| Magnetostratigraphy | ||||
Background to stratigraphy across the PTB in RussiaKnowledge of the end-Permian mass extinction has advanced recently thanks to improved understanding of stratigraphy in marine sections. The base-Triassic Global Stratotype Section and Point (GSSP) was established in 2001 at Meishan in China, corresponding to the the first appearance datum of the conodont Hindeodus parvus (Yin et al. 2001). Read more about this GSSP and how it was established in a pdf article from Episodes, and the official Chinese PTB GSSP web site.
Attempts to establish a stratigraphic system for the terrestrial Middle and Upper Permian based on carbon isotopes have proved difficult because of the non-global nature of some carbon isotope excursions (Corsetti et al. 2005). Further, there are no direct links from the tetrapod biozones of the Karoo to marine beds with conodonts (conodonts do not occur in fresh waters. New, and as yet unpublished work, in the Karoo may provide partial answers - there are some radiometric exact dates to come. Most studies of the late Permian mass extinction event in the terrestrial realm have focussed on South Africa (e.g Smith and Ward 2001). To test whether trends and patterns of extinction recorded in these studies are truly global, we have studied the Permian-Triassic record of northern palaeohemisphere sections in Russia (Benton 2003; Tverdokhlebov et al. 2003, 2005). However, in order to integrate the rich and diverse fossil tetrapod assemblages of this region into global estimates of species loss at this critical extinction event, it is necessary to demonstrate unequivocally how the Russian sections correlate with the global stratigraphic scheme. There is in fact hope for lateral correlation of Russian continental redbed sequences with marine Permo-Triassic in the Arctic regions of Russia. Work on the Kanin Peninsula (Grunt 2006) has revealed age-diagnostic marine fossils which provide links to the Chinese global standard, but with the possibility of linking through to non-marine units further south. It has long been assumed that the Russian Tatarian is equivalent to the Late Permian (e.g. Efremov 1937; Olson 1957; Chudinov 1965; Benton et al. 2004). However, we had the shock of our lives when we opened the latest edition of the Cambridge timescale project, the 2004 volume (Gradstein et al. 2004), and found that the the uppermost Permian, including the PTB, of European Russia is deemed to be missing. Instead the Tatarian and Kazanian are compressed and correlated with the Guadalupian stage rather than with the Lopingian (Gradstein et al. 2004). If correct, this would imply a significant 9-10 myr gap in the Russian stratigraphic record, as well as in the South African succession (correlated roughly by shared tetrapod genera). This would lead to the extraordinary situation of genera that cross the PTB, such as Lystrosaurus, but that had supposedly existed for 9-10 myr virtually unchanged. Also, of course, the hundreds of palaeontologists who thought they had been documenting tetrapod evolution through the PTB in Russia and South Africa would turn out to have been deluded! Fortunately, the error has been rectified in the most recent version of the time scale (Ogg et al. 2008), in which the old Russian 'Upper Permian' (Ufimian, Kazanian, Tatarian) is stretched to occupy most of the Middle Permian and all of the Upper Permian.
Magnetostratigraphy, in the fieldMagnetostratigraphy is an attractive alternative to these approaches as it utilises the globally synchronous nature of magnetic reversals and is, essentially, a facies-independent technique. It does however rely upon the construction of a coherent and composite magnetostratigraphic record, linked to biostratigraphy, and significant progress has been made toward a global, composite Permian-Triassic Boundary (PTB) record (Gallet et al. 2000; Scholger et al. 2000; Molostovskii 2005; Steiner 2006; Szurlies 2007).The magnetostratigraphy of Permian-Triassic sections in Russia has a long history of study commencing in the late 1950s and includes the work of Khramov, Molostovskii, Borisov, and Burov and their colleagues. However, much of the work has been published within Russia, and often in regional conference volumes, and remains difficult to access; furthermore there have been significant concerns about the demagnetisation techniques employed (Bazhenov 2008). The most comprehensive and recent information (in English) is that for the Volga and Kama areas (Burov et al. 1998) some 600-700 km N-NW of our study area. In addition to a composite and individual magnetostratigraphic sections, this summary of the available information also presents basic data for each section, which helps to assess the reliability of the information.
We collected numerous samples of red-bed sandstones during the 2006 Expedition, and these have been analysed for their original magnetization, whether 'normal' or 'reversed', to check their location in magnetostratigraphic scales, and also to determine the orientation of the Late Permian north pole, and so to check palaeogeographic reconstructions. Our study (Taylor et al. 2009) of the Permian-Triassic boundary of the southern Cis-Urals aims to (a) resolve the issue of whether or not there is a major temporal gap below the PTB in this part of Russia, and (b) contribute to the global magnetostratigraphic record of this crucial interval in Earth history. This study therefore has focussed upon sampling across the supposed PTB, concentrating in particular on the uppermost Tatarian deposits of the Vyatskian Gorizont immediately below the locally recognised PTB.
Our resultsOur new paper (Taylor et al. 2009) presents the evidence that there is no long hiatus spanning the Upper Permian in Russia, nor in other areas correlated with the Russian Ufimian, Kazanian, and Tatarian zones:The palaeomagnetic data yield a distinct series of polarity zones that provide clear local and regional correlation and are readily tied to a recently compiled global magnetostratigraphic record. On the basis of this correlation the sampled sections span the upper Guadalupian to Induan stages without any obvious break, so confirming the traditional view that the Tatarian is Late Permian in age. Anomalies in the magnetic inclination are consistent with sediment compaction (inclination shallowing, a common phenomenon of red beds) but declination anomalies between these sites and elsewhere in Russia may suggest localised vertical axis rotation.The most complete section sampled was at Boyevoya Gora, spanning Upper Permian and Lower Triassic, and this was used as the reference with which to compare the other sections ( figure below.
The polarity sequences for the five sections have been integrated into a single composite section (Fig. 8). in essence, correlated with this one. With the exception of Tuyembetka, the locally postulated PTB (defined on facies and fossil evidence) falls close to a polarity transition from R to N polarities, but always within the lower part of the N interval. This positioning is consistent with previous Russian studies and present global correlations (Burov et al., 1998; Gallet et al., 2000; Molostovskii, 2005; Steiner, 2006; Szurlies, 2007). The PTB is recognised to fall in the lower part of this N polarity chron and beneath a short R event within the N chron (Steiner, 2006). This short-lived R event (t1 of figure below) is present in the Boyevaya Gora, Tuyembetka and Krasnogor sections and may also be present in the other two sections where reversed polarity was detected in isolated samples at or near the top of the sampled sections.
Correlation of the studied sections and a composite section for the Orenburg region.
References
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Our understanding of the extinction event in the terrestrial realm is, however, relatively poor.
A key problem has been to date the terrestrial deposits, and to correlate their ages with the
marine timescale. Hitherto, the main stratigraphic system for Middle and Late Permian continental
beds has been to erect vertebrate biozones (Lucas 2006) stemming mainly from the Karoo successions
in South Africa (Rubidge 1995). The famous tetrapod zones of the Karoo (see Section left), the Dicynodon
Assemblage Zone, the Lystrosaurus Assemblage Zone, the Cynognathus Assemblage Zone,
and so on were founded on their commonest vertebrate fossils, and so could be recognized readily
by field collectors. However, there was minimal evidence for linking these tetrapod biozones to
the global marine standard.
Right: Collection of cores taken from different sandstones, catalogued, numbered,
and marked with a way-up. These were taken back to the geomagnetism lab in Plymouth, where
their north/ south normal/ reversed magnetizations were measured.
Composite log for the Boyevaya Gora section, including uppermost Permian (Severodvinian,
Vyatkian) and earliest Triassic (Vokhmian). The columns from left to right are as follows:
simplified lithology (mudstone, sandstone and paleosols are shown as light grey, white and black
fills respectively; Q, the quality factor (based on the noisiness of the geophysical data); the
Virtual Geomagnetic Pole latitude calculated from the measured declination, inclination and site
location in situ (grey lines mark the latitude of the mean pole in Normal (N) and Reverse (R)
polarities); the intensity of total remanence and volume susceptibility units.
