Anoxic oceanic conditions during the late Permian mass extinction-evidence from the Chutani formation, Bolivia

Highlights

• The transgressive of the basin exhibits an upwards decreasing trend of δ¹³C values while regression is marked by an increase.

• The negative δ¹³C shifts are caused by meteoric, volcanic or hydrothermal fluid interaction with rock.

• Significant variation in isotope values among neighbouring samples, especially during transgression.

• Heavier values during regression may reflect more evaporitic and anoxic conditions towards the Permian-Triassic boundary.

Abstract

We analyze diagenesis of carbonate rocks from the Late Permian Chutani Formation of western Bolivia (San Pablo de Tiquina section) in the southern Lake Titicaca zone, which is a sedimentary succession of semiarid tidal flat comprised of mixed carbonate and siliciclastic units. The diagenetic study includes petrographic analysis (conventional petrography and cathodoluminescence) and geochemical analysis (carbon and oxygen isotopes and minor element chemistry). An integrated study of lithofacies and isotope stratigraphy of carbonates shows a succession of five types of depositional environments: tidal barrier, tidal flat, shoal coastal and shoreface. The Chutani Formation was subjected to different diagenetic processes including micritization, cementation, mechanical compaction, dissolution, neomorphism, dolomitization and dedolomitization that occurred during marine to shallow burial stages. Carbon isotope (δ¹³C) values range between −7 and 2.9‰ (VPDB) with variations linked to stratigraphic changes. The transgressive stage of the basin exhibits an upwards decreasing trend of δ¹³C values whereas regression is marked by an increase in such values. The oxygen isotope values (δ¹⁸O) vary from −16.6 to −1‰ VPDB with lighter values towards the top of the stratigraphy. The transgressive trend may reflect mixing of meteoric water and/or volcanic-hydrothermal fluids with seawater or progressive oxygenation with enhanced circulation conditions. Heavier values during regression may reflect more evaporitic and anoxic conditions towards the Permian-Triassic boundary. Significant variation in isotope values among neighbouring samples is observed, especially during trangression, which may be the result of different diagenetic processes.

Introduction

Permian was a time of global change in the Earth's history. Middle and late Permian are marked by mass extinction events (Shen et al., 2010), which significantly affected global ecosystems, wiping out an estimated ~95% of all marine species (Raup, 1979) and 70% of all vertebrate families (King, 1991; Maxwell, 1992). Mechanisms for these extinctions are not clear, but possible causes are (1) major sea level changes (Chen et al., 1998; Haq and Schutter, 2008; Wignall et al., 2009a), (2) repeated carbon cycle (Chen et al., 1991; Jin et al., 2000; Limarino and Spalletti, 2006; Shen et al., 2011), (3) marine anoxia reaching unusually shallow depths (Wignall and Hallam, 1992; Isozaki, 1997; Wignall et al., 2009a), and high-latitude warming (Svensen et al., 2009; Shen et al., 2011; Joachimski et al., 2012). Interestingly, an extensive magmatic activity in the Emeishan and Siberian Large Igneous Provinces are dated at end-Guadalupian (Zhou et al., 2002; Wignall et al., 2009b; Sun et al., 2010) and Late Permian (Campbell et al., 1992; Renne et al., 1995; Kamo et al., 2003; Svensen et al., 2009). The association between the volcanic events and biotic crisis has led to assume a cause-effect relationship between volcanism and mass extinction, due to high emissions of greenhouse gases, destabilization of gas hydrates (Retallack and Jahren, 2008), and/or contact metamorphism in organic carbon-rich sediments (Ganino and Arndt, 2009; Svensen et al., 2009). All these processes could potentially have contributed to higher atmospheric pCO₂ and pCH₄ levels (Chen et al., 2013), which would affect life on Earth.

Stable isotope compositions of marine precipitates record climatic and environmental changes (Buggisch et al., 2015). Carbon and oxygen isotope ratios of seawater have considerably fluctuated during Late Paleozoic times (Veizer and Hoefs, 1976; Veizer et al., 1999), and shifts in δ¹³C are suggested to have been caused mostly by burial and re-oxidation of ¹²C-enriched organic matter (Schidlowski and Aharon, 1992; Kump and Arthur, 1999). Most carbon is tied in sediments in the lithosphere, mainly marine limestones, and therefore carries a relatively high δ¹³C signature (Anderson and Arthur, 1983; Hundson, 1977). Therefore, in any diagenetic process that involves rock-water interaction such as solution-reprecipitation under burial conditions, the relatively heavy rock carbon tends to dominate and to buffer low δ¹³C contributions from biological processes (Moore and Wade, 2013). Shifts in δ¹³C of seawater-dissolved inorganic carbon (DIC) have successfully been reconstructed by analyzing whole-rock carbonates which may preserve their original δ¹³C signature (Buggisch et al., 2015). The δ¹⁸O in the ocean is controlled generally by the waning and waxing of ¹⁶O-enriched continental glaciers (Railsback, 1990; Buggisch et al., 2015), but larger variations on multimillion year time scales may have been related to variable degrees of interaction of seawater with the meteoric water of the lithosphere (Gregory, 1991), likely driven by tectonic forces (Veizer et al., 1997, 1999; Wallmann, 2001; Veizer and Mackenzie, 2004) and altered during diagenetic stabilization (Weissert et al., 2008). The collection of brachiopod shells composed of low-magnesium calcite have been preferentially used as a suitable material that retains the carbon and oxygen isotopic composition and is relatively resistant to diageneic alteration (Compston, 1960; Lowenstam, 1961; Popp et al., 1986; Veizer et al., 1986, 1999). However, these fossil samples are not always free of diagenetic overprint due to local variations in environmental conditions (Grossman et al., 2008).

The western margin of Gondwana comprises enough Upper Paleozoic basins of South America for the reconstruction of the paleocontinent's history and exhibit complete successions of the Late Paleozoic (Limarino and Spalletti, 2006). During Late Devonian to Early Carboniferous, tectonism had a major control over sedimentation (Limarino and Spalletti, 2006). An important feature of the Late Devonian to Early Carboniferous orogeny was the uplift of the Protoprecordillera fold and thrust belt that separated arc-related basins of Chile and western Argentina from the more stable retroarc Paganzo Basin (Limarino and Spalletti, 2006). Sempere (1995) also proposed uplift during formation of the Early Carboniferous Huarina fold and thrust belt, located close to the eastern border of the Bolivian Altiplano (Limarino and Spalletti, 2006). Isaacson and Diaz Martínez (1995) highlighted that the uplift period was followed by intense subaerial erosion and nondeposition in the Altiplano of Bolivia and Peru during the Serpukhovian and Bashkarian. Therefore, a Middle Carboniferous unconformity would mark the culmination of important paleogeographic changes that led to mountain belt formation, such as the Protocordillera and Huarina (Gohrbandt, 1992).

Late Permian recorded a sea level drop associated with semiarid and locally arid conditions that progressively replaced marine sediments by shallow, transitional and/or continental deposits (Rohn, 1994). Sedimentation in the western arc-related basins in Peru and Bolivia were almost entirely replaced by widespread acidic volcanism in some places (Limarino and Spalletti, 2006). In Bolivia this volcanism supplied numerous ash beds in the Titicaca area (Grader et al., 2008).

Biostratigraphic schemes developed from study of fossiliferous assemblages are used in regional correlations between Gondwana basins, as they are an important tool to constrain the timing of geological events (Archangelsky et al., 1996; Díaz-Martínez et al., 2000). Late Permian Gondwana palynoflora in Bolivia includes: Glossopteris, Pecopteris, Luekisporites teaniaeformis, Corisaccites alutas and Weylandites magnus (Sempere et al., 1992; Vieira et al., 2004). Additionally, vertebrate chodrychthyes and ostheichthyes remains are described in these continental deposits (Sempere et al., 1992). We present new carbon and oxygen isotope data for the Late Permian Chutani Formation (San Pedro de Tiquina section, Fig. 1) in western Bolivia to investigate changes in marine systems during that time. The Chutani Formation displays a good relationship between climate and sea levels changes throughout its formation. Here we describe the diagenetic processes of the Chutani Formation, the stratigraphical sequence and identify depositional environments. The data supports anoxic conditions in the investigated marine systems, which fits global Late Permian conditions, and adds evidence to the temporal correlation between anoxic oceanic conditions and mass extinction events.