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McLean, D. M., 1991, Impact winter in the global K/T extinctions: no definitive evidences, in Levine, J. S., ed., Global biomass burning: atmospheric, climatic, and biospheric implications: Cambridge, MIT Press, p. 493-503.

Impact Winter in the Global K-T Extinctions:
No Definitive Evidences

Dewey M. McLean

Department of Geological Sciences
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061


About 66 million ago, Earth experienced a global extinction event so profound that it marks the boundary between the Mesozoic and Cenozoic eras and, on a finer scale, the Cretaceous (K) and Tertiary (T) periods. Long the topic of scientific inquiry and debate, the Cretaceous/Tertiary, or K/T, extinctions are cited as one of the top 10 to 20 unsolved mysteries in science. In the past decade, the debate has attracted scientists from may disciplines, and has expanded into one of the truly great debates in the history of science.

The K/T debate is a classic example of conceptual polarities, or antitheses, emerging from a common data base. The most fundamental polarities involve (1) extraterrestrial asteroid/comet impact versus terrestrial volcanism as causative factors in the extinctions, (2) sudden inpact-induced catastrophe spanning a few months to years versus gradual volcanic-induced bioevolutionary turnover spanning several hundreds of thousands of years, and (3) "impact winter" via impact dust blasted into the stratosphere blocking out sunlight, and plunging Earth into darkness and refrigeration versus "greenhouse" warming via volcanic CO2 release into the atmosphere that triggered climatic warming, marine chemistry changes, and ecosystem collapses of a CO2-induced "greenhouse." The K/T debate can only be resolved by eliminating polarities.

This chapter addresses the latter polarity, and suggests that if a K/T boundary impact winter occurred, it was too transitory, or feeble, to be recorded in the geological record, and not of sufficient magnitude to trigger global biological catastrophe. On the other hand, a major long-duration K/T transition carbon cycle perturbation associated with coeval climatic warming is indicated in the record. Throughout, I will discriminate between (1) short-duration K/T boundary impact-type phenomena that begin at the K/T contact and persist for a few months to a few years, and (2) long-duration K/T transition volcanic-type phenomena that begin in the Upper Cretaceous and persist into Early Tertiary, spanning several hundreds of thousands of years.

K-T Boundary Climatic Cooling/Warming Background

Climatic cooling as a factor in the K-T extinctions, and especially those of the dinosaurs, has long been espoused. Through the 1970s, Dale Russell related K-T climatic cooling to a supernova explosion in the vicinity of the solar system (Russell, 1977, 1979a, 1979b). According to Russell (1977), evidences for a supernova explosion in the K-T extinctions were: (1) the extinctions themselves, and (2) evidences of an enormous ring of interstellar neutral hydrogen (Lindblad's ring), some 18,000 billion miles in diameter near the solar system, whose speed of expansion indicates an explosion relatively close to the sun about 65 million years ago. Supposedly, climatic effects are due to increase in visible light from the supernova, and to changes in ionizing radiation which would disturb the radiation of the upper atmosphere. Russell (1979a) suggests that such atmospheric perturbations would have triggered climatic cooling.

The modern K-T cooling/warming antithesis dates from the late 1970s when I proposed CO-2-induced greenhouse warming as a factor in the K-T extinctions (McLean, 1978). Initially, I evoked the failure of the dominant planktonic marine algae, the coccolithophorids, as the cause of the greenhouse. Those algae served as a major sink for the uptake and storage of CO2. Prior to the K-T extinctions, the coccolithophorids had been so abundant that their photosynthesis and CaCO3 shell production had drawn down atmospheric pO2, causing antigreenhouse climatic cooling. Their K-T failure would have triggered a carbon cycle perturbation. Later, I evoked mantle CO2 degassing perturbation of the carbon cycle via the Deccan Traps volcanism in India as the cause of coccolithophorid failure, and the combination of volcanism and coccolithophorid failure for a major trans-K-T greenhouse (McLean, 1981, 1982a, b; see also McLean, 1985a, 1985b, 1985c). Recently, I subsumed the Deccan Traps volcanism, and extinctions, into a broader theory involving thermal evolution of the earth (McLean, 1988).

K-T boundary climatic cooling was reinforced by Alvarez et al. (1980) who proposed that 66 million years ago a giant asteroid 10 ± 4 km diameter struck earth, blasting dust into the stratosphere and blocking sunlight for several years until the dust settled to earth, plunging earth into darkness and refrigeration. Loss of sunlight suppressed photosynthesis, collapsed most food chains, and triggered global extinctions. Theoretically, the asteroid was rich in the element iridium, and the impact dust settled out globally as a thin, iridium-rich clay layer that marks the K-T boundary. The K-T boundary clay was taken as primary evidence of a sunlight-blocking dust shroud. A K-T impact winter was reinforced by discovery of elemental carbon, mostly in the form of soot, in the K-T boundary clay (Wolbach et al., 1985). Theoretically, asteroid impact ignited global wildfires that consumed most of earth's forest biomass, injecting soot into the atmosphere simultaneously with the impact dust, enhancing the dark and cold of the K-T impact winter. The soot can also be taken as primary evidence of a K-T impact winter.

The notion of a K-T boundary impact winter was further reinforced by the proposals of Wolfe and Upchurch (1986, 1987) that plant physiognomy-climate relationships in the Rocky Mountain paleobotanical record supports pronounced, short-duration K-T boundary climatic cooling. Assuming that physiognomy-climate relationships are valid as pertains to the K-T paleofloral record, a postulated short-duration K-T boundary cooling can also be taken as primary evidence of a K-T impact winter.

In this paper, I will examine validity of the primary evidences supporting the concept of a K-T boundary impact winter against the geobiological record. These involve: (1) genesis of the iridium-rich K-T boundary clay, (2) genesis of K-T boundary clay soot enhancement that provide the basis of K-T wildfires, and (3) the K-T paleofloral record that is claimed to indicate sharp, pronounced, K-T cooling supportive of an impact-induced "impact winter."

K-T Boundary Iridium Clay Implication in Impact Winter

K-T Boundary Interval Clays (Western North America)

The picture presented by K-T impact-oriented literature is that dust blasted into the stratosphere by the Alvarez asteroid settled out as a simple dust layer that marks the global K-T boundary. In fact, the K-T boundary sedimentary record is too complex to be explained so simply.

The American Rocky Mountain region contains the most complete, and examined, terrestrial K-T sections known. Figure 1 (adapted from Izett, 1987) shows that the K-T boundary interval is not a simple unit, but instead, a complex couplet composed of a lower "K-T Boundary Claystone" (mostly kaolinite), and an overlying "K-T Boundary Impact Layer" (mostly smectite). Those two units are separated by a hiatus (Izett, 1987; Fastovsky et al., 1989). The Impact Layer is itself directly overlain by the Boundary Coal Bed.

Figure 1. Composite Rocky Mountain K-T boundary interval showing the boundary clay couplet composed of the K-T Boundary Clay and the K-T Impact Layer. These units are separated by a depositional hiatus. The pollen-based K-T contact occurs at the top of the Boundary Clay.

K-T Boundary Claystone (abstracted from Izett, 1987). This 1 to 2 cm thick, largely kaolinitic unit, displays no evidence of an impact origin. It lacks both iridium and shock-metamorphosed mineral grains. Its lithology indicates slow accumulation by normal sedimentary processes rather than rapidly by fallout of impact material. Horizontal vitrinite laminae, blebs of amber, degraded plant material, fossil twigs, fossil roots, angular and rounded claystone fragments, rounded fragments of woody material, and carbonaceous structures that flare out at the contact with the overlying Impact Layer indicate a complex origin. It lacks the angular shard shapes and broken crystals typical of impact fallout material; it is not altered tektite glass. Authigenic spherules of kaolinite, goyazite, and jarosite, resembling objects in marine K-T boundary clays described as impact melt droplets, are likely not of impact origin. Its interpretation as a widespread continuous layer may have been misinterpreted as it has been found only at about 25 localities in low-energy coal-swamp environments in north-trending Rocky Mountain coal basins. It has not been found outside of these coal basins. It underwent pedogenesis; its upper surface is a depositional hiatus.

K-T Boundary Impact Layer (abstracted from Izett, 1987). This 4.0 to 8.0 mm thick, largely smectitic unit, contains the maximum iridium abundance in the Rocky Mountain K-T boundary interval, and also shock-metamorphosed grains; the latter constitute no more than 2.0 percent by volume, and are not concentrated at the base of the unit as would primary air-fall materials. Its microlaminated structure contrasts with the underlying Boundary Clay. Microlaminations are warped over and under clastic mineral grains reflecting soft sediment compaction. Microlaminae surfaces display impressions of macerated plant material, suggesting slow accumulation during normal depositional processes. Formed mostly of locally derived sediment, and not from altered volcanic or impact glass, the unit reflects slow accumulation of clay particles. Ubiquitous pellets of white cryptocrystalline kaolinite are common. That it does not contain a large component of asteroidal materials is attested by low nickel and cobalt contents. It is overlain at most localities by the Boundary Coal which, itself, ranges from 4-16 cm in thickness.

Izett (1987) states that shocked minerals and iridium of the Impact Layer settled out upon a Boundary Clay paleosurface that was intermittently covered with water and local floras. Impact upset the hydrologic regime via climate change which triggered a new but amazingly gentle erosional-depositional cycle during which shocked minerals were reworked gently into a slowly accumulating deposit of clay and decaying plant material. The presence of only one K-T boundary shocked mineral spike indicates but one impact event, and not multiple impacts as suggested by some authors. Izett (1987) notes that the K-T boundary impact event was considerably smaller than that proposed by Alvarez et al. (1980).

Comment: The Impact Layer, which reflects slow accumulation by normal processes of locally derived materials, contains a miniscule amount of shocked mineral grains. They are distributed throughout the Impact Layer, and not at the base as they would be if they represented simple air-fall of impact debris. Microlamination indicates lack of bioturbation; thus, it is difficult to see how the shocked mineral grains mixed throughout the Impact Layer were "reworked" into it. Most grains illustrated in publications have rounded edges, suggesting transport into the coal basins via fluvial processes. Grains settling directly from an impact related air fall would be sharply angular. Most plausibly, the shocked mineral grains were transported into topographically-low coal basins by natural sedimentary processes together with other sediments that make up the Impact Layer.

Iridium generally peaks in the Impact Layer, but also occurs in carbonaceous-rich layers either above or below it (Izett, 1987). Izett assumes that the iridium was originally concentrated in the Impact Layer, and was later mobilized and transported to adjacent carbonaceous sediments. The Impact Layer is directly overlain by the Boundary Coal. The latter represents a return to coal-producing conditions after a long coal-barren interval, and seems to have developed as a part of the flooding of the region during the K-T transition (Fastovsky et al., 1989). Iridium existing about on the landscape as a result of either impact or volcanism (see section "K-T Boundary Iridium and Deccan Traps Volcanism" below) would have been mobilized by abundant humic acids and transported into, and concentrated in low-lying swamp areas (McLean, 1985b), along with the shocked minerals. The Impact Layer could be underclay-equivalent base of the Boundary Coal.

Conclusion. The terrestrial Rocky Mountain K-T boundary clays do not definitively support a K-T boundary blockage-of-sunlight-induced impact winter.

K-T Boundary Clay (Marine)

Impact Origin. Alvarez et al. (1980) proposed that the K-T boundary clay represents impact dust that settled out as a global iridium-rich clay unit. Kastner et al. (1984) noting that smectite, the dominant clay mineral in the K-T boundary at Stevns Klint, Denmark, is rich in magnesium, proposed that the K-T boundary clay is unique, and dissimilar from other smectites in the rock record. They concluded that the unusual Mg-smectite originated via alteration of impact ejecta.

Terrestrial Origin. Most authors propose an earthly source for the marine K-T boundary clay. Because of the high smectite content, Christensen et al. (1973) suggested a volcanic origin for the K-T boundary clay. Elliott et al. (1989) contend that the Mg-smectite is not unique to the K-T boundary, and that it was not derived by a bolide impact, but by alteration of volcanic ash. Hansen et al. (1988) note that smectite represents devitrification of volcanic materials; their study of chromite chemistry of the Danish boundary clays indicate a terrestrial ultrabasic origin. Rampino and Reynolds (1983) indicated that the K-T boundary clay is neither mineralogically exotic, nor distinct from locally derived clays above and below it. The Schmitz (1985) study of metal enrichment in the K-T boundary clay in Denmark indicated that the most likely source for the metals is dissolved detritus in the underlying chalks which may be volcanic debris with some inputs of terriginous and meteoritic material. Schmitz (1988) concludes that the "impact scenario of Alvarez et al. (1980) in which a world-encompassing dust cloud led to suppression of photosynthesis and extinction, is not supported by chemical and mineralogical characteristic of the marine K/T boundary clays."

K-T Marine Clay Deposition Time. The long duration of deposition of the K-T marine boundary clay argues against rapid air-fall from an impact event. Smit (1982) notes that the K-T boundary clay represents 5,000-15,000 years deposition. Joseph Hazel (personal communication, 1989) indicates from sophisticated time-rock relationships that the K-T boundary clay represents about 4,000 years deposition.

Conclusion. Studies of the K-T boundary clay layer from various parts of the world indicate that it represents altered volcanic materials, and not a short-duration air-fall of impact debris. The marine K-T boundary clay does not support a K-T boundary dustshroud-induced impact winter.

K-T Boundary Refrigeration via Impact Dust

A K-T boundary refrigeration of magnitude to trigger global catastrophic extinctions would be recorded in the oxygen isotope record as a sharp positive excursion precisely at the K-T boundary. In fact, a negative excursion begins below the boundary and continues well into the Early Tertiary coeval with the Deccan Traps volcanism in India.

Global Blackout via Impact Dust

There are no definitive evidences of an impact-induced K-T boundary global blackout. A test of a global blackout exists via the marine algal record. Microscopic, planktonic, algae such as the diatoms, dinoflagellates, and coccolithophorids, depend upon light for photosynthesis. A 6 month to 1 year blackout would have triggered massive extinctions of many species precisely at the level of the K-T boundary iridium clay.

Diatoms. The K-T event did not much affect the diatoms. Harwood (1988), based on studies from Seymour Island, eastern Antarctic Peninsula, the first to record siliceous microfossil assemblages across a K-T boundary sequence, notes that diatom survivorship across the K-T boundary was above 90 percent. Resting spores increase from 7 percent below to 35 percent across the K-T boundary.

Dinoflagellates. Dinoflagellates also were little affected by the K-T event (Bujak and Williams, 1979). Brinkhuis and Zachariasse (1988) record no accelerated rates of extinction across the K-T boundary in Tunisia. Nor does Hultberg (1986) in Scandinavia. Danish dinoflagellates responded more by appearance of new species than by extinctions (Hansen, 1977), as did Seymour Island assemblages (Askin, 1988).

Coccolithophorids. Marine calcareous microplankton, the coccolithophorids and planktonic foraminifera (discussed below), were hit hardest of all by the K-T event. Thierstein (1981) proposes that the coccolithophorids extinctions were the most severe plankton extinction event in geologic history; via a "deconvolution" process, Thierstein (1981, 1982) reduced a Cretaceous-Tertiary "transition," in which Cretaceous assemblages were replaced by "new" Tertiary taxa, to an instantaneous catastrophe.

Comment. Perch-Nielsen et al. (1982) note that the "catastrophic event"at the K-T boundary did not result in geologically instant extinction of the calcareous nannoplankton, and that most Cretaceous species survived the event. At DSDP Site 524, a sample above the K-T boundary contains 90 percent Cretaceous species. Isotopic analyses indicated that the Cretaceous species were not reworked specimens, but represented survivors of the K-T event that continued to reproduce in the earliest Tertiary oceans. The relict species became extinct some tens of thousands of years after K-T boundary time, probably via environmental stresses.

Antia and Cheng's (1970) work on survival times of phytoplankton species in complete darkness indicate that 1 month of complete blackout would produce 13 percent extinction, 3 months 68 percent extinction , and 6 months 81 percent. Thus, the 6 month to 1 year global blackout predicted by Wolbach et al. (1985) would have obliterated diatoms, dinoflagellate, and coccolithophorids precisely at the K-T boundary. A blackout event is not reflected in the algal record. The calcareous coccolithophorids and foraminifera were likely affected by pH change of the marine mixed layer via CO2 mantle degassing by the Deccan Traps volcanism.

Conclusion. The marine algal record does not support a K-T boundary global blackout.

Foraminifera. Studies at El Kef, Tunisia, and Brazos River, Texas (Keller, 1988a, 1988b, 1989a, 1989b; Keller and Lindinger, 1989) , indicate that : (1) an extended period of species extinctions spans the K-T boundary from about 300,000 years below to 200,000-300,000 years above the boundary in shallow continental shelf sections, (2) apart from the K-T boundary, there are two distinct extinction episodes beginning at about 300,000 years below the boundary, and at about 50,000 years above the boundary, (3) at El Kef, the K-T event affects only 26 percent of the planktic foraminiferal species; 29 percent disappear earlier, 11 percent disappear 15 cm above the K-T boundary, 18 percent disappear near Zone P0/P1a about three m above the K-T boundary, and the remaining Cretaceous survivors disappear gradually 150,000-300,000 years after the K-T boundary, (4) at Brazos River, there are no species extinctions or measurable faunal changes associated with the K-T boundary. There are two extinction events. The first, with 46 percent of the species extinct, occurs 27-35 cm (about 310,000 years ) below the K-T contact. The second, with 45 percent of the species extinct, occurs 25 cm (about 50,000 years ) above the K-T contact, at the Zone P0/P1a boundary. No species extinctions, or major faunal assemblage changes occur at the Brazos River K-T boundary.

Keller and Lindinger (1989) note that primary productivity in the oceans was drastically reduced for at least 230,000 years. CaCO3 percent, benthic and fine fraction C-13 values reach near pre-K-T levels about 300,000-400,000 years after the K-T boundary. This period was coeval with Strangelove ocean conditions that were coeval with the Deccan Traps volcanism.

Conclusion. Foraminifera, the organisms most affected by K-T event, do not reflect a single catastrophe precisely at the K-T boundary but, instead, a gradual process that affected them over a long K-T transition interval of time. The Deccan Traps volcanism, that began erupting below the K-T boundary, and whose main activity was in Early Tertiary would have produced long-duration K-T transition pH change of the mixed layer, and the "Strangelove" oceans implicated in low Early Tertiary marine productivity.

Reptilian Extinctions and Impact Winter

Carpenter (1983) notes that because of the assumption that all Latest Cretaceous dinosaurian taxa persisted to the end of the Cretaceous before becoming extinct permeates the popular and scientific literature, it is no wonder that single-cause catastrophic hypotheses abound.

Russell (1975, 1979a, 1979b, 1982, 1984), who contends that there was no decline in reptilian diversity toward the end of the Cretaceous, and that over half of earth's species became extinct in a terminal Mesozoic crisis,.has advanced the idea of a K-T asteroid impact. He visualizes 18 families of dinosaurs, three of pterosaurs, one of ichthyosaurs, three of plesiosaurs, and one of mosasaurs becoming extinct at the K-T boundary.

Sullivan (1987), who has developed the first global survey of reptilian species-level diversity spanning the K-T transition, contradicts Russell. Sullivan notes that of the 44 reptilian families existing before the K-T boundary, 13 died out before the boundary, 9 died out at the boundary, and 22 survived it. The extinction rate at the boundary is thus about 20 percent. For the 19 dinosaurian families existing in the last 20 million years of the Cretaceous, most disappeared before K-T boundary time, and eight at the K-T boundary. The eight families are represented by 12 species in the final 3 million year of the Cretaceous, and most of the species by only a few specimens (two to 10, average 5.7).

One turtle family died out prior to the K-T, and six survived the K-T boundary event. Twelve lizard and snake families survived the K-T event; one, the mosasaurs, had died out at the beginning of the Maastrichtian. The four crocidilian families survived seemingly unaffected. Of two pterosaur families, one died prior to the K-T, and one at the K-T boundary. The single families of ichthyosaurs and plesiosaurs died out in the Campanian long before K-T boundary time.

Sloan et al. (1986) noted that the dinosaurian extinctions were a gradual process beginning 7 million years before the end of the Cretaceous, and accelerating rapidly in the final 0.3 million years. They propose that in Montana, the last dinosaurs occur stratigraphically above (about 40,000 years after) the K-T iridium anomaly. Of the 30 dinosaur genera living in the area 8 million years before the end of the Cretaceous, 12 genera were still living just before the K-T boundary event, and between seven to 11 survived into the Paleocene. Only one to three genera became extinct at the K-T boundary. They propose that the dinosaurs survived well into the early Paleocene in tropical India, the Pyrenees, Peru, and New Mexico.

Conclusion. Evidence exists that the dinosaurs could tolerate periods of darkness and cold. Rich et al. (1988) indicate from work in the high latitudes of southeastern Australia that cold and darkness may not have been a prime factor in the dinosaurian extinction of dinosaurs at the K-T boundary unless the duration exceeded 3 to 5 months. Dinosaurs living near the poles was an enduring event in which they had coped with high latitudes for at least 65 million years. Brouwers et al. (1987) note for the high latitudes of the Alaskan North Slope 66-76 million years ago, that dinosaurs remained at high latitudes the year-round, challenging the idea that short-term periods of darkness and cold via bolide impact triggered the dinosaurian extinctions. The reptilian record does not support a K-T boundary impact winter.

K-T Boundary Soot Implication via Wildfire-Induced Impact Winter

K-T Boundary Wildfires and Soot. Wolbach et al. (1985) and Wolbach et al. (1988) reported the presence of graphitic carbon, primarily in the form of soot, at several K-T boundary sites from various localities: Stevns Klint and Nye Klov, Denmark; Woodside Creek and Chancet Rocks, New Zealand; and Caravaca, Spain. They attribute the soot to impact-ignited global wildfires that burned down most of earth's forests. Theoretically, radiation from the impact fireball and cloud of rock vapor can ignite wildfires more than 1,000 km away; once started, fires can spread over entire continents, distributing soot worldwide; trees dried by the impact via heat and winds exacerbated ignition and combustion. The authors note that because soot enrichment begins in the first 0.3 cm of the K-T boundary clay, wildfires started before the impact rock dust had settled. Because soot absorbs sunlight more effectively and settles more slowly than does rock dust, it would have caused darkness and cold to last longer than would rock dust alone.

Fusinite as a Test for K-T Wildfires. Charcoal (structured fusinite of petrographic usage, after Cope and Chaloner, 1985) provides evidences of wildfire. K-T global wildfires of the magnitude envisioned by Wolbach et al. (1985) and Wolbach et al. (1988) would have produced a global layer of fusinite (charcoal) precisely at the K-T boundary in the Rocky Mountain region and, because of continental runoff, at the K-T boundary in marine sections as well. Fusinite is not recorded in the Rocky Mountain K-T boundary sediments in sufficient quantities to support the idea of K-T wildfires. Abundant vitrinite (coalified plant tissue) is recorded.

Marine Algae as a Test for K-T Soot-Induced Blackout. Same as for the K-T boundary impact rock dust blackout discussed above.

Terrestrial Origin of K-T Soot. Hansen et al. (1987) note that carbon black occurs frequently from Devonian to present, and is generally interpreted as soot from forest fires or burning of grasslands. In Danish sections, carbon black enrichments begin 3.5 m below the boundary clay, or about 50,000 years earlier than the iridium anomaly. The carbon black is consistent with an earthly origin from reduced carbon or carbonados; carbonados are amorphous lumps of pure carbon associated with kimberlites. The Danish carbon black reflects pulsed or multistage deposition lasting for about 50,000 years .

Hansen et al. (1988) note that elemental carbon in the Danish K-T boundary clays reflects terrestrial origin, unrelated to meteoritic carbon. Isotopic evidence, and the long duration of carbon enrichment, indicate that it cannot originate from world forest fires, and that ultrabasic volcanism is the likely origin of the carbon black.

K-T Clay Sedimentation Rates. For the duration time of deposition of the boundary clay, Wolbach et al. (1988) assume that the clay represents impact ejecta that could not have stayed aloft for more than six months. Thus, they propose that the soot "enrichment" in the K-T clay resulted from short-duration global wildfires. In fact, Smit (1982) indicates a 5,000-15,000 year deposition time for the K-T boundary clay. Joseph Hazel (personal communication, 1989) indicates on the basis of sophisticated time-rock relationships a 4,000 year deposition time.

Figure 2a of Wolbach et al. (1988) shows iridium and soot peaking in abundance in the K-T boundary clay; iridium and elemental carbon concentrations rise sharply at the boundary by factors of 1,500 and 210 compared with the Cretaceous values, and soot by 3,600. However, both iridium and soot begin increasing in abundance below the K-T clay, and remain above Cretaceous values well into the Early Tertiary. At the time of K-T clay deposition, CaCO3 deposition had nearly ceased. I have argued (e.g., McLean, 1985b) that K-T cessation of CaCO3 sedimentation would allow clay particles that are ordinarily admixed in CaCO3 to become concentrated as a clay layer at the K-T boundary. Slow accumulation of the K-T clay would allow iridium and soot to become concentrated in it. By this mechanism, soot from natural forest fires becoming concentrated via slow accumulation of the K-T clay would create the illusion of rapid fallout of soot over a short period.

Conclusion. The K-T boundary soot enrichment represents concentration via normal earthly sedimentary processes, and is not supportive of K-T impact winter blackout and refrigeration.

K-T Boundary Paleobotanical-Climate Implications in Impact Winter

Physiognomy-Climate Relationships. According to Wolfe (1971, 1981), foliar physiognomy provides one of the firmest bases for evaluating the paleoclimatic significance of fossil plants. Wolfe and Upchurch (1986, 1987) propose that because vegetation and climate can be directly inferred from the physiognomy of leaves, and because leaf species typically represent low taxonomic categories, leaf floras of western North America can be used to infer K-T climatology. They propose that those floras support a K-T boundary impact winter with a 1-2 month mean temperature near 0oC. They note that sharpness and magnitude of vegetational and floral changes at the K-T boundary are dramatic, and attribute mass-kill, ecological disruption, climatic change, and the K-T iridium to bolide impact.

Wolfe and Upchurch follow the physiognomy-climate methodology of Bailey and Sinnott (1915, 1916). The latter reported that in mesic, tropical environments, the percentage of dicotyledonous species with entire (untoothed) margins is high, but low in mesic, temperate environments. Wolfe and Upchurch (1987) note that in mesic east Asia environments, the percentage of dicotyledonous species with entire margins closely parallels the mean annual temperature. Reportedly, an increase of 3 percent of entire-margined species correlates with an increase of 1oC in mean annual temperature. Eastern Asia is their standard by which all other foliar physiognomic analyses should be judged.

However, the physiognomic-climate relationships utilized by Wolfe and Upchurch are more complex than they present, open to other interpretations, and fraught with flaws. According to Dolph (1990), Wolfe's work contain fatal flaws that bring his climate inferences based upon foliar physiognomy into question. Dolph notes that several crucial questions must be answered before leaf margin analysis can be used to precisely estimate paleoclimate; most basic is whether or not the foliar physiognomy of eastern Asia should be used at a standard by which all other physiognomic analyses are judged. Dolph (1990) notes that analysis of Wolfe's data on the foliar physiognomy of eastern Asia shows that that area is unacceptable as a standard for foliar physiognomic studies. He points out fatal flaws both in Wolfe's choice of eastern Asia as the standard, and in methodological errors. Other readings on problems associated with physiognomy-climate interpretations may be found in Dolph and Dilcher (1979), and Dolph (1984, 1987).

Comments. Wolfe and Upchurch (1987; after Wolfe, 1971) base their physiognomy-climate analyses on the assumption that their assemblages represent nonsuccessional, climax vegetation. Wolfe believes that more than 1 million years must pass before the vegetation takes on its climax characteristics; vegetation of disturbed habitats can give anomalously low temperature estimates because of the high proportion of toothed-leaf species in disturbed versus "climax" vegetations. This assumption bears upon their interpretation of a K-T impact winter. During the K-T transition, flooding in the Rocky Mountain region would have caused continual migration of plant biotas, impeding development of climax vegetational patterns, and producing temperature estimates based on physiognomy-climate relationships lower than they actually were.

Palynological K-T Boundary: Claims that Rocky Mountain paleofloras reflect an impact winter must be based on evidence that the American K-T boundary is isochronous with European K-T boundary stratotypes. Such isochroneity has not been convincingly established. Use of a K-T iridium spike is not definitive in that some localities display multiple iridium spikes. For example, Lattengebirge, Bavarian Alps, has three iridium anomalies, below, at, and above, the K-T boundary (Graup and Spettel, 1989). The oldest anomaly antedates the K-T boundary by 14,000-9,000 years. To which of these anomalies might the Rocky Mountain "K-T boundary" iridium spike be isochronous, if any?

The pollen-based K-T boundary currently used in the Rocky Mountain region was arbitrarily picked at the top of the Boundary Clay (Izett, 1987). Izett notes that the K-T boundary, placed by Tschudy et al. (1984) and Tschudy and Tschudy (1986) at the top of the Boundary Clay, was an arbitrary choice because only rare palynomorphs have been recovered from the Boundary Clay, and these might have been reworked from underlying Cretaceous sediments.

K-T Fern Spike: An abrupt increase in the ratio of fern spores to angiosperm pollen at this stratigraphic level has been attributed to recolonization of a devastated landscape following asteroid impact (Tschudy et al., 1984; Tschudy and Tschudy, 1986). However, Sweet (1988) notes that absence of the fern spore spike in most western Canadian localities may denote rapid flooding rather than recolonization after a catastrophic destruction of vegetation. Fastovsky et al. (1989) note that in eastern Montana, the earliest Paleogene sediments are mainly ponded water deposits, and that the K-T transition was concommitant with extensive flooding of the landscape, indicated by coal deposition and the ponding. I have previously discussed problems relating to chroneity of Tschudy's pollen-based Rocky Mountain K-T boundary relative to European K-T stratotypes (McLean, 1985c).

Canadian Floras at K-T: Sweet and Jerzykiewicz (1985) note that many species in western Canada range across the K-T boundary and provide a strong sense of continuity; evolutionary trends also continue uninterrupted across the boundary. Those authors note that they cannot personally add support to Dr. Alvarez' hypothesis of an extraterrestrial catastrophic event. Lerbekmo et al. (1987) note that western Canada iridium anomalies and palynological floral events do not support a simple, North American-wide catastrophic destruction of vegetation resulting in extinctions coincident with an iridium event at the time of initiation of swamp condition in western Canada.

Seymour Island Terrestrial Floras: The high latitudes of Seymour Island, Antarctic Peninsula, supported humid temperate coniferous forests during the Maastrichtian and Paleocene (Askin, 1988). Over that time interval, plant associations remain essentially unchanged except for slight differences in relative abundances which may be facies related. The only changes in terrestrial palynomorphs species are among angiospersm. Of 51 species recorded, only eight species disappear, and not simultaneously but over 20 m. Less than 10 percent of the total preserved terrestrial species disappeared near the K-T boundary, and these were supplanted by new species. The apparent lack of disturbance in the Seymour terrestrial vegetation, similar to K-T successions throughout much of the world, is consistent with a gradual K-T transition. Askin notes that a K-T event that caused the demise of many marine and nonmarine taxa at the end of the Cretaceous had little effect on the terrestrial floras of the Seymour Island region. The nonmarine palynomorph succession of Seymour Island is more compatible with gradual climate change. A K-T impact winter blackout would likely have had major impact upon those high latitude floras.

Conclusion: The Wolfe and Upchurch proposal that Rocky Mountain K-T indicate an impact winter supportive of bolide impact seemingly requires reinterpretation. More plausibly, floral change are a consequence of flooding during the K-T transition (Fastovsky et al.,1989; Sweet (1988). In fact, Wolfe and Upchurch note that a transgression of the North America epeiric sea had reached the Western Interior by the early Paleocene. They further note that the period of instability encompassed by the mid-Maastrichtian regression and the Paleocene transgression might have contributed to climatic and extinction events associated with the boundary. In light of documented flooding in the Rocky Mountain region during the K-T transition, it seems unnecesary to evoke bolide impact as cause of floral perturbations.

K-T floras in the Rocky Mountain region do not support a K-T impact winter.

K-T Boundary Iridium and Deccan Traps Volcanism

Whether K-T iridium represents impact or volcanism has been controversial. Alvarez et al. (1980) attributed it to an "abnormal influx of extraterrestrial material." Early in the debate, I attributed it to the Deccan Traps volcanism in India at the 1981 Toronto AAAS national meeting, the 1981 Ottawa K-TEC II, and the Snowbird I Conference. Later, Zoller et al. (1983) discovered iridium in the gas phase of the modern Kilauea volcano, and suggested that the Deccan Traps was of sufficient magnitude to have produced the K-T iridium. Recently, Toutain and Meyer (1989) reported that the hotspot volcano that produced the Deccan Traps is yet liberating iridium.

The Deccan Traps, whose lavas yet cover much of northern and western India, is part of hotspot track produced by northward drift of the Indian lithospshere plate over a deep-origin mantle plume. Courtillot et al. (1986) note that the hotspot now located under Reunion Island was beneath the Deccan Traps when they erupted. Southward from India, the hotspot track system later generated the Chagos-Laccadive ridge, the Mascarene plateau, and then the Mauritus and Reunion islands (Toutain and Meyer, 1989).

The Deccan Traps volcanism was the major K-T boundary event capable of perturbing earth's outer physiochemical spheres sufficiently to trigger biological overturn in the biosphere. Morgan (1971, 1972a, b) originally related the Deccan Traps to the Reunion hotspot, and estimated that duration of the Deccan Traps was within 1 millioin years (1981). Subbarao and Sukheswala (1981) cite it as possibly the greatest volume of continental basalt on earth's surface. Alexander (1981) notes that of the major episodes of flood basalt volcanism, the Deccan Traps has the greatest volume of all and was erupted over the shortest time span. Courtillot et al. (1986) and Courtillot and Cisowski (1987) cite it as one of the greatest episodes of flood basalt volcanism in earth history, and Courtillot et al. (1988) as the largest volcanic eruption during the Mesozoic and Cenozoic eras. Duncan and Pyle (1988) cite it as one of the most remarkable volcanic provinces on earth in sheer extent and volume, and that it can be correlated in time with events at the K-T boundary.

Today, after extensive erosion, Deccan Traps lavas cover about 5 x 105 km2 of western and central India to a thickness of about 2000 m in western India to 100-200 m in central India (Bose, 1972). Pascoe (1964) suggested that the original lava coverage and related volcanics was greater than 2.6 x 106 km2. Eruption was extremely rapid, perhaps spanning 0.5 million years (Courtillot and Cisowski, 1987). Eruptive rates for the Deccan Traps of 1.5 km3 were far greater than for the later Reunion hotspot track of about 0.04 km3 (Richards et al., 1989). Vogt (1972) had visualized the Deccan Traps as part of globally synchronized mantle plume activity in the K-T transition.

The hotspot volcano Piton de la Fournaise on Reunion Island, that produced the Deccan Traps, is still producing iridium. Toutain and Meyer (1989) reported that sublimates deposited at 250-450oC contain 7.5 ppb iridium, and the lavas 0.25 ± 0.03 ppb. They noted that because Piton de la Fournaise is related to the hotspot track that generated the Deccan Traps that volcanic gaseous iridium can be related to the K-T iridium anomaly. Rocchia et al. (1988) calculated that iridium amounts three times less than in lavas of Piton de la Fournaise could have produced the K-T iridium anomaly.

Multiple iridium spikes at and about the K-T boundary suggest volcanic source. An almost complete K-T section at Lattengebirge, Bavarian Alps, has three iridium bearing events over an extended period from latest Maastrichtian into early Danian (Graup and Spettel, 1989). The oldest spike predates the K-T boundary by 14,000-9,000 years. Geochemically those spikes display the same signature as the K-T boundary layer, and should have the common source. The Brazos River, Texas, locality has two iridium spikes, one at the K-T boundary, and one below (Ganapathy et al., 1981). At Gubbio, Italy, both the iridium and shocked minerals do not occur as a sharp K-T spike; enrichment begins 2 m below the boundary and extends 2 m above it for a total of about 400,000 years (Crockett et al, 1988; Rocchia et al., 1989). Koeberl (1989) reported iridium concentrations up to 7.5 ppb in volcanic dust bands in blue ice fields from Antarctica.

Comment. Asteroid impact has been suggested as initiator of the Deccan Traps volcanism. However, White (1989) notes that even gigantic impacts are not capable of generating large quantities of basaltic melt by removing the crust and allowing the underlying mantle to decompress. The kinetic energy of a bolide could generate some melt but, for realistic bolide sizes, is inadequate to explain observed flood basalt volumes. Morgan (1979) noted that the Reunion hotspot was beneath the northern part of India 80 million years ago. If an impact event triggered eruption of the Deccan Traps, then an extraterrestrial-origin iridium spike would have to be 80 million old, and not 66 milliion years.

K-T iridium was the basis for the Alvarez et al. (1980) impact theory. However, a suitable crater of the right age and magnitude to trigger the K-T extinctions has not been located. The Manson crater in Iowa has been suggested as the K-T impact site (French, 1984; Izett and Pillmore, 1985; Hartung et al. ,1986). Kunk et al. (1989) argue that its age, 65.7 ± 1.0 million years, is indistinguishable from the K-T boundary, estimated to be 66.0 million years. However, Cisowski (1988; personal communication, 1989) notes that the Manson structure is in a normal magnetic polarity chron, whereas the K-T boundary is in a reversed polarity chron (R29). The Manson structure--if it is an impact event--was too small to have triggered an impact winter.

The Kara and Ust-Kara impact structures in the USSR are too old for a 66 million year K-T boundary event (Koeberl et al., 1990). Proposed impact sites in Cuba (Bohor and Seitz, 1990), and the Colombian Basin of the Caribbean Sea (Hildebrand and Boynton, 1990) are too tentative, and open to terrestrial tectonic and volcanic interpretations at this point, to be currently considered at the K-T impact site.

Conclusion. The Deccan Traps volcanism was the major documentable biospheric perturbing event of the K-T transition, and is of the correct age. The Reunion hotspot volcano that produced the Deccan Traps is yet producing iridium after 66 million years. Originating near earth's core-mantle boundary, it could have liberated siderophiles in cosmical proportions onto earth's surface. India's near-equatorial location during the K-T, and the fact that thermal plumes above flood basalt type volcanos can penetrate to the stratosphere, could have carried iridium bearing dust particle into both the northern and southern hemispheres.

Having a documentable terrestrial source of iridium negates having to evoke an exotic exterrestrial source for the K-T boundary iridium, and further weakens the concept of a K-T impact winter.

Stable Isotope Record

Climatic cooling associated with a K-T impact winter would be reflected as a positive excursion of the oxygen isotope record in the K-T clay on a global scale. In fact, a general pattern of a negative excursion beginning below the K-T boundary, and extending well above it into the Early Tertiary, is emerging. This general signal of K-T transition warming is coeval with both the Deccan Traps volcanism and a major carbon cycle perturbation. Scholle and Arthur, (1980) and Perch-Nielsen et al. (1982) note that a major negative shift in carbon isotopes has been detected at every K-T boundary sequence examined.

Mount et al. (1986) note that several oxygen and carbon isotopic excursions occurred before and during the K-T boundary event. Each involved synchronous negative shifts in O-18 and C-13 values of about 2 per mil. They note that whereas some workers have related those excursions to an impact event, they are not significantly different from several other that precede it. They further suggest that the K-T boundary excursion is not unique; the excursions are interpreted as episodic warming events. Extinctions of some marine invertebrates were associated with surface-water warming events and decreases in productivity beginning before the K-T boundary event.

Conclusion. Globally, the C-13 and O-18 negative excursions begin below the K-T boundary, and extend above it into the Early Tertiary, coevally with the duration of the Deccan Traps volcanism. Because mantle CO2 is depleted in C-13, I have proposed that the trans-K-T C-13 excursion is reflective, at least in part, of mantle CO2 degassing associated with the Deccan Traps volcanism (McLean, 1985a, b, c). The O-18 excursion is reflective of trans-K-T climatic warming. The combination of major carbon cycle perturbation and climatic warming would seem to indicate a trans-K-T greenhouse. Stressed conditions in the oceanic mixed layer that persisted for about 500,000 years into the Early Tertiary, referred to as "Strangelove" oceans (Hsu and McKenzie, 1985), were coeval with the Deccan Traps mantle CO2 degassing.

The isotope record does not support a K-T boundary impact winter.

K-T Shock-Metamorphosed Minerals

Shock-metamorphosed quartz grains displaying multiple intersecting sets of planar lamellae discovered at the K-T boundary by Bohor et al. (1984) were proposed to be definitive evidences of a K-T boundary impact event. Supposedly, such a high degree of shock metamorphism can only be produced by the hyperevelocity impact events. Shocked minerals remain the most compelling evidences of a K-T boundary impact event. However, Carter et al. (1986) have described microstructures in the Toba Tuff of Sumatra, proposing the shock metamorphism can be produced by explosive volcanism. The origin of the K-T shocked mineral is currently the topic of lively debate.

Shocked mineral grains are most abundant at western North American sites, but have been reported from several sites in Europe, a core from the north-central Pacific Ocean, and a site in New Zealand (Bohor et al., 1987). Izett (1987) notes that the grains outside North America are exceedingly rare.

Bohor et al. (1987) note that the ubiquitous presence in the K-T boundary layer of shocked quartz, feldspar, and composite siliceous grains argues against an oceanic asteroid impact and, instead, support the idea of a large asteroid impacting into a continental crustal terrain. In this light, an impact crater estimated to range in diameter up to 200 km (Alvarez et al, 1980) ought to be plainly visible somewhere on the North American continent.

Conclusion. Shocked mineral grains at the K-T boundary remain the most convincing evidence of a possible K-T impact event.

[After this paper was published, the Chicxulub structure on Yucatan was proposed as a K-T impact site.]


Little definitive evidence supports the concept of a bolide-induced K-T boundary impact winter. Thus, impact advocates must seek another killing mechanism capable of triggering global extinctions. A K-T transition greenhouse, that is supported by the record, is a possibility.

The recent Williamsburg "Chapman Conference on Global Biomass Burning: Atmospheric, Climate, and Biospheric Implications," provided stimulating thinking on the possibility of a combined impact-volcano unification of the K-T record via a CO2-induced greenhouse. A small impact event, by starting fires that injected greenhouse gases into the atmosphere, could have reinforced an ready existing volcano-induced greenhouse, pushing the biosphere "over the edge" at the K-T boundary.

A combination model would have a short-duration K-T boundary impact-induced greenhouse superimposed upon, and intensifying, a long-duration volcano-induced K-T transition greenhouse. Such unification would accommodate the K-T boundary shocked minerals, and intensification of ecological stresses, within the long-duration K-T transition carbon cycle and bioevolutionary perturbations that are preserved in the record. This unification, which accords with the actual record, offers a step forward in isolating the cause of the extinctions while other details are being sorted out down through the years. Theory coupling greenhouse warming to embryogenesis dysfuntion in global extinctions has been developed (McLean, 1988). In fact, some impactors have advocated greenhouse conditions as a K-T killing mechanism in their models (Emiliani et al., 1981; O'Keefe and Ahrens, 1988, 1989; Hsu et al., 1982).


I thank Dr. Joel Levine, Senior Scientist, Atmospheric Sciences Division of NASA Langley Research Center, Hampton, VA, for the invitation to participate in the timely and educational "Chapman Conference on Global Biomass Burning: Atmospheric, Climate, and Biospheric Implications" held at Williamsburg, VA, March, 1990, and to write this paper.


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