PALEOZOIC (CAMBRIAN THROUGH DEVONIAN) Anoxltropic BIOTOPES
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Palaeogeography, Palaeoclimatology, Palaeoecology, 74 (1989): 3-13 3 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands PALEOZOIC (CAMBRIAN THROUGH DEVONIAN) ANOXlTROPIC BIOTOPES W. B. N. BERRY, P. WILDE and M. S. QUINBY-HUNT Marine Sciences Group, Department of Paleontology, University of California, Berkeley, CA 94720 (U.S.A.) (Received April 27, 1989) Abstract Berry, W. B. N., Wilde, P. and Quinby-Hunt, M.S., 1989. Paleozoic (Cambrian through Devonian) anoxitropic biotopes. Palaeogeogr., Palaeoclimatol., Palaeoecol., 74:3 13. The oxygen-poor to denitrified waters of modern oceans provide an analog for open ocean conditions which contained the Early Paleozoic "graptolite" biotope. Certain euphausiids, other zooplankton and fish, as demonstrated in the Eastern Tropical Pacific, are found concentrated in and about denitrified waters. The boundary zone between oxic and denitrified waters show intense organismal growth due to the increased chemical availability of nutrients as reduced nitrogen compounds. Such waters are areally limited in the present cold and well-ventilated modern ocean. The distribution patterns showing attraction to low or oxygen-depleted waters (anoxitropy) in these modern open marine faunas suggest that Early Paleozoic plankton and nekton, preserved in lithofacies formed under low oxygen or anoxic coditions, could have been similarly attracted to the more extensive ancient denitrified and anoxic waters. The major Paleozoic faunal groups, in order of their occupation of the niche, include agnostid and olenid trilobites, graptoloid graptolites, styliolinids, thin-shelled bivalves, small orthocone nautiloids, and early ammonoids. The expansion of low-oxygen to anoxic waters across the shelves during warm climates in the Early Paleozoic may be correlated with major evolutionary radiations in the graptolites and possibly in the early ammonites. Introduction mental conditions of these biofacies and litho- facies are difficult to study using modern In Early into Middle Paleozoic oceans, two analogs. Accordingly, the origins of the grap- facies dominate: the shelly facies with mostly tolitic biofacies and its related black shale benthic organisms requiring oxygenated lithofacies have been the subject of specula- waters generally associated with sandstones, tion (Bulman, 1970; Pettijohn, 1975). Bulman limestones, and bioturbated shales; and the (1970) in his analysis of the environmental graptolite facies with planktic or passive conditions under which the "graptotite facies" organisms found in laminated unbioturbated accumulated, suggested that ~the essential black shales. Fossils found in the shelly facies condition is the complete lack of bottom rarely occur in the graptolite facies; although circulation so that dissolved oxygen, soon graptolite facies organisms may be found as a exhausted, cannot be replenished; while a high minor component of certain shelly facies rocks. proportion of decaying organic matter may be This separation implies distinct environmental contributed by animal and plant remains conditions for each facies. Due to extinctions falling from superficial aerated layers." and replacements of most of the graptolite Erdtmann (1982) noted that preservation of facies organisms and the lack of extensive delicate graptolite peridermal structures sug- modern black shale lithofacies, the environ- gests that both living conditions and sites of 0031-0182/89/$03.50 @ 1989 Elsevier Science Publishers B.V. fossilization were removed from vigorous wave Wilde and Berry (1982, 1984, 1986), Wilde (1987) action and from oxygenated bottom waters have applied fundamental principles of physi- which could have permitted active predation cal and chemical oceanography, based upon and scavenging by benthic organisms. This observations of modern oceanic environments view is supported by the evidence that most to distinguish those oceanic environments in black graptolite-bearing shales commonly are which strata typical of the '~graptolite facies" characterized by fine-scale laminations, ab- could have formed. This series of papers sence of evidence of bioturbation, and locally indicates that the Early Paleozoic oceans were abundant pyrite (Pettijohn, 1975, p. 282). There anoxic beneath the surface wind-mixed layer also is a spatial separation of these facies as and when sea level rose during warm climates the occurrences of graptoloid or planktic anoxic waters invaded the outer shelf. During graptolites indicates that they were most glacial intervals in the Late Ordovician or cool abundant in outer shelf and slope waters climates in the Devonian, the waters beneath (Berry, 1974); whereas the shelly biofacies are the pycnocline were ventilated by deep circula- near-shore shallow water to inter-tidal. tion driven by the sinking of cold oxic waters That the graptolite lithofacies is not re- at high latitudes. A similar mechanism drives stricted to the life-span of the floating or the ventilation and circulation in the modern graptoloid graptolites has been discussed by deep ocean. An oceanographic model is pro- Berry (1962, 1974) and House (1975) who posed here for the environmental conditions in recognized black shales of the graptolite facies which graptolite facies organisms lived in the type in Cambrian and Devonian stratal se- Lower Paleozoic. This model offers an explana- quences respectively. Cambrian ~graptolite tion for the separation of both graptolite facies" strata bear primarily agnostid trilo- biofacies and black shale lithofacies from that bites (Berry, 1974; Ludvigsen et al., 1986). Lud- of the shelly facies. vigsen et al. (1986) found that dark, laminated mudstones that formed on Late Cambrian Modern environmental analogs of the continental slopes contain almost exclusively graptolite facies olenid and agnostid trilobites. The agnostids are considered planktic; whereas certain olen- Chemical oceanography of transitional (oxic to ids probably were swimmers. Berry (1974) anoxic) waters noted that the planktic graptolites appear in black, thinly laminated shales with agnostids Potential modern analogs of oceanic en- in the Cambro-Ordovician transition interval. vironments in which the '~graptolite facies" Ultimately, planktic graptolites replaced agnos- formed are limited in extent because the tid and olenid trilobites in that facies formed on modern oceans are well ventilated and circula- the continental slope in the Ordovician. The tion is rapid in the modern inter-glacial Devonian "graptolite facies" strata bear, in climate (Munk, 1966). The modern ocean is layers stratigraphically above those containing truly anoxic in very limited areas such as the last graptoloid graptolites, styliolinids (no- fjords, small basins in the Baltic, the Black tably Nowakia, Lutke, 1979), slender orthocone Sea, and the Cariocao Trench (Richards, 1965; nautiloids, some ammonoids (House, 1975), thin- Deuser, 1975; Demaison and Moore, 1980). shelled bivalves, and certain ostracods (Rabien, Such anoxic areas are formed due to special 1956). Thus, the Paleozoic "graptolite facies" conditions blocking extensive interchange rocks bear remains of planktic organisms other with the aerated open ocean usually by shal- than graptolites in strata both younger and low sills. However, there are relatively exten- older than those formed in the life span of the sive areas of the modern open ocean that have planktic graptolites. little or no oxygen and are transitional be- Berry and Wilde (1978), Berry et al. (1987), tween oxic and anoxic waters. In such waters slowly metabolizing organisms or migrating core of what now is the oxygen minimum zone transient higher organisms survive. Areas of probably was anoxic. In this case denitrified modern oceans in which an oxic surface layer waters were transitional between the surface is underlain by near anoxia in the pycnocline oxic water and sulfidic rich anoxic waters in are present in the Eastern Tropical Pacific the pycnocline (Wilde and Berry, 1986; Wilde, (ETP) (Anderson, 1982), the northern Arabian 1987). Thus, in the Paleozoic, denitrified waters Sea in the Indian Ocean (Qasim, 1982), and may contain an attractive food source just seasonally in areas of coastal upwelling as off overlying the toxic sulfide rich waters of truly the western coasts of California (Mullins et al., anoxic oceanic environments. This suggests 1985) and Namibia (Calvert and Morris, 1977). not only the possibility of an unique biotope In these waters, oxic chemical reactions cease there, but also explains the preservation of at oxygen concentrations below 0.22 ml/1 (De- planktic organisms as fossils in the underlying vol, 1978). Below this concentration and to an black shale lithofacies. negative oxygen equivalent of about -1 ml/1 (Wilde, 1987) nitrogen compounds are con- Biological oceanography in modern denitrified sumed as an "oxidant" in chemical reactions in waters the process of denitrification (Stumm and Morgan, 1970; Curtis, 1983; Hashimoto et al., Both the oceanographic and biological con- 1983. Dugdale et al. (1977) described denitrifi- ditions have been studied extensively in the cation in the ETP as a process of reduction of denitrified waters of the Eastern Tropical NO 3 to N z or N20 by bacteria and the use of Pacific. Wooster and Cromwell (1958) and nitrate as a hydrogen acceptor when oxygen Wyrtki (1967) described the physical condi- concentrations are too low for its use as an tions. Eppley et al. (1968), Longhurst (1967), oxidant (see also discussion in Richards, 1965; Brinton