Evolution of terrestrial and marine environments during the late Early Triassic: (a) early Smithian, (b) late Smithian thermal maximum (LSTM), (c) Smithian–Spathian boundary, and (d) early Spathian. This model integrates changes in subaerial weathering rates and oceanic productivity and redox conditions documented in this study with data regarding paleoclimate variation, terrestrial floral assemblages, and marine biodiversity patterns from other sources (cited in text). We infer that the modeled environmental changes were ultimately due to variation in the eruption rate of the Siberian Traps, although this has not been proven to date. See text for further discussion. 

Contexts in source publication

Context 1

... underscore the fundamental significance of the SSB, which represents the termination of the Early Triassic hyper-greenhouse climate and the reinvigoration of global-ocean overturning circulation (Fig. 6). Oceanographic changes at the SSB had a major effect on contemporaneous marine biotas. Several invertebrate clades, including ammonoids, conodonts, and foraminifera, appear to have suffered severe losses of biodiversity at this time (Orchard, 2007; Stanley, 2009; Song et al., 2011). Ammonoids diversified greatly during the Griesbachian to Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were widespread and characteristic of the SSB. We propose that all of the changes in our model (Fig. 6) were due to a cooling event that commenced following the LSTM and that continued strongly across the SSB. In particular, we infer that cooling led to the reinvigoration of global-ocean overturning circulation. It should be noted that we are not envisioning complete ocean stagnation during the preceding Griesbachian–Smithian interval, which is unlikely, based on physical oceanographic principles (e.g., Kiehl and Shields, 2005), but, rather, a strong slowing of overturning circulation that led to a buildup of nutrients in the deep ocean (Fig. 6). The reinvigoration of global-ocean circulation at the SSB flushed this buildup of nutrients back into the ocean-surface layer, triggering a transient increase in marine productivity and an expansion of thermoclinal anoxia that lasted until this deepwater nutrient source was depleted. The brevity of the SSB anoxic event at Shitouzhai, which lasted ∼ 75–150 kyr, is consistent with such a mechanism. This mechanism also accounts for the abrupt, large positive shift in δ 13 C carb at the SSB, which was due to a productivity-related increase in organic carbon burial rates (Fig. 6). The ultimate cause of the SSB event is uncertain. Given that the onset of the Permian–Triassic boundary crisis has been firmly linked to the initiation of the main eruptive phase of the Siberian Traps large igneous province (STLIP) (Renne et al., 1995; Kamo et al., 2003) and that the Early Triassic was an interval of repeated environmental distur- bances (Algeo et al., 2011; Retallack et al., 2011) and elevated global temperatures (Sun et al., 2012; Romano et al., 2013) linked to volcanogenic greenhouse gas emissions (Retallack and Jahren, 2008; Black et al., 2012), the obvi- ous explanation for the SSB is a reduction in the intensity of magmatic activity in the STLIP source region (Fig. 6). The available radiometric age data for the Siberian Traps, although sparse, are consistent with this possibility. U–Pb dating of perovskites in the early Arydzhangsky flow and zircons from the late Delkansky silicic tuff of extrusive suites in the Maymecha–Kotuy region suggests that the STLIP flood basalt eruptions commenced at 251.7 ± 0.4 Ma and ended at 251.1 ± 0.3 Ma, i.e., representing an interval of ∼ 600 kyr (Renne et al., 1995; Kamo et al., 2003). However, an Ar–Ar date of 250.3 ± 1.1 Ma was obtained for the final stage of extrusive volcanism at Norilsk, the core area of the STLIP (Reichow et al., 2009; see also the review of evidence for a late eruptive stage by Ovtcharova et al., 2006). The more critical issue, in any case, is the duration not of flood basalt eruptions but of intrusive magmatism in the West Siberian Coal Basin, which was probably the main source of volcanogenic greenhouse gases (Retallack and Jahren, 2008; Black et al., 2012). Reichow et al. (2009) reported ages for STLIP-related intrusives spanning several million years, which is consistent with the hypotheses that large-scale intrusive activity continued at least until the SSB and that cessation of most such activity at the SSB was responsible for contemporaneous climatic cooling (Sun et al., 2012; Romano et al., 2013). Further work on the chronology of the STLIP will be needed to conclusively evaluate controls on the SSB event. The SSB event (late Early Triassic) was investigated at Shitouzhai, Guizhou Province, South China, using a multidisci- plinary approach combining carbonate carbon ( δ 13 C carb ) and carbonate-associated sulfate sulfur isotopes ( δ 34 S CAS ) , rare earth elements, and elemental paleoredox and paleoproductivity proxies. The Shitouzhai section exhibits a large ( + 4 ‰) positive δ 13 C carb shift across the SSB similar to that seen in other SSB sections globally, reflecting enhanced marine productivity and organic carbon burial. Various elemental and isotopic proxies also document a major decrease in chemical weathering intensity and detrital sediment input, a shift toward a better-ventilated oceanic thermocline, and a dimin- ished burial flux of reduced sulfur. All of these changes co- incided with a large cooling of sea-surface temperatures that terminated the Early Triassic hothouse regime. The extreme temperatures of the late Smithian thermal maximum (LSTM) may have triggered a biocrisis just prior to the SSB. Ma- rine biotas did not recover immediately in response to climatic and environmental amelioration at the SSB, however, but underwent a stepwise recovery during the early to middle Spathian. The cause of the SSB event is uncertain but may have been related to a reduction in intrusive magmatic activity in the Siberian Traps large igneous province. The study section is located at Shitouzhai village (GPS: 25 ◦ 45 9.6 N, 106 ◦ 6 29.7 E), about 3 km east of Ziyun County town in southern Guizhou Province, South China (Fig. A1). During the Early to Middle Triassic, the Ziyun area was located on the southern margin of the Yangtze Platform, to the north of the Nanpanjiang Basin (Enos et al., ...

Context 2

... Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were widespread and characteristic of the SSB. We propose that all of the changes in our model (Fig. 6) were due to a cooling event that commenced following the LSTM and that continued strongly across the SSB. In particular, we infer that cooling led to the reinvigoration of global-ocean overturning circulation. It should be noted that we are not envisioning complete ocean stagnation during the preceding Griesbachian–Smithian interval, which is unlikely, based on physical oceanographic principles (e.g., Kiehl and Shields, 2005), but, rather, a strong slowing of overturning circulation that led to a buildup of nutrients in the deep ocean (Fig. 6). The reinvigoration of global-ocean circulation at the SSB flushed this buildup of nutrients back into the ocean-surface layer, triggering a transient increase in marine productivity and an expansion of thermoclinal anoxia that lasted until this deepwater nutrient source was depleted. The brevity of the SSB anoxic event at Shitouzhai, which lasted ∼ 75–150 kyr, is consistent with such a mechanism. This mechanism also accounts for the abrupt, large positive shift in δ 13 C carb at the SSB, which was due to a productivity-related increase in organic carbon burial rates (Fig. 6). The ultimate cause of the SSB event is uncertain. Given that the onset of the Permian–Triassic boundary crisis has been firmly linked to the initiation of the main eruptive phase of the Siberian Traps large igneous province (STLIP) (Renne et al., 1995; Kamo et al., 2003) and that the Early Triassic was an interval of repeated environmental distur- bances (Algeo et al., 2011; Retallack et al., 2011) and elevated global temperatures (Sun et al., 2012; Romano et al., 2013) linked to volcanogenic greenhouse gas emissions (Retallack and Jahren, 2008; Black et al., 2012), the obvi- ous explanation for the SSB is a reduction in the intensity of magmatic activity in the STLIP source region (Fig. 6). The available radiometric age data for the Siberian Traps, although sparse, are consistent with this possibility. U–Pb dating of perovskites in the early Arydzhangsky flow and zircons from the late Delkansky silicic tuff of extrusive suites in the Maymecha–Kotuy region suggests that the STLIP flood basalt eruptions commenced at 251.7 ± 0.4 Ma and ended at 251.1 ± 0.3 Ma, i.e., representing an interval of ∼ 600 kyr (Renne et al., 1995; Kamo et al., 2003). However, an Ar–Ar date of 250.3 ± 1.1 Ma was obtained for the final stage of extrusive volcanism at Norilsk, the core area of the STLIP (Reichow et al., 2009; see also the review of evidence for a late eruptive stage by Ovtcharova et al., 2006). The more critical issue, in any case, is the duration not of flood basalt eruptions but of intrusive magmatism in the West Siberian Coal Basin, which was probably the main source of volcanogenic greenhouse gases (Retallack and Jahren, 2008; Black et al., 2012). Reichow et al. (2009) reported ages for STLIP-related intrusives spanning several million years, which is consistent with the hypotheses that large-scale intrusive activity continued at least until the SSB and that cessation of most such activity at the SSB was responsible for contemporaneous climatic cooling (Sun et al., 2012; Romano et al., 2013). Further work on the chronology of the STLIP will be needed to conclusively evaluate controls on the SSB event. The SSB event (late Early Triassic) was investigated at Shitouzhai, Guizhou Province, South China, using a multidisci- plinary approach combining carbonate carbon ( δ 13 C carb ) and carbonate-associated sulfate sulfur isotopes ( δ 34 S CAS ) , rare earth elements, and elemental paleoredox and paleoproductivity proxies. The Shitouzhai section exhibits a large ( + 4 ‰) positive δ 13 C carb shift across the SSB similar to that seen in other SSB sections globally, reflecting enhanced marine productivity and organic carbon burial. Various elemental and isotopic proxies also document a major decrease in chemical weathering intensity and detrital sediment input, a shift toward a better-ventilated oceanic thermocline, and a dimin- ished burial flux of reduced sulfur. All of these changes co- incided with a large cooling of sea-surface temperatures that terminated the Early Triassic hothouse regime. The extreme temperatures of the late Smithian thermal maximum (LSTM) may have triggered a biocrisis just prior to the SSB. Ma- rine biotas did not recover immediately in response to climatic and environmental amelioration at the SSB, however, but underwent a stepwise recovery during the early to middle Spathian. The cause of the SSB event is uncertain but may have been related to a reduction in intrusive magmatic activity in the Siberian Traps large igneous province. The study section is located at Shitouzhai village (GPS: 25 ◦ 45 9.6 N, 106 ◦ 6 29.7 E), about 3 km east of Ziyun County town in southern Guizhou Province, South China (Fig. A1). During the Early to Middle Triassic, the Ziyun area was located on the southern margin of the Yangtze Platform, to the north of the Nanpanjiang Basin (Enos et al., 2006). The paleogeographic configuration of the Ziyun area changed from a platform-margin reef system in the latest Permian to a platform-ramp environment in the Early Triassic (Feng et al., 1997). In this area of the Nanpanjiang Basin in southern Guizhou Province, the Upper Permian successions usually comprise bioclastic rocks, which are collectively assigned to the Wujiaping Formation. However, unlike the same formation exposed elsewhere in South China, which is confined to the Wuchiapingian Stage of the late Permian, the Wujiaping Formation ...

Context 3

... However, there is an even larger increase in Mn / Th at this level (Fig. 4). Under reducing conditions, Mn 2 + is highly soluble and does not ac- cumulate in substantial amounts in marine sediments. However, suboxic to oxic conditions commonly result in Mn en- richment through the accumulation of Mn(II) in carbonates or Mn(III) in oxyhydroxides (Okita et al., 1988). Strong Mn enrichment is thus common on the margins of reducing deep water masses (Landing and Bruland, 1987). Mn enrichment in carbonates is accepted as a good indicator of suboxic conditions (Rue et al., 1997; Pakhomova et al., 2007). At Shitouzhai, the Mn / Th profile suggests dominantly anoxic conditions below the SSB (0–18 m) and suboxic conditions above it (20–37 m), although with a brief in- terlude of more reducing conditions during the early Spathian (28–32 m; Fig. 4). Cerium (Ce) is the only REE that is affected by oxidation- reduction processes in the Earth-surface environment. Under reducing conditions, Ce 3 + has the same valence as other REEs and, therefore, is not fractionated relative to them, yielding Ce / Ce* ratios of ∼ 1.0 (German and Elderfield, 1990). Under oxidizing conditions, Ce 4 + is preferentially removed from solution, yielding local sedimentary deposits with Ce / Ce* > 1.0, whereas the Ce / Ce* ratio of seawater and of any hydrogenous deposits incorporating REEs from seawater is < 1.0 (e.g., 0.3–0.4 in the modern ocean). Thus, Ce is potentially a good proxy for marine paleoredox conditions, provided that a hydrogenous signal can be measured (Wright et al., 1987). Terrigenous influence (e.g., addition of REEs from clay minerals) will generally cause Ce / Ce* ratios to converge on 1.0, which is by definition the value for average upper crustal rocks. In the study section, Ce / Ce* ratios vary from 0.79 to 0.88 (Fig. 4). These moderately high values are nominally indicative of suboxic conditions. However, the Ce / Ce* ratio was probably heavily influenced by REEs from the clay fraction of the sediment, making the Ce / Ce* ratio of any hydrogenous contribution uncertain. Th / U ratios are useful for paleoredox analysis, owing to the redox-dependent behavior of U. Under oxidizing conditions, U(VI) tends to form stable carbonate complexes in seawater (Langmuir, 1978; Algeo and Maynard, 2004). Under reducing conditions, U(IV) is readily removed to the sediment. Th, however, is not subject to the influence of redox condition, resulting in higher Th / U ratios under reducing conditions as aqueous U is lost (Wignall and Myers, 1988). In the study section, a distinct decrease in the Th / U ratio at the SSB indicates a shift toward more oxygenated conditions, which was sustained into the early Spathian (Fig. 4). These results are consistent with dominantly oxic to suboxic conditions in the study area following the SSB (Fig. 6). Seawater sulfate δ S rose sharply from ∼ + 15 ‰ in the late Permian to > + 30 ‰ in the Middle Triassic (Claypool et al., 1980; Kampschulte and Strauss, 2004), although the pattern of increase during the Early Triassic has only recently be- gun to be worked out (Song et al., 2014). The present study provides the most comprehensive analysis of δ 34 S CAS variation at the SSB of any study to date. The Shitouzhai section exhibits a distinct, ∼ 10–15 ‰ negative shift in δ 34 S CAS that is paired with a ∼ 4 ‰ positive shift in δ 13 C carb (Fig. 4). Both shifts are limited to a narrow interval around the SSB, probably representing no more than ∼ 75–150 kyr based on average sedimentation rates for the study section (Fig. 3a). These two features (i.e., negative covariation and a short event interval) impose significant constraints on the under- lying causes of the isotopic shifts. Most of the Early Triassic is characterized by positive δ 13 C carb – δ 34 S CAS covariation, a pattern that is consistent with control by sediment burial fluxes, i.e., co-burial of organic carbon and pyrite, linked to variations in marine productivity and/or redox conditions (Luo et al., 2010; Song et al., 2014). In contrast, negative δ 13 C carb – δ 34 S CAS covariation during a short-term event at the SSB is indicative of oceanographic controls. Specifically, we hypothesize that cooling-driven reinvigoration of oceanic overturning circulation led to stronger upwelling, mixing nutrient- and sulfide-rich deep waters upward into the ocean- surface layer and causing both enhanced marine productivity (hence higher δ 13 C DIC ) and oxidation of advected H 2 S (hence lower δ 34 S sulfate ) (Fig. 6). Such an oceanographic process was inherently transient, lasting only until the nutrients and sulfide that had accumulated in the deep ocean during the Griesbachian–Smithian interval of intense oceanic stratifica- tion (Song et al., 2013) became depleted. The same process was inferred for the latest Spathian by Song et al. (2014), an interval also characterized by short-term negative δ 13 C carb – δ 34 S CAS covariation (Song et al., 2014, their figure 6) and linked to global climatic cooling (Sun et al., 2012). These considerations underscore the fundamental significance of the SSB, which represents the termination of the Early Triassic hyper-greenhouse climate and the reinvigoration of global-ocean overturning circulation (Fig. 6). Oceanographic changes at the SSB had a major effect on contemporaneous marine biotas. Several invertebrate clades, including ammonoids, conodonts, and foraminifera, appear to have suffered severe losses of biodiversity at this time (Orchard, 2007; Stanley, 2009; Song et al., 2011). Ammonoids diversified greatly during the Griesbachian to Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were ...

Context 4

... (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were widespread and characteristic of the SSB. We propose that all of the changes in our model (Fig. 6) were due to a cooling event that commenced following the LSTM and that continued strongly across the SSB. In particular, we infer that cooling led to the reinvigoration of global-ocean overturning circulation. It should be noted that we are not envisioning complete ocean stagnation during the preceding Griesbachian–Smithian interval, which is unlikely, based on physical oceanographic principles (e.g., Kiehl and Shields, 2005), but, rather, a strong slowing of overturning circulation that led to a buildup of nutrients in the deep ocean (Fig. 6). The reinvigoration of global-ocean circulation at the SSB flushed this buildup of nutrients back into the ocean-surface layer, triggering a transient increase in marine productivity and an expansion of thermoclinal anoxia that lasted until this deepwater nutrient source was depleted. The brevity of the SSB anoxic event at Shitouzhai, which lasted ∼ 75–150 kyr, is consistent with such a mechanism. This mechanism also accounts for the abrupt, large positive shift in δ 13 C carb at the SSB, which was due to a productivity-related increase in organic carbon burial rates (Fig. 6). The ultimate cause of the SSB event is uncertain. Given that the onset of the Permian–Triassic boundary crisis has been firmly linked to the initiation of the main eruptive phase of the Siberian Traps large igneous province (STLIP) (Renne et al., 1995; Kamo et al., 2003) and that the Early Triassic was an interval of repeated environmental distur- bances (Algeo et al., 2011; Retallack et al., 2011) and elevated global temperatures (Sun et al., 2012; Romano et al., 2013) linked to volcanogenic greenhouse gas emissions (Retallack and Jahren, 2008; Black et al., 2012), the obvi- ous explanation for the SSB is a reduction in the intensity of magmatic activity in the STLIP source region (Fig. 6). The available radiometric age data for the Siberian Traps, although sparse, are consistent with this possibility. U–Pb dating of perovskites in the early Arydzhangsky flow and zircons from the late Delkansky silicic tuff of extrusive suites in the Maymecha–Kotuy region suggests that the STLIP flood basalt eruptions commenced at 251.7 ± 0.4 Ma and ended at 251.1 ± 0.3 Ma, i.e., representing an interval of ∼ 600 kyr (Renne et al., 1995; Kamo et al., 2003). However, an Ar–Ar date of 250.3 ± 1.1 Ma was obtained for the final stage of extrusive volcanism at Norilsk, the core area of the STLIP (Reichow et al., 2009; see also the review of evidence for a late eruptive stage by Ovtcharova et al., 2006). The more critical issue, in any case, is the duration not of flood basalt eruptions but of intrusive magmatism in the West Siberian Coal Basin, which was probably the main source of volcanogenic greenhouse gases (Retallack and Jahren, 2008; Black et al., 2012). Reichow et al. (2009) reported ages for STLIP-related intrusives spanning several million years, which is consistent with the hypotheses that large-scale intrusive activity continued at least until the SSB and that cessation of most such activity at the SSB was responsible for contemporaneous climatic cooling (Sun et al., 2012; Romano et al., 2013). Further work on the chronology of the STLIP will be needed to conclusively evaluate controls on the SSB event. The SSB event (late Early Triassic) was investigated at Shitouzhai, Guizhou Province, South China, using a multidisci- plinary approach combining carbonate carbon ( δ 13 C carb ) and carbonate-associated sulfate sulfur isotopes ( δ 34 S CAS ) , rare earth elements, and elemental paleoredox and paleoproductivity proxies. The Shitouzhai section exhibits a large ( + 4 ‰) positive δ 13 C carb shift across the SSB similar to that seen in other SSB sections globally, reflecting enhanced marine productivity and organic carbon burial. Various elemental and isotopic proxies also document a major decrease in chemical weathering intensity and detrital sediment input, a shift toward a better-ventilated oceanic thermocline, and a dimin- ished burial flux of reduced sulfur. All of these changes co- incided with a large cooling of sea-surface temperatures that terminated the Early Triassic hothouse regime. The extreme temperatures of the late Smithian thermal maximum (LSTM) may have triggered a biocrisis just prior to the SSB. Ma- rine biotas did not recover immediately in response to climatic and environmental amelioration at the SSB, however, but underwent a stepwise recovery during the early to middle Spathian. The cause of the SSB event is uncertain but may have been related to a reduction in intrusive magmatic activity in the Siberian Traps large igneous province. The study section is located at Shitouzhai village (GPS: 25 ◦ 45 9.6 N, 106 ◦ 6 29.7 E), about 3 km east of Ziyun County town in southern Guizhou Province, South China (Fig. A1). During the Early to Middle Triassic, the Ziyun area was located on the southern margin of the Yangtze Platform, to the north of the Nanpanjiang Basin (Enos et al., 2006). The paleogeographic configuration of the Ziyun area changed from a platform-margin reef system in the latest Permian to a platform-ramp environment in the Early Triassic (Feng et al., 1997). In this area of the Nanpanjiang Basin in southern Guizhou Province, the Upper Permian successions usually comprise bioclastic rocks, which are collectively assigned to the Wujiaping Formation. However, unlike the same formation exposed elsewhere in South China, which is confined to the Wuchiapingian Stage of the late Permian, the Wujiaping Formation in the Nanpanjiang Basin yields biotas of Wuchiapingian and Changhsingian age. This means that, in the study area, the Changxing Formation of Changhsingian age cannot be separated on the basis of lithology from the Wujiaping Formation. In most areas of the Nanpanjiang Basin, the contact of Upper Permian limestones with the overlying Lower Triassic Luolou Formation is conformable, although karstic phenomena may occur locally due to the end-Permian regional regression that affected the entire South China Block (Yin et al., 2014). At Shitouzhai, Upper Permian to Middle Triassic ...

Context 5

... 0.98 and 1.42, with relatively higher and stable values below the SSB and more variable values above the SSB (Fig. 4). Th / U ratios range from 0.34 to 1.56, with values mostly > 1.0 below the SSB and mostly < 1.0 above it (Fig. 4; Appendix C). Ce / Ce* ratios range from 0.73 to 0.88, with higher values below the SSB than above it. The chemical index of alteration (CIA) values range from 0.69 to 0.78 but are consistently higher below the SSB (> 0.75) than above it (< 0.73). Mn / Th ratios are uniformly low (< 300) below the SSB but more variable and generally higher (to ∼ 1900) above the SSB (Fig. 4). Sr concentrations range from 508 to 2160 ppm, and Mn concentrations range from 230 to 3776 ppm (Appendix C). Mn / Sr values are uniformly < 1 below the SSB and range from 0.24 to 3.8 with a median value of 2.1 above the SSB (Fig. B1). All of these elemental proxies exhibit a significant excursion at or close to the SSB (Fig. 4). Bulk accumulation rates (BAR) are higher in the Smithian ( ∼ 11 g cm − 2 kyr − 1 ) than in the Spathian ( ∼ 5 g cm − 2 kyr − 1 ) (Fig. 3b). Carbonate mass accumulation rates (MAR carb ) fluctuated in the range of 7–9 g cm − 2 kyr − 1 below the SSB and declined to 4–5 g cm − 2 kyr − 1 above the SSB. Clay mass accumulation rates (MAR clay ) fluctuated in the range of 2–4 g cm − 2 kyr − 1 below the SSB and declined to < 1 g cm − 2 kyr − 1 above the SSB. On a fine scale below the SSB, MAR carb , and MAR clay varied inversely because carbonates and clays are the two main components of the study section and, hence, produced dilutive effects of one compo- nent by the other. Studies of both modern and ancient carbonates show that a primary seawater signature is characterized by low REE and relative HREE enrichment (Webb et al., 2009). However, carbonate sediments containing even a minor amount of clay minerals tend to acquire a terrigenous REE signal characterized by high REE and strong LREE or MREE enrichment (Sholkovitz and Shen, 1995; Bright et al., 2009). At Shitouzhai, REE exhibits a strong positive correlation with Th ( r = + 0.97; Fig. 5a), indicating that REEs came from the detrital clay fraction not the hydrogenous (seawater) fraction (Zhao et al., 2013). Moreover, the clay fraction (as estimated from Th / Th*) is substantial, ranging from ∼ 10 to 30 % of the total sample, which reflects the argillaceous/muddy char- acter of carbonates in the study section. All samples at Shitouzhai yield Y / Ho ratios of ∼ 30–35 (Appendix C), which are closer to terrestrial values ( ∼ 25– 30) than to seawater values (44–74) (Bau, 1996; Webb et al., 2009). REE also exhibits a modest negative correlation with Y / Ho ( r = − 0.65; Fig. 5b). Thus, a large compo- nent of the REEs in the study section is terrestrially derived, probably through release from clay minerals during diagenesis. Nearly all Eu / Eu* ratios are in the range of 0.9–1.0 (Appendix C), which are typical of crustal rocks and are consistent with the uptake of REEs from clay minerals (McLennan, 2001). MREE enrichment is rather strong (most samples yield Sm N / Yb N > 1.0; Fig. 4), suggesting the presence of phosphate in the sediment or the influence of pore waters previously in contact with phosphate (Kidder and Eddy- Dilek, 1994; Bright et al., 2009). All of the detrital proxies from the study section provide evidence of a major decrease in weathering intensity at the SSB. The age–depth model for the study section (Fig. 3a) shows that the SSB is characterized by a large decline in linear sedimentation rates (LSRs) from 43 to 21 m Myr − 1 and a proportional decrease in bulk accumulation rates (BAR) from 10.7 to 5.3 g cm − 2 kyr − 1 (Fig. 3b). The mass accumulation rates (MAR) of both clays and carbonate also declined across the SSB, although the decline was larger for clays ( ∼ 80– 90 %) than for carbonate ( ∼ 30–40%; Fig. 3b). These proportional differences reflect the greater concentration of clays in Smithian beds relative to Spathian beds. The sharp decline in REE concentrations near the SSB (Fig. 4) is also evidence of a decrease in clay-mineral content upsection. The CIA has been widely used as a proxy for chemical weathering intensity in sediment source regions (Nesbitt and Young, 1982; Goldberg and Humayun, 2010). The abrupt decline in CIA values at Shitouzhai, from ∼ 0.76–0.78 to ∼ 0.70–0.72 (Fig. 4), probably indicates a major decrease in chemical weathering intensity at the SSB. This interpretation is supported by strong correlations of CIA with many detrital proxies, including Al ( r = + 0.87), REE ( r = + 0.81), Th / Th* ( r = + 0.81), and LSR ( r = + 0.93). Although changes in CIA potentially can be due to changes in sediment prove- nance (e.g., Price and Velbel, 2003), the weak correlation of CIA to Eu / Eu* ( r = − 0.21) argues against this interpretation. All detrital proxies for the Shitouzhai section are thus consistent in documenting a major decrease in both chemical and physical weathering intensity at the SSB (Fig. 6). These changes are reflected in lower CIA values, greatly reduced clay-mineral production, and more limited transport of siliciclastics to shallow marine systems. Lower bulk sediment fluxes merely reflect a return to more typical long- term values, however, as the Griesbachian–Smithian inter- val of the Early Triassic was characterized by exceptionally high sediment fluxes and chemical weathering rates (Algeo and Twitchett, 2010). These weathering-related changes at the SSB are likely to have been due to a sharp, ∼ 5 ◦ C temperature decrease in the tropics (Sun et al., 2012; Romano et al., 2013). Even the decline in carbonate flux may have been a consequence of reduced riverine inputs of Ca 2 + and CO 2 3 − ions to marine systems, although other factors such as climatic cooling or changes in oceanic thermohaline circulation may have influenced marine carbonate production. The concentrations of redox-sensitive trace elements (e.g., Mo, U, and V) are low (i.e., close to detrital background values) in all samples from the study section, although there is a slight increase around the SSB, especially on a Th- normalized basis (Appendix C). However, there is an even larger increase in Mn / Th at this level (Fig. 4). Under reducing conditions, Mn 2 + is highly soluble and does not ac- cumulate in substantial amounts in marine sediments. However, suboxic to oxic conditions commonly result in Mn en- richment through the accumulation of Mn(II) in carbonates or Mn(III) in oxyhydroxides (Okita et al., 1988). Strong Mn enrichment is thus common on the margins of reducing deep water masses (Landing and Bruland, 1987). Mn enrichment in carbonates is accepted as a good indicator of suboxic conditions (Rue et al., 1997; Pakhomova et al., 2007). At Shitouzhai, the Mn / Th profile suggests dominantly anoxic conditions below the SSB (0–18 m) and suboxic conditions above it (20–37 m), although with a brief in- terlude of more reducing conditions during the early Spathian (28–32 m; Fig. 4). Cerium (Ce) is the only REE that is affected by oxidation- reduction processes in the Earth-surface environment. Under reducing conditions, Ce 3 + has the same valence as other REEs and, therefore, is not fractionated relative to them, yielding Ce / Ce* ratios of ∼ 1.0 (German and Elderfield, 1990). Under oxidizing conditions, Ce 4 + is preferentially removed from solution, yielding local sedimentary deposits with Ce / Ce* > 1.0, whereas the Ce / Ce* ratio of seawater and of any hydrogenous deposits incorporating REEs from seawater is < 1.0 (e.g., 0.3–0.4 in the modern ocean). Thus, Ce is potentially a good proxy for marine paleoredox conditions, provided that a hydrogenous signal can be measured (Wright et al., 1987). Terrigenous influence (e.g., addition of REEs from clay minerals) will generally cause Ce / Ce* ratios to converge on 1.0, which is by definition the value for average upper crustal rocks. In the study section, Ce / Ce* ratios vary from 0.79 to 0.88 (Fig. 4). These moderately high values are nominally indicative of suboxic conditions. However, the Ce / Ce* ratio was probably heavily influenced by REEs from the clay fraction of the sediment, making the Ce / Ce* ratio of any hydrogenous contribution uncertain. Th / U ratios are useful for paleoredox analysis, owing to the redox-dependent behavior of U. Under oxidizing conditions, U(VI) tends to form stable carbonate complexes in seawater (Langmuir, 1978; Algeo and Maynard, 2004). Under reducing conditions, U(IV) is readily removed to the sediment. Th, however, is not subject to the influence of redox condition, resulting in higher Th / U ratios under reducing conditions as aqueous U is lost (Wignall and Myers, 1988). In the study section, a distinct decrease in the Th / U ratio at the SSB indicates a shift toward more oxygenated conditions, which was sustained into the early Spathian (Fig. 4). These results are consistent with dominantly oxic to suboxic conditions in the study area following the SSB (Fig. 6). Seawater sulfate δ S rose sharply from ∼ + 15 ‰ in the late Permian to > + 30 ‰ in the Middle Triassic (Claypool et al., 1980; Kampschulte and Strauss, 2004), although the pattern of increase during the Early Triassic has only recently be- gun to be worked out (Song et al., 2014). The present study provides the most comprehensive analysis of δ 34 S CAS variation at the SSB of any study to date. The Shitouzhai section exhibits a distinct, ∼ 10–15 ‰ negative shift in δ 34 S CAS that is paired with a ∼ 4 ‰ positive shift in δ 13 C carb (Fig. 4). Both shifts are limited to a narrow interval around the SSB, probably representing no more than ∼ 75–150 kyr based on average sedimentation rates for the study section (Fig. 3a). These two features (i.e., negative covariation and a short event interval) impose significant constraints on the under- lying causes of the isotopic shifts. Most of the Early Triassic is characterized by positive δ 13 C carb – δ 34 S CAS ...

Context 6

... analysis, owing to the redox-dependent behavior of U. Under oxidizing conditions, U(VI) tends to form stable carbonate complexes in seawater (Langmuir, 1978; Algeo and Maynard, 2004). Under reducing conditions, U(IV) is readily removed to the sediment. Th, however, is not subject to the influence of redox condition, resulting in higher Th / U ratios under reducing conditions as aqueous U is lost (Wignall and Myers, 1988). In the study section, a distinct decrease in the Th / U ratio at the SSB indicates a shift toward more oxygenated conditions, which was sustained into the early Spathian (Fig. 4). These results are consistent with dominantly oxic to suboxic conditions in the study area following the SSB (Fig. 6). Seawater sulfate δ S rose sharply from ∼ + 15 ‰ in the late Permian to > + 30 ‰ in the Middle Triassic (Claypool et al., 1980; Kampschulte and Strauss, 2004), although the pattern of increase during the Early Triassic has only recently be- gun to be worked out (Song et al., 2014). The present study provides the most comprehensive analysis of δ 34 S CAS variation at the SSB of any study to date. The Shitouzhai section exhibits a distinct, ∼ 10–15 ‰ negative shift in δ 34 S CAS that is paired with a ∼ 4 ‰ positive shift in δ 13 C carb (Fig. 4). Both shifts are limited to a narrow interval around the SSB, probably representing no more than ∼ 75–150 kyr based on average sedimentation rates for the study section (Fig. 3a). These two features (i.e., negative covariation and a short event interval) impose significant constraints on the under- lying causes of the isotopic shifts. Most of the Early Triassic is characterized by positive δ 13 C carb – δ 34 S CAS covariation, a pattern that is consistent with control by sediment burial fluxes, i.e., co-burial of organic carbon and pyrite, linked to variations in marine productivity and/or redox conditions (Luo et al., 2010; Song et al., 2014). In contrast, negative δ 13 C carb – δ 34 S CAS covariation during a short-term event at the SSB is indicative of oceanographic controls. Specifically, we hypothesize that cooling-driven reinvigoration of oceanic overturning circulation led to stronger upwelling, mixing nutrient- and sulfide-rich deep waters upward into the ocean- surface layer and causing both enhanced marine productivity (hence higher δ 13 C DIC ) and oxidation of advected H 2 S (hence lower δ 34 S sulfate ) (Fig. 6). Such an oceanographic process was inherently transient, lasting only until the nutrients and sulfide that had accumulated in the deep ocean during the Griesbachian–Smithian interval of intense oceanic stratifica- tion (Song et al., 2013) became depleted. The same process was inferred for the latest Spathian by Song et al. (2014), an interval also characterized by short-term negative δ 13 C carb – δ 34 S CAS covariation (Song et al., 2014, their figure 6) and linked to global climatic cooling (Sun et al., 2012). These considerations underscore the fundamental significance of the SSB, which represents the termination of the Early Triassic hyper-greenhouse climate and the reinvigoration of global-ocean overturning circulation (Fig. 6). Oceanographic changes at the SSB had a major effect on contemporaneous marine biotas. Several invertebrate clades, including ammonoids, conodonts, and foraminifera, appear to have suffered severe losses of biodiversity at this time (Orchard, 2007; Stanley, 2009; Song et al., 2011). Ammonoids diversified greatly during the Griesbachian to Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were widespread and characteristic of the SSB. We propose that all of the changes in our model (Fig. 6) were due to a cooling event that commenced following the LSTM and that continued strongly across the SSB. In particular, we infer that cooling led to the reinvigoration of global-ocean overturning circulation. It should be noted that we are not envisioning complete ocean stagnation during the preceding Griesbachian–Smithian interval, which is unlikely, based on physical oceanographic principles (e.g., Kiehl and Shields, 2005), but, rather, a strong slowing of overturning circulation that led to a buildup of nutrients in the deep ocean (Fig. 6). The reinvigoration of global-ocean circulation at the SSB flushed this buildup of nutrients back into the ocean-surface layer, triggering a transient increase in marine productivity and an expansion of thermoclinal anoxia that lasted until this deepwater nutrient source was depleted. The brevity of the SSB anoxic event at Shitouzhai, which lasted ∼ 75–150 kyr, is consistent with such a mechanism. This mechanism also accounts for the abrupt, large positive shift in δ 13 C carb at the SSB, which was due to a productivity-related increase in organic carbon burial rates (Fig. 6). The ultimate cause of the SSB event is uncertain. Given that the onset of the Permian–Triassic boundary crisis has been firmly linked to the initiation of the main eruptive phase of the Siberian Traps large igneous province (STLIP) (Renne et al., 1995; Kamo et al., 2003) and that the Early Triassic was an interval of repeated environmental distur- bances (Algeo et al., 2011; Retallack et al., 2011) and elevated global temperatures (Sun et al., 2012; Romano et al., 2013) linked to volcanogenic greenhouse gas emissions (Retallack and Jahren, 2008; Black et al., 2012), the obvi- ous explanation for the SSB is a reduction in the intensity of magmatic activity in the STLIP source region (Fig. 6). The available radiometric age data for the Siberian Traps, although sparse, are consistent with this possibility. U–Pb dating of perovskites in the early Arydzhangsky flow and zircons from the ...

Context 7

... H 2 S (hence lower δ 34 S sulfate ) (Fig. 6). Such an oceanographic process was inherently transient, lasting only until the nutrients and sulfide that had accumulated in the deep ocean during the Griesbachian–Smithian interval of intense oceanic stratifica- tion (Song et al., 2013) became depleted. The same process was inferred for the latest Spathian by Song et al. (2014), an interval also characterized by short-term negative δ 13 C carb – δ 34 S CAS covariation (Song et al., 2014, their figure 6) and linked to global climatic cooling (Sun et al., 2012). These considerations underscore the fundamental significance of the SSB, which represents the termination of the Early Triassic hyper-greenhouse climate and the reinvigoration of global-ocean overturning circulation (Fig. 6). Oceanographic changes at the SSB had a major effect on contemporaneous marine biotas. Several invertebrate clades, including ammonoids, conodonts, and foraminifera, appear to have suffered severe losses of biodiversity at this time (Orchard, 2007; Stanley, 2009; Song et al., 2011). Ammonoids diversified greatly during the Griesbachian to Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were widespread and characteristic of the SSB. We propose that all of the changes in our model (Fig. 6) were due to a cooling event that commenced following the LSTM and that continued strongly across the SSB. In particular, we infer that cooling led to the reinvigoration of global-ocean overturning circulation. It should be noted that we are not envisioning complete ocean stagnation during the preceding Griesbachian–Smithian interval, which is unlikely, based on physical oceanographic principles (e.g., Kiehl and Shields, 2005), but, rather, a strong slowing of overturning circulation that led to a buildup of nutrients in the deep ocean (Fig. 6). The reinvigoration of global-ocean circulation at the SSB flushed this buildup of nutrients back into the ocean-surface layer, triggering a transient increase in marine productivity and an expansion of thermoclinal anoxia that lasted until this deepwater nutrient source was depleted. The brevity of the SSB anoxic event at Shitouzhai, which lasted ∼ 75–150 kyr, is consistent with such a mechanism. This mechanism also accounts for the abrupt, large positive shift in δ 13 C carb at the SSB, which was due to a productivity-related increase in organic carbon burial rates (Fig. 6). The ultimate cause of the SSB event is uncertain. Given that the onset of the Permian–Triassic boundary crisis has been firmly linked to the initiation of the main eruptive phase of the Siberian Traps large igneous province (STLIP) (Renne et al., 1995; Kamo et al., 2003) and that the Early Triassic was an interval of repeated environmental distur- bances (Algeo et al., 2011; Retallack et al., 2011) and elevated global temperatures (Sun et al., 2012; Romano et al., 2013) linked to volcanogenic greenhouse gas emissions (Retallack and Jahren, 2008; Black et al., 2012), the obvi- ous explanation for the SSB is a reduction in the intensity of magmatic activity in the STLIP source region (Fig. 6). The available radiometric age data for the Siberian Traps, although sparse, are consistent with this possibility. U–Pb dating of perovskites in the early Arydzhangsky flow and zircons from the late Delkansky silicic tuff of extrusive suites in the Maymecha–Kotuy region suggests that the STLIP flood basalt eruptions commenced at 251.7 ± 0.4 Ma and ended at 251.1 ± 0.3 Ma, i.e., representing an interval of ∼ 600 kyr (Renne et al., 1995; Kamo et al., 2003). However, an Ar–Ar date of 250.3 ± 1.1 Ma was obtained for the final stage of extrusive volcanism at Norilsk, the core area of the STLIP (Reichow et al., 2009; see also the review of evidence for a late eruptive stage by Ovtcharova et al., 2006). The more critical issue, in any case, is the duration not of flood basalt eruptions but of intrusive magmatism in the West Siberian Coal Basin, which was probably the main source of volcanogenic greenhouse gases (Retallack and Jahren, 2008; Black et al., 2012). Reichow et al. (2009) reported ages for STLIP-related intrusives spanning several million years, which is consistent with the hypotheses that large-scale intrusive activity continued at least until the SSB and that cessation of most such activity at the SSB was responsible for contemporaneous climatic cooling (Sun et al., 2012; Romano et al., 2013). Further work on the chronology of the STLIP will be needed to conclusively evaluate controls on the SSB event. The SSB event (late Early Triassic) was investigated at Shitouzhai, Guizhou Province, South China, using a multidisci- plinary approach combining carbonate carbon ( δ 13 C carb ) and carbonate-associated sulfate sulfur isotopes ( δ 34 S CAS ) , rare earth elements, and elemental paleoredox and paleoproductivity proxies. The Shitouzhai section exhibits a large ( + 4 ‰) positive δ 13 C carb shift across the SSB similar to that seen in other SSB sections globally, reflecting enhanced marine productivity and organic carbon burial. Various elemental and isotopic proxies also document a major decrease in chemical weathering intensity and detrital sediment input, a shift toward a better-ventilated oceanic thermocline, and a dimin- ished burial flux of reduced sulfur. All of these changes co- incided with a large cooling of sea-surface temperatures that terminated the Early Triassic hothouse regime. The extreme temperatures of the late Smithian thermal maximum (LSTM) may have triggered a biocrisis just prior to the SSB. Ma- rine biotas did not recover immediately in response to climatic and environmental amelioration at the SSB, ...

Context 8

... Ca 2 + and CO 2 3 − ions to marine systems, although other factors such as climatic cooling or changes in oceanic thermohaline circulation may have influenced marine carbonate production. The concentrations of redox-sensitive trace elements (e.g., Mo, U, and V) are low (i.e., close to detrital background values) in all samples from the study section, although there is a slight increase around the SSB, especially on a Th- normalized basis (Appendix C). However, there is an even larger increase in Mn / Th at this level (Fig. 4). Under reducing conditions, Mn 2 + is highly soluble and does not ac- cumulate in substantial amounts in marine sediments. However, suboxic to oxic conditions commonly result in Mn en- richment through the accumulation of Mn(II) in carbonates or Mn(III) in oxyhydroxides (Okita et al., 1988). Strong Mn enrichment is thus common on the margins of reducing deep water masses (Landing and Bruland, 1987). Mn enrichment in carbonates is accepted as a good indicator of suboxic conditions (Rue et al., 1997; Pakhomova et al., 2007). At Shitouzhai, the Mn / Th profile suggests dominantly anoxic conditions below the SSB (0–18 m) and suboxic conditions above it (20–37 m), although with a brief in- terlude of more reducing conditions during the early Spathian (28–32 m; Fig. 4). Cerium (Ce) is the only REE that is affected by oxidation- reduction processes in the Earth-surface environment. Under reducing conditions, Ce 3 + has the same valence as other REEs and, therefore, is not fractionated relative to them, yielding Ce / Ce* ratios of ∼ 1.0 (German and Elderfield, 1990). Under oxidizing conditions, Ce 4 + is preferentially removed from solution, yielding local sedimentary deposits with Ce / Ce* > 1.0, whereas the Ce / Ce* ratio of seawater and of any hydrogenous deposits incorporating REEs from seawater is < 1.0 (e.g., 0.3–0.4 in the modern ocean). Thus, Ce is potentially a good proxy for marine paleoredox conditions, provided that a hydrogenous signal can be measured (Wright et al., 1987). Terrigenous influence (e.g., addition of REEs from clay minerals) will generally cause Ce / Ce* ratios to converge on 1.0, which is by definition the value for average upper crustal rocks. In the study section, Ce / Ce* ratios vary from 0.79 to 0.88 (Fig. 4). These moderately high values are nominally indicative of suboxic conditions. However, the Ce / Ce* ratio was probably heavily influenced by REEs from the clay fraction of the sediment, making the Ce / Ce* ratio of any hydrogenous contribution uncertain. Th / U ratios are useful for paleoredox analysis, owing to the redox-dependent behavior of U. Under oxidizing conditions, U(VI) tends to form stable carbonate complexes in seawater (Langmuir, 1978; Algeo and Maynard, 2004). Under reducing conditions, U(IV) is readily removed to the sediment. Th, however, is not subject to the influence of redox condition, resulting in higher Th / U ratios under reducing conditions as aqueous U is lost (Wignall and Myers, 1988). In the study section, a distinct decrease in the Th / U ratio at the SSB indicates a shift toward more oxygenated conditions, which was sustained into the early Spathian (Fig. 4). These results are consistent with dominantly oxic to suboxic conditions in the study area following the SSB (Fig. 6). Seawater sulfate δ S rose sharply from ∼ + 15 ‰ in the late Permian to > + 30 ‰ in the Middle Triassic (Claypool et al., 1980; Kampschulte and Strauss, 2004), although the pattern of increase during the Early Triassic has only recently be- gun to be worked out (Song et al., 2014). The present study provides the most comprehensive analysis of δ 34 S CAS variation at the SSB of any study to date. The Shitouzhai section exhibits a distinct, ∼ 10–15 ‰ negative shift in δ 34 S CAS that is paired with a ∼ 4 ‰ positive shift in δ 13 C carb (Fig. 4). Both shifts are limited to a narrow interval around the SSB, probably representing no more than ∼ 75–150 kyr based on average sedimentation rates for the study section (Fig. 3a). These two features (i.e., negative covariation and a short event interval) impose significant constraints on the under- lying causes of the isotopic shifts. Most of the Early Triassic is characterized by positive δ 13 C carb – δ 34 S CAS covariation, a pattern that is consistent with control by sediment burial fluxes, i.e., co-burial of organic carbon and pyrite, linked to variations in marine productivity and/or redox conditions (Luo et al., 2010; Song et al., 2014). In contrast, negative δ 13 C carb – δ 34 S CAS covariation during a short-term event at the SSB is indicative of oceanographic controls. Specifically, we hypothesize that cooling-driven reinvigoration of oceanic overturning circulation led to stronger upwelling, mixing nutrient- and sulfide-rich deep waters upward into the ocean- surface layer and causing both enhanced marine productivity (hence higher δ 13 C DIC ) and oxidation of advected H 2 S (hence lower δ 34 S sulfate ) (Fig. 6). Such an oceanographic process was inherently transient, lasting only until the nutrients and sulfide that had accumulated in the deep ocean during the Griesbachian–Smithian interval of intense oceanic stratifica- tion (Song et al., 2013) became depleted. The same process was inferred for the latest Spathian by Song et al. (2014), an interval also characterized by short-term negative δ 13 C carb – δ 34 S CAS covariation (Song et al., 2014, their figure 6) and linked to global climatic cooling (Sun et al., 2012). These considerations underscore the fundamental significance of the SSB, which represents the termination of the Early Triassic hyper-greenhouse climate and the reinvigoration of global-ocean overturning circulation (Fig. 6). Oceanographic changes at the SSB had a major effect on contemporaneous marine biotas. Several invertebrate clades, including ammonoids, conodonts, and foraminifera, appear to have suffered severe losses of biodiversity at this time (Orchard, 2007; Stanley, 2009; Song et al., 2011). Ammonoids diversified greatly during the Griesbachian to Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and ...

Context 9

... REE concentrations near the SSB (Fig. 4) is also evidence of a decrease in clay-mineral content upsection. The CIA has been widely used as a proxy for chemical weathering intensity in sediment source regions (Nesbitt and Young, 1982; Goldberg and Humayun, 2010). The abrupt decline in CIA values at Shitouzhai, from ∼ 0.76–0.78 to ∼ 0.70–0.72 (Fig. 4), probably indicates a major decrease in chemical weathering intensity at the SSB. This interpretation is supported by strong correlations of CIA with many detrital proxies, including Al ( r = + 0.87), REE ( r = + 0.81), Th / Th* ( r = + 0.81), and LSR ( r = + 0.93). Although changes in CIA potentially can be due to changes in sediment prove- nance (e.g., Price and Velbel, 2003), the weak correlation of CIA to Eu / Eu* ( r = − 0.21) argues against this interpretation. All detrital proxies for the Shitouzhai section are thus consistent in documenting a major decrease in both chemical and physical weathering intensity at the SSB (Fig. 6). These changes are reflected in lower CIA values, greatly reduced clay-mineral production, and more limited transport of siliciclastics to shallow marine systems. Lower bulk sediment fluxes merely reflect a return to more typical long- term values, however, as the Griesbachian–Smithian inter- val of the Early Triassic was characterized by exceptionally high sediment fluxes and chemical weathering rates (Algeo and Twitchett, 2010). These weathering-related changes at the SSB are likely to have been due to a sharp, ∼ 5 ◦ C temperature decrease in the tropics (Sun et al., 2012; Romano et al., 2013). Even the decline in carbonate flux may have been a consequence of reduced riverine inputs of Ca 2 + and CO 2 3 − ions to marine systems, although other factors such as climatic cooling or changes in oceanic thermohaline circulation may have influenced marine carbonate production. The concentrations of redox-sensitive trace elements (e.g., Mo, U, and V) are low (i.e., close to detrital background values) in all samples from the study section, although there is a slight increase around the SSB, especially on a Th- normalized basis (Appendix C). However, there is an even larger increase in Mn / Th at this level (Fig. 4). Under reducing conditions, Mn 2 + is highly soluble and does not ac- cumulate in substantial amounts in marine sediments. However, suboxic to oxic conditions commonly result in Mn en- richment through the accumulation of Mn(II) in carbonates or Mn(III) in oxyhydroxides (Okita et al., 1988). Strong Mn enrichment is thus common on the margins of reducing deep water masses (Landing and Bruland, 1987). Mn enrichment in carbonates is accepted as a good indicator of suboxic conditions (Rue et al., 1997; Pakhomova et al., 2007). At Shitouzhai, the Mn / Th profile suggests dominantly anoxic conditions below the SSB (0–18 m) and suboxic conditions above it (20–37 m), although with a brief in- terlude of more reducing conditions during the early Spathian (28–32 m; Fig. 4). Cerium (Ce) is the only REE that is affected by oxidation- reduction processes in the Earth-surface environment. Under reducing conditions, Ce 3 + has the same valence as other REEs and, therefore, is not fractionated relative to them, yielding Ce / Ce* ratios of ∼ 1.0 (German and Elderfield, 1990). Under oxidizing conditions, Ce 4 + is preferentially removed from solution, yielding local sedimentary deposits with Ce / Ce* > 1.0, whereas the Ce / Ce* ratio of seawater and of any hydrogenous deposits incorporating REEs from seawater is < 1.0 (e.g., 0.3–0.4 in the modern ocean). Thus, Ce is potentially a good proxy for marine paleoredox conditions, provided that a hydrogenous signal can be measured (Wright et al., 1987). Terrigenous influence (e.g., addition of REEs from clay minerals) will generally cause Ce / Ce* ratios to converge on 1.0, which is by definition the value for average upper crustal rocks. In the study section, Ce / Ce* ratios vary from 0.79 to 0.88 (Fig. 4). These moderately high values are nominally indicative of suboxic conditions. However, the Ce / Ce* ratio was probably heavily influenced by REEs from the clay fraction of the sediment, making the Ce / Ce* ratio of any hydrogenous contribution uncertain. Th / U ratios are useful for paleoredox analysis, owing to the redox-dependent behavior of U. Under oxidizing conditions, U(VI) tends to form stable carbonate complexes in seawater (Langmuir, 1978; Algeo and Maynard, 2004). Under reducing conditions, U(IV) is readily removed to the sediment. Th, however, is not subject to the influence of redox condition, resulting in higher Th / U ratios under reducing conditions as aqueous U is lost (Wignall and Myers, 1988). In the study section, a distinct decrease in the Th / U ratio at the SSB indicates a shift toward more oxygenated conditions, which was sustained into the early Spathian (Fig. 4). These results are consistent with dominantly oxic to suboxic conditions in the study area following the SSB (Fig. 6). Seawater sulfate δ S rose sharply from ∼ + 15 ‰ in the late Permian to > + 30 ‰ in the Middle Triassic (Claypool et al., 1980; Kampschulte and Strauss, 2004), although the pattern of increase during the Early Triassic has only recently be- gun to be worked out (Song et al., 2014). The present study provides the most comprehensive analysis of δ 34 S CAS variation at the SSB of any study to date. The Shitouzhai section exhibits a distinct, ∼ 10–15 ‰ negative shift in δ 34 S CAS that is paired with a ∼ 4 ‰ positive shift in δ 13 C carb (Fig. 4). Both shifts are limited to a narrow interval around the SSB, probably representing no more than ∼ 75–150 kyr based on average sedimentation rates for the study section (Fig. 3a). These two features (i.e., negative covariation and a short event interval) impose significant constraints on the under- lying causes of the isotopic shifts. Most of the Early Triassic is characterized by positive δ 13 C carb – δ 34 S CAS covariation, a pattern that is consistent with control by sediment burial fluxes, i.e., co-burial of organic carbon and pyrite, linked to variations in marine productivity and/or redox conditions (Luo et al., 2010; Song et al., 2014). In contrast, negative δ 13 C carb – δ 34 S CAS covariation during a short-term event at the SSB is indicative of oceanographic controls. Specifically, we hypothesize that cooling-driven reinvigoration of oceanic overturning circulation led to stronger upwelling, mixing nutrient- and sulfide-rich deep waters upward into the ocean- surface layer and causing both enhanced marine productivity (hence higher δ 13 C DIC ) and oxidation of advected H 2 S (hence lower δ 34 S sulfate ) (Fig. 6). Such an oceanographic process was inherently transient, lasting only until the nutrients and sulfide that had accumulated in the deep ocean during the Griesbachian–Smithian interval of intense oceanic stratifica- tion (Song et al., 2013) became depleted. The same process was inferred for the latest Spathian by Song et al. (2014), an interval also characterized by short-term negative δ 13 C carb – δ 34 S CAS covariation (Song et al., 2014, their figure 6) and linked to global climatic cooling (Sun et al., 2012). These considerations underscore the fundamental significance of the SSB, which represents the termination of the Early Triassic hyper-greenhouse climate and the reinvigoration of global-ocean overturning circulation (Fig. 6). Oceanographic changes at the SSB had a major effect on contemporaneous marine biotas. Several invertebrate clades, including ammonoids, conodonts, and foraminifera, appear to have suffered severe losses of biodiversity at this time (Orchard, 2007; Stanley, 2009; Song et al., 2011). Ammonoids diversified greatly during the Griesbachian to Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported ...

Context 10

... of Mn(II) in carbonates or Mn(III) in oxyhydroxides (Okita et al., 1988). Strong Mn enrichment is thus common on the margins of reducing deep water masses (Landing and Bruland, 1987). Mn enrichment in carbonates is accepted as a good indicator of suboxic conditions (Rue et al., 1997; Pakhomova et al., 2007). At Shitouzhai, the Mn / Th profile suggests dominantly anoxic conditions below the SSB (0–18 m) and suboxic conditions above it (20–37 m), although with a brief in- terlude of more reducing conditions during the early Spathian (28–32 m; Fig. 4). Cerium (Ce) is the only REE that is affected by oxidation- reduction processes in the Earth-surface environment. Under reducing conditions, Ce 3 + has the same valence as other REEs and, therefore, is not fractionated relative to them, yielding Ce / Ce* ratios of ∼ 1.0 (German and Elderfield, 1990). Under oxidizing conditions, Ce 4 + is preferentially removed from solution, yielding local sedimentary deposits with Ce / Ce* > 1.0, whereas the Ce / Ce* ratio of seawater and of any hydrogenous deposits incorporating REEs from seawater is < 1.0 (e.g., 0.3–0.4 in the modern ocean). Thus, Ce is potentially a good proxy for marine paleoredox conditions, provided that a hydrogenous signal can be measured (Wright et al., 1987). Terrigenous influence (e.g., addition of REEs from clay minerals) will generally cause Ce / Ce* ratios to converge on 1.0, which is by definition the value for average upper crustal rocks. In the study section, Ce / Ce* ratios vary from 0.79 to 0.88 (Fig. 4). These moderately high values are nominally indicative of suboxic conditions. However, the Ce / Ce* ratio was probably heavily influenced by REEs from the clay fraction of the sediment, making the Ce / Ce* ratio of any hydrogenous contribution uncertain. Th / U ratios are useful for paleoredox analysis, owing to the redox-dependent behavior of U. Under oxidizing conditions, U(VI) tends to form stable carbonate complexes in seawater (Langmuir, 1978; Algeo and Maynard, 2004). Under reducing conditions, U(IV) is readily removed to the sediment. Th, however, is not subject to the influence of redox condition, resulting in higher Th / U ratios under reducing conditions as aqueous U is lost (Wignall and Myers, 1988). In the study section, a distinct decrease in the Th / U ratio at the SSB indicates a shift toward more oxygenated conditions, which was sustained into the early Spathian (Fig. 4). These results are consistent with dominantly oxic to suboxic conditions in the study area following the SSB (Fig. 6). Seawater sulfate δ S rose sharply from ∼ + 15 ‰ in the late Permian to > + 30 ‰ in the Middle Triassic (Claypool et al., 1980; Kampschulte and Strauss, 2004), although the pattern of increase during the Early Triassic has only recently be- gun to be worked out (Song et al., 2014). The present study provides the most comprehensive analysis of δ 34 S CAS variation at the SSB of any study to date. The Shitouzhai section exhibits a distinct, ∼ 10–15 ‰ negative shift in δ 34 S CAS that is paired with a ∼ 4 ‰ positive shift in δ 13 C carb (Fig. 4). Both shifts are limited to a narrow interval around the SSB, probably representing no more than ∼ 75–150 kyr based on average sedimentation rates for the study section (Fig. 3a). These two features (i.e., negative covariation and a short event interval) impose significant constraints on the under- lying causes of the isotopic shifts. Most of the Early Triassic is characterized by positive δ 13 C carb – δ 34 S CAS covariation, a pattern that is consistent with control by sediment burial fluxes, i.e., co-burial of organic carbon and pyrite, linked to variations in marine productivity and/or redox conditions (Luo et al., 2010; Song et al., 2014). In contrast, negative δ 13 C carb – δ 34 S CAS covariation during a short-term event at the SSB is indicative of oceanographic controls. Specifically, we hypothesize that cooling-driven reinvigoration of oceanic overturning circulation led to stronger upwelling, mixing nutrient- and sulfide-rich deep waters upward into the ocean- surface layer and causing both enhanced marine productivity (hence higher δ 13 C DIC ) and oxidation of advected H 2 S (hence lower δ 34 S sulfate ) (Fig. 6). Such an oceanographic process was inherently transient, lasting only until the nutrients and sulfide that had accumulated in the deep ocean during the Griesbachian–Smithian interval of intense oceanic stratifica- tion (Song et al., 2013) became depleted. The same process was inferred for the latest Spathian by Song et al. (2014), an interval also characterized by short-term negative δ 13 C carb – δ 34 S CAS covariation (Song et al., 2014, their figure 6) and linked to global climatic cooling (Sun et al., 2012). These considerations underscore the fundamental significance of the SSB, which represents the termination of the Early Triassic hyper-greenhouse climate and the reinvigoration of global-ocean overturning circulation (Fig. 6). Oceanographic changes at the SSB had a major effect on contemporaneous marine biotas. Several invertebrate clades, including ammonoids, conodonts, and foraminifera, appear to have suffered severe losses of biodiversity at this time (Orchard, 2007; Stanley, 2009; Song et al., 2011). Ammonoids diversified greatly during the Griesbachian to Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were widespread and characteristic of the SSB. We propose that all of the changes in our model (Fig. 6) were due to a cooling event that commenced following the LSTM and that continued strongly across the SSB. In particular, we infer that cooling led to the reinvigoration of global-ocean ...

Context 11

... 2010; Song et al., 2014). In contrast, negative δ 13 C carb – δ 34 S CAS covariation during a short-term event at the SSB is indicative of oceanographic controls. Specifically, we hypothesize that cooling-driven reinvigoration of oceanic overturning circulation led to stronger upwelling, mixing nutrient- and sulfide-rich deep waters upward into the ocean- surface layer and causing both enhanced marine productivity (hence higher δ 13 C DIC ) and oxidation of advected H 2 S (hence lower δ 34 S sulfate ) (Fig. 6). Such an oceanographic process was inherently transient, lasting only until the nutrients and sulfide that had accumulated in the deep ocean during the Griesbachian–Smithian interval of intense oceanic stratifica- tion (Song et al., 2013) became depleted. The same process was inferred for the latest Spathian by Song et al. (2014), an interval also characterized by short-term negative δ 13 C carb – δ 34 S CAS covariation (Song et al., 2014, their figure 6) and linked to global climatic cooling (Sun et al., 2012). These considerations underscore the fundamental significance of the SSB, which represents the termination of the Early Triassic hyper-greenhouse climate and the reinvigoration of global-ocean overturning circulation (Fig. 6). Oceanographic changes at the SSB had a major effect on contemporaneous marine biotas. Several invertebrate clades, including ammonoids, conodonts, and foraminifera, appear to have suffered severe losses of biodiversity at this time (Orchard, 2007; Stanley, 2009; Song et al., 2011). Ammonoids diversified greatly during the Griesbachian to Smithian but underwent a major evolutionary turnover at the SSB, fol- lowed by a stepwise increase in biodiversity in the early to middle Spathian (Brayard et al., 2009). Conodonts show a similar pattern, with a rapid radiation in the early to middle Smithian terminated by a severe extinction at the SSB, fol- lowed by a second radiation in the early to middle Spathian (Orchard, 2007). Changes in biodiversity were mirrored by changes in body size. Chen et al. (2013) documented a brief but significant size reduction among conodonts, coin- ciding with the late Smithian thermal maximum (Sun et al., 2012), based on bulk sample analysis from an outcrop section in Guizhou Province, southwestern China. Conodonts remained diminutive during the SSB transition and the earliest Spathian and then underwent a stepwise size increase during the early to middle Spathian (Chen et al., 2013). Although literature surveys show that marine clades such as conodonts, ammonoids, and foraminifera experienced a sharp decline in diversity at the SSB (Orchard, 2007; Stanley, 2009; Song et al., 2011), this pattern may be biased by data binning effects. In fact, an examination of the stratigraphic distribution of these marine clades in actual geolog- ical sections suggests that diversity losses occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were widespread and characteristic of the SSB. We propose that all of the changes in our model (Fig. 6) were due to a cooling event that commenced following the LSTM and that continued strongly across the SSB. In particular, we infer that cooling led to the reinvigoration of global-ocean overturning circulation. It should be noted that we are not envisioning complete ocean stagnation during the preceding Griesbachian–Smithian interval, which is unlikely, based on physical oceanographic principles (e.g., Kiehl and Shields, 2005), but, rather, a strong slowing of overturning circulation that led to a buildup of nutrients in the deep ocean (Fig. 6). The reinvigoration of global-ocean circulation at the SSB flushed this buildup of nutrients back into the ocean-surface layer, triggering a transient increase in marine productivity and an expansion of thermoclinal anoxia that lasted until this deepwater nutrient source was depleted. The brevity of the SSB anoxic event at Shitouzhai, which lasted ∼ 75–150 kyr, is consistent with such a mechanism. This mechanism also accounts for the abrupt, large positive shift in δ 13 C carb at the SSB, which was due to a productivity-related increase in organic carbon burial rates (Fig. 6). The ultimate cause of the SSB event is uncertain. Given that the onset of the Permian–Triassic boundary crisis has been firmly linked to the initiation of the main eruptive phase of the Siberian Traps large igneous province (STLIP) (Renne et al., 1995; Kamo et al., 2003) and that the Early Triassic was an interval of repeated environmental distur- bances (Algeo et al., 2011; Retallack et al., 2011) and elevated global temperatures (Sun et al., 2012; Romano et al., 2013) linked to volcanogenic greenhouse gas emissions (Retallack and Jahren, 2008; Black et al., 2012), the obvi- ous explanation for the SSB is a reduction in the intensity of magmatic activity in the STLIP source region (Fig. 6). The available radiometric age data for the Siberian Traps, although sparse, are consistent with this possibility. U–Pb dating of perovskites in the early Arydzhangsky flow and zircons from the late Delkansky silicic tuff of extrusive suites in the Maymecha–Kotuy region suggests that the STLIP flood basalt eruptions commenced at 251.7 ± 0.4 Ma and ended at 251.1 ± 0.3 Ma, i.e., representing an interval of ∼ 600 kyr (Renne et al., 1995; Kamo et al., 2003). However, an Ar–Ar date of 250.3 ± 1.1 Ma was obtained for the final stage of extrusive volcanism at Norilsk, the core area of the STLIP (Reichow et al., 2009; see also the review of evidence for a late eruptive stage by Ovtcharova et al., 2006). The more critical issue, in any case, is the duration not of flood basalt eruptions but of intrusive magmatism in the West Siberian Coal Basin, which was probably the main source of volcanogenic greenhouse gases (Retallack and Jahren, 2008; Black et al., 2012). Reichow et al. (2009) reported ages for STLIP-related intrusives spanning several million years, which is consistent with the hypotheses that large-scale intrusive activity continued at least until the SSB and that cessation of most such activity at the SSB was responsible for contemporaneous climatic cooling (Sun et al., 2012; Romano et al., 2013). Further work on the chronology of the STLIP will be needed to conclusively evaluate controls on the SSB event. The SSB event (late Early Triassic) was investigated at Shitouzhai, Guizhou Province, South China, using a multidisci- plinary approach combining carbonate carbon ( δ 13 C carb ) and carbonate-associated sulfate sulfur isotopes ( δ 34 S CAS ) , rare earth elements, and elemental paleoredox and paleoproductivity proxies. The Shitouzhai section exhibits a large ( + 4 ‰) positive δ 13 C carb shift across the SSB similar to that seen in other SSB sections globally, reflecting enhanced marine productivity and organic carbon burial. Various elemental and isotopic proxies also document a major decrease in chemical weathering intensity and detrital sediment input, a ...

Context 12

... occurred slightly prior to the SSB (Zhao et al., 2007; Song et al., 2011; Zakharov and Popov, 2014) and were probably associated with the late Smithian thermal maximum (Sun et al., 2012; Romano et al., 2013; Fig. 6) rather than the Smithian–Spathian boundary itself. The affected marine clades also did not recover immediately when climatic and environmental conditions ameliorated abruptly at the SSB but, rather, underwent a stepwise recovery during the early to middle Spathian (Orchard, 2007; Stanley, 2009; Brayard et al., 2009). The SSB was characterized by a major change in terrestrial flora. Lycopsid-dominated assemblages were replaced by conifer-dominated or mixed lycopsid-conifer vegetation, as indicated by palynological data from Pakistan (Hermann et al., 2011), Norway (Galfetti et al., 2007; Hochuli and Vigran, 2010), and central Europe (Kurscher and Herngreen, 2010). A similar floral change was reported from the Spathian– Anisian boundary in Hungary (Looy et al., 1999), suggesting some variation in the timing of terrestrial floral recovery in different regions of the world. Macrofloral fossil evidence indicates a more volatile record of vegetation change, with multiple short-term expansions of lycopsids from tropical regions temporarily displacing conifers during the Olenekian (Retallack et al., 2011; Hochuli et al., 2010; Looy et al., 2001). These inferences are supported by biomarker and bio- geochemical studies. Saito et al. (2013) reported that sediments of Griesbachian to Smithian age yield carbon/nitrogen (C / N) ratios < 10 and contain abundant retene, simonel- lite, and dehydroabietan, which are interpreted to have been sourced from lycopsids and/or bryophytes. After the SSB, sediments yield C / N ratios > 10 and exhibit a large increase in pimanthrene abundance, suggesting the dominance of terrestrial floras by conifers. As a result, a highly diverse conif- erous flora became widely reestablished around the SSB, re- placing the lycopsid- and fern-dominated disaster-type vegetation that had dominated the Griesbachian to Smithian interval (Saito et al., 2013; Fig. 6). The SSB was also characterized by major environmental changes. Strong climatic cooling has been inferred from both faunal (Galfetti et al., 2007) and oxygen-isotope evidence (Sun et al., 2012; Romano et al., 2013). Changes in oceanic circulation appear to have occurred at the same time. Saito et al. (2013) interpreted an increase in extended tricyclic ter- pane ratios (ETR) around the SSB as being due to a shift from limited to vigorous overturning circulation (Fig. 6). These climatic and oceanographic changes were probably linked: an increase in the intensity of global meridional circulation would have been a natural consequence of climatic cooling (e.g., Rind, 1998), leading to more vigorous deepwater formation in high-latitude regions (Kiehl and Shields, 2005). The environmental and climatic changes documented at Shitouzhai reinforce observations made in other SSB sections globally and, thus, serve to demonstrate that these changes were widespread and characteristic of the SSB. We propose that all of the changes in our model (Fig. 6) were due to a cooling event that commenced following the LSTM and that continued strongly across the SSB. In particular, we infer that cooling led to the reinvigoration of global-ocean overturning circulation. It should be noted that we are not envisioning complete ocean stagnation during the preceding Griesbachian–Smithian interval, which is unlikely, based on physical oceanographic principles (e.g., Kiehl and Shields, 2005), but, rather, a strong slowing of overturning circulation that led to a buildup of nutrients in the deep ocean (Fig. 6). The reinvigoration of global-ocean circulation at the SSB flushed this buildup of nutrients back into the ocean-surface layer, triggering a transient increase in marine productivity and an expansion of thermoclinal anoxia that lasted until this deepwater nutrient source was depleted. The brevity of the SSB anoxic event at Shitouzhai, which lasted ∼ 75–150 kyr, is consistent with such a mechanism. This mechanism also accounts for the abrupt, large positive shift in δ 13 C carb at the SSB, which was due to a productivity-related increase in organic carbon burial rates (Fig. 6). The ultimate cause of the SSB event is uncertain. Given that the onset of the Permian–Triassic boundary crisis has been firmly linked to the initiation of the main eruptive phase of the Siberian Traps large igneous province (STLIP) (Renne et al., 1995; Kamo et al., 2003) and that the Early Triassic was an interval of repeated environmental distur- bances (Algeo et al., 2011; Retallack et al., 2011) and elevated global temperatures (Sun et al., 2012; Romano et al., 2013) linked to volcanogenic greenhouse gas emissions (Retallack and Jahren, 2008; Black et al., 2012), the obvi- ous explanation for the SSB is a reduction in the intensity of magmatic activity in the STLIP source region (Fig. 6). The available radiometric age data for the Siberian Traps, although sparse, are consistent with this possibility. U–Pb dating of perovskites in the early Arydzhangsky flow and zircons from the late Delkansky silicic tuff of extrusive suites in the Maymecha–Kotuy region suggests that the STLIP flood basalt eruptions commenced at 251.7 ± 0.4 Ma and ended at 251.1 ± 0.3 Ma, i.e., representing an interval of ∼ 600 kyr (Renne et al., 1995; Kamo et al., 2003). However, an Ar–Ar date of 250.3 ± 1.1 Ma was obtained for the final stage of extrusive volcanism at Norilsk, the core area of the STLIP (Reichow et al., 2009; see also the review of evidence for a late eruptive stage by Ovtcharova et al., 2006). The more critical issue, in any case, is the duration not of flood basalt eruptions but of intrusive magmatism in the West Siberian Coal Basin, which was probably the main source of volcanogenic greenhouse gases (Retallack and Jahren, 2008; Black et al., 2012). Reichow et al. (2009) reported ages for STLIP-related intrusives spanning several million years, which is consistent with the hypotheses that large-scale intrusive activity continued at least until the SSB and that cessation of most such activity at the SSB was responsible for contemporaneous climatic cooling (Sun et al., 2012; Romano et al., 2013). Further work on the chronology of the STLIP will be needed to conclusively evaluate controls on the SSB event. The SSB event (late Early Triassic) was investigated at Shitouzhai, Guizhou Province, South China, using a multidisci- plinary approach combining carbonate carbon ( δ 13 C carb ) and carbonate-associated sulfate sulfur isotopes ( δ 34 S CAS ) , rare earth elements, and elemental paleoredox and paleoproductivity proxies. The Shitouzhai section exhibits a large ( + 4 ‰) positive δ 13 C carb shift across the SSB similar to that seen in other SSB sections globally, reflecting enhanced marine productivity and organic carbon burial. Various elemental and isotopic proxies also document a major decrease in chemical weathering intensity and detrital sediment input, a shift toward a better-ventilated oceanic thermocline, and a dimin- ished burial flux of reduced sulfur. All of these changes co- incided with a large cooling of sea-surface temperatures that terminated the Early Triassic hothouse regime. The extreme temperatures of the late Smithian thermal maximum (LSTM) may have triggered a biocrisis just prior to the SSB. Ma- rine biotas did not recover immediately in response to climatic and environmental amelioration at the SSB, however, but underwent a stepwise recovery during the early to middle Spathian. The cause of the SSB event is uncertain but may have been related to a reduction in intrusive magmatic activity in the Siberian Traps large igneous province. The study section is located at Shitouzhai village (GPS: 25 ◦ 45 9.6 N, 106 ◦ 6 29.7 E), about 3 km east of Ziyun County town in southern Guizhou Province, South China (Fig. A1). During the Early to Middle Triassic, the Ziyun area was located on the southern margin of the Yangtze Platform, to the north of the Nanpanjiang Basin (Enos et al., 2006). The paleogeographic configuration of the Ziyun area changed from a platform-margin reef system in the latest Permian to a platform-ramp environment in the Early Triassic (Feng et al., 1997). In this area of the Nanpanjiang Basin in southern Guizhou Province, the Upper Permian successions usually comprise bioclastic rocks, which are collectively assigned to the Wujiaping Formation. However, unlike the same formation exposed elsewhere in South China, which is confined to the Wuchiapingian Stage of the late Permian, the Wujiaping Formation in the Nanpanjiang Basin yields biotas of Wuchiapingian and Changhsingian age. This means that, in the study area, the Changxing Formation of Changhsingian age cannot be separated on the basis of lithology from the Wujiaping Formation. In most areas of the Nanpanjiang Basin, the contact of Upper Permian limestones with the overlying Lower Triassic Luolou Formation is conformable, although karstic phenomena may occur locally due to the end-Permian regional regression that affected the entire South China Block (Yin et al., 2014). At Shitouzhai, Upper Permian to Middle Triassic strata are assigned to the Wujiaping, Luolou, and Xinyuan formations, in ascending order (Ding and Huang, 1990). The upper Wujiaping Formation consists largely of massive sponge reef limestone and yields the fusulinid Paleofusulina sinse and the conodont Clarkina changxingensis , both of which point to a late Changhsingian age (Ding and Huang, 1990; Shen and Xu, 2005; Wu et al., 2010). The Luolou Formation is composed of thin-bedded calcareous mudstone, muddy limestone, and vermicular limestone with interbeds of breccia, from which conodont zones of definite Early Triassic age have been established (Ding and Huang, 1990). The lower Xinyuan Formation is also well exposed and consists of thin- ...