Australia: The Land Where Time Began
Permian-Triassic Neo-Tethyan Margin of Gondwana – Catastrophic Environmental transition
According to Ghosh et al. in order to interpret the global consequences of the mass extinction at the Permian-Triassic (P-Tr) requires that examination across palaeogeographic realms of Pangaea. Trails of this environmental catastrophe in the Neo-Tethys Ocean are preserved in the Spiti Valley, India, as a remnant of the peri-Gondwanan shelf. In this study Ghosh et al. document new sedimentological observations and high-resolution concentrations of trace elements and carbon, lead, oxygen, isotope data in Spiti across the Permian-Triassic boundary. Framboidal pyrites, fossils and laminated lithology of shales dating to the Late Permian indicate a deeper anoxic depositional environment while 13Corg concentration excursions of 2.4 ‰ and 3.1 ‰ in Atargu and Guling outcrops, respectively, identify the Permian-Triassic transition across a clayey, partly gypsiferous ferruginous layer. Subaqueous oxidation of shallow marine sediments on a regional scale is indicated by sedimentological similarities of this layer to other Neo-Tethyan sections from Transcaucasia and Iran. Shales dating to the Late Permian enriched in light rare earths and with Cerium – Europium anomalies reflect their source in the adjacent Panjal Trap basalts (about 289 Ma) of Kashmir. At the Permian-Triassic boundary sediments continental crust Niobium, Tantalum and Zirconium – Hafnium anomalies appear, and prevail through the overlying Early Triassic carbonates. The volcanic source of the shales dating to the Late Permian are distinguished from the siliciclastic signature of the carbonates dating to the Early Triassic by original lead isotope ratios, as well as an increasing abundance of lead closer to the Permian-Triassic boundary. In this study the 13Corg trace element and lead isotope record from Spiti indicate catastrophic changes in sediment source and facies, with effects on the carbon cycle and are consistent with an abrupt episode of marine regression and erosional forcing, also observed elsewhere along northern Gondwana. Ghosh et al. propose that simultaneous eruption of Siberian volcanics and bolide impact in the Paraná Basin, Brazil, and elsewhere, combined, left catastrophic local to regional imprints on sea level, climate, anoxia in the ocean and tectonic stability connecting the Permian-Triassic crisis across terrestrial and marine realms of the peri – Gondwanan region.
The largest mass extinction event of the Phanerozoic was recorded by the extinction event of the end-Permian (end-Changhsingian), that was calibrated geochronologically to 251.941 ± 0.037 Ma (Burgess et al., 2014) at the Permian-Triassic (P-Tr) Global Stratigraphic Section and Point (GSSP) in Meishan, China (Yin et al., 2001). As a result of this mass extinction event approximately 90 % of marine species and about 70 % of contemporary terrestrial vertebrate families were lost. Variable estimates of the age of the Permian-Triassic boundary (Bowring et al., 1998; Mundil et al., 2004; Shen, S.Z. et al., 2011) and uncertainty about the cause of such a massive extinction event have been the subject of much scientific speculation, in spite of the severity of the consequences of this event. Permian-Triassic studies carried out in
· China (Payne et al., 2004; Zhang et al., 2007; Zhou & Kyte, 1988),
· Vietnam (Algeo et al., 2008; Algeo et al., 2007a),
· India – northern margin (e.g. Algeo et al., 2007b; Baud et al., 1996; Brookfield et al., 2003; Kummel & Teichert, 1970; Nakazawa et al., 1975),
· Tibet (Bruwiler et al., 2009; Garzanti et al., 1998; Shen et al., 2010; Wignall & Newton, 2003; Yuan et al., 2014),
· Canadian Arctic (Algeo et al., 2012; Beauchamp et al., 2009; Grasby & Beauchamp, 2008; Grasby & Beauchamp, 2009),
· Southern Alps (Fenninger, 1991; Holser et al., 1989),
· Iran/Oman (Korte et al., 2004; Richoz et al., 2010) and
· Antarctica (Retallack et al., 2005; Sheldon et al., 2014)
All of which represent sections of the supercontinent Pangaea, the Panthallasic Ocean, and the Tethys Ocean, invoke many hypotheses to explain mass mortality. It is considered that Siberian flood basalt volcanism (Renne & Basu, 1991; Renne et al., 1995) and a subsequent climate change leading to global marine anoxia Hallam & Wignall, 1999; Isozaki, 1997; Wignall & Twitchett, 1996) are considered to be prime targets for this massive palaeoenvironmental disaster. There are a number of other causes of the mass extinction event at the close of the Permian that have been proposed, such as:
· wildfires (Shen, W. et al., 2011),
· methane hydrates (Heydari & Hassanzadeh, 2003; Majorowicz et al., 2014; Retallack & Krull, 2006; Ryskin, 2003),
· rapid changes in sea level (Erwin, 1994; Grasby & Beauchamp, 2009; Hallam & Wignall, 1999) and
· extraterrestrial bolide impacts (Basu et al., 2003; Bhandari et al., 1992; Retallack et al., 2005; Retallack et al., 1998; Tohver et al., 2012).
It is evident that at this geological transition, whatever the cause, there was a prolonged disturbance in both the marine and terrestrial environments of biological productivity. Several studies, with the exception of carbon isotope studies, that have attempted to constrain the palaeosedimentary geochemistry of this geological transition that was so critical (e.g. Algeo et al., 2007b; Fio et al., 2010; Holser et al., 1989; Shen et al., 2013; Yoshida et al., 2014; Zhou & Kyte, 1988) could resolve the conundrum of discriminating between terrestrial and extraterrestrial causes of the mass extinction of the Late Permian. The need to characterise geologically multiple Permian-Triassic boundary sections from different palaeogeographic realms has been emphasised repeatedly (e.g. Payne et al., 2004). Such a study would enable the synthesis of global ramifications across the Permian-Triassic boundary.
The Spiti Valley, India, accorded as the Museum of Indian Geology, has preserved fossiliferous sequences from the Neoproterozoic to Cretaceous, which include the rock succession from the Permian-Triassic. In the Spiti Valley, Indian Himalaya, the Permian-Triassic boundary sections are promising outcrop candidates for the characterisation of palaeoenvironmental changes in the southern margin of the Neo-Tethys Ocean. The sections viz., Atargu and Guling in this valley were chosen by Ghosh et al. for the study, the results of which are presented in this paper. Detailed studies of general and specific lithostratigraphy and biostratigraphy of the Spiti Valley sections (Bhargava, 1987, 2008; Bhargava et al., 2004; Krystyn et al., 2004; Srikantia & Bhargava, 1998) continue appearing in recent literature, beginning with the pioneering study of Griesbach. 1981).
Stable carbon isotope data on a few samples from the Spiti Valley sections were reported in an early study (Ghosh et al., 2002). According to Ghosh et al. a comprehensive Chemostratigraphic, biostratigraphic and lithostratigraphic profiling were overdue to pinpoint the extinction interval, as well as possible cause, pattern and mechanisms of this global catastrophe across the southern margin of the Tethys Ocean.
Geochemical studies from the region (Bhandari et al., 1992; Shukla et al., 2002) together with sparse trace element data on the select samples were interpreted as assessing volcanic and extraterrestrial hypotheses. It was suggested by investigations in the nearby Guryul Ravine, Kashmir (Algeo et al., 2007b) that marine anoxia with increasing clay content of seismite and tsunamite deposits (Brookfield et al., 2013) during the Late Permian Event Horizon (LPEH). Trace element data and applied weathering indices from the Atargu Permian-Triassic section in Spiti were utilised (Williams et al., 2012) to conclude that the shales from the Late Permian did not undergo diagenetic alteration and therefore preserve the primary signatories of sedimentary deposition.
In this paper Ghosh et al., based on close-spaced sampling, report geochemical changes across the Permian-Triassic boundary in Spiti Valley, India. They have documented and correlate new sedimentary and palaeontological observations to detailed high resolution (about cm scale) carbon and oxygen isotope data of organic and carbonate phases from 2 Permian-Triassic sections, Atargu and Guling in the Spiti Valley. Ghosh et al. also examine the trace element and Pb isotopic composition of the Atargu outcrop, which is comprised of shales from the Permian, a transitional ferruginous layer and carbonates from the Early Triassic. The evidence for changes in the sedimentary provenance related to volcanic or impact, and their possible link to abrupt marine regression in this shallow continental shelf of the Neo-Tethys Ocean, was tested by the combination of the abundance of trace elements and lead isotope ratios. This study attempted to understand the significance of variations, regionally and locally, in the Peri-Gondwanan region, by the use of these data, in the context of the global sequence of events leading up to the greatest cataclysm in the geological and evolutionary record.
Geological setting and field observations of the shale and limestone beds in the Spiti Valley
Sections at the localities in the Spiti Valley that are mentioned above are located between 77o38’E – 78o36’E and 31o42’N – 32o29’N represent a Neo-Tethyan continental shelf environment dating to the Late Palaeozoic to the Early Mesozoic. Earlier workers (Shanker et al., 1993; Bhargava & Bassi, 1998; Bhargava, 2008), provided a broad sedimentological interpretation of the Gungri Formation, in which they suggested the black Gungri shale was formed in a quiet mid-shelf depositional environment where there was only limited circulation much below the wave base. The overlying limestones of the Mikin Formation in contrast, contain thick-shelled fauna at the base and thin-shelled packstone in the overlying part, which implied an initially shallow marine environment with rapid deepening. Several shoaling cycles were distinguished which ranged from lower to normal wave base with bottom water currents that were stronger (Bhargava, 1987; 2008). As part of the study reported in this paper extensive field-based micro-palaeontological and sedimentological studies have been carried out in the Gungri formation. Alternating shale and shaly-siltstone horizons comprise the Gungri Formation. The shale unit is persistent laterally with lensoidal to tubular siltstone beds and contains the ichnogenera Zoophycos abundantly, and a brachiopod faunal assemblage (Productus sp. indet, Lamnimargus himalayensis, Waagenoconcha sp. indet), crinoids and ammonoids (Cyclolobus oldhami).
In the 709638 boundary sections of Spiti, the Gungri Formation from the Late Permian (Wuchiapingian-early Changhsingian) is located between the underlying Gechang Formation of Asselian-Sakamarian age and the overlying Induan Limestone Member (Miki Formation) of the Lilang Supergroup (Bhargava et al., 2004). The Gungri Formation transitions up from black carbonaceous shale to dark grey silty and occasionally micaceous shales. Closer with the contact with the limestone dating to the Triassic these shales alternate with calcareous and phosphatic nodule-rich, shaly limestone layers (Bhargava, 2008). The alternating siltstone and shale beds are 3-5 m thick, lenticular to tabular in geometry and have gradational to sharp bases with the shale. At the top of the Guling Formation between 14.5-17.3 m there is a 2.80 m thick shaly siltstone unit that has an eroded contact with the black silty shale that underlies it. Fossils of cephalopoda, foraminifera, crinoids and brachiopods were found in an iron-rich ferruginous layer rich in clay that was previously thought to contain no fossils was included in the shaly siltstone layer (Shukla et al., 2002). This layer is ripple cross-bedded and parallel laminated, which indicate traction transport, and it contains pyrite and rare phosphatic nodules. The top 10 cm is richly fossiliferous, though fossils are present in the lower and upper part of this bed. The fossil bands are about 1-2 cm thick.
There are ripple forms at the top of the bed and it becomes muddier as it marks the Permian-Triassic Event Horizon (PTEH), without, however the Late Changhsingian fauna. An Early Changhsingian age for these Upper Permian shales (Bhargava et al., 2004; Shukla e al., 2002), and possibly also the overlying ferruginous layer, is indicated by the occurrence of Wuchiapingian cephalopods Cyclolobus oldhami, Cyclolobus walkeri, brachiopod Lamnimargus himalayensis and ammonoid Xenodiscus carbonarius in the lower middle parts of the Gungri Formation. An Early Changhsingian upper age limit for the Gungri Formation may be suggested by the presence of the brachiopod Waagenoconcha sp. indet and the ammonoid Cyclolobus in the upper part. Prolific development of Zoophycos, in particular associated with shaly units (Bhargava et al., 1985) as well as Rhizocorallium and Skolithos (that were identified during the study this paper reports on) in siltstone-shale beds are known from the Gungri Formation.
A fossiliferous packstone (limestone) unit that is 62 cm thick, the Mikin Formation, marks the beginning of the Triassic period, and the base of this unit is sharp, probably erosional, and appears to on-lap the underlying shale of the Gungri Formation. The limestone beds are persistent laterally and their thickness decreases towards the top. The top 10 cm of this unit is characterised by syn-sedimentary deformation with ripple cross-laminations. The Mikin Formation was divided into 4 members (Bhargava et al., 2004):
a) Lower Limestone Member,
b) Limestone Shale Member,
c) Niti Member (Nodular Limestone Member) and
d) The Upper Limestone Member.
At the base of the Mikin Formation of the Lower Limestone Member, that has been investigated mineralogically and geochemically in the study that is reported in this paper, is the P-Tr boundary.
The richly fossiliferous Mikin Formation is typically comprised of ammonoids and conodonts. Included among the ammonoids are Otoceras woodwardi, Ophiceras tibeticum, Discophiceras, Pleurogyronites planidorsatus and the conodonts are represented by Hindeodus parvus, Neogondolella nassichucki and Isaricella staeschei (Krystyn, et al., 2004). Beneath the Otoceras zone these conodonts occur in the basal most 20 cm. In the base to almost 50 cm from the mud section in Spiti Neogondolella carinata and N. krystyni have been found (Krystyn et al., 2004). In this study identical palaeontological observations were made for the Guling section and therefore can be extended for Atargu section as well.
An Induan age is represented by the basal part of the Mikin Formation, and the Induan-Olenekian boundary is located about the base. At the base of the Mikin Formation in Spiti the identification of Hindeodus parvus (Krystyn et al., 2004) defines the P-Tr boundary, which corresponds to the first known appearance of Hindeodus parvus in the Meishan GSSP section.
There is a distinct ferruginous layer 15-20 cm thick along of the contact of the Gungri Formation with the Mikin Formation. It is indurated and bedded in nature and has a composition that is silty-clayey and partly calcareous. There are 2 beds, each of which is 7-10 cm thick and of a sheet like geometry that are separated by grey shale partings that are 1 cm thick. The unit is differentially ferruginised and mottled, with the result that there are yellow-red mottles on a grey background. Mud clasts/balls of 1 mm size that are present in this layer display calcareous content. A Late Permian (Changhsingian) age is clearly indicated for the underlying ferruginous layer by the co-occurrence of the conodont Hindeodus parvus from the Triassic and Otoceras woodwardi from the Permian at the base of the Mikin Formation (Krystyn et al., 2004), provided the submarine sedimentary break is of short duration.
During this investigation a preliminary microscopic study, in transmitted light, of the ferruginous layer at Guling-1 revealed the presence of cephalopoda and foraminifera fossils. The ferruginous layer was previously believed to not contain fossils. Ghosh et al. say more palaeontological research is needed to age these fossils precisely and to determine whether these are reworked from the underlying shales with fossil elements from the Wuchiapingian or in situ. The ferruginous layer is present in both the Atargu and the Gulin-1 section. Ghosh et al. characterised this transition by the ferruginous layer and in tandem with the lack of fossils from the Changhsingian, they have assumed the transition to be a short sedimentological sub-marine (Bhargava, 2008). Ghosh et al. have interpreted this break to be representative of the event horizon (LPEH) of the Late Permian in the northern Indian margin. This is also the zone of the Late Permian Mass Extinction (LPME) with respect to the global P-Tr boundary sections.
Ghosh et al. have provided the first detailed geological characterisation of the extinction event boundary across the defined biostratigraphic P-Tr boundary, by using sedimentary-palaeontological observations, and trace element analysis, stable isotope and lead isotope data from the P-Tr boundary sections in the Spiti Valley. The sharp facies change of the Spiti sections from organic rich shale to a limonitic ferruginous layer, in tandem with an organic carbon flux change, witnessed shallowing that occurred rapidly, and caused rapid ventilation of the continental shelf under sub-aqueous conditions. It is clearly suggested that there was a rapid sea level drop on a regional scale by red clays that were similarly observed as well as fossil assemblages that were reported from Transcaucasia, which included sections from Azerbaijan, Armenia, Julfa and Naxcivan (Kotljar et al., 1983), to Tibet (Shen et al., 2010; Bruhwiler et al., 2009). The need for further biostratigraphic control on the Upper Permian of the Spiti Valley sections is demonstrated by the discovery by Ghosh et al. of fossils in the Gungri Shale and the ferruginous layer in this study. In spite of a possible reworking of sediments from the Spiti Valley, the reduction of the organic carbon isotopic composition and the discovery of fossils in the ferruginous layer validates a marine geochemical change, reaffirming the global nature of the extinction event at the end-Permian across continental and oceanic realms, that extended from the Boreal Realm to the Peri-Gondwanan region. A direct link between marine and terrestrial ecological crisis through soil erosion and acidification during the end-Permian have been proposed based on recent studies (Stephen et al., 2005; Sephton et al., 2015) considered an integrated volcano-impact scenario for the Cretaceous-Palaeogene boundary. Ghosh et al. have proposed, based on their study in the Spiti Valley that is reported in this paper, that catastrophic environmental conditions prevailed at the P-Tr boundary, which were characterised by an arid climate and increased erosion. Ghosh et al. suggest that volcanism in Siberia (e.g. Renne et al., 1995) simultaneously triggered such catastrophic conditions in the Neo-Tethyan shelf and the Araguainha in Brazil (Tohver et al., 2013; Tohver et al., 2012). Though it is not clear what the cause of the sudden marine regression was, the possibility of the impact of a bolide (Kaiho et al., 2006; Basu et al., 2003) which was followed by destabilisation of surface deposits by tsunami and seismic events (Brookfield et al., 2013; Baud & Bhat, 2014) needs to be ascertained. A significant addition to the available database on extinction at the P-Tr, and showing the Permian-Triassic extinction was indeed a much wider global phenomenon that extended into the Peri-Gondwanan region.
Ghosh et al. concluded, based on their analysis of sedimentological and geochemical-isotope data from the Spiti Valley that is reported in this paper that:
1) It is indicated from this study that the sediments from the Late Permian were derived from Panjal volcanics and were deposited in a shallow shelf region under oxic to sub-oxic conditions.
2) The change in the composition of carbon isotopes in the organic matter occurred synchronously with a rapidly dropping sea level of the Neo-Tethys as well as change in the depositional environment, as is evidenced by the sharp change in facies, composition of trace elements and the lead isotope ratios.
3) The preceding impact-release of large volumes of thermogenic methane from the Paraná Basin exacerbated the outcome of the Siberian flood basalt volcanism on mass extinction in the Spiti Valley or the Neo-Tethys Ocean. Arid, unstable environmental conditions, that led to the mass extinction event of the end-Permian and a prolonged recovery in the Triassic, would have been caused by such a large volume of release of CO2 and CH4 into the atmosphere, as well as the increase in decomposition of biomass in water and on the land.
4) In the mid-shelf environment of the Neo-Tethys Ocean the marine anoxia in the Late Permian was limited to deeper waters and did not extend into the upper column, to influence primary biological productivity, as is suggested by P-Tr sections worldwide. The positive Cerium anomaly in the rare earth element pattern of the shales from the Late Permian supports this observation, and does not appear to be in tandem with the deeper environments of the Palaeo-Tethys or Panthallasic Ocean, which implies that shallow marine anoxia may not have been the only cause of extinction, at least along the margins of the continents.
5) The sharp change in facies, with their distinct element and Pb isotope compositions, revealed by catastrophic marine regression, in the Neo-Tethys Ocean at the close of the Permian exposed the pyritised iron-rich shales to subaqueous oxidation. The marine transgression that followed in the Early Triassic led to transgressive lag that formed the ferruginous layer as a result of the expulsion of iron-rich water from the Gungri shales by overloading the Mikin limestones (Bhargava, 2008).
6) Chaotic sediment mixing, and amplification of the erosional input from the Peri-Gondwanan continent into the Neo-Tethys Ocean, was caused by arid environmental conditions and abrupt changes in sea level.
7) The global P-Tr catastrophe of this magnitude was brought about by the combined impact of these processes, not in isolation.
Ghosh, N., A. R. Basu, O. N. Bhargava, U. K. Shukla, A. Ghatak, C. N. Garzione and A. D. Ahluwalia (2016). "Catastrophic environmental transition at the Permian-Triassic Neo-Tethyan margin of Gondwanaland: Geochemical, isotopic and sedimentological evidence in the Spiti Valley, India." Gondwana Research 34: 324-345.
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