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Effects of Past Global Change on Life
More zoogeographic provinces among benthic faunas would have resulted as a consequence of slowed surface circulation. Sheehan and Coorough (1990) noted that the Early Silurian brachiopod faunas were characterized by more provinces than the Hirnantian Stage brachiopod faunas.
Deep ocean circulation probably increased significantly during glacial maximum. Deep ocean circulation would have been driven by both evaporation and cooling of large volumes of ocean water situated around the margins of a glaciated Gondwanaland. Cold, dense water formed in this manner sinks and flows along the ocean floor. Such water bears more oxygen than warmer surface water. Thus, during glacial maximum, the deep oceans would have received a significant and prolonged increase in oxygen. As deep water flowed north, the positions of the plates (aligned essentially east-west in the tropics and south of the tropics) would have created baffles against which deep ocean water would have advected upward. A consequence of that advection would have been a general toward-the-surface movement of the thermocline and of the zone of oxygen-depleted waters or oxygen minimum zone waters. Upward motion of oxygen-depleted water would have forced waters that contained metal ions and other substances toxic to many organisms into the mixed layer. Breaking of internal waves, as well as upwelling of waters bearing unconditioned nutrients (nutrients in proportions or in oxidation states such that organisms cannot take them in) or toxic trace metals, would have resulted in episodic incursion of water toxic to nearly all phytoplankton and zooplankton into those waters inhabited by most plankton and nekton, the mixed layer. If glacial development waxed and waned, as suggested by Vaslet (1990) and Brenchley et al. (1991), then these incursions may have had a greater or lesser effect on organisms, depending on the influence of bottom-water generation, which was related to glacial developments. Such episodic stronger and weaker incursions of toxic waters into plankton and nekton habitats could have had a greater impact on the long-term survival or extinction of many organisms than a single incursion. Wilde and Berry (1984) and Wilde et al. (1990) described how regional- to global-scale vertical advection of deep ocean waters into near-surface mixed layer water can create an environmental change crisis for many marine organisms. Such vertical advection into the mixed layer can result in the following (Wilde et al., 1990): (1) direct toxicity of mixed layer water; (2) modification or reduction of nutrients and food resources through inhibition of photosynthesis; (3) chronic debilitation through continued contacts with toxic waters; and (4) increased predation by more adapted organisms. Such environmental crises for most organisms could also result in new ecologic opportunities for organisms that had been ecologically suppressed under prior environmental conditions.
Whereas deep circulation was vigorous during glaciation, it slowed markedly with the onset of deglaciation. A characteristic of Pleistocene glacial to interglacial change is rapid development of deglaciation (W. Broecker, Lamont-Doherty Earth Observatory, oral communication, 1990). Relatively rapid deglaciation results in rapid change in deep ocean circulation. Marked vertical advection of the glacial maximum was followed by ocean conditions in which the zone of oxygen-depleted water expanded and descended somewhat in the ocean. Upward vertical advection from that zone diminished as a result. Mixed zone water expanded downward and rapidly became more hospitable to life. Sea-level rose as a consequence of deglaciation. As sea-level rose and oxygen-depleted waters expanded, these waters spread anoxia across the outer parts of shelves and platforms.
Vertical advection of toxic waters during glaciation would have created inhospitable environments not only for nektic and planktic organisms, but also for many benthic organisms. Reduction or absence of vertical advection of toxic waters during deglaciation would have reopened many environments in the mixed layers to resettlement by organisms.
GEOCHEMICAL EVIDENCE OF DEEP OCEAN VENTILATION
Although the ocean surface and deep circulation suggested for the Late Ordovician glacial and subsequent nonglacial interval is essentially speculative and derived from proposed paleogeographic reconstructions, some direct geochemical evidence has been developed to support the proposed model. Quinby-Hunt et al. (1989) summarized the results of approximately 300 neutron activation analyses of dark shales. Many of the samples came from the Late Ordovician-Early Silurian succession at Dob's Linn (Wilde et al., 1986; Quinby-Hunt et al., 1989). Dark shales in which the calcium concentration is less than 0.4% were selected for close scrutiny. The low calcium concentration in such samples allows the assumption that the Fe and Mn contained in them are in oxides and sulfides that reflect reducing conditions. The Fe and Mn in such low-calcium rocks are not bound in carbonates. Accordingly, Fe and Mn concentrations may be used as indicators of the intensity of reducing conditions.
Under oxic conditions (environments in which oxygen is relatively plentiful), Fe and Mn concentrations are relatively high. As oxygen availability diminishes to a condition in which it is no longer present, Mn is reduced before Fe and becomes more soluble. As a consequence, Mn concentration diminishes because it may form oxides and sulfides. Manganese diminishes in the early stages of onset of reducing conditions. As reducing conditions be