RICERCA ITALIANA     

Stratigraphic-paleogeographic and geochemical-petrographic characterization of the events around the Triassic-Jurassic boundary: an integrated approach

Università degli Studi di Milano

Abstract

The Late Triassic was a period of intense biological changes, involving both marine and non-marine biota, culminating at the Triassic/Jurassic (T/J) boundary in one of the five largest extinction events of the Phanerozoic. The biotic crisis was synchronous with a profound reorganisation of mantle and lithospheric plates dynamics related to the incipient phases of Pangea break-up. The opening of the Central Atlantic Ocean and of the Alpine Tethys in the Jurassic were preceded, at the T/J boundary, by the emplacement of the Central Atlantic Magmatic Province (CAMP), one of the largest magmatic provinces of the Phanerozoic.
Aim of this project is to contribute to explain timing and causes of the T/J extinction event. A multidisciplinary approach on selected stratigraphic sections will be adopted, combining lithostratigraphy, biostratigraphy, magnetostratigraphy, chemostratigraphy, geochemistry, geochronology, petrography, and paleogeography. The research fields of this project are outlined below:
a) identification of major physical and biological events occurring across the T/J boundary in sedimentary successions deposited in different paleogeographic settings, evaluating their local as opposed to global significance;
b) age determination, duration assessment, and correlation of the stratigraphic events recognized in the selected sections; radiometric (Ar/Ar on magmatic rocks), biostratigraphic, and magnetostratigraphic dating are planned;
c) analysis of the geodynamic evolution of lithospheric plates during the opening of the Atlantic Ocean and nearby basins (Alpine Tethys); reconstruction of the spatial and volumetric distribution of the CAMP; evaluation of the quantity of gas released in the atmosphere by the CAMP activity and their impact on the environment;
d) geochemical characterisation of the volcanic and sedimentary successions using stable isotope analysis; evaluation of the variations in the isotopic signature in time;
e) study of the duration, age, and main characteristics of the environmental changes that occurred across the T/J boundary;
f) reconstruction of the position of plates during the various phases of Pangea break-up, integrating stratigraphic, paleoenvironmental, and paleomagnetic data.
The selected stratigraphic successions are located in the Atlantic and Tethyan domains. In the Atlantic domain, sections from Europe and North Africa (Portugal and Morocco), and South America (Bolivia and Brazil) will be studied. These successions are characterized by the interfingering of CAMP-related lava flows and fossiliferous shallow marine or continental sediments. In the Tethyan domain, marine successions will be studied. In detail, we will analyze and compare shallow water carbonate facies (Brenta, Tarvisiano, Dolomites, Dinarids, Central Apennines) and subtidal successions (Southern Alps, Austroalpine, Northern and Central Apennines). The Peri-Tethyan successions, transitional to the Atlantic province, will be studied in North Africa (Tunisia and Morocco). In addition, the analysis of volcanic units that developed around the T/J boundary in selected South European localities (France and Dinarids) will be carried out in order to verify their possible relations with the CAMP and, as a consequence, to recognize the eastern boundary of the magmatic province itself.
The Research Units of the project will operate when necessary in mutual cooperation during data acquisition phases. All Units will participate actively to eventual data interpretation and modelling. The obtained results will be presented in a conclusive workshop and in peer-reviewed scientific journals. <<<

Principal Investigator

Flavio JADOUL Università degli Studi di MILANO

Research Objectives

FOREWORD
The recent years experienced a general improvement of the biostratigraphic, geochemical, litostratigraphic, and magnetostratigraphic knowledge of key stratigraphic successions deposited in the Central Atlantic and Tethyan domains around the Triassic/Jurassic (T/J) boundary, which is one of the five major biologic crises of the Phanerozic. Nevertheless, a general model able to relate all different events, both biotic and abiotic, which occurred across the boundary, has not yet been proposed. Our multidisciplinary project aims at filling this gap of knowledge. In particular, our project will attempt to determine absolute and/or relative age of all major biotic and abiotic events that occurred across the T/J boundary in different paleoenvironmental settings.

STUDY TOPICS
The following aspects will be studied:
a) Identification and correlation of the physical and biological events in sedimentary successions deposited in different paleogeographic settings, evaluating their local or global significance. Selected taxa (mainly palynomophs and nanoplankton) will be studied in order to unravel their evolution throughout the T/J boundary assess their recovery after the crisis;
b) dating of the stratigraphic events: radiometric (Ar/Ar on magmatic rocks), biostratigraphic (mainly palynological), and magnetostratigraphic dating are planned in order to unravel age, duration, and correlation of the different events recognized in the selected sections. Particular attention will be paid to the dating of the CAMP event, in order to establish its temporal links with the T/J crisis;
c) geodynamic setting: detailed reconstruction of the events related to the opening of the Atlantic Ocean and nearby basins (i.e. Alpine Tethys); reconstruction of the areal and volumetric distribution of the CAMP and, as a consequence, of the quantity of gases released in the atmosphere and their possible effects on the environment;
d) geochemistry: geochemical characterization of the sedimentary and volcanic successions, the formers studied by means of stable isotope analysis; evaluation of the variations in the isotopic signature in time;
e) paleoenvironment: study of the duration, age, and characteristics of the environmental changes across the T/J boundary;
f) paleogeography: reconstruction of the position of lithospheric plates during the disruption of Pangea, integrating stratigraphic, paleoenvironmental, and paleomagnetic data.
This integrated approach will produce a general framework able to explain time and causal relations among the different phenomena recorded across the T/J boundary in the studied successions.


STUDY AREA
Stratigraphic successions pertaining to the Central Atlantic and Tethyan domains will be studied.
In the Atlantic domain, sections from Europe and North Africa (Portugal, Morocco) and South America (Bolivia and Brazil) will be analyzed. These successions are characterized by the interfingering of CAMP-related lava flows and fossiliferous sediments (both shallow marine and continental). A biostratigraphic and magnetostratigraphic correlation of our data with data from dated reference sections in North America (i.e., Newark Basin) will be carried out.
In the Tethyan domain, where the break-up of Pangea is also recorded by the opening of the Alpine Tethys, marine successions will be studied. In detail, we will analyze and compare shallow water carbonate platform facies (Brenta, Dolomites, Friuli Prealps, Dinarids, Central Apennines) and subtidal successions (western Southern Alps of Lombardy, Austroalpine, Northern and Central Apennines). The Peri-Terhyan successions, transitional to the Atlantic province, will be studied in North Africa (Tunisia, Morocco). In addition, the study of volcanic units developed around the T/J boundary in selected South European localities (France, Dinarids) will be carried out in order to verify their possible relations with the CAMP and, as a consequence, to recognize the eastern boundary of this magmatic province.
The analysis of (literature) information from the Boreal Domain (Great Britain) will also allow a comparison with data directly retrieved form the Tethyan and Atlantic domains. On most of the sections, a biostratigraphic study (based on palynomorphs and, where possible, foraminifera, nannoplankton and radiolarians) and paleomagnetic study will be carried out. Detailed magnetostratigraphic and chemostratigraphic analyses will allow correlation of these sections with the global geomagnetic reference scale. Paleomagnetic analyses will also focus on the reconstruction of the apparent polar wander path of all major circum-Atlantic plates with the aim at reconstructing the paleogeographic scenario across the T/J boundary.


METHODS
A multidisciplinary approach will be adopted. Members of this project have a long scientific experience in the field of stratigraphy, geochemistry, and petrology as documented by the scientific publications of the last years. In detail, the disciplines that will contribute to the success of this project are: 1) stratigraphy, with facies, microfacies, petrofacies, and organic facies (palynofacies) analyses of sedimentary units; 2) paleomagnetism, with magnetostratigraphic analyses of selected sections and determination of paleomagnetic poles for paleogeographic reconstructions; 3) petrography, with detailed studies of the composition of the CAMP; 4) geochemistry, with the study of the stable isotope composition of the sedimentary successions as well as of the volcanic units of the CAMP; geochronology (Ar-Ar) of the CAMP products; 5) biostratigraphy, with the analysis of selected taxa (palynomorphs, foraminifera, dasycladdacee, pelecypods, radiolarians).


CONCLUSIONS
This project will contribute to give an answer to questions such as: which where the timing and causes of the biologic crisis at the T/J boundary? How important was this crisis? Was it triggered by the CAMP event? are there evidences for a meteorite impact, or the crisis was controlled by the release of large amounts of methane? We will try to answer these questions by studying age and characteristics of major chemical, physical, and biological events occurring in selected stratigraphic sections whose position within the global distribution of plates is known, also with respect to climate belts assuming an uniformitarian climate model. <<<

Timescale

24 months

National and international background

INTRODUCTION
The mass extinction that occurred about 200 Ma at the Triassic/Jurassic (T/J) boundary (Palfy et al., 2000; Hallam, 2002; Tanner et al., 2004) is one of the five main extinction events of the Phanerozoic (Raup & Sepkoski, 1982). Bivalves, gastropods, corals and brachiopods, which had flourished during the Carnian-Norian, suffered progressive specific reduction during the Rhaetian and became, after a first Late Norian crisis, largely extinct by the end of the Triassic. In the pelagic domain, several families of organisms underwent complete extinction. On the contrary, calcispheres and calcareous nannofossils first appeared (or secreted for the first time a calcareous shell) during the Late Triassic and became a relevant source of carbonate productivity by the Jurassic. The Triassic/Jurassic boundary is marked by a series of biostratigraphic events involving Dynoflagellates and a few shallow water fauna (references in Galli et al., 2005; Barattolo e Romano 2005)
Among vascular plants, microflora analysis showed that the T/J boundary transition was apparently gradual in the Tethyan and Boreal domains (Buratti, 2003), in apparent contradiction with evidences for an abrupt turnover at the boundary in the Newark basins of North America (e.g., Fowell et al. 1994).

PALEOGEOGRAPHY AND GEODYNAMICS
The Norian-Hettangian time interval marks the fragmentation of Pangea, the supercontinent formed by the Variscan coalescence of Gondwana and Laurasia during the Carboniferous. At the longitudes of the circum-Atlantic continents, highly subsiding extensional basins on continental crust formed during the early stages of Pangea break-up along North American and NW African margins. The eastern prosecution of the Atlantic opening took place between Adria (the African promontory) and Eurasia. Lacustrine or evaporitic sedimentation characterized the Afro-American rift basins, whereas marine carbonates dominated the Tethyan and Peri-Tethyan basins.

THE SEDIMENTARY SUCCESSIONS
Widespread carbonate platform deposition characterized the tropical latitudes of the Western Tethys during the Norian (Dolomia Principale-Haupdolomit, references in Cirilli et al. 1999). At the close of the Triassic, shallow-water deposition was tipically mixed terrigenous-carbonatic (Cirilli et al. 1994; Jadoul et al. 1994; Jadoul et al. 2004). These Rhaetian mixed facies were characterized by a diagenetic fingerprint markedly different from that of the Norian carbonate platform facies (Iannace & Frisia, 1994). Bahamian-type carbonate platforms replaced the Rhaetian platform ramps in the Early Jurassic (McRoberts, 1994; Jadoul et al., 2000). Mud resulting from the micritization of, e.g., oolites and peloids was exported to deeper basinal settings that formed as a consequence of the Liassic opening of the Central Atlantic Ocean (references in Bertotti et al., 1993). Across the T/J boundary, the contribution of recifal organisms to primary productivity was reduced (Galli et al. 2005).In the pelagic domain, the development during the Late Triassic of the first conspicuous nannoplankton communities led to the formation of expanded, well-bedded limestone successions.

EUSTATIC OSCILLATIONS
The Late Triassic was characterized by an anomalously low number of depositional sequences / eustatic oscillations compared to the late Rhaetian-Hettangian (4 third order cycles, Jadoul et al. 1994; Jadoul et al. 2004) , which experienced higher frequency sea level oscillations superposed to a long-term increase of relative sea level that caused the widespread transgressions onto large portions of the Pangea supercontinent.

CLIMATE
Apparently, the fragmentation of Pangea was contemporaneous with the transition from a Triassic zonal climate characterized by a narrow humid equatorial belt and expanded arid intertropical belts (Kent & Olsen, 2000) to a Jurassic climate with progressively less pronounced latitudinal gradients that led to the global expansion of the zonal humid belts and the formation of the first oceanic anoxic events of the Mesozoic. The fragmentation of Pangea as a consequence of the opening of the Central Atlantic ocean at the Triassic/Jurassic transition may have controlled climate regionally by enhancing plates' motion across Earth's zonal climate bands and modifying the oceanic and atmospheric general circulation patterns.

THE CENTRAL ATLANTIC MAGMATIC PROVINCE
The Central Atlantic magmatic province (CAMP) is roughly contemporaneous with the T/J boundary mass extinction (Dunning & Hodych, 1990; Marzoli et al., 1999), and is one of the largest magmatic provinces of the Phanerozoic (Marzoli et al., 1999), comparable in size (and composition) with the Siberian or Deccan Traps. The CAMP consists of intrusives, dyke swarms, and lava flows of basaltic tholeiitic composition that were emplaced rapidly (1-2 m.y.) around 200 Ma during the early stages of Pangea break-up. The CAMP basalts are well exposed on land in North America, South America, Africa, and Europe over an area of more than 7 millions km2. The "tethyan CAMP" instead in only locally represented by magmatismo and volcanism (Francia, Croazia) but is not well understood as age, volumes and geodynamic meaning.

MAGNETOSTRATIGRAPHY AND ASTROCHRONOLOGY
The Late Triassic-earliest Jurassic Newark astrochronological polarity time scale (APTS) (Kent & Olsen 1999) was constructed by anchoring a Milankovitch chronostratigraphy to the palynological T/J boundary located a few meters below the oldest basalt flow of the basin's extrusive zone with collective ages of about 199-202 Ma. A 199.6+/-0.4 Ma U/Pb age for the T/J boundary was also found in ammonite-bearing marine strata from North America. Based on continental palynology and astrochronology relative to a T/J boundary age of 202 Ma, the Newark Norian/Rhaetian boundary was correlated to within magnetozone E18 at about 208 Ma. In the pelagic domain of the Western Tethys, a large number of limestone sections were magnetostratigraphically studied and correlated to the Newark APTS; among others, the Late Triassic expanded sections from Sicily (Muttoni et al., 2001; in press) yielded the most complete record of magnetic polarity reversals across a consistent portion of the Late Triassic time interval. Recent magnetostatigrahy of St. Audrys' Bay (reduced Norian-hettangian succession of Southern England) has been proposed by Hounslow et al. (2004), its correlation with Newark is still problematic.

CHEMOSTRATIGRAPHY
Stable isotope stratigraphy in the Late Triassic-Early Jurassic time interval is scarcely known. Negative isotopic shifts were reported at the T/J boundary in Hungary, British Columbia, the UK, and the Southern Alps (Palfy et al., 2001; Ward et al., 2001; Hesselbo et al., 2002; Galli et al., 2005), and were related to a massive CO2 release from volcanic degassing and/or dissociation of gas hydrates. However, Ward et al. (2001) observed that the isotopic shift at the T/J boundary was contemporaneous with the extinction of radiolarians in the marine domain, and suggested that an impact event may have caused a sudden collapse of the eutrophic chain in the oceans. In the Late Triassic, Sephton et al. (2002) described a complex isotopic excursion at the Norian/Rhaetian boundary in British Columbia. According to these authors, the Rhaetian was a period of recurring extinctions and perturbations of the global carbon cycle. Moreover, preliminary data from the Pizzo Mondello section from the Sicani basin of Sicily revealed the occurrence of a carbon isotope shift across the Carnian/Norian boundary (Muttoni et al. 2004). We conclude that the isotope signature during the Late Triassic and across the T/J boundary was much more complex than previously assumed.

CAUSES OF MASS EXTINCTION AT THE T/J BOUNDARY
The following causes/mechanisms to explain the T/J boundary mass extinction have been variably proposed in the literature.

CO2 perturbations and CAMP volcanism
The T/J boundary marks at a gross scale the transition between the end of the Variscan orogenic cycle and the beginning of the Alpine cycle. The Variscan and post-Variscan tectonic activity of Late Paleozoic-Permian age generated a large collisional mountain chain at equatorial latitudes that was presumably subjected to intense weathering during the Permian-Early Triassic. The process of weathering of silicatic rocks involves atmospheric CO2 sequestration, which has the effect of cooling Earth's climate globally. In absence of continental masses at high latitudes, polar ice caps probably did not develop in the Triassic. In any case, the post-Variscan global cooling should have had the effect of expanding globally the intertropical arid belts. By the very beginning of the Jurassic, this long-term cooling trend was presumably inverted by the introduction in the ocean/atmosphere system of massive amounts of CO2 of volcanic (CAMP) origin (about 8000 Gt; Beerling & Berner, 2002), which may have caused global temperatures to increase by 3-4 °C (McElwain et al., 1999). In addition to the CAMP event, tectonic instability related to the break-up of Pangea may have also triggered episodes of release of CO2 of biogenic origin (methane) previously stored in porous sediments deposited along the newly formed circum-Atlantic continental margins. To this respect, a new theory based on methane-driven oceanic eruptions has been recently proposed to explain mass extinctions (Ryskin 2003). In any case, abrupt perturbations of the long-term global carbon cycle may trigger major faunistic and floristic turnovers or mass extinctions such as that at the T/J boundary. In fact, the rapid increase of global CO2 as consequence of the CAMP event was directly related to the T/J mass extinction by a number of authors (Hesselbo et al. 2002; Beerling & Berner 2002; Palfy 2003; Huynh & Poulsen , 2005 ). However, additional data are required to evaluate the amount of CO2 effectively produced by the CAMP event.

Extraterrestrial bolide
Scientists have found possible evidence of an impact of an asteroid at the T/J boundary. At this boundary, a number of species/genera were wiped out similar to the number of species/genera that went extinct at the Cretaceous/Tertiary boundary by the Chicxulub asteroid (Hildebrand et al. 1991). Levels containing iridium, a metal that rarely occurs naturally on Earth's surface and is proven mostly to be of extraterrestrial origin, were found at the Triassic-Jurassic boundary at two distinct sections (referenze in Olsen et. al. 2002). However, no impact craters of appropriate age and size has been found yet (not surprisingly as the Earth is mostly covered by oceans). Dating of the best candidate impact structure, the Manicouagan crater, would place the impact event well before the Triassic/Jurassic boundary.

Eustatism and anoxia
Potentially, a sea-level fall resulting in a loss of echosystem availability, followed by rapid sea-level rise resulting in generalized marine anoxia may explain extintion events (Hallam, 2000).

CONCLUSIONS
The following conclusions can be drawn from what exposed above:
1) a major biologic crisis, which affected both the marine and continental realms, occurred at the Triassic/Jurassic boundary;
2) the Triassic/Jurassic boundary crisis seems to represent the culmination of a much longer period spanning the Late Triassic characterized by generalized instability.
3) causes and mechanisms that triggered the Triassic/Jurassic boundary crisis are not yet univocally determined. <<<