Vol. 5, No. 3 March 1995 INSIDE GSA TODAY • Geopals Program, p. 46 • 1995 GeoVentures, p. 48 A Publication of the Geological Society of America • 1994 Medals and Awards, p. 51 Late Devonian Oceanic Anoxic Events and Biotic Crises: “Rooted” in the Evolution of Vascular Land Plants? Thomas J. Algeo, H. N. Fisk Laboratory of Sedimentology, Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013 Robert A. Berner, Department of Geology and Geophysics, Yale University, New Haven, CT 06511 J. Barry Maynard, H. N. Fisk Laboratory of Sedimentology, Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013 Stephen E. Scheckler, Departments of Biology and Geological Sciences, Virginia Polytechnic Institute, Blacksburg, VA 24061 ABSTRACT Evolutionary developments among vascular land plants may Figure 1. Paleobotanic have been the ultimate cause for reconstructions of oceanic anoxic events, biotic crises, (A) an Early Devonian global climate change, and geochem- coastal delta, (B) an Early ical and sedimentologic anomalies of Devonian upland flood Late Devonian age. The influence of plain, (C) a Late Devonian vascular land plants on weathering coastal delta, and (D) a processes and global geochemical Late Devonian upland cycles is likely to have increased flood plain. Early Devonian substantially during the Late Devon- plants: Pr = Pertica, Ps = Psilophyton, Sc = Sciado- ian owing to large increases in root phyton, and Sw = Sawdonia; biomass associated with develop- Late Devonian plants: A = ment of (1) arborescence (tree-sized Archaeopteris, B = Barino- stature), which increased root pene- phyton, L = tree lycopod, tration depths, and (2) the seed R= Rhacophyton, and habit, which allowed colonization S = seed plant. Data from of drier upland areas. We hypothe- Scheckler (1986), Gensel size that rapidly increasing root mass and Andrews (1984, 1987), led to transient intensification of the and P. G. Gensel (personal rate of soil formation and to perma- commun.). nent gains in the thickness and areal extent of deeply weathered soil pro- files. In the short term, greater pedo- genesis caused increased sediment yields owing to episodic disturbance of vascular land plants, in the biomass 1983), whereas high-latitude and cold- chemical records. Laminated black of developing soils and to increased and complexity of floral communities, water species were less affected (Cop- shales indicate episodic development nutrient fluxes to the oceans as a and in the geographic distribution of per, 1986). The two extinction maxima of widespread oceanic anoxia in many result of enhanced chemical weather- terrestrial vegetation (Fig. 1; Scheckler, of widest taxonomic impact occurred cratonic sequences during this interval ing. Long-term effects included 1986; Gensel and Andrews, 1987). In at or near the Frasnian-Famennian (F-F) (Fig. 3, B–D). Deposition of organic-rich increased landscape stabilization, this paper, we propose that evolution- and Devonian-Carboniferous (D-C) shales and coals during the Devonian- drawdown of atmospheric CO 2 ary innovations among vascular land boundaries and are known as the Kell- Carboniferous transition sequestered through enhanced uptake in silicate plants were the ultimate cause of both wasser and Hangenberg events, respec- large quantities of isotopically light car- weathering and burial of organic car- the Late Devonian biotic crisis and a tively (Fig. 3A; Talent et al., 1993). bon in the sedimentary reservoir, caus- bon, and global cooling. Coeval ter- variety of coeval sedimentologic and The origin of the Late Devonian ing an enrichment of marine carbonate restrial paleobotanic developments geochemical anomalies. The main lines biotic crisis has been the subject of δ13C values of about 4‰ (Fig. 2B; and marine anoxic and extinction of evidence supporting this hypothesis considerable debate. Much recent Lohmann, 1988; Berner, 1989). A events are likely to have been linked are (1) close temporal relations between research has sought evidence of a combination of increased burial of causally through transient nutrient Late Devonian paleobotanic develop- bolide impact, an idea stimulated by organic carbon and enhanced silicate pulses that caused eutrophication of ments and major episodes of oceanic proposals for such a catastrophic mech- weathering by vascular plants drew semirestricted epicontinental sea- anoxia and mass extinction, and (2) a anism at the Cretaceous-Tertiary (K-T) down atmospheric CO levels from ways, stimulating marine algal 2 model that successfully links these boundary (Alvarez et al., 1980). ~12–16 present atmospheric level (PAL) blooms. Correlativity of black shale paleobotanic developments to the Late Although minor iridium anomalies in the early–middle Paleozoic to ~1 PAL horizons and episodes of extinction Devonian biotic crisis and coeval sedi- (Geldsetzer et al., 1987; Wang et al., in the Carboniferous and Permian (Fig. of tropical marine benthos impli- mentologic and geochemical anomalies 1993) and small concentrations of 2C; Berner, 1994). Evidence of lowered cates oceanic anoxia rather than through changes in pedogenic rates microspherules (Wang, 1992; Claeys atmospheric CO is provided by global cooling as the proximate 2 and processes. et al., 1992) have been identified close changes in soil carbonate δ13C (Mora et cause of the Late Devonian biotic to the F-F and D-C boundaries at sev- al., 1991) and by a marked decline crisis. LATE DEVONIAN BIOTIC eral locales, siderophile element ratios in dolomite abundance across the D-C INTRODUCTION CRISIS AND ANOMALIES are incompatible with those of mete- boundary (Fig. 2D). Marine evaporites orites, and these anomalies have been of this age exhibit a +8‰ to +10‰ During the Late Devonian biotic The origin of the Late Devonian interpreted as resulting from concen- δ34S excursion as a consequence of crisis (Frasnian-Famennian extinction), biotic crisis is a subject of continuing tration of metals by cyanobacteria or large-scale bacterial reduction of dis- about 21% of families and 50% of gen- debate. Although various causes have changes in redox conditions (Playford solved sulfate in association with burial era among marine organisms disap- been proposed, including bolide et al., 1984; Wang et al., 1993). Other of organic carbon (Fig. 2E; Holser et al., peared (Sepkoski, 1986). This event was impacts, oceanic overturn, sea-level causes proposed for the Late Devonian 1989). Drawdown of atmospheric CO unusual in three respects (1) duration, 2 changes, and global climate change biotic crisis include climate change initiated global cooling, recorded as a ~20 m.y. (beginning in the Givetian, or (Copper, 1986; Geldsetzer et al., 1987; associated with global tectonics (Cop- about +3‰ enrichment of abiotic late Middle Devonian); (2) episodicity, McGhee, 1991; Claeys et al., 1992), per, 1986), oceanic overturn (Geldsetz- marine carbonate δ18O values across comprising at least eight separate none has gained general acceptance. er et al., 1987), and sea-level elevation the D-C boundary (Fig. 2F; Lohmann, episodes of extinction (House, 1985); Few, if any, of these theories have changes (McGhee, 1991), but none of 1988), and resulted in continental and (3) selectivity, disproportionately attempted to link Late Devonian ex- these fully accounts for the duration, glaciation by the late Famennian eliminating tropical marine benthos tinctions to coeval paleobotanic events; episodicity, and selectivity of the crisis. (Fig. 3E; Caputo, 1985). (Bambach, 1985; Sepkoski, 1986). however, the Givetian–Famennian The Late Devonian is also charac- Extinctions were particularly severe epochs are characterized by important terized by an unusual combination of among the middle Paleozoic reef paleobotanic developments, including major excursions or permanent shifts community, dominated by stromato- large increases in the maximum size in a variety of sedimentologic and geo- poroids and corals (Fig. 2A; James, Plants continued on p. 64 Plants continued from p. 45 rapidly with the appearance of seed plants (Fig. 3E; Beck, 1981; Gensel and VASCULAR LAND PLANT Andrews, 1984; Scheckler, 1986). Seed EVOLUTION plants spread rapidly during the latest Famennian owing to the advantages Although land plants appeared in conferred by seeds, including ability to the Late Ordovician or Early Silurian adapt to diverse ecological conditions and vascular plants diversified in the and to occupy drier upland habitats Figure 2. Phanerozoic Late Silurian and Early Devonian (Fig. 3E; Gillespie et al., 1981; Rothwell records exhibiting Late (Edwards and Berry, 1991), full colo- et al., 1989). Devonian anomalies: nization of land surfaces is likely to Close temporal relations exist (A) dominant Phanero- have been a protracted process that zoic reef-building between Late Devonian anoxic and continued throughout the Devonian groups (James, 1983); extinction events and these paleo- and later. Initially, the impact of land (B) marine carbonate botanic developments. First, the onset plants on their physical environment δ13C (Berner, 1989); of a protracted late Middle–Late Devo- was negligible owing to small size, (C) atmospheric CO2 nian interval of widespread oceanic limited biomass, shallow rooting, and (RCO2 is the ratio of anoxia (Fig. 3, B–D) followed closely CO at a given time in restriction to moist lowland habitats. 2 the advent of secondary vascular sup- the past to that at As land plants increased in size and porting tissues (Fig. 3E) and coincided present; Berner,