Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis

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Abstract

Following the Cambrian Explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans, the Ordovician and Silurian periods witnessed three subsequent major biotic events: the Great Ordovician Biodiversification Event, the end-Ordovician extinction (the first animal extinction and second largest of the five mass extinctions of the Phanerozoic), and the Early Silurian post-extinction recovery. There are currently no simple explanations for these three major events. Combined extrinsic (geological) and intrinsic (biological) factors probably drove the biodiversifications and radiations, and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography, palaeoclimate and sea-water chemistry. The present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the Ordovician and Silurian, including sea-level changes, geochemical proxies (δ13C, δ18O, 87Sr/86Sr) of the ocean waters, and the evolution of the atmosphere (oxygen and carbon dioxide content).

Introduction

During the Ordovician and Silurian, profound changes occurred in the planet's ecosystems. Marine life was characterised by a major diversification, the Great Ordovician Biodiversification Event (GOBE), a major extinction, the end-Ordovician event and a subsequent recovery during the Early Silurian. These events are part of a continuum from the evolution of the first metazoans at least by the Ediacaran, the skeletalization of animals during the late Neoproterozoic, the explosion of body plans during the Early to Mid Cambrian, and the massive diversification of benthic marine life during the Ordovician, consequently with demersal and nektonic organisms radiating during the Devonian (Klug et al., in press). Pivotal to this process was the GOBE, but there is currently no single explanation (Servais et al., 2009, Servais et al., 2010, Zhang et al., 2010b). Perhaps a coincidence of biological and geological factors combined to help drive and encourage the biodiversification. Irrespective of its causes, the diversification changed the oceans forever and set a new agenda for marine life (Harper, 2006). The cascading increase in biodiversity at species, genus and family hierarchies was apparent at global levels with the high provincialism of Early to Mid Ordovician faunas, at regional levels with the development of new community types, particularly in deeper water and in and around reefs, and thirdly at local levels where more animals were squeezed into existing communities. The oceans were no longer sterile expanses of water, being filled now by phyto- and zooplankton, punctuated by blooms, and including larvae and animals such as the graptolites. Community structures were better organised and more densely packed with the expansion of the number of so called ecological guilds, signalling a range of new feeding strategies and life modes. Tiering structures developed both above and within the substrates while the bioerosion and encrustation of hard surfaces offered a new range of ecological opportunities. The Palaeozoic evolutionary fauna was relatively stable, surviving the end-Ordovician and late Devonian extinctions for some 200 million years. The end-Permian extinction event virtually destroyed its suspension feeding networks and a new ecosystem, based on the detritus feeding ecosystem of the Modern evolutionary fauna and a more explicit arms race, the escalated interactions between predators and prey, diversified and intensified during the Triassic. Additionally during the Ordovician and Silurian, widespread biogenic carbonate factories were established through the generation of heavily skeletalised organisms together with metazoan reefs with consequences for the longer term function of the carbon cycle and the planet's climate.

The biological signals for these events have become well established during the last two decades, however, their relationships to chemical and physical processes occurring in the world's ocean and atmosphere are far from clear. Nevertheless through biological, physical and geochemical proxies, the roles of sea–water chemistry and sea level on the planet's climate and evolution are now being more accurately investigated, not least through a range of new techniques and carefully collected field data. In particular ocean geochemistry and sea-level change is making a huge impact on our understanding of the palaeoclimate and palaeogeography of the Ordovician and Silurian together with the evolution of their biotas. These advances are in part a tribute to the networks and results of two major international projects, the most recent, IGCP 503 with a specific focus on the climate and geography of the two periods.

The International Geoscience Programme (IGCP) 503 ‘Ordovician Palaeogeography and Palaeoclimate’ commenced in 2004 and was completed in 2009. It was based on the previous, highly successful IGCP project focused on the Ordovician, IGCP 410 ‘The Great Ordovician Biodiversification Event’ that extended from 1997 to 2002 (see Webby et al., 2004). The main objectives of IGCP 410 were to construct diversity curves for all marine invertebrates during the Ordovician biodiversification, but also to establish a new stratigraphical standard that would permit intercontinental correlation, not only of strata, but also of palaeodiversity patterns and trends of all fossil groups during the radiation.

Following project 410, the new programme IGCP 503 focused specifically on the search for the biological and geological triggers of the Ordovician biodiversification. The main goals were, as indicated in the title of the project, to understand the influence of changing geography and climate on the Ordovician radiation. However, because such changes in the Ordovician could only be understood within a broader frame, many Cambrian and Silurian workers also participated in the programme. Project 503 was therefore not limited to the Ordovician Period, and most meetings and field trips covered the entire Lower Palaeozoic. The scientific output of the project, comprising several hundred published papers is, of course, difficult to summarise. Servais et al., 2009, Servais et al., 2010 have reviewed many results of the project, indicating that a continuous sea-level rise between the Early Cambrian and the early Late Ordovician broadly matches the diversification of the marine invertebrates during these periods. Palaeogeography can also be linked to the Early Palaeozoic radiation, as it coincides with the breakup of the supercontinent Rodinia in the late Precambrian. The formation of numerous smaller continents triggered the biodiversification, with seafloor spreading and continental dispersal at their maxima during the Ordovician, together with the greatest extension of tropical shelves of the entire Phanerozoic. Several special issues have published results of project 503, with many of them including some of the major advances presented at the main annual meetings (Munnecke and Servais, 2007, Owen, 2008, Servais and Owen, in press). The present special issue highlights climate and sea-level changes during the Early Palaeozoic and the range of investigative techniques currently available for their study. The present paper reviews our knowledge of these parameters and their relationships to the Great Ordovician Biodiversification Event, the end-Ordovician extinction, and the subsequent Silurian radiation.

The Ordovician and Silurian world witnessed three major biotic events, the Great Ordovician Biodiversification Event (Webby et al., 2004, Harper, 2006), the end-Ordovician extinction (Barnes, 1986, Rong and Chen, 1986, Rong and Harper, 1988, Barnes et al., 1995, Sheehan, 2001a) and the Early Silurian recovery (Rong and Harper, 1999). These three events helped develop the complexity of the Palaeozoic evolutionary fauna and established the pattern of marine life in the Ordovician and Silurian world (Sheehan, 2001b). In general terms Palaeozoic oceans were characterised by short trophic chains dominated by suspension-feeding organisms, evolved during persistent intervals of greenhouse climate. This ecosystem contrasted with that of the subsequent Mesozoic and Cenozoic eras, dominated by deposit-feeding communities linked to more bioturbated substrates, complex community structures driven by a more pervasive arms race. Early Palaeozoic biodiversifications in most marine groups were spectacular and sustained, setting the agenda for subsequent marine life on the planet. The majority of metazoan groups appeared first at the base of the Palaeozoic (Budd, 2008), increasing in diversity during the Cambrian Explosion and Ordovician Radiation to establish an ecosystem that survived some 250 million years of Earth history. Nevertheless Cambrian ecosystems were probably quite different from those to follow during the Ordovician and Silurian periods, characterised by relatively few megaguilds, poorly-structured communities and a relatively sterile water column.

The transition from the Cambrian to Ordovician worlds was a major turning point in Earth history. Much evidence now suggests that the late Cambrian was characterised by warm oceans with widespread anoxia and dysoxia and probably low saturation states for calcite and aragonite (Pruss et al., 2010). Despite the appearance of calcified skeletons in both solitary and colonial organisms in the late Neoproterozoic (Wood et al., 2002), the Cambrian carbonate factory was dominated by physical and microbial processes rather than by biogenic material. Carbonate build-ups and reefs were rare following the virtual extinction of the archaeocyathans in the mid Cambrian.

The Ordovician Period experienced a truly massive rise in marine biodiversity (Sepkoski, 1981) accompanied by an increase in the biocomplexity of marine life (Droser and Sheehan, 1997) marking ‘The Great Ordovician Biodiversification’ as one of the two most significant radiation events in the history of marine life. The unique environmental conditions through the Ordovician Period have been emphasised in a number of publications (e.g., Jaanusson, 1984). Extensive, epicontinental seas developed during sea-level high stands (Algeo and Seslavinski, 1995, Pratt and Holmden, 2008), driven by an extended greenhouse climate, were associated with virtually flat seafloors and restricted land areas, many probably represented only by occasional, emergent archipelagos. Sea levels were most probably the highest of the Palaeozoic and possibly the highest of the entire Phanerozoic (Hallam, 1992, Miller et al., 2005, Haq and Schutter, 2008), and there are no modern analogues to the epicontinental seas of the Ordovician Period. Magmatic and tectonic activity was intense and persistent with rapid plate movements and widespread volcanic activity. Possibly even mantle plumes were associated with climatic and faunal changes (Barnes, 2004, Lefebvre et al., 2010). Island arcs and mountain belts provided sources for clastic sediment in competition with the carbonate belts associated with the platforms on most of the continents. The continents were widely dispersed (Cocks, 2001) driving provincialism. Such biogeographical differentiation was extreme, affecting plankton, nekton and benthos, and climatic zonation, particularly in the southern hemisphere, was pronounced. Provincial differentiation amongst the benthos was also marked with biogeographic differences persisting until near the end of the period (Williams, 1973), when these were disrupted by the end-Ordovician glaciation (Rong and Harper, 1988, Owen et al., 1991). Together these conditions were, nevertheless, clearly ideal for allopatric (geographic) speciation processes together with opportunities for canalization of ecological niches (Harper, 2006).

Climate and environmental proxies for this interval, that are advancing our understanding of the background to the diversification, extinction and recovery of biotas, are in a rapid stage of development. Isotope shifts in carbon, oxygen and strontium are providing vital clues to the cycling of carbon, temperature fluctuations and the input of terrigenous material associated with orogenic activity. Moreover, sea-water chemistry is proving essential to our understanding of skeletal secretion. The link between sea-level change (e.g., McKerrow, 1979) and climate change together with tectonic activity has been known for some time but refined regional sea-level curves (see Section 2; Fig. 1, Fig. 2) are providing a more accurate and precise assessment of these global processes and their influence on biotic evolution at taxonomic, community and ecosystem levels.

Today, it is more and more clear that the Great Ordovician Biodiversification Event was an accumulation of biodiversification events (taking place at different times on different continents within different phyla) that covered the entire Ordovician and were part of a wider Cambrian–Ordovician radiation (Servais et al., 2010). This longterm radiation was interrupted by the first of the ‘Big Five’ mass extinctions: the end-Ordovician extinction, considered as being the second most important extinction of Phanerozoic marine life (the Permian–Triassic extinction being the most severe, e.g., Sepkoski, 1981). This end-Ordovician extinction was considered for many years to be abrupt and directly correlated with the Hirnantian glaciation, with the disappearance of about 85 % of marine species (e.g., Sheehan, 2001a). Two pulses of extinction have been recorded, and discussed in the frame of the glacial intervals in the Late Ordovician. The first pulse was related to the beginning of the glaciation with an important sea-level fall. The second pulse of the extinction was related to the end of the glaciation when sea level started to rise again and oceanic circulation stagnated, marking the end of a long interval of ecologic stasis (Ecologic-Evolutionary Unit) (Sheehan, 2001b, Brenchley et al., 2003). However, in the last few years, several authors noted that the global cooling at the end of the Ordovician was not as abrupt as previously thought (e.g., Saltzman and Young, 2005). Temperatures (and sea level) were decreasing since the middle part of the Late Ordovician, accompanied by a decrease of biodiversity in many fossil groups that apparently began much earlier than the Hirnantian glaciation (e.g., Servais et al., 2008). The Hirnantian glaciation probably only marked the final Ordovician phase of a long interval of overall global cooling.

By contrast the Silurian was considered a relatively short but calm period most noted for the beginning of the greening of the land and the radiation of the gnathostome and theledont fishes (see numerous articles in Holland and Bassett, 1989, Bassett et al., 1991, Landing and Johnson, 1998, Landing and Johnson, 2003). On a broader scale, the Silurian is sandwiched in between the Late Ordovician ice-house climate and Devonian extreme greenhouse conditions. Similar to the Ordovician, it is characterised by an archipelagic distribution of several continents in low latitudes (Laurentia, Baltica, Siberia, Kazakhstania), a vast north polar ocean, and the supercontinent Gondwana extending from equatorial latitudes to the South Pole. The sea level was high, large shallow epicontinental seas were widely distributed, and the continents had a low relief. Terrestrial plants were quantitatively insignificant and thus had little influence on the global carbon cycle. During the Silurian, the Iapetus Ocean, that separated Laurentia and Baltica, was closed leading to the Caledonian orogeny. Major extinction events comparable to those of the Ordovician or Devonian periods were unknown (Kaljo et al., 1995), and, except for the Malvinokaffric realm (the southern temperate zone typically represented by the low-diversity Clarkeia (brachiopod) fauna from Gondwanan Africa and South America), reefs were widely distributed. Reefs are reported more or less throughout the entire Silurian, but their distribution through time seems to be clustered. The earliest Llandovery is characterised by the near absence of reefs. The first Silurian reefs appeared in the mid-Aeronian (Li and Kershaw, 2003). Intervals with higher abundances of reefs are the mid to late Aeronian, latest Telychian to early Sheinwoodian, late Homerian, late Gorstian to early Ludfordian, and mid-late Ludfordian (Brunton et al., 1998, Copper, 2002). However, since Silurian biostratigraphy is mainly based on graptolites, which normally are very rare in reefal limestones, the precise stratigraphic correlation of many reef deposits with respect to the graptolite biostratigraphy is still somewhat uncertain.

In the past two decades our picture of the apparently ‘calm’ Silurian has changed dramatically (see review in Calner, 2008). Investigations of stable carbon and oxygen isotopes suggest a highly volatile ocean–atmosphere system (e.g., Samtleben et al., 1996, Saltzman, 2001, Kaljo et al., 2003). The presence of four major positive stable carbon isotope excursions in the Silurian (early Wenlock, late Wenlock, late Ludlow, Silurian–Devonian boundary) suggest that fundamental changes in the global carbon cycle were much more frequent in the comparatively short Silurian Period than in any other system of the Phanerozoic (see Section 3). The amplitudes of the Silurian stable isotope excursions are extremely high compared to Mesozoic and Cenozoic excursions, and there is no general agreement on the palaeoenvironmental changes responsible for these excursions (see Section 3). The carbon isotope excursions are also characterised by elevated oxygen isotope values, and are closely correlated with extinction events and with lithological changes (summarised in Munnecke et al., 2003). At the very beginning or even prior to the increase of C- and O-isotope values, many groups of organisms were affected. Especially conodonts, graptolites and trilobites, but also acritarchs, chitinozoans, ostracods, brachiopods, and corals show extinctions, sometimes of a step-wise nature; organisms living in hemipelagic environments were more strongly affected than organisms occupying shallow-water settings. Munnecke et al. (2003) postulated similar but unknown controlling mechanisms for the major Silurian isotope excursions based on their lithological, palaeontological, and geochemical similarities.

Section snippets

Evolution of the atmosphere

In this section, a brief summary of our current knowledge of the oxygen and carbon dioxide composition of the atmosphere is presented. In contrast to the previous section, which focused on the presentation of measured data, there is as yet no geochemical proxy, which can be directly and unequivocally related to atmospheric gas composition; and thus the concentrations reported must be critically assessed. We are not aware of a single paper published dealing exclusively with either the Ordovician

Ordovician climate

Critical to our understanding of Ordovician and Silurian ecosystems and environments is the development of an accurate model for climate change and climatic conditions during the two periods. Some authors have linked climate change directly to evolution (see Harper, 2009). There are a number of challenges not least that surface water temperatures across the globe have a range of about 30 °C and much of the planet is subjected to significant seasonal variations in temperature. Traditionally a

Conclusion

Studies on the Ordovician and Silurian systems have, during the last two decades, advanced dramatically through the acquisition of new data from many new and previously poorly-studied sections throughout the world. More importantly, however, a range of new geochemical techniques and sea-level proxies applied to existing and new sections are rapidly developing our knowledge of ocean chemistry and sea level change. The results of IGCP 410 and 503 have sharpened our focus on the significance of

Acknowledgments

This paper summarises many of the activities of IGCP 503 (“Ordovician Palaeogeography and Palaeoclimate”) that extended from 2004 to 2009. We acknowledge all the participants of the project who discussed many aspects of the present manuscript with us, and in particular our three co-leaders Alan Owen (Glasgow, Scotland), Li Jun (Nanjing, China) and Peter Sheehan (Milwaukee, Wisconsin, USA). AM acknowledges funding from the German Research Foundation (DFG Mu 2352/1), and DATH financial support

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