Silurian global events - At the tipping point of climate change

Abstract

Mass extinction events affect a wide breadth of ecosystems and are one of the major driving mechanisms behind evolution, origination, and diversification of taxa. Such dramatic turnovers have therefore played a significant role in the history of life. The 'major five' mass extinctions (Raup and Sepkoski 1982) have received intense attention over the last 25 years, starting with the K-Pg impact hypothesis launched by Luis Alvarez and others in Science in 1980. By comparison, few studies have concerned the about sixty smaller-scale bioevents that are scattered throughout the last ca 543 million years (Barnes et al. 1995), although their architecture often resembles that of the large scale mass-extinctions and they therefore may hold important information relevant to events in general. The Silurian Period is the shortest time period of the entire Phanerozoic (~443-416 Ma) but yields a significant record of ocean-atmospherebiosphere changes. The period has traditionally been described as an environmentally and faunally stable period in Earth history - a greenhouse period with a moderate latitudinal climate gradient and impoverished marine faunas slowly recovering from the end-Ordovician mass extinction. It has more recently, however, been shown that the Silurian was no quieter than any other time interval during the Phanerozoic Eon, rather the opposite. The results of an immense number of studies, primarily in the fields of taxonomy, biostratigraphy and stable isotope stratigraphy clash fundamentally with the old view. The new studies have unequivocally shown that repeated marine biodiversity crises took place, affecting e.g. graptolites, conodonts, chitinozoans, acritarchs, brachiopods and reefs, and that these turnovers were closely linked to abrupt and significant changes in oceanography and the global carbon cycle. Accordingly, today the Silurian can be regarded as one of the most volatile periods of the entire Phanerozoic when considering the ocean-atmosphere system (Cramer and Saltzman 2005). In order to understand these changes we need to see beyond the defined stratigraphic boundaries of the Silurian System and treat the time interval in a broader context. This is because the Silurian global events form part of a series of ocean-atmosphere-biosphere changes ranging in time from the Late Ordovician and through the Early Devonian (Fig. 1). In the authors opinion the Silurian global events require fundamental changes in Earth's climate, which are best explained by a series of glaciations. The lack of sedimentary evidence such as tillites from middle and younger Silurian formations has so far hindered the acceptance of this hypothesis. This is the first thematic publication in more than ten years in which the Silurian global events are treated in close context with the major five extinction events of the Phanerozoic. The previous publication was the result of the IGCP Project 216 'Global Biological Events in Earth History' in which the Silurian bioevents were treated by Kaljo et al. (1995). This chapter calls attention to the tremendous development of Silurian stratigraphy achieved in this last decade and how it generates a fundamentally new view of the Silurian Period as a time interval of repeated global change. The three main events - the Early Silurian Ireviken Event, the Middle Silurian Mulde Event, and the Late Silurian Lau Event - are reviewed with regard to biodiversity changes and how these relate to carbon and oxygen stable isotope evolution and sea-level change. Much of these data presented herein come from the island of Gotland (Sweden; Fig. 2) were a series of stacked carbonate platform generations, ranging in age from the latest Llandovery through the Ludlow, are preserved. Accordingly, this contribution should be seen as 'a view from a carbonate platform' situated on the paleocontinent Baltica in the Silurian tropics, although other areas are referred to as far as possible. In order to stimulate research, facilitate the identification of the Silurian global events elsewhere, and establish a coherent working frame, this paper also serves to present and define the stratigraphical extent of these three events, in terms of first and last appearance datums (FAD's and LAD's), and their expression in shallow-water environments. As the reader will learn, the comparably small-scale Silurian global events share unexpectedly many similarities with the great mass extinctions (Munnecke et al. 2003; Calner 2005a). © 2008 Springer-Verlag Berlin Heidelberg.