Elsevier

Chemical Geology

Volume 550, 20 September 2020, 119696
Chemical Geology

Geochemistry of the new Permian-Triassic boundary section at Sitarička Glavica, Jadar block, Serbia

https://doi.org/10.1016/j.chemgeo.2020.119696Get rights and content

Abstract

The new Permian-Triassic boundary section at Sitarička Glavica is unusual for the Alpine-Dinaride area, in that it is dominantly detrital in the early Triassic. It shows a change from late Permian limestones and shales with a diverse algal-brachiopod-echinoid-molluscan biota deposited in quieter normal marine water (unit 1), into similar sediments, with coarser calcisiltites and a reduced fauna (unit 2), into a post-extinction early Triassic calcareous micaceous siltstones and silty limestones (unit 3) with only molluscan fossils. The major element ratios to Al, reported previously, change little through the section, but with slightly higher ratios just above the extinction horizon (except for K) at the top of unit 2, and show moderate chemical weathering, and change to slightly less mature sediments above. Trace element ratios to Al show little consistent change, except for higher values of most ratios at the contact of units 1 and 2, where slight but temporary increases in TOC, δ13Corg and δ15Norg, and C/N, and decrease in δ34Spyrite occur. Low HFSE (Zr, Nb, REE, Th, U, Ta) values and ratios to other elements indicate that the resistant mineral containing these elements were somehow removed prior to deposition in residual lags by preferential wind erosion. Paleoredox indicators are inconsistent but anoxia seems present in unit 1 and 2 with mostly oxic conditions in unit 3 above. The geochemistry of this section does not show the concentrations of Ni and other elements attributed to Siberian Trap sources and evidence of volcanic input to the sediments is confined to below and above the extinction horizon. The main geochemical changes occur at the top of unit 1, corresponding with a faunal diversity decrease, but not extinction, and at the +5 metre horizon, which seems to- have no faunal changes associated with it. The carbon and other organic-related elements and isotopes indicate a terrestrial input of freshwater and organic matter just before the main negative spike in carbon isotopes, which greatly reduced the diversity of the fauna, and was shorty followed by an input of intermediate volcanic ash. The actual extinction and paleontological Permian-Triassic boundary occurred at the top of unit 2, and has little geochemical changes associated with it. These events can be recognized in other Alpine-Dinaride sections -. All Alpine-Dinaride sections also show a latest Permian ostracod-dominated horizon with a negative spike in organic carbon and sulphur isotopes that indicate an input of freshwater and land plant material, reducing the surface waters to almost brackish, followed by a return to normal salinities with the return of a reduced latest Permian fauna, accompanied by a gradual negative shift in carbonate carbon isotopes accompanied by input of intermediate volcanic material. The main extinction is accompanied by a drop in the difference between carbon and organic carbon isotopes, indicating less land plant input; but otherwise there is little geochemical change at this horizon.

Introduction

The greatest extinction in the geological record took place at the end of the Permian around 252 Ma (Baresel et al., 2017), supposedly killing over 90% of all marine species (though this may be may be 81% according to Stanley, 2016), and about 70% of all terrestrial species (Erwin, 2006). Both gradual environmental and catastrophic events have been used to explain the extinction, but the detailed timing and progression of such events, and the causes of the extinction are still not established (Wignall, 2015). One hypothesis is that massive and short-lived basaltic volcanism of the Siberian Traps directly outgassed vast amounts of water, carbon dioxide, sulphur dioxide, hydrogen sulphide, together with particulates enriched in nickel and other metals into the atmosphere (Saunders and Reichow, 2009), and caused contact metamorphism of organic carbon-rich sediments to further inject these and other materials, like Ni and Hg, into the atmosphere and ultimately oceans (Retallack and Jahren, 2008; Svensen et al., 2009; Rampino et al., 2017; Sial et al., 2019). This explanation, however, explains neither its rapidity, nor the extreme negative shift of carbon isotopes just below the extinction - volcanic carbon is insufficiently light given estimates of trap volumes (Brand et al., 2012). Furthermore, current radiometric dating suggest that the Siberian Trap eruptions occurred in multiple pulses, and that the most intense eruptions happened after the end-Permian extinction had ended (Ivanov et al., 2013).

Another hypothesis is that an extraterrestrial impact caused environmental changes severe enough to disrupt Earth systems -sufficiently to cause major extinctions. This hypothesis has lacked any convincing geochemical evidence so far, such as extraterrestrial platinum group element (PGE) anomalies and shocked quartz at the extinction level (Koeberl et al., 2004; Farley et al., 2005; Brookfield et al., 2010). But extraterrestrial spherules and a 3He isotope spike has been found just at the boundary, and correlates- with a δ13Corg negative spike reported here and in other western Tethyan sections (Iwahashi et al., 1991; Korchagin et al., 2010; Onoue et al., 2019). So, it is still worth ongoing consideration.

Estimated durations of the extinction, based on radiometric dating in South China, range from less than one million years (Bowring et al., 1998), to 61,000 ± 48,000 years (Burgess et al., 2014; Shen et al., 2011a; Baresel et al., 2017). Astronomical cycle tuning gives comparable estimates of 700,000 years (Huang et al., 2014), 83,000 years (Wu et al., 2013) and <40,000 years (Li et al., 2016). Milankovitch cyclostratigraphy across a continuous Permian-Triassic boundary section in the southern Alps constrains the extinction interval within an interval of <60,000 years (possibly <8000 years with the accompanying sharp negative global carbon-isotope shift within <30,000 years) (Rampino et al., 2000).

To explain the very negative carbon isotope shift, destabilization and oxidation of methane hydrates by global warming during trap eruptions is envisaged (Brand et al., 2012). Even if most Trap eruptions postdated the extinction, individual trap eruptions could provide enough carbon dioxide to produce global warming sufficient to destabilize hydrates. This could not only reduce atmospheric oxygen and produce further carbon dioxide, sufficient to cause anoxia, but also decrease respiratory rates and increase carbon dioxide poisoning (hypercapnia) in organisms (Rothman et al., 2014; Brand et al., 2016; Penn et al., 2018). The high burial rate of organic carbon in late Carboniferous to early Permian coals produced a significant drawdown of atmospheric carbon dioxide to very low values (100 ± 80 ppm) for the earliest Permian, accompanied by a significant increase in atmospheric oxygen (Feulner, 2017). Current studies place the development of ocean anoxia just prior to the extinction (Brennecka et al., 2011).

To test the various hypotheses, it is essential to separate and correlate individual events that take a geologically short time to occur. This requires detailed sampling and analysis of thick continuous sections. But there are relatively few such sections. At the Global Stratotype Section and Point (GSSP) for the Permian-Triassic boundary at Meishan, Southeast China, the extinction occurs within <0.3 m of a section with erosion surfaces (Yin et al., 2001). In other, thicker, sections, like Shangsi (South China), East Greenland, and Kashmir (India), distinct events can be better separated than at Meishan (Nakazawa et al., 1975; Wignall et al., 1995; Stemmerik et al., 2001; Algeo et al., 2007; Brookfield et al., 2020). By analyzing closely spaced (sometimes continuous) samples in apparently complete sections with accurately located fossils, successive changes can be more precisely defined.

This paper summarizes the geochemistry of an apparently continuous Permian-Triassic boundary section, discovered in 2011 by M. Brookfield, M. Sudar, D. Jovanovic and J. Williams in southern Serbia at Sitarička Glavica, about 90 km SW of Belgrade near the town of Valjevo, primarily to test for evidence of contributions from the Siberian Trap at an equatorial site. The Sitarička Glavica section differs from other Serbian boundary sections in that it seems not only continuous but also has a mixed detrital/carbonate early Triassic section. For example, Komarić, 20 km to the northwest, is entirely carbonate with a dolomite micro-breccia at the boundary (Sudar et al., 2007, Sudar et al., 2010). Ongoing stratigraphical, paleontogical, sedimentological and some preliminary geochemical observations at Sitarička Glavica were reported in Sudar et al. (2018): but this did not include most geochemical data or much evaluation. Here, we report and interpret elemental and isotopic geochemical studies on this section as a contribution to the data base on changes across the Permian-Triassic boundary, and evaluate the changes in terms of hypotheses advanced to explain the end Permian extinction.

If anyone wishes to do other geochemical analyses on our samples please contact M. Brookfield.

The Sitarička Glavica section is on the Jadar block, a fragment of continental crust overlain by Carboniferous to early Triassic continental and shelf sediments (Filipović, 2005). The Jadar block lies within the Vadar Zone, an ophiolitic suture zone with early Triassic and younger oceanic basalts overlain by Triassic radiolarites which mark the opening of the Neothethys Ocean in the Dinarides (Ozsvárt et al., 2012) (Fig. 1). The tectonics of this area are extremely complex due to deformations associated with the opening and closing of the Neothys from Triassic time onwards (Stampfli and Kozur, 2006). The Jadar block may be a continental fragment rifted off the northern Gondwana margin during the opening of the Neotethys, like some fragments in Central Iran and elsewhere (Omrani et al., 2013). Or it may be a part of the northwestern margin of Adria that was detached and transported southward in response to right-lateral strike–slip during the mid-Cretaceous or early Cenozoic (Karamata et al., 2000; Robertson et al., 2009). In any case, the Jadar block lay at the extreme western end of the Neotethys (Fig. 2) as part of an extensive tropical Permian-Triassic carbonate environment, possibly analogous to the Cenozoic Great Bahama Bank (Newell and Rigby, 1957; Newell et al., 1959; Brookfield, 2020), the Reed Bank of the South China Sea (Ding et al., 2014), or the Miocene Cyrenaican platform (Amrouni, 2015).

Section snippets

Section description

The Sitarička Glavica section is exposed 10 km west of Valjevo in a relatively new large roadcut where the beds dip steeply to the northeast (Fig. 3). The section was measured from a distinct buff clay which separates grey from buff weathering rocks (Fig. 4).

The lower unit 1 has about nine meters of interbedded calcisiltites and dark grey shales and is dominated by a diverse fauna of crinoids, algae, small foraminifera, molluscs, bryozoans, brachiopods and foraminifera, similar to the

Geochemical analysis

Samples were taken every 1 m above and below the calcareous silty mudstone datum, as well as at shorter intervals in the first metre above and below the datum and their lithologies described (Supplementary Table S1). The samples were visually inspected and any veins and weathered surfaces were removed prior to grinding in a tungsten ball mill to a fine powder for analysis.

Major and trace elements, C, N, S and isotopes were analyzed at the Environmental Analytical Facility at the University of

Results

The Sitarička Glavica sediments contain appreciable amounts of silica in excess of that required for the clay minerals and this is in the form of fine quartz sand and silt and not siliceous fossils such as radiolarians and diatoms. The Si/Al, Ti/Al, Fe/Al and Mn/Al ratios change little through the section, except that there are higher Si/Al, Ti/Al, Fe/Al, Mn/Al, Ca/Al just above the extinction horizon (+3 m). Mg/Al, and Na/Al rise as the lithologies change to less carbonate-dominated

Provenance

The composition of clastic sediments and rocks is controlled by source rock composition, chemical weathering, abrasion, sorting during transport, and diagenesis (Johnsson, 1993). Sitarička Glavica sediment clastic grain sizes are mostly too small to identify individual minerals and use petrography to infer provenance and climate.

The normative compositions calculated with Kackstaetter's (2014) methods allow original mineralogical compositions to be inferred, though calcareous sediments can give

Reworking and transport

Identifying reworked sedimentary is very important in working out sources and transport mechanisms. Though water transport is often simply assumed, wind transport is important in dry climates where the finer particles of soils and sediment can undergo deflation and be transported far from their sources (Li et al., 2009). During wind transport, minerals softer than quartz and those with good cleavages, get ground down to dust while harder minerals, with no cleavages do not. Heavy minerals

Volcanic input

Though volcanic ash layers (bentonites) can usually be identified in ancient sediments, there are few studies relating Quaternary element geochemistry of sediments to input of dispersed volcanic ash (Scudder et al., 2016); though this should be a basic requirement for interpretation of such possible inputs to ancient sediments. Various studied have used specific element and isotope variations to infer Siberian Trap aerosol input to the atmosphere and then into Permian-Triassic sediments; for

Climate

Climate change in source regions, without source composition changes, can be determined by fluctuations in Ti/Al, Ti/K and Ti/Sc ratios. During Quaternary, sharp changes in these ratios correlate with glacial/interglacial cycles, with higher ratios in warmer periods due to increased chemical weathering (Wei et al., 2003). In the Arabian Sea and eastern Mediterranean, higher Ti/Al and Ti/Zr ratios in Saharan dust from the surrounding land during dry climates phases alternate with lower ratios

Paleoenvironments from geochemical proxies

Proxies are things that can be measured in the rock record, and that have responded systematically to changes in variables, such as turbulence, temperature and salinity which are otherwise unrecorded (Henderson, 2002). Selected trace elements and element ratios are useful as environmental proxies and can be used as geochemical proxies for some environmental variables, such as temperature (Mg/Ca, Sr/Ca), salinity (B/Be; B/Ga), productivity (Ba/Al, Cd/Ca) and redox conditions (V, Cr, Mo, U, Re,

Summary

The elemental geochemistry of the Sitarička Glavica section show little change in source and transport characteristics. All samples, except that at +5 and +5.3 m, show the same REE distribution indicating that source of material did not change throughout the boundary. Other elements and ratios, however, suggest that the geochemistry of Sitarička Glavica PT section varies somewhat from the lower unit 1, into the middle unit 2, and upper unit 3, with four main changes; two across the 0 m clay

Comparison with other sections

The extreme tectonic fragmentation of the western Tethyan Permian-Triassic carbonate environments makes it difficult to assign sections anywhere near their original depositional sites (Pesic et al., 1986; Kovács et al., 2011). For example, the Permian-Triassic boundary sections in the Jadar block are so different as to require significant tectonic displacements to juxtapose them (Pantič-Prodanovic, 1989–90). A detailed summary of all Alpine-Dinaride-Carpathian Permian-Triassic boundary sections

Conclusions

The Permian-Triassic boundary section at Sitarička Glavica has three transitional units. The first has alternating calcilutites and grey shales with a diverse algal-brachiopod-echinoid-molluscan biota deposited in quieter normal marine water: it resembles the Bellerophon Formation of the Alps. The second unit is similar but with coarser calcisiltites and similar biota, but without brachiopods: it resembles the Tesero horizon of the Alpine Werfen Formation. The third unit has calcisiltites and

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We greatly appreciate discussions and guidance in the field from Milan Sudar and Divna Jovanović, and the support of Robyn Hanigan during the fieldwork and analyses. We gratefully acknowledge funding of geochemical analyses at the Environmental Analytical Facility at University of Massachusetts Boston (NSF Award # 09-42371; DBI: MRI-RI2; to Robyn Hannigan and Alan Christian). Alan Stebbins also gratefully acknowledges support by the National Science Foundation Graduate Research Fellowship

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