Abstract

The Early Triassic record, from the Smithian stratotype, shows that the organic carbon isotope record from northwest Pangea closely corresponds to major fluctuations in the inorganic carbon records from the Tethys, indicating truly global perturbations of the carbon cycle occurred during this time. Geochemical proxies for anoxia are strongly correlated with carbon isotopes, whereby negative shifts in δ13Corg are associated with shifts to more anoxic to euxinic conditions, and positive shifts are related to return to more oxic conditions. Rather than by a delayed or prolonged recovery, the Early Triassic is better characterized by a series of aborted biotic recoveries related to shifts back to ocean anoxia, potentially driven by recurrent volcanism.

INTRODUCTION

Starting with the latest Permian extinction (LPE), ca. 252 Ma, the Permian-Triassic biotic crisis (PTBC) persisted through the following 5 m.y. of Early Triassic time (Lehrmann et al., 2006) before substantial biotic recovery occurred (Bottjer, 2001; Erwin, 2006). However, recent evidence suggests that rather than a prolonged biotic recovery, the Early Triassic is better characterized by several failed recoveries of reefs, microfossils, ammonoids, and conodonts (Brayard et al., 2011; Song et al., 2011; Stanley, 2009; Orchard, 2007). The mechanism causing these recoveries to fail remains uncertain. Profound fluctuations in stable carbon isotopes through the PTBC suggest that significant disruption of global biogeochemical cycles occurred during this time (Galfetti et al., 2007a; Horacek et al., 2007a; Payne et al., 2004). We show that in northwest Pangea there were also pronounced fluctuations in ocean redox state throughout the PTBC. These redox shifts closely corresponded to variations in the carbon isotope record, suggesting that recurrent ocean anoxia played a significant role in delaying the Early Triassic recovery.

STUDY AREA

We examined a marine geochemical record through the PTBC from northwest Pangea, in the Sverdrup Basin, a Carboniferous–Paleogene depocenter of the Canadian High Arctic (Figs. 1A and 1B). The Sverdrup Basin is where the Lower Triassic stratotypes for the Griesbachian, Dienerian, Smithian, and Spathian substages are located (Fig. 1B) (Tozer, 1967). From late Carboniferous to Early Triassic time, the Sverdrup Basin was characterized by a subtropical (35° to 40°N) deep basinal area surrounded by a peripheral shallow shelf that was dominated by carbonate and chert until the LPE (Beauchamp and Grasby, 2012; Ogg and Steiner, 1991), and by clastic sediments thereafter (Embry and Beauchamp, 2008). Sedimentation occurred across the LPE horizon in the distal basin-axial area (Grasby and Beauchamp, 2008). Located at a similar paleolatitude as and downwind of the Siberian Traps, the basin was directly impacted by volcanic eruptions (Fig. 1C) (Grasby et al., 2011; Sanei et al., 2012).

We studied two locations: (1) the uppermost Permian to lower Griesbachian record at Buchanan Lake, eastern Axel Heiberg Island, Nunavut, as previously reported (Grasby and Beauchamp, 2008, 2009), and (2) the Lower Triassic record at Smith Creek (stratotype of the Smithian substage) on Svartfjeld Peninsula, northern Ellesmere Island, Nunavut (see Table DR1 in the GSA Data Repository1 for our new analytical results) (Fig. 1B). Buchanan Lake represents a distal deep-water bathyal setting, and Smith Creek a distal slope to outer shelf setting (Beauchamp et al., 2009). The Smith Creek section starts in the upper Griesbachian and extends through the upper Spathian (Baud et al., 2008; Beauchamp et al., 2009), including strata belonging to the Dienerian and Spathian substages, as defined at nearby Diener and Spath Creeks (Fig. 1B) (Tozer, 1967). Lower Griesbachian strata are absent in the Smith Creek area (Beauchamp et al., 2009) but basal Griesbachian strata are present in the upper portion of the Buchanan Lake section (Grasby and Beauchamp, 2008, 2009). The Griesbachian stratotype itself (at Griesbach Creek, Axel Heiberg Island) is thermally altered, making it unsuitable for geochemical analysis. The two sections studied here are placed in time sequence at the same vertical scale in Figure 2. Total organic carbon (TOC) in Figure 2A represents changes in total organic matter deposition through the PTBC. Optical petrography of our samples indicates that small organic particles (macerals) during the Early Triassic are dominantly normal marine organics, such that terrestrial organic matter has limited influence on TOC and the organic carbon isotope record studied here.

RESULTS

Carbon Isotope Record

In the Sverdrup Basin, δ13Corg values show an initial stepwise negative shift below the LPE horizon (Fig. 2B), followed by a 5‰ decline across the LPE horizon. The LPE event is followed by a prolonged period of progressive and continued recovery of δ13Corg through the Griesbachian and Dienerian to preextinction values by the middle Dienerian. There was then a much larger, 9‰ negative δ13Corg excursion in the mid- to late Smithian, followed by recovery to pre-LPE values in the basal Spathian, and then a subsequent drop to low δ13Corg values in the middle Spathian. δ13Corg values recover again to a pre-LPE state toward the late Spathian.

Anoxia Record

Increased concentrations of Mo, both absolute and normalized to Al (correcting for variations in terrigenous [clastic] mineral input), are strong indicators of marine anoxia (e.g., Tribovillard et al., 2006). The Mo/Al ratio has a strong negative correlation with δ13Corg throughout the PTBC (Fig. 2C), suggesting that episodic disruptions of the carbon cycle throughout the PTBC are linked to changes in ocean anoxia. Based on temporal changes in Mo/Al (Fig. 2C), along with other indicators of anoxic to euxinic conditions (Fepyrite concentrations, and pyrite framboid <7 μm distribution; Fig. 2D), we show that the Sverdrup Basin shifted from initial oxic to anoxic conditions in the Changhsingian, during periods of volcanic venting and associated coal fly ash and toxic metal deposition (Grasby et al., 2011; Sanei et al., 2012), to fully euxinic conditions at the LPE boundary (Grasby and Beauchamp, 2009). The Dienerian represents a period of progressive recovery to preextinction oxic conditions by the Dienerian-Smithian boundary (as shown by decreasing Mo/Al along with increasing TOC), during a time of rapid worldwide recovery of conodonts and ammonoids (Brayard et al., 2006; Orchard, 2007) and microfaunal communities (Song et al., 2011) along with transient reef development (Brayard et al., 2011). This period is also marked by the appearance of abundant benthic bryozoan fauna at Smith Creek (Baud et al., 2008). This initial biotic recovery from the LPE event failed as the Sverdrup Basin returned back to anoxic to euxinic conditions in the late Smithian (marked also by the 9‰ decline in δ13Corg values and abundance of pyrite framboids <7 μm). This shift back to anoxic conditions is coincident with a severe Smithian ammonoid and conodont extinction (Brayard et al., 2006; Orchard, 2007), and coincides with evidence for renewed Siberian Trap volcanism (Paton et al., 2010) and cyanobacteria blooms (Xie et al., 2010). A second recovery to more oxic conditions in the early Spathian is followed by yet another switch back to anoxic conditions and decline in δ13Corg through the middle Spathian.

DISCUSSION

The profound shifts in δ13Corg through the PTBC that we observed in northwest Pangea are consistent with inorganic carbon isotope records from the Tethys (Horacek et al., 2007a; Payne et al., 2004) and Panthalassa (Horacek et al., 2009). The strong correlation with inorganic carbon isotope records from the Tethys (Galfetti et al., 2007b; Horacek et al., 2007b) are illustrated in Figure 3. These results demonstrate that disruption of the carbon cycle through the PTBC was indeed global, and that there are parallel impacts on both the organic and inorganic marine carbon cycle through the Early Triassic.

Development of anoxic conditions has been commonly invoked as a contributing cause to the LPE event (e.g., Wignall and Twitchett, 1996), and previous workers have suggested that prolonged anoxic conditions contributed to a delayed Early Triassic biotic recovery (e.g., Hallam, 1991; Woods et al., 1999; Wignall and Twitchett, 2002). However, our results show that the Early Triassic is better characterized by recurrent anoxic conditions. The global nature of these recurrent anoxic events remains uncertain, however the close correspondence with globally recognized fluctuations in the Early Triassic carbon cycle suggests broader occurrence of environmental disturbance following the LPE. As well, the swings between oxic and anoxic conditions coincide with transient recovery and subsequent collapse of marine ecosystems.

The overall driver for the recurrent anoxia remains unclear. Data increasingly show a correspondence between Siberian Trap volcanism and major extinctions at the LPE boundary (Shen et al., 2011; Grasby et al., 2011; Sanei et al., 2012; Black et al., 2012; Reichow et al., 2009) as originally suggested by Campbell et al. (1992). Evidence for episodic volcanism subsequent to the initial Siberian Trap eruptions has also been provided by Paton et al. (2010) and may thus be a cause of the disturbed biogeochemical cycles along with the aborted biotic recoveries through the PTBC (Galfetti et al., 2007a). Volcanic emissions of CO2 can have significant impact on carbon isotope records (Korte et al., 2010), and model results demonstrate that volcanic emissions can account for observed carbon isotope fluctuations through the PTBC (Payne and Kump, 2007; Sobolev et al., 2011). Episodic volcanism would also account for the parallel trends in both organic and inorganic carbon pools. The negative correlation between δ13Corg and Mo/Al shown here may suggest that major volcanic events directly drive development of ocean anoxia. This is consistent with the HEATT (haline euxinic acidic thermal transgression) model (Kidder and Worsley, 2010), which suggests that normal Phanerozoic greenhouse states can be pushed to hothouse conditions by volcanic eruptions. Significant CO2 emissions would drive global warming that, in turn, would lead to diminished thermohaline circulation and oxygen solubility. However, modeling by Ozaki et al. (2011) and Meyer and Kump (2008) indicate this would not in itself be sufficient to develop euxinic conditions, and increased nutrient flux to the oceans is required, potentially related to enhanced continental weathering following the LPE (Sephton et al., 2005; Beauchamp and Grasby, 2012). The suggested link between cyanobacteria blooms and volcanism during the PTBC (Xie et al., 2010), and the link between coal fly ash loading and anoxia (Grasby et al., 2011), also suggest volcanic ash fall causes nutrient loading that drives eutrophication.

CONCLUSIONS

The Sverdrup Basin on northwest Pangea shows strong evidence for development of recurrent anoxic to euxinic conditions that strongly parallel fluctuations in the carbon isotope record throughout the 5 m.y. of the Permian-Triassic biotic crisis. Rather than by a prolonged or delayed recovery, the Early Triassic is better characterized by a series of aborted recoveries coincident with shifts back to anoxic conditions. The initial biotic recovery following the LPE was likely much more rapid than previously considered, and the delayed Early Triassic recovery was a factor of repeated environmental stress rather than the severity of the LPE event itself. While the ultimate driver of both changes in ocean redox state and major perturbations to global biogeochemical cycles remains uncertain, recent models and evidence point to the potential of recurrent volcanic events as a possible cause. This suggests that following the LPE event, life was not able to make a final sustained recovery until volcanism diminished by Middle Triassic time.

Field assistance and sample preparation by Gennyne McCune and Shannon Provencher is greatly appreciated. We appreciate helpful comments by three anonymous reviewers. This is Geological Survey of Canada contribution 20120200.

1GSA Data Repository item 2013043, Table DR1 (analytical results for shale analyses used in this study), is available online at www.geosociety.org/pubs/ft2013.htm, or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.