Richard J. Twitchett

Oceanic Anoxia and the End Permian Mass Extinction

Data on rocks from Spitsbergen and the equatorial sections of Italy and Slovenia indicate that the worlds oceans became anoxic at both low and high paleolatitudes in the Late Permian. Such conditions may have been responsible for the mass extinction at this time. This event affected a wide range of shelf depths and extended into shallow water well above the storm wave base.

Rapid and synchronous collapse of marine and terrestrial ecosystems during the end-Permian biotic crisis

A newly studied Permian-Triassic (P-Tr) boundary section in Jameson Land, East Greenland, contains an abundant and well-preserved marine fauna as well as terrestrial palynomorphs. For the first time it is possible to compare the biotic crises of the marine and terrestrial realms using the same samples from the same section. The sediments record a negative excursion in δ 13 C carb values of 8‰–9‰, and in δ 13 C org values of 10‰–11‰. The presence of the conodont Hindeodus parvus, combined with the δ 13 C carb record, enables correlation with the proposed global stratotype section at Meishan. This shows that the Greenland section is the most expanded P-Tr section known. Collapse of the marine and terrestrial ecosystems took between 10 and 60 k.y. It took a further few hundred thousand years for the final disappearance of Permian floral elements. Collapse of the terrestrial and marine ecosystems began at the same stratigraphic level and preceded the sharp negative excursion in the δ 13 C record.

Life in the end-Permian dead zone.

The fossil record of land plants is an obvious source of information on the dynamics of mass extinctions in the geological past. In conjunction with the end-Permian ecological crisis, ≈250 million years ago, palynological data from East Greenland reveal some unanticipated patterns. We document the significant time lag between terrestrial ecosystem collapse and selective extinction among characteristic Late Permian plants. Furthermore, ecological crisis resulted in an initial increase in plant diversity, instead of a decrease. Paradoxically, these floral patterns correspond to a “dead zone” in the end-Permian faunal record, characterized by a paucity of marine invertebrate megafossils. The time-delayed, end-Permian plant extinctions resemble modeled “extinction debt” responses of multispecies metapopulations to progressive habitat destruction.

Anomalous Early Triassic sediment fluxes due to elevated weathering rates and their biological consequences

Analysis of 16 marine Permian-Triassic boundary sections with a near-global distribution demonstrates systematic changes in sediment fluxes and lithologies in the aftermath of the end-Permian crisis. Sections from continent-margin and platform settings exhibit higher bulk accumulation rates (BARs) and more clay-rich compositions in the Griesbachian (earliest Triassic) relative to the Changhsingian (latest Permian). These patterns, which largely transcend regional variations in tectonic setting, sequence stratigraphic factors, and facies, are hypothesized to have resulted from a substantial (average ∼7×) increase in the flux of eroded material from adjacent land areas owing to accelerated rates of chemical and physical weathering as a function of higher surface temperatures, increased acidity of precipitation, and changes in landscape stability tied to destruction of terrestrial ecosystems. This sediment surge may have been a contributory factor to the latest Permian marine biotic crisis as well as to the delayed recovery of Early Triassic marine ecosystems owing to the harmful effects of siltation and eutrophication. Contemporaneous deep-sea sections show no increases in sediment flux across the Permian-Triassic boundary owing to their remoteness from continental siliciclastic sources and location below the paleo–carbonate compensation depth.

Rapid marine recovery after the end-Permian mass-extinction event in the absence of marine anoxia

A new Early Triassic marine fauna is described from the Central Oman Mountains. The fauna is Griesbachian in age, on the basis of abundant conodonts and ammonoids, and was deposited in an oxygenated seamount setting off the Arabian platform margin. It is the first Griesbachian assemblage from a well-oxygenated marine setting and thus provides a test for the hypothesis that widespread anoxia prevented rapid recovery. The earliest Griesbachian (parvus zone) contains a low-diversity benthic fauna dominated by the bivalves Promyalina and Claraia. A similar level of recovery characterizes the immediate postextinction interval worldwide. However, the middle upper Griesbachian sedimentary rocks (isarcica and carinata zones) contain an incredibly diverse benthic fauna of bivalves, gastropods, articulate brachiopods, a new undescribed crinoid, echinoids, and ostracods. This fauna is more diverse and ecologically complex than the typical middle to late Griesbachian faunas described from oxygen-restricted settings worldwide. The level of postextinction recovery observed in the Oman fauna is not recorded elsewhere until the Spathian. These data support the hypothesis that the apparent delay in recovery after the end-Permian extinction event was due to widespread and prolonged benthic oxygen restriction: in the absence of anoxia, marine recovery is much faster.

Palaeoenvironments and faunal recovery after the end-Permian mass extinction

Abstract Strata of the Lower Triassic Werfen Formation (Dolomites, northern Italy) record deposition in a wide range of palaeoenvironments from peritidal to outer ramp settings. This enables accurate assessment of faunal recovery following the end-Permian mass extinction by eliminating facies-related effects. Extinction of the shelly taxa appears to be rapid and occurs in the latest Permian Tesero Oolite Horizon. Disappearance of the various trace fossil taxa appears to be more gradual. Following the extinction, the lower and middle Griesbachian are characterised by laminated sediments containing low diversity bedding plane assemblages. Ichnological, geochemical and sedimentological data show evidence of widespread low oxygen conditions. Normally oxygenated waters return in the late Griesbachian and there is a moderate increase in tiering above and below the surface. Macrofaunal and ichnofossil diversity decrease markedly during the Smithian, with the appearance of more terrigenous strata possibly reflecting an increased run-off and more brackish conditions. Complete recovery begins in the basal Spathian with an increase in tiering above and below the substrate. Crinoids and ammonoids appear for the first time in the Werfen Formation, and the ichnogenus Rhizocorallium re-appears. Global correlation is hindered by poor biostratigraphy in the Dolomites region. However, on a broad scale it appears as though many of the palaeoenvironmental changes are repeated worldwide. Current available data suggest that the re-appearance of some trace fossil taxa (e.g. Diplocraterion, Rhizocorallium) may be globally synchronous.

Trace fossils and the aftermath of the Permo-Triassic mass extinction: evidence from northern Italy

Abstract The duration of early Triassic anoxia in the carbonate ramp settings of the Werfen Formation (Dolomites, northern Italy) has been constrained using ichnofabric and sedimentological data. This includes fine lamination and abundant pyrite framboids commonly reworked into lags by storm events. The “anoxic” event spanned the entire Griesbachian Stage and extended from deep waters into extraordinarily shallow water settings; evidence for oxygen deficiency can be found in mid and inner ramp facies. The extent and duration of this event may at least partially explain the magnitude of the Permo-Triassic mass extinction. Normal benthic conditions returned rapidly in the Dienerian Stage and pervasively bioturbated distal ramp facies testify to the ventilation of the deeper water areas of the region in post-Griesbachian times. A post-mass extinction radiation is not particularly well displayed in either trace fossil or body fossil diversity compilations from the Dolomites. Moderate/low levels of trace fossil diversity are maintained throughout the post-Griesbachian Werfen Formation whilst the body fossil data shows a moderate increase from the mid-Smithian Stage. There is thus a curious lag period spanning the Dienerian to mid-Smithian when environmental conditions were apparently normal but the marine fauna failed to show any recovery.

A unique Permian^Triassic boundary section from the Neotethyan Hawasina Basin, Central Oman Mountains

In the Wadi Wasit area (Central Oman Mountains), Dienerian breccias are widespread. These breccias consist mostly of Guadalupian reefal blocks, often dolomitised, and some rare small-sized blocks of lowermost Triassic bivalve-bearing limestones. A unique block, with a size of about 200 m3, including Permian and earliest Triassic faunas has been studied in detail. The so-called Wadi Wasit block consists of three major lithological units. A basal unstratified grey limestone is rich in various reef-building organisms (rugose corals, calcareous sponges, stromatoporoids) and has been dated as Middle Permian. It is disconformably overlain by well- and thin-bedded light grey to yellowish coloured limestones rich in molluscs. Two major lithologies (Coquina Limestone respectively Bioclastic Limestone unit) characterise the shelly limestones, their contact seems gradual. These two units are well-dated; they are of Griesbachian age and contain three conodont zones, the Parvus Zone, the Staeschei Zone and the Sosioensis Zone, and two ammonoid zones, the Ophiceras tibeticum Zone and an ‘unnamed interval’. The third unit consists of a grey marly limestone containing Neospathodus kummeli (basal Dienerian). It is the first record of well-dated basal Triassic sediments in the Arabian Peninsula. The Coquina Limestone is dominated by the bivalve Promyalina with some Claraia and Eumorphotis. This bivalve association is interpreted as a pioneering opportunistic assemblage. Towards the top of the Bioclastic Limestone unit, the faunal diversity increases and contains probably more than 20 taxa of bivalves, microgastropods, crinoids, brachiopods, ammonoids, echinoid spines, ostracods and conodonts. The generic diversity of this biofacies exceeds by far any other Griesbachian assemblage known. Our data give new evidence for the geodynamical history for the distal carbonate shelf bordering the Hawasina Basin. A break in the sedimentation characterises the Late Permian. The basal Triassic shows a steady transgression and the breccias may record a distinct gravitational collapse of platform margins linked with sea-level low stand at the end of Induan time (late Dienerian–basal Smithian). δ13Ccarb isotopic analyses were performed and yield typical Permian values of around 4‰ for the Reefal Limestone, with a strong negative shift across the Permian–Triassic boundary. During the Griesbachian values shift positively from 0.5 to 3.1‰ parallel to an increase in faunal diversity and probably primary productivity. The detailed faunal analysis and the discovery of an unexpected diversity give us a new understanding of the recovery of the Early Triassic marine ecosystem.

Trace fossils in the aftermath of mass extinction events

Abstract Ichnology has great potential to advance our understanding of mass extinction events and yet is currently an underutilized resource in such studies. Here we review published ichnological studies for the Ordovician-Silurian, Permian-Triassic and Cretaceous-Tertiary extinction-recovery intervals. In addition, new information regarding the Triassic-Jurassic ichnological record from England, Austria and the western USA is presented. Trace fossils provide important information on the ecological response of the benthic community at such times. In the immediate post-extinction aftermath, the ichnodiversity, burrow size, depth of bioturbation, and ichnofabric index of the sediments are all much reduced. There is an increase in all these parameters through the post-extinction recovery period. In some cases, the stepwise reappearance of certain distinctive ichnotaxa (e.g. Diplocraterion, Rhizocorallium and Thalassinoides) may be of some stratigraphic use. Evidence from Permian-Triassic studies indicates that recovery took longer at low (tropical) palaeolatitudes than mid-high palaeolatitudes. Trace fossils also provide important information on palaeoenvironmental change through the extinction-recovery interval. The application of ichnology to mass extinction studies is in its infancy, but should prove a valuable tool in future research.

Unique marine Permian‐Triassic boundary section from Western Australia

A unique marine Permian‐Triassic boundary section containing rich oil source rocks has been continuously cored in a petroleum borehole from the Perth Basin of Western Australia. Such sequences, which provide a biostratigraphic and environmental record at the time of the largest extinction event of the past 500 million years, are globally rare, and this is the first to be documented in Australia. Throughout geological history there have been periods of global marine anoxia that commonly resulted in the widespread deposition of petroleum source rocks, most notably in the mid‐Cretaceous and Late Jurassic. An apparent paradox is that, previously, source rocks have not been recognised in association with the Permian‐Triassic boundary, despite widespread marine anoxia at this time. The Perth Basin source rocks contain abundant and unusual biomarkers, apparently related to the highly specialised and limited biota that flourished in the aftermath of the end‐Permian extinction event. Local conditions may have favoured source‐rock development, either due to higher productivity resulting from coastal upwelling or through enhanced preservation under strongly anoxic conditions.