Over the past decades, much research has focused on the mid-Cretaceous greenhouse climate, the formation of widespread organic-rich black shales, and cooling intervals from low- to mid-latitude sections. Data from the High Arctic, however, are limited. In this paper, we present high-resolution geochemical records for an ∼1.8-km-thick sedimentary succession exposed on Axel Heiberg Island in the Canadian Arctic Archipelago at a paleolatitude of ∼71°N. For the first time, we have data constraints for the timing and magnitude of most major Oceanic Anoxic Events (OAEs) in brackish-water (OAE1a) and shelf (OAE1b and OAE2) settings in the mid-Cretaceous High Arctic. These are consistent with carbon-climate perturbations reported from deep-water records of lower latitudes. Glendonite beds are observed in the upper Aptian to lower Albian, covering an interval of ∼6 m.y. between 118 and 112 Ma. Although the formation of glendonites is still under discussion, these well-dated occurrences may support the existence of cool shelf waters in the High Arctic Sverdrup Basin at this time, coeval with recent geochemical data from the subtropical Atlantic indicating a drop in sea-surface temperature of nearly 4 °C.
Although major progress in Cretaceous (145–66 Ma) paleoclimate and paleoceanography has been made during the past decade (e.g., Föllmi, 2012), high-latitudinal environmental change has been less studied relative to low- and mid-latitude marine and terrestrial environments (e.g., Herman and Spicer, 1996; Jenkyns et al., 2004). As an alternative to drilling the Arctic Ocean, which is challenging and expensive, the Canadian Arctic Sverdrup Basin provides excellent exposures on land (Embry and Beauchamp, 2008). Still, the exact timing of massive carbon perturbations such as Oceanic Anoxic Events (OAEs) in relation to global warming and cooling periods and their consequences for the evolution of the marine mid-Cretaceous Arctic realm are poorly constrained. Importantly, records of the OAEs are lacking in the High Arctic, except for OAE2. This event has been previously documented based on positive carbon isotope excursions from Ellef Ringnes Island (Pugh et al., 2014) and May Point on Axel Heiberg Island (Lenniger et al., 2014) in Nunavut, Canada. Here we present multi-proxy records from exceptional exposures of Cretaceous sediments on the southern part of Axel Heiberg Island (Fig. 1) at a Cretaceous paleolatitude of ∼71°N. Our work provides a unique view into the mid-Cretaceous paleoenvironmental history of the Arctic in a fluvial-deltaic, brackish-water to outer-shelf setting. We present new sedimentological, whole-rock organic δ13C geochemistry, and U-Pb zircon geochronology data, which we compare with a newly tuned low-latitude carbonate δ13Ccarb record compiled from the literature. Our detailed stratigraphic framework allows the correlation between High Arctic and low- to mid-latitudes. Therefore, major paleoceanographic events known from low latitudes can be compared with the Sverdrup Basin.
Samples were collected at Glacier Fiord on Axel Heiberg Island (78°37.787′N, 89°52.123′W). Total organic carbon (TOC, in %) and δ13Corg measurements were performed on 454 samples. TOC measurements were conducted with a LECO RC-412 with reproducibility of 0.01%. δ13C analysis of total organic carbon was performed using a Flash Elemental Analyzer 1112, connected to the continuous flow inlet system of a MAT 253 gas source mass spectrometer. Samples and standards both reproduced within ±0.2‰ and are reported relative to Vienna Peedee belemnite standard. The U-Pb geochronology used the chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) method. All zircon fractions were prepared using the annealing and chemical leaching technique (CA-TIMS) modified from that described by Mattinson (2005). Following the approach of Jarvis et al. (2006), who developed a calibrated carbonate-based δ13Ccarb reference curve for the Cenomanian–Campanian based on sections of the English Chalk, we extended this record to the Barremian (Fig. 2) by adding published high-resolution δ13Ccarb records from southeast France (Herrle et al., 2004; Gale et al., 2011) and Italy (Erba et al., 1999). The GSA Data Repository1 provides more details on our methodologies and age interpretations used for U-Pb geochronology and the δ13Ccarb reference curve.
MID-CRETACEOUS HIGH ARCTIC CHEMO- AND CHRONOSTRATIGRAPHY
The age model for our studied section is based on lithostratigraphy and biostratigraphy including benthic foraminifera, dinoflagellates, and macrofossils (Schröder-Adams et al., 2014, and references therein). This is integrated with new U-Pb ages and refined with carbon isotope chemostratigraphy. The δ13Corg record of Axel Heiberg Island plotted against stratigraphic depth is marked by several negative and positive excursions of >1‰ and distinctive intervals identified with letters a to i (Fig. 2). Based on correlation of the δ13Corg records from Axel Heiberg Island with the low-latitude composite δ13Ccarb record, most major paleoceanographic and paleoclimatic events (e.g., OAEs and late Aptian–earliest Albian cooling) previously described from low- to mid-latitude sedimentary successions (e.g., Leckie et al., 2002) can be precisely identified in a single sedimentary succession of the Canadian High Arctic at Glacier Fiord (Fig. 2). Using our composite δ13C stratigraphic approach, we are able to define the Barremian-Aptian, Aptian-Albian, and Cenomanian-Turonian boundaries and substages on Axel Heiberg Island, following the Gradstein et al. (2012; GTS2012) age assignments (Fig. 2). Furthermore, we are able to refine the ages of the Canadian High Arctic lithostratigraphic formations and members as well as recognize disconformities on Axel Heiberg Island.
The U-Pb age dating of the Invincible Point Member of the Christopher Formation using bentonite zircons allows the assignment of 111.74 ± 0.26 Ma to the middle part of the section that correlates well with the δ13Corg curve using the GTS2012 age-calibrated composite δ13Ccarb curve (Fig. 2). Therefore, the U-Pb age confirms the GTS2012 definition of the Aptian-Albian boundary at ca. 113 Ma based on the first occurrence of the calcareous subcircular nannofossil taxon Praediscosphaera columnata in southeast France (Herrle et al., 2004). The first appearance datum (FAD) of P. columnata is located in the range of the break point between the end of the uppermost Aptian positive δ13C excursion and the onset of the long-term negative shift of δ13C values in southeast France below the widespread black shale of the Niveau Kilian (Herrle et al., 2004; Fig. 2). It is also consistent with the U-Pb age assignment of 113.1 ± 0.3 Ma in the Lower Saxony Basin (Selby et al., 2009). Thus, our High Arctic U-Pb age assignment and δ13C stratigraphy indicate a chronostratigraphic age of ca. 111.2 Ma and 112.5 Ma for the OAE1b and Kilian events, respectively. Both black shale intervals are potential marker beds for defining the Aptian-Albian boundary (Kennedy et al., 2014). In contrast, our age assignment of 106.58 ± 0.27 Ma in the upper part of the Invincible Point Member does not support the observed δ13C fluctuation of the Axel Heiberg Island record and the GTS2012 composite δ13C record. We speculate that the transition of the shale to a sandstone interval in the upper Invincible Point Member is probably marked by undefined hiatuses and/or that the GTS2012 needs to be improved for the middle to late Albian.
The Rondon Member of the Isachsen Formation has been previously assigned to the Barremian based on dinoflagellate assemblages (e.g., Nøhr-Hansen and McIntyre, 1998). However, the observed major negative (>4‰) and positive (>6‰) δ13Corg changes in the Rondon and lower Walker Island Members is unique within the Cretaceous Period, likely reflecting a major perturbation of the global carbon cycle and the onset of the globally observed early Aptian OAE1a period. Our Arctic record shows the same character and amplitudes known from low-latitude δ13Corg records at Cismon, Italy (Menegatti et al., 1998). Thus, our δ13C stratigraphic age assignment indicates an early Aptian age for the Rondon and lower Walker Island Members (upper part of interval b to lower part of interval c, Fig. 2). Based on our δ13C stratigraphy we propose a Barremian age for the fluvial-deltaic sandstone-dominated interval of the upper Paterson Island Member of the Isachsen Formation (interval a). The transition from the uppermost deltaic Isachsen Formation into the silty, muddy Christopher Formation is marked by two major omission horizons indicated by Rhizocorallium sandstone beds (Cotillon, 2010) interbedded with reddish mudrock (base of interval e). These two horizons are interpreted to represent a condensed section on Axel Heiberg Island covering an interval of ∼4 m.y. (Fig. 2), likely correlative with the global sequence boundary SB Ap5 (Al-Husseini and Matthews, 2010). Thus, the major long-term negative δ13C shift between 122 Ma and 118 Ma of >3‰ as shown in the composite δ13Ccarb record is probably not recorded in the sedimentary succession on Axel Heiberg Island (Fig. 2). The Bastion Ridge Formation has been previously assigned to the late Albian, based on terrestrial and marine palynomorphs (Núñez-Betelu and Hills, 1994). In contrast, our δ13C stratigraphy indicates a middle to late Cenomanian age, supported by the occurrence of the Gaudryina irinensis Zone at the base of this formation (Fig. 2).
MID-CRETACEOUS HIGH ARCTIC OAEs AND ENVIRONMENTAL AND CLIMATE EVENTS
Based on the unique δ13Corg shifts, we interpret the Rondon Member to be the sedimentary expression of OAE1a on Axel Heiberg Island, which is characterized by a TOC-rich (average of 2.8%), mudrock-dominated transgressive unit with large iron concretions (>0.5 m) and coalified wood sandwiched within deltaic sandstones. The foraminiferal assemblage of OAE1a is dominated by the agglutinated benthic genus Miliammina, a brackish-water indicator (e.g., Tibert and Leckie, 2004). Thus, the High Arctic OAE1a expression occurs within a transgressive unit marked by a restricted brackish marine, shallow-water environment.
Using the δ13C stratigraphy, the Kilian and OAE1b climate events can be identified within the middle part of the Invincible Point Member by intercalated reddish siltstone to sandstone beds within a lower shoreface to offshore transition with TOC values of up to 6%. Thus, in the Sverdrup Basin, the Kilian and OAE1b events occur in a transgressive unit with restricted bottom-water conditions (Schröder-Adams et al., 2014), comparable with findings from the subtropical Vocontian Basin in France (Herrle et al., 2003).
The lowermost Kanguk Formation contains finely laminated organic-rich black shales (paper shales) marked by a significant positive δ13Corg excursion of >2‰ that corresponds to the latest Cenomanian OAE2 (interval i, Fig. 2). This excursion is used to place the Cenomanian-Turonian boundary in the lower part of the Kanguk Formation at the top of the paper shale. The OAE2 event spans an interval of ∼20 m, with TOC values of up to 10% (Fig. 2). Dynamic paleoenvironmental conditions during this interval are marked by fluctuations between dysoxic and anoxic bottom waters, indicated by the presence and absence of benthic foraminifera, and fluctuating surface-water productivity as indicated by varying hydrogen indices (Schröder-Adams et al., 2014). Thus, our δ13C record supports previous findings on OAE2 in terms of structure and thickness of the same event recently described at May Point on Axel Heiberg Island by Lenniger et al. (2014). Their absolute δ13C values are ∼0.4‰ lighter compared to our locality, which might reflect a slightly more terrestrial influence. However, benthic foraminiferal assemblages indicate that OAE2 is marked by varying dysoxic to anoxic conditions at Glacier Fiord, in contrast to the prevailing anoxic conditions at May Point. This suggests a complex biotic response to OAE2 within the Polar Sea.
The Invincible Point Member of the Christopher Formation is marked by up to 20 glendonite beds, including the level of the Jacob black shale within the second late Aptian positive δ13C excursion (interval e to middle of interval g, Fig. 2). The Jacob event was originally described from the Vocontian Basin as a response to enhanced delivery of terrestrial-derived organic matter during a sea-level lowstand, causing bottom-water anoxia (e.g., Bréhéret, 1994). Glendonites represent calcite pseudomorphs that favor marine settings of elevated alkalinity and dissolved phosphate with near-freezing bottom-water temperatures (∼0–7 °C) and methane seeps (e.g., Selleck et al., 2007). The now-confirmed age of the glendonite-rich interval based on our δ13C stratigraphy in our section corresponds well with the late Aptian to early Albian surface-water cooling of ∼4 °C observed in the subtropical Atlantic Ocean, as well as the occurrence of the Boreal calcareous nannofossil species Repagulum parvidentatum in the low latitudes (McAnena et al., 2013). If the glendonite occurrences on Axel Heiberg Island are the Arctic representation of the late Aptian to early Albian cold snap (Kemper, 1987), the mid-Cretaceous cooling would have lasted for at least 6 m.y. based on our composite δ13Ccarb stratigraphy. In the Glacier Fiord section, the youngest glendonite bed occurs at ca. 112.8 Ma (lowermost Albian) corresponding to a return to warm temperatures as recorded in the subtropical Atlantic Ocean (McAnena et al., 2013) and black shale formation of the Niveau Kilian event (Herrle et al., 2003).
By using a chemostratigraphic approach, we demonstrate that the major mid-Cretaceous OAEs and paleoclimatic events previously described from the mid- to low latitudes are recorded in a single TOC-rich marine sedimentary succession at Glacier Fiord on Axel Heiberg Island in the High Arctic Sverdrup Basin. Depositional environments range from fluvial-deltaic to shallow, brackish to offshore distal shelf settings. We demonstrate that the occurrence of upper Aptian to lowermost Albian glendonite beds within the Polar Sea are coeval with a subtropical Atlantic drop in sea-surface temperatures of ∼4 °C. Thus, our data support the presence of cool shelf waters in the High Arctic at this time.
We thank the Polar Continental Shelf Program of Natural Resources Canada for logistical support; K. Littler, M. Leckie, E. Thomas, and one anonymous reviewer for their constructive reviews; A. Embry, T. Wagner, and S. Grasby for discussions; and J. Fiebig and B. Schminke for stable isotope measurements. Financial support to Herrle was provided by the German Research Foundation (DFG) (HE 3521/6), support to Schröder-Adams was provided by a Natural Sciences and Engineering Research Council (NSERC) Collaborative Research and Development Grant with GSC (GEM Program) and ConocoPhillips as partners, and support to Galloway was provided by an NSERC fellowship.