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Global Change Leading to Biodiversity Crisis in a Greenhouse World: The Cenomanian-Turonian (Cretaceous) Mass Extinction

Earle G. Kauffman

University of Colorado

ABSTRACT

The Cenomanian-Turonian (C-T) mass extinction occurred during a peak global greenhouse interval, with eustatic sea-level elevated nearly 300 m above present stand; atmospheric CO2 at least four times present levels; and global warm, more equable climates reflecting low thermal gradients from pole to equator, and from the top to the bottom of world oceans. Despite development of an oceanic anoxic event (OAE II), marine diversity was at a Cretaceous high just prior to the extinction interval; many lineages had evolved narrow adaptive ranges over millions of years under greenhouse conditions. Marine biotas were thus extinction prone. Tropical reef ecosystems experienced widespread extinction beginning near the early-middle Cenomanian boundary; major lower Cretaceous lineages of reef-building rudistid bivalves were largely extinct by middle-late Cenomanian time. Within 520,000 yr of the C-T boundary, nontropical late Cenomanian biotas experienced 45-75% species extinction, depending on the group, through a series of discrete, ecologically graded, short-term events, or steps, beginning with subtropical and warm temperate stenotopic biotas, and terminating with more broadly adapted cool temperate biotas. These extinction events were closely linked or coeval with abrupt, large-scale perturbations in the ocean-climate system, as evidenced by major fluctuations in trace elements (including Ir), stable isotopes, and organic carbon values; the rate and magnitude of these chemical and thermal perturbations progressively exceeded the adaptive ranges of various components of the marine ecosystem as the effects of late Cenomanian environmental perturbations became compounded through time. Two possible catalysts for these abrupt environmental changes are (1) expansion of the oceanic oxygen minima zone(s) to intersect both the deep ocean floor, and deeper continental shelf and epicontinental sea habitats, initiating trace element advection and chemical stirring of the oceans; and (2) oceanic impacts of meteorites and/or comets as part of the Cenomanian impact shower. Evidence is presented for both hypotheses, and a multicausal explanation for C-T mass extinction is



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Effects of Past Global Change on Life 3 Global Change Leading to Biodiversity Crisis in a Greenhouse World: The Cenomanian-Turonian (Cretaceous) Mass Extinction Earle G. Kauffman University of Colorado ABSTRACT The Cenomanian-Turonian (C-T) mass extinction occurred during a peak global greenhouse interval, with eustatic sea-level elevated nearly 300 m above present stand; atmospheric CO2 at least four times present levels; and global warm, more equable climates reflecting low thermal gradients from pole to equator, and from the top to the bottom of world oceans. Despite development of an oceanic anoxic event (OAE II), marine diversity was at a Cretaceous high just prior to the extinction interval; many lineages had evolved narrow adaptive ranges over millions of years under greenhouse conditions. Marine biotas were thus extinction prone. Tropical reef ecosystems experienced widespread extinction beginning near the early-middle Cenomanian boundary; major lower Cretaceous lineages of reef-building rudistid bivalves were largely extinct by middle-late Cenomanian time. Within 520,000 yr of the C-T boundary, nontropical late Cenomanian biotas experienced 45-75% species extinction, depending on the group, through a series of discrete, ecologically graded, short-term events, or steps, beginning with subtropical and warm temperate stenotopic biotas, and terminating with more broadly adapted cool temperate biotas. These extinction events were closely linked or coeval with abrupt, large-scale perturbations in the ocean-climate system, as evidenced by major fluctuations in trace elements (including Ir), stable isotopes, and organic carbon values; the rate and magnitude of these chemical and thermal perturbations progressively exceeded the adaptive ranges of various components of the marine ecosystem as the effects of late Cenomanian environmental perturbations became compounded through time. Two possible catalysts for these abrupt environmental changes are (1) expansion of the oceanic oxygen minima zone(s) to intersect both the deep ocean floor, and deeper continental shelf and epicontinental sea habitats, initiating trace element advection and chemical stirring of the oceans; and (2) oceanic impacts of meteorites and/or comets as part of the Cenomanian impact shower. Evidence is presented for both hypotheses, and a multicausal explanation for C-T mass extinction is

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Effects of Past Global Change on Life probable. Development of an integrated real-time scale for the Cenomanian-Turonian extinction interval, blending new 40Ar-39Ar ages from volcanic ashes (bentonites), with 100,000- and 41,000-yr Milankovitch climate cycle deposits across the boundary, allows a precise timetable for environmental perturbation and C-T mass extinction to be developed at a resolution comparable to Quaternary studies of global change. The Cenomanian-Turonian mass extinction may thus serve as a model for the rates, patterns, causes, and consequences of a global biodiversity crisis, leading to mass extinction, in a greenhouse world. INTRODUCTION The modern Earth is undergoing a geologically "instantaneous" transformation, or global change, characterized by increasing concentrations of atmospheric greenhouse gases, ozone depletion, global warming, environmental deterioration, habitat destruction, and the mass extinction of species, resulting in a global biodiversity crisis (Wilson, 1988). These extraordinarily rapid global perturbations are related largely to the overpopulation of a single species, Homo sapiens, whose numbers may already exceed the resource-carrying capacity of the Earth. Of the estimated 30 million species of plants and animals (Wilson, 1988) that probably existed on Earth prior to man's recent population explosion, more than half live within complex, easily perturbed, tropical ecosystems (e.g., rain forests and reefs) that are currently threatened by human activity. The predicted destruction of these ecosystems may result in the loss of more than half of global biodiversity. We have thus entered an early phase of a global mass extinction, but at a rate that exceeds that for nearly all well-documented ancient mass extinction events. It is imperative that we develop integrated physical, chemical, and biological data that will help us understand the processes and consequences of global change and biodiversity decline, not just from the familiar but geologically atypical icehouse world of the Quaternary, but also from past greenhouse worlds lacking permanent polar ice and cold climates. Greenhouse worlds are characterized by higher sea-level; warmer, more equable, maritime-influenced climates; expanded tropics; and a largely stenotopic global biota delicately perched on the verge of extinction. This is a world that we may soon be entering through accelerated global warming. The focus of global change research on Quaternary history is built on three main premises: (1) there is an urgent need to understand the natural evolution of Quaternary Earth systems as a baseline for assessing the staggering impact of the human species on global environments and ecosystems during the past 9000 to 15,000 yr, but especially during the past 3000 yr (the agrarian and industrial "revolutions"); (2) the physical, chemical, and biological processes characterizing Quaternary Earth history can largely be observed, interpreted, and modeled from modern observations—the Uniformitarian approach; and (3) the preservation and resolution of Quaternary physical, chemical, and biological data relevant to understanding the dynamics of global change are unparalleled in the geologic record. These are justifiable approaches to an urgent problem. Yet, when considering the current rate of global warming associated with modern environmental changes and the expanding biodiversity crisis, it is necessary to refocus some of our global change research to understanding the greenhouse intervals that characterized so much of Earth history and to study in detail not only the processes, but also the ecological and genetic consequences, of global mass extinction. Research focused on ancient global change characteristically depends on achieving a resolution among physical, geochemical, and paleobiological data that is adequate for interpretation and predictive modeling of modern global change phenomena. In searching for a geological test case for the study of ancient global change with relevance to the modern Earth, the Cretaceous Period emerges as one of the best candidates. In particular, the middle Cretaceous presents a unique opportunity to document and model dynamic changes in ocean-climate systems associated with a global mass extinction (Cenomanian-Turonian boundary bioevent) in a greenhouse world, and then to compare these with the environmental and ecological crisis on the modern Earth as it potentially moves from an icehouse to a greenhouse state. This chapter demonstrates that (1) resolution of middle Cretaceous physical, chemical, and biological data are sufficient to make comparisons with major trends in Quaternary ocean-climate systems, and biodiversity decline; (2) systems of chronology and regional correlation of environmental and biological changes in the Cretaceous are comparable to those used in Quaternary global change studies; (3) significant similarity in patterns of environmental change and biological response exists between Cretaceous and Quaternary extinction intervals to allow construction of predictive models from Cretaceous observations that are applicable to our understanding of the potential long-term consequences of the modern biodiversity crisis.

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Effects of Past Global Change on Life HIGH-RESOLUTION APPROACH TO DOCUMENTING ANCIENT ENVIRONMENTAL CHANGE Quaternary analyses of global environmental changes, and biological responses to them, are conducted at scales of years (varves) to tens of thousands of years with nearly continuously represented data (Figure 3.1), commonly sampled at the 1- to 10-cm stratigraphic scale. Excellent preservation of original sedimentological, geochemical, and paleobiological materials allows integrated environmental and ecological analyses through small increments FIGURE 3.1 A comparison between the resolution of Quaternary data and time scales used in global change studies, and that possible from high-resolution Cretaceous paleoenvironmental studies in fine-grained marine facies. Quaternary data taken from Oeschger and Arquit (1991): Lower field of small black circles represent CO 2 values obtained from centimeter-scale (100- to 200 yr) intervals in ice cores at Byrd Station, Antarctica; the thin line graph above it connects d18O values from the same core and sample set. The triangles connected by a heavy dashed or solid line and superimposed on the Quaternary data set represent a typical high-resolution, centimeter-scale (1000-yr) sample interval for middle Cretaceous global change and mass extinction research, applied to the same data set. Note that all major climate trends shown by high-resolution Quaternary data are also shown by the data set representing Cretaceous sample intervals, and only the smallest fluctuations are lost between the 100-yr and 1000-yr scales. of time. These analyses reveal the dynamics and mechanisms of global change at several scales (e.g., Thompson, 1991; Webb, 1991). However, claims that older stratigraphic data sets are not completely enough preserved or highly enough resolved to contribute significantly to global change research and predictive modeling, are rejected. High-resolution event stratigraphic methodology (Kauffman, 1988a; Kauffman et al., 1991) provides interdisciplinary data, at 100- to >1000-yr Quaternary scales, for Phanerozoic strata. Rock accumulation rates (RARs) for marine strata, which may preserve the most continuous and diverse record of Phanerozoic environmental changes, range from <1 cm/1000 yr (basinal fine-grained facies) to >1 m/1000 yr (e.g., in coarse-grained turbiditic, slope fan, shoreface, foreshore, and estuarine channel facies). Whereas more rapidly deposited strata allow finer stratigraphic time divisions to be sampled easily, these facies commonly reflect episodic high-energy sedimentation events separated by erosive intervals and do not preserve a long, continuous record of environmental change. More slowly but more continuously deposited basinal marine and lacustrine sequences characterized by shales, mudstones, and biogenic pelagic or hemipelagic facies provide the best Phanerozoic record of global change at scales comparable to long-term Quaternary records. Such data do exist and have been gathered largely through the application of methods inherent in high-resolution event stratigraphy (HIRES: Kauffman, 1988a; Kauffman et al., 1991). HIRES focuses the analysis of stratigraphic sections on the centimeter-scale in search of event and cyclic stratification (see papers in Einsele et al., 1991). These events are expressed as physically unique surfaces or thin intervals; as short-term geochemical excursions from background values; as short-term evolutionary and ecological phenomena; and as depositional cycle and hemicycle boundaries, all with regional to interregional extent. Thus, stratigraphic deviations from background patterns are emphasized, and data from various disciplines are integrated into holistic interpretations of these depositional events. Initially, these data comprise a working chronostratigraphy for regional correlations at very high levels of resolution (days to hundreds of years per event surface or thin stratigraphic interval, typically spaced at intervals hundreds to thousands of years apart; Kauffman, 1988a). The correlation potential for HIRES exceeds that of the best biozonation, geochronology, or magnetostratigraphy (Kauffman et al., 1991). Ultimately, a diverse, high-resolution physical, chemical, biological, and cyclostratigraphic data base collected continuously over a significant interval of Phanerozoic time will enhance integrated analysis of dynamic changes in regional to global environments, and biological responses

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Effects of Past Global Change on Life 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

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Effects of Past Global Change on Life 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. (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). 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

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Effects of Past Global Change on Life 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 d13C, d18O, Corg, and trace element values. The broad positive global excursion of the d13C 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)

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Effects of Past Global Change on Life 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. 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). 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

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Effects of Past Global Change on Life curs mainly within the early part of the d13C 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,

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Effects of Past Global Change on Life 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.

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Effects of Past Global Change on Life 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.). 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 d13C 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 d18O 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. 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 d13C, d18O, Corg, CaCO3, and trace elements. The extinction occurs

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Effects of Past Global Change on Life 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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Effects of Past Global Change on Life lutionary rate. However, the same scale deviates by 440,000 to 510,000 yr from ages calculated by the same method for early Turonian ammonite biozones. This suggests that postextinction ammonite evolution takes place at much more variable rates than found among pre-extinction ammonite lineages. In both Milankovitch cycle-based time scales, therefore, the assumption that ammonite biozones have approximately equal PBC 4, and relatively stable evolutionary rates through the C-T mass extinction-recovery interval is not supported. The calibrated time scale based on integrating new 40Ar-39Ar ages with the 100,000-yr Milankovitch cycle history, with calculations starting from a radiometric age in the middle of the cyclic depositional sequence, is thus the most accurate. This scale is used below to calibrate the physical, chemical, and biological components of the Cenomanian-Turonian mass extinction-recovery interval. Even finer resolution for dating and correlation can be obtained by utilizing physical and chemical event deposits/horizons within the same section. Most of the 112 event beds/horizons in the Bridge Creek Limestone are bentonites, trace element, stable isotope, and organic carbon enrichment levels (spikes), and sea-level or climate cycle boundaries with regional to interregional dispersion. Within the time scales developed for this interval from radiometric and climate cycle data, the event-bounded intervals of the Bridge Creek Limestone Member, spanning the C-T boundary, have an average duration of 17,000 yr. This highly refined C-T chronology for the Western Interior Cretaceous Basin, integrating radiometric, event/ cycle chronostratigraphic, and biostratigraphic data, closely parallels common time scales utilized in Quaternary studies of global change (Figure 3.1), ecosystem destruction, and biodiversity loss (e.g., see papers in Bradley, 1991). A TIMETABLE FOR CENOMANIAN-TURONIAN MASS EXTINCTION The integration of high-resolution stratigraphic, geochemical, and paleobiological data depicting late Cenomanian-early Turonian global change, with a real time scale combining 40Ar-39Ar volcanic ash ages with Milankovitch climate cycle deposits of known time duration, makes it possible to evaluate the rates, patterns, and timing of the C-T mass extinction and associated environmental perturbations within a greenhouse world (see Appendix). Figures 3.3 and 3.4 show the integrated high-resolution data for the study of the C-T mass extinction at Pueblo, Colorado. Figure 3.5 shows the derivation of the real time scale from this data base. Together, these allow derivation of the timetable for environmental perturbations and mass extinction in the C-T boundary interval at Pueblo (shown in the Appendix). From oldest (1) to youngest (22), these divisions are based collectively on physical, chemical, and biological characteristics that are unique to each interval. Figure 3.6 summarizes these relationships through time. These data can be used to model the patterns of extinction, survival, and recovery in a greenhouse world, and to enhance the development of predictive models for the modern global crisis. INTERPRETATIONS AND CONCLUSIONS The Cenomanian-Turonian mass extinction is among the best-studied biotic crises in Earth history, and is unusual in that it occurred during one of the greatest eustatic highstands of the Phanerozoic, in a relatively equable, warm, greenhouse world usually considered favorable for the diversification of life. High sea-level and widespread development of epicontinental seas in this interval have ensured the preservation of complete C-T boundary sections worldwide. In most sections, there is little or no sedimentologic expression of the extinction interval, which is preserved primarily within relatively monotonous, fine-grained neritic, hemipelagic, and pelagic facies characterized by well-defined Milankovitch bedding cycles. However, integrated geochemical and paleobiological data, obtained through very high-resolution (1 to 10 cm) stratigraphic sampling, reveal dynamic environmental changes in ocean-climate systems during this global mass extinction interval. The C-T boundary interval initiates with a series of tropical extinctions among rudistid bivalves and other reef-associated taxa, beginning at the end of the early Cenomanian (95.9 Ma) and culminating 0.5-1.0 m.y. below the C-T boundary (about 93.9 to 94.4 Ma); dating of this final tropical extinction event and its correlation with well-studied temperate records, however, is imprecise. Johnson and Kauffman (1990) recorded a loss of three rudistid genera (0.1 per taxon extinction rate), and 32 species (0.2 per taxon extinction rate), which comprise 43% of the lower Cenomanian rudistid species, 26% of the middle Cenomanian species, and 47% of the upper Cenomanian rudistid species in the Caribbean Province. Tropical extinction patterns are similar in various well-studied Italian and French sections, where no reefs are known within 3 to 10 m of the C-T boundary. In the Western Interior of North America, where the best integrated physical, geochemical, and temperate to subtropical biological record of the C-T extinction interval has been compiled, an integrated dating matrix of 40Ar-39Ar radiometric ages, and bedding rhythms, representing the 41,000-yr and 100,000-yr Milankovitch climate cycles, has allowed development of a relatively precise timetable for Cenomanian-Turonian boundary extinctions and their probable causal mechanisms. The Pueblo, Colorado, ref-

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Effects of Past Global Change on Life erence section is utilized to illustrate this chronology and its application to interpreting rates, patterns, and probable causes of C-T mass extinction. The first geochemical perturbations associated with the C-T extinction interval in this setting occur 880,000 yr below the C-T boundary; the first molluscan extinction event (step) occurs 520,000 yr below the C-T boundary, coeval with initiation of a series of 10 successive trace element enrichment horizons, probably representing an oceanic advection sequence. The first five of these enrichment intervals have Ir peaks that are two to five times greater than background levels, and are precisely coeval with the first five C-T extinction steps; two of these levels have recently yielded possible microtektites and rare shocked quartz grains in Colombia and Colorado, respectively. Similar trace element enrichment patterns occur at many sections in North America, western Europe, Colombia, and North Africa. The first C-T extinction events at Pueblo also mark the first extraordinary peaks of stable isotope and organic carbon fluctuations, relative to background values. An overall enrichment of Corg across the C-T boundary records development of an oceanic anoxic event (OAE II). Ocean-climate systems are therefore highly perturbed, with the rates and magnitude of change in temperature and ocean chemistry well above Cretaceous background levels. In the North American Interior, extinction among subtropical to temperate lineages spans 1.46 m.y. from the latest Cenomanian through the middle-early Turonian, and appears punctuated or stepwise in nature based on regionally composited, robust range-zone plots of both macro- and microfossil species. The major part of the extinction interval spans 520,00 yr of the latest Cenomanian and comprises five steps of macrofaunal (mainly molluscan) extinction and two microfossil extinction levels. Extinction events occur successively, therefore, about every 100,000 yr. Three smaller macrofossil and two microfossil extinction events occur 230,000 to 940,000 yr above the C-T boundary within the survival and early recovery intervals. Given that Cenomanian marine organisms over much of the world had evolved over millions of years into a warm, more equable greenhouse world characterizing Cretaceous eustatic sea-level rise, it is probable that most lineages had relatively narrow adaptive ranges with sharp ecological thresholds, especially in terms of thermal range and water chemistry, and that they were thus extinction prone. These biotas would have been easily stressed by even small-scale environmental perturbations of the water column. The very close correlation of mass extinction steps with short-term trace element enrichment events (advection events?), and with rapid, large-scale fluctuations in the temperature, salinity, and oxygen of the water column, suggests that the extraordinary rate and magnitude of these environmental shifts directly caused the series of ecologically graded C-T extinction events, or steps, as they progressively breached the narrow adaptive ranges of Cenomanian-Turonian species, and ecosystems, adapted to an equable greenhouse world. Predictably, extinction initiated among tropical reef ecosystems, and terminated with more temperate-adapted and cosmopolitan groups. Yet the cause of these marine environmental perturbations remains an enigma; several viable hypotheses exist. Hallam (1984) and others have suggested that many mass extinctions, including the C-T boundary event, resulted from oxygen depletion in oceans and relatively deep epicontinental seas associated with expanding oxygen minima zones and the establishment of oceanic anoxic events. This mechanism would be especially effective during global greenhouse conditions, when ocean circulation was sluggish and may have been driven largely by low-oxygen warm saline bottom waters derived from marginal tropical evaporite belts, and when productivity was high. Widespread dysoxic to anoxic conditions would ecologically stress global normal marine biotas and could have contributed to C-T mass extinction. However, this hypothesis is rejected as the primary cause for C-T extinction because (1) widespread oxygen depletion in the oceans characterized most of the time interval between the Aptian Mass Extinction and the C-T boundary interval, including the long Albian OAE Ic, without causing mass extinction. If anything, these were longer and more severe intervals of oxygen depletion than the one that coincided with C-T extinction. Instead, C-T extinction comes at the end of the dysoxic to anoxic interval (Hay, 1988, Figure 3.3), during a set of dynamic geochemical changes leading to reoxygenation of deeper portions of the oceans. C-T extinction events were more likely related to the rate and magnitude of these changes than to oxygen depletion itself. (2) Because of the long middle Cretaceous interval of oceanic oxygen depletion associated with global greenhouse warming and elevated sea-level, a large proportion of deeper water marine biotas became adapted to low-oxygen conditions and are commonly associated with organic-rich black shales (e.g., many Inoceramidae and other bivalves; Kauffman and Sageman, 1990; Sageman et al., 1991, 1994). Yet many of these same lineages became extinct in the C-T boundary interval as conditions shifted rapidly toward more oxygenated oceans in the basal Turonian. If anything, a case for oxygen poisoning might be made for the extinction of some low-oxygen adapted taxa. A second explanation for C-T marine mass extinction might lie with trace element poisoning associated with advection events from the deep ocean and from epicontinental basins, as suggested for Paleozoic mass extinctions by Berry et al. (1990) and Wilde et al. (1990). Potentially toxic trace elements were greatly enriched in the oceans

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Effects of Past Global Change on Life and deeper portions of epicontinental seas during accelerated outgassing associated with large-scale middle Cretaceous plate reorganization and development of the Pacific superplume (Larson, 1991a,b). These trace elements could have been concentrated as oxides and carbonates in basinal Cretaceous sediments in the presence of at least moderate amounts of benthic oxygen, especially during high sea-level and offshore sediment starvation; trace elements could also have been sequestered in solution in low-oxygen to anoxic water masses (OMZs). Expansion and benthic touchdown of the oceanic oxygen minimum zone during the Cenomanian-Turonian OAE would have remobilized sequestered trace elements from the seafloor and caused progressive advection of potentially toxic chemicals through the water column, with profound effects on the global marine biota. If the base of the food chain was affected by these advection events, it would have caused a far-reaching set of negative ecological feedback loops within the trophic web. The precise correlation of the first five C-T extinction steps, and two of the succeeding steps, with trace element enrichment horizons in epicontinental and continental shelf deposits of four continents, strongly supports the hypothesis of advection and trace element poisoning as a partial cause of C-T mass extinction. The relatively shallow water trace element enrichment layers found to be associated with C-T extinction events in high-resolution stratigraphic analysis, probably represent depositional sites of advecting trace elements from deeper ocean sources, situated at and above the oceanic redoxcline between the OMZ and the mixing zone. A third hypothesis for C-T mass extinction, which draws on events described in the two previous hypotheses, focuses on the extraordinary rates and magnitudes of ocean-climate changes associated with the 1.46-m.y.-long C-T boundary interval, and their effects on a global biota narrowly adapted for the most part to the equable greenhouse environments that had been developing throughout the early and middle Cretaceous. Data from Pueblo, Colorado (Figures 3.3 and 3.4) are characteristic of many global boundary sections and show a series of exceptionally rapid, large-scale shifts in organic carbon values (representing rapid shifts in benthic oxygen levels); in d13C values (representing changes in carbon cycling) within the global positive d13C excursion; in d18O values (possibly representing rapid salinity and/or temperature changes); and in trace element values (probably representing one or more oceanic advection sequences). Virtually all late Cenomanian extinction events, and some lesser ones in the early Turonian, are correlative with one or more of these rapid, large-scale geochemical fluctuations. Whereas these geochemical signals can be strongly modified by diagenetic processes, the fact that similar changes occur in virtually all well-studied global C-T boundary sections suggests that they represent a primary ocean-climate signal. Major changes in ocean chemistry and temperature around the C-T boundary could well have been the primary cause of extinction events as they progressively exceeded the narrow adaptive ranges of many stenotopic Cenomanian-Cretaceous lineages. Of special interest in this theory is the general correlation of many apparently rapid environmental fluctuations with bedding rhythms representing 41,000- and 100,000-yr Milankovitch climate cycles (Barron et al., 1985; Fischer et al., 1985; Kauffman, 1988a; Glancy et al., 1993; Figures 3.3 and 3.5 herein). On the one hand this may suggest diagenetic modification of a primary signal in carbonate-rich versus carbonate-poorer facies of the C-T boundary interval, but to the degree that it represents a primary signal, it suggests that the Milankovitch climate cyclicity may have acted as an independent catalyst that drove an environmentally perched, greenhouse ocean-climate system to even greater levels of change, at rates dictated by the climate cycles themselves. Finally, the possibility of extraterrestrial influences on the C-T mass extinction cannot be ruled out. The precise correlation of the first five late Cenomanian extinction events, or steps, with Ir enrichment of two to four or five times background levels leaves open the possibility of extraterrestrial sources for the iridium. Orth et al. (1989, 1990, 1993) have been cautious in suggesting extraterrestrial origins, pointing out instead an apparent similarity of the overall C-T trace element suite to those originating from deep mantle outgassing, and the fact that the C-T boundary interval was also a time of major plate rearrangement and superplume development (Larson, 1991a,b). On the other hand, four temporally clustered late Albian to late Cenomanian terrestrial impact craters are known with age error bars that overlap the C-T boundary (Grieve, 1982), suggesting an impact storm, or shower (sensu Hut et al., 1987); this is a conservative estimate of terrestrial impacts, when considering the amount of Cenomanian surface that has been subducted or is now covered by younger sediments/strata, vegetation, ice, and especially water. Because impacting is predictably spatially random (Grieve, 1982), and the late Cenomanian world was 80 to 82% covered by water near eustatic highstand, it is likely that at least four of five potential impactors would have fallen in the world seas and oceans during this impact shower. This statistically projects at least 20 impacts during the Cenomanian and early Turonian interval, the majority of which would be aquatic. Oceanic impacting would cause repeated, short-term stirring events. This hypothesis further predicts an extended duration for the Cenomanian impact shower, with high probability that it would overlap the C-T mass extinction interval. Recent discoveries of possible microtektites (Colombia) and multilamellate shocked quartz grains (Colorado) at two

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Effects of Past Global Change on Life levels within the latest Cenomanian extinction interval (discussed in text) support this hypothesis. These new finds suggest at least a partial extraterrestrial origin for late Cenomanian iridium, and for dramatic changes in ocean chemistry associated with some of the major early extinction events in the Cenomanian-Turonian interval. If the shocked quartz, at least, represents a younger Cenomanian terrestrial impact than previously known, for which a crater has not yet been found, a minimal number of five Cenomanian terrestrial impacts is now known, and these predict at least 20 additional aquatic impacts during the Cenomanian impact shower. Is there a linkage among these varied data, their interpretations, and the causal mechanisms of mass extinction, especially the abruptness with which large-scale geochemical fluctuations and correlative extinction events initiate and perpetuate in the late Cenomanian and into the early Turonian? There is a high probability that such a relationship exists, that the C-T mass extinction was multicausal, with meteor/comet impacts into Cenomanian oceans—already highly stratified, largely oxygen-depleted, and rich in sequestered trace elements—causing rapid stirring, overturn, and advection events, and dramatically changing the thermal and chemical regimes of the water column up into the mixing zone. Each successive oceanic impact would set in motion a series of dynamic feedback processes expressed as rapid, large-scale, geochemical and thermal fluctuations such as those associated with individual mass extinction events, or steps, throughout the C-T boundary interval. These ocean-wide perturbations, acting on a largely stenotopic, extinction-prone marine biota, were apparently the direct causes of the ecologically graded C-T extinction events as they progressively exceeded the adaptive ranges of, first, lineages within tropical ecosystems, secondly subtropical to warm temperate lineages, and eventually more temperate lineages. Between impacting events, oceanic feedback processes, perhaps driven independently in part by Milankovitch climate cycles, might continue for perhaps thousands of years, seeking equilibrium. Yet each successive oceanic impact within the Cenomanian shower would reset the perturbation clock, driving extinction for hundreds of thousands of years and, in the end, cumulatively affecting more extinction resistant, temperate ecosystems. This hypothesis best fits the available high-resolution data base for the C-T boundary interval, one of the best-studied mass extinction intervals in the world. However, it is a hypothesis in need of extensive testing, especially in the recognition of impacting events in the deep ocean through sedimentological, paleobiological, and geochemical sensing. Ongoing research on the C-T mass extinction interval thus provides us with an extensive integrated data base, blending sedimentologic, geochemical, and paleobiological data at a level of stratigraphic resolution that equals that of broader-scale (100- to 1000-yr observational scale) Quaternary studies of global change. This study reveals the complex dynamics of change within ocean-climate systems, and biological response to them, at peak development of a greenhouse world—a world toward which we may be heading rapidly as a result of modern global warming and ozone depletion. Of greatest importance is the ability, in older Cretaceous rocks, to develop a timetable for extinction, survival, and recovery spanning one of the major global mass extinctions. From these data, we can develop predictive models, within a high-resolution temporal framework, for other mass extinctions, including the one currently in progress as a result of overpopulation, habitat and ecosystem destruction, and resource depletion at the hands of the human species. It should be of concern to us that modern rates of biodiversity loss exceed the extinction rates of well-studied Cretaceous mass extinctions, including those clearly associated with large bolide impacts. Even more frightening is the prediction from the fossil record that the complex tropical ecosystems, reefs, and rain forests of today, which contain more than half of the global biodiversity, are commonly the first to disappear in ancient mass extinctions and the last to ecologically recover, some 2 to 10 m.y. later (Kauffman and Fagerstrom, 1993). These tropical ecosystems are the harbingers of the mass extinction process and long-term biological crisis on Earth. They alone, once gone, account for enough global biodiversity loss to reach mass extinction levels (>50% of global species). At the rate of global destruction today, tropical rain forests and to a lesser degree tropical reefs will be largely decimated by the year 2500, and certainly by A.D. 3000. Simberloff (1984) estimated that only 2.5% of the neotropical rainforests would remain by 2050, with a resultant loss of more than 50% of the biodiversity of the Western Hemisphere, at current rates of destruction. This may be conservative. Even so, if research on ancient mass extinctions can act as a model for the future of the modern biodiversity crisis, it may be millions of years, without human perturbation, before similar ecosystems will become reestablished in the tropics. Research on ancient mass extinction, survival, and recovery (radiation) intervals can play an important role in understanding the consequences of modern global change phenomena, and in making long-term predictions concerning the present biodiversity crisis-which is already spiraling toward mass extinction levels. REFERENCES Alvarez, W., and R. A. Muller (1984). Evidence from crater ages for periodic impacts in Earth, Nature 308, 718-720.

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Effects of Past Global Change on Life APPENDIX Rates, Patterns, and Timing of the C-T Mass Extinction and Associated Environmental Perturbations (see Figure 3.6) LATE CENOMANIAN BACKGROUND CONDITIONS The extinction of Caribbean reef communities reached its peak prior to the events described below for the subtropical to temperate Western Interior Province. Precise correlation between the tropics and the north temperate realm is difficult at present. Oxygen isotope values are relative to the Pedee belemnite standard (PDB). 150,000-yr duration: A moderate negative shift in d13Corg to lowest Cenomanian values (-27.7%o); positive shifts in d18O and total organic carbon (TOC; wt%) to average late Cenomanian values (-8%o and 3.5-4 wt%, respectively). Several moderately elevated peaks in molluscan diversity and abundance. Organic carbon storage occurs in the benthic zone of dysoxic stratified seas, but with dynamic, episodically oxygenated bottom waters. Falling sea-level culminates in a third-order sequence boundary at the top of this interval. 50,000-yr duration: A moderate positive shift in d13Corg to average late Cenomanian values (-26.5 to -27%o) a negative shift in d18O values to a late Cenomanian low of -10.5%o, and major decrease in TOC to 1.2 wt%. An oxygenated interval in the benthic zone during early sea-level rise leads to moderate benthic molluscan diversity and sequestering of Mn in benthic sediments. A modest extinction among low-oxygen adapted benthic molluscs and ammonites is followed by an origination and/or immigration event among benthic molluscs adapted to somewhat higher oxygen levels. 60,000-yr duration: Return of d18O values to normal late Cenomanian values; a major increase in TOC to a late Cenomanian high of 4.5 wt%, and sharp increase in U-Th values, both indicating strong benthic oxygen restriction and a stratified water column during late sea-level rise. High surface productivity leads to organic carbon storage. Despite this, a significant origination event occurs, especially among low-oxygen adapted benthic molluscs and pelagic ammonites, leading to a modest increase in molluscan diversity. 120,000-yr duration: Relatively stable oceanographic conditions associated with third-order sea-level highstand and active volcanism in the basin. Organic carbon levels remain very high, reflecting a stratified, dysoxic middle to lower water column. An origination interval among mainly benthic molluscs leads to moderate abundance and diversity values. 50,000-yr duration: Two abrupt, major trace element (Ti, V, U) enrichment events occur against otherwise stable background geochemistry during a modest third-order sea-level fall. These trace element spikes are associated with major short-term increases in molluscan abundance and diversity, and a moderate origination event, mainly among benthic bivalves. Biological trends possibly reflect elevated nutrient levels. 30,000-yr duration: Stable oceanographic background conditions are associated with late phases of a third-order relative sea-level fall and a molluscan origination event. A third-order sequence boundary, reflecting maximum relative sea-level fall, caps the sequence. 90,000-yr duration: Relatively stable oceanographic background conditions during early sea-level rise are associated with active explosive volcanism and ash falls, and with an abrupt U-Th enrichment. Significant increases in molluscan diversity and abundance may reflect nutrient enrichment. 90,000-yr duration: The final stable phase of oceanographic background conditions prior to the C-T perturbed interval is associated with active explosive volcanism; numerous ash falls: and successive, short-term, trace element enrichment events (Mn-Co followed by U). An unexplained decrease in molluscan diversity and abundance (extinction?) is followed by a modest origination event. Planktonic foraminifers show a marked diversity increase (10-11 species) after a long period of stable background levels. Lowering of relative sea-level continues. 220,000-yr duration: Initiation of the geochemically perturbed interval of the C-T boundary sequence, 880,000 yr below the boundary, is associated with increased volcanism and rapid northward immigration of southern warm water masses; major positive excursions of d13Corg (initiating a global oceanographic event), d18O, and TOC reflect a trend toward normalization of marine surface waters in the basin, but significant stratification of the underlying water column, lowering of benthic oxygen levels, and possible immigration and expansion of the oceanic oxygen-minimum zone. These events mark initiation of the global oceanic anoxic event (OAE II) that characterizes the C-T boundary interval worldwide. They are associated with rapid northward immigration of warm-temperate to subtropical molluscs and microbiotas from Gulf Coast and Caribbean sources, a marked origination event among shallow water benthic and pelagic lineages, and the first reoccur-

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Effects of Past Global Change on Life rence of benthic foraminifers after >700,000 yr of environmental exclusion from the benthic zone. There is a general decrease in benthic molluscan diversity. 140,000-yr duration: Oceanic environments continued to show dynamic fluctuations as southern water masses expanded their rapid northward immigration, and global sea-level approached its highest Mesozoic-Cenozoic stand. The value of d13Corg reached its first of four positive late Cenomanian peaks (-24.5%o), d18O declined sharply, U-Th became enriched in benthic sediments, and TOC decreased rapidly to near zero after a million years at high levels, reflecting a major short-term oxygenation event in the benthic zone. Moderate increases in molluscan and planktic foraminiferal diversity, a major diversification among benthic foraminifers, and a final origination event among warm water molluscs, characterize this interval. Thus, conditions for life seemed highly favorable, with increasing benthic and pelagic diversity, just prior to the abrupt initiation of the C-T mass extinction interval during the next 50,000 yr interval. LATE CENOMANIAN MASS EXTINCTION High-resolution geochemical and paleobiological data suggest that, after the widespread elimination of reef ecosystems in the Caribbean Province by middle late Cenomanian time (Johnson and Kauffman, 1990), the last 520,000 yr of the Cenomanian was characterized by chaotic, short-term, large-scale perturbations in the ocean-climate system. The rate and magnitude of these changes exceeded the adaptive ranges of diverse, extinction-prone marine organisms, narrowly adapted to an equable, maritime-dominated, warm greenhouse world, near one of the highest peaks of Phanerozoic eustatic sea-level rise and global warming. These successive perturbations, most of less than 100,000-yr duration, caused steps of mass extinction that were ecologically graded, first affecting tropical to subtropical taxa, subsequently warm temperate lineages, and finally the cooler temperate and more cosmopolitan elements of marine ecosystems. These environmental perturbations, and resultant C-T extinction events, are as follows. 50,000-yr duration: Stable isotope and TOC values are similar to (10) and atypical of late Cenomanian background conditions. This interval is characterized by the first of 11 successive phases of trace element enrichment (Ir, C, Ni, Sc peaks) above background levels; these are interpreted as a dramatic series of oceanic advection events associated with benthic touchdown of the oceanic oxygen minimum zone during OAE II, remobilization of sequestered trace elements in oxygen-deficient waters, and their reprecipitation on oxygenated substrates above the redoxcline. This trace element enrichment and the numerous volcanic ash falls changed marine chemistry and nutrient levels. Nutrient enhancement may have initially caused increases in diversity and abundance of planktic and benthic foraminifers, and molluscs. The initial regional step of the C-T mass extinction (MX1A), however, eliminated several subtropical to warm-temperate molluscan lineages, and was correlative with the first trace element enrichment peak, including iridium. Contouring of trace element values at numerous localities in the basin (Orth et al., 1993) shows a proto-Caribbean source, with values diminishing northward into Canada. A second extinction event, characterized by the abrupt loss of keeled rotaliporid foraminifers (PFX), occurs at the boundary between this and the succeeding interval, with initiation of a second zone of Ir and other trace element enrichment (see 12), and possible microtektite concentrations (in Colombia). 80,000-yr duration: A second short-term trace element spike (C, Mn, Ni, Sc advection) characterizes this interval. An abrupt positive d18O excursion is coupled with further drop in TOC values and continued active volcanism. Nutrient enrichment may have led to increases in benthic foraminifer diversity and abundance, but cannot explain decline in both molluscs and planktic foraminifers, reflecting the effects of the planktic foraminifer extinction (PFX) event (''Rotalipora extinction") at the base of the interval. 40,000-yr duration: One of the most geochemically dynamic intervals of the C-T extinction, associated with high sea-level and active volcanism. Two successive trace element enrichment levels, the first characterized by Ir and C, the second by Ni and Sc spikes, may record a rapid trace element advection during downward expansion of OAE II to the seafloor. Modest negative excursions of d13C and d18O, and a small positive Corg spike, are associated with the trace element excursions, describing dynamic ocean-climate systems. Significant increases in abundance and diversity of molluscs, and planktic and benthic foraminifers, may reflect increased nutrient levels. A short-term extinction step (MX1B), during which some subtropical and warm temperate molluscs disappeared, was associated with iridium enrichment. 20,000-yr duration: Rapid, large-scale geochemical fluctuations continue as perturbations of the ocean-climate system intensify. Two short-term trace element enrichment levels, with enhanced Ir, Pt, Sc and C followed by Mn and C enrichment, record additional trace element advection from the seafloor. A second major positive d13C spike within the global d13C excursion reaches maximum Cretaceous levels in this region (-24.2%o), and is associated with sharp positive followed by rapid negative d18O

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Effects of Past Global Change on Life excursions, and depletion of Corg to near zero. Benthic foraminifers show a significant diversity increase: a modest origination event (OM9) occurs among molluscs. The major effects of these geochemical perturbations are two successive, large-scale mass extinction events—a first among subtropical and warm temperate molluscan lineages (MX2), directly correlative with the Ir-rich level, and shortly thereafter (15), a major extinction among southern, warm water elements (MX3). 20,000-yr duration: As in the stratigraphically lower sections (13, 14), rapid geochemical fluctuations in trace elements, stable isotopes, and Corg levels describe a highly perturbed, unstable ocean-climate system. Significant positive excursions occur in d13C, d18O, and Corg values, and a final major trace element enrichment layer (advection event) containing Ir, Pt, and C is directly associated with one of the largest warm temperate molluscan extinction events of the C-T interval (MX3). This is followed rapidly by a modest molluscan radiation (OM10), but the diversity and abundance of molluscs and benthic foraminifers decrease sharply through the interval. 170,000-yr duration: Geochemical fluctuations wane, but a strong positive d13C spike marks the last regionally correlative spike within the global positive d13C interval. It is associated with sharp negative d18O and Corg excursions, reduction in molluscan and planktic foraminifer diversity and abundance, and a moderately strong temperate molluscan extinction step (MX4). HIGHEST CENOMANIAN EVENTS 140,000-yr duration: Dynamic changes in the oceanclimate system return, with very rapid positive to negative d13C excursions, negative to positive d18O excursions, very high global sea-level, and a moderate U, Th, and trace element enrichment level. The rapidity and large scale of these oceanographic fluctuations exceeded the adaptive ranges of a large diversity of warm temperate molluscan genera and species. This produced the largest (terminal) Cenomanian extinction event in the C-T mass extinction interval, predominantly among temperate and cosmopolitan ammonite and bivalve lineages (MX5), which disappeared at or just below the Cenomanian-Turonian boundary. BASAL TURONIAN EVENTS 80,000-yr duration: The last extinction steps of the CT boundary interval occur in association with population expansion among surviving clades, and early radiations of new lineages in the basal Turonian (18-20 herein). This interval is characterized by active volcanism, a sharp negative excursion in d13C, and rapid negative-positive-negative excursions in d18O. Marked decline in molluscan abundance and diversity is followed shortly by the first Turonian origination event (OM12). Sea-level is nearly at its highest Mesozoic stand (>300 m above present stand). 150,000-yr duration: Maximum eustatic highstand (>300 m above present stand) was accompanied by sharp increase, then decrease in d18O values; a sharp decrease, then increase in TOC levels; and the final two trace element enrichment levels (Co, U, Th, followed by Mn, U, and V enrichment) of the C-T oceanic advection interval. A modest buildup in diversity and abundance among temperate molluscs was interrupted by a small extinction event (MX6). FINAL PHASES OF THE C-T MASS EXTINCTION 230,000-yr duration: Strong negative excursions of d13C and TOC, and a strong positive excursion of d18O, record the final major perturbations of the C-T boundary interval. Molluscan diversity and abundance fluctuate wildly within the interval, from low to high values. An important molluscan radiation (OM13) is sandwiched between a major nannoplankton extinction (NX) and a moderate temperate molluscan extinction step (MX7). Above this, extinction events no longer cancel out radiation events, and overall diversity builds gradually as ecosystems recover. EARLY RECOVERY INTERVAL 110,000-yr duration: Despite active volcanism and ash fall, sharp reductions in d13C and d18O values, and a major increase in TOC, the low diversity survivor and recovery faunas of the early Turonian remain fairly stable, with exceptionally low diversity levels noted only among benthic foraminfers. 370,000-yr duration: The final phases of geochemical perturbations associated with the C-T mass extinction interval occur at this level. The d13C and d18O values show strong positive excursions, whereas TOC drops dramatically to near zero. Increases in abundance and diversity among molluscs reflect two major origination events (OM14, OM15) during broad early Turonian radiations. These are associated with the final small extinction step (MX8) of the 1.5- to 2-m.y. long C-T mass extinction interval including the paleotropics). This small extinction affects predominantly mid- to north-temperate and cosmopolitan molluscan lineages. END OF SAMPLING INTERVAL