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IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 39 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
IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 40 come somewhat more intense, Fe3+ is reduced to Fe2+, possibly as a result of resolution of oxyhydroxides (Libes, 1992, p. 196). Much of the reduced Fe will form iron sulfide. These developments in Mn and Fe, which follow from a change from oxic to mildly reducing conditions in depositional environments, suggest the existence of three basic environmental situations that reflect the change from oxic to reducing conditions: (1) relatively high concentrations of Fe and Mn in oxic environments; (2) relatively low concentration of Mn and high concentration of Fe in mildly reducing conditions; and (3) relatively low concentrations of both Mn and Fe in slightly more highly reducing conditions. Based on Fe and Mn concentrations reflective of the change from oxic to reducing environments, the low-calcium dark shale sample analyses were studied to ascertain if their Mn and Fe concentrations reflected depositional environment. The Mn and Fe concentrations in about 200 low-calcium dark shale samples revealed three clusters (see Quinby-Hunt et al., 1988, 1990). As indicated in Table 2.2, the three clusters appear to reflect the degree of oxidation in the above three environmental situations. Also indicated in Table 2.2, a fourth cluster is present in the samples analyzed. That cluster of samples is characterized by relatively low concentrations of manganese and iron but high concentrations of vanadium. Samples from modern oxic and anoxic depositional environments were analyzed by using neutron activation to ascertain if Mn and Fe concentrations in them were closely similar to those characteristic of any of the clusters recognized among the ancient shales. Sediment samples from the Santa Barbara and Santa Monica Basins, housed at the University of Southern California, were analyzed. Oxygen content of waters approximately a meter above these sediment had been measured (D. Gorsline, oral communication, 1988.). The Mn and Fe concentrations of samples of basin sediment beneath waters in which the oxygen content varied from 0.1 ml/l to undetectable were comparable to cluster 2. Sediments beneath waters that had an oxygen content of 0.5 ml/l or greater had relatively high concentrations of Mn and Fe (similar to cluster 1). TABLE 2.2 Chemical Clustering of ~200 Low-Calcium Dark Shales Cluster Mn (ppm) Fe (ppm) 1 1300 56,000 2 310 52,000 3 176 19,000 4 Similar to cluster 3 but V ranges from 350 to 1500 ppm SOURCE: Quinby-Hunt et al., 1988, 1990. Petroleum source-rock samples from the Miocene Monterey Formation recovered from a producing oil well in California also were examined with neutron activation. These source-rock samples had low Mn and Fe and high V concentrations, the unique geochemical signature of cluster 4. Kastner (1983) pointed out that the highly organic-rich, petroleum source-rock shales of the Monterey Formation formed under highly reducing, methanogenic conditions. The analyses of modern sediments from the Santa Barbara and Santa Monica Basins and those of the organic-rich shales from the Monterey Formation suggest certain geochemical aspects of the depositional environments in which dark shales in clusters 1, 2, and 4 accumulated. Berner's (1981) discussion of a geochemical classification of sediments indicates possible geochemical conditions in the depositional environment of cluster 3. Berner (1981) described four primary geochemical categories of environmental conditions under which sediment accumulates: (1) oxic; (2) post-oxic, nonsulfidic; (3) sulfidic or sulfate reducing; and (4) methanogenic. Categories 2, 3, and 4 are indicative of an increasingly greater degree of reducing conditions in the depositional environment (Berner, 1981). The geochemical data from analyses of ancient dark shales and modern sediment are consistent with Berner's (1981) geochemical categories for clusters 1 and 2. If cluster 4 is indicative of Berner's methanogenic zone, then cluster 3 is likely, at least in part, to be a product of sediment accumulation in Berner's sulfidic category. In view of the lack of direct comparison with samples from modern environments, some cluster 3 samples could have been derived from sediment that accumulated in Berner's post-oxic, nonsulfidic interval. Bacterial sulfate reduction generates hydrogen sulfide that may react with iron to form pyrite (Berner, 1981). Pyrite occurs in many dark, graptolite-bearing shales that have Mn and Fe concentrations characteristic of cluster 3 (Quinby-Hunt et al., 1989). Shale samples were taken from closely spaced stratigraphic intervals in the Late Ordovician-Early Silurian shales that comprise the Ordovician-Silurian boundary stratotype at Dob's Linn, southern Scotland. These samples were analyzed by neutron activation (Wilde et al., 1986). The Fe, Mn, and V concentrations (Wilde et al., 1986) in the Ordovician-Silurian boundary interval shales at Dob's Linn are consistent with the assignment of each sample to one of the four clusters reflective of oxic and the sequence of increasingly more reducing anoxic depositional environments (Figure 2.3). The Mn and Fe concentrations in gray shales that bear rare graptolites or are unfossiliferous are similar to the Mn and Fe concentrations in Santa Barbara and Santa Monica Basin sediments that accumulated
IMPACT OF LATE ORDOVICIAN GLACIATION-DEGLACIATION ON MARINE LIFE 41 under waters with some oxygen. Dob's Linn section shales bearing numerous graptolites representative of numbers of different taxa commonly have Mn, Fe, and V concentrations characteristic of clusters 3 or 4 (Quinby-Hunt et al., 1989). Shales having Mn and Fe concentrations characteristic of cluster 2 contain fewer graptolites than those with Mn and Fe concentrations typical of clusters 3 and 4. Shales with Mn and Fe concentrations indicative of oxic depositional environments (cluster 1) contain only specimens of climacograptids of C. miserabilis and C. normalis groups (normalograptids) in the Dob's Linn Ordovician- Silurian boundary interval. These graptolites were survivors of the Late Ordovician mass mortality among graptolites (Berry et al., 1990). The shales with Mn and Fe concentrations indicative of oxic depositional environments, containing only normalograptids, were deposited during the Late Ordovician glaciation in the Southern Hemisphere. That is, they were deposited during the Hirnantian Stage of the post-D. anceps into the G. persculptus zone interval. Deep ocean water circulation would have been at its maximum during glacial maximum, and the oxygen content of that water would have been as its greatest. Figure 2.3 Diagram illustrating the results of the neutron activation analyses of shales in the Ordovician-Silurian boundary section at Dob's Linn, southern Scotland. The Mn and Fe concentrations of cluster 1 indicate relatively oxic depositional environments. Concentrations of Mn and Fe suggest that cluster 2 is mildly anoxic. Mn and Fe concentrations of clusters 3 and 4 are indicative of relatively highly anoxic depositional environments. Shales with Mn and Fe concentrations suggestive of oxic conditions formed during glaciation, an interval during which deep ocean circulation was strong and ocean water ventilation great. The data indicate that samples bearing extraordinarius zone graptolites accumulated under anoxic conditions, perhaps during an interglacial. Anoxic environments developed in the area during persculptus zone time, probably reflecting the onset of deglaciation and global warming. Oxic conditions appeared in the depositional environment during sedgwicki zone time, possibly reflecting shallowing of the basin and/or influx of currents with oxic water. Orth (see Wang et al., 1990) analyzed closely spaced black shale samples from the Ordovician-Silurian boundary interval at two localities in south China using neutron activation. The Mn, Fe, and V concentrations from the south China samples reflect a pattern of change from anoxic to oxic and a return to anoxic conditions in the deposi