Palaeogeography, Palaeoclimatology, Palaeoecology 363–364 (2012) 1–10
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Late Permian to Early Triassic environmental changes in the Panthalassic Ocean: Record from the seamount-associated deep-marine siliceous rocks, central Japan Hiroyoshi Sano ⁎, Takuya Wada 1, Hiroshi Naraoka Department of Earth and Planetary Sciences, Kyushu University, Fukuoka, 812‐8581 Japan
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Article history: Received 11 January 2012 Received in revised form 9 July 2012 Accepted 20 July 2012 Available online 10 September 2012 Keywords: Permian–Triassic boundary Panthalassic Ocean Radiolarian chert Anoxia Mino terrane
a b s t r a c t In order to infer the late Late Permian to early Early Triassic environmental changes in a pelagic realm of the Panthalassic Ocean, the stratigraphic variations of TOC (wt.%) and δ13Corg (‰) of the PTB siliceous rock section (~3.5 m in thickness) were examined. The study section crops out in the Mino terrane, central Japan, and consists of the upper Upper Permian chert (Changhsingian) and lower Lower Triassic (Induan) black claystone intermittently with thin chert beds. The succession is reconstructed as sediments on the lower slope of a mid-Panthalassic seamount. The major extinction event of Permian radiolarians (MEE) occurs at the top of the Changhsingian chert. Our analysis shows that the TOC content is markedly higher in the Induan black claystone than the Changhsingian chert. TOC values rapidly increase across the PTB. The onset of the increase in TOC values corresponds to MEE. The rapid and profound increase in TOC values implies the sudden onset of oceanic anoxia as well as the rapid increase in primary production at the PTB. δ13Corg values are generally higher in the Changhsingian chert than the Induan claystone and chert. The stepwise drop of δ13Corg values characterizes their excursion of the study section. The largest-scale drop occurs prior to MEE. The minimum δ13Corg value is recorded at the base of the Induan claystone. The flux of δ13C-depleted gasses related to the Siberian Traps could have significantly contributed to the marked drop of δ13Corg values. Due to the weakened ocean circulation under the predominant condition of global warming, oceanic anoxia suddenly took place at the end of the Permian. The anoxic condition in the surface layer was the most likely kill-mechanism for Permian radiolarians. The anoxic condition persisted into the Induan, but was intermittently weakened. The episodic relaxation of the anoxic condition in the Induan resulted in intermittent and transient blooming of radiolarians and deposition of radiolarian chert with lowered TOC values. These biotic and depositional records of the study section imply the mid-Panthalassic Ocean was under the more hospitable conditions favored by the earlier recovery of radiolarians, relative to the Tethys Ocean and Pangean marginal seas. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The Permian–Triassic boundary (PTB: ca. 251.0 Ma: Ogg et al., 2008; ca. 252.3 Ma: Mundil et al., 2010; ca 252.17 Ma: Shen et al., 2011a, 2011b) is known as a period when the most profound collapse both of marine and terrestrial ecosystems and the global environmental devastation occurred (Erwin, 1994). The palaeontological studies of fossil records show that approximately 90% of marine and terrestrial species went extinct at the end of the Permian (Sepkoski, 1984). This profound extinction resulted in the largest and most abrupt biotic turnover of the Phanerozoic (Erwin, 1994). Concurrently with the end-Permian biotic
⁎ Corresponding author at: Department of Earth and Planetary Sciences, Kyushu University, 6-10-1 Hakozaki, Fukuoka, 812‐8581 Japan. Tel.: +81 92 642 2606; fax: +81 92 642 2686. E-mail address:
[email protected] (H. Sano). 1 Present address: Hiji Branch of Oita Bank Co. Ltd., 2982 Hiji, Oita 879‐1506, Japan. 0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.palaeo.2012.07.018
crisis, O2-depleted and sulfidic conditions expanded both in shallowmarine (e.g., Wignall and Hallam, 1992; Nielsen and Shen, 2004; Grice et al., 2005; Cao et al., 2009; Bond and Wignall, 2010; Shen et al., 2011a, 2011b) and deep-marine settings (e.g., Isozaki, 1997a; Wignall and Newton, 2003). A great deal of palaeontological, sedimentological, chronological, and geochemical studies on the PTB biotic and environmental crises have been carried out mostly in shallow-marine carbonate and siliciclastic facies on the Tethyan platforms and peri-Pangean shelves (e.g., Yin et al., 2001; Krull et al., 2004; Wignall et al., 2005; Orchard, 2007; Korte and Kozur, 2010; Mundil et al., 2010; Shen et al., 2010) and in continental facies (e.g., Retallack and Jahren, 2008; Krassilov and Karasev, 2009; Hochuli et al., 2010). Also PTB oceanographic and atmospheric conditions have been numerically simulated (e.g., Kidder and Worsley, 2004; Kiehl and Shields, 2005; Roscher et al., 2011; Winguth and Winguth, 2012). These studies significantly enhanced our understanding of the PTB biotic and environmental crises.
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By contrast, PTB events in the Panthalassic Ocean, especially those in its pelagic setting, are less well known despite its vast extent. This is mainly due to the scarcity of stratigraphically intact, age-constrained PTB sections from this region. The lack of knowledge on PTB events in the Panthalassic Ocean may bias our understanding of PTB biotic and environmental changes. In order to enhance our understanding of the PTB environmental changes, we present high-resolution geochemical profiles of the Panthalassic upper Upper Permian to lower Lower Triassic siliceous rock section of the Mino terrane, central Japan. We analyzed total organic carbon (TOC) content and organic carbon isotopic values (δ 13Corg). As the study section is precisely dated on the basis of radiolarian and conodont fossils by Sano et al. (2010), we also discuss the biotic and depositional responses to the environmental changes in the Permian–Triassic transition. The examined siliceous rocks are reconstructed as pelagic, deepmarine sediments resting upon the lower flank of a seamount (Sano et al., 1992), inferred to have been located in a low-latitude zone of the mid-Panthalassic Ocean in Late Permian time (Mino seamount: Fig. 1-1). The study section crops out in the Mt. Funabuseyama area in the western part of the Mino Massif, central Japan (Fig. 1-2). The examined rocks correspond to the upper part of the Lower Permian to Lower
Triassic Hashikadani Formation characterized by pelagic, deep-marine siliceous rocks (Sano, 1988; Kuwahara et al., 2010).
2. Geologic setting The Mino terrane comprises a Jurassic subduction-generated accretionary complex of unmetamorphosed sedimentary rocks with subordinate volcanic rocks. Its major exposure areas are in central Japan (Fig. 1-2). According to Wakita (1988), the accretionary rocks of the Mino terrane are divided into seven, fault-bounded tectonostratigraphic units, including two coherent units and five mélange units. The coherent units comprise a deep-marine oceanic plate stratigraphy consisting of Lower Triassic siliceous claystone, Lower Triassic to lowest Cretaceous radiolarian chert and related siliceous rocks, and upper Middle Jurassic to lowest Cretaceous trench-fill turbidites. The mélange units are characterized by the chaotic mixing of blocks and slabs of the Permian to Jurassic oceanic assemblage with the matrix of the Early Jurassic to earliest Cretaceous scaly mudstone. The Mt. Funabuseyama area is chiefly underlain by the Permian oceanic rocks and the Jurassic mudstone of the Funabuseyama Unit
Fig 1. Maps for the location of the Mino seamount and study section. (1) Mino seamount in a low latitude zone of the western Panthalassic Ocean, shown on the Late Permian palaeogeographic map courtesy of R. Blakey (http://jan.ucc.nau.edu/~rcb7/globaltext2.html). Convergent margins modified from C. R. Scotese (http://www.scotese.com/ newpage5.htm). Location of Mino seamount from Isozaki (1997b). (2) Study section in the western part of the Mino Massif indicated on the base map illustrating an approximate distribution of the Mino terrane and its equivalents in central Japan (Nakae, 2000).
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Section NF 1212C consists of the lower unit of grey chert, middle unit of black to dark grey chert with a chocolate brown-weathered silty layer at the top, and upper unit of black claystone with thin, black chert beds (Fig. 3). The total thickness reaches approximately 3.5 m (Fig. 4-1). Grey, dark grey, and black chert of the lower and middle units is thinly bedded with clayey partings (Fig. 4-2, ‐3). The individual chert beds range in thickness from 2 to 5 cm. The color of the clayey partings matches that of the alternating chert beds. The chocolate brown-weathered bed at the top of the middle unit is inferred to be a pyrite-rich sediment (Fig. 4–3), although its pristine lithologic composition has been lost owing to intense chemical weathering (Sano et al., 2010). An abrupt facies change marks the base of the upper unit (Fig. 4‐1, ‐3). Black claystone of the upper unit is extremely fine-grained and fissile, highly carbonaceous, and contains thin, black chert beds at several levels with sharply defined top and bottom surfaces (Fig. 4-4). Faint, grey parallel laminae occur in the black claystone. According to Sano et al. (2010), grey and black chert of the study section contains radiolarian remains with minor siliceous sponge spicules and scattered tiny pyrite grains. Black claystone of the upper unit consists of a mixture of clays minerals and cryptocrystalline quartz with a great admixture of carbonaceous matter. All the examined PTB rocks of section NF 1212C lack both coarse-grained terrigenous clastic material and calcitic skeletal debris. This is consistent with deposition in a pelagic setting at great water depths in the open ocean setting far beyond the reach of coarse terrigenous grains.
3.2. Biostratigraphy and age Fig. 2. Locality of the study section, NF 1212C, indicated on the simplified geologic map of the Mt. Funabuseyama area after Sano (1988). A = Amanokawara plateau, N = Neohigashidanigawa River, K = Kanzakigawa River. The mapped area is indicated in Fig. 1.
The chert succession of the lower and middle units corresponds to the Neoalbaillella optima Zone (Kuwahara et al., 1998) correlated with the upper Changhsingian (Fig. 3: Sano et al., 2010). The basal part of the upper unit is referable to the Hindeodus parvus Zone indicative of the basal Triassic (Yin et al., 2001). The remaining part of the upper unit is correlated broadly with the lower Lower Triassic (Induan). The
(Fig. 2), defined as the Middle Jurassic mélange unit (Wakita, 1988). Sano (1988) divided the Permian oceanic rocks in the Mt. Funabuseyama area into the Funabuseyama Formation of shallowmarine limestone facies, Amanokawara Formation of carbonate breccia facies, and Hashikadani Formation of deep-marine chert facies, all of which are commonly underlain by basaltic rocks. The Funabuseyama and Amanokawara formations are correlated with the upper Lower to upper Middle Permian and upper Lower Permian, respectively. The Hashikadani Formation ranges from the middle Lower Permian to the upper Lower Triassic (Sano, 1988; Kuwahara et al., 2010). Sano et al. (1992) interpreted the Funabuseyama, Amanokawara, and Hashikadani formations as sediments at the top, upper slope, and lower flank of a seamount in a mid-oceanic setting, respectively. The geochemical attributes of the basaltic rocks indicate their hotspot origin (Jones et al., 1993), supporting the facies interpretation by Sano et al. (1992). 3. Materials 3.1. Lithology The examined PTB siliceous rock section, NF 1212C, crops out along the logging road on Amanokawara plateau in the northern part of the Mt. Funabuseyama area (Fig. 2). The study section corresponds exactly to section NF 1212R, lithostratigraphically and biostratigraphically studied by Sano et al. (2010). However, as we found additional black chert beds in the upper part of section NF 1212R, we partly amended the lithostratigraphy established by Sano et al. (2010).
Fig. 3. Lithology, fossil zones, and age of the study section. The fossil zones and age follow Sano et al. (2010). See Fig. 2 for the locality.
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Fig. 4. Outcrop views of the examined PTB rocks of the study section. 1. 2. 3. 4.
Entire view of section NF 1212C comprising the intact succession of the upper Upper Permian (Changhsingsian) chert (P) and lower Lower Triassic (Induan) black claystone (T). The yellow and blue rectangles indicate the enlarged areas in panels 2 and 3, respectively. Upper Upper Permian ribbon-bedded grey chert with grey clayey partings of the lower unit. The area of the view is shown by the yellow rectangle in panel 1. Close-up view of the PTB, indicated by an arrow, corresponding to the sharp lithologic boundary between the chocolate brown-weathered, probably pyrite-rich layer (CB) and the basal Triassic black claystone (T), the base of which bears the PTB-indicative conodont, Hindeodus parvus. Thin black chert beds, indicated by arrows, intercalated in the lower Lower Triassic black claystone.
age of the chocolate brown layer immediately above the upper Changhsingian chert of the middle unit remains uncertain. However, we presume that this layer is correlated with the uppermost Permian, because fist-sized pyrite nodules occur embedded in the chert bed near the top of the Changhsingian of the PTB siliceous rock section comparable with the study section (Sano et al., 2012). Sano et al. (2010) positioned the PTB at the base of the upper unit, which corresponds to the marked lithologic boundary between the middle unit and the upper unit (Fig. 3).
We confirmed that the carbon content of all the samples is invariant through the processing by using HCl. Also, according to the results of the petrographic observation by Sano et al. (2010), neither carbonate nor sulfate minerals are contained in the PTB siliceous rocks of section NF 1212C. These mean that carbon in the analyzed samples is due to organic carbon. 5. Results of geochemical analyses 5.1. Total organic carbon (TOC)
4. Analytical methods We collected 63 samples of chert and black claystone from section NF 1212C. Each chert bed was collected and black claystone samples were collected at 10-cm stratigraphic intervals. Little weathered and veined 50 samples were selected for analyses of total organic carbon (TOC), including 34 samples of the Changhsingian chert, 11 samples of the Induan black claystone, and 5 samples of the Induan chert. Carbon isotope measurements (δ 13Corg) were performed for 48 samples including 32 samples of the Changhsingian chert, 11 samples of the Induan claystone, and 5 samples of the Induan chert. Weathered parts of each sample were carefully removed using a rotary handpiece, and then the samples were crushed and powdered. Before the analyses, the powdered samples were soaked with ca. 6 M HCl, followed by leaving out in a container with concentrated HCl (~12 M) overnight to remove carbonate carbon, and dried in a desiccator with NaOH pellets. The acid-processed samples were analyzed for TOC contents by using the elemental analyzer (Fisons Instrumental NA 1500 NCS) at Kyushu University. The δ 13Corg value was determined using the Thermo Finnigan DELTA-S mass spectrometer coupled with the elemental analyzer in on-line mode at Kyushu University.
Concentration of TOC in the upper unit is markedly higher than that of the lower and middle units; 0.20 to 2.37 wt.% in the former, while 0.01 wt.% or less to 0.08 wt.% in the latter (Fig. 5‐1). The rapid and profound increase of the TOC content occurs across the PTB (Fig. 5‐1). The higher TOC content in the upper unit is consistent with the carbonaceous property of black claystone and black chert. The marked increase in TOC across the PTB is comparable to that in the Ubara section and probably with that in the Gujo-hachiman section (Algeo et al., 2010b). The profound TOC increase in these sections commonly corresponds to the rapid lithologic change from the grey chert or siliceous claystone into the overlying carbonaceous black claystone. The TOC content in the lower and middle units of the study section is generally low and stable (Fig. 5‐1). However, a closer look at the profile shows that the slight increase of TOC commences in the upper part of the lower unit, ca. 0.76 m below the PTB (Fig. 5‐2). The black to dark grey color of the chert in this part is consistent with its increased TOC contents (Fig. 4‐3). This slight increase in TOC values is comparable with that in the PTB siliceous rocks section from another Jurassic accretionary complex in northern Japan (Takahashi et al., 2009). These authors showed the onset of the slight increase approximately 0.5 m below the marked lithologic boundary
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Fig. 5. Geochemical profiles of the PTB siliceous rocks of the study section. 1. 2. 3.
Concentration of total organic carbon for the entire section (TOC, wt.%). Concentration of total organic carbon exaggerated for the Changhsingian (upper Upper Permian) chert section (wt.%). Organic carbon isotope values (δ13Corg, VPDB ‰) with division of the carbon isotope zone (CIZ) at the right margin. See Fig. 6 for the explanation of the abbreviations MEE and PTB. The age-assignment from Sano et al. (2010).
between the Changhsingian siliceous claystone and the overlying black claystone. The TOC values largely fluctuate in the upper unit (Fig. 5‐1). The fluctuation is due to the much lower TOC values in the intercalated black chert relative to the black claystone. The TOC concentration of the black chert beds of the upper unit is even higher than that of the chert of the lower and middle units. 5.2. Organic carbon isotopic values (δ 13Corg) δ13Corg values are generally higher in the lower and middle units than the upper unit (Fig. 5‐3). They range in the lower, middle, and upper units from −27.95‰ to −19.10‰ (−24.60‰ in average), –28.74‰ to −22.62‰ (−25.04‰ in average), and −30.06‰ to −26.15‰ (−27.84‰ in average), respectively. The study section exhibits the large fluctuation of δ 13Corg values and records their stepwise decrease (Fig. 5‐3). On the basis of the stratigraphic variation of δ 13Corg values, we divided their excursion into the five zones in the study section (carbon isotope zone: CIZ 1 to CIZ 5 of Fig. 5‐3). CIZ 1 (~ 137-cm-thick zone above the base of the section) nearly corresponds to the entire of the lower unit and is characterized by the slow increase of δ 13Corg values with a few drops (e.g., ~ 4‰ drop 42 cm above the base of the section). CIZ 1 ends with ca. 5.5‰ rapid positive shift near its top, which is followed by the sudden and profound negative shift of CIZ 2 (~ 11 cm thick), ca. 8. 8‰ drop from − 19.10‰ down to − 27.95‰. It is noted that the negative shift in CIZ 2 is larger-scale than the two major negative shifts in CIZ 4 near the PTB. CIZ 2 is succeeded by CIZ 3 (~ 60 cm thick) characterized by the positive shift of δ 13Corg values (~ 5.3‰) with the negative shift in the upper part. CIZ 4 (~ 25 cm thick) is characterized the large fluctuation of δ 13Corg values including two sharp negative shifts that are separated by one positive shift. The first negative shift (~ 6.1‰ from − 22.62‰ down to − 28.74‰, the minimum value in
the Changhsingian chert) occurs immediately below MEE (major extinction event: Fig. 6). This negative shift near the top of the Changhsingian chert is followed by ca. 4.2‰ positive shift at MEE, which is, in turn, followed by the second negative shift of CIZ 4 (~ 5.5‰ drop) across the PTB. The δ 13Corg value rapidly decreases to − 30.06‰, the minimum of the study section near the bottom of the Induan black claystone. CIZ 5 (~ 120 cm thick) is defined by the slow increase of the δ 13Corg values in the lower part of the upper unit, which is followed by the nearly constant δ 13Corg values (− 26 to − 27‰) in the upper part of the upper unit.
6. Diversity change of Permian radiolarians and correlation with geochemical signals Sano et al. (2010) showed the stratigraphic distribution of 56 species of 37 genera of moderately preserved Late Permian radiolarians in the Changhsingian chert succession of section NF 1212R, which corresponds exactly to the lower and middle units of the study section. On the basis of Sano et al. (2010), we examined the stratigraphic variation of the species number of the Permian radiolarians as a proxy for their temporal biodiversity change (Fig. 6). The change in the number of species with the stratigraphic position shows the profound diversity loss at the top of the Changhsingian chert succession, ca. 10 cm below the PTB (Fig. 6). The major extinction event of Permian radiolarians no doubt occurred at the end of Permian (MEE in Fig. 6). It is noted that MEE took place synchronously with the marked increase of the TOC content at the end of the Permian (Figs. 5, 6). This close correlation implies the causal link between the major diversity loss of the Permian radiolarians and the rapid environmental change represented by the sudden increase of TOC. On the other hand, no clear correlation is recognized between the major diversity loss of Permian radiolarians and the δ 13Corg excursion (Figs. 5, 6). For instance,
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thermal metamorphism, corresponding to the post-oil generation stage. Taking into account the loss of organic matter during thermal maturation, these authors estimated that the organic matter content in the Lower Triassic black claystone was ca. 3 wt.% before thermal maturation, comparable to TOC values of sediments in modern active upwelling regions. Suzuki et al. (1998) suggested that the high TOC values of the Lower Triassic black claystone resulted from elevated primary productivity mainly due to a rapid bloom of planktons with an additional contribution of bacterial activity, caused by the upwelling of nutrient-rich deep-waters. Both of the Lower Triassic black claystones examined by the present study and Suzuki et al. (1998) occur in Jurassic accretionary complexes which have commonly suffered from low-grade metamorphism, usually pumpellyite–actnolite facies to prehnite–pumpellyite facies or less (e.g., Takami and Itaya, 1996). Also, these Lower Triassic black claystones are similar in lithofacies and TOC concentration to each other. These similarities imply that both of these Lower Triassic claystones experienced the comparable thermal maturation and consequent loss of organic carbon. Thus, the organic matter content before the thermal maturation of the Induan black claystone of the study section is inferred to be approximately 3 wt.%, comparable to that for the Lower Triassic black claystone estimated by Suzuki et al. (1998). 7.2. Oceanic anoxia Fig. 6. Stratigraphic variation of the number of Permian radiolarian species in the PTB siliceous rock section NF 1212R drawn from Fig. 8 of Sano et al. (2010). Abbreviations MEE and PTB stand for the main extinction event horizons of Permian radiolarians and Permian–Triassic boundary, respectively.
the largest negative shift of δ 13Corg values of the study section (CIZ 2 in Fig. 5‐3) occurs much prior to MEE. 7. Discussion 7.1. Primary productivity On the basis of the rapid rise of TOC values in conjunction with MEE and the marked lithologic change at the PTB of the study section (Fig. 5‐1), we infer the increase in primary productivity across the PTB concurrently with the decrease of biogenic silica supply due to the marked decline of radiolarian production. Lowered TOC values of the Induan chert beds represent the episodic decrease of primary production and short-lived blooming of radiolarians. The slight increase of the TOC concentration in the upper part of the lower unit implies that an increase in primary production in the PTB transition slowly commenced precursorily as early as the late Late Permian (Fig. 5‐2). Several authors have investigated the stratigraphic variation of TOC in the Japanese PTB siliceous rock sections (Tenjinmaru section: Kajiwara et al., 1994; Ashimidani: Suzuki et al., 1998; Akkamori: Takahashi et al., 2009; Ubara and Gujo-hachiman: Algeo et al., 2010a, b). The patterns of the stratigraphic variation of TOC in these sections are similar to one another and also to our study section. Low TOC values in the Changhsingian chert (0.01 to 0.06 wt.%: Takahashi et al., 2009; ~0.2 ± 0.1 wt.%: Algeo et al., 2010a; 0.13 ± 0.06 wt.%: Algeo et al., 2010b) and increased TOC values in the overlying black claystone (e.g., 2.2 wt.% in average: Suzuki et al., 1998; 1.06–3.31 wt.%: Takahashi et al., 2009; 1.2 ±0.5 wt.%: Algeo et al., 2010a; 0.52 ± 0.35 wt.%: Algeo et al., 2010b) characterize the PTB siliceous rocks in Japanese accretionary terranes. Using biogeochemical parameters for the thermal maturity of organic matter, Suzuki et al. (1998)) showed the Lower Triassic black claystone experienced a maximum temperature of ca. 140 °C during
Rapidly elevated TOC values in the Induan black claystone of the study section imply the sudden onset of oceanic anoxia as well as the rapid increase in primary productivity at the PTB. Anoxic conditions in bottom waters contributed to the enhanced preservation of organic carbon. Represented by the slight increase in TOC values (Fig. 5‐2), weakly anoxic or suboxic conditions may have commenced precursorily as early as in the late Late Permian, prior to the rapid onset of the major anoxic event at the end of the Permian. The anoxic conditions and elevated primary productivity persisted into the Induan, although episodically relaxed, and resulted in the deposition of highly carboniferous black claystone. The episodic relaxation of the anoxic conditions caused the intermittent deposition of radiolarian chert with lowered TOC values. The intermittent environmental relaxation event may correspond with brief and episodic oxidizing events during the early Early Triassic inferred from the lowered δ 34S (Kajiwara et al., 1994). Takahashi et al. (2009) attributed high TOC values of the uppermost Permian to upper Lower Triassic black claystone to the enhanced preservation of organic carbon under deep-sea anoxic conditions. These authors inferred that primary productivity was maintained or slightly increased in the PTB transition. Algeo et al. (2010b) also considered the high TOC concentration in black claystone as a result of the increase in primary productivity and intensified euxinic conditions in the oxygen-minimum zone within the intermediate water mass. The redox changes in the Panthalassic Ocean are also investigated by using other geological and geochemical proxies of the accreted PTB siliceous rocks in Japan. With an emphasis upon the lithofacies change as well as the stratigraphic variation of colors of the siliceous rocks, Isozaki (1997a) hypothesized the progressive shift of redox conditions from oxic through suboxic to anoxic–euxinic conditions during the early Late Permian (Wuchiapingian) to early Middle Triassic (Anisian) interval. Anoxic conditions are thought to have expanded from the deep ocean to surface environments in the Changhsingian, resulting in the extinction of Permian radiolarians and the cessation of chert deposition, and to have culminated during late Changhsingian to Dienerian time (Isozaki, 1997a). The Mösbauer spectroscopic examination of iron reduction status (Matsuo et al., 2003) and the quantitative analyses of the major, trace, and rare earth elements (Kato et al., 2002) have supported the hypothesis by Isozaki (1997a). On the basis of the stratigraphic change of pyritic sulfur isotopic ratios (δ 34S), Kajiwara et al. (1994) suggested
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stagnant and stratified ocean and anoxic deep water conditions (higher δ 34S) in the early Late Permian to Early Triassic, which were disturbed by brief and episodic oxidizing events (lower δ 34S) during the early Early Triassic due to massive oceanic mixing. The recent study of the petrography and size-analysis of pyrite suggests that weakly suboxic conditions in the latest Permian changed to the euxinic to anoxic conditions at the PTB, which was followed by early Early Triassic weakly suboxic conditions punctuated by euxinic to anoxic episodes (Wignall et al., 2010).
Mundil et al., 2010), we suggest that the flux of 13C-depleted gasses due to the Siberian Trap volcanism, especially the release of significantly 13C-depleted metamorphic carbon gasses (ca. −30 to −25‰: Svensen et al., 2009) could have contributed to the stepwise drop of δ13Corg values of the study section.
7.3. Comparison of δ 13C profile
Geological records as well as numerical modeling show that the late Late Permian warm climate was coupled with an increase of the atmospheric CO2 levels (Taylor et al., 1992; Crowley and Berner, 2001; Retallack et al., 2003; Montenegro et al., 2011; Roscher et al., 2011). The Late Permian to Early Triassic pronounced polar warming weakened the meridional overturning ocean circulation (Kidder and Worsley, 2004; Kiehl and Shields, 2005) and generated ocean stratification, which led to extensive anoxic conditions (Hotinski et al., 2001). Although the warm climate mode was punctuated by the end-Permian transient cooling episodes due to the injection of aerosols and ash from the Siberian Traps to the atmosphere (Rampino and Self, 1984; Campbell et al., 1992; Krassilov and Karasev, 2009), the late Late Permian warmth is the significant forcing condition in terms of biotic and environmental crises during the PTB transition (Joachimski et al., 2012). The coincidence of MEE and the onset of the marked increase of TOC (Fig. 5‐1) means that photic-zone anoxia generated by the weakened meridional ocean circulation due to the Late Permian to Early Triassic pronounced polar warming was the most likely kill mechanism of Permian radiolarians. Kiessling and Danelian (2011) also hypothesized that the photic zone anoxia was the essential trigger of the extinction of radiolarians. Radiolarians are considered to be have been less affected by hypercapnia (Knoll et al., 2007) and ocean acidification (Payne et al., 2007) than calcifying organisms.
The study section exhibits the stepwise decrease of δ 13Corg values by ca. 11‰ within the ca. 86 cm-thick-stratigraphic interval of the PTB transition (CIZ 2 to CIZ 4 in Fig. 5‐3). It is noted that the largest-scale negative shift of δ 13Corg took place much prior to MEE. The comparable stepwise drop of δ13Corg is reported from PTB sections of many regions (Panthallasic Ocean: Musashi et al., 2001; Takahashi et al., 2010; Tethyan platforms and Pangean shelf seas: Cao et al., 2002; Korte and Kozur, 2010; Shen et al., 2010; Hermann et al., 2010; Luo et al., 2011). For example, Takahashi et al. (2010) showed the δ13Corg excursion pattern in the Panthalassic deep-marine siliceous rock section highly comparable with that of our study section. The δ13Corg excursion shown by Takahashi et al. (2010) includes the variation within the Changhsingian chert comprising the gradual increase, first negative shift, and positive shift in ascending order, which is followed by the second drop across the marked lithologic boundary between the Changhsingian chert and the overlying black claystone and then a gradual increase in the black claystone. The first and second drops are correlated with the negative shifts in CIZ 2 and CIZ 4 of the study section. It is noted that the negative shift of the first step is larger in scale than that of the second step. Luo et al. (2011) showed the negative shift of δ13Ccarb in two steps in the four PTB shallow-marine carbonate sections in South China. Hermann et al. (2010) documented a stepwise 8‰ decline to the first minimum around the PTB in Trøndelag and Finnmark Platform, Norway. The stepwise decline is followed by a subsequent positive shift and then a rapid negative shift to the second minimum in the basal Induan. According to these authors, the negative shift of the first step occurs prior to the main biotic crisis event and is larger in magnitude than that of the second step around the main crisis. Causes for the marked decrease of δ13Corg values in the PTB transition have been long discussed by many researchers in the context of the perturbation of the global carbon cycle. The major hypotheses include erosion of organic matter and soils (Baud et al., 1989), diminished transport of organic matter to deep-waters due to the decline of marine and terrestrial primary productivity (Wang et al., 1994; Rampino and Caldeira, 2005), mixing of 13C-depleted anoxic-euxinic deep waters into shallow levels (Knoll et al., 1996; Hotinski et al., 2001; Kump et al., 2005; Riccardi et al., 2007), dissociation of isotopically light methane hydrates in shelf sediments and permafrost soils (Erwin, 1993; Krull and Retallack, 2000), release of 13C-depleted volcanic (Grard et al., 2005; Hansen, 2006) and metamorphic (Retallack and Jahren, 2008; Ganino and Arndt, 2009; Svensen et al., 2009) carbon gasses related to the Siberian Traps (Kamo et al., 2003; Reichow et al., 2009; Buslov et al., 2010), and the combined effects of all these events. Among these hypothesized causes, enhanced erosion of marine and on-land organic matter and collapse of methane hydrates are unlikely causes, because of the mid-Panthalassic palaeoposition and great water-depths of the depositional site of the study section. We do not rule out the possibility of the marked decline of the biomass in the Panthalassic Ocean. However, the marked rise of TOC commenced later than the first negative shift of δ13Corg (Fig. 5‐1, ‐3). This implies that the large-scale drop of δ13Corg was unlikely caused by the change of the primary productivity. With an emphasis upon the essentially synchronous onset of the main Siberian Traps volcanic activity with the age of the PTB (Kamo et al., 2003;
8. Biotic and depositional responses to PTB environmental changes 8.1. Diversity decline of Permian radiolarians
8.2. Early Triassic recovery of radiolarians and chert sedimentation Several authors have documented the early biotic recovery in the aftermath of the end-Permian crisis. For an instance, Brayard et al. (2009) showed that the rapid diversity recovery of ammonoid less than 2 m y after the PTB on the basis of the analysis of a global diversity data set of ammonoid genera. Also, Brayard et al. (2011) reported the significantly earlier occurrence of sponge-serpulid reefs under the temporary favorable conditions in the late Induan (early Smithian) of the western USA. Song et al. (2011) showed the onset of the diversity recovery of foraminifers in South China as early as the late Induan (early Smithian), ca. 1 m y after the end-Permian crisis. These paleontological records of some marine taxa from the Tethys and marginal Pangean regions have questioned claims for an approximately 5 m y delay in the biotic recovery from the end-Permian crisis. The further earlier recovery of radiolarians is recently reported from the Panthalassic region. Sano et al. (2010) reported the occurrence of the Mesozoic-type nassellarian species (Hozmadia sp.) with several types of sphaeroid spumellarians from the Induan chert of the study section. In addition, Sano et al. (2012) recognized the occurrence of Triassospongocyrtis? sp. of a primitive Mesozoic nassellarian species from the Hindeodus parvus-bearing, lowest Induan chert in the Mino terrane. Also at Arrow Rocks, New Zealand, the Mesozoic-type nassellarians are recognized in the upper Induan chert (Kamata et al., 2007). Thus, the Mesozoic-type radiolarians are inferred to have appeared much earlier in the Panthalassic Ocean than previously documented in Tethyan and marginal Pangean PTB sections (e.g., late Olenekian: O'Dogherty et al., 2010). Related to the profound decline of radiolarians and siliceous sponges, the Early Triassic is known as the period of the approximately 5-million year-long global disappearance of chert deposition (Early
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Triassic chert gap: Racki, 1999; Kidder and Erwin, 2001; Beauchamp and Baud, 2002). The Early Triassic chert gap is thought to be one of the important sedimentary events in the PTB transition mainly on the basis of studies in the Tethys Ocean and Pangean marginal basins. The chert in the upper unit of the study section, however, was deposited during the interval of the Early Triassic chert gap. Near the study section, Sano et al. (2012) also reported the occurrence of the Hindeodus parvus‐bearing, lower Induan chert intercalated in the black claystone of the PTB siliceous rock section comparable with the study section. The lower Induan radiolarian chert sedimentation is also reported from the PTB siliceous rock sections with the Panthalassic affinity at Arrow Rocks, New Zealand (Aita and Spörli, 2007; Takemura et al., 2007; Yamakita et al., 2007). On the basis of the discovery of diverse and ecologically complex, early Induan (middle to late Griesbachian) marine faunas mainly of mollusks and brachiopods from an oxygenated seamount setting in the southern margin of the Tethys Ocean, Twichett et al. (2004) concluded the marked delay in the biotic recovery from the end-Permian crisis was due to extensive and long-lived anoxia. Their conclusion implies that marine ecosystems are possible to recover earlier under less anoxic harsh conditions. Thus, the intermittently lowered TOC values of the Induan chert of the study section are inferred to represent episodically weakened anoxic conditions. The surface layer in the mid-Panthalassic Ocean was intermittently under less ecologically stressed, more hospitable conditions, which possibly generated the transient and episodic blooming of radiolarians and their deposition during early Induan, than the Tethys Ocean and Pangean marginal seas. The temporary environmental relaxation event may correspond with brief and episodic oxidizing events during the early Early Triassic inferred from the lowered δ34S (Kajiwara et al., 1994). 9. Summary The stratigraphic variations of TOC (wt.%) and δ 13Corg (‰) of the PTB siliceous rock section were analyzed to infer late Late Permian to Early Triassic environmental changes in the mid-Panthalassic Ocean. The study section crops out in the Mino terrane, central Japan. The examined PTB succession comprises the lower unit of gray chert (~ 1.6 m thick), middle unit of black chert (~ 0.7 m), and upper unit of highly carbonaceous black claystone (~ 1.2 m) with thin black chert beds at several levels. The lower and middle units are correlated with the Changhsingian and the upper unit is refered to the Induan. The PTB succession is reconstructed as sediments on the lower slope of a seamount in the mid-Panthalassic Ocean. The PTB is placed at the bottom of the upper unit that marks the rapid and sharp lithologic change. The major diversity loss of Permian radiolarians occurs at the top of the Changhsingian chert. The TOC values are generally higher in the upper unit than the lower and middle units. The rapid onset of the increase in TOC occurs across the PTB. The upper unit exhibits a large fluctuation of TOC, which is due to the much lower TOC values in the intercalated black chert relative to the black claystone. The stratigraphic variation of δ 13Corg values in the study section is characterized by their stepwise decrease in the PTB transition. The first and rapid negative shift (~ 8.8%) occurs at the top of the lower unit and records the largest magnitude of the drop in the study section. The second and third negative shifts are recorded immediately below MEE and across the PTB, respectively. The δ 13Corg values decrease down to the minimum (− 30.06%) in the basal part of the Induan black claystone, which is followed by the slow increase in the upper unit. Rapidly elevated TOC values in the Induan black claystone of the study section imply the sudden onset of oceanic anoxia at the end of the Permian as well as the rapid increase in primary productivity across the PTB. The coincidence of the marked diversity loss of
Permian radiolarians and the sudden onset of the increase in the TOC content means that photic-zone anoxia generated by the weakened meridional ocean circulation due to the Late Permian to Early Triassic pronounced polar warming was the most likely kill mechanism of Permian radiolarians. The anoxic conditions persisted into the Induan, but were intermittently weakened. The temporary relaxation of the anoxic conditions resulted in episodic and transient deposition of radiolarian chert with lowered TOC values. We suggest that the flux of 13C-depleted gasses due to the Siberian Trap volcanism and related thermal metamorphism could have significantly contributed to the stepwise drop of δ 13Corg values. The sedimentary and palaeontological records of the study section imply that the radiolarians and their accumulation recovered earlier in the pelagic realm of the Panthalassic Ocean than Tethys Ocean and Pangean marginal seas. We infer that the mid-Panthalassic Ocean was under the more hospitable conditions, favored by the earlier recovery of radiolarians with temporal environmental relaxation, than the Tethyan and Pangean seas. Acknowledgments The authors express our sincere thanks to Neo Kaihatsu Limited Company (Motosu City, Gifu) for their kind permission for fieldwork along the logging road on the Amanokawara plateau. Special thanks go to Lynn Soreghan, who much improved the English of the draft. Ms. S. Ukon and Mr. R. Sakamoto assisted a part of the geochemical analyses. We thank two anonymous reviewers for their constructive suggestion. References Aita, Y., Spörli, K.B., 2007. Geological framework for the pelagic Permian/Triassic oceanic sequence of Arrow Rocks, Waipapa Terrane, Northland. In: Spörli, K.B., Takemura, A., Hori, R.S. (Eds.), The oceanic Permian/Triassic boundary sequence at Arrow Rocks (Oruatemanu), Northland, New Zealand: GNS Science Monograph, 24. Institute of Geological & Nuclear Sciences Ltd., Lower Hutt, pp. 1–16. Algeo, T.J., Hinnov, L., Moser, J., Maynard, J.B., Elswick, E., Kuwahara, K., Sano, H., 2010a. Changes in productivity and redox conditions in the Panthalassic Ocean during the latest Permian. Geology 38, 187–190. Algeo, T.J., Kuwahara, K., Sano, H., Bates, S., Lyons, T., Hinnov, L., Elwood, B., Moser, J., Maynard, J.B., 2010b. Spatial variation in sediment fluxes, redox conditions, and productivity in the Permian–Triassic Panthalassic Ocean. Palaeogeography Palaeoclimatology Palaeoecology 308, 65–83. Baud, A., Magaritz, M., Holser, W.T., 1989. Permian–Triassic of the Tethys: carbon isotope studies. Geologische Rundschau 78, 649–677. Beauchamp, B., Baud, A., 2002. Growth and demise of Permian biogenic chert along northwest Pangea: evidence for end-Permian collapse of thermohaline circulation. Palaeogeography Palaeoclimatology Palaeoecology 184, 37–63. Bond, D.P.G., Wignall, P.B., 2010. Pyrite framboid study of marine Permian–Triassic boundary sections: a complex anoxic event and its relationship to contemporaneous mass extinction. Geological Society of America Bulletin 122, 1265–1279. Brayard, A., Escarguel, G., Bucher, H., Monnet, C., Brühwiler, T., Goudemand, N., Galfetti, T., Guex, J., 2009. Good genes and good luck: ammonoid diversity and the endPermian mass extinction. Science 325, 1118–1121. Brayard, A., Vennin, E., Oliver, N., Bylund, K.G., Jenks, J., Stephen, D.A., Bucher, H., Hofmann, R., Goudemann, N., Escarguel, G., 2011. Transient metazoan reefs in the aftermath of the end-Permian mass extinction. Nature Geoscience 4, 693–697. Buslov, M.M., Safonova, I.Yu., Fedoseev, G.S., Reichow, M.K., Davies, K., Babin, G.A., 2010. Permo-Triassic plume magmatism of the Kuznetzk Basin, Central Asia: geology, geochronology, and geochemistry. Russian Geology and Geophysics 51, 1021–1036. Campbell, I.H., Czamanske, G.K., Fedorenko, V.A., Hill, R.I., Stepanov, V., 1992. Synchronism of the Siberian Traps and the Permian–Triassic boundary. Science 258, 1760–1763. Cao, C., Wang, W., Jin, Y., 2002. Carbon isotope excursions across the Permian–Triassic boundary in the Meishan section, Zhejian Province, China. Chinese Science Bulletin 47, 1125–1149. Cao, C., Love, G.D., Hays, L.E., Wang, W., Shen, S., Summons, R.E., 2009. Biogeochemical evidence for euxinic oceans and ecological disturbance presaging the end-Permian mass extinction event. Earth and Planetary Science Letters 281, 188–201. Crowley, T.J., Berner, R.A., 2001. CO2 and climate change. Science 292, 870–872. Erwin, D.H., 1993. The Great Paleozoic Crisis: Life and Death in the Permian (Critical Moments in Paleobiology and Earth History Series). Columbia Univ, Press, New York. Erwin, D.H., 1994. The Permo‐Triassic extinction. Nature 367, 231–236. Ganino, C., Arndt, N.T., 2009. Climate changes caused by degassing of sediments during the emplacement of large igneous provinces. Geology 37, 323–326.
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