Organic Carbon Isotope Chemostratigraphy of Late Jurassic Early Cretaceous Arctic Canada

University of Plymouth PEARL https://pearl.plymouth.ac.uk

Faculty of Science and Engineering School of Geography, Earth and Environmental Sciences

Finding the VOICE: organic carbon isotope chemostratigraphy of Late Jurassic Early Cretaceous Arctic Canada

Galloway, JM http://hdl.handle.net/10026.1/15324

10.1017/s0016756819001316 Geological Magazine Cambridge University Press (CUP)

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AQ1: The distinction between surnames can be ambiguous, therefore to ensure accurate tagging for indexing purposes online (e.g. for PubMed entries), please check that the highlighted surnames have been correctly identified, that all names are in the correct order and spelt correctly. AQ2: Please check that affiliations of all the authors and the corresponding author details are correctly set. Geological Magazine Finding the VOICE: organic carbon isotope 1 – 2 www.cambridge.org/geo chemostratigraphy of Late Jurassic Early Cretaceous Arctic Canada 3

1,2 3 4 1 Original Article Jennifer M. Galloway , Madeleine Vickers , Gregory D. Price , Terence Poulton , 4 , Stephen E. Grasby1, Thomas Hadlari1 , Benoit Beauchamp5 and Kyle Sulphur1 5 AQ1 Cite this article: Galloway JM, Vickers M, 5 Price GD, Poulton T, Grasby SE, Hadlari T, 1Geological Survey of Canada/Commission géologique du Canada, Natural Resources Canada/Ressources 6AQ2 Beauchamp B, and Sulphur K. Finding the 2 VOICE: organic carbon isotope naturelles Canada, 3303 33rd St N.W., Calgary, Alberta T2L 2A7, Canada; Aarhus Institute of Advanced Studies, 7 3 chemostratigraphy of Late Jurassic – Early Aarhus University, Høegh-Guldbergs Gade 6B 8000 Aarhus C, Denmark; Faculty of Science, Geology Section, 8 4 Cretaceous Arctic Canada. Geological Magazine University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark; School of Geography, Earth & 9 https://doi.org/10.1017/S0016756819001316 Environmental Sciences, University of Plymouth, Drake Circus, PL4 8AA, UK and 5Department of Geosciences, 10 University of Calgary, Calgary, AB T2N 1N4, Canada 11 Received: 4 April 2019 Revised: 6 September 2019 Accepted: 7 October 2019 Abstract 12 A new carbon isotope record for two high-latitude sedimentary successions that span the 13 Keywords: Jurassic–Cretaceous boundary interval in the Sverdrup Basin of Arctic Canada is presented. 14 Arctic; Jurassic–Cretaceous; Canada; carbon isotopes This study, combined with other published Arctic data, shows a large negative isotopic 15 δ13 ‰ − ‰ excursion of organic carbon ( Corg)of4 (V-PDB) and to a minimum of 30.7 in the 16 Author for correspondence: probable middle Volgian Stage. This is followed by a return to less negative values of 17 Jennifer M. Galloway, c. −27‰. A smaller positive excursion in the Valanginian Stage of c.2‰, reaching maximum 18 Emails: [email protected]; − ‰ [email protected] values of 24.6 , is related to the Weissert Event. The Volgian isotopic trends are consistent 19 δ13 with other high-latitude records but do not appear in Tethyan Tithonian strata Ccarb records. 20 δ13 In the absence of any obvious definitive cause for the depleted Corg anomaly, we suggest 21 several possible contributing factors. The Sverdrup Basin and other Arctic areas may have 22 experienced compositional evolution away from open-marine δ13C values during the 23 Volgian Age due to low global or large-scale regional sea levels, and later become effectively 24 coupled to global oceans by Valanginian time when sea level rose. A geologically sudden 25 δ13 increase in volcanism may have caused the large negative Corg values seen in the Arctic 26 Volgian records but the lack of precise geochronological age control for the Jurassic– 27 Cretaceous boundary precludes direct comparison with potentially coincident events, such 28 as the Shatsky Rise. This study offers improved correlation constraints and a refined C-isotope 29 curve for the Boreal region throughout latest Jurassic and earliest Cretaceous time. 30

1. Introduction 31 The Jurassic–Cretaceous boundary interval was characterized by significant fluctuations in 32 Earth system processes (Hallam, 1986; Ogg & Lowrie, 1986; Sager et al. 2013; Price et al. 33 2016) that resulted in the extinction of many marine invertebrates (Hallam, 1986; Alroy, 34 2010; Tennant et al. 2017). Despite its importance in Earth history, the precise radiometric 35 age and correlations of the Jurassic–Cretaceous boundary interval are poorly understood com- 36 pared with those of other Phanerozoic environmental crises. This is partly because of the 37 ongoing lack of a robust, global chronostratigraphic framework for the boundary (Zakharov 38 et al. 1996; Wimbledon et al. 2011). After long debate, the Berriasian Working Group of the 39 International Subcommission on Cretaceous Stratigraphy has voted to adopt the base of 40 the Calpionella alpina Subzone as the primary marker for the base of the Berriasian Stage in 41 the Tethyan faunal realm (Wimbledon, 2017). At this time, a stratotype section has not 42 been formally designated. This potential Global Boundary Stratotype Section and Point 43 (GSSP) level cannot be traced biostratigraphically into Arctic areas (e.g. Wimbledon, 2017, 44 fig. 1). Palaeomagnetic reversal data may provide direct Boreal–Tethyan correlation for the 45 Tithonian–Berriasian boundary eventually, but data from the Boreal Nordvik section (Houša 46 et al. 2007; Bragin et al. 2013; Schnabl et al. 2015) remain to be confirmed in other Arctic 47 sections. Alternative options for the placement of the Jurassic–Cretaceous boundary continue 48 to find support. 49 Although the international chronostratigraphic terminology for the Jurassic–Cretaceous 50 © Cambridge University Press 2019. boundary interval (Tithonian and Berriasian stages) is increasingly being used in Canadian 51 Arctic studies, interpretations of the correlations of the substages and fossil zones entailed in 52 these Tethys-based stages into the Arctic vary among global workers. Particularly contentious 53 and significant is how much of the upper Volgian Stage is time-equivalent with the lower 54 Berriasian Stage. Our usage in this report of the roughly equivalent Boreal (Volgian, Ryazanian) 55 2 JM Galloway et al.

56 and Tethyan nomenclature follows that of the relevant original lit- in the Sverdrup Basin, Arctic Canada, is presented here. 118 57 erature cited. Our data do not contribute to, or require, discussion Geochemical trends are compared with data from other high- 119 58 of their detailed correlations or about the common but potentially latitude successions as well as with Tethyan sections to evaluate 120 59 misleading use of the term Boreal for some NW European their palaeoceanographic and palaeoclimatic importance and 121 60 Sub-boreal sequences. potential for stratigraphic correlation. In the absence of any 122 61 The numerical age of the Jurassic–Cretaceous boundary is obvious definitive cause for VOICE, several possible contributing 123 62 also under debate. The International Commission of factors, both regional and distant, are considered and discussed. 124 63 Stratigraphy (Cohen et al. 2013, updated 2018/08) places the 64 Jurassic–Cretaceous boundary at c. 145 Ma following Mahoney 2. Study area 125 65 et al.(2005), who suggest a minimum age for the boundary based 66 on mean 40Ar–39Ar ages of 144.6 ± 0.8 Ma, although recent U–Pb The Sverdrup Basin is a 1300 × 350 km palaeo-depocentre in the 126 67 studies by Aguirre-Urreta et al.(2019) and Lena et al.(2019) Canadian Arctic Archipelago that contains up to 13 km of nearly 127 68 provide new U–Pb ages that suggest that the numerical age of continuous Carboniferous–Palaeogene strata (Figs 1, 2; Balkwill, 128 69 the boundary could be as young as 140–141 Ma. 1978; Embry & Beauchamp, 2019). 129 70 A small change to lower δ13Cvaluesoccurswithin Basin subsidence began following rift collapse of the Ellesmerian 130 71 Magnetozones M18–M17, and within the B/C Calpionellid Zone Orogenic Belt during early Carboniferous time (Embry & 131 72 (Weissert & Channell, 1989), that contrast with more positive Beauchamp, 2019). Rifting of the Sverdrup Basin continued 132 73 values obtained from the Valanginian Stage (Lini et al. 1992; during the late Carboniferous Period and led to widespread 133 74 Price et al. 2016). Such variation suggests that carbon isotope flooding of the rift basin and increasingly open-marine connections 134 75 anomalies may be useful to characterize the Jurassic–Cretaceous with Panthalassa and North Greenland and the Barents Sea (Embry 135 76 boundary interval (e.g. Michalík et al. 2009; Dzyuba et al. 2013). &Beauchamp,2019). After the first rift phase, marine deposition 136 77 A recent global stack compiled by Price et al.(2016) that included persisted through the Permian and Triassic periods. A second 137 78 data from many sites spanning a range of mainly southerly lati- phase of rifting began in the Early Jurassic Period, continued 138 79 tudes, and was therefore considered representative of the global through the Late Jurassic – earliest Cretaceous interval, and then 139 13 80 carbon isotopic signal, showed that the composite δ Ccarb curve ceased in the Sverdrup Basin when seafloor spreading began in 140 81 from the base of the Kimmeridgian to the base of the the adjacent proto-Amerasia Basin to form the Arctic Ocean 141 82 Valanginian stages has no major perturbations. However, there (Hadlari et al. 2016). Deposition in the Sverdrup Basin ended 142 83 is a paucity of published δ13C data from Arctic regions and, in inthePalaeogenePeriodduetoregionalcompressionandwide- 143 84 those that do exist, there is notably greater variation in high- spread uplift associated with the Eurekan Orogeny (Embry & 144 δ13 85 northern-latitude Corg (e.g. Hammer et al. 2012) than in Beauchamp, 2019). 145 86 better-studied middle- to low-latitude carbonate records In the Late Jurassic Period, the Sverdrup Basin was one of 146 δ13 δ13 87 ( Ccarb) (Price et al. 2016)orin Ccarb records from belemnites many rift basins that formed during the break-up of Pangea 147 88 in Arctic successions (Žák et al. 2011). and affected palaeoceanographic connections between the 148 δ13 89 Hammer et al.(2012) present Corg data for the Upper western Tethys and Panthalassa in northern latitudes. 149 90 Jurassic – lowermost Cretaceous systems of central Spitsbergen. Deposition of the Deer Bay Formation during latest Jurassic – 150 91 This record shows a middle Volgian excursion of c.5‰ that they earliest Cretaceous time marked a rift climax in the Sverdrup 151 92 term the Volgian Isotopic Carbon Excursion (VOICE). Koevoets Basin prior to the break-up of the adjacent proto-Amerasia 152 93 et al.(2016) documented a middle Volgian negative excursion Basin, manifested as a sub-Hauterivian break-up unconformity 153 δ13 ‰ 94 in Corg of c.3 in the Agardhfjellet Formation of central in the Sverdrup Basin (Embry, 1985a;Gallowayet al. 2013; 154 95 Spitsbergen. Records from northern Siberia also document a Hadlari et al. 2016;Fig.2).TheDeerBayFormationistherefore 155 δ13 96 Corg excursion to isotopically lighter values in the upper middle a lithostratigraphic unit of interest from both a tectonostrati- 156 97 Volgian (Exoticus Zone; Zakharov et al. 2014), but with no parallel graphic and palaeoceanographic perspective; its study may 157 δ13 98 trend in Ccarb measured in belemnite rostra from the same provide insight into both regional and global changes at this 158 99 section (Žák et al. 2011); this is possibly because carbon isotopes dynamictimeinEarth’shistory. 159 100 preserved in belemnite rostra may not be in equilibrium with The Deer Bay Formation is a succession of mudstone with 160 101 ambient seawater (Wierzbowski & Joachimski, 2009). Turner interbeds of siltstone and very-fine-grained sandstone deposited 161 δ13 102 et al.(2018) report a Corg curve from the 6406/12-2 drill core in pro-delta to offshore shelf environments across the Sverdrup 162 103 from the Norwegian Sea that spans the interval from the base of Basin during the Volgian to late Valanginian ages (Heywood, 163 104 the Pallasioides Zone to the top of the Rotunda Zone in the lower 1957; Balkwill, 1983; Embry, 1985b, c). The Deer Bay Formation 164 105 middle Volgian Stage. A negative isotopic excursion occurs in the reaches a maximum thickness of 1375 m on eastern Ellef 165 106 Pallasioides Zone that the authors relate to VOICE. Further south, Ringnes Island and 920 m on Axel Heiberg Island (Balkwill, 166 107 Morgans-Bell et al.(2001) examined the carbon isotope stratigra- 1983). Offshore shelf mudstones of the Deer Bay Formation 167 108 phy of organic matter preserved in the Wessex Basin. Their record conformably overlie either the shallow-shelf sandstones of the 168 109 extends into the Upper Jurassic System but does not continue Awingak Formation or the Ringnes Formation, its offshore-shelf 169 110 through to the lowest Berriasian Stage. This curve shows a trend mudstone equivalent (Fig. 2). Deer Bay mudstones grade conform- 170 δ13 111 of declining Corg of much greater magnitude than the time- ably into delta-front and fluvial-deltaic sands of the overlying 171 112 equivalent carbonate curve. Isachsen Formation along the axis of Sverdrup Basin (Fig. 2; 172 113 Alternative correlation tools, such as geochemical anomalies Balkwill, 1983; Embry, 1985b), but these formational contacts 173 114 in marine strata, may therefore aid with future correlations of are disconformable on basin margins (Hadlari et al. 2016; 174 115 Jurassic–Cretaceous strata, particularly in high northern lati- Embry & Beauchamp, 2019). The Deer Bay Formation is undivided 175 δ13 – 116 tudes. A new Corg record from Upper Jurassic Lower except for the designation of the c. 40 m sandstone-dominated 176 117 Cretaceous argillaceous strata from two stratigraphic sections Glacier Fiord Member in its upper part on southern Axel 177 Jurassic–Cretaceous carbon isotope stratigraphy 3

Siberia

North China Sverdrup Baltica Rift Basin South Sverdrup China Proto-Amerasia Basin Basin North America Teth yan Trench

Central Atlantic Panthalassic Trench Tethys

Panthalassa South America

PANGEA Late Jurassic

160°0'0"W 140°0'0"W 120°0'0"W 100°0'0"W 80°0'0"W 60°0'0"W 40°0'0"W

Area enlarged

CANADACANADA 80°0'0"N

Axel Heiberg Island

75°0'0"N

400 km

NEOGENE PEARYA TERRANE CANADIAN SHIELD Sands and gravels MID-PROTEROZOIC - SILURIAN Volcanic, deep water and Crystalline basement shallow water strata AREA WITH RIFT BASINS JURASSIC - CRETACEOUS Sandstone and shale

SVERDRUP BASIN CARBONIFEROUS - EOCENE SALT-CORED STRUCTURES LOCATION OF STUDY Carbonate, sandstone Diapirs and shale Buchanan Lake IGNEOUS ROCKS Geodetic Hills FRANKLINIAN MARGIN NEOPROTEROZOIC - DEVONIAN

Subsurface extent of sills inferred from seismic

Fig. 1. (Colour online) Upper: palaeogeographic map of Pangea at c. 150 Ma (Tithonian; modified from Scotese, 2014), with modifications from Amato et al.(2015), Midwinter et al.(2016) and Hadlari et al.(2016, 2017, 2018). Arc and microcontinental terranes that had not yet docked with the North American and Siberian accretionary margins are not illustrated in the palaeo-Pacific Ocean (Panthalassa). Lower: map of the Sverdrup Basin showing location of stratigraphic sections studied at Geodetic Hills and Buchanan Lake, Axel Heiberg Island, Nunavut. After Dewing et al.(2007). 4 JM Galloway et al.

Sverdrup Basin 3. Materials and methods 199 A total of 154 samples were collected every c. 1.5–2 m throughout 200 a 255 m exposure of the Deer Bay Formation at Buchanan Lake 201 (79° 22 0 0.47″ N, 87° 46 0 9.03″ W), and 92 samples were collected 202 every c.3–4 m from a 388 m exposure of the Deer Bay Formation 203 at Geodetics Hills (79° 48 0 57.20″ N, 89° 48 0 20.41″ W), Axel 204 Heiberg Island (Fig. 1). Bivalves, belemnites and ammonites were 205 collected from the Buchanan Lake section; macrofossils were not 206 93-84 Ma ash (4) observed at the Geodetic Hills section. All samples are stored in 207 95-91 Ma gabbro, granitoids, and permanent collections of the Geological Survey of Canada. 208 diabase (3,5,6) Mudstone samples were pre-treated with 10% HCl to remove 209 Bastion Ridge 13 MacDougall Point Mbr 105 Ma ash (1) carbonates, and then δ C analysis of organic carbon was 210 106 Ma ash (2) Invincible Point Mbr 112 Ma ash (2) performed using a Elemental VarioEL Cube Elemental Analyser fol- 211 117 Ma diabase (5) lowed by a trap-and-purge separation and online analysis by continu- 212 Walker Island Mbr 120 Ma diabase (5) 121 Ma diabase (1,5) ous flow with a DeltaPlus Advantage isotope ratio mass spectrometer 213 *Rondon Mbr 125.0 127 Ma gabbro (1) coupled with a ConFlo III interface at the GG Hatch Stable Isotope 214 129.4 Paterson Island Mbr Laboratory, University of Ottawa. Results are reported as ‰ relative 215 132.9 to Vienna Peedee belemnite (V-PDB) and normalized to internal 216 139.8 standards calibrated to the international standards IAEA-CH-6 217 (–10.4‰), NBS-22 (−29.91‰), USGS-40 (−26.24‰)andUSGS- 218 41 (37.76‰). Long-term analytical precision is based on blind analy- 219 sis of the internal standard C-55 (glutamine; −28.53‰)notusedfor 220 calibration, and is routinely better than 0.2‰.FortheBuchananLake 221 dataset (n = 154), 14 quality control duplicate analyses were run (rep- 222 older than 200 Ma = detrital zircon (3) resenting 9% of the samples). For the Geodetic Hills dataset (n 92), 223 12 quality control duplicate analyses were run (12%) (online 224 Supplementary Material available at http://journals.cambridge. 225 org/geo). Average relative percent difference (RPD) was 226 0.13 ± 0.10‰ SD (n = 14) for the Buchanan Lake samples and 227 0.55 ± 0.42‰ SD (n = 12) for the Geodetic Hills material. The 228 Only U-Pb zircon or baddeleyite ages are blind standard C-55 was run in triplicate for each of the three 229 considered for the igneous events, sources are: Mudstone-Siltstone Dominant (1) Evenchick et al. (2015); (2) Herrle et al. (2015); batches to assess accuracy. The average RPD between the measured 230 (3) Omma et al. (2011); (4) Davis et al. (2016); (5) and expected value of the standard was 0.18 ± 0.13‰ SD (n = 9). 231 Dockman et al. (2018); (6) Kingsbury et al. (2018) Organiccarbonisotopiccompositioncanbeinfluencedbythetype 232 and maturity of organic matter; Rock-Eval pyrolysis was therefore 233 Fig. 2. (Colour online) Mesozoic lithostratigraphy of Sverdrup Basin (after Hadlari et al. 2016). The International Chronostratigraphic Chart (ICS) v 2018/08 (Cohen conducted on all samples. Total organic carbon (TOC, wt%) was 234 et al. 2013; updated) is used for absolute ages. Note that intrusive ages should be determined by Rock-Eval 6 (Vinci Technologies, France) pyrolysis 235 younger than the intruded strata, and that detrital zircon ages can be older. as the sum of organic matter during pyrolysis (pyrolysable carbon, 236 100–650°C) and oxidation (residual carbon, 400–850°C) on all sam- 237 ples. Analyses of standard reference materials (IFP 160000, Institut 238 Français du Pétrole; internal 9107 shale standard, Geological 239 178 Heiberg Island, south of the study area (Embry, 1985b). In other Survey of Canada, Calgary; Ardakani et al. 2016) was run every fifth 240 179 localities, this member is absent from shale facies or because sample demonstrating a < 1% relative standard deviation (RSD) for 241 180 of truncation below an intra- or sub-Isachsen unconformity TOC, < 3% RSD for S1 and S2, and 11% RSD for S3. The lower accu- 242 181 (Embry, 1985b). Concretions of various compositions, size and racy for S3 in bulk samples was expected due to poor peak integration 243 182 shape occur throughout the Deer Bay Formation, with large (up and distinction between S3 organic matter and S3 carbonates that may 244 183 to 5 m long) calcitic and sideritic mudstone concretions common occur because of the presence of siderite in standards (Ardakani et al. 245 184 in its lower portion. Glendonites occur in multiple horizons that 2016). Duplicate analyses were conducted for assessment of analytical 246 185 range in thickness from 2 to 20 m throughout the Deer Bay precision. In the Buchanan Lake dataset 22 duplicate samples were 247 186 Formation and are most common in its upper Valanginian portion run, and in the Geodetic Hills dataset two duplicate samples were 248 187 (Kemper, 1975, 1983, 1987;Kemper&Jeletzky,1979;Selmeier& run (online Supplementary Material available at http://journals. 249 188 Grosser, 2011; Grasby et al. 2017). This upper interval is further char- cambridge.org/geo). Samples from both sections comprised the ana- 250 189 acterized by finely laminated siltstones and fissile shales that host lytical batch from which quality control duplicate samples were ran- 251 190 rare thin rusty-weathering calcareous layers and irregularly distrib- domly selected. Average RPD for TOC (wt%) was 16.75 ± 26.93, S1 252 191 uted intervals of calcareous concretions (Heywood, 1957; Kemper, is 13.21 ± 15.34, S2 is 9.56 ± 13.67 and S3 is 11.02 ± 14.30 (n = 24). 253 192 1975; Balkwill, 1983). The biostratigraphic framework of the 193 glendonite-bearing Valanginian succession was described by 194 Kemper (1975, 1977, 1987) based on ammonites in successions 4. Results 254 195 exposed on Amund Ringnes (lower Valanginian) and Ellef 4.a. Macrofossils and age of strata 255 196 Ringnes (upper Valanginian) islands. These strata also contain 197 age-diagnostic marine bivalves, including Buchia keyserlingi Macrofossils were found during this study in the middle and upper 256 198 (Lahusen) and belemnites (Jeletzky, 1973;Kemper,1977). parts of the Deer Bay Formation in the Buchanan Lake section and 257 Jurassic–Cretaceous carbon isotope stratigraphy 5

Fig. 3. All fossils are stored in the National Type Invertebrate Collection of the Geological Survey of Canada. The size of all figures can be judged by the 1 cm scale bar, except (f) which is half the scale of the others and of the scale bar. (a, b) Nikitinoceras kemperi (Jeletzky). GSC 140515 (figured specimen number) from GSC locality C-626163 (GSC curation number), lateral and ventral views. (c) Buchia sp. cf. inflata (Toula). GSC 140516 from GSC locality C-626163. (d) Buchia okensis (Pavlow). GSC 140517 from GSC locality C-626165. (e–g) Borealites (Pseudocraspedites) sp. (e, f) GSC 140518 from GSC locality C-626176, macroconch phragmocone fragment, lateral view and cross-section (at adoral preserved end; size reduced ×1/2) views of septate inner case; and (g) ventral view of part of inner whorl. Another larger phragmocone fragment, with outer shell surface, is septate to a whorl height of at least 7 cm. (h, i) Borealites sp. GSC 140519 from GSC locality C-626176, lateral and ventral views, outer shell surface.

258 were not seen in the Geodetic Hills section. The Buchanan Lake (Fig. 3e, f) and of Borealites s.l. (Fig. 3g, h) occur 143 m below 276 259 macrofossils are, from top of the section to the base: (1) small the Isachsen Formation (C-626176, 15-GTA-A80) and are of 277 260 impressions of Buchia sp., 76 m below the base of the Isachsen early Ryazanian age. Poorly preserved, unidentifiable fossil 278 261 Formation (GSC loc. C-626162); age, undeterminable within the fragments occur in still lower beds and above the carbon isotope 279 262 late Oxfordian – Valanginian interval; (2) several fragments of anomaly. Mikhail Rogov (pers. comm., 2019) has assisted us 280 263 ammonite Nikitinoceras kemperi (Jeletzky) (Fig. 3a, b), bivalve with our identification of the specimens we have assigned to 281 264 Buchia sp. cf. inflata (Toula) (Fig. 3c) and belemnites Acroteuthis? Nikitinoceras and Borealites Klimova. 282 265 and Cylindroteuthis? (C-626163) occur 75.5 m below the base of The Borealites specimens are the lowest in our collections and 283 266 the Isachsen Formation; age, early Valanginian; (3) numerous provide a youngest age limit for the lower negative δ13C anomaly 284 267 impressions of Buchia okensis (Pavlow) or B. sp. aff. okensis (sensu at Buchanan Lake. A previous fossil collection from perhaps the 285 268 Jeletzky 1964, 1984) occur 77 m below the base of the Isachsen same level as our Borealites fauna and in a similarly prolific horizon 286 269 Formation (C-626165; Fig. 3d);age,earlyRyazanian(i.e. (GSC loc. 26171, 316 feet = 96.3 m above the base of the Deer Bay 287 270 Berriasian, but probably not earliest Berriasian equivalent); Formation according to Souther, 1963, p. 438) contains ammonites 288 271 (4) fragments of bivalves 125 m below the base of the Isachsen closely similar to ours. They were initially reported as Valanginian 289 272 Formation including Buchia sp. aff. okensis, Mclearnia?, Oxytoma? (Frebold, in Souther, 1963) but were figured, together with associ- 290 273 and Meleagrinella?, with unidentified gastropods and the belemnite ated Buchia okensis, as lower Berriasian Tollia (Subcraspedites) 291 274 Acroteuthis (C-626172); of probable early Ryazanian age; and (5) sev- aff. suprasubditus (Bogoslovsky) by Jeletzky (1964, plate I–III), 292 275 eral fragments of relatively large Borealites (Pseudocraspedites) as Craspedites (Subcraspedites) by Jeletzky (1973, plate 6, from 293 6 JM Galloway et al.

“ — ” δ13 ‰ 294 136.6 140 metres above base of the formation, which we take A negative Corg excursion, with a magnitude of c.4 and 354 295 to be mistaken) and as Tollia (Subcraspedites) aff. suprasubditus by reaching minimum values of −29.8‰ at Buchanan Lake and 355 296 Jeletzky (1984,p.223,atthe“95m level”). Jeletzky (1973, 1984) also −30.7‰ at Geodetic Hills, is observed within the lower Deer Bay 356 297 reported similar faunas at higher levels, but acknowledged confusion Formation. All of the recovered macrofossils from the Buchanan 357 298 about their stratigraphic levels and noted re-assignment of the Lake section occur stratigraphically above the negative carbon 358 299 ammonites to Praetollia (Pseudocraspedites) and P. (Praetollia), isotope excursion, dating the overlying strata as late Volgian or 359 300 now included in Borealites (Wright et al. 1996), and thought Ryazanian in age and younger in the Buchanan Lake section. 360 δ13 301 Craspedites (Taimyroceras?) canadensis Jeletzky to occur below them. This negative Corg excursion is followed by a return to less 361 302 We did not find fossils to control the older age limit for the neg- negative values of c. −27‰. A small negative shift of c. 1.5‰ occurs 362 303 ative δ13Canomaly in the sections studied. However, Jeletzky in strata that are likely late middle Volgian or early late Volgian in 363 304 (1984, p. 221, GSC loc. 26156) reported generically indeterminate age, and this is followed by an interval of generally increasing 364 305 dorsoplanitid ammonites and large Buchia fischeriana (d’Orbigny) values across the interpreted Jurassic–Cretaceous boundary until 365 306 from “an 8 m bed commencing 31 m” above the base of the Deer the upper Valanginian part of the Deer Bay Formation. A positive 366 307 Bay Formation along the Awingak River, that is, near or within carbon isotope excursion is evident in its upper part in both 367 308 our Buchanan Lake section. The collection has not been relocated sections, with a magnitude of c. 1.5‰ (interpreted here as the 368 309 but, if the fossils are correctly determined, they imply a middle, Weissert Event; Erba et al. 2004). Carbon-13 isotope ratios reach 369 310 perhaps early middle, Volgian age for this interval, which would maximum values of −24.6‰ at Buchanan Lake and −24.9‰ at 370 311 fall at about the maximum depletion point of the δ13C curve. Geodetic Hills during this event (Fig. 4). 371 312 Dorsoplanitid ammonites and various associated Buchia species 313 including B. fischeriana (d’Orbigny) are widespread on nearby 4.c. Rock-Eval 6 pyrolysis 372 314 Ellesmere Island (Jeletzky, 1984; Schneider et al. 2019) and indicate 315 a middle Volgian age for the lower Deer Bay Formation and its TOC measured by Rock-Eval 6 pyrolysis on samples of the 373 316 initial transgression event throughout eastern Sverdrup Basin. Buchanan Lake section (median TOC 1.16 wt%; range 0.09– 374 317 Jeletzky (1984, p. 223) also reported other unidentifiable ammon- 4.36 wt%; n = 154) and Geodetic Hills section (median TOC 375 318 ites and bivalves in lower parts of the Buchanan Lake succession. 1.48 wt%; range 0.48–5.87 wt%, n = 92) are typical for high- 376 319 Two reports of Buchia mosquensis (von Buch) from Amund latitude Upper Jurassic and Lower Cretaceous mudrock succes- 377 320 Ringnes Island (Jeletzky, in Balkwill et al. 1977, p. 1136) may be sions (cf. Hammer et al. 2011). The TOC range indicates poor 378 321 early Volgian, rare indicators of this interval in the more axial por- to excellent source rock (see online Supplementary Material avail- 379 322 tion of the basin, or they may be late Kimmeridgian in age. able at http://journals.cambridge.org/geo). Thermal alteration of 380 323 Stratigraphically close juxtaposition of early Ryazanian and early material indicated by Tmax (the temperature corresponding to 381 324 Valanginian fossils supports the interpretation of a strongly con- maximum S2 during pyrolysis) ranges from 427 to 499°C in sam- 382 325 densed interval or basinal disconformity at the Buchanan Lake local- ples collected from the Buchanan Lake section and from 436 to 383 326 ity near the depocentre of the Sverdrup Basin. The apparent absence 448°C in samples collected from the Geodetic Hills section; the 384 327 of diagnostic fossils of late Berriasian age across the Sverdrup Basin majority of samples from both sections are in the oil window. 385 328 has been used previously to suggest a widespread sub-Valanginian The S2 values (amount of hydrocarbons generated by thermal 386 329 disconformity (Jeletzky, 1973;Kemper,1975;Embry,2011). cracking of organic matter) and S3 (the amount of CO2 released 387 330 The Valanginian strata in the northern and eastern parts of during thermal breakdown of kerogen) range from 0.15 to 388 331 Sverdrup Basin, as across the Arctic, are replete with glendonites 2.42 mg HC/g and 0.27–2.41 mg HC/g at Buchanan Lake, respec- 389 332 (Kemper & Schmitz, 1975; Grasby et al. 2017; Rogov et al. tively (n = 154). S2 and S3 range from 0.22 to 6.13 mg HC/g TOC 390 333 2017), but minor occurrences of ‘stellate nodules’ or ‘carbonate and 0.13–1.27 mg HG/g TOC, respectively, at Geodetic Hills 391 334 crystal rosettes’ have been reported in upper Oxfordian or lower (n = 92). The hydrogen index (HI = S2/g TOC) and oxygen index 392 335 Kimmeridgian strata to Berriasian strata in the western Sverdrup (OI = S3/g TOC) suggest that organic matter is predominantly 393 336 Basin (Poulton, 1994, p. 183), northern Yukon (Poulton, 1996, Type III kerogen at Buchanan Lake and a mixture of Type II 394 337 p. 285), and the Northwest Territories (Mountjoy & Procter, 1969). and III kerogen in the Geodetic Hills samples (Fig. 5). The 395 338 While their appearance in only the upper 104 m of the Buchanan Geodetic Hills locality was more distal and in a deeper part of 396 339 Lake section of the Deer Bay Formation at Buchanan Lake might the basin during latest Jurassic – earliest Cretaceous times than 397 340 suggest pre-Valanginian ages for the underlying strata, the interval the Buchanan Lake locality, and this is reflected in the higher pro- 398 341 with glendonites overlap with strata containing Buchia okensis,or portion of Type III kerogen at Buchanan Lake. Samples with very 399 342 B. cf. and aff. okensis, collected in this study and reported by low TOC resulted in HI or OI values > 200 (Buchanan Lake A22, 400 343 Jeletzky (1984, p. 221, 223). They may indicate an age for the asso- A43, A56, A65, A76, A82, A121 and A124) and are not plotted on 401 344 ciated glendonites as old as early Ryazanian, although it is possible the Van Krevelen diagram (Fig. 5) or stratigraphically (Fig. 6). 402 345 that they developed within the lower Ryazanian strata exposed on Stratigraphic trends in TOC, HI and OI are shown in Figure 6. 403 346 the sea floor during Valanginian time. In both the Buchanan Lake and Geodetic Hills sections, TOC 404 increases near the top of the Deer Bay Formation. Trends in HI 405 and OI are also similar between the two sections, with marginally 406 347 4.b. Carbon isotopes higher HI values near the base of the Deer Bay Formation. 407 δ13 − − ‰ ’ 348 Measured Corg values fall within a range of 30.7 to 24.6 Spearman s rank correlation was conducted to evaluate rela- 408 = = δ13 349 (V-PDB) for both sections (n 92 Geodetic Hills section; n 154 tionships between Corg and organic matter source and maturity. 409 δ13 350 Buchanan Lake section; see online Supplementary Material available In both sections, Corg is significantly related to TOC (Buchanan 410 δ13 = < = 351 at http://journals.cambridge.org/geo). Two outliers (A124, A21) in Lake Corg:TOC rs 0.3, P 0.001, n 146 with outliers A22, 411 352 the Buchanan Lake dataset were removed. Without further evi- A43, A56, A65, A76, A82, A121 and A124 removed; Geodetic 412 δ13 = < = 353 dence, we disregard these values as outliers due to contamination. Hills Corg:TOC rs 0.43, P 0.001, n 92). In the Buchanan 413 Jurassic–Cretaceous carbon isotope stratigraphy 7

ICS Boreal ICS Boreal

)

e Buchanan Lake Geodetic Hills

tag

eight (m

ub-s

tage tage

tage ub-s

eries

ormation

ithology

Stage

Lithology

Series Height (m)

Formation

S S

S tag S

H –30 –28 –26 –24 –22 F –30 –28 –26 –24

370 ISACHSEN 0 ISACHSEN 360 350 10 340 20 330 320 30

inian 310

40 lang 300

alanginian

inian inian

Weissert Event Va V Weissert Event 290 50

Valang 280 Valang

60 270 s

aceous 260

70 retaceous C-626162 250 C-626163

r Cretaceou 80 C-626165 Lower Cret 240 Lower C 230

Lowe 90 220

Upper 210 er 100

200 Upp 110 190

DEER BAY

yazanian

Ryazanian

Berriasian

rriasian 120 180 R

Y C-626172 170 Low

130 Lower 160

DEER BA C-626176 150 140 140 150 130

pper

r 120 U 160

Uppe 110 170 100

90 c 180 80

Volgian

70 Middle

olgian 190 r Jurassi

thonian

60 Ti

Uppe

200 er Jurassic

VOICE Tithoni 50 VOICE

Upp 210 40 30 220 20

Lower 10 230 er Midd 0

Low 240 cl si vf f m c –30 –28 –26 –24 δ13

AK C[‰] 250 org Kimm. Kimm.

AWINGAK 260 AWING cl si vf f m c –30 –28 –26 –24 –22 δ13 C[‰]org

KEY ammonite ironstone beds bivalve C-626162- Buchia sp. impressions (late Oxfordian to Valanginian ) c sandstone-conglomerate C-626163 -Nikitinoceras kemperi(Jeletzky), Buchia sp. cf. inﬂata (Toula), belemnitesAcroteuthis ? and Cylindroteuthis ? ( early Valanginian) belemnite C-626165 - numerous impressions ofBuchia okensis (Pavlow)orB . sp. aff. okensis ( sensu Jeletzky 1964, 1984) (early Berriasian) vf-m sandstone wood C-626172 - fragments of bivalves includingBuchia sp. aff. okensis , Mclearnia ?, Oxytoma ?, and Meleagrinll a? with unidentiﬁable gastropods and the belemnite Acroteuthis (early Berriasian) siltstone glendonite horizon C-616176 - fragments ofSubcraspedites sp. ( early Berriasian) organic-rich mudstone bioturbation

13 Fig. 4. (Colour online) Buchanan Lake and Geodetic Hills stratigraphy and δ Corg. The datum for measurement of the Buchanan Lake section was the Isachsen – Deer Bay formational contact; the datum for measurement of the Geodetic Hills section was the Awingak – Deer Bay formation contact. The International Chronostratigraphic Chart (ICS) v 2018/08 (Cohen et al. 2013; updated) and Boreal Stage and Sub-stage (after Shurygin & Dzyuba et al. 2015) are shown.

δ13 < 414 Lake samples, Corg is also significantly (P 0.001) correlated category as Type III on a van Krevelen-type plot (Tyson, 1995); a 429 = − = = − 415 with S1 (rs 0.34), S3 (rs 0.33) and HI (rs 0.3), but these change in organic matter source from dominantly terrestrial 430 416 relationships are insignificant in the Geodetic Hills samples. In (Type III) to marine (Type II) may therefore not be recognizable 431 δ13 – 417 both sections the relationships between Corg, Tmax and S2 are in an HI OI cross-plot/van Krevelen-type diagram if the organic 432 418 insignificant (P > 0.05). While statistically significant, the relation- matter became highly thermally degraded. However, the reproduc- 433 δ13 419 ships between Corg and organic matter parameters (TOC in tion of the carbon isotope curve in two stratigraphic sections, and 434 420 both sections, S1 and S3 for Buchanan Lake) are weak as shown consistency with curves from other Arctic areas, lends confidence 435 421 by the low values of rs, suggesting that the influence of organic mat- to the hypothesis that the signals are not overly influenced by 436 δ13 422 ter source, diagenesis and thermal maturation on the Corg values changes in organic matter source. 437 – 423 is limited. The high thermal maturity (Tmax, 427 499°C Buchanan 424 Lake and 436–448°C in Geodetic Hills) of the material could com- 5. Discussion 438 425 plicate interpretations of the Rock Eval pyrolysis data. Thermal 13 426 degradation may disguise a change in organic matter course as The δ Corg and TOC curves across Upper Jurassic – Lower 439 427 heating pushes kerogen types to low HI (Hunt, 1996). Degraded, oxi- Cretaceous strata from the Buchanan Lake and Geodetic Hills 440 428 dized, residual ‘dry-gas-type’ kerogen (Type IV) falls into the same sections show similar trends, and this permits confidence in 441 8 JM Galloway et al.

1000 excursion in DSDP site 534A in the ?Tithonian strata (western cen- 472 TYPE I KEROGEN Oil Prone tral Atlantic; Katz et al. 2005). Hammer et al.(2012) conclude that 473 δ13 900 the lower middle Volgian negative excursion seen in their Corg 474 Buchanan Lake record from Spitsbergen is consistent with carbonate records from 475 Geodetic Hills elsewhere in the Boreal and High Boreal realms, the central 476 800 Atlantic and, ‘to a lesser degree’ with the western Tethys. 477 Koevoets et al.(2016) also examined the organic carbon isotope 478 700 record preserved in the Upper Jurassic – Lower Cretaceous 479 Agardhfjellet Formation of central Spitsbergen. A marked negative 480 ‰ 600 excursion of c.4 is measured and dated as middle Volgian. 481 TYPE II KEROGEN Koevoets et al.(2016) argue that the VOICE is also recognized 482 Oil Prone δ13 in Ccarb curves from the Russian Platform (Price & Rogov, 483 500 2009). Zakharov et al.(2014) document an irregular but overall 484 δ13 Ž decline in Ccarb (as determined in belemnite rostra; ák et al. 485 400 2011) throughout Upper Jurassic strata from the Nordvik section 486 – that they relate to a gradual increase in CO2 in the atmosphere 487 ocean system, and that may have led to warming based on coeval 488 Hydrogen Index (mg HC/g TOC) (mg HC/g Hydrogen Index 300 Mixed Type II / III KEROGEN changes in a belemnite oxygen isotope record. They also present a 489 Oil / Gas Prone δ13 ‰ Corg record that shows a negative excursion of c.3 within the 490 200 Exoticus Zone and extending into the basal part of the [Craspedites] 491 Okensis Zone (late middle Volgian – early late Volgian). Trends 492 δ13 100 observed in the Corg at this locality are not observed in the 493 δ13 Ž TYPE III KEROGEN Ccarb of belemnite rostra from the same section ( ák et al. 494 Gas Prone 2011; Zakharov et al. 2014). Morgans-Bell et al.(2001) examined 495 0 – 0 20 40 60 80 100 120 140 160 180 200 the Kimmeridgian Berriasian interval of the Wessex Basin from 496

Oxygen Index (mg CO2/g TOC) Dorset, UK. A prominent middle Tithonian negative excursion of 497 13 δ Corg is not apparent in their record, although a short-lived excur- 498 Fig. 5. (Colour online) Van Krevelen diagram of hydrogen index v. oxygen index from sion may be related to the VOICE (Turner et al. 2018). Turner et al. 499 the Buchanan Lake and Geodetic Hills sections. δ13 (2018) also interpret a short-lived decline in Corg values in the 500 lower middle Volgian Pallasioides Zone in Core 6406/12-2 from 501 δ13 442 extrapolating fossil age control from the Buchanan Lake section the Norwegian Sea as the VOICE. The composite Ccarb curve 502 443 to the Geodetic Hills section. A marked negative excursion of up from the base of the Kimmeridgian to the base of the Valanginian 503 444 to −4‰,reachingto−30‰ (Fig. 4), occurs in probable middle sections, based mostly on Tethyan data, shows no major negative 504 445 Volgian strata of the lower Deer Bay Formation. This is followed carbon isotope events (Fig. 7;Priceet al. 2016). 505 δ13 446 by a return to less negative values near −27‰,abriefnegative Decoupling of high-latitude Corg records and Tethyan 506 447 excursion of an additional c.1.0–1.5‰ that may be late records, the latter based mostly on carbonates, suggests either that 507 448 Volgian in age, an interval of generally increasing values and then pools of organic carbon and dissolved inorganic carbon were effec- 508 449 a relatively positive carbon isotope excursion in strata of tively decoupled during this time, or that there was latitudinal 509 450 Valanginian age of the upper part of the Deer Bay Formation. decoupling between the Arctic and Tethyan seas. Typically, covari- 510 δ13 δ13 ant marine Ccarb and Corg are seen and interpreted as evidence 511 that both carbonate and organic matter were originally produced in 512 451 5.a. VOICE the surface waters of the ocean and retained their original δ13C com- 513 δ13 452 Trends in Corg fromtheBuchananLakeandGeodeticHills position (e.g. Kump & Arthur, 1999; Meyer et al. 2013). Coupled ter- 514 453 sections of the Deer Bay Formation are consistent with other restrial organic (e.g. derived from fossil wood or charcoal) and 515 δ13 – 454 Corg curves spanning the Jurassic Cretaceous boundary carbonate records suggest strong coupling of the ocean–atmosphere 516 455 interval in the High Arctic (Hammer et al. 2012;Zakharov system (e.g. Gröcke et al. 2005;Vickerset al. 2016), whereas 517 δ13 δ13 456 et al. 2014;Koevoetset al. 2016;Fig.7). In those records, rela- decoupled Ccarb and Corg records have been interpreted as 518 457 tively positive carbon isotope values of c. −28‰ are observed in evidence for diagenetic alteration (Meyer et al. 2013;Hanet al. 519 458 the Kimmeridgian and lowest Volgian strata and are followed by 2018). In these latter examples, a large negative excursion in 520 – ‰ δ13 459 an up to 4 6 morenegativeexcursioninthemiddleVolgian Ccarb is typically not accompanied by a large response in the 521 δ13 460 strata. This event is followed by a return to relatively more Corg record (e.g. Fike et al. 2006). Alternatively, Bodin et al. 522 461 positive values during late Volgian and Ryazanian time. (2016) have recently suggested lithological control on decoupling 523 δ13 δ13 462 Hammer et al.(2012) term the negative excursion they document between Ccarb and Corg records during Early Jurassic time, 524 δ13 463 in lower middle Volgian strata of the Slottsmøya Member whereby Ccarb signatures were affected by regional variation in 525 464 (Agardhfjellet Formation) the Volgian Isotopic Carbon Excursion carbonate composition. As the Arctic middle Volgian negative event 526 465 (VOICE). Hammer et al.(2012) correlate the VOICE with a lower is observed in organic carbon records from Canada (this study), 527 δ13 466 middle Volgian broad minimum in the Ccarb record from belemn- Spitsbergen and Siberia (Fig. 7), it is unlikely that diagenesis or 528 467 ite rostra of Žák et al.(2011) that spans the Oxfordian–Ryazanian regional differences in the composition of bulk organic carbon are 529 468 interval at the Nordvik Peninsula, Siberia. Hammer et al.(2012) also significant factors in explaining the contrast with its absence from 530 δ13 469 relate the VOICE to a negative excursion in Ccarb from Helmsdale, lower-latitude areas. Instead, the absence of the negative excursion 531 470 Scotland in the Sub-boreal lower middle Volgian Rotunda–Fittoni from lower-latitude carbonate records may be explained by decou- 532 δ13 471 ammonite zone (Nunn & Price, 2010) and a negative Ccarb pling of high-northern-latitude regions from the global carbon pool. 533

Jurassic–Cretaceous carbon isotope stratigraphy 9

ge ub-s Sta wer Lo ower L id e Middl er Upp r Uppe

Valanginian

Stage Ryazanian olgian Vo

Boreal Stage Berriasian Tithonian nginian Vala

Series

taceous Lower Cre Lower pper Jurassic pper U

ICS

OC)

T 50 100 150

O/g CO

g g

m (

OI TOC) curves are shown in yellow. The International Chronostratigraphic

50 100 150 200

HC/g

(mg org HI C 13 δ 4.0 8.0 Weissert Event Weissert

Geodetic Hills

C( VOICE TO c

vf f m

y Litholog

cl si

(m) Height Height

90 80 70 60 50 40 30 20 10 110 370 360 350 340 330 320 310 300 290 280 270 260 250 240 230 220 210 200 190 180 170 160 150 140 130 120 100

ion Format K WINGA AW DEER BAY DEER

ISACHSEN

ge ub-s Sta

Lower iddle oe M Lower pper U

Upper ) are shown.

inian Valang

nian Ryaza Volgian Stage

Kimm.

Boreal

Stage Berriasian onian Tith Valanginian 2015

Kimm.

s Serie

r Cretaceous r Lowe Cretaceous Lower Lower

Jurassic Upper sian Berria Tithonian Valanginian Acroteuthis Cylindroteuthis ICS et al 150 B okensis sensu

or . sp. aff. ( Jeletzky 1964, 1984)

OC)

/g 50 100 200 2

I O (mg CO (mg O Buchia okensis Mclearnia Oxytoma Meleagrinll Acroteuthis 150 sp. ( Buchia okensis late Oxfordian to Valanginian (Jeletzky), sp. cf. (Toula), early Subcraspedites

50 100

OC)

gT / Nikitinoceras kemperi Buchia inﬂata

- Buchia HI (mg HC (mg HI C-626162C-626163 -C-626165 - numerous impressions ofC-626172 - fragments of bivalves including sp. impressions (C-616176 - fragments of sp. aff. (Pavlow) , early Berriasian) ?, ) ?, and with unidentiﬁable gastropods and the belemnite a? belemnites (early Berriasian) (early Berriasian) ? and ? ( Valanginian) Weissert Event Weissert 4.0 8.0

VOICE TOC (wt. %) (wt. TOC ; updated) and Boreal Stage and Sub-stage (after Shurygin & Dzyuba Buchanan Lake wood glendonite horizon ammonite bioturbation bivalve belemnite 2013 c . C-626162 C-626163 C-626165 C-626172 C-626176

et al

vf f m Lithology

cl si Height (m) Height 0 10 20 30 40 50 60 70 80 90

240 250 260 100 110 120 130 140 150 160 170 180 190 200 210 220 230

c sandstone-conglomerate vf-m sandstone siltstone organic-rich mudstone ironstone beds

Formation WINGA A K Y EER BA EER D KEY ISACHSEN (Colour online) Stratigraphic trends in Rock Eval parameters TOC, HI and OI from the Buchanan Lake and Geodetic Hills sections. Events recognized in Fig. 6. Chart (ICS) v 2018/08 (Cohen 10

International Festningen,Svalbard Chronostratigraphic (Vickers et al. 2019) Sub-Boreal 13 δ C(‰)organic Chart (ICS) Polarity Ammonite -26 -25 -24 -23 -22 -21 Boreal Boreal (Siberian) Period Stage Chron zones Stages Ammonite zones DSDP Site 416 M5 DSDP Site 535 (Wortmann and Weissert 2000) Discofalcatus

n (Cotillion and Rio 1984) δ13 Tethyan Carbonate Composite 13 C(‰)organic Versicolor M6 δ C(‰)organic Rio Corna, Italy (Lini et al. 1992) -26 -25 -24 (Price et al. 2016) M7 Gottschei -28 -27 -26 δ13C(‰) Wollaston Forland M8 Staffi organic -28 -27 -26 -25 -24 (Pauly et al. 2013) Geodetic Hills Buchanan Lake 13 M9 Aegocriceras δ C(‰)organic ()This study ()This study -26 -25 -24 -23 -22 -21 Polyptychloides M10 Regale

Hauterivia Noricum Amblygonium Boyarkensis M10N Paucinodum

Tuberculatum ginian Bidichotomus Dichotomites M11

Valan Prodichotomites Weissert Event M11A Ramulicosta M12 Polyptychites 1406/3-4, North Sea M12A Platylenticeras (Turner et al. 2018) Astieriptychus

Valanginian M13 13 δ C(‰)organic -32 -31 -30 -29 -28 Albidum Quadriﬁdus Cretaceous M14 33/12, North Sea Klimovskiensis M15 Stenomphalus (Turner et al. 2018) 13 Tolli δ C(‰)organic -32 -31 -30 -29 Mesezhnikowi Icenii Analogus M16 anian Boreholes DH2 and DH5R, Svalbard Kochi Kochi (Koevoets et al. 2016) δ13C(‰) organic Ryaz -31 -30 -29 -28 -27 Sibericus Runctoni M17 Lower Upper

Berriasian Janusfjellet, Svalbard (Hammer et al. 2012) 13 Chetae M18 Lamplughi δ C(‰)organic -31 -30 -29 -28

Preplicomphalus

Upper M19 Taimyrensis Primitivus Oppressus Okensis Anguiformis Exoticus M20 Kerberus Okusensis Variabilis Excentricus Glaucolithus VOICE Albani an Maximus Fittoni Ilovaiskyi Rotunda

Middle Volgi Iatriensis M21 Pallasioides

Tithonian Pectinatus Pectinatus -30 -29 -28 Hudlestoni 13 δ C(‰)organic Wheatleyensis Subcrassum Jurassic M22 6406/12-2, Norwegian Sea -29 -28 -27 -26 Scitulus 13 Lower (Turner et al.201)9 δ C(‰)organic Elgens Magnum Nordvik, Siberia M22A Autissiodorensis (Zakharov et al. 2014) -30 -29 -28 -27 -26 13 -30 -29 -28 -27 -26 δ C(‰)organic M23 -30 -29 -28 -27 -26 Eudoxus 13 δ C(‰)organic 13 δ C(‰)organic

Kimm. 7120/2-3, Barents Sea Kimm. M24 Mutabilis (Turner et al.2091) -29 -28 -27 13 δ C(‰)organic -28 -27 -26 -25 -24 0.0 1.0 2.0 3.0 4.0 5.0 13 δ13C (‰) δ C(‰)organic 16/1-14, North Sea carbonate Dorset, UK (Turner et al. 2019) (Morgans-Bell et al. 2001)

Fig. 7. (Colour online) Summary of published data for Late Jurassic – Early Cretaceous organic carbon isotope data from Atlantic and Tethyan sections, the global stack of Tethyan carbonate records and the new Arctic curves. Sub-boreal ammonite zones from Mutterlose et al.(2014) and Turner et al.(2018). Boreal (Siberian) ammonite zones after Zakharov et al.(1997), Baraboshkin (2004) and Shurygin & Dzyuba (2015). The International Chronostratigraphic Chart (ICS) v. 2018/08 (Cohen et al. 2013; updated) and Boreal Stage and Sub-stage (after Shurygin & Dzyuba et al. 2015) are shown. MGalloway JM tal et . Jurassic–Cretaceous carbon isotope stratigraphy 11

534 The organic carbon isotope record is influenced by a number of early thermal steepening of the geothermal gradient and/or 596 535 environmental factors (Kump & Arthur, 1999) and, as such, can be tectonic activity associated with the initial phases of High Arctic 597 536 difficult to interpret (Jenkyns et al. 2002). Organic carbon isotope Large Igneous Province (HALIP) activity (Maher, 2001; Hammer 598 537 composition is strongly controlled by the type of organic matter et al. 2011). HALIP, a major magmatic event, may therefore be 599 538 (marine v. terrestrial) and, therefore, by both local and regional relevant to the VOICE carbon isotope record, although the currently 600 539 variables such as sea level, productivity and climate. Burial rate known ages of the HALIP intrusives are younger than those of the 601 540 of organic matter enriched in 12C is also important, as more heavy VOICE, ranging from 95–91 Ma to c. 127 Ma (Omma et al. 2011; 602 541 carbon would remain in the global carbon pool. This process leads Evenchick et al. 2015;Dockmanet al. 2018;Kingsburyet al. 2018; 603 542 to a positive isotopic shift in both carbonates and organic matter. Fig. 2). Seep carbonates are also found in the Janusfjellet section 604 543 A decline in the δ13C value involves a relative increase in 12C in the of Spitsbergen; these are of late Volgian – earliest Valanginian age 605 544 oceanic carbon reservoir (Price & Gröcke, 2002). This could occur (Wierzbowski et al. 2011), and are therefore younger than the carbon 606 545 through a combination of mechanisms, including decreased car- isotope excursion documented in Sverdrup Basin. 607 546 bon burial rate as a result of decreased preservation (e.g. deep basin Eustatic sea-level fall was invoked by Nunn & Price (2010)to 608 δ13 547 ventilation), decreased sea-surface productivity (Weissert & explain a general trend towards more negative Ccarb values in 609 548 Channell, 1989; Weissert & Erba, 2004), increased flux of 12C into their belemnite record from Helmsdale, Scotland, in the 610 549 surface waters by upwelling of 12C-rich bottom waters (Küspert, Tithonian Stage. A sea-level fall could result in enhanced release 611 550 1982) or intensified weathering and riverine input of dissolved of 12C from weathering, erosion and oxidation of organic-rich 612 551 inorganic carbon (Weissert & Mohr, 1996). A geological rapid sub-aerially exposed rock (Voigt & Hilbrecht, 1997; Price & 613 552 release of 12C into the atmosphere associated with volcanism, Gröcke, 2002) as well as compositional deviation away from 614 553 methane release from dissociation of gas hydrates or combustion open-marine δ13C values in relatively isolated epeiric seas (e.g. 615 554 of organic matter associated with emplacement of large igneous Holmden et al. 1998; Immenhauser et al. 2003). ‘Local’ depletion 616 13 555 bodies are other mechanisms that can cause a negative excursion in C is caused by isotopically light CO2 input from respiration of 617 556 in δ13C (Dickens et al. 1995; Hesselbo et al. 2000; Padden et al. marine organisms, as well as oxidation of terrestrial organic matter 618 557 2001; Schröder-Adams et al. 2019). and input of isotopically light riverine dissolved inorganic carbon 619 558 A geologically sudden increase in volcanism could potentially (Patterson & Walter, 1994; Holmden et al. 1998). Progressive 620 13 559 explain the large negative δ Corg values seen in the middle oxidation of organic matter to CO2 (‘sea water aging’, Holmden 621 13 560 Volgian Arctic records and an absence from δ Ccarb records et al. 1998), which then forms dominantly bicarbonate in sea water, 622 561 (Price et al. 2016). As modelled by Kump & Arthur (1999), an is greatest during a long residence time of water masses in shallow, 623 562 increase in volcanism sufficient to perturb atmospheric pCO2 poorly circulated settings (Patterson & Walter, 1994). The uptake 624 563 levels could drive down the carbon isotopic value in the ocean– of this bicarbonate in carbonates or marine organic matter in 625 δ13 564 atmosphere system. However, any trend in Ccarb could be rela- isotopic equilibrium with dissolved inorganic carbon results in 626 565 tively quickly countered as burial of anomalously depleted organic carbonate or organic materials with depleted δ13C values. 627 566 matter may overcompensate for additional input of depleted The Deer Bay Formation is the result of regional marine trans- 628 567 volcanic CO2 (Kump & Arthur, 1999). Notwithstanding this, the gression that was preceded by a sea-level lowstand in Sverdrup 629 568 Shatsky Rise, a vast shield volcano with a surface area of Basin (Embry & Beauchamp, 2019), with restricted marine con- 630 569 c. 480 000 km2, formed in the NW Pacific Ocean at about the nections and a large number of restricted environments (e.g. 631 570 Jurassic–Cretaceous boundary (Sager et al. 2013). Recent 40Ar/ Ziegler, 1988; Hardenbol et al. 1998). The Deer Bay rift climax 632 571 39Ar age determinations of basaltic lava samples from Tamu of the Sverdrup Basin occurred during this time and basin sub- 633 572 Massif, the oldest and largest edifice of the submarine Shatsky sidence was associated with contemporaneous rift margin uplift. 634 573 Rise, provide an age of c. 144 Ma (Geldmacher et al. 2014), similar Due to low global sea-level during the Tithonian Age, the only 635 574 to the widely used c. 145 Ma 40Ar/39Ar minimum age for the direct connection between the North Atlantic and the Sverdrup 636 575 Jurassic–Cretaceous boundary proposed by Mahoney et al.(2005). Basin was the narrow and shallow Norwegian–Greenland Seaway, 637 576 However, new U–Pb ages from Argentina and Mexico suggest that which was more than 1500 km long and only 200–300 km wide 638 577 the numerical age of the Jurassic–Cretaceous boundary may lie (Ziegler, 1988;Dore,1991). Connections between the western 639 578 between 140.7 and 140.9 Ma; this evidence would place an age of Sverdrup Basin and Panthalassa were similarly constricted prior to 640 579 c. 145 Ma (the current ICS age for the base of the Berriasian stage) rift-opening of the Canada Basin in the Hauterivian Age (e.g. 641 580 into the middle of the Tithonian age (Lena et al. 2019), whether the Embry, 1991). The Sverdrup Basin and other high-latitude Boreal 642 581 base of the Tithonian is of age 152.1 Ma (Cohen et al. 2013;updated basins (e.g. Dypvik & Zakharov, 2012) could have experienced 643 582 2018/08) or 148 Ma (Lena et al. 2019) or somewhere between. Sub- compositional evolution away from global marine δ13C values dur- 644 583 aerial volcanism and summit weathering and/or erosion of the ing middle Volgian time, but effectively became re-coupled by 645 584 emergent phase of the Shatsky Rise is thought to have occurred as Valanginian time due to global sea-level rise. The hypothesis of 646 585 early as during the Valanginian age (Yasuhara et al. 2017), suggesting restriction of Sverdrup Basin water masses during Volgian time, 647 586 possible further complications in the interpretation of significance of followed by more open circulation during Valanginian time, is 648 587 the age of the sills associated with the Shatsky Rise. The ages of the consistent with global sea-level fluctuations (Haq et al. 2017), 649 588 base of the Tithonian and Berriasian stages are yet to be established and may be supported by the greater number of known ammonite 650 589 (e.g. Ogg & Hinnov, 2012; Aguirre-Urreta et al. 2015). occurrences in the Valanginian part of the Deer Bay Formation, 651 590 Hydrocarbon seeps are widely distributed in Upper Jurassic and the greater similarity of inter-marine faunas between the 652 591 and Jurassic–Cretaceous boundary beds in Spitsbergen. Seeps Arctic and Europe at this time. Embry (1991, p. 408, 414) noted 653 592 characterized by authigenic carbonates in the uppermost three transgressive–regressive cycles during the Kimmeridgian – 654 593 Jurassic Slottsmøya Member of the Agardhfjellet Formation in late Berriasian interval in the Sverdrup Basin, a gradual decline 655 594 the Sassenfjorden area of central Spitsbergen (Hammer et al. in sediment supply and a shift of the basin axis to the west, with 656 595 2011) may be related to the release of gas hydrates (Kiel, 2009), sandstones occupying the basin margins. Sea-level rise during 657 12 JM Galloway et al.

658 Early Cretaceous time would have increased ventilation of the University. JMG received funding from the AIAS-COFUND II fellowship 716 659 incipient Arctic Ocean and thus coupled the carbon dynamics programme that is supported by the Marie Skłodowska-Curie actions under 717 660 of the Sverdrup Basin to the open-marine system. This interpreta- the European Union’s Horizon 2020 (grant agreement no. 754513) and the 718 661 tion would imply a similar oceanographic restriction to explain Aarhus University Research Foundation. We are grateful to Dr Keith Dewing 719 662 the middle Volgian negative δ13C events in Svalbard and Siberia. for project management and Dr Lisa Neville (Calgary, AB) and Pilipoosie 720 Iqaluk (Hamlet of Resolute Bay, NU) for assistance with sample collection and 721 663 It might also partly explain and support the ongoing difficulties field logistics. We acknowledge the logistics support provided by the Polar 722 664 with correlating Tethyan and Boreal marine faunas, especially Continental Shelf Program (NRCan) and UHL Helicopters (Pilot Lorne Pike). 723 665 if exacerbated by concurrent climate-influenced biogeographic We are grateful for the staff of the Environment and Climate Change Canada 724 666 differentiation. Eureka Weather Station and, in particular, Station Manager André Beauchard. 725 Dr Mikhail Rogov particularly, as well as Drs Aleksandr Igolnikov and 726 667 5.b. Weissert Event Victor Zakharov, offered important advice concerning the current taxonomic 727 assignments and ages of the ammonites and a Buchia specimen. Glen 728 668 A particularly prominent feature of Early Cretaceous global carbon Edwards produced the photographs and the fossil plate. This publication 729 669 isotope records is the Valanginian (Weissert) δ13C positive excur- represents NRCan Contribution Number/Numéro de contribution de 730 670 sion (Lini et al. 1992; Price et al. 2016). This isotope event is widely RNCan 20190001. We are grateful for the comments of Dr Manuel Bringué 731 671 documented globally in marine carbonates, fossil shell material, (Geological Survey of Canada) for his internal review and to Dr Øyvind 732 672 terrestrial plants and marine organic matter (e.g. Lini et al. Hammer and Dr Mikhail Rogov for detailed external reviews that greatly 733 673 1992; Gröcke et al. 2005; Aguirre-Urreta et al. 2008; Price et al. improved this contribution. We also thank the Editor Dr Bas Van de 734 674 2016). Marine organic matter (Lini et al. 1992; Wortmann & Schootbrugge for comments and suggestions. 735 675 Weissert, 2000) typically shows a c.2‰ excursion. Despite the Supplementary material. To view supplementary material for this article, 736 676 noisy pattern seen in these published records, which possibly relate please visit https://doi.org/10.1017/S0016756819001316. 737 677 to changes in the composition of the bulk organic carbon, the shape 678 of the δ13C curve is characterized by a rapid rise from the pre- 679 excursion background, a plateau and a less steep decline to a References 738 680 new steady state that is slightly more positive than prior to the 681 event. Only in the record from Greenland is the Valanginian Aguirre-Urreta MB, Lescano M, Schmitz MD, Tunik M, Concheyro A, 739 682 (Weissert) δ13C positive excursion less clear, possibly due to high Rawson PF and Ramos VA (2015) Filling the gap: new precise Early 740 683 condensation of the strata and related sample density, or a hiatus in Cretaceous radioisotopic ages from the Andes. Geological Magazine 152, 741 557–64. 742 684 the sedimentary record (Pauly et al. 2013). 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Geosphere 11, 1530–64. 756 695 The Volgian isotopic trends are consistent with other high-latitude Ardakani OH, Sanei H, Snowdon LR, Outridge PM, Obermajer M, Stewart 757 δ13 696 records but are decoupled from Tethyan Ccarb records. The R, Vandenberg R and Boyce K (2016) The accepted values for the internal 758 697 globally recognized isotopically positive Weissert Event in the Geological Survey of Canada (GSC) 9107 Rock-Eval 6® standard (Upper 759 698 Valanginian Stage is also recognized in the Canadian Arctic Cretaceous Second White Speckled Shale, Colorado Group), western Canada. 760 699 sections. The Sverdrup Basin and other Arctic basins may have Geological Survey of Canada, Open File no. 8043, 9 p. doi: 10.4095/298729 761 Balkwill HR (1978) Evolution of Sverdrup Basin, Arctic Canada. 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