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During the Late Ordovician, greenhouse conditions prevailed but during the latest stage (Hirnantian), a short glaciation of approximately 0.5-Ma duration occurred (Brenchley 2001; Sheehan and Harris 2004). The glaciation probably started when Late Ordovician orogenic activities led to extensive exposure and weathering of silicate terrains and, therefore, CO2 consumption. Under falling CO2 levels, ice sheets started to grow and albedo feedback led to an extensive Gondwana glaciation. The ice cover in turn inhibited silicate weathering, and CO2 levels rose again. After a threshold was reached, greenhouse conditions returned and the ice caps melted quickly (Kump et al. 1999). It is now well documented that the mass extinction during the Late Ordovician was a two-phase event and the first extinction pulse at the base of the Hirnantian extraordinarius zone corresponded to the initiation of the Gondwana glaciation and the second pulse in the first third of the persculptus zone to the rapid decay of the polar ice cap.
During the first extinction event, various groups suffered heavy losses. Thereafter, an impoverished and remarkably cosmopolitan benthic fauna was present on most shelves from high latitudes to the tropics. This so-called Hirnantia fauna can be regarded as a cool-adapted, opportunistic community that spread after the extinction removed the hitherto dominant species (Brenchley 2001; Sheehan 2001b; Jia-Yu et al. 2002). Primary productivity was obviously much reduced during the Hirnantian.
The second extinction phase was again sharp and hit most of the groups that suffered already during the first extinction pulse. It eliminated much of the benthic Hirnantia fauna (Hallam and Wignall 1997), but the recovery interval did not last very long. It is most surprising that the faunal turnover during the Late Ordovician mass extinction was acompagnied by very little ecological changes, and the structure of Silurian communities is remarkably similar to those of the Late Ordovician (Droser et al. 2000; Bottjer et al. 2001; Sheehan 2001b).
The causes of the two extinction pulses were certainly linked to the rapid onset and the later abrupt termination of the Gondwana glaciation during an otherwise warm climatic mode (Fortey and Cocks 2005). Global cooling of the oceans of perhaps as much as 8°C together with a loss of benthic habitat due to regression might have, in part, been responsible for the first extinction (Berry and Boucot 1973; Brenchley et al. 1994; Armstrong 1996). Yet pelagic forms also suffered heavy losses, and changing circulation patterns in the oceans were probably crucial (Hallam and Wignall 1997). The widespread deep anoxic waters, the extensive dysoxic zone, and the nutrient-rich surface waters vanished during the onset of the glaciation when cold, deep water led to intensified ocean circulation. During the second pulse, the termination of cold, deep water production led again to widespread stratified oceans with anoxic deep and intermediate dysoxic waters. Black shales accumulated again, and the transgressing dysoxic waters eliminated most of the benthic Hirnantia fauna and other benthic organisms not resistant to oxygen-poor conditions (Hallam and Wignall 1997).
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