RICERCA ITALIANA     

The end and the beginning: loss and recoveries from the end-Ordovician mass extinction in the North Gondwana

Università degli Studi di Modena e Reggio Emilia

Abstract

The second major extinction event in life history occurred in the Late Ordovician. It was followed by an important recovery in the Early Silurian: the end and the beginning, precisely.
After having cleared up all "inorganic" elements of this crisis (geodynamic evolution, climate change, ocean circulation patterns, regional anoxia, O2/CO2 ratio), how did marine faunas respond to this dramatic episode? How could they react and trigger recovery processes? How important were palaeogeographic, palaeoclimatic and palaeoecological frameworks?
We will try to give an answer to the previous questions with the aid of sole pelagic faunas (conodonts, graptolites and cephalopods), main victims of the extinction, in a specific palaeogeographic setting, the North Gondwana. Biodiversity change evaluation, as well as palaeoclimatic information obtained by means of geochemical analyses, will allow the precise and detailed monitoring of marine biotic response to the Late Ordovician extinction event. <<<

Principal Investigator

Annalisa FERRETTI Università degli Studi di MODENA e REGGIO EMILIA

Research Objectives

Only very recently scientists are suggesting that the sixth great mass extinction, involving present world, has already started.
Major crises of the life history are being riexamined with renewing interest in order to obtain new data on the dynamics and causes of these episodes and to have a better comprehension of what is happening nowadays.
The Big Five mass extinctions classically identified in the Phanerozoic have greatly attracted scientists since the pioneer works of Newell (1967) and Raup & Sepkoski (1982). The Late Ordovician crisis represents the second severest extinction episode. The majority of research carried out on this topic has been mostly devoted to unravelling the biotic response of benthic faunas, which are more strictly dependent on ecological features. Pelagic faunas, on the contrary, have been often ignored. In a time slice for which palaeogeographic elements are continuosly moving and palaeoclimatologic data are contributing to define a more and more precise Ordovician "scenario", is the chapter on the Late Ordovician extinction already finished?
On this basis, we intend to propose the project: THE END AND THE BEGINNING: LOSS AND RECOVERIES FROM THE END-ORDOVICIAN MASS EXTINCTION IN THE NORTH GONDWANA.
The aim of the project is a study of the distinctive extinction/radiation pattern of pelagic faunas during and after the end-Ordovician crisis in order to unravel the variety of mechanisms, intensities and magnitudes of the biotic response. Furthermore, the comparison of taxonomic and ecological relationships and biodiversity levels of pre- and post-extinction biotic associations coupled with the geochemical signature of both biota and sediments, will be taken in a larger time span than the event itself. The Northern Gondwana region has been chosen as the study area, being affected by both sharp and recurrent climatic fluctuations (e.g. glaciation and related sea-level changes) and having a dynamic history of continental block dispersal.
The multiple causes of extinctions are the subject of much controversy and are beyond the objective scope of this project.
To approach the issue, the project will focus as follows (see also, for details, # 2.5 of "Modello B" of this project):

1) Tools. Primary tools are represented by pelagic marine fossil biota (conodonts, graptolites and cephalopods). More specifically high-resolution palaeontological databases will be provided through detailed sampling along stratigraphic sections and systematic palaeontological and biostratigraphic analyses.
2) Methods. Starting from original databases of selected marine biota and sedimentary rocks, the end-Ordovician mass extinction will be investigated in terms of its biotic, lithological and geochemical fabrics. Particular attention will be given to the analysis of:
- extinction intensity;
- post-extinction recovery (biodiversity rebound after extinctions and nature of delayed recoveries);
- biodiversity change;
- environmental setting (sea-level, temperature, anoxia);
- palaeoecological change (population dynamic and community analysis);
- geochemical signature;
- climatic evolution.
3) Palaeobiogeographic regulation. The palaeogeographic configuration of the selected time interval is characterized by the dispersal of continental blocks from the Gondwanan margin, which is best recorded in the selected study area. The definition of peripheral blocks of the Northern Gondwana margin is getting more and more precise with the recent introduction of several microplates (f.e. Perunica and Carnica) located at lower latitudes. The palaeogeographic configuration has significant implications for oceanic circulation, climate change, and lastly for the bioprovincial pattern and the response of marine biota.
4) Palaeoclimatic constraints. The Ordovician-Silurian time slice is characterized by a complex interplay of succeeding intervals of "greenhouse" and "icehouse", fluctuating warming and widespread anoxia.

In this context, we will try to answer to the following questions:

1) What was the response of the selected marine pelagic biota before, during and after the two phases of the Late Ordovician crisis in terms of their extinction intensity, recovery and biodiversity change? Are there any similarities between the two episodes?
2) What were the palaeoecological and palaeoenvironmental changes across the events?
3) How significant were the palaeoclimatic setting and the palaeobiogeographical regulation (if any)? <<<

First Results

The main expected partial result consists in the collection of data in the field, for all the time intervals included in the present proposal. The systematic and biostratigraphical study of the selected fossil groups will be carried out as well. In detail:

- mapping, measurements and sampling of already mentioned sections;
- lithostratigraphic and sedimentological analysis of the sections under study;
- lab processing (acid treatment for conodonts, thin-sections and preparation of peels for microfacies analysis);
- systematic identification of the investigated fossil groups;
- biostratigraphical analysis;
- palaeoecological analysis.

The general meeting scheduled for October 2005 with foreign collaborators will verify the results so far obtained in comparison with the scheduled objectives.The expected results are the final results of the project. The details are explained in the second phase. The basic purpose of the project is to assess biotic response to global change in the Northern Gondwana area during the second severest extinction of life history, the Late Ordovician event. It is strongly desirable that our study, focused on some pelagic taxa often ignored, might produce new data for improving the already existing information about this topic so as to understand why these extinction events did occur, in which way they developed and how and why marine faunas responded to them. <<<

Timescale

24 months

National and international background

Compilations of global marine biodiversity through the Phanerozoic (Sepkoski 1979, 1981, 1990) have identified intervals of progressive increase and long-term plateaux punctuated by five sharp events of mass extinction, the so called Big Five (Raup & Sepkoski 1982). Of these, the end-Ordovician and end-Permian are the two most severe crises. Since the hypothesis of Raup & Sepkoski (1984) about the cyclicity and similarity of mechanisms of major extinction events, only a few studies have been carried out on the comparison of the five crisis showing a correlation in the pattern of biotic response (Hallam & Wignall 1997; Harries & Little 1999; Rong & Shen 2002). In fact, most palaeontologists chiefly analyse patterns of extinction and survival that characterise each mass extinction. As regards the Late Ordovician episode, indeed, many elements still need a clear definition, first of all a precise palaeoclimatic background as well as a definite palaeogeographic "scenario" if a final answer on the Late Ordovician extinction is really expected.
The major features and general state of the art about the Late Ordovician great event are outlined by the answers to the following questions.

-HOW SEVERE WAS THE ORDOVICIAN EXTINCTION? Taxonomic loss during the Late Ordovician extinction was particularly heavy at lower levels. An estimated 85% of species disappeared, 61% of genera (Jablonski 1991) and 12-24% of families (Sepkoski 1997; Benton 1995) but few o no orders. All major taxonomic and ecological groups were affected. Some 60-75% of genera became extinct in the mainly benthic groups (brachiopods, echinoderms, corals, trilobites and bivalves) whilst about 85% of the pelagic graptolites disappeared (Underwood 1998) and 89% of the conodonts (Sepkoski 1995). The extinction occurred in two phases c. 0,5-1 million years apart. A rapid and weaker extinction occurred at the Rawtheyan/Hirnantian boundary and a stratigraphically fairly poorly constrained, stepped series of extinctions occurred within and at the end of the Hirnantian (Owen & Robertson 1995).

-WHAT WAS THE PALAEOGEOGRAPHIC PICTURE? Despite substantial advances in plate tectonic modelling in the last three decades, the position of the North-Gondwanan terranes postulated for the Palaeozoic has seldom been validated by faunal data. Palaeomagnetic and lithological data locate the Ordovician Northern Gondwana margin in latitudinal positions close to 80° S (Scotese & McKerrow 1990). This assumption has been strongly criticised by recent faunistic information (Havlicek et al. 1994; Vennin et al. 1998; Villas et al. 1999; Ferretti et al. 2000; Villas et al. 2002; Brett et al. 2004) which suggests lower latitudes as well as the existence of peripheral blocks detaching from North Gondwana.

-WHAT DO WE KNOW ABOUT THE OCEAN CIRCULATION PATTERNS? A detailed palaeogeographic reconstruction enables us to reconstruct the pattern of the oceanic currents. Baltica, after the accretion of Avalonia, was placed in the Ordovician at intermediate latitudes NW of the Northern Gondwana margin and could have deflected southward the South Equatorial current, enabling it to reach the margin and establish warm-water faunas. This situation strongly resembles the present geography of the South Pacific, where Australia deflects the warm South Equatorial current towards New Zealand (Villas et al. 2002).

-WHAT WAS THE PALAEOCLIMATIC EVOLUTION? A Late Ordovician glaciation is well-established both on the basis of sedimentary evidence (Brenchley et al. 1991 and references therein) and isotopic data (Middleton et al. 1991; Brenchley et al. 1994). The glaciation was brief and confined to the Hirnantian (1-0,5 million years; Harland et al. 1990; Barnes 1992). A climatic amelioration suddenly occurred on the Northern Gondwana margin, testified by deposition of disconnected temperate limestones with a lateral extent of more than 2000 km, and overlying several thousand meters of Lower-Middle Ordovician siliciclastic sediments. The latter represent temperate to cold environments which predominated during ca. 45 m.y. and lack distinct episodes of carbonate production (Villas et al. 2002). The change from pre-Hirnantian "greenhouse" climates to Hirnantian "icehouse" conditions was rapid and not preceded by climatic fluctuations which might have helped acclimatise the biota to climatic variations (Brenchley et al. 1994).

-WHAT WAS THE ROLE OF CO2 AND O2 IN THE OCEAN-ATMOSPHERE SYSTEM? Hirnantian glaciation has aroused a keen interest during the last decade as it occurred during times of very high levels of the greenhouse gas CO2 (14-18 times the present atmospheric value) and not low values, as possibly expected. According to Villas et al. (2002) the onset of the glaciation is explained as the result of the accumulation of great volumes of carbonates in the pre-Hirnantian Late Ordovician in regions where these deposits were previously absent. These carbonates are considered as the sink of the atmospheric C02 which was extracted from the atmosphere causing a remarkable lowering of its values at the start of the Hirnantian. The glacioeustatic lowering of the sea level would have stopped the sedimentation of carbonate due to the retreat of the oceans from the platforms, closing this CO2 sink simultaneously with the growth of the ice cap. The pre-glacial CO2 levels would then be reattained due to volcanic and metamorphic CO2 outgassing. After the subsequent melting of the ice cap, the oceanic circulation did not recover its previous strength, resulting in strong stratification of ocean waters and precluding the recovery of extensive carbonate deposition. Brenchley et al. (1994) had proposed a similar approach to the glaciation in considering the event closely related to the carbon cycle, but introducing an earlier Hirnantian increment in production and/or sedimentation rates of organic carbon as the major sink for the atmospheric CO2. At the end of the Hirnantian, ice cap melting resulted in a rapid, eustatic sea-level rise and in the development or spread of anoxic and low-oxygen waters on the shelves (Rong & Harper 1988; Owen & Robertson 1995). The latter would have caused further extinctions of the benthos but might have promoted the radiation of the graptoloids (Owen & Robertson 1995).

-WHAT ABOUT THE RECOVERY? Mass extinction is an extreme biodiversity crisis that had not only evolutionary consequences arising from the termination of clades, but potentially also had severe ecological effects. The mass extinctions terminated periods of ecological stability and preceded a period of recovery during which a new ecological structure was established (Sheehan 1996).
The Early Llandovery record of several groups is incomplete judging by the number of "Lazarus" taxa missing and there could therefore have been even further minor extinctions in the Early Silurian (Owen & Robertson 1995). Three global extinction events were documented for Llandovery graptolites by Melchin et al. (1998).
The Early Llandovery shows all the hallmarks of a post-mass extinction interval and many of the features are seen again following the end-Permian event. Thus, reefs are absent, level-bottom communities consist of depauperate but cosmopolitan faunas and many survivors temporarily disappear from the fossil record. Also remarkable is the slow pace of radiation in the initial 5 m.y.; it was not until the Wenlock, with the reappearance of Lazarus taxa and reefs and the re-establishment of faunal provinces, that the marine ecosystem could be said to have fully recovered. The ecological structures that had become established in the earlier part of the Ordovician became re-established during the Silurian recovery (Droser & Sheehan 1995). The surprisingly limited range of adaptive innovation may be partly because no major groups became extinct and released a large and distinctive volume of ecospace to be filled.

-WHAT ABOUT THE CAUSE? The Big Five mass extinctions may vary in the extent and nature of their causal mechanisms and in their effects on the course of evolution. Cause and effect are difficult to establish (Jablonski 1996), particularly when there have been several environmental changes nearly coincident with the extinctions. The close correlation between the Ordovician extinction and the glaciation suggests climatic change as the proximate cause. However, the extinction was probably a complex event and a sea-level fall and rise (Sheehan 1973), changes in oceanic structure (Wilde & Berry 1984; Wilde et al. 1990), nutrient fluxes (Brenchley et al. 1995) and development of anoxia (Briggs et al. 1988; Fortey 1989) were all ultimately related to climatic change and may have been contributing factors. <<<