Steve Kershaw

Microbialites and global environmental change across the Permian-Triassic boundary: a synthesis

Permian-Triassic boundary microbialites (PTBMs) are thin (0.05-15 m) carbonates formed after the end-Permian mass extinction. They comprise Renalcis-group calcimicrobes, microbially mediated micrite, presumed inorganic micrite, calcite cement (some may be microbially influenced) and shelly faunas. PTBMs are abundant in low-latitude shallow-marine carbonate shelves in central Tethyan continents but are rare in higher latitudes, likely inhibited by clastic supply on Pangaea margins. PTBMs occupied broadly similar environments to Late Permian reefs in Tethys, but extended into deeper waters. Late Permian reefs are also rich in microbes (and cements), so post-extinction seawater carbonate saturation was likely similar to the Late Permian. However, PTBMs lack widespread abundant inorganic carbonate cement fans, so a previous interpretation that anoxic bicarbonate-rich water upwelled to rapidly increase carbonate saturation of shallow seawater, post-extinction, is problematic. Preliminary pyrite framboid evidence shows anoxia in PTBM facies, but interbedded shelly faunas indicate oxygenated water, perhaps there was short-term pulsing of normally saturated anoxic water from the oxygen-minimum zone to surface waters. In Tethys, PTBMs show geographic variations: (i) in south China, PTBMs are mostly thrombolites in open shelf settings, largely recrystallised, with remnant structure of Renalcis-group calcimicrobes; (ii) in south Turkey, in shallow waters, stromatolites and thrombolites, lacking calcimicrobes, are interbedded, likely depth-controlled; and (iii) in the Middle East, especially Iran, stromatolites and thrombolites (calcimicrobes uncommon) occur in different sites on open shelves, where controls are unclear. Thus, PTBMs were under more complex control than previously portrayed, with local facies control playing a significant role in their structure and composition.

?Microbialites in the Permian-Triassic boundary interval in central China: Structure, age and distribution

SummaryA carbonate crust, with a principally digitate structure, caps latest Permian reef complexes in east Sichuan, China. The crust contains the conodontHindeodus parvus, confirming its age as earliest Triassic; it therefore closely postdates the end-Permian mass extinction, and is related to associated palaeoenvironmental change. Although its branches have lobate margins, and internal structures of radial fabrics and lobate fabrics in different specimens, an organic origin cannot be confirmed, because crust fabric is largely recrystallised. Therefore we apply the term ?microbialite to reflect uncertainty of its nature. The crust is co-eval (within theparvus Zone) with confirmed microbial biostromes and mounds in Guizhou Province, south of Sichuan. The sum of evidence, assembled by workers in several sites worldwide, indicates a sea-level rise occurred in the boundary interval, and this is corroborated by facies of the Sichuan crust. Abrupt appearance and disappearance of the crust, formed by precipitated carbonates, in east Sichuan, represents short-lived unusual post-extinction marine conditions which were switched abruptly on, then off. Microbial deposits overlying the P/T boundary in other locations in the Tethys Ocean (Iran and Japan) support the view that the unusual oceanic conditions had at least a regional distribution. Because the crust abruptly terminates, and is not succeeded by fossil-rich deposits, application of the disaster biota concept is inappropriate; an environmentally-driven control on carbonate precipitation is better supported by the evidence, whether or not it was biotically-mediated.

Erosional truncation of uppermost Permian shallow-marine carbonates and implications for Permian - Triassic boundary events: Comment.

[Payne et al. (2007)][1] suggest that there is widespread evidence for a carbonate dissolution surface at the level of the end-Permian mass extinction, and that this crisis was at least partly due to ocean acidification that preferentially affected heavily calcified marine invertebrates. These ideas

Ostracods (Crustacea) and water oxygenation in the earliest Triassic of South China: implications for oceanic events at the end-Permian mass extinction

Ostracods (Crustacea) are benthic inhabitants well known for their consistent qualities as paleoenvironment markers. In particular, they are reliable indicators of water oxygenation level: filter feeders are more common in poor oxygen conditions, contrasting with deposit feeders, which are abundant in well-oxygenated settings. In the Permian/Triassic (P/Tr) boundary transition in the Great Bank of Guizhou, ostracod species are dominated by deposit feeders, showing well-oxygenated conditions from the latest Permian, through the extinction level into the earliest Triassic. These results are consistent with ostracod faunas from northwest Guangxi Province. However, these two examples are in contrast with coeval ostracods from Sichuan, which show lower-than-normal oxygen levels in the earliest Triassic. The Great Bank of Guizhou forms an isolated platform in the large Nanpanjiang Basin on the south side of the South China Block; northwest Guangxi is nearby, in a marginal setting: both faced the Panthalassa Ocean through the P/Tr boundary times according to several published paleogeographic reconstructions. In contrast, P/Tr boundary transition rocks in Sichuan Province, located ∼600 km north of the Great Bank of Guizhou, lie on the Tethyan side of the South China Block. Both the Great Bank of Guizhou and the Sichuan sites have earliest Triassic microbialites, but these are profoundly different in structure and composition. The difference between the two areas may reflect contrasts in the nature of circulating ocean waters, with reduced levels of oxygenation in the Tethys (Sichuan), associated with modelled slow circulation, in contrast to better circulated Panthalassa ocean waters (Great Bank of Guizhou and northwest Guangxi). This also may be an argument to show that low oxygenated, or even anoxic, waters were not the only reason for the P/Tr boundary crisis.

Holocene sea-level change in Sicily and its implications for tectonic models: new data from the Taormina area, northeast Sicily

The northeast coast of Sicily shows emergent marine features that have been uplifting during the Holocene along the footwalls of two major regional fault systems, the Malta Escarpment and Messina fault system. Previously, uplift rates were interpreted as up to about 1.8 mm/mm a−1. New dates on shelly remains, collected close to sea-level, from the Taormina area north of Mount Etna, and amended sea-level curves, are used to show that uplift over the past 6000 years has been proceeding at a slower rate of about 1.4 mm a−1. However, over a longer time period, from the Tyrrhenian Oxygen Isotope Stage 5.5 (about 125 ka) to the present day, the uplift rate has been yet slower, at about 1 mm a−1. Northeast Sicily lies in a complex plate boundary region whereas, in contrast, the rest of Sicily appears to have been stable throughout the later Quaternary. Further comparisons show that the French Mediterranean coast [Lambeck, Bard (2000) Earth Planet. Sci. Lett., 175, 202–222] is a region of crustal stability, where movement is dominated by subsidence of the outer portion of the proglacial forebulge of the last glaciation. There the coastline has been progressively submerged during the Holocene, and sea level has never been higher than at present. Northeastern Sicily uplift is therefore more likely controlled by plate processes that mask most of the effects of glacio-hydro-isostatic adjustment.

Microbial response to limited nutrients in shallow water immediately after the end-Permian mass extinction.

Previous work indicates that a variety of microbes bloomed in the oceans after the end-Permian faunal mass extinction, but evidence is sporadically documented. Thus, the nature and geographic distribution of such microbes and their associations are unclear, addressed in this study using a series of biomarker groups. On the basis of microbial biomarker records of the 2-methylhopane index, evidence is presented for cyanobacterial blooms in both the western and eastern Tethys Sea and in both shallow and deep waters, after the mass extinction. The enhanced relative abundance of C(28) (expressed by the C(28) /C(29) ratio of) regular steranes suggests a bloom of prasinophyte algae occurred immediately after the end-Permian faunal extinction, comparable with those observed in some other mass extinctions in Phanerozoic. Significantly, cyanobacteria and prasinophyte algae show a synchronized onset of bloom in the shallow water Bulla section, north Italy, inferring for the first time their coupled response to the biotic crisis and the associated environmental conditions. However, in Meishan of Zhejiang Province in south China, the bloom declined earlier than in Bulla. The association of increased 2-methylhopane index with a negative shift in the nitrogen isotope composition infers a scenario of enhanced nitrogen fixation by cyanobacteria immediately after the faunal mass extinction. N(2) fixation by cyanobacteria is here interpreted to have provided prasinophyte algae with ammonium in nutrient-limited shallow waters, and thus caused their associated blooms.

Microbialites as disaster forms in anachronistic facies following the end-Permian mass extinction: a discussion

‘Anachronistic facies’ and ‘disaster forms’ are interpretive terms applied from the early 1990s to sedimentary deposits and biotas in the aftermath of mass extinctions; both terms have been used especially for the deposits formed directly after the end-Permian mass extinction. Microbial carbonates (disaster forms) are abundant in the earliest Triassic and often considered as a return to environmental conditions typical of Neoproterozoic to Cambro-Ordovician times. However, this view does not take into account: (i) the growing evidence that microbialites are stimulated by bicarbonate-supersaturated waters irrespective of mass extinction; (ii) the potential oceanic and climatic effects of the Siberian Traps volcanics; and (iii) the unique global plate-tectonic setting of Pangaea at that time. The configuration of land masses led to near-isolation of Tethys from Panthalassa, with modelled slow circulation and accumulation of anoxic deep water in Tethys. Evidence of catastrophic overturn of the Tethys Ocean reflects instability, possibly driven by climate changes, which released anoxic bicarbonate-rich waters to the surface. Items (ii) and (iii) are features of the Permian–Triassic boundary transition and are not parallels of earlier episodes of Earth history. Taking the argument wider, not all mass extinctions are followed by widespread anachronistic facies and disaster biotas. Therefore, it may be argued that application of anachronism and disaster biota concepts is an oversimplification of mass extinction processes in general, and the Permian–Triassic boundary extinction in particular. Continued use of these terms generates a narrowed view of processes and hinders development of comprehensive interpretations of changes of facies and biotas in mass extinction research.

Spatial spectral variations of microtremors and electrical resistivity tomography surveys for fault determination in southwestern Crete, Greece

The horizontal to vertical spectral ratio (HVSR) technique using microtremors and electrical resistivity tomography (ERT) surveys reveal a potentially seismic active source in southwestern Crete located within the outer forearc of the Hellenic subduction zone in one of the most seismically active deformed regions in Europe. The combined approach is applied on the Pahia Ammos coast southwest of the Paleohora peninsula and reveals an almost E–W-striking fault crosscutting the dense populated area. Spatial HVSR variations in the fundamental frequencies and HVSR shapes using microtremors pattern the effects of surface and subsurface structure on seismic ground motion and are capable of delineating fault zones. One clear HVSR peak in the low frequencies is related to the thickness of the alluvial deposits. Two amplified frequencies are attribute to lateral heterogeneities/irregularities induced by the fault zone and thickness variations of the geological column overlying the lateral irregularities of near-subsurface structure. Dipole–dipole and Wenner–Schlumberger configuration arrays are conducted to model the surface and subsurface structure variations. The identified fault zone striking E–W inland is capable of enhancing ground seismic motion and significantly contributes to the seismic hazard assessment of the studied area. Geophysical results are cross-correlated, verifying the validity of the research outcome.

Modern Black Sea oceanography applied to the end-Permian extinction event

Abstract The modern Black Sea has a mixed upper layer in the top 150–200 m of the water column, below which the water is anoxic, separated from the mixed layer by a redox boundary. There is limited vertical movement of water. Pyrite framboids form in the water column of the anoxic zone, then have been traditionally interpreted to sink immediately and accumulate in the sediments of the Black Sea. Thus the occurrence of framboids in sediments in the rock record is widely interpreted to indicate poorly oxygenated to anoxic conditions in ancient environments. However, in the Permian–Triassic boundary (PTB) microbialites of South China, which formed in shallow marine conditions in contact with the atmosphere, the published occurrence of framboids is inconsistent with abundant gastropod and ostracod shells in the microbialite. Furthermore, in the modern Black Sea, (a) framboids may be suspended, attached to organic matter in the water column, thus not settle to the sea floor immediately after formation; and (b) the redox zone is an unstable complex area subject to rapid vertical water movement including occasional upwelling. The model presented here supposes that upwelling through the redox zone can lead to upward transport of suspended pyrite framboids into the mixed layer. Advective circulation could then draw suspended framboids onto the shelf to be deposited in oxygenated sediments. In the Permian–Triassic transition, if framboids were up-welled from below the redox boundary and mixed with oxygenated waters, sediment deposited in these conditions could provide a mixed signal for potentially misleading interpretations of low oxygen conditions. However, stratigraphic sampling resolution of post-extinction microbialites is currently insufficient to demonstrate possible separation of framboid-bearing layers from those where framboids are absent. Profound differences between microbialite constructors and sequences between the western and eastern Tethys demonstrate barriers to migration of microbial organisms. However, framboid occurrences in both areas indicate upwelling and emphasize vertical movement of water from the lower to upper ocean, yet the mixed layer advective motion may not have been as effective as in modern oceans. In the modern Black Sea, such advection is highly effective in water mixing, and provides an interesting contrast with the PTB times.

Reef reconstruction after extinction events of the latest ordovician in the Yangtze platform, South China

SummaryEarly Silurian reef reconstruction on the Yangtze Platform, in the northern part of the South China Block, is preceded by a combination of regional and global processes. During most of Ashgill time (Late Ordovician), the area was dominated by Wufeng Formation deep water graptolitic black shales. Reefs largely disappeard in the middle of the Ashgill Stage, from the northwestern margin of Cathaysian Land (southeastern South China Block), in advance of the Late Ordovician glaciation and mass extinction, due to regional sea-level changes and regional uplift, unrelated to the mass extinction itselt. Late Ordovician microbial mudmound occurrence is also found in the western margin of the Yangtze Platform, its age corresponding to theDicellograptus complexus graptolite biozone of pre-extinction time. On the Yangtze Platform, a thin, non-reef-bearing carbonate, the Kuanyinchiao Formation (=Nancheng Formation in some sites), thickness generally no more than 1m, occurs near several landmasses as a result of Hirnantian regression. Reappearance of the earliest Silurian carbonates consisting of rare skeletal lenses in the upper part of Lungmachi Formation, are correlated to theacensus graptolite biozone, early Rhuddanian of Shiqian, northeastern Guizhou, near Qianzhong Land. Carbonate sediments gradually developed into beds rich in brachiopods and crinoids in the lower part of Xiangshuyuan Formation, middle Rhuddanian. In the middle part of Xiangshuyan Formation, biostromes, containing abundant and high diversity benthic faunas such as corals, crinoids and brachiopods, show beginnings of reconstruction of reef facies. Substantial reef recovery occurred in the upper part of Xiangshuyuan Formation, lower Aeronian, as small patch reefs and biostromes. During the late Aeronian, carbonate sediments, especially reefs and reef-related facies, expanded on the upper Yangtze Platform, and radiation of reefs occurred in Ningqiang Formation, upper Telychian. The long period of reef recovery, taking several million years, remains difficult to explain, because redistribution of any refugia faunas would be expected to take place soon after the extinction. Reefs and reef-related facies subsequently declined after Telychian time due to regional uplift of the major portion of the Yangtze Platform. Carbonate facies are therefore uncommon in South China during the rest of Silurian time.