern and timing of the F–F biotic crises, and it mixed with platform-derived carbonates in slope and basin settings permits inferences to be made about the global carbon cycle during (Fig. 3; Oliver and Cowper 1963, Stoakes 1980, Switzer et al. 1994, the studied catastrophic events. Whalen et al. 2000b, Wendte and Uyeno 2005). During most of the The stepwise F–F events are commonly referred to as the Upper and Frasnian, carbonate-platform growth kept pace with prolonged Lower Kellwasser events (UKE and LKE, respectively) based on second-order (,10 Myr) and third- or fourth-order (0.5–5.0 and characteristic organic-rich shales deposited during these intervals at 0.1–0.5 Myr, respectively) sea-level rises and outpaced basinal many locations in Europe (Buggisch 1991, Joachimski and Buggisch sedimentation (Johnson et al. 1985, 1996; Day et al. 1996; Day 1993). The LKE and UKE are characterized by positive carbon 1998; Whalen et al. 2000b). The platforms record a series of isotope excursions (Fig. 1) commonly interpreted to result from depositional sequences that developed in response to eustatic sea-level carbon burial in low-oxygen marine environments (Joachimski et al. events during the Late Devonian (Johnson et al. 1985, 1996; Whalen 2001). Strata representing the events are usually relatively thin and and Day 2010). Two of these sea-level events (IId2 and IId3, Fig. 3) locally condensed, and the LKE interval is commonly thinner than the coincide with the F–F LKE and UKE (Joachimski et al. 2009; Stigall

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TABLE 1.—Thicknesses of the LKE and UKE intervals at localities around the world. The thicknesses of the LKE and UKE were determined by the extent of the documented isotropic excursion in each cited publication (see text for further explanation). Note that those sections with very thick event intervals (.10 m for LKE and .20 m for UKE) are based on relatively coarse sampling.

Location LKE (m) UKE (m) Source

Section C, W. Canada 7.5 13.5 This study Section W4, W. Canada 8.0 13.0 This study Fuhe, S. China 2.8 8.2 Chen et al. (2005) Baisha, S. China 4.0 6.7 Chen et al. (2005) Carnic Alps, Austria 0.2 1.2 Joachimski and Buggisch (1993) Coumiac, France 1.2 1.2 Joachimski and Buggisch (1993) Benner, Germany 1.0 1.2 Joachimski and Buggisch (1993) Schmidt, Germany 0.5 1.2 Joachimski and Buggisch (1993) Vogelsberg, Germany 1.8 1.3 Joachimski and Buggisch (1993) Kowala, Poland 1.5 8.0 Joachimski et al. (2001) Budesheimer,¨ Germany 6.6 5.0 Joachimski et al. (2002) Bou Ounebdou, Morocco 0.5 0.7 Joachimski et al. (2002) Devils Gate, Nevada 5.0 30.0 Joachimski et al. (2002) West Valley core, New York 20.0 30.0 Murphy et al. (2000) HS core, W. Australia 10.0 42.0 George et al. (2014) Oscar Range, W. Australia 28.3 13.0 Stephens and Sumner (2003) Dino Gap, W. Australia 28.3 45.6 Stephens and Sumner (2003) Windjana Gorge, W. Australia 28.3 30.4 Stephens and Sumner (2003)

2012; Whalen and Day 2008, 2010) and are linked to the Late of Upper Devonian sections, and the Kellwasser events are no Devonian biotic crisis. exception (Buggisch 1991, Joachimski and Buggisch 1993). Pertur- The Miette and Ancient Wall platforms, in the western part of the bations of the carbon cycle appear to have been globally synchronous Western Canada Sedimentary Basin, are 400 to 500 m thick, and fundamental to the ongoing ecosystem changes (Buggisch 1991, respectively (Figs. 2, 3). Ancient Wall was larger than Miette, and Joachimski et al. 2002). Therefore, they constitute the most suitable both were much smaller than the Southesk-Cairn and other attached proxy for evaluating the pattern and timing of the F–F events. We reef complexes further to the east (Fig. 2; Geldsetzer 1989, Mountjoy adopt a similar procedure as the one that has been employed in the 1989). Miette and Ancient Wall remained isolated platforms through evaluation of other major positive carbon isotope excursions such as the Frasnian until their demise, along with Devonian metazoan reefs Steptoean positive carbon isotope excursion (SPICE) (Saltzman et al. worldwide, during the F–F biotic crisis (McLaren 1982, Stearn 1987, 1998), Hirnantian carbon isotope excursion (HICE) (Bergstrom et al. McGhee 1996, Whalen et al. 2000a, Wood 2000, Copper 2002). 2006), or the Irivekian (Cramer and Saltzman 2007), Mulde (Cramer Facies associated with these buildups were deposited in platform, et al. 2006), or Hangenberg excursions (Cramer et al. 2008). In all of margin, slope, and basin environments (Whalen et al. 2000a, 2000b). these cases, the onset and termination of the carbon isotope excursion Platform margins are locally characterized by a rim of stromatoporoid mark the beginning and the end of the event, respectively (Buggisch biostromes surrounding a typical back-reef interior with subtidal and 1991; Saltzman et al. 1998; Joachimski and Buggisch 1993; peritidal, meter-scale, shoaling-upward cycles (Klovan 1964; Mount- Joachimski et al. 2002; Bergstrom et al. 2006; Cramer et al. 2006, joy 1965; Mountjoy and Mackenzie 1974; Kobluk 1975; Wendte 2008; Cramer and Saltzman 2007). Therefore, in our analysis, the 1992, 1994; Whalen et al. 2000a, 2000b; Whalen and Day 2008). onset of the Kellwasser positive excursions indicates the initiation of Seaward of the platform margin, slope and basin deposits consist of a the event. The decline of d13C values to a background level, which mix of coarse- and fine-grained, platform-derived, nonskeletal may differ from the pre-event background, marks the termination of carbonate, bioclastic material and fine-grained siliciclastic material the event. (Stoakes 1980, McLean and Mountjoy 1993, Whalen et al. 2000b). The main stratigraphic sections sampled for this study are located METHODS along the southeast margins of the Miette and Ancient Wall platforms (Fig. 2). The section from Miette is atop the platform, thus recording Sequence Stratigraphy and Biostratigraphy platform-interior depositional environments within the Ronde Mem- ber (Mbr.), Southesk Formation (Fm.), and the Sassenach Fm. The Previous high-resolution sequence stratigraphic analysis of the section at Ancient Wall records upper-slope and prograding outer- Miette and Ancient Wall platforms identified a series of Frasnian ramp facies of the Simla Mbr., Southesk Fm. and the Sassenach Fm. depositional sequences and documented sea-level events that (Fig. 4). controlled their deposition (van Buchem et al. 1996; Whalen et al. 2000a, 2000b). In this project, we employed conodont biostratigra- Defining the Kellwasser Events phy (Klapper 1989; Ziegler and Sandberg 1990; Klapper et al. 1995; Day et al. 1996; Klapper 1989, 1997, 2007) and magnetic The Upper Devonian Kellwasser horizons were originally defined susceptibility (MS) stratigraphy (Crick et al. 1997, 2001, 2002; by the loss of biodiversity and the stratigraphic extent of black shales Ellwood et al. 1999, 2000, 2001) in conjunction with sequence common in the Upper Devonian sections of Europe (Buggisch 1991). stratigraphic methods (Handford and Loucks 1993, Posamentier and Carbon isotopic data are one of the most commonly reported records Allen 1999) to better constrain temporal correlations. Whalen and

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Day (2008, 2010) documented the MS stratigraphy and conodont biostratigraphy according to the Montange Noire (MN) or Frasnian zonation (FZ) of Klapper (1989), with refinements to that zonation by Girard et al. (2005). Details of the sequence stratigraphic framework for these rocks were documented by van Buchem et al. (1996) and Whalen et al. (2000a, 2000b), and a thorough discussion of the biostratigraphic and MS data within the sequence stratigraphic framework was outlined by Whalen and Day (2008, 2010). Both the outer-ramp to slope succession at section C and the platform-interior section, W4, record seven parasequences over the F–F interval (Figs. 4, 5). Five parasequences make up sequence 8, and two para- sequences are within the lowstand systems tract (LST) and transgressive systems tract (TST) of sequence 9 (Figs. 4, 5). Section C appears to record continuous deposition, but it is characterized by some redeposited carbonates, especially in the lower part of the section, and it also includes a thin black shale interval at the F–F boundary that is probably condensed (Figs. 4, 5). Section W4 records subtidal parasequences with normal marine fauna in most of sequence 8 that become more peritidal during regression with minor truncation indicating subaerial exposure (Figs. 4, 5), while sequence 9 is dominated by peritidal facies (Whalen et al. 2000a, 2000b). The cyclostratigraphic framework (Fig. 6), based on the MS data of the studied sections (De Vleeschouwer et al. 2012b), provides insight into the chronology of the Kellwasser events during the Late Devonian.

Magnetic Susceptibility

MS represents the degree of magnetization of a material in response to an applied magnetic field. The MS of a sediment varies in proportion to the concentration of detrital-dominated paramag- netic and ferrimagnetic minerals and forms a proxy for the rate of supply of impurities delivered to the sedimentary environment (Ellwood et al. 2000). In the studied section, large-scale MS variations can be ascribed to relative sea-level changes and million- year-scale climate variations due to plate tectonics or biosphere innovations (Mabille and Boulvain 2007; Da Silva et al. 2009; Whalen and Day 2008, 2010). On a shorter timescale, climate, through variations in precipitation and wind, determines the flux of magnetic minerals to the ocean through riverine or eolian processes (Hladil et al. 2006, Riquier et al. 2010). Sampling for MS was conducted at half-meter intervals, and all samples are keyed to stratigraphic sections measured with a Jacob’s staff or tape measure and compass (Whalen and Day 2008). MS samples were weighed to within 0.001 g and measured on a KLY-3 Kappa bridge MS meter at Brooks Ellwood’s laboratory (Louisiana State University). MS values represent an average of three measurements of mass- normalized bulk MS reported in units of m3/kg, for each sample. Due to the high carbonate content of most samples, the mass- normalized bulk MS signature is weakly positive. Whalen and Day (2010, Fig. 14 therein) demonstrated that the MS signal in the studied section is mainly driven by detrital inputs, and, consequently, the role of diagenesis is considered negligible.

FIG. 2.—A) Palinspastically restored Middle Frasnian paleogeographic map, illustrating the locations of Upper Devonian isolated and attached carbonate platforms in the Western Canada Sedimentary in western Alberta and eastern British Columbia, showing the Basin. Buildups located west of the easternmost barbed line, locations of the Miette and Ancient Wall platforms (after Mountjoy indicating the eastern limit of Laramide thrusting, are exposed in 1965). Lettered stars indicate the location of measured stratigraph- the Canadian Rocky Mountains (after Geldsetzer 1989, Mountjoy ic sections: C ¼ Thornton Creek, W4 ¼ Poachers Creek. 1989, Switzer et al. 1994). B) Location map of the overthrust belt

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FIG. 3.—Upper Devonian stratigraphy of western Canada (after Switzer et al. 1994). Platform units are light gray, while basinal facies are white. Note that there is different stratigraphic terminology for platform as opposed to basinal units. The right side of the diagram illustrates relative transgressive–regressive sea-level events (after Johnson et al. 1985; Day et al. 1996, Whalen and Day 2008, 2010) The LKE and UKE strata are located within the marked study intervals.

Stable C Isotopic Analysis Time-Series Analysis

Stable carbon isotopic analyses on organic matter were conducted at In this paper, power spectra of the studied MS signals were the Alaska Stable Isotope Facility at the University of Alaska– computed using the multitaper method (MTM; Thomson 1982) with Fairbanks. Samples were prepared by acidifying 10-g subsamples of 3-2p prolate tapers, which is the same method used in De powdered material with an excess of 1 M HCl at 508 C for 24 hours. Vleeschouwer et al. (2012b). However, in this paper, we used a different red noise model (conventional AR[1]; Gilman et al. 1963) The acid-insoluble residues were rinsed, freeze-dried, and analyzed for compared to the original paper (‘‘robust’’ AR[1]; Mann and Lees their C contents using a Costech Elemental Analyzer (ECS 4010). 1996), because Meyers (2012) demonstrated that the ‘‘robust’’ AR(1) Isotopic ratios were then measured using a Conflo III interface with a approach often imposes false positives within the low-frequency (i.e., DeltaþXP mass spectrometer, and ratios are reported using conven- eccentricity) portion of the spectrum. Also, we applied two tional delta (d) notation in per mil (%) (Coplen 2011) relative to the complementary techniques: the MTM harmonic F-test (Thomson international Vienna Peedee belemnite (VPDB) standard (Gonfiantini 1982) and the average spectral misfit method (ASM; Meyers and 1978; Fig. 4). Typical instrumental precision is ,0.2%. Sageman 2007), as implemented in the R package ‘‘astrochron’’

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13 FIG. 4.—d Corg data from sections W4 and C placed within a lithostratigraphic, biostratigraphic, and sequence stratigraphic framework. Diagram also illustrates lithostratigraphic units, stage and biostratigraphic information (MN or Frasnian zonation of Klapper 1989; standard zonation of Ziegler and Sandberg 1990), Late Devonian transgressive–regressive sea-level events (T–R IId2, IId3) of Johnson et al. (1985, 1996), as modified by Day et al. (1996), Day (1998), and Whalen and Day (2008), and Alberta depositional sequences (7, 8) of Whalen and Day (2008, 2010). Abbreviations: LKE ¼ Lower Kellwasser event, UKE ¼ Upper Kellwasser event, l. rh. ¼ Lower rhenana Zone, up. rhen. ¼ Upper rhenana Zone, lin. ¼ linguiformis Zone, triang. ¼ triangularis Zone, Famen. ¼ Famennian, Sass. ¼ Sassenach Fm., m ¼ meters, M ¼ mudstone, W ¼ wackestone, P ¼ packstone, G ¼ grainstone, F/R ¼ framestone/rudstone. LST, TST, HST ¼ lowstand, transgressive, and highstand systems tract, respectively.

(Meyers 2012). The ASM was calculated for the Simla Mbr. of section with different accumulation rates, cycles of equal duration are C using frequencies that exceeded the 90% F-test confidence level expressed by a sediment package of different thickness. However, if (Fig. 6). The ASM method evaluates the misfit between the selected one assumes that accumulation rate within the same sedimentary frequencies and the expected astronomical spectrum for the Late environment is quasi-constant, the above-listed techniques can be Devonian, with 405- and 100-kyr eccentricity, 32.25-kyr obliquity, carried out separately for different sedimentary environments. This and 19.95- and 16.85-kyr precession (Berger et al. 1992), for a wide assumption seems acceptable given that Whalen and Day (2010) range of plausible sedimentation rates. The null hypothesis of ‘‘no demonstrated the absence of significant unconformities and only astronomical influence’’ was quantitatively evaluated using Monte slight variations in sedimentation rate within depositional environ- Carlo simulation. This approach permits a null hypothesis test in the ments. absence of high-resolution biostratigraphic or radioisotopic age constraints. RESULTS It should be noted, however, that the above-listed methods are designed for depositional settings characterized by quasi-constant Biostratigraphy and Sequence Stratigraphy accumulation rates. Hence, some assumptions and restrictions must be explicitly stated and incorporated. On carbonate platforms and ramps, The study intervals for sections C and W4 record the LST, TST, and sediment production rate is, above other factors, a function of highstand systems tract (HST) of Alberta sequence 8 and the LST and carbonate productivity. The main trend is a decrease in the carbonate TST of sequence 9 (Whalen and Day 2008, 2010). Deposits of productivity with increasing depth, with maximum carbonate sequence 8 record a very late Frasnian sea-level deepening event that production in reefal environments (James 1997, Da Silva et al. resulted in progradation of ramp facies of the Ronde Mbr. and upper- 2009). Given that section C contains a transition between slope and slope facies of the Simla Mbr. (Figs. 4, 5; Whalen et al. 2000a, outer-ramp facies, the accumulation rate cannot be considered 2000b). This deepening occurred at or near the top of Frasnian Zone constant between the two lithologies. In depositional environments (FZ) 12, as documented by the first appearance of Polygnathus

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FIG. 5.—MS data from sections W4 and C placed within a lithostratigraphic, biostratigraphic, and sequence stratigraphic framework. Parasequences within sequences 8 and 9 are illustrated along with band-pass filters representing the 100-kyr (meter-scale) and 405-kyr (decameter-scale) eccentricity cycles. Note the positions of the LKE and UKE with respect to the eccentricity cycles, with the LKE initiating in LEC 15 and the UKE in LEC 17. Also note that LEC 16 in section W4 is out of phase due to the minor unconformity at the top of LEC 16. Parasequence boundaries coincide with either the downward or upward climbing limb of the 405-kyr eccentricity cycles, both of which coincide with 100-kyr eccentricity maxima. See Figure 4 caption for key to abbreviations. lodiensis (Klapper 2007) at 21.5 m in the Simla Mbr. at section C (Fig. environment (section C), the base of sequence 8 is a 15-m-thick 7; Table 2). Conodont samples from the platform-top environment of package of redeposited lithoclastic rudstone interpreted as the LST. the Ronde Mbr. in section W4 contain few age-diagnostic forms, and Overlying parasequences of the TST and HST are 3 to 9 m thick and biostratigraphic zonation remains questionable (Figs. 4, 5). are characterized by flooding surfaces overlain by calcareous shale or The base of FZ 13 is determined by the first occurrence of wackestone, followed by redeposited carbonate float–rudstones or Polygnathus brevicarina; Palmatolepis bogartensis, the index taxon packstones with corals and lithoclasts. for FZ 13A, is first found higher (Fig. 7; Table 2). Palmatolepis Deposits of sequence 9 include the Sassenach Fm. (Figs. 3–6). linguiformis, marking the base of FZ 13B, was recovered at 34 m in The tops of the extinct Frasnian platforms in western Alberta were section C (Table 2). At Miette, late Frasnian faunas in the upper part emergent during a latest Frasnian–earliest Famennian sea-level of section W4 are from the Polygnathus biofacies and thus far do not lowstand, although subtidal conditions persisted in off-reef settings. allow recognition of FZ 13. The deepening event recorded by The early Famennian conodont fauna from an oncoid debris flow sequence 8 coincides with a major global sea-level rise designated as bed (Whalen et al. 2002), 1.8 m above the base of the Sassenach North American cratonic Devonian T–R cycle IId2 (Day 1998), and Fm. stratotype at section C (Table 2, sample 48.8), indicates that the LKE was initiated during the TST of this sea-level cycle (Fig. 4). initial latest Frasnian transgression of T–R cycle IId3, and The TST contains five parasequences, and the LKE initiated near resumption of carbonate deposition on the flanks and tops of the maximum transgression during deposition of the fourth parasequence. older extinct Frasnian platforms, occurred during the Lower to The remainder of the LKE is within the lower portion of the HST of Middle Palmatolepis triangularis Zone (Figs. 4–6). The Sassenach sequence 8 (Figs. 4–6). Parasequences in the LST of sequence 8 on the Fm. spans the interval of the Lower–Middle Pa. triangularis Zone platform top (section W4) are 6 to 8 m thick and sharp based, and they (Table 2) through the Lower Palmatolepis crepida Zone, following thicken and coarsen upward from silty bioclastic wackestones to float– brachiopod-based correlations by Raasch (1989) (Famennian zones rudstones containing brachiopods and corals (Fig. 5). In the last DFM1 to DFM3), and conodont faunas documented by Johnston parasequence of the TST, a 14-m-thick cycle shoals upward from silty and Chatterton (1991), which place the base of the overlying mudstones into laminated, fenestral peritidal wackestones with an Palliser Fm. within the Lower Pa. crepida Zone. Only two undulating truncated upper surface (Fig. 5). The HST is an 8-m-thick parasequences were documented in the lower part of sequence 9. bed of relatively massive bioclastic wackestone. In the slope At section C, the lower parasequence is 2.4 m thick, appears to be

This is an author e-print and is distributed freely by the authors of this article. Not for resale. 192 ..WAE,D EVESHUE,JH AN,JE A,DJ OVER, D.J. DAY, J.E. PAYNE, J.H. VLEESCHOUWER, DE D. WHALEN, M.T. This is an author e-print and is distributed freely by the authors of this article. AND Not .CLAEYS P. for resale. FIG. 6.—Astronomical calibration of MS data from western Canada. A) Cyclostratigraphic correlation of the Frasnian of western Canada (modified from De Vleeschouwer et al. 2012b). MS stratigraphy (black) and band-pass filtered signal of the MS stratigraphy at the frequencies of the 405-kyr eccentricity cycles (E1, brown and orange lines). White and gray boxes indicate 16.5 405-kyr eccentricity cycles over the entire Frasnian stage, implying a duration of 6.7 6 0.6 Myr (the counting error is estimated to be 1 E1-cycle). B, C) Astronomical calibration of Upper Frasnian sections W4 (B) and C (C) that span the LKE and UKE intervals. The orange band-pass filters represent 405-kyr eccentricity cycles (LEC), and the blue band-pass filters represent 100-kyr eccentricity cycles. D) MTM power spectrum and harmonic F-test of the MS data from the Ronde Mbr., section W4, with astronomical interpretation. E) MTM power spectrum and harmonic F-test of MS data from the Simla Mbr., section C. F) The astronomical interpretation of this spectrum, here refined by the result of the average spectral misfit method, suggests an optimal sedimentation rate of 3.43 cm/kyr. This value is close to the sedimentation rate (3.2 cm/kyr) previously obtained by De Vleeschouwer et al. (2012b). PATTERN AND TIMING OF LATE DEVONIAN BIOTIC CRISIS, WESTERN CANADA 193

condensed, and consists of black shale overlain by a redeposited oncoid rudstone (Figs. 4, 5). The second parasequence is about 10 m thick, has a thin black shale at the base, and consists of interbedded black to gray shale, and silty mud–wackestone. The first parasequence is interpreted as the LST, and its base records the initiation of the UKE, while the second records deepening and the beginning of the TST. On the platform top, the base of sequence 9 consists of two parasequences, 8 and 5 m thick, respectively, that contain bioturbated silty mud–wackestone that grade upward into laminated or ripple cross-bedded wacke–packstones, all deposited within a peritidal environment (Figs. 4, 5). The first parasequence is interpreted as the LST and contains the initiation of the UKE, which continues into the overlying TST (Figs. 4, 5).

Magnetic Susceptibility

MS data are reported for sections C and W4 that span the F–F boundary (Figs. 5, 6). The mean MS value for samples from the part of section C reported here is 2.63 3 108 m3/kg (n ¼ 362 for 189 m of section; Fig. 6). The mean MS value for samples from the portion of section W4 reported is 1.073108 (n ¼ 136 for 67 m of section; Fig. 6). In section C, the MS values are relatively low at the bottom of the interval and increase upward for about 10 m (Figs. 5, 6), with two relatively high MS peaks (3.41 3 108 m3/kg and 3.14 3 108 m3/kg) near the top of the LKE interval. MS values drop to their lowest point in the studied interval (8.27 3 1010 m3/kg) within the uppermost Simla Mbr. and then gradually increase again in the overlying Sassenach Fm., with several peaks over the next 15 m of section (maximum value of 4.09 3 108 m3/kg). This second relatively high MS interval corresponds to the UKE (Figs. 5, 6). In section W4, MS is very low (1.41 3 109 m3/kg) in the lower part of the studied interval, corresponding to the LKE strata in the uppermost Ronde Mbr. (Figs. 5, 6). MS levels increase precipitously by over an order of magnitude (2.48 3 108 m3/kg) at the Ronde– Sassenach contact, marking the base of the UKE interval. MS values fluctuate but stay relatively high through most of the UKE but gradually drop in the upper part (Fig. 5, 6).

13 d Corg Section C Stable isotopic analyses were only conducted on samples straddling the F–F interval. In section C, this zone spans 34 m in the upper part 13 of the section (Fig. 4). The d Corg values record two positive excursions of similar magnitude of about 4% (Fig. 4). The first is a 4.2% (29.7 to 25.5%) excursion between 351 and 359 m, and it marks the position of the LKE. There are two minor positive fluctuations at the base of the interval (351–354 m), and values progressively drop to more negative levels of ~28% at 359 m. There are minor fluctuations above, until a large positive excursion of 4.39% (27.7 to 23.3%) between 368.5 and 373 m, marking the lower part FIG. 7.—SEM images of conodonts from section C. Scale ¼ 0.1 mm. of the UKE interval (Fig. 4). This second positive excursion is Specimens are in the Paleontological Collection, Department of followed by a decrease and a return to more negative values of Geological Sciences, State University of New York–Geneseo. (1) ~27% at 380.5 m, marking the top of the UKE (Fig. 4). Palmatolepis triangularis (Sannemann 1955), section C, 48.5 m. 13 (2) Palmatolepis boogaardi morph cf. Pa. nasuta (Muller¨ 1956), d Corg Section W4 section C 21.5 m. (3) Palmatolepis linguiformis (Muller¨ 1956), In section W4, the interval spanning the F–F boundary covers 25 m section C, 34 m. (4) Palmatolepis boogaardi (Klapper and Foster 13 of strata near the top of the measured section (Fig. 4). The d Corg data 1993), section C, 34 m. (5) Palmatolepis bogartensis (Stauffer are much more variable than in section C (Fig. 4). The lower portion 1938), section C, 34 m. (6) Polygnathus lodinensis (P¨olser, 1969), of the isotopic curve displays two positive peaks. The first, from 41 to section C, 35.1 m. 42 m records a 2.2% excursion (28.1 to 25.9%). Values then drop back to 27.4 before the second 1.7% positive excursion (27.4 to 25.7%) from 42.5 m to 48.0 m. The strata from 41 to 48 m, which record these two positive excursions, are interpreted as the LKE interval (Fig. 4).

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TABLE 2.—Conodont biostratigraphic data from section C, Ancient Wall. Table illustrates first occurrences of conodont taxa, conodont zones (Frasnian—Klapper 1989, 2007; Girard et al. 2005; Famennian—Ziegler and Sandberg 1990), stratigraphic units, and depositional sequences (Whalen and Day 2008, 2010).

Conodont sequence in section C: Ancient Wall

Sample interval (m) 0.5 13 16 21.5 23 25.5 34 35 35.1 35.7 36 37 37.2 37.4 38.1 38.3 39 45.5 47 48.8 68.4 Conodont zone FZ 12 FZ 13A FZ 13B FZ 13C L-M triang. Palmatolepis winchelli XX X XX X Polygnathus politus XXX X X Mehlina sp. X X X X X Po. imparilis XX X X X X XX X X Po. samueli XX Pa. sp. X X X X X Ancyrognathus aff. An. altus X Pa. rhenana XX X Pa. boogaardi XX X X X Po. iodiensis XX XX Po. pacificus X Pelekygnathus planus X Ancyrodella ioides XX [homeomorph] Ad. hamta XX Po. brevicarina XXXXX X Pa. boogaardi cf. Pa. nasuta XX Po. sp. X X X X Po. unicornis XX Pa. linguiformis X? Pa. youngquisti X Po. n. sp. R of Klapper X and Lane Pa. bogartensis XX X Ancyrognathus amana X Polygnathus ettremae X Pa. sp. cf. Pa hassi X Po. aspelundi X Po. sp. cf. P. unicornis X An. cf. An. asymmetricus X Icriodus alternatus XXXX I. iowaensis XX Po. decorosus XX Pa. ultima X Pa. triangularis X Po. brevilaminus XX Po. praecursor? X Pelek. sp. X Stratigraphic unit Mt. H Simla Member, Southesk Formation Sassenach Depositional sequence Sequence 7 Sequence 8 Seq. 9 Upper Devonian stage Upper Frasnian Substage Famennian Ad. ¼ Ancyrodella; An. ¼ Ancyrognathus; I. ¼ Icriodus; Pa. ¼ Palmatolepis; Pelek. ¼ Pelekygnathus; Po. ¼ Polygnathus.

Above the LKE, there is a series of four relatively large and 61.5 and 64.5 m. We interpret the relatively abrupt decline, to a closely spaced fluctuations that vary between a maximum of 23.9% slightly higher background level, above this positive excursion to and minimum of 28.1% over 4.5 m (Fig. 4). The UKE interval mark the top of the UKE interval. spans 52 to 65 m. The base of the UKE is taken as the first significant positive excursion of 3% (27 to 24%) beginning at 52 Time-Series Analysis m. Above the closely spaced fluctuations, the isotopic signature records a significant 3% negative excursion (25.7 to 28.7%) We applied the cyclostratigraphic framework for the studied between 57.5 and 61.5 m in the section. Above this, there is a sections C and W4 (Fig. 6), as presented in De Vleeschouwer et al. pronounced positive excursion of 3.7% (28.7 to 25%) between (2012b), to assess the timing and pacing of the F–F transition. The

This is an author e-print and is distributed freely by the authors of this article. Not for resale. PATTERN AND TIMING OF LATE DEVONIAN BIOTIC CRISIS, WESTERN CANADA 195 interval examined in section C spans the upper Mount Hawk Fm., DISCUSSION the Simla Mbr. of the Southesk Fm., and the lower Sassenach Fm. (279–469 m), and it includes the uppermost part of FZ 11, 12, and Determination of the timing of geologic and paleontologic events in 13, and the early Famennian Lower and Middle triangularis zones the rock record is one of the most vexing issues in sedimentary (Figs. 4, 5) (Whalen and Day 2008). A strong low-frequency (9–25- geology, especially in deep time. The LKE and UKE were originally m period) cycle that is persistent throughout the Mount Hawk Fm. defined by the occurrence of distinctive black shale facies in the and the Simla Mb., but loses spectral power in the Famennian Upper Frasnian of Europe (Buggisch 1991, Joachimski and Buggisch Sassenach Fm., is associated with the 405-kyr eccentricity cycle 1993). Since both the LKE and UKE record biotic crises, the bases of (orange line on Fig. 6). The higher-frequency blue line (2.5–5-m these intervals are defined biostratigraphically and occur at the top of filter on Fig. 6) indicates a periodic component in the MS data that is FZ 12 (the base of the Upper Palmatolepis [Pa.] rhenana Zone) and associated with 100-kyr eccentricity. While we are aware that there near the top of FZ 13b (upper part of the Lower Pa. linguiformis are other longer-term eccentricity cycles (Laskar et al. 2011), as a Zone), respectively (Girard et al. 2005). Gaps in the geographic short hand we will refer to the 100-kyr eccentricity as ‘‘short’’ and distribution of Pa. linguiformis lead to difficulty in identifying the the 405-kyr eccentricity as ‘‘long’’ eccentricity cycles (LECs). Both UKE; therefore, proxy taxa are often employed as a work-around low- and higher-frequency cycles show a thickening of the cycles (Girard et al. 2005). Girard and Renaud (2007) developed a throughout these late Frasnian and earliest Famennian deposits. quantitative conodont-based approach that appears to be capable of Moreover, neither low- nor high-frequency cycles can be identified distinguishing the positions of the event intervals in a variety of above 420 m in section C, suggesting that the input of coarser- lithofacies. grained siliciclastic sediments during the early Famennian hindered 13 the development of astronomical cycles (Fig. 6). The thickening of d C Excursions and Defining the F–F Events cycles is in line with geological expectations, as a relatively higher sedimentation rate is expected in the prograding slope and outer- In addition to the faunal changes, both the LKE and UKE are 13 ramp facies of the Simla Mbr. compared to the deeper-slope facies of characterized by significant positive d C excursions usually on the the Mount Hawk Fm. order of 4 to 5% (Joachimski and Buggisch 1993, Joachimski et al. Section W4 includes the Ronde Mbr. of the Southesk Fm. and the 2002, Racki 2005). These isotopic excursions appear to occur lower Sassenach Fm. It spans FZ 13a to 13c and the early Famennian synchronously around the globe (Fig. 1), and they provide a method Lower and Middle triangularis Zone (Figs. 4, 5) (Klapper 1989, of evaluating the pattern and timing of the F–F events, when they are Girard et al. 2005, Whalen and Day 2008). Similar to section C, placed within a cyclostratigraphic framework. Astrochronology decameter-scale cyclicity with a period between 10 and 20 m is provides a geochronometer of unprecedented resolution with which ascribed to long eccentricity, and meter-scale cyclicity between 3 and to evaluate the pacing of events such as LKE and UKE (Hinnov 2013, 4.5 m is attributed to short eccentricity (Fig. 6). De Vleeschouwer and Parnell 2014). To evaluate the timing of the F–F biotic crisis, we must codify the stratigraphic record of the events so that they can readily be compared F–F Eccentricity Cycles and Events at many sections (Fig. 1). While the LKE and UKE are first and foremost biotic crises (McGhee 1988, Bambach et al. 2004, Stigall De Vleeschouwer et al. (2012b) analyzed data for several 2012), assembling detailed species richness data from many sections continuous and composite sections in western Canada and document- would be labor intensive. Wide varieties of geochemical data are ed 16 Frasnian LECs. After comparison with the contemporaneous available across the F–F interval, including elemental data that serve Kowala section in Poland (De Vleeschouwer et al. 2013), it was as proxies for detrital input, productivity, and redox conditions (Over apparent that one half-long eccentricity cycle needed to be added, et al. 1997; Riquier et al. 2006, 2007; Whalen and Day 2008; Boyer et bringing the total up to 16.5 Frasnian LECs (Fig. 6). This corresponds al. 2011; Whalen et al. 2015). However, many of these proxies vary to a duration of 6.7 Myr for the Frasnian (De Vleeschouwer et al. with depositional setting, hampering their use as global markers 2012b). The 405-kyr LECs and the cyclostratigraphic correlation of (Algeo and Rowe 2012). Carbon isotopic data are one of the most the sections (Fig. 6) are consistent with biostratigraphic- and sequence commonly reported records from Late Devonian sections (Fig. 1). stratigraphic-based correlations (Whalen and Day 2008, 2010). This Perturbations of the carbon cycle appear to have been globally cyclostratigraphic correlation also indicates that part of LEC 16 in synchronous and fundamental to the ongoing ecosystem changes section W4 is likely missing due to a minor exposure-related (Joachimski et al. 2002, Buggisch and Joachimski 2006). Therefore, unconformity (Figs. 4, 5). Subsequently, the stratigraphic positions 13 they constitute the most suitable proxy for evaluating the pattern and of the d C excursions are indicated to pinpoint the LKE and UKE timing of the F–F events. As discussed earlier, we define the onset of within the astrochronologic framework (blue and orange boxes on Fig. the carbon isotope excursion to mark the beginning of the events, 6). The onset of LKE is within Frasnian LEC 15 (close to the while the decline of d13C values to a background level, which may maximum of that cycle), and it terminates within the lower part of differ from the pre-event background, marks the termination of the LEC 16. The onset of the UKE is in the lower part of the last Frasnian event (Fig. 4), similar to other positive excursions documented in the LEC 17 (Figs. 4, 5). In section W4, the minor unconformity at the top geologic record (Buggisch 1991; Joachimski and Buggisch 1993; of the parasequence underlying the onset of the UKE hampers the Saltzman et al. 1998; Joachimski et al. 2002; Bergstrom et al. 2006; cyclostratigraphic assessment of the onset of the UKE. It seems that Cramer et al. 2006, 2008; Cramer and Saltzman 2007). the UKE onset appears a bit lower (within LEC 16) in section W4 than Surveying the shape structure of the d13C excursions, recorded in in section C, but we consider section C to reflect the cyclostratigraphic both carbonates and organic matter, the LKE is commonly patterns better and therefore place the onset of the UKE in the lower characterized by an abrupt onset (i.e., most of the d13C excursion part of Frasnian LEC 17 (Figs. 5, 6). LEC 17 also contains the F–F occurs over less than 1 m of stratigraphy), one or two positive peaks, boundary, which implies that only the lower part of the cycle is and a gradual decline to background values. This is the pattern Frasnian in age, while the upper part is Famennian. Consequently, documented at the Benner, Budesheimer¨ Bach, Steinbach Schmidt, Frasnian LEC 17 and Famennian LEC 1 refer to the same 405-kyr and Vogelsberg localities in Germany; Coumiac, France (Joachimski eccentricity cycle. According to this definition, the UKE clearly and Buggisch 1993, Joachimski et al. 2002); Baisha and Fuhe terminates in the uppermost part of Famennian LEC 1 (Figs. 5, 6). sections, south China (Chen et al. 2005); Windjana Gorge, Australia

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(Stephens and Sumner 2003); as well as in the studied section W4 have had a duration of about one full LEC, i.e., about twice the from western Alberta, Canada. Studied section C records a more duration of the LKE (Figs. 5, 6). Several other lines of evidence also complicated pattern of generally decreasing d13C after the initial rapid suggest a longer duration for the UKE. Joachimski et al. (2004, 2009) excursion. provided d18O data from conodont apatite, and van Geldern et al. The UKE carbon isotopic records are more variable and (2006) reported d13C and d18O data from brachiopods from many complicated compared to the LKE. Many sections appear to record localities that record a positive isotopic excursion over a very narrow a relatively abrupt onset of the UKE d13C excursion, such as those in interval for the LKE, while the excursion associated with the UKE is south China (Chen et al. 2005); New York State, USA (Murphy et al. spread out over a thicker stratigraphic interval. Data from these studies 2000); western Canada (Wang et al. 1996, this volume); Budesheimer¨ are rather coarse and result in duration estimates on the order of 0.5 Bach, Germany; Kowala, Poland; Nevada, USA (Joachimski et al. and .1 Myr for the LKE and UKE, respectively, which are similar to 2002); and western Australia (Stephens and Sumner 2003). However, duration estimates based on the extent of carbon isotope excursions 13 the d Ccarb record at Kowala, Poland, appears to record a bit more (Godd´erisand Joachimski 2004) and conodont biostratigraphy from protracted onset to the excursion, and several other localities share this the Benner and Schmidt quarry sections in Germany (Joachimski and more protracted onset, including Bou Ounebdou, Morocco (Joachim- Buggisch 1993), the latter of which provides the only late Frasnian ski et al. 2002), and the classic face at Windjana Gorge and the Oscar radiometric tie point in the most recent geologic timescale (Becker et Range, Australia (Stephens and Sumner 2003). The UKE interval at al. 2012). several localities records multiple positive and negative fluctuations of The cyclostratigraphic estimates from western Canada (this study), 2to3%, such as in south China, Windjana Gorge, Kowala, Poland, southern China (Chen et al. 2005), and Poland (De Vleeschouwer et and section W4, western Canada. These excursions may be related to al. 2013) agree quite well, given the different methods used to define relative sea-level change, that will be discussed later herein. In many and delineate the events and the varying accumulation rates and sections, picking the termination of the UKE is difficult, because the depositional settings between localities. These data, along with d13C isotopic background following the event is 1 to 2% higher than that trends from other localities, indicate a relatively rapid initiation and before the onset of the event. Here, we define the top of the UKE 13 termination of the LKE, while the UKE locally records rapid initiation interval as either the location where d C values return to background, but requires a longer time for termination of the d13C excursion. Post- or at the top of the Middle triangularis conodont zone if no UKE d13C background levels remain higher than pre-event values, discernible return to background levels was recorded. The thickness of indicating continued carbon burial into the Famennian (Figs. 1, 4). the LKE and UKE intervals is strongly influenced by sedimentation The association of these d13C excursions with interpreted changes in rates, condensation, and omission surfaces or unconformities (Table sea level and tropical sea-surface temperature (from d18O of conodont 1). apatite; Joachimski et al. 2004, 2009) implies a relationship with climate change (Fig. 4). Implications for Timing of the F–F Events Relationship Between Sea Level, Climate, and The LKE interval in section C is 8 m thick, while the UKE interval is 13.5 m thick (Fig. 4; Table 1). In section W4, the LKE interval is 7 Astronomical Forcing m thick, and the UKE interval 13 m thick (Fig. 4; Table 1). The onset of the LKE occurs within Frasnian LEC 15, about half (section W4) to The LKE and UKE both appear to have been intimately related to two thirds (section C) of the way through the cycle (Figs. 4–6). This is climate and associated sea-level changes. Alberta sequence 8 and the in good agreement with the cyclostratigraphic framework that has LST and TST of sequence 9 comprise seven parasequences, each been constructed for the Kowala section in Poland, where the LKE indicating higher-frequency sea-level events (Fig. 5). Sequence 8 was black shale, which corresponds to an abrupt positive isotope initiated during T–R cycle IId2, while sequence 9 was deposited excursion, occurs in the middle of LEC 15 (De Vleeschouwer et al. within cycle IId3 (Figs. 4, 5) (Johnson et al. 1985, 1996; Day 1998; 2013). The end of the LKE isotope excursion in the studied sections Whalen and Day 2010). occurs about one quarter into Frasnian LEC 16. This interpretation is Sections C and W4 in western Canada record upper-slope–outer- solely based on section C, as it is likely that part of LEC 16 in section ramp and platform-top depositional environments, respectively. The W4 is missing due to a minor exposure-related unconformity (Fig. 5). different depositional processes and the likelihood of subaerial The onset of the UKE occurs close to the start of Frasnian LEC 17. exposure of the platform top likely influence the records of these The UKE isotope excursion roughly covers this entire cycle, which two events. Deposition of the Simla Mbr. along the slope and outer can also be referred to as Famennian LEC 1. In the Kowala section ramp at section C appears to have been continuous, although the thin (Poland), the UKE black shale also occurs right at the onset of the last black shale interval that marks the base of the Sassenach Fm., F–F Frasnian LEC. Hence, the cyclostratigraphic position of the onset of boundary, and the UKE (Fig. 6) is very likely condensed compared to the UKE isotope excursion in western Canada is in perfect agreement the rest of the section. The Ronde Mbr. at section W4 was deposited with the position of the UKE black shale in Kowala, Poland (De under initially subtidal environments that shoaled to peritidal, and it Vleeschouwer et al. 2013). However, De Vleeschouwer et al. (2013) displays evidence of subaerial exposure, including minor truncation of suggested that the time difference between the onset of the LKE and beds and an undulating erosional surface at the top of one UKE amounts to ~800 kyr. Based on the results presented here, this parasequence in the middle of the LKE interval (Figs. 4, 5). The estimate should be revised to 500 to 600 kyr. This new estimate is in UKE at section W4 is within the Sassenach Fm. and is entirely very good agreement with a cyclostratigraphic study from south China peritidal facies. The d13C record at section W4 was likely influenced by Chen et al. (2005). These authors indicated that the LKE represents by episodes of subaerial exposure, as parasequence boundaries usually ~250 kyr, the UKE represents ~500 kyr, and the time difference record a positive shift, likely influenced by exposure-related meteoric between the onset of both events was ~650 kyr. In the Kowala section diagenesis (e.g., Immenhauser et al. 2003), followed by a negative 13 (Poland), d Corg takes ~600 kyr to return to background values after swing that could be related to transgressive reworking and oxidation the UKE (De Vleeschouwer et al. 2013). of previously deposited organic matter (Fig. 4). The LKE isotope excursion, as delineated in the studied sections, The record of the LKE at section C records a relatively abrupt initial occurs within one LEC and appears to have been ~250 kyr in duration positive excursion followed by a gradual decline to background levels. (Figs. 5, 6). The UKE, as delineated in the studied sections, appears to Section W4 records a small initial abrupt positive excursion followed

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by a gradual positive excursion that ends abruptly. This variability is data and numerical climate modeling results suggest an important role likely related to the increasingly peritidal conditions in section W4 of astronomical forcing in bringing the climatic and environmental (Fig. 4). system, prone to anoxic bottom water conditions, past its tipping point The record of the UKE at section C is relatively well defined, with (Fig. 16 in De Vleeschouwer et al. 2014b). an abrupt initial positive excursion and a gradual drop to less positive values (Fig. 4). The UKE in section W4 displays much more CONCLUSIONS variability, with significant closely spaced positive and negative shifts in the lower portion of the UKE interval (Fig. 4). Again, we interpret The Late Devonian records some of the most profound geologic, this difference as reflecting the different depositional settings between climatic, and biotic events of the Paleozoic, including a greenhouse- the two sections. The outer-ramp to slope environment at section C did to-icehouse transition, eustatic sea-level changes, orogenesis, wide- not experience exposure-related meteoric diagenesis, and thus it does spread marine anoxia, and a severe stepwise biotic crisis that ranks as not display successive positive–negative isotopic shifts, as recorded in one of the ‘‘big five’’ mass extinctions. peritidal facies in section W4. We combine records of sequence stratigraphy, stable carbon The similarity in the astronomical interpretation for the two sections isotopic data, and time-series analysis of high-resolution MS data to reveals comparable patterns indicating that the record at W4, while provide insight into the pattern and timing of the F–F events. Placing influenced by exposure events, is largely complete. We note one the isotopic excursions in a cyclostratigraphic framework (De incidence where a minor unconformity is indicated by truncated beds Vleeschouwer et al. 2012b), we demonstrate that the LKE began and an undulating erosional surface within the LKE interval, but it within Frasnian LEC 15 and terminated in the lower portion of appears that only a portion of LEC 15 was lost during exposure- Frasnian LEC 16, implying a duration of 200 to 250 kyr. The onset of related weathering. the UKE occurs in the lower part of Frasnian LEC17 and terminates at The LKE and UKE both begin during LST or TST, and their onsets the end of that same cycle in the earliest Famennian (Frasnian LEC 17 occur close to a minimum in their respective Frasnian LECs (Figs. 5, and Famennian LEC 1 are synonymous). Therefore, the duration of 6). This is in excellent agreement with a Late Devonian numerical the UKE isotope excursion in western Canada is estimated to be ~400 climate modeling study, suggesting that an eccentricity minimum is a kyr. According to this new estimate, the onset of the LKE isotope favorable astronomical configuration for a relatively cool and dry excursion and the onset of the UKE isotope excursion are separated by global climate (De Vleeschouwer et al. 2014b). Hence, we suggest 500 to 600 kyr. Our analysis is supported by cyclostratigraphy from a that rapid climatic warming and relative sea-level rise, incited by an coeval section—the cyclostratigraphic position of the onset of the increasing eccentricity of Earth’s orbit, drove the Kellwasser events. UKE isotope excursion in western Canada is in perfect agreement Several parasequence boundaries in the latest Frasnian interval with its position within black shale in Kowala, Poland (De correlate with the descending limbs of LECs (13, 14, 15, and 16), Vleeschouwer et al. 2013). indicating eccentricity minima during sea-level lows (Fig. 6). This analysis, along with d13C trends from other localities, Maximum transgression is associated with maximal eccentricity provides a view of a relatively rapid initiation and termination of the during both Kellwasser events (De Vleeschouwer et al. 2013). LKE. The UKE locally records rapid initiation but required a longer According to our cyclostratigraphic framework, the UKE is clearly time for termination of the d13C excursion, and background levels related to the 405-kyr eccentricity maximum of Frasnian LEC 17 remained higher than pre-event values, indicating continued carbon (Figs. 5, 6). The LKE on the other hand, which is of shorter duration, burial during the early Famennian. The associated changes in was probably related to a 100-kyr eccentricity maximum within tropical sea-surface temperature (from d18O of conodont apatite) Frasnian LEC 15 (Figs. 5, 6). imply a relatively rapid rise and drop in temperature during the LKE, Late Devonian global warming was also associated with a wetter followed by a rapid rise and protracted drop in temperature global climate and thus with higher rates of terrestrial weathering and associated with the UKE (Joachimski et al. 2004, 2009; van Geldern delivery of detritus to epeiric seas (Riquier et al. 2006, Whalen et al. et al. 2006). 2015). This, in conjunction with the increase in vegetated land area The cause of the F–F biotic crisis remains contentious, but an and total root mass and concomitant effects on weathering rates from emerging consensus proposes that it was linked to the development of the Frasnian into the Famennian (Algeo and Scheckler 2010), resulted terrestrial forests and associated changes in root and soil development in detrital-driven nutrification, high rates of primary productivity, and that influenced weathering and, through eutrophication, the redox state development of low-oxygen conditions, all contributing to the biotic of many epeiric seas around the world (Algeo et al. 1998; Joachimski crisis (Joachimski et al. 2001, Riquier et al. 2006, Bond and Wignall et al. 2001, 2002; Algeo and Scheckler 2010). Relatively abrupt 2008, Whalen et al. 2015). Subsequent cooling during the early warming intervals and transgression of low-oxygen waters into Famennian was the result of a net carbon transfer from atmospheric shallow platform environments during these two latest Frasnian sea- CO2 towards the types of organic-rich shales that exemplify the level events appear to have been important drivers of the biotic crisis Kellwasser events. The d18O records from conodonts and brachiopods (Hallam and Wignall 1999, Joachimski et al. 2002, Buggisch and lend support to this climatic interpretation (Joachimski et al. 2004, Joachimski 2006, Bond and Wignall 2008, Stigall 2012). Transgres- 2009; van Geldern et al. 2006). The two events, however, were sive events also potentially influenced the biotic diversity trends qualitatively different in that the LKE records demonstrate relatively during the F–F interval by facilitating the emigration of invasive rapid warming and cooling, whereas the UKE indicates rapid warming species, i.e., ecological generalists that outcompeted specialists, followed by protracted cooling into the early Famennian (Joachimski reducing overall diversity (Stigall 2012). This points to the complex et al. 2004, 2009; van Geldern et al. 2006). This chronology is interrelationships among climate, weathering, oceanography, and supported by our cyclostratigraphic analysis as well as the general ecosystem dynamics that were severely perturbed during the F–F thickness trends for the positive isotopic excursions documented biotic crisis. globally (Table 1). Although the main triggering mechanism for the Kellwasser events ACKNOWLEDGMENTS remains under debate, the growing consensus on the relative timing and duration of these events demonstrates that astronomical climate Acknowledgment is made to the donors of The Petroleum Research forcing played a major role in the timing and pacing of these Fund, administered by the American Chemical Society (Whalen), and environmental perturbations. Indeed, a comparison between isotopic to the National Geographic Society–Foundation for Exploration and

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Research (Day) for partial support of this research. Part of this Chen D, Qing H, Li R. 2005. The Late Devonian Frasnian–Famennian (F/F) 13 13 87 86 research was also supported by a PhD fellowship from the Research biotic crisis: Insights from d Ccarb, d Corg and Sr/ Sr isotopic Foundation–Flanders (FWO) assigned to David De Vleeschouwer. systematics. Earth and Planetary Science Letters 235(1–2):151–166. Philippe Claeys thanks the Vrije Universiteit Brussel research fund for Chen D, Tucker ME, Shen Y, Yans J, Preat A. 2002. Carbon isotope excursions support. Small grants and scholarships, including the University of and sea-level change: Implications for the Frasnian–Famennian biotic crisis. Alaska–Fairbanks (UAF) BP–Amoco and Brian Zelenka Memorial Journal of the Geological Society, London 159:623–626. Awards, the American Association of Petroleum Geologists Wanek Claeys Ph, Casier J-G, Margolis SV. 1992. Microtektites and mass extinctions. Award, an Alaska Geological Society scholarship, and a Geological Evidence for a Late Devonian impact. Science 257:1102–1104. Society of America student research grant, helped fund Payne’s MS Coplen TB. 2011. Guidelines and recommended terms for expression of stable- research. Thanks go to the Department of Geology and Geophysics isotope-ratio and gas-ratio measurement results. Rapid Communications in and the Geophysical Institute, UAF, for their support. We appreciate Mass Spectrometry 25:2538–2560. M. Sliwi´nskifor´ comments on an earlier version of this manuscript, as Copper P. 1986. Frasnian/Famennian mass extinction and cold-water oceans. well as constructive comments from Brooks Ellwood, Paul Mont- Geology 14:835–839. gomery, Ted Playton, and Nicholas Sullivan. This research would not Copper P. 2002. Reef development at the Frasnian/Famennian mass extinction have been possible without the cooperation of Parks Canada for boundary. 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