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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.
Conclusions
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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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |