RICERCA ITALIANA     

A multiproxy approach to the study of environmental changes: numerical simulation of sedimentary wedges evolution during the two global extintion events at the P-T and Fr-Fm boundaries, time-constrained and calibrated by the faunal dynamics

Università degli Studi di Bologna

Abstract

The main goal of the multidisciplinary research is quantitative evaluation of important climate-forced parameters (i.e. sea-level curve) which changed as a consequence of global environmental changes during two of the "Big Five" events: the Permian-Triassic (P-T) and the Frasnian-Famennian (Fr-Fm) global extinction events. Both are elegantly expressed in stratigraphic sequences in the Southern Alps.
We will model the architecture of P-T and Fr-Fm sedimentary wedges using a physically-based, parameter-rich model ("Basin"), calibrated by good regional stratigraphic data.
Analysis of the P-T boundary will be a pilot study, taking advantage of availability of a rich dataset on lithostratigraphy and facies, occurring as a near-continuum over a large area (Southern Alps), and well represented by the Bulla section.
In this work we plan to use conodonts and associated fossil groups such as palynomorphs for detailed analysis of evolutionary patterns of conodonts and associated fossil groups in relation to the pattern of environmental change. Interpretation of faunal dynamics in the investigated sequences will be based on taxonomic, biostratigraphic and sedimentologic data. Isotopic data from carbonate matrices and organic matter—specifying changes in ocean chemistry— will provide an additional dimension to the palaeontologic and sedimentologic data.
The modelling will be constrained by chronostratigraphic (radiometric dating) and palaeomagnetic data obtained from coeval sequences, including the P-T stratotype in China. We will use these time-steps to run the back-analysis simulations constrained by the observed lithofacies architecture and geometry: these runs will yield calibrated estimates of parameters such as heat flux, geometry of basal transgressive surface, rates of tectonic subsidence and sediment compression-dissolution, water table and eustatic fluctuations.
The same research method will be applied to the Fr-Fm boundary carbonate sequence in the Carnic Alps. <<<

Principal Investigator

Enzo FARABEGOLI Università degli Studi di BOLOGNA

Research Objectives

-The main goal is to quantitatively evaluate some of the climatic parameters which showed pronounced variation before. during and after the major global extinction events associated with the Permian-Triassic (P-T) and Frasnian-Famennian (Fr-Fm) boundaries <<<

First Results

-Sampling and measurement of stratigraphic sections at the Frasnian-Famennian and Permian-Triassic boundaries.

-Preparation of samples collected in continuum - providing polished slabs and thin sections for microfacies and diagenesis analyses as <<<

Timescale

24 months

National and international background

Historically the boundaries of the main chronostratigraphic intervals of the geological time-scale were made to coincide with phases of extinction and/or rapid increase of taxonomic diversity. Changes in biodiversity, when seen in detail, are complex. Qualitative and quantitative studies of variation at low taxonomic level and on local or regional scale are necessary to verify how they fit in with or depart from gross global trends, and how they relate to changes in environmental parameters (climate, sea-level fluctuations, volcanic activity, tectonics, and oceanic circulation) (Valentine 1985, Signor 1990).
Conodonts, as prominent components of the Palaeozoic fauna, are known to reflect major changes in biodiversity (Sweet, 1988).
Five grand-scale extinction events (the "Big Five") have been discriminated within the Phanerozoic. These include the major biological crises at the Frasnian-Famennian (Fr-Fm) and Permian-Triassic (P-T) boundaries. A plethora of global causes have been postulated for these events and other life crises, among them transgression-regression events, climate change connected with orbital variations, tectonics, massive volcanism, and extraterrestrial causes (meteorites, comets). The last has been the ruling hypothesis for the massive extinction event at the Cretaceous-Paleocene boundary.
The Late Devonian extinction at the Frasnian-Famennian (Fr-Fm) boundary has been intensively investigated, particularly in the last decade (House et al. 2000; Racki & House, eds, 2002). That event affected most of the marine biota with 15 % of families disappearing at the end of the Frasnian. The low latitude reefs that characterised the Middle Devonian up to the middle Frasnian underwent a dramatic decline (Cooper 2002). Among benthic organisms brachiopods were reduced by 86% (Johnson 1971, 1974; Johnson & Boucot 1973; Mc Ghee 1981; Talent et al. 1993) and among nektic organisms 88% of ammonoid species went on extinction (House 1983, 1985, 2002, Becker 1993, Brcker & House 1994). Conodont and phytoplankton (Streel et al. 1987, Streel & Vanguestaine 1988) were drastically reduced. Two anoxic or dysoxic events, often associated with deposition of black shales , occurred in pelagic environments in the upper part of the Frasnian, the Lower and the Upper Kellwasser (LKW, UKW) events, respectively near the base of the Upper rhenana Zone and at the top of the linguiformis Zone of the conodont zonation. The base of the Famennian, established with its GSSP at Coumiac in the Montagne Noire of S France, corresponds to the lower boundary of the Lower triangularis conodont Zone (Klapper et al., 1994; House et al., 2000). This corresponds to the upper boundary of the Upper Kellwasser (UKW) extinction event discriminated in Germany, France, Morocco (Schindle, 1990), Kirghizistan and South China (J.A. Talent, pers. comm., 2003) in all of which it is characterised by presence of a 1-2 m black shale interval reflecting an anoxic event. In several sequences this shale interval occurs interbedded among light grey cephalopod limestones. The UKW event is less readily identified where it occurs in basinal shale sequences or where a shale interval is absent, e.g. in the Canning Basin of NW Australia, where the event is nevertheless identifiable from carbon and strontium isotope excursions, and from faunal changes (G. Klapper pers. comm., 2003).
Biotic, isotopic, magnetosusceptibility, and sea-level changes have been reported across the Fr-Fm boundary without consensus as to the cause (or causes) of the major extinction event (or events): extra-terrestrial and/or terrestrial. This, the UKW extinction event nevertheless happened in a relatively brief interval at the end of the last of the Frasnian conodont zones. Stratigraphic and sedimentologic studies carried out across the UKW in various regions suggest a regressive phase followed by a rapid dysoxic transgressive phase, both associated with severe faunal extinctions. For example, ammonoids, declined at the beginning of the regression, whereas some trilobite families became extinct during the transgression and anoxia. On the basis of orbital Milankovich rhythms, Chen & Tucker (2003) with analyses of deep water sediments in China estimated a duration of about 400 kyr for the first major extinction event and of about 50 kyr for the second. The extinction event was followed by a rapid adaptive radiation of the marine biota, including conodonts, with development of numerous species from the few survivors. Diversification during the Famennian conspicuously affected the genus Palmatolepis (important in the standard biozonation of the Famennian), beginning from the single species Pa. triangularis at the base of the Famennian (Ziegler 1962, Helms & Ziegler 1981).
In the Carnic Alps, recent biostratigraphic studies on conodonts (unpublished data) have enabled discrimination of the Fr-Fm Boundary in two sections--Pramosio A and Freikofel T--in limestone sequences with most levels medium to highly bioturbated. In neither section are black shales present. The shallow-water Pramosio-A section has numerous depositional sequences reflecting sea- level changes; these extend through the Fr-Fm boundary. A significant disconformity is present at the base of the Pa. triangularis Zone (Fr-Fm boundary); several transgressive-regressive sequences with decimetric to metric thickness are also present. Preliminary isotopic analyses seem to coincide with sea level fluctuations and to reflect climatic variation. Cathodeluminescence analyses on thin sections indicate a very complex precocious to tardy diagenetic imprint.
The Fr-Fm boundary crisis was followed by another important extinction, c. 10 millions of years later, at the Devonian-Carboniferous boundary. The two crises together were responsible for a dramatic impoverishment of the marine biotas. A marked evolutionary recovery during the Carboniferous and Permian regenerated the life in the oceans to again reach a high level of diversity; this also occurred in terrestrial environments.
Diversity collapsed again at the end of the Permian, another of the "Big Five"global life crises to be probed in this project. Disappearance at the end of the Permian of a large part of the marine and terrestrial biotas was so dramatic (immediately visible in the field) that the P-T boundary was made to coincide with this event—. The record of marine organisms (more complete than for terrestrial organisms) indicates that 75-96% of species and 65% of families became extinct Similar dramatic changes affected terrestrial plants and animals.Two distinct extinction phases are believed to have taken place during c. 6 million years—in the Dzhulfian and at the end of the Changhsingian respectively. Between these two phases, some groups diversified only to collapse again the second phase of the crisis (Erwin 1993, Hallam & Wignall 1997) when the Rugosa, Trilobita and Fusulinida disappeared completely and other groups almost passed into extinction. As had happened in previous Paleozoic crises, tropical reef communities were decimated and tropical carbonate productivity was reduced. It nevertheless exhibits several similarities with the Late Devonian crisis.
The stratigraphic succession spanning the P-T boundary in the Southern Alps, the westernmost area of the Tethys, has been investigated by several researchers—including people from the Bologna Unit of this proposal. Former views as to the position of the boundary have been modified following establishment of the stratotype of the P-T boundary in the Meishan D section in China, defined by first appearance of the conodont Hindeodus parvus (Yin et al 2001). The zonation across the boundary is now firmly based on species of the conodont genera Hindeodus and Isarcicella.
In the Southern Alps, the P-T boundary was formerly assumed, on sedimentologic and palaeontologic criteria, to coincide with the lithostratigraphic boundary between the Bellerophon and Werfen formations. As conseguence of the discovery of faunas with "Permian affinities" (Pasini 1985) in the lowermost part of the Werfen Formation, the P-T boundary was located in different positions depending by the criteria chosen. Biostratigraphic analysis, and specifically the appearance of H. parvus (Farabegoli & Perri 1998, Perri & Farabegoli 2003) in the Bulla Section in the Dolomites has enabled identification of the P-T boundary at 1.30 m from the base of the Werfen Fm. In the Bulla section, the middle-high rate of sedimentation has enabled discrimination of the pattern of evolution from Hindeodus to Isarcicella. Sedimentologic analyses show that the succession immediately below the P-T boundary includes two unconformities; one of these, of regional extent, was first recognized by Assereto et al. (1973). The succession above the boundary consists of small calcareous-terrigenous (predominantly calcareous) sequences that can be identified in coeval sections elsewhere in the Southern Alps. This notwithstanding, the Bulla and correlative sections are superior in several respects to the very condensed stratotype section in China. Having 40 m of sequence at Bulla correlating with only 1.5 m at Meishan has enabled detailed elaboration of the hindeodid-isarcicellid evolutionary plexus, as well as discrimination of six conodont biozones having wide applicability (Perri & Farabegoli 2003).
Though there have been numerous stratigraphic, sedimentologic and geochronologic studies on the Fr-Fm and P-T boundaries, and much speculation regarding the causes (terrestrial and/or extraterrestrial) of these global life crises, neither event has been probed quantitatively based on finest-scale sampling. This is essential if highest resolution regarding what happened through these events (and their sub-events) is to be obtained and, hopefully, a significant advance towards understanding the causes of the two events.

Data accumulated from many years of research through the targeted interval, coupled with the relevant competence of the investigators, have equipped them appropriately for undertaking the proposed project, and to do so from various perspectives. Specialised laboratories for microfossil preparation, sedimentology, photography and SEM are available in both departments involved in the project. External laboratories will perform specific analyses, such as isotopic analyses. <<<