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IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 44 ventilation; advection upward of waters potentially toxic to many organisms into the lower part of the ocean mixed layer; the cooling and then warming of ocean surface water temperatures; and changes in nutrient availability in many nearshore marine environments. The most prominent organismal (Table 2.3) mass mortality in the Late Ordovician took place at or close to the Rawtheyan-Hirnantian Stage boundary. Many graptolites, brachiopods, trilobites, corals, and chitinozoans became extinct or were markedly reduced in numbers and taxonomic diversity at that time. Isotopic studies of brachiopod shells from Sweden (Middleton et al., 1988; Marshall and Middleton, 1990) using 12C and 13C indicate that a significant sequestering of 12C in sediment took place at about the Rawtheyan-Hirnantian Stage boundary. Hirnantian brachiopod shells are enriched in 13C. Wang Xiaofeng and Chai Zhifang (1989) described 13C enrichment in the Hirnantian in their study of dark shales in the Ordovician-Silurian boundary interval in south China. Faunal and geochemical studies appear to be consistent in indicating a marked biomass change near the Rawtheyan-Hirnantian boundary. Slowed rates of origination characterized most organismal stocks during the Hirnantian Stage and Glytograptus persculptus zone. Extinction rates slowed in post-Rawtheyan-Hirnantian Stage boundary interval time, but slow origination rates during the Hirnantian into earliest Silurian resulted in marked faunal changes between the Late Ordovician and Early Silurian. Recovery and reradiation were slow until sea-level rose significantly such that many shelf sea habitats not only reopened but became stable during the early part of the Silurian. Much of the Hirnantian and subsequent Early Silurian Rhuddanian was typified by environmental instabilities resulting from glaciation followed by relatively rapid global warming. Each major organismal stock responded to these environmental changes somewhat specifically, depending on its tolerances for the environmental changes. TABLE 2.3 Significant Physical Environmental Changes, Organismal Mass Extinctions, and Clade Turnovers in the Latest Ordovician Stage Zone Event Rhuddanian acuminatus â Hirnantian persculptus Conodont turnover Graptolite reradiation Sea-level rise Onset of glacial melting Brachiopod turnover extraordinarius Glacial maximum Rawtheyan pacificus Trilobite mass extinction Brachiopod major extinction complexus NOTE: Prominent depletions in 13C (Wang Xiaofeng and Chai Zhifang, 1989; Marshall and Middleton, 1990) have been noted in samples from near the Rawtheyan-Hirnantian boundary and close to the base of the persculptus zone. Significant numbers of brachiopod extinctions may have taken place throughout the late Rawtheyan to early Rhuddanian, although the majority seemingly occurred at the levels indicated. Prominent trilobite mass mortalities appear to have taken place near the end of the Hirnantian Stage in the tropics and at the Rawtheyan-Hirnantian boundary outside the tropics. Conodont mass mortality or faunal turnover occurred at about the same time the graptolites commenced reradiation during the time of the persculptus zone. The prominent graptolite mass mortality took place close to the end of the Rawtheyan, as did that of the chitinozoa. Mass mortality among corals occurred in the tropics near the Rawtheyan- Hirnantian boundary as sea-levels dropped significantly. The patterns in mass mortality and reradiation differ from organism to organism, depending on their mode of life and tolerance to change in the physical environment. 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