Effects of Past Global Change on Life

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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

TABLE 2.3 Significant Physical Environmental Changes, Organismal Mass Extinctions, and Clade Turnovers in the Latest Ordovician

Stage	Zone	Event
Rhuddanian	acuminatus	—
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. As Wilde and Berry (1984) proposed, significant faunal changes took place near both the beginning and the end of glaciation. Organisms responded to major changes in ocean circulation and thermohaline density stratification at those times.

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.

REFERENCES

Barnes, C. R., and S. M. Bergstrom (1988). Conodont biostratigraphy of the uppermost Ordovician and lowermost Silurian, in A Global Analysis of the Ordovician-Silurian Boundary, L. R. M. Cocks and R. B. Rickards, eds., British Museum (Natural History) Bulletin 43 (Geology Series), pp. 325-343.

Barnes, C. R., and S. H. Williams, eds. (1991). Advances in Ordovician Geology, Geological Survey of Canada Paper 909, 336 pp.

Berner, R. A. (1981). A new geochemical classification of sedimentary environments, Journal of Sedimentary Petrology 51, 359-365.

Berry, W. B. N., and A. J. Boucot (1973). Glacio-eustatic control of Late Ordovician-Early Silurian platform sedimentation and faunal change , Geological Society of America Bulletin 84, 275-284.

Berry, W. B. N., P. Wilde, and M. S. Quinby-Hunt (1987). The oceanic non-sulfidic oxygen minimum zone: A habitat for graptolites? Geological Society of Denmark Bulletin 35, 103-114.

Berry, W. B. N., P. Wilde, and M. S. Quinby-Hunt (1990). Late Ordovician mass mortality and subsequent Early Silurian reradiation, in Extinction Events in Earth History, E. G. Kauffman and O.H. Walliser, eds., Springer-Verlag, Berlin, pp. 115-123.

Beuf, S., B. Biju-Duval, O. de Chapperal, R. Rognon, O. Gariel, and A. Bennacef (1971). Les Gres du Paleozoique inferieur au Sahara—Sedimentation et discontinuities, evolution structurale d'un Craton, Institute Francais Petrole—Science et Technique du Petrol 18, 464 pp.

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