ABSTRACT:
The vast majority of oceanic biomass lives in the surface wind-mixed layer (0-100 m) of the ocean, trophically dependent on light or primary production based on photosynthesis. Waters from the main pycnocline (100-1000 m) or deeper naturally contain decay products from sinking organic matter as a function of the oxidation state of the waters. Such products, in proper concentrations, can inhibit photosynthetic growth or are toxic or debilitating to respirors. Usually, physical oceanographic processes of vertical circulation are slow or volumetrically small enough to permit "conditioning" or mixing of toxicants of the deep waters with surface waters so that the deleterious effects of deep water are neutralized or localized. However, rapid global to regional scale vertical advection of deep waters into the surface mixed layer could create an ecologic crisis for various marine groups through a combination oh (1) direct toxicity; (2) reduction or modification of nutrient and food supplies through inhibition of photosynthesis; 3) chronic debilitation caused by contact with such toxic waters; or (4) increased predation by more adaptive or less effected taxa. Such events are not necessarily universally deleterious as they could offer new opportunities for taxa ecologically restricted under prior conditions. During cool climates with oxic deep waters, a crisis may be caused by upwelling of metals concentrated with depth and resulting in reduced primary productivity, as well as metal toxic and/or chronic reactions in higher groups. During warm climates with anoxic to dysaerobic waters in the pycuocline, a crisis may result from contact with anoxic waters with a maximum effect on respirors and a minimal to enhanced effect on phytoplankton. Upwelling may come from three redox zones: I- oxic; II- nitric; and III- sulfatic. Each zone would be the source of waters of differing chemistries that could be advected into the photic zone. The effect on specific taxa will be selective as a function of the depth and volume of source water, as different organisms have different tolerance limits or preadaptive capabilities. In the geologic record, significant upwelling events would be recorded initially as a general reduction in diversity, followed by mass extinctions in some groups and the possibility of rapid radiation in opportunistic groups. The ecologic requirements of both the extinct taxa and the newly enhanced taxa might be used to identify the type of any given major upwelling event.
INTRODUCTION
Wilde and Berry (1982,1986) discussed the oceanographic conditions which could contribute to rapid changes in marine communities and to apparent mass mortalities. They proposed physical oceanographic processes which bring chemically different waters from depth into the life range of fossil-producing organisms. Wilde and Berry (1982) concentrated on the physical mechanisms required to produce rapid overturn of deep water. Expanding on this theme, Wilde and Berry (1986) showed that chemical variability in the water column with depth, produced by both oxic and anoxic decomposition of surface-produced organic matter, when introduced into surface waters, could trigger rapid changes in the marine biota. This paper discusses how the scale of vertical advection, both in depth and areal extent, influences the living conditions of marine biotas ranging from essentially marked enhancement to rapid extinction.
GENERAL CHEMICAL CONSIDERATIONS
The ocean is generally stratified with respect to density (temperature and salinity) and light. Most organisms live in the photic zone. Organisms are exposed to and adapt to different chemical environments, depending on time, geographical location and where they live in the water column. In the modern ocean, phytoplankton and other organisms in the surface layers consume and sequester various metals and compounds keeping the surface waters low in nutrients (Figs 1 a-d) and dissolved metal ions (Vinogradov, 1953; Goldberg, 1957; Knauer and Martin, 1983). At depth, as organisms decay, nutrient species are released causing their concentrations to increase.The oygen concentration in the oceanic water column varies significantly with depth (Fig. 1 d). At the surface the oygen concentration is a function of the atmospheric concentration, temperature and salinity. Through the photic zone, oygen is consumed or released by different organisms resulting in a subsurface maximum. Below the photic zone, as a result of the consumption of oxygen by oxidation of decaying organic material, the concentration declines dramatically. Below the pycnocline, the oxygen concentration in the modern ocean increases due to an injection of high latitude deep and bottom waters. Oxic waters are defined as having chemically active dissolved oxygen as the primary oxidant. If sufficient organic matter is present, all dissolved oxygen will be consumed and other agents win be used to oxidize organic matter. In the oceans, the thermodynamic sequence of available oxidants after free oxygen are the oxygenated nitrogen species followed by oxidized sulfur species, principally sulfate. Fig. 2 shows the areas of the modern ocean with dissolved oxygen below 0.4 mL/L. In the modern ocean the result of oxidation by oxygenated nitrogen species may be seen in the eastern tropical Pacific (Fig 3).
In the nitric zone, nitrate and nitrite act as oxidizing agents producing nitrite, nitrogen and ammonia. The concentration of nitrite in the nitric zone initially increases with depth, then decreases as nitrate is consumed, and subsequently, nitrite assumes the role as oxidant (Fig 3). In the sulfatic zone, sulfate is the oxidizing agent resulting in production of sulfides and ammonia. The sulfatic zone exists today in the Cariaco Trench, the Black Sea and in some fjords, such as Lake Nitinat. In the ancient oceans, particularly in warm, calm conditions, the extent of the nitric and sulfatic zones would have been greatly expanded.
In the photic One, various metals and compounds are taken out of sea water during primary production and released back into sea water below the photic zone by oxidation of sinking organic matter (Redfield, Ketchum and Richards, 1963). The distribution of such metals in the water column generally follow the concentrations of dissolved nutrients with depth, but as a function of their solubility (Fig 3).
For oxic conditions seen in the modern ocean, Quinby-Hunt and Wilde (1987) reported that the concentrations of Cr, Ni, Se, Sr, Cd, I, and Ba correlate quantitatively with the concentrations of the nutrients N. P. or Si. In addition, Be, Mn, Fe, Co, Cu. Ga, As, Pd. rare earth elements, Hg, Rn, and Ra also are partially nutrient-related, but no relatively simple expression has been developed relating these elements with nutrients. As the concentration of the nutrient-related elements generally increases with depth in oxic waters, their effect on organisms living in the photic zone as a function of concentration can be related to the depth of vertical advection or upwelling. The concentrations of some of these elements, notably Fe and Mn, also controlled by scavenging and redox considerations, tend to decrease below the mixed layer, although the concentration may increase to some degree in the oxygen-minimum zone.
Under anoxic conditions, the distribution of nutrient-related elements may differ from that under oxic conditions. If the waters are from the nitric zone (Wilde, 1987), such as the denitrification zone of the Eastern Tropical Pacific and the Northern Arabian Sea of the Indian Ocean, then the concentrations of many elements may be expected to increase. Notably, the concentrations of Fe and Mn might be expected to increase because neither the 0=, or S=, which tend to precipitate these elements are present. Other elements whose concentration in the oxic and suffatic zones is solubility controlled win also increase.
If anoxia has reached the stage of sulfate reduction, those elements in sea water which form sulfides win form mineral complexes and be removed from the water column. Thus, Cu. Cd, and Zn are depleted below the redoxocline in the Black Sea (Brewer & Spencer, 1974) and the Cariaco Trench (Jacobs et al., 1987) (Figs 4b, c). In anoxic waters, metals that form sulfides are depleted. In the sulfatic zone, Fe is more depleted than in the nitric zone, but appears in significantly higher concentrations than in the oxic zone (Fig 4a) due to greater solubility of the iron sulfide vs ferric hydroxide. Manganese occurs in greater con- centrations in the nitric and sulfide zones than in the oxic zone (Fig 4d).
PHYSICAL PROCESSES OF VERTICAL ADVECTION
Wilde and Berry (1986, p. 81) discussed three major geographical scales of upwelling produced by several physical processes: I. Planetary (Oceanic); II. Regional; and III. Local. Upwelling on a oceanic scale may be the result of divergence of water by Ekman transport produced by change in direction of zonal surface winds at boundaries of major climatic belts. The effective maximum depth for upwelling in this case is that of mixed layer (ca. 100 meters); the vertical rise would be 10 to 80 meters/month. (Apel 1987, p. 270, 10-6 m/sec = 30 m/yr, equatorial). Divergence due to Ekman transport occurs at the equator and the temperate/polar boundary (60 N and 60 S).Vertical advection on an oceanic scale also may occur as a result of displacement by continual renewal of water masses at source. In this case, the vertical rise is of the order of millimeters/day. Thus, one mixing cycle occurs about every 1000 years (3 m/yr: Apel 1987). Overturn of deep water to the surface could also occur on an oceanic scale. The vertical rise probably would be rapid greater even than for Ekman transport. Horizontal movement is from high to lower latitudes. Regional (100 to 1000 km) upwelling may be due to seasonal Ekman transport: for example, due to atmospheric high pressure off western coasts. Regional upwelling also may be due to off-shore advection, that is, entrainment by major oceanic currents moving off-shore or by vertical Coriolis deflection and surfacing of Equatorial undercurrents as eastward flowing undercurrents rise.
Wilde and Berry (1986, p. 81-82) listed some of the physical processes that can produce local (< 100 km) upwelling. Obstruction of horizontal current flow by banks or seamounts produce vertical advection in Taylor columns along the leading edge of the obstruction. Closed cyclonic eddy circulation, that is, oceanic Weathers spin-off, and migration of cold-core eddies from Rossby waves in major currents may cause vertical advection of deep waters causing local upwelling. Bernoulli uplift, by flow through constricted straits; breaking internal waves or internal surf also cause local upwelling.
BIOLOGICAL IMPLICATIONS OF THE VARIATION OF CONCENTRATION WITH DEPTH AND REOX CONDITIONS
The effects of rapid global to regional vertical advection of deep waters into the mixed layer could cause ecologic crisis due to: (1) direct toxicity or enrichment; (2) reduction/modification of nutrient or food supply, (3) chronic debilitation due to contact with deep waters; and (4) increased predation by taxa adapted to the new water mass chemistry.The effect of a dissolved chemical on a biotic community is a function of the concentration of that chemical the temperature and pH of the water, and other constituents present in the water column. A particular compound may be inert (no effect), limiting (required at a certain minimum concentration), inhibiting (also referred to as sublethal or chronic, that is debilitating for short exposures or with life- shortening consequences), or toxic (lethal at certain concentrations). Elemental concentrations do not always indicate whether the above effects win occur. Chelating agents, antagonistic or synergistic elements (antagonistic elements reduce the effect of an element; synergistic elements enhance the effect), redox state, pH or temperature modify the actual chemical configuration or activity experienced by the organism (Bowers & Yeats, 1977; Anderson & Morel 1978, 1982; Thomas et al., 1980). Thus, the activity of the chemical species rather than the absolute concentration is the important consideration (Jackson & Morgan, 1978).
Fe, Mn, and Zn are limiting for a number of organisms. Diatoms, coccoliths, dinoflagellates, and cyanobacteria are Fe-limited in neritic waters for activities less than 10-7 M and in pelagic ocean waters at activities less than 10-9 M (Brand et al., 1983). The concentration of dissolved Fe in the open North Pacific Ocean is much less than 10-9 (Gordon et al., 1982). The same groups of organisms are Mn-limited at activities less than 10-1 M in neritic waters. In the oceanic pelagic realm, ad but coccoliths are limited at less than 10-1 M. In the open ocean, dissolved Mn is about 0.24 x 10-9 in the mixed layer (Landing & Bruland, 1980; Martin & Knauer, 1984). Concentrations of both Mn and Fe are sufficiently low in the open ocean that they may be controlling for some protic species (Martin & Gordon, 1988; Foster & Morel, 1982).
In neritic waters, Brand et al. (1983) reported that certain neritic diatoms are Zn-limited at activities less than 10-11.5 5 M; but in the pelagic oceans, other phytoplankton (including diatoms, coccoliths, dinoflagellates and cyanobacteria) are not limited even at activities as low as 10-13 M (Brand et al., 1983). Although metals such as Zn and Cu are limiting at low activities, they may be toxic at high activities (Sunda & Guillard, 1976). For example, the deleterious effects of copper on the diatom, Thallassiosira pseudonana, are chronic above activities of 3 x 10-11 M, and toxic above 5 x 10-9 M. The concentrations of Cu in the upper open ocean are of the order of 10-9; therefore organisms with sensitivities similar to T. pseudonana exist in conditions where Cu concentrations, even if strongly chelated are inhibiting to growth.
Not all of the nutrient-related elements have either a plastic or catalytic role (sense Dietrich, 1963, p. 246) in the biology of marine organisms, particularly those in higher trophic levels. Some elements or compounds are sequestered passively in the lipids of living organisms (for example, the highly toxic methyl mercury, Boney, 1975, p. 103). For lower trophic levels, such concentrations may be harmless. But by concentration in successive trophic levels, certain elements or compounds may reach chronic or toxic levels detrimental to organisms in higher trophic levels. Provasoli (1963) found such a situation for the toxicity of dinoflagellates to higher trophic level during Red Tide upwellings off the southwest coast of Africa. Morel (1986) has noted that for algae, which in the ocean are the major primary producers, Cd, Pb, and Hg (ad non-essential elements) are toxic at low concentrations.
Accordingly, a crisis could be produced by upwelling of enriched metals or other compounds from deeper water into the surface layers. Upwelled toxic waters that inhibit photosynthesis could cause a crisis in groups with short food chains by reduction of primary production. During warm climates and potentially anoxic waters in the pycnocline below the mixed layer, the crisis could be due to low levels of oxygen and its related chemistry so that more active organisms would be affected more than sessile or low-oxygen tolerant organisms.
Conversely, upwelling of nutrient species including those trace metals whose concentrations are limiting, could result in enhanced living conditions for some species, resulting in a bloom. In either case, the species composition may change (Sunda et al., 1981). Sunda et al. (1981) hew shown that in water from off the North Carolina coast in which the diatom Nitzschia and the green flagellate Chaetoceros where common elements of pelagic communities, Cu additions caused a shift in species dominance from the diatoms to the flagellate.
POTENTIAL IMPACTS OF UPWLLLING EVENTS
The effects of upwelling on biotas depend on the depth of upwelling which win in turn control the physico-chemical conditions of the upwelled water. For modern open ocean conditions with well-ventilated deep waters, generally oxic conditions occur regardless of the depth of upwelling. In restricted locations where a major denitrification zone occurs, such as the Eastern Tropical Pacific (see discussion in Berry et al., 1987), waters representative of the nitric zone might be vertically advected during enhanced upwelling.In basins such as the Cariaco Trends, Black Sea, or fjords such as Lake Nitinat, upwelling of sulfatic waters could occur. Brongersma-Sanders (1957) and Richards (1965) have given various incidences of mass mortalities due to the introduction of sulfides into the water column.
For the Phanerozoic, Wilde (1987) proposed that three redox zones: oxic; (2) nitric, i.e. anoxic, with no sulfide production; and sulfatic, i.e. anoxic, with sulfate-reduction, existed in the open ocean on a regional to planetary scale. This zonation assumes a wind-mixed oxygenated surface layer of 50 to 100 meters below the surface. Variations in redox occurred in the underlying pycnocline as a function of the efficacy of deep ventilation (Wilde & Berry, 1982), which, presumably, was related to climate. Upwelling of increasing depth would result in penetration into the superimposed oceanic redox zones.
Thus, for an ocean with a discrete anoxic layer below the surface mixed layer, such as proposed by Wilde (1987; Wilde & Berry, 1982), there is a potential for upwelling from all three zones at the same location, depending on the depth of upwelling. Upwelling, in order to influence extinction or killing events, must be sufficiently rapid that chemical equilibrium is not attained during the rise.
Upwelling from the Oxic Zone
With duration of upwelling and/or increasing depth of the source of upwelled water, snore decay products would be brought into the photic zone as oxidized metals. Therefore, with increasing depth, increased quantities of the dissolved micronutrients, N (as nitrate), P. and Si will be brought to the surface layer. Increased levels of Cd, Cu, Zn, Co, Ni, Se, Cr. Ba, Ge, As, Pd, Te, I, REE and organic decay products are also expected. Lower concentrations of dissolved oxygen would be brought to the surface from the oxygen minimum zone. Fe and Mn concentrations generally decline with depth, although there can be increased concentrations in the oxygen-minimum zone. Thus, Cu and Cd algal toxicity would increase. However, the availability of nitrate, phosphate and silicate would increase. The upwelling of Mn-poor waters, such as those of the Sargasso Sea, may result in acute Mn deficiency (Sunda & Huntsman, 1983). The conjunction of waters low in Mn and higher in Cu could result in enhanced toxicity as Cu is antagonistic to Mn-usage in some diatoms (Sunda & Huntsman, 1983). A similar relationship exists between Cd+2 and Fe+3: Cd toxicity can be reversed if there is sufficient Fe+3 present (Poster & More, 1982). Upwelling of deep oxic waters depleted in Fe+3 or Mn, but with high levels of Cd+2 or Cu, could have greater toxicity for some species than would nitric zone waters that would have dramatically increased levels of Fe and Mn.Due to the influx into the photic zone of unchelated metal ions, such as Cu or Cd (Barber et al., 1971; Terry & Caperon, 1982) and low levels of O2, photosynthesis would initially be suppressed even though additional nutrients also would be entrained. If the depth of upwelling is great enough, suppression might continue to drastically reduce primary productivity. This would be particularly critical in middle to high latitude locations if the upwelling occurs during the normal time of plankton growth when seasonal light levels also are critical. With the suppression of primary productivity, effects would be felt in the higher trophic levels in the water column and eventually among the benthic seston feeders.
Under most situations involving upwelling of oxic waters, a phytoplankton bloom (due to increased levels of nutrients), and an increase in zooplankton grazers would be expected to follow the initial suppression. These upwelling conditions could be advantageous to benthos above the oxygen minimum zone. If the oxygen demand of the bloom is excessive, eutrophication may occur causing death of respirors and benthos if anoxia reaches the bottom. Blooms of certain dinoflagellates may be inherently toxic to predators.
Upwelling from the Nitric Zone
In the nitric zone (where nitrate is the oxidant), concentrations of micronutrients (P as phosphate, Si as silicate and N as nitrite and ammonia) increase with increasing depth. Certain trace metal concentrations, Fe, Mn, Cd, Cu. increase with depth because of the lack of precipitating anions such as sulfide. The nitric zone occurs at relatively shadow depths at the top of the pycnocline, about 100 m. The increased concentrations of micronutrients, as wed as reduced nitrogen compounds (Eppley et al., 1969) and the limiting trace metals, Fe and Mn, would increase productivity and may suppress Cu or Cd toxicity.In the nitric zone, there would be reduced nektonic activity, and little or no benthic megafauna. The oxygen-poor waters would inhibit respiring organisms, especially nekton. Such conditions would give advantage to specialized low-oxygen tolerant types, such as the copepods Euphausia distinguenda and E. eximia (Brinton, 1980), which occur in the eastern tropical Pacific off Peru (Fig. 2). The sequence of biological events as a consequence of upwelling from the nitric zone initially would be enhanced primary productivity, but rapid reduction in active grazers and nekton. Increased productivity would secondarily add to the oxygen demand in the water column and cause a reduction in benthic populations, even with additional seston food supply.
Upwelling from the Sulfatic Zone
In the sulfatic zone, micronutrient concentrations, N (as ammonia), phosphate, and silicate, increase with depth due to continuing decay of organic matter. Concentrations of Fe would be greater with depth than in oxic ocean waters, though probably less than in nitric zone waters (Fig 4a). Mn concentrations are greater in the sulfatic zone than in the oxic zone, and similar to those in the nitric zone, based on observations from the Cariaco Trendh (Fig 4d). The concentrations of Cu. Cd, Zn, and any cations that form sulfides win decline due to sulfide precipitation (Fig 4b, 4c). Ni is unchanged with depth. Sulfatic zone waters are oxygen-depletecL and with depth hew increasingly higher concentrations of toxic hydrogen sulfide.During upwelling of sulfatic zone water, primary productivity win increase due to increased levels of ammonia, particularly for those phytoplankton that are Fe- or Mn-limited. On the other hand, primary productivity may decrease among other phytoplankton due to low levels of trace nutrients, such as Zn and Cu. For animals, acute hydrogen sulfide toxicity win cause losses of nekton and benthic megafauna with an excellent chance of preservation of large organisms in anoxic bottoms. Thus, upwelling from sulfatic waters would be characterized by enhanced primary productivity and elimination of nekton and respiring benthos.
POTENTIAL EXAMPLES IN GEOLOGIC RECORD
Evidence of extinctions or killing-events in the geological record would appear as reductions in diversity, mass extinction of some taxa, followed by rapid radiations among surviving and newly adapted organisms if conditions remained stable. Survivors could return if preexisting conditions were reestablished. Evidence for mass mortality/radation events due to vertical advection from the oxic, nitric or sulfatic zones may be found in the rock record and are discussed below. Possible examples of vertical advection from the oxic zone are found in sequences from the end of the Ordovician and from the end of Triassic. A mass mortality/ra&ation event in the late Wenlock could be due to upwelling from the lower nitric zone. The extinctions evidenced in the early Jurassic Toarcian shales may be the result of upwelling from the sulfatic zone.
Advection from the Oxic Zone
At the end of the Ordovician, graptolites suffered mass mortality at various tropical localities, inducting Dob's Linn, Scotland, Anhui, China, and Mirny Creek, USSR (Berry et al., this volume). The latest Ordovician graptolites were restricted to limited areas in the tropics of the time. Each incidence of near extinction was a consequence of a local event. At each site, the environment changed from one inhabited by graptolites to one in which graptolites were virtually excluded. That change involved introduction of oxic waters into the oxygen-poor waters inhabited by graptolites resulting in graptolite mass mortality. Such oxygenation could be the result of vertical addiction of pycnoclinal waters ventilated during the glaciation that occurred at the end of the Ordovician. Advection of colder, more oxygenated and more metalliferous water may hew affected the graptolites themselves or their food supply.In the late Triassic, House (1985) has shown a massing, world-wide decline of ammonites in the interval of the latest Norian through Rhaetian, the latest part of the Triassic. By the close of the Triassic, ammonites had become virtually extinct. Triassic ammonites lived primarily on the margins of the tropical shelves of the time. They seemingly lived primarily in low oxic (dysaerobic) waters under similar environmental conditions as those inhabited by graptolites at an earlier time. During the late Triassic, major marine transgressions began across many of the low-lying continental areas (Sellwood, 1978). Anderton et al. (1979) described a major transgression in Britain that began in the south and spread initially over lagoons. Volcanism occurred in southern Europe, and rocks from the Aquitaine and New England suggest that an initial phase of North Atlantic rifting began in the late Triassic (Anderson et al., 1979). Accordingly, the new ocean became connected to the world ocean with better circulation. As the Atlantic opened, the rise in sea level presumably was accompanied by advection of more oxic waters into ammonite habitats. Introduction of these waters resulted in ammonite extinctions that took place over a several minion year interval. The length of time of the extinctions suggests that the demise of the ammonites may have resulted from a long-term chronic condition. Throughout the Jurassic, ammonite diversity increased notably in species better adapted to oxygenated waters. Apparently, as a result of the opening of the Atlantic, new patterns of vertical addiction supported the radiation of these species.
Advection from the Nitric Zone
In the late Wenlock, mass mortalities of tropical graptolites (Rickards et al., 1977) occurred concurrently with development of massing carbonate reefs after a long interval of limited reef growth (Berry & Boucot, 1970; Ziegler et al., 1974), as did the development of land plants with vascular tissue (Cooksonia) (Edwards & Feehan, 1980). Both marine events were of relatively short duration. The marine events took place in water oceanward from the shallow subtidal zone. The oldest vascular tissue was found in early Ludlow strata in Wales, which was tropical in the Silurian. These events can be explained by expansion of the nitric zone toward the surface. Upwelled waters from the deep nitric zone would contain no oxygen and would be elevated in ammonia (see above). Vertical advection of anoxic waters could have caused mass mortalities of graptolites via a number of mechanisms. Mortalities might be caused by lack of sufficient oxygen to support either graptolites or their food. If graptolites were forced into upper levels of the mixed layer, they may have been exposed to greater predation. By the time ammonia was advected into shallow waters, it would have been Saluted sufficiently to be a nutrient rather than a toxicant. This diluted ammonia would hew increased productivity, thereby providing increased food supply for the reef-building corals.Increased levels of ammonia could also increase the alkalinity sufficiently to favor carbonate precipitation necessary for coral reef building. levels of ammonia in tidal waters may have provided the necessary fertilization for the development of vascular land plants in the nearshore environment. As upwelling from this zone ceased, the graptolites reradiated, and coral reefs receded.
Advection from the Sulfatic Zone
Hallam (1967, 1975, 1977, 1981), Jenkyns (1985), Wilde et al. (1986), and Riegel et al. (1986) discussed the Lower Jurassic Toarcian extinctions in terms of physical conditions relating oceanic redox conditions with transgression and regression. The extinctions can be explained in terms of an upwelling of sulfidic waters. Toarcian rocks in Great Britain and Europe are organic-rich shales with a non-bituminous argillaceous facies above and below. The fauna is characterized by nektonic ammonites, belemnites and fish scales; and microfossils: ostracods, foraminifera, and radiolarians (Kauffman, 1978, 1981). The benthic fauna are of limited diversity with large populations. Massive mortalities, consistent with upwelling from the sulfidic zone, are observed in Yorkshire at the base of the Jet Rock (Hallam, 1967; Jenkyns, 1985) and in Germany, in the Posidonia Shale (Riegel et al., 1986). In the Posidonia shale, pyritic concretions, indicative of the presence of sulfidic waters, are common in the lower billions zone (Riegel et al., 1986).
C ONCLUSIONS
Vertical advection or upwelling is a mechanism that could cause selecting extinctions or killing events. By this mechanism, step-wise or gradual extinctions are readily explained as are survivals. The impact of upwelling is a function of depth, temperature and the chemical properties of the source water. The redox properties, that is whether the water is oxic (oxygen as oxidant), nitric (oxygenated-nitrogen species as oxidant), or sulfatic (sulfate as oxidant) control the effect of such waters on the biota. The chelating tendencies of the water, organismal tolerances, and the presence or absence of antagonistic or synergistic elements may determine whether an organism thrives, exists or dies. Impacts equally depend on the taxa present, their tolerance limits and preadaptive capabilities. The results of massive upwelling events would be characterized in the geologic record by decreased diversity of species, and, possibly, certain areas depauperate of species followed by radiations.
Acknowledgements
The authors wish to thank Profs. A. Boucot, W. Holser, E. Kauffman and O. Walliser for useful discussions and encouragement at Boulder. M. Krup drafted the figures and coordinated the preparation of the manuscript with her usual efficiency. This is contribution number MSG-88-003 of the Marine Sciences Group, University of California, Berkeley.
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