National Academies Press: OpenBook

Effects of Past Global Change on Life (1995)

Chapter: THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD

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Suggested Citation:"THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
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Suggested Citation:"THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 51
Suggested Citation:"THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 52
Suggested Citation:"THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 53
Suggested Citation:"THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 54
Suggested Citation:"THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 55
Suggested Citation:"THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 56
Suggested Citation:"THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION - AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD." National Research Council. 1995. Effects of Past Global Change on Life. Washington, DC: The National Academies Press. doi: 10.17226/4762.
×
Page 57

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GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 50 (CRETACEOUS) MASS EXTINCTION to them, at Quaternary time scales of hundreds to tens of thousands of years. As examples, Figure 3.1 shows typical analyses of Quaternary climate change, as measured by stable isotopes and CO2 data depicting temperature changes over the past 50,000 yr; the values for 1000-yr intervals (the typical duration of 1-2 cm of shale or limestone in Phanerozoic basinal facies) are highlighted by triangles and connected by a bold line. The thousand-year Phanerozoic level of analysis effectively replicates the significant patterns of global change in the more finely analyzed Quaternary data. Further, interregional correlation of Quaternary global change data relies heavily on the isotopic record of glaciation and sea-level fluctuations, as regulated through Milankovitch climate cyclicity. Milankovitch cyclostratigraphy is also widely recognized in ancient sedimentary sequences representing nonglacial greenhouse intervals (e.g., for the Cretaceous; Barron et al., 1985; Fischer et al., 1985; Kauffman, 1988a; Glancy et al., 1993), and extensively used for regional correlation as a part of HIRES chronostratigraphy (Hattin, 1971, 1985; Elder, 1985; Kauffman, 1988a; Kauffman et al., 1991). Quaternary and older rocks can thus be correlated at the same scales. Further, event chronostratigraphic resolution in the older stratigraphic record commonly exceeds that possible from Quaternary climate cyclostratigraphy. Whereas Quaternary scientists have the advantage of finer time scales of observation, students of older geological intervals have the advantage of longer continuous records of global change. Both groups have much to contribute to the documentation, interpretation, and prediction of global change phenomena and their short- and long-term effects on Earth ecosystems. An example of high-resolution stratigraphic analysis of an interval of significant past global change and biodiversity crisis is drawn from the Cretaceous record of North America, the Cenomanian-Turonian mass extinction interval (92 to 94 million years ago (Ma)). THE CENOMANIAN-TURONIAN (C-T) MASS EXTINCTION—AN ANCIENT GLOBAL BIODIVERSITY CRISIS IN A CHAOTIC GREENHOUSE WORLD An abrupt change in the global marine biota at the Cenomanian-Turonian boundary was first noted by d'Orbigny (1842-1851), who named these stages (Hancock, 1977). Raup and Sepkoski (1984, 1986) and Sepkoski (1990, 1993) statistically determined that the Cenomanian-Turonian boundary interval was a secondary mass extinction on the basis of both familial and generic-level data. They recorded a 5% loss of families and a 15% loss of genera (per-genus extinction rate 0.026), utilizing stage-level data with an average duration of 6 million years (m.y.); this broad resolution did not, however, allow rates and patterns of Cenomanian extinction to be determined. For example, a review of global Cenomanian ammonite extinction data (Collom, 1990) resolved to substage level showed several discrete extinction events among these molluscs within the Cenomanian. A number of highly detailed regional studies of Cenomanian-Turonian extinction patterns, and associated environmental perturbations, have been conducted during the last three decades which shed light on the dynamic nature of this interval. Of particular interest are the new data from the Western Interior of North America (Figure 3.2), from western and central Europe, from North Africa, and from northern South America (Colombia, Venezuela, and Brazil). These studies do not support the idea of a single "catastrophic" boundary extinction among ecologically and genetically diverse taxa, but rather a 1.46-m.y. interval of ecologically graded extinction characterized by several discrete, short-term events with global expression ("steps" of Kauffman, 1988b), most of them in the last 520,000 yr of the Cenomanian. Each of these discrete extinction events is associated with geochemical evidence for environmental chaos in the ocean-atmosphere system—short- term, large-scale perturbations in temperature, ocean chemistry, and carbon and nutrient cycling in the sea, upon which are superimposed a somewhat exaggerated expression of Milankovitch climate cyclicity at 41,000-and 100,000-yr intervals (subsequent discussion). Collectively, these integrated, high-resolution (centimeter to meter scale) paleobiological, geochemical, and sedimentological studies provide a global perspective of the rates, patterns, causes, and consequences of the C-T mass extinction interval. In the Western Interior Basin of North America (Figure 3.2), bed-by-bed stratigraphic and paleontologic studies by Cobban and Scott (1972) and later Cobban (1985, 1993) documented an abrupt change in ammonite assemblages across the C-T boundary; they noted the loss of typical late Cenomanian ammonites at five to six discrete levels within 4 m of the C-T boundary, spanning the biozones of Sciponoceras gracile (Vascoceras diartanum and Euomphaloceras septemseriatum subzones) and, above it, Neocardioceras spp. (Eoumphaloceras navahopiensis, Neocardioceras juddi, and Nigericeras scotti subzones, ascending). Cobban (1985, 1993) and Kennedy and Cobban (1991) defined the C-T boundary between the latest Cenomanian Neocardioceras biozone, Nigericeras scotti subzone, and the overlying Watinoceras biozone, W. devonense subzone. Kauffman (1975) and Kauffman et al. (1976, 1993) also noted a major change in Bivalvia at the ammonite-based C- T boundary of North America and Europe, with the disappearance of latest Cenomanian Inoceramus pictus and I. tenuistriatus lineages, overlain by the first common occurrence of early Turonian Mytiloides

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 51 (CRETACEOUS) MASS EXTINCTION (M. hattini zone). To date, microfossils have not been successfully used to identify the ammonite-based C-T boundary. The boundary falls within the Whiteinella archeocretacea planktic foraminifer biozone (Caron, 1985; Eicher and Diner, 1985, 1989; Leckie, 1985) and within the upper Lithraphidites acutum nannoplankton biozone (Watkins, 1985; Watkins et al., 1993). Figure 3.2 Paleogeographic map of the Western Interior Basin of North America during the Cenomanian-Turonian boundary interval, showing the extent of the interior epicontinental seaway (defined by bold lines enclosing sedimentary rock symbols) during maximum eustatic sea-level highstand. Note the large aerial extent of fine- grained basinal facies (limestone, marl, shale symbols), which preserve relatively complete C-T boundary sections, and from which the high-resolution sedimentologic, geochemical, and paleo- biologic data were obtained for this study of Cretaceous global change associated with a mass extinction within a greenhouse world. Studies of the rates and patterns of marine extinction across the C-T boundary in North America began with the work of Koch (1977, 1980), who noted an overall loss of nearly 70% of known late Cenomanian molluscan species occurring in the Sciponoceras gracile and Neocardioceras biozones; the most extinction-prone molluscs were those with limited facies associations and biogeographic range. Kauffman (1984b) integrated diverse late Cenomanian molluscan data from detailed stratigraphic sections to demonstrate that the C-T extinction occurred as a series of discrete events (later called "steps"; Kauffman, 1988b). Elder (1985, 1987a,b) conducted the most detailed (10-cm scale) high-resolution stratigraphic and paleontologic analysis of the late Cenomanian macrofaunal extinctions. He recorded extinction levels as follows: 13% of 61 genera and 51% of 84 species; ammonites and inoceramid bivalves accounted for 85% of the extinction; ammonites suffered 33% extinction among 24 genera, and 74% extinction among 31 species; inoceramids suffered 92% extinction among 11 species, but no generic extinction. Harries and Kauffman (1990) and Harries (1993) extended these high-resolution studies through the early Turonian, at numerous sections across the Western Interior Basin of North America. Eicher and Diner (1985, 1989) and Leckie (1985) noted two important extinction events, or steps, among a few species of planktonic foraminifera across the C-T boundary; the late Cenomanian Rotalipora extinction is coincident with the

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 52 (CRETACEOUS) MASS EXTINCTION Figure 3.3 Compilation of high-resolution sequence stratigraphic, cyclostratigraphic, volcanic (bentonite) event- stratigraphic, and geochemical data for the Cenomanian-Turonian boundary interval at the proposed C-T boundary stratotype section (Kennedy and Cobban, 1991), along the north cuts of the Arkansas River just west of Pueblo, Colorado. Note that sea level reaches a second-order maximum flooding peak (300 m above present stand), after a series of third-order fluctuations, just above the C-T boundary. Note also the intense interval of C-T volcanism, and the rapid, large-scale geochemical perturbations of the ocean-climate system as depicted by extraordinary fluctuations in δ13C, δ18O, Corg, and trace element values. The broad positive global excursion of the δ13C record reflects a major change in oceanic carbon cycling associated with development of oceanic anoxic event II (see Arthur et al., 1985, and references therein). Trace element enrichment levels are represented in the right-hand column by right-facing arrows and abbreviations of the primary trace elements characterizing each enrichment horizon. An Ir curve is provided to show the enrichment interval, and the two major spikes, all correlative with C-T mass extinction events (from Orth et al., 1988). The figure is plotted at the same scale as Figure 3.4, and the two may be placed together to show the entire high-resolution data set for the C-T extinction interval and adjacent background conditions. first group of molluscan extinction events within a zone of Ir and other trace element enrichment (Figures 3.3 and 3.4) (Kauffman, 1988b, Figure 3.2), and the extinctions of the Whiteinella boundary fauna and the Lithraphidites acutum nannoplankton assemblage (Watkins, 1985; Watkins et al., 1993) occur within a lower Turonian interval that also includes macrofossil extinction step 6 (Figure 3.4) of Kauffman (1988b). These studies revealed an even more complex stepwise pattern of molluscan extinction, based on robust integrated range data, than originally noted by Kauffman (1984a); these data are now being tested for sampling biases to compensate for the Signor-Lipps (1982)

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 53 (CRETACEOUS) MASS EXTINCTION effect, by calculating predicted ranges for taxa at 50, 75, and 95% confidence intervals, using the Koch and Morgan (1988) and Marshall (1991) statistical tests. Initial results support the stepwise extinction hypothesis (Harries, Kauffman, and Elder, in manuscript). Figure 3.4 Compilation of ammonite, inoceramid bivalve, planktonic foraminifer and nannofossil biozones for the C-T boundary interval (left side), as they are defined at the proposed C-T boundary stratotype just west of Pueblo, Colorado (see Figure 3.3 explanation: both figures are plotted at the same scale). To the right is a compilation of the C-T boundary ecostratigraphy at Pueblo, consisting of diversity and abundance curves, stratigraphically concentrated origination (OM-1-OM-15) and extinction events, or steps (MX, MX-1, to MX-8 for molluscs; NX for nannoplankton extinctions; PFX for the Rotalipora planktonic foraminifer extinction), and to the far right, a compilation of ecozone and biofacies boundaries, reflecting ecological response to abrupt changes in benthic environments. Note the focus of extinction events around the C-T boundary, mainly in the latest Cenomanian, following a long interval of origination and buildup of diversity. Note also unexplained abrupt increase in benthic foraminifer diversity, after a long interval without benthics on the seafloor, at the initiation of macrofossil and planktic foraminifer extinction. The global aspect of the Cenomanian-Turonian mass extinction and its stepwise pattern are demonstrated by similar high-resolution stratigraphic and paleobiologic studies in England and France, and in Colombia, South America. Jeffries (1961, 1963) first documented the details of diverse macrofossil extinction patterns in the Plenus Marls of England; ammonite extinction patterns have more recently been studied by Kennedy (1971) and Wright and Kennedy (1981). Whereas these studies have not been directed specifically at mass extinction phenomena, they do show similarly distributed, stepwise disappearance of late Cenomanian macrofossils. Gale et al. (1993) have further shown that the macrofossil extinction interval oc

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 54 (CRETACEOUS) MASS EXTINCTION curs mainly within the early part of the δ13C global excursion in the same way as it does in the Western Interior Basin of North America, and elsewhere. The most detailed study of diverse microfossil extinction patterns yet conducted for western Europe are those in Jarvis et al. (1988); these patterns can be correlated precisely to North America for cosmopolitan planktic foraminifera and nannofossils. In Colombia, Villamil and Arango (1994) have recently completed a high-resolution, integrated stratigraphic analysis for the C-T boundary interval in the Magdalena Basin region, and have shown by graphic correlation nearly identical geochemical fluctuations (stable isotopes, trace elements including Ir, organic carbon), in the same stratigraphic order as found in Europe and North America, suggesting oceanic sources for these chemical perturbations. Within this geochemical framework, two of the latest late Cenomanian molluscan extinction events have been recognized, as well as the same planktic foraminifer extinctions below (Rotalipora extinction) and above (Whiteinella extinction) the C-T boundary. Microtektite-shaped (glassy?) spheres were recently discovered at two levels in the latest Cenomanian extinction interval of Colombia by Arango and Villamil (in manuscript), one level associated with the Rotalipora extinction, and a higher level associated with the first major iridium spike in the sequence, about 2 m below the C-T boundary. An apparent ecological gradation is shown by successive extinction events across the C-T mass extinction interval, suggesting varying sensitivities of different marine ecosystems to large-scale ocean-climate perturbations, and the cumulative effect of multiple perturbations through time. Johnson and Kauffman (1990) showed that extinction of tropical rudistid- dominated reefs in the Caribbean Province was initiated in the latest early and middle Cenomanian, and was largely completed by the time the extinction process began in the North Temperate Realm (e.g., western Europe and the Western Interior Basin of North America), some 520,000 yr below the C-T boundary (Kauffman, 1984a, 1988b; Elder, 1985, 1987a, b) (Figures 3.4 to 3.6). Philip (1991) observed a similar pattern of early extinction for many Cenomanian rudists in southern Europe, but with the terminal rudistid extinction apparently closer to the C-T boundary than in the Caribbean. Near or at the end of the Cenomanian reef extinctions in the Caribbean Province, marginal tropical and subtropical molluscan taxa and warm water calcareous plankton that had rapidly immigrated into the Western Interior Basin of North America along with a warm water mass from the proto-Gulf of Mexico and Caribbean basins (Kauffman, 1984b), largely became extinct 450,000 to 520,000 yr below the C-T boundary (late late Cenomanian). More temperate and cosmopolitan taxa disappeared in the latest Cenomanian and on into the basal Turonian in both North America and Europe. The C-T mass extinction occurs in association with an extraordinary interval of rapid global environmental change, and is further unusual in that it is associated with elevated temperatures (Hay, 1988) and the highest sea-level stand of the late Phanerozoic, reaching nearly 300 m above present stand (Pitman, 1978; McDonough and Cross, 1991) (see Figure 3.2 for extent of North American interior seaway). These are conditions usually considered favorable to the diversification of life. The extinction occurs at the peak of the last great greenhouse interval, with atmospheric CO2 at least four times present levels (Berner, 1991, 1994), and mean global sea surface temperatures 2°C (tropics) to 17°C (poles) higher than today (Hay, 1988, Figure 1). Thermal gradients were thus relatively low from pole to equator, and from surface to bottom waters of the global ocean system (Hay, 1988, Figure 1). This, in turn, promoted sluggish ocean circulation not dominantly driven by polar water masses as today, but rather by warm, oxygen-poor, hypersaline water masses (Hay, 1988, Figure 5) derived from subtropical evaporite belts (Barron,

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 55 (CRETACEOUS) MASS EXTINCTION Figure 3.5 Diagram showing the various methods of establishing a real-time scale for the study of the C-T mass extinction and its causes. The proposed C-T boundary stratotype section (Kennedy and Cobban, 1991) of the Bridge Creek Limestone Member, Greenhorn Formation, at Pueblo, Colorado, is shown on the left, and standard Western Interior ammonite biozones are shown in the next column. Radiometric ages (with asterisks) are from Obradovich (1993) and based on new single crystal 40Ar-39Ar age analyses. The calculated time scale based on assigning equal time durations to ammonite biozones between dated levels (third column) yields a duration for the Bridge Creek Limestone Member at Pueblo of 1.9 m.y. In the fourth and fifth columns (A,B), a time scale is calculated by assigning 100,000-yr intervals to prominent limestone beds (first and sixth columns) interpreted to represent the 100,000-yr Milankovitch orbital eccentricity cycle. The fourth column (A) starts with a radiometrically dated cycle in the middle of the series, and calibrates ages both up and down the column; this is the best scale, and most closely matches other radiometric ages and the Cenomanian biozone-based scale. The fifth column (B) starts at the top of the series and counts 100,000-yr intervals downward through the major Milankovitch cycles; this method is less accurate than A. By using this scale (fourth column, A), the total number of Milankovitch bedding rhythms (n = 46) is divided into the duration of the Bridge Creek Limestone Member (1.9 m.y.), yielding an average duration of 41,300 yr per bedding rhythm-the Milankovitch axial obliquity cycle. The same scale is used to document the average duration of regional event-bounded stratigraphic intervals (17,000 yr), the highest resolution of regional chronostratigraphic correlation across the C-T mass extinction interval.

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 56 (CRETACEOUS) MASS EXTINCTION 1993). Collectively, these conditions resulted in expansion of the oceanic oxygen minima zones, in some cases extending to the ocean floors, and the development of an oceanic anoxic event (OAE II of Schlanger and Jenkyns, 1976; Arthur et al., 1985). A globally documented positive shift in δ13C values (Scholle and Arthur, 1980; Pratt, 1985; see Figure 3.3) over the 880,000-yr span of OAE II denotes a shift in global oceanic carbon cycling and increased sequestering of organic carbon in ocean basins and epicontinental seas. A strong but dynamically fluctuating increase in total organic carbon values and petroleum source rock development resulted from these paleoceanographic events. In many parts of the world, rapid, large-scale fluctuations in marine δ18O also characterize this interval (e.g., Arthur et al., 1985; Pratt, 1985), denoting extraordinary temperature and/or salinity fluctuations, although the extent of these primary signals is difficult to calculate because of widespread diagenetic overprints in pelagic and hemipelagic sediments spanning the C-T boundary interval. A series of closely spaced, short-term, compositionally distinct, trace element enrichment events is associated, in North and South American, European, and North African data, with the early phases of large-scale, stable isotope and organic carbon fluctuations early in OAE II. These trace element spikes, in similar stratigraphic order and including at least two major iridium excursions (Orth et al., 1988, 1990, 1993) (see Figure 3.3), possibly represent a complex trace element advection sequence (sensu Berry et al., 1990; Wilde et al., 1990) associated with benthic touchdown of the Cenomanian OAE, and remobilization of sequestered trace elements from the seafloor. However, an extraterrestrial source for the iridium cannot be ruled out (Orth et al., 1988, 1990, 1993, and subsequent discussion). Collectively, these geochemical data suggest a major change in latest Cenomanian-earliest Turonian ocean-climate systems, from one of stability to one of great instability characterized by high-frequency, large-scale, ocean-climate perturbations acting on an extinction-prone, predominantly stenotopic global marine biota. These perturbations may have been the primary causes for the series of late Cenomanian extinction events seen worldwide, as they progressively exceeded the adaptive ranges of, first, tropical and, eventually, temperate taxa and ecosystems. Figure 3.6 Summary diagram: a timetable for C-T mass extinction, integrating all of the high-resolution sedimentologic, geochemical, and paleobiological data (see Appendix) collected at the Pueblo, Colorado, boundary section. The data extend through the pre-extinction, mass extinction, and survival/recovery intervals. Divisions of the data into time packages, utilizing the time scale in column A of Figure 3.5, are based on unique stratigraphic combinations of physical, chemical, and biological events (from Figures 3.3 and 3.4). Right-facing arrows denote an abrupt, major increase in a value, and left-facing arrows a major decrease. Key: SL = sea level, with standard sequence stratigraphic abbreviations; VOLC = volcanic activity (ACT) or relative quiescence, as measured by the number and thickness of volcanic ashes per unit time; BF = benthic foraminifers; PF = planktonic foraminifers; M = molluscan taxa; OM = stratigraphically focused molluscan origination level; MX = focused molluscan extinction level, or step; NX = nannoplankton extinction event; PFX = planktonic foraminifer extinction level (Rotalipora spp.). For example, using compilations (Kauffman, 1984a, 1988b; Elder, 1985, 1987a) of molluscan range data, and integrating these with stable isotope data of Arthur et al. (1985) and Pratt (1985), and trace element data of Zelt (1985) and Orth et al. (1987, 1990, 1993) for the Western Interior Basin of North America, Kauffman (1988b) noted a close correlation of the eight C-T extinction events, or steps, of Elder (1985, 1987a), and ocean-climate perturbations expressed as short-term excursions of the stable isotope and trace element record above Cenomanian background levels. In particular, the first five mass extinction steps are coincident with iridium and other noble metal trace element excursions, suggesting a cause-effect relationship. Further, the entire C-T mass extinction record in the Western Interior of North America and elsewhere occurs within the 1.2-m.y.-long stratigraphic interval of extraordinarily rapid, large-scale, fluctuations in δ13C, δ18O, Corg, CaCO3, and trace elements. The extinction occurs

GLOBAL CHANGE LEADING TO BIODIVERSITY CRISIS IN A GREENHOUSE WORLD: THE CENOMANIAN-TURONIAN 57 (CRETACEOUS) MASS EXTINCTION against a background of exceptionally well defined 41,000- and 100,000-yr Milankovitch climate cyclicity and increasing oxygen restriction in the world's oceans (OAE II) with expansion and possibly benthic touchdown of the oceanic oxygen minimum zone(s). Yet the question remains: What initiated these dynamic changes on such a short time scale? Whereas Orth et al. (1987, 1990, 1993) have cautiously suggested that the enriched late Cenomanian trace element suite, including Ir, was probably derived from oceanic sources (e.g., mantle outgassing during rapid Cenomanian-Turonian plate rearrangements, and/or from the last phases of outgassing of the Pacific superplume; Larson, 1991a,b), extraterrestrial sources cannot be ruled out. Whereas impacting is a stochastic process, and predictably spatially random (Grieve, 1982), it is not always temporally random. For example, Grieve (1982) has noted four large middle Cretaceous terrestrial impact craters with age error bars overlapping the Cenomanian-Turonian boundary: these are the Deep Bay Crater (100 ± 50 Ma), the Logoisk Crater (100 ± 20 Ma), the Steen River Crater (95 ± 7 Ma), and the West Hawk Crater (100 ± 50 Ma). Two additional craters have late Albian to Cenomanian ages of 100 and 100 ± 5 Ma. Alvarez and Muller (1984) and Strothers and Rampino (1990) have both noted that these craters comprise a statistical impact cluster, one of several with a proposed periodicity of 30 to 34 Ma. The late Albian-early Turonian (but predominantly Cenomanian) cluster of crater ages may therefore represent an impact storm, or shower (sensu Hut et al., 1987). Kauffman (1988b) pointed out that not only was the terrestrial impact record conservative for any temporal cluster of craters because of loss of record by subduction; weathering and erosion; the extent of younger sedimentary, vegetative, and ice cover on land; as well as the fact that many areas are still poorly explored (Grieve, 1982), but also that most extraterrestrial objects hitting Earth would fall into the sea. During greenhouse intervals (Fischer and Arthur, 1977) with elevated global sea-level, as in the Cenomanian-Turonian boundary interval, up to 80% or more of the Earth was covered by water. Thus, for any single terrestrial impact recorded at these times, an additional four would be predicted to have landed in the sea, and the timespan of the impact shower would be expanded over that of the terrestrial impacting record by at least a million years. These predictions are conservative because of the loss of the shallow water and terrestrial impact record through subduction, weathering, erosion, and younger sedimentary cover. However, if they are correct, the predominantly late Albian-Cenomanian impact shower would have extended across the C-T boundary. A series of meteorites or comets impacting in the sea over a relatively short period would probably result in massive evacuation of water (as steam) and debris along the impact path, major shock waves with their resultant marine tectonic effects, compression and heating of the surrounding oceanic water mass, giant tsunamis, and ultimately rapid mixing of oceanic water masses (Melosh, 1982). Oceanic feedback from such a geologically instantaneous perturbation might produce overturn of oceanic water masses, chemical advection events, and dramatic changes in the thermal and chemical character of these water masses. These changes would be preserved in the sedimentary record as dramatic excursions in the stable isotope, trace element, and Corg record beyond background levels (Kauffman, 1988b). Such oceanic perturbations would be expected to result in a prolonged and complex series of feedback processes (loops) as the ocean system sought equilibrium after each impact event. Could the dramatic chemical and thermal changes in the oceans across the Cenomanian-Turonian extinction interval have been initiated by oceanic impact, as would be predicted by the known impacting record? Emerging new data support this hypothesis. Latest Cenomanian bulk sediment samples collected by Michael Rampino and the author north of Boulder, Colorado, have yielded a few shocked quartz grains, some with multiple shock lamellae (Rampino et al., 1993; M. Rampino, personal communication, August, 1993), within an interval also characterized elsewhere by trace element (including iridium) enrichment (Figure 3.3). Further, Tomas Villamil and Claudia Arango (University of Colorado) have collected numerous microtektite-shaped microspheres, possibly glass, from at least two latest Cenomanian horizons in the Villeta Group of central Colombia, one associated with the Rotalipora foraminifer extinction, the other with a major iridium spike and molluscan extinction event. The composition of these grains is currently being analyzed; the results will bear heavily on the hypothesis of impacting as a catalyst for dramatic C-T ocean-climate perturbations and resultant steps of mass extinction. The high-resolution biological, geochemical, and sedimentological data base developed internationally for the C-T mass extinction interval is one of the most comprehensive data sets available for the study of an ancient biodiversity crisis. These global data provide a wealth of information that can be correlated precisely, at a high level of resolution, among different continental margins, epicontinental seas, and ocean basins through lineage and assemblage biostratigraphy, event and cycle chronostratigraphy, and on a broader scale, geochronology and sea-level history. We are now in a position to study and model the rates, patterns, causal mechanisms, and ecological and evolutionary aftermath of an ancient (C-T) mass extinction in a greenhouse world, at a very fine level of temporal resolution. This will, in turn, help us to understand the possible consequences of the modern biodiversity crisis on Earth.

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What can we expect as global change progresses? Will there be thresholds that trigger sudden shifts in environmental conditions—or that cause catastrophic destruction of life?

Effects of Past Global Change on Life explores what earth scientists are learning about the impact of large-scale environmental changes on ancient life—and how these findings may help us resolve today's environmental controversies.

Leading authorities discuss historical climate trends and what can be learned from the mass extinctions and other critical periods about the rise and fall of plant and animal species in response to global change. The volume develops a picture of how environmental change has closed some evolutionary doors while opening others—including profound effects on the early members of the human family.

An expert panel offers specific recommendations on expanding research and improving investigative tools—and targets historical periods and geological and biological patterns with the most promise of shedding light on future developments.

This readable and informative book will be of special interest to professionals in the earth sciences and the environmental community as well as concerned policymakers.

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