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Volcanism, Mass Extinction, and Carbon Isotope Fluctuations in the Middle Permian of China

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Science  29 May 2009:
Vol. 324, Issue 5931, pp. 1179-1182
DOI: 10.1126/science.1171956

Abstract

The 260-million-year-old Emeishan volcanic province of southwest China overlies and is interbedded with Middle Permian carbonates that contain a record of the Guadalupian mass extinction. Sections in the region thus provide an opportunity to directly monitor the relative timing of extinction and volcanism within the same locations. These show that the onset of volcanism was marked by both large phreatomagmatic eruptions and extinctions amongst fusulinacean foraminifers and calcareous algae. The temporal coincidence of these two phenomena supports the idea of a cause-and-effect relationship. The crisis predates the onset of a major negative carbon isotope excursion that points to subsequent severe disturbance of the ocean-atmosphere carbon cycle.

The temporal link between mass extinction events and large igneous province volcanism is one of the most intriguing relationships in Earth’s history, with the end-Permian extinction–Siberian Traps association being the most celebrated (1, 2), but the causal link is far from resolved. A major problem is that the site of volcanism can rarely be directly correlated with the marine extinction record (3), and so comparison can only be achieved with the use of geochronological bio- and chemostratigraphic correlation techniques, with their inherent timing inaccuracies. To clarify some of these relations, we have studied the Emeishan flood basalt province in southwest China, where Middle Permian platform limestones pass up into a volcanic pile with interbedded limestones. These record both a marine extinction record and a major C isotope excursion. Thus, we were able to document multiple phenomena associated with the Middle Permian mass extinction within the same geological sections.

Middle Permian (Guadalupian) platform carbonate rocks of the Maokou Formation are widespread throughout south China. In western Guizhou, southern Sichuan, and Yunnan Provinces they pass laterally into the flows of the Emeishan large igneous province (Fig. 1). The original size of the province is difficult to estimate because much has been eroded (scattered outcrops of contemporaneous volcanic rocks are found up to 300 km from the main sections, Fig. 1), but its main outcrops cover 2.5 × 105 km2 in southwest China; the original volume was probably substantially less than 1 × 106 km3 (4). Despite their relatively small size, the coincidental timing of the Emeishan eruptions with the Guadalupian mass extinction has led to suggestions that they may be implicated in this environmental calamity (2, 5).

Fig. 1

Outcrop map (red) of the Emeishan large igneous province in southwest China (4).

Sections from both within the volcanic province and around its margins record a prolonged phase of stable carbonate platform deposition before its termination by abrupt base-level changes. To the north of the province, in northern Sichuan, the Maokou limestones are capped by a karstic surface dated by conodont studies to an interval within the early Capitanian Stage (6). In contrast, within the outcrop area of the igneous province, shallow-water carbonate rocks persisted longer but were abruptly drowned, whereupon deep-water radiolarian-spiculite chert became established. Conodont biostratigraphic control at the Xiong Jia Chang section, in western Guizhou (Fig. 2) and other sections (7), shows that the deepening occurred around the beginning of the Jinogondolella altudaensis zone in the mid-Capitanian. The brief phase of deep-water chert deposition was terminated by the onset of Emeishan volcanism. At Xiong Jia Chang, the basal volcanic rocks consist of a 70-m-thick composite unit comprising aphyric basalt and interbedded mafic volcaniclastic rocks. The latter are dominated by vesicular basalt clasts, many of which display chilled or glassy margins, which testify to an explosive style of mafic volcanism. Some clasts show a fiammé texture, typical of terrestrial emplacement, whereas others are dominated by palagonite-rimmed clasts, indicating subaqueous emplacement (8). Lithoclasts of platform carbonate, spiculite, and isolated marine fossils are also common (8), indicating that seafloor erosion accompanied emplacement. Such violent phreatomagmatic-style volcanism is typical of much of the early to mid-stages of Emeishan volcanism (9).

Fig. 2

Xiong Jia Chang section (26.5°N, 105.7°E), 40 km west of Zhijin, western Guizhou Province, showing the transition from Maokou Formation carbonates to volcanics at the base of the Emeishan province. Range charts for foraminifers and calcareous algae show the extinction to occur immediately below the first eruptive unit. These phenomena also coincide with a sharp negative carbon isotope excursion.

The initial phase of volcanism was followed by the reestablishment of diverse carbonate facies. Thus, at Xiong Jia Chang, the lowest volcanic pile is overlain by deep-water chert, with euxinic framboid populations (10); this unit is then abruptly succeeded by carbonate beds belonging to the J. prexuanhanensis/J. xuanhanensis assemblage zone, with a shallow-water microbiota (Fig. 2). Intertrappean limestone developments at other sections include Tubiphytes sponge reefs up to 30 m thick (11) and steep slope facies with allodapic limestones (12). The volcanic interregnum was terminated by the return of spectacular pyroclastic-phreatomagmatic volcanism and the deposition of individual mafic volcaniclastic beds approaching 200 m in thickness (11).

Fossil range data show that the collapse of carbonate platform deposition, shortly before the onset of eruptions, coincides with the disappearance of a shallow-water marine biota dominated by foraminifers and calcareous algae. Similar platform facies reappear above the basal volcanic pile, but many of the lost taxa do not (Fig. 2): The species-level composition of the calcareous algae shows complete turnover. The foraminifers also show extinctions, with the loss of the distinctive keriotheca-walled Schwagerinidae and Neoschwagerinidae and their replacement with assemblages consisting of small, long-ranging lagenides, the miliolinid Hemigordius, and small fusulinaceans (Codonofusiella and Reichelina).

This mid-Capitanian age for the mass extinction is substantially earlier than previous estimates (13, 14). C isotope data (15) from Xiong Jia Chang reveal a 5 to 6 per mil (‰) negative shift above the lowest volcanic bed (Fig. 2). This big excursion postdates the extinction interval, which is within a phase of stable values around the J. shannoni/J. altudaensis zonal boundary. Similar faunal turnover is seen in the carbonate platform sections at Gouchang (Fig. 1), approximately 50 km east of the Emeishan volcanic margin, and here too it coincides with stable δ13C values below a major negative excursion (Fig. 3). A similar large-amplitude negative excursion was reported from the mid-Capitanian in northern Sichuan Province, to the north of the Emeishan volcanic province (6). This excursion is now seen to be widespread (sites are up to 1000 km apart) and from several types of depositional environment (16), which suggests that it is a global signal. A further large negative excursion has been reported at the end of the Capitanian Stage (17), indicating that the post-extinction interval was marked by several large fluctuations.

Fig. 3

Gouchang section, near Ziyun, central Guizhou, developed 50 km east of the eastern margin of the Emeishan province, showing the coincidence of the mass extinction level seen in range truncations of schwagerinids and calcareous algae. The post-extinction neoschwagerinids are fragmented and abraded specimens interpreted to be reworked from pre-extinction strata, but they may suggest that the interval of extinction is longer than depicted. A major negative shift of δ13C isotope values (analyzed in whole-rack carbonate) is seen to occur after the extinction. Several gaps in this otherwise continuous road section may reflect the presence of shale and/or volcanic ash levels.

The close temporal link between the onset of eruptions and extinction suggests a cause-and-effect scenario. Cooling and acid rain (caused by SO2 effusion and sulfate aerosol formation) and consequent environmental deterioration are candidates for this link (18, 19). The dominance of pyroclastic volcanism (rather than more quiescent-style flood basalt eruptions) in the initial eruptive stages of the Emeishan province and the large scale of the flows (30 to 200 m thick) suggest that such effects are likely to have been severe. The subsequent negative shift of C isotope values is too large to be attributed to relatively heavy volcanic CO213C = –5‰), but it may record the release of much lighter thermogenic C from the site of volcanism (20). This was in the aftermath of the biotic crisis, but the high diversity of the post-extinction biota suggests that the light C flux is not linked to any prolongation of the environmental stress that caused the extinction.

Our study of the volcano-sedimentary record of southwest China reveals that the Middle Permian marine crisis precisely coincided with the onset of Emeishan volcanism. This provides evidence for a potential link between mass extinction and the eruption of this igneous province, although the absolute time scale for the event is not yet known. The subsequent negative δ13C excursion implies that the C cycle was destabilized for some time after the extinctions, perhaps by C release from thermogenic sources.

Supporting Online Material

www.sciencemag.org/cgi/content/full/324/5931/1179/DC1

Fig. S1

References and Notes

  1. Composite sections around Pingdi, on the Guizhou-Yunnan border, show the development of Maokou Formation platform carbonates overlain by deeper-water radiolarian cherts and slumped carbonates, with a conodont fauna spanning the J. shannoni/J. altudaensis zones. These are overlain by the lowest flood basalts of the Emeishan volcanic pile.
  2. See supporting material on Science Online.
  3. Pyrite framboid populations from deep-water cherts in the basal J. prexuanhanensis/J. xuanhanensis assemblage zone at Xiong Jia Chang are dominated by framboids with a mean diameter of less than 7 μm, a characteristic of populations formed in anoxic bottom water (21).
  4. A section on the eastern access road to Lugu Lake (27°41.711′N, 100°58.788′E) in northern Yunnan shows the lower Emeishan volcanic pile to be dominated by thick, mafic volcaniclastic flows. These are overlain by a 30-m-thick Tubiphytes sponge-cement reef, a further mafic volcaniclastic unit approaching 200 m thick, another limestone approximately 20 m thick, and finally a continuous section of basalt lavas, in excess of 1 km thick. These contain flows showing well-developed pillows.
  5. The Pingchuan section (27°40.779′N, 101°53.104′E), a roadside section developed immediately to the east of the small town of Pingchuan (southern Sichuan), shows the main succession of Emeishan flood basalts to be underlain by alternations of clast-supported breccia beds (containing clasts of Maokou limestone, basalt, and dacite) and laminated cherty micrites yielding goniatites.
  6. Carbonate-C isotopes were measured at the stable isotope laboratory at the University of Leeds on CO2 generated by the addition of anhydrous phosphoric acid to around 20 mg of powdered whole rock in a vacuum (22). Values are corrected with standard methods (23) and reported relative to the Vienna Pee Dee belemnite (VPDB) standard. The analytical precision for this analysis, based on replicate analyses of an in-house strontium carbonate standard, is estimated at 0.1‰.
  7. The negative δ13C excursion at Xiong Jia Chang (analyzed in whole-rock carbonate) occurs within a section showing abrupt deepening from platform carbonate to deep-water, calcareous chert deposition associated with the development of the Zhijin Basin, a north-south rift structure developed immediately before volcanism (24). The excursion at Gouchang occurs within a shallow-water platform carbonate succession. A potential third example of the excursion is seen in the upper part of unit 2 of the Maokou Formation at the Chaotian section in northern Sichuan, where it occurs within lagoonal carbonates on the southern margin of the North Yangtze Basin (6).
  8. We thank for funding the Natural Environment Research Council (grant no. NE/D011558/1), Natural Science Foundation of China (grants nos. 40872002, 406210022, and 40232025), Chinese State Administration of Foreign Experts Affairs (grant B08030), and Hong Kong Research Grant Council (grant no. HKU700204).
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