Phanerozoic O2 variation, fire, and terrestrial ecology

Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 75 (1989): 223-240 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

223

PHANEROZOIC O 2 VARIATION, FIRE, AND TERRESTRIAL ECOLOGY J.M. ROBINSON Earth System Science Center and Department Geography, 302 Walker Building Pennsylvania State University, University Park, PA 16802 (U.S.A)

(Received December 14, 1988; revised and accepted February 14, 1989)

Abstract Robinson, J.M., 1989. Phanerozoic 0 2 variation, fire, and terrestrial ecology. Palaeogeogr., Palaeoclirnatol., Palaeocol. (Global Planet. Change Sect.), 75: 223-240. Sedimentary evidence suggests that 02 levels have varied greatly in the Phanerozoic. Fire is sensitive to 02. The effects of fire on modern ecosystem structure and on plant morphology are reviewed and used as a basis for predicting how the fossil record might reflect high 02 fire regimes. It is observed that fire is a strong agent of natural selection, and that if 02 has been unstable, the waxing and waning of fire stress has probably been a major force in the evolution, ecology, and biogeochemistry of high 02 periods. The broad features of terrestrial ecology in the putative high 02 periods (the Late Cretaceous and the Carboniferous to Permian) are observed. Hints of agreement between the observed trends and the predicted effects of 02 history are found. Much stronger testing of the hypothesis of 02 variation can be done through the combination of experimental studies and focused observation of the fossil evidence.

Introduction This paper examines the link from the burial of o r g a n i c c a r b o n t o t h e m i x i n g r a t i o of 0 2 in the atmosphere, to the average propensity to burn (pyricity), to ecology and natural selection. T h e o p e n i n g s e c t i o n o u t l i n e s t h e r e a s o n s for s u p p o s i n g t h a t t h e 0 2 level of E a r t h ' s a t m o sphere has varied over Phanerozoic time, and p o i n t s o u t t h e d i s c r e p a n c y b e t w e e n 02, a s p r e dicted by sediment models, and presumed limits t o a t m o s p h e r i c 02, a s b a s e d o n a r g u m e n t s r e l a t i n g t o t h e e f f e c t s of fire. T h e r e a f t e r i t is ass u m e d , h y p o t h e t i c a l l y , t h a t 0 2 level h a s v a r i e d over the Phanerozoic. Two questions are raised: (1) H o w w o u l d v e g e t a t i o n a n d e c o s y s t e m s h a v e r e s p o n d e d t o c h a n g i n g p y r i c i t y ? (2) I s t h e fossil record consistent with the predicted response? Although they would be unlikely to raze Earth's g r e e n m a n t l e , i t is o b s e r v e d t h a t 0 2 - d r i v e n fires would have forced major changes in spatial 0921-8181/89/$03.50

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organization, energy partitioning, and reproduct i v e s t r a t e g i e s of p l a n t s , a n d m a y p u t a p r e m i u m on c h a n g e s in m o i s t u r e r e l a t i o n s a n d t i s s u e c h e m i s t r y . I n s h o r t , fire is a s t r o n g a g e n t of n a t u r a l s e l e c t i o n , a n d if 0 2 h a s b e e n u n s t a b l e , t h e w a x i n g a n d w a n i n g of fire s t r e s s h a s p r o b a b l y b e e n a m a j o r f o r c e i n t h e e v o l u t i o n , ecology, a n d b i o g e o c h e m i s t r y of h i g h 0 2 p e r i o d s . Discussion should be prefaced with the obs e r v a t i o n t h a t n e i t h e r fire i n geologic h i s t o r y , nor 0 2 variation, has been treated seriously by the scientific community. Experience predisposes u s t o a s s u m e t h a t a t m o s p h e r i c 0 2 is cons t a n t . T h e i m p o r t a n c e of l o w 0 2 for p r e - D e v o n i a n p l a n t s h a s b e e n c o n s i d e r e d ( K n o l l e t al, 1986), a n d t h e p h y s i o l o g y of a n o x i a h a s b e e n s t u d i e d (e.g., H o o k a n d C r a w f o r d , 1978). B u t 02 is o m i t t e d f r o m lists of t h e e n v i r o n m e n t a l v a r i a bles to which plants respond even by authors w h o a r e w e l l a w a r e of i t s p h y s i o l o g i c a l i m p o r t a n c e (e.g. K n o l l a n d N i k l a s , 1987), a n d a p a r t

224 from occasional speculation on the effects of high O2 on large vertebrates (see Budyko et al., 1987), paleontologists have given Phanerozoic O2 variation little thought. Because both plant and animal metabolism are dominated by reactions with 02, changes in 02 abundance are apt to have far-reaching implications for the biosphere and for biologically induced cycling of energy and matter. To appreciate the difficulty we have conceptualizing O2 enhancement, imagine the difficulty we would have had comprehending glaciation if science had emerged in the Cretaceous. Being unfamiliar with natural ice, we would be inclined to conclude that freezing is deadly to life. Comprehension of the geography and chronology of ice advances and retreats would come slowly, with great effort. Many aspects of freezing, e.g., the varied ramifications of the fact that water expands on freezing, would at first be missed. The prospect of high O2 is no less complex than t h a t of low temperature. Concerning ancient fire, the literature is very sparse, especially in light of the unquestionable and extensively documented importance of fire in modem plant communities. A standard forest ecology text book states (Spurt and Barnes, 1980, p. 421): "Fire is the dominant fact in forest history. The great majority of the forests of the world ... have been burned over at more or less frequent intervals for many thousands of years." Strong correspondence between charcoal layers and changes in floristic composition have been found in many sites (e.g. Swain, 1973; Nichols, 1975; Green, 1983; Kershaw, 1984, 1985). It is well established th a t human use of fire has greatly altered natural vegetation on a scale t h a t approaches continental magnitude (Bartlett, 1956; Stewart, 1956). By contrast few articles treat ancient fire (Harris, 1958, 1981; Komarek, 1973; Kemp, 1981; Cope and Chaloner, 1980, 1985). These present convincing evidence t h a t fire was present in the geologic past, but stop short of asserting t hat fire was significant to ecology or evolution. Paleo fire ecology has been neglected because all concerned, including those writing on ancient fire, have presumed t hat the potency of fire as a

force shaping the landscape is strictly modem, and related to the ascent of man. The attitude has been reinforced by scorn shown by a leading geologist for the concept that fusain is of fire origin (Schopf, 1975), and seems to be backed by the sequences of charred carbon deposition in Pleistocene lake sediment cores (Kershaw, 1984, 1985; Singh et al., 1981) and by deep sea cores (Herring, 1977, 1985). If 02 has been high, the objections to paleo fire must be reexamined. I know of no prior work on the ramifications of high 02 for the biosphere. The following is a first attempt, restricted to the fire-related effects of 02 variation. It probably contains misconceptions and overlooks important factors. It does, however, offer a hypothesis t hat is open to testing and modification, and t hat may shed new light on the coevolution of the atmosphere and the biosphere.

Variable 02, variable pyricity The present atmosphere is 21% 02 . T he proposition t hat the 02 fraction of Earth's atmosphere has, at times, exceeded 25-30% is neither new, nor easily dismissed. Life creates and replenishes Earth's pool of free oxygen as follows 1: photosynthesis fixes carbon from CO2 and releases 02; and thereafter, organically fixed carbon (Corg) is buried, which prevents rapid recombination of C with 02. This allows 02 to accumulate in the atmosphere. In the 1950s and 1960s it was noted (Rubey, 1951; Berkner and Marshall, 1965; Tappan, 1968) t h a t Corg burial rates have varied greatly over Phanerozoic time and, thus, it was concluded t h a t there may have been periods in E art h history where 02 was much higher, and much lower, than at present. This concept has since been refined through models (e.g. Budyko, 1982; Berner, 1987; Budyko et al., 1985, 1987; Shackleton, 1985; Bexmer and Canfield, 1989) t hat infer the 02 mass of the atmosphere from the geologic record of C and S deposition under varying assumptions about the

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Time (my) Fig. 1. Predicted O2 history in relation to postulated 02 ceiling and floor values. Based on variable land mass scenario 1, Berner and Canfield (1989). Upper and lower bounds represent assumed initial (570 m.y.) Oz values of, respectively, 23 and 8× 10~s moles. See source for full model description. D, C, P, Tr, J, K, T indicate Devonian, Carboniferous, Permian, Triassie, Jurassic, Cretaceous, and Tertiary 2.

r a t e at which the weathering of crustal materials takes up to O 2. Figure 1 shows the outcome of t h a t is probably the most advanced of such models, as documented by Berner and Canfield (1989). Other models v a r y in the timing and amplitude of 02 peaks and troughs. For example, in the work of B u d y k o et al., in contrast to

1 T h i s is one of a series of sensitivity tests, based on altern a t e a s s u m p t i o n s a b o u t p a r a m e t e r s a n d s t r u c t u r a l features t h a t c a n n o t be specified precisely, a n d should be viewed in a qualitative to s e m i - q u a n t i t a t i v e m a n n e r . I have s h o w n Berner a n d Canfield's variable land m a s s scenario, r a t h e r t h a n their " m o s t likely" scenario (in w h i c h 02 peaks are of lesser amplitude) because t h e a s s u m p t i o n of variable land m a s s : (a) is m o r e realistic t h a n t h a t of c o n s t a n t land m a s s (as m a d e in t h e " m o s t likely" scenario; R.A. Berner, pers. comm., 1988); a n d (b) unlike t h e " m o s t likely" scenario, does n o t produce t h e biologically incongruous result of a n 02 m a x i m u m placed well p a s t t h e K / T b o u n d a r y . 2 T i m e scale follows t h a t of t h e s e d i m e n t a r y d a t a on which calculations were based (e.g. B u d y k o et al., 1987). T h e app a r e n t l y long Triassic, for example, is n o t accidential.

the trend shown in Fig. 1, the Cretaceous 02 peak is higher t h a n t h a t of the Carboniferous, and it begins in the Jurassic and drops off sharply in the early Cretaceous. These differences arise from differences in procedures and assumptions t h a t have been discussed elsewhere (Berner and Canfield, 1989; Berner, 1989), and will not be gone into here. For present purposes, three observations will suffice. First, where model time horizons have extended into periods of high Corg deposition, t h a t is, the C a r b o n i f e r o u s - P e r m i a n and the Cretaceous-Paleocene, models commonly show O 2 mixing level reaching or exceeding 25%. In periods of low Corg deposition, such as the Triassic, t h e y tend to show significant declines in 02, relative to present levels. Second, precise predictions cannot be made a b o u t either the magnitude or the timing of 02 changes; and the fine-scale ( < 5 m.y.) features of 02 variation are likely to remain obscure. This is because the 02 source is not computed directly. Rather, 02 release is inferred from mass balance calculations for C or S in different pools (e.g. carbonate, organic carbon, and C02) in a context in which m a n y pools are inexactly known (Hay, 1985) and 02 turnover is rapid compared to t h a t of the sedimentary masses of C and S ( K u m p and Garrels, 1986; Berner, 1989). Third, O 2 instability is forced by Corg and sulfur deposition rates, which, in turn, are computed based on observational data. 02 variation is therefore a robust and persistent model outcome, which cannot be altered without fundamental structural modifications, such as addition of feedback t h a t increases the rate at which 02 is weathered from the atmosphere as the 02 level rises (Kump, 1988). M a n y are unconvinced by the arguments for 02 variability. For example, physiologists (see Gilbert, 1981), noting t h a t changes in 02 tension adversely affect the circulatory and respiratory systems of higher animals, have concluded t h a t O 2 tension probably has not undergone significant variation. This argument is less t h a n covincing because it fails to address the evolutionary time scale, and because m a n y higher animals, e.g. water breathers, a c c o m m o d a t e large

226

changes in 02 level. Others h a v e used the fact t h a t combustion is hypersensitive to 02 mixing ratio 1 to t r y to bound the range of possible variation in Phanerozoic 02 mixing ratios. T h i s has produced various estimates of O 2 ceiling and floor values (see below), as plotted in Fig. 1. As show, predicted 02 exceeds the ceiling in the Carboniferous and the Cretaceous, and approaches the floor in the Triassic and Jurassic. A t t e m p t s to resolve the issue by retrieving 02 content from presumed fossil air t r a p p e d in amber (Berner and Landis, 1988) have been controversial (Hopfenberg et al. and others, 1988). Before dismissing the model results, let us examine the basis for the floor and the ceiling.

The ceiling In setting the stage for the Gala hypothesis, Lovelock and I ~ d g e (1972, p. 577) proposed t h a t " a 25 per cent oxygen a t m o s p h e r e would be incompatible with standing vegetation even in rain forests." No reference is given. T h e statem e n t seems to rest on a general observation t h a t at 25% 02 , plant materials burn explosively, even when fairly moist, and fires are easy to ignite and almost impossible to extinguish, In later writings (Lovelock, 1979; W a t s o n et al., 1978), Watson (1978) is cited as evidence for a fire-related 02 ceiling, and the f u r t h e r assertion is m a d e t h a t at 25% 02, " v e g e t a b l e m a t t e r a t the fibre s a t u r a t i o n point has a significant probability of ignition ... a fire could be kindled even during a rainstorm." (Watson et al., 1978, p. 295). 2

1 As opposed to the partial pressure of 0 2, which does not strongly effect combustion over the relevant range of concentration. If partial pressure were limiting, fire behavior would be very sensitive to altitude. 2 T h e point is not to attack the Gaian foundation. The hypothetical Gala does not demand O e stability. If life could survive the transformation from a reducing to an oxidizing atmosphere "in the flexible Gaian way, by adapting to change and converting a murderous intruder into a powerful friend" (Imvelock, 1979, p. 31), it could certainly make relatively minor adjustments needed to adapt to flamboyant ecology.

Watson (1978) staged hundreds of small burns under conditions of controlled 02 mixing ratio and fuel moisture, and t h e r e b y defined curves for the probability of ignition b y electrical discharge, fire spread rate, and the moisture required to extinguish a flame as a function of 02 level over the range 16-40% 02 . T h e results d e m o n s t r a t e t h a t 02 strongly s t i m u l a t e s fire and counteracts the d a m p i n g effect of moisture. While providing a clear, graphic example of the sensitivity of fire to 02 , this in no way proves the existence of an O z ceiling. T h e responses, t h o u g h steep, are continuous. N o t h i n g special h a p p e n s at 25% O 2. Extension to the biosphere is problematic. In m o s t cases, W a t s o n used p a p e r for fuel. P a p e r is a poor surrogate for the biosphere. Chemically, it contains too little lignin to represent terrestrial plant m a t t e r , and it m a y contain additives t h a t strongly influence combustion. Structurally, p a p e r has a surface to volume ratio like t h a t of a thin leaf, b u t unlike living cells, it cannot retain w a t e r b y osmotic pressure. W a t s o n ' s experiments stop a t 80% fuel moisture (defined as the ratio of wet weight to oven dry weight) because p a p e r cannot hold more water. Green leaves and sapwood, on the other hand, generally contain 100-200% fuel moisture ... sometimes more (Brown and Davis, 1973; T i l l m a n et al., 1981). For a group of 53 Amazonian forest trees, average fuel moisture values for t r u n k wood were found to be slightly over 100%, and in one case (Ceiba pentandra, Bombacaceae), nearly 250% ( c o m p u t e d from I B D F / C N P q , 1981). Decomposing p l a n t tissue, likewise, exceeds p a p e r in its capacity to hold water. Peat, for example, m a y hold over ten times its dry weight in w a t e r ( F a r n h a m , 1982). In sum, neither W a t s o n ' s experiments, nor a n y other research done to date, satisfactorily addresses the c o m p o n e n t s of p l a n t s t r u c t u r e t h a t are m o s t limiting to fire spread and m o s t imp o r t a n t in allowing vegetation to survive fire ... green, living tissues, and thick, bark-sheathed, trunks. Watson, b y the way, does not claim to have established an absolute 02 limit (pers. comm., 1988), and has expressed the opinion (1978, p. 241) t h a t the Carboniferous a t m o sphere m a y have been 25% 02 .

227 T h e problem extends beyond the choice of fuels. If 02 surged overnight, much would go up in flames. However, the processes of sediment a r y deposition t h a t control the 02 source are not rapid ( K u m p and Garrels, 1986). 02 buildup, if it occurred, would have taken millions of years, and the pressures t h a t it exerted would probably have been less acute t h a n those faced by modern ecosystems in adjusting to the agency of man. Given tens of thousands of years, a biota with reasonably diverse genetic resources can re-associate to form viable communities even in the face of radical environmental change: For example, take the drastic case t h a t 02 level rose to the point where green leaves burned freely. If this happened, individual plants and plant organs would be transformed into little bombs, photosynthesizing and adding to their charges until they were d e t o n a t e d b y lightning or b y energy released b y detonation of a neighboring energy store. However, so long as t h e y kept far enough a p a r t to prevent pyric contagion, plants would survive. Spatial barriers to contagion can operate on scales ranging from t h a t of p a t c h mosaic (several times the bole diameter of the d o m i n a n t life form) to branch structure (e.g., diffuse foliage, as seen in m a n y leguminous shrubs of tropical savannas ). Based on observations of the plants t h a t grow on munitions test ranges in seasonally dry climates, I would expect t h a t the resulting vegetative cover would be generally thinned and locally denuded, b u t not barren. Dense cover cannot be ruled out in environments, such as perennially wet swamps, whose structure is inimicable to fire, or in insular systems where lightning and volcanism were rare or absent. Life's struggles with fire are abetted by the fact t h a t fire is ultimately controlled by fuel a m o u n t and condition. Where fire is common, fire history m a y influence fuel condition every bit as strongly as do biological productivity, litter accumulation, and weather (drying). In recently-burned systems, fuel loadings tend to be fight, and there is no basis for large, intense, or catastrophic fires. As well documented by Olson (1981), where fire return interval is short, low intensity burns prevail, and ecosystems come

to absorb fire as a routine, n o n - t r a u m a t i c event. I t has also been suggested (Komarek, 1965) t h a t fire-related heating is mutagenic to seeds. If so, high pyricity may, so to speak, p u n c t u a t e equilibria, and hasten genetic evolution. Moreover, in fire-prone environments, the chemical and morphological features of m a n y species suggest t h a t t h e y have evolved to promote fire in were on the brink of a fire catastrophe, plants would reduce their probability of reproductive success b y producing highly flammable compounds, and vegetative formations, such as the California chaparral, in which plants copiously produce highly volatile organic compounds (Rundel, 1981) would not be viable.

The floor T h e evidence for an 0 2 floor (the minimum 02 value at which fire will occur) is stronger t h a n t h a t for the ceiling. Cope and Chaloner (1980) first placed the 02 floor at 0.3 of the present level, or 6-7%, based on studies of the combustion of CO in nitrogen atmospheres. L a t e r t h e y revised the figure upward to 13%, based on the observations of diffusion flames in biotic fuels and work of Rashbash and Langford (1968) in burning cribs of woody material in O2-depleted atmospheres. Watson (1978) found t h a t fire will not spread in dry, shredded paper below about 16% 02 . As it stands, the floor concept makes no distinction between flaming and glowing combustion. This distinction m a y be important. I t is known from the manufacture of charcoal t h a t 02 deprivation shifts the balance from flaming to smoldering combustion and increases charcoal yield, and t h a t charcoal kilns continue smoldering even under tight restriction of air supply. Russian calculations and experiments (Gundar, 1976) indicate peat can sustain fire u n d e r 02 levels as low as 5-10%. This is especially pertinent because fusain, which has been used to test for the presence of fire, probably originated in the smoldering combustion of dry peat. T h u s the presence of fusain cannot presently be used to exclude the possibility of 0 2 levels dropping below ten percent.

228 Biotic response

If 02 has varied, and the biosphere has responded gradually to associated changes in global pyricity, major structured changes should be evident in the fossil record. T h e hypothesis of high 02 m a y be rejected for geologic periods in which fire-intolerant forms were common. Dominance of forms showing fire defense is necessary, but not a sufficient condition of a high 02 atmosphere. Stronger evidence is provided by the appearance of fire defenses in organisms adapted to environments t h a t would be too wet to burn at current 02 levels. T h e question is, what morphological features indicative of fire tolerance or sensitivity are to be sought in the fossil record? This is a novel question, and careful answers to it will not come quickly. T h e r e is a massive literature on fire and fire ecology (extensive works include: Batchelder and Hirt, 1966; Daubenmire, 1986; Gill et al., 1981, Komarek, 1961-1976; Pyne, 1984; Heinselman and Wright, 1973; Kozlowsky and Ahlgren, 1974; Mooney et al., 1981; Mooney and Conrad, 1977; Rothermel, 1983; Wein and McLean, 1983; Wright and Bailey, 1983). I t seems likely physical fire stresses will select for convergent morphology among communities and organisms subject to similar fire regimes. T h u s we can guess at the structure of high O 2 communities on the basis of vegetative changes brought about by human intensification of fire regimes. Because both the Cretaceous and the Carboniferous were much warmer than the present, and because the fossil plant record is so much richer in swamps t h a n on dry land, structural observations from modern tropical systems (e.g. Hopkins, 1965; Coutinho, 1982; Trollope, 1982; Fatubarin, 1987) and from swamps (e.g. Gundarson, 1984; Hamilton, 1984) are especially valuable. T h e fossil record must be approached cautiously. T h e plant kingdom of putative high 02 periods was radically different from t h a t of the present, and patterns of fire tolerance are often counterintuitive, especially when they extend across large differences in climate or taxonomy, or to poorly understood features such as but-

tress roots and Cypress "knees" (Brown, 1984). For example, examination of angiosperm foliage could easily lead to the conclusion t h a t fern-like leaves are fire sensitive. Because cycads are very slowgrowing, and tree ferns and trees with buttressed roots are associated with climates antithetical to fire (at least at present 02 levels), the uninitiated would not expect them to tolerate burning. Yet, bracken (Ptericliurn aquilinium) and m a n y other ferns thrive under routine burning; tree ferns, e.g., Cyathea sp., Dicksonia sp. (Duncan and Isaac, 1986; Cortlett, 1987) are often among the few components of the forest understory t h a t survive when tropical moist forest is cleared and burned; cycads proliferate under fire disturbance in Florida, South Africa, and Australia; and it is not u n c o m m o n to see large buttressed trees as lone survivors in pastures t h a t have been newly formed by clearing and burning tropical forest (pers. obs.). M a n y ecosystems subject to frequent or severe burning are now dominated by gymnosperms, including pines, cypress, redwoods, and araucarias. T h e systematic and historical aspects of fire tolerance in gymnosperms have not been studied, to say nothing of less dominant groups, such as ferns, lycopods, sphenopsids, relict conifers, and cycads. S t u d y of wood as a material (e.g. Siau, 1983) and as a fuel (e.g. Tillman et al., 1981), applied in conjunction with observations of the a n a t o m y of fossil woods (Carlquist, 1975; Cichan, 1985a, b, 1986a-c; Knoll and Niklas, 1987), m a y permit relatively rigorous deductions about paleo fire ecology, b u t to date no work has been done on this line. Woody tissue is composed primarily of cellulosic elements (polysaccharides) and lignin (phenolics). Of the two, lignin has the higher heat content, but is less easily volatilized, has higher energy of activation, and is more inclined to smolder t h a n to flame. Conifers have higher lignin concentrations than angiosperms. For example, among N o r t h American species used for pulp manufacture, the man lignin content for gymnosperms was 29.1% ( N = 39, std dev = 3.48), while mean lignin content for angiosperm species was 22.2% ( N = 2 4 , std d e v = 2 . 6 1 ) (computed from Isenberg, 1980). High moisture contents

229 are found earlywood than in latewood, and in sapwood (outer growth rings) than in heartwood (Saiu, 1983). In high 02 conditions, this might impede combustion of upright stems. Density may also be critical. Dense wood is more constant in its flammability than is lighter wood; being less porous, it holds less water, and its moisture content is less variable. Woods with higher densities are generally more difficult to ignite, but have higher heat contents per unit volume than lighter woods. By contrast, lighter woods tend to be flashy when dry and unburnable when wet (Tillman et al., 1981). Possession of fibrous wood and ability to maintain high water content, as found in both African and New World Bombacaceae (baobab and sumah ma or ceiba), and many palms, seems to be associated with fire resistance. Wet and dense materials have high thermal conductivity (Siau, 1983), which would put the cambial tissue (an absolutely vital layer of cells lying just below the bark in angiosperms and gymnosperm trees) at risk. Thick bark (periderm) offers protection to the cambial layer, and is strongly associated with fire tolerance. Thus the fossil record of bark m a y be even more revealing than that of wood. There is ample fossilbark, but it has never been systematically studied.

The predicted convergent pattern In general, pyric stress, and modification to it, is horizontally stratified. Fuels accumulate and become continuous along the ground (< 1 m). Ground fires eliminate delicate and fire prone structures in the mid-canopy (one to several meters) and tend to prune back, if not eliminate, mid- and upper canopy. However, understory response is notoriously complex (Mount, 1974). Low intensity fires, confined to regions near the ground, as is the case in many coniferous canopies (Christensen, 1981), often allow the survival of less sensitive components of mid-canopy. It is rare, in any forest, for the upper stratum of tree crowns to get sufficiently continuous and sufficiently dry to carry a fire front. Hence crown fires are uncommon. Mid canopy "ladders," i.e., fine, dry plant material that con-

nects the upper canopy to the ground, greatly increases the extent to which ground fires "crown out." Most trees die when their crowns burn, although a few, for example many eucalypts (Cochrane, 1968; Mount, 1974) and some palms and oaks (pers. obs.; Trabaud, 1984), apparently thrive, despite periodic defoliation by fire; many resprout from roots. Many communities, including boreal forest (Heinselman, 1981), Nothofagus (Veblen and Lorenz, 1987), and many eucalypts (Cochrane, 1968, 1969), have adapted to crown fire regimes by seeding prolifically after a killing fire. This typically produces single-aged stands. If crown a n d / o r ground fires are too frequent, seedlings fail to achieve sexual maturity, and the upper strata are eliminated. 02 dirven pyricity would increase the hazard of crown fires by allowing wetter fuels to burn. This would especially threaten communities, such as chaparral, which form dense canopies near the ground, and which rely for fire protection on the high moisture content of verdant stands. Fire stress tends to favour clonal plant reproduction (e.g. Keeley, 1981; Trabaud, 1984; Fatubarin, 1987) a n d / o r colonization through copious seeding. Where fire recurrence is too frequent to allow plants to set seed, seed reproduction fails. Angiosperms are unique in the rapidity with which they set seeds, and in their ability to complete a generation within a year. Thus, it is likely that strategies for fire tolerance shifted from clonal toward seed-based strategies in the Cenozoic. Where recolonization is achieved by seeding, seeds must be able to survive fire. Alternatives include retention of seeds by the plant in locations that are not strongly exposed to fire, and fireproof seeds or seed capsules (legumes, many gymnosperm cones). Fires often initiate patterns of dynamic community succession (seres). "Fire weeds" often proliferate after burning. In high latitude forests, an herbal weed generation is often succeeded by woody transients, e.g. first brambles (e.g. Rubus ), then aspen ( Populus tremuloides ), which in turn are succeeded by climax forest species. In cyprus (Taxodium) swamps, the succession goes from herbaceous "prairie" (water

230 lilly, arum, sedges, sphagnum) through cyprus swamp, with or without a bush understory, through mixed communities of cypress and other species. Periodic burning tends to prevent the climax mixed community from developing (cause a retrograde sere) (Hamilton, 1984). The vegetative history of Northern Europe suggests t hat burning of moist forest may result in extensive bog formation (Eyre, 1968). Tropical forests also exhibit seral patterns (Kellman, 1984; Corlett, 1987; Uhl et al., 1988), but these a r e , in general, poorly understood, and, like the tropical flora itself, are likely to be diverse in both morphology and taxonomy. For example, slash-andburn agriculture in tropical forest may be succeeded by stump sprouts, bamboos, Cecropia sp. (Americas), grasses, viney thickets (often Lantana in East Africa and Central America), bushes, or fern brakes. Which groups succeed, why, where, in what sequence and proportion remain open questions. If fire set off seral progressions, but in fossilization, these were interpreted as a single stratum, it is likely that early successional groups would be interpreted as understory. This would mask the effects of fire pruning by creating the illusion of a multi-stratal canopy. There are ways around this problem. For example, seral progressions, apparently representing sere durations of hundreds of years, have been detected through analysis of the fine scale vertical stratigraphy of spore deposition in bituminous coal (Chaloner and Muir, 1968; Smith, 1968), and a recent study of root penetration sequence in coal balls demonstrated the existence of well defined sere in coal balls from the Pennsylvanian (Raymond, 1988). Features anticipated in high-O 2 floras include: absence of long-lived, fire-tender plants in mid-canopy layers; upper canopy often lacking, and where present, open and free from downward hanging vines or fronds that would allow ground fire to move into the canopy; leaves on low and mid-height plants growing as dispensable appendages t h a t can grow quickly and photosynthesize in fire-free intervals; perenniating buds kept below ground (many ferns, cycads, and equisetes), heavily sheathed (cycads), or

restricted to high canopy positions with a low probability of scorch (savanna trees); abundance of roots, rhizomes, and lignotubers, t hat store energy and facilitate regrowth; and thick, dry, and possibly lignin-rich bark t h a t provides insulation while preventing burning or retarding fire. Tufted foliation t hat draws the heat of flaming combustion away from the plant's central axis and protects the apical meristem, as found in palms, tree ferns, fire-tolerant grasses, agave, and cycads, are highly viable under frequent burning (Gill, 1981). Further advantage is gained by putting this form atop a columnar trunk, wrapped in cork-like tissue. In total, high 02 landscapes were probably complex mosaics of one and two-tiered vegetative formations. These would have had lower biomass loadings and less complete foliar coverage than their climatic counterparts in undisturbed modern, low 02 systems, but may resemble systems subjected to intense fire stress through the agency of man. It may be noted t hat routine disturbance would violate the assumptions t h a t underlie the inference of paleoctimates from foliar morphology (e.g., Upchurch and Wolfe, 1987). In my observations, small to medium sized leaves with serrate margins are common in secondary vegetation in disturbed tropical moist forest. If this observation holds, leaf morphology studies will, in periods where disturbance by fire was common, tend to classify regions that are actually mesic (moist) as seasonal or semi-arid. Anomalies are likely. Fire response is affected by microclimate, drainage, and topographic factors. Lightning and other sources of ignition may have limited the presence of fire in some regions and periods. If high latitude forests had adopted wide spacing intervals between trees to increase light interception at low illumination angles (Creber and Chaloner, 1985), they may have been pre-adapted to avoid the risk of crown fires. The fossil record

Reconstruction of 02 history (Fig. 1) suggests t hat post-Devonian 02 levels, with the exception of an 02 dip in the interval from the Late

231 Permian to the Triassic, have generally been higher than those of the present, with a strong rise in the Carboniferous and Lower Permian, and a broader but lower rise centered over the Cretaceous. The plant record is incomplete, and strongly biased toward swamp environments. The putative low-O 2 periods are thought to have been dry, and are poorly represented. For putative high-O 2 intervals, especially for the Pennsylvanian peak, swamp systems have left abundant (e.g. coal) and remarkably detailed (e.g. petrifications in the form of coal balls) fossil traces. Swamp fossils are hard to interpret because of the prevailing influence of groundwater hydrology on fire behavior. To appreciate the difficulty, imagine trying to reconstruct present global fire regimes based on char horizons in the Okefenokee Swamp, the Everglades, and Indonesian peat swamps. The upland plant record is a hodgepodge of occasional, discontinuous, in situ preservations, plus a relatively continuous, but still incomplete, background of spores, leaf fragments, and other materials t h a t were transported into lowland sedimentary depositional environments. Transported material is problematic. Much of it probably originates from riparian ecosystems (Spicer and Greer, 1986). Spores say little about whole plant morphology; leaf fragments are little better. Thus reconstructions of upland plants and communities are highly conjectural. Moreover, because paleobotany has tended to emphasize taxonomy over physiology, morphology, and biochemistry, the delineation of the diagnostics for pyricity, such as bark thickness, geometry of fusain deposition, tissue chemistry, and canopy structure will in many cases require reexamination of actual fossil materials. The present scope is less ambitious; and is confined to a speculative search for pattern in the putative high O2 periods, focusing first on the Carboniferous-Permian, and then on the Cretaceous- Paleocene.

The Carboni[erous The Carboniferous has yielded magnificent fossils of swamp plants. I find the upland record

is inadequate to support evaluation of fire trends, and thus, despite the hazards of doing so, have only reviewed the swamp record. The resulting picture is very incomplete. It is thought that Carboniferous swamp and upland floras were floristically distinct (DiMichele et al., 1985), and there is no basis for assuming t h a t upland and swamp communities manifested similar trends. Arborescent lycopods strongly dominated swamp floras of the Lower and Middle Pennsylvanian. They occur with a flora of lower stature. I n the mid-height range (< 12 m), this featured tree-fern like pteridosperms and marattialean ferns, sphenopsids (distant relatives of modern Equisetum), and lianes (vine-like plants), including, among others taxa, Lyginopter/s, and Cordiates, a group t h a t had some parallels, e.g., in wood structure, to modern conifers. Rhizomatous herbaceous lycopods and ferns as well as non-vascular plants grew near the ground. The relative dominance (tissue volume of fossil material) of arborescent lycopods vs. smaller plants varied, with the smaller groups becoming more prominent in what appear to have been drier periods. High fusain content and evidence of greater oxidation, such as lower ratios of shoot to root tissue, are used as evidence of drying (Phillips et al., 1985). In the Upper Pennsylvanian the arborescent lycopods became extinct over most of their range, presumably due to drying a n d / o r tectonic events that restricted the extent of suitably wet habitats, and were replaced by a taxonomically more diverse flora. Fragmentary fossil evidence suggests that gymnosperm groups, including the Gondwanan Glossopteris flora (Archangelsky, 1986) and the cordiates, prevailed in uplands and in colder climes. Keeping in mind prior cautions regarding swamp evidence, the Carboniferous evidence can be interpreted to support the hypothesis of high 02 according to the following reconstruction. Arborescent lycopods, which relied on thick bark, rather than woody tissue, for their structural support, and whose unbranched and tall (often 30 m) trunks are reminiscent of fire-pruned araucariads (Veblen, 1982a) or palm stands, functioned as fortresses protecting sensitive

232 vascular systems and apical tissue from fire. T h i s allowed t h e m to form a climax under periodic burning. As is observed in modern cypress s w a m p s (Gunderson, 1984) in times of low water, fire m a y h a v e burned deeply into the peat, killing roots, and d e v a s t a t i n g the c o m m u n i t y , and initiating a sere. T h e r e is evidence of seral p a t t e r n s in the Coal Measures. For example, a s t u d y of root penetration events in Mid-Pennsylvanian coal from Iowa ( R a y m o n d , 1988) has identified a progression from cordiataleans to Psaronius and Medullosa and then to arborescent lycopods. Separation of the effects of w a t e r level from those of fire is not simple, b u t there is evidence t h a t fire was involved in a t least some such seres. For example, the cordinates and arborescent ferns and seed ferns achieve their greatest dominance in the periods t h a t contain more fusain and fewer arborescent lycopods (Phillips et al., 1985); and fusain exhibits p a t t e r n e d behavior in relation to fine-scale vertical s t r a t i g r a p h y and palynology in bituminous coal (Chaloner and Muir, 1968). I t seems likely that, under high 02, fire would be present t h r o u g h o u t the sere. Judging from the fire tolerance of m a n y m o d e r n ferns, morphology cannot rule out fire tolerance in Carboniferous groups with fern-like a t t r i b u t e s such as Psaronius and Medullosa. Modern Equisetum is almost u n b u r n a b l e (pers. obs.), p e r h a p s due to high silica content 1; the sphenopsids m a y have used the same defense; in addition, their growth habits, including hollow s t e m s and growth from extensive systen~s of rhizomes, are suggestive of traits t h a t allow modern b a m b o o s to recover quickly and assert post-burn dominance after their above ground tissue has been killed by burning (Veblen, 1982b).

i There is mixed evidence on silica. Wildland studies consider silica to be less important than other cations (Rundel, 1981) in retarding fire. On the other hand, the unusually high silica content of rice straw is given as an explanation for the very great difficulty of burning rice straw, either as fuel, or in cleaning up agricultural waste (Barnard and Kristofersen, 1985).

T h e a p p a r e n t propensity to fire tolerance t h a t runs t h r o u g h g y m n o s p e r m t a x a m a y be a carryover from the role of fire in n a t u r a l selection during the period in which the g y m n o s p e r m s originated. O t h e r seemingly sensitive plants m a y have been transients, or denizens of fire refuges created by networks of open w a t e r in the s w a m p system. S o m e cordiates, for example were probably mangroves. Lyginopteris w e n t extinct in the Lower Pennsylvanian, p e r h a p s because its vine-like m o r p h o l o g y did not p e r m i t it to a d a p t to routine burning. In the aggregate, the Carboniferous s w a m p floras seem to fit the predicted c o n v e r g a n t p a t tern for high 02 in t h a t t h e y invested heavily in the production of b a r k and belowground reproductive and storage tissues. B a r k and roots, as opposed to foliage and wood, are b y far the d o m i n a n t tissue c o m p o n e n t s in the Pennsylvanian coal balls (DiMichele et al., 1986). T h i s has been i n t e r p r e t e d as a t a p h o n o m i c effect, b u t also seems to reflect propensities toward thick b a r k and toward storage of tissue below ground. As shown in Fig. 2, the e s t i m a t e d family level diversity of clonal, r h i z o m a t o u s plants, after declining in the Mississippian, increased in the Pennsylvanian (Tiffney and Niklas, 1985). T h e wood s t r u c t u r e of Carboniferous plants (Cichan, 1986a) shows t h a t m a n y understory groups were at least the equals of modern plants with respect to their c a p a c i t y to t r a n s p o r t water. This, along with high tables, m a y suggest a sort of " w e t b l a n k e t " fire defense. T h e a b o v e evidence is n o t conclusive. Arborescent lycopods have no m o d e r n analogs. T h e i r growth seems to have been determinate. T h a t is, their tissue differentiation seems to have followed a p r e - p r o g r a m m e d order, as in higher animals or flowers, r a t h e r t h a n being free-form, and in the normal branching p a t t e r n of living trees. D e t e r m i n a t e growth m a y h a v e left plants quite vulnerable to disturbance. T h e thick b a r k of arborescent lycopods m a y be an unavoidable consequence of their evolutionary history. T h e a p p a r e n t fact t h a t some arborescent lycopods reproduced only once a lifetime m a y have been an intolerable liability u n d e r high pyricity. And the i n t u i t i v e sense t h a t the

233

[ ] Sil [ ] Dev [ ] Miss

• Penn [ ] Jur [ ] Paleo • Perm [ ] LowK [ ] Neo [ ] Trias • Up K

Cioa,ioi,, r

Herb

I!iiiiiiiii

~i i

0

20

40

60

80

100

0

2

4

Li~ne.'lik~ i i i i

6

8

10 12 14 16 18 20

Number of Families Fig. 2. Indicated distribution of p l a n t reproductive strategies over geologic time. R e d r a w n from e s t i m a t e s given in Wiffney a n d Niklas (1985). P u t a t i v e h i g h O 2 periods ahown in darker hues. T h e s e d a t a should be considered suggestive only; t h e fossil record often does n o t p e r m i t precise determinations. Refer to original source for detail.

drooping, large-leaved, and apparently succulent fern-like groups would have been fire-sensitive may be correct. The Cretaceous A fiery Cretaceous would add a new twist to the debates over the origin of the angiosperms and the evolution and demise of the dinosaurs (It also has implications for the hypothesized impact event at the C r e t a c e o u s / T e r t i a r y boundary, see Appendix). It is now generally accepted t h a t the angiosperms originated in the Cretaceous, probably as weedy herbs and shrubs in stressed and disturbed, semiarid, upland (non-swamp) habitats (Doyle and Hickey, 1976; Doyle, 1977). Fire, like the disturbances proposed in the theory of upland origin, would have placed a premium on the ability to set seeds rapidly and colonize new habitats. Moreover, it would extend the domain of the " u p l a n d " disturbed sites into relatively mesic (moist) habitats. This eliminates two objections to the theory of upland origin ... the observations t h a t

(Hughes, 1976) in modern systems, plants adapted to the relatively stressful upland environments are generally weak competitors in lowland habitats, and t hat scaliform perforation plates, a feature of the conductive vessels in plant tissue t hat is associated with older and more primative angiosperm groups, is strongly associated with mesic conditions (Carlquist, 1975).

V O L U M

E %

10 9 8

7 6 5 4 3 2 1 0 Carboniferous Paleocene Cretaceous Eocene

All

Fig. 3. Fusinite a n d semi-fusinite c o n t e n t of coals of different ages. D a t a from P S C S B D B (1988). S a m p l e size are n o t e d on bars. D a t a includes all n o n b r o w n coals in t h e P S C S B D B .

234 In the Early Tertiary, declining O 2 would have reopened the mid canopy niches eliminated by high pyricity. In keeping, it is t h o u g h t that, " t h e family-level modernization of angiosperm Floras in the Early T e r t i a r y was due, in part, to the expansion of multistratal vegetation over large geographical areas ... Foliar physiognomy indicates t h a t lianas and understory plants are more abundant and diverse in the Early Tertiary than in the L a t e Cretaceous (Upchurch and Wolfe, 1987)." As shown above in Fig. 2, the family level diversity of all groups increased through the Cretaceous. T h e most significant radiation was t h a t of non-clonal angiosperms (Tiffney and Niklas, 1985), with herbs and vines, which had been rare or absent, becoming r a t h e r common.

Trophic considerations (Coe et al., 1987) suggest t h a t terrestrial herbivorous dinosaurs achieved very high population densities, and thus t h a t upland vegetation was at least regionally, prolific and stressed by heavy browsing. This implies a situation of intense interaction among fire, tetrapod fauna, and plants; a situation not unlike t h a t found in modern African savanna (Beuchner and Dawkins, 1961). Heavy browsing m a y have created gaps and hence fire breaks, thereby reducing the size of fires and decreasing the frequency of burning. Fire presumably increased the supply of succulent vegetation along the ground while depleting the lower canopy. This could have contributed to the regional demise of the high-browsing sauropods and stegosaurs at the J u r a s s i c / C r e t a c e o u s boundary (Bakker, 1978) and favored their succession first by smaller, low-browsing dinosaurs, and eventually, by mammals and birds. Aquatic tetrapods should be relatively immune to fire. Unlike land tetrapods, freshwater tetrapods do not show mass extinction, although marine aquatic tetrapods suffered extinctions at the Jurassic-Cretaceous and Cretaceous-Tertiary boundaries, in phase with b o t h the highbrowsing dinosaurs and dinosaurs in general. T h e latter, while not disproving the fire hypothesis, does serve as a reminder t h a t changing fire regimes are likely to have been part of a suite of environmental changes.

T a x o n o m y and patterns of evolution m a y provide further evidence. T a x a surviving periods of high pyricity must have been able to survive fire. For example, the more primitive members of angiosperm taxa of Cretaceous origin should be fire-adapted, as should taxa t h a t survived the Carboniferous and Permian. Moreover, if relaxation of the constraints imposed by fire increased niche space and fostered radiation and diversification in the Cretaceous-Paleocene, it should also have done so in the Carboniferous-Permian. With some imagination, the 0,~ decline of the Permian can be related to the radiation and diversification of the gymnosperms, while the O 2 decline of the T e r t i a r y is related to the radiation and diversification of the angiosperms. Differences would be expected because the former period headed into the glaciated, and generally arid times of the Pangean climate (Kutzback and Gallimore, 1989), while the latter headed into the warm and moist times following the separation of Gondwana. Fusain T h e fusinite 1 present in coal preserves a hist o r y of swamp fires. Figure 3 compares the abundance of fusinite and semi-fusinite in coals of different ages, based on d a t a in the Penn S t a t e Coal Sample Bank and D a t a Base (PSCSBDB, 1988). Of the periods represented, the Carboniferous, Cretaceous and Paleocene have been proported t o have high 02 (Fig. 1), while the Eocene does not. As shown, the average fusinite and semi-fusinite contents for the Eocene are less t h a n half those for other periods. This evidence is far from decisive. T h e r e is much scatter in the data, the Eocene is less well represented t h a n other periods, and because fusinite is relatively inert, its concentration would be expected to increase somewhat, though not dou-

, Fusinite and semi-fusinite (partially fusinized material) include both pyro- and degradohminite, This former is distinguished by clear, brittle, charcoal-like cell-wall structures (Harris, 1981). Degradofusinite is relatively uncommon in Carboniferous and Cretaceous coals (A. Davis, pers. comm., 1988).

235 ble, over time. Furthermore, 02 enhancement would be expected to increase both the rate of burning, and hence charcoal formation, and the rate of charcoal destruction by burning. It is thus likely, but not guaranteed, that 02 enhancement increased the rate of charcoal deposition. Some coals contain astounding quantities of fusinite. In around 10% of the samples listed in the PSCSBDB (1988), the combination of fnsinite and semi-fusinite exceeds 15% of sample volume. Indeed, the difficulty in explaining how so much charcoal got into swamp peat lead early researchers to the conclusion t h a t fusinite could not be of pyric origin (White, 1925). The Okefenokee Swamp, which is thought to be a good analog for Tertiary coal swamps, is subject to frequent fires and contain abundant charcoal (Cohen, 1984). Carboniferous coal beds, on the other hand, are thought to have originated in raised, megathermal peat bogs. The presumed closest modern analogs of such swamps, the swamps of Indonesia and Malaysia, are largely char-free (B. Cecil pers. comm., 1988), although the combination of drought and human disturbance lead to burning of up to 0.5 m of peat in the 1982-1983 ENSO event (Goldammer, 1987).

Conclusions The terrestrial biosphere can tolerate high pyricity. The ecological consequences of fires resulting from 02 levels approaching or exceeding 25% would probably be no more drastic than those of anthropogenic activity in modern years, except t h a t they would be sustained for millions of years. If 02 has varied, the fossil record should show, not a catastrophic fire event, but rather a complex pattern of interrelated, gradual ecological changes brought about by changing fire regimes. Initial searches for such patterns provide hints of agreement between major taxonomic and morphological changes and predicted trends in 02 history. However, reconstructions leave so much to the imagination that final verdict is a matter of judgement.

Many of the ambiguities in the fossil record can be ameliorated by experimental studies and focused observations. In particular, research is needed on the effects of 02 on fire and on the mechanisms by which plants may have countered stress arising from those effects. For example, it is not known whether or under what circumstances green foliage can burn; nor do we have a sense of the extent to which maintenance of high moisture content or changes in organic chemistry can provide defence against high O 2. The avenues for extracting 02 history from the biotic record have barely been explored, and the biological and geochemical implications of 02 variability are essentialy unstudied. The fusain record has not been worked. The effects of 02 enhancement on char formation is unknown and its effects on the formation of peats and coals has not been considered; the diagenesis of charcoal is poorly studied (Robinson, 1987). In closing, I should like to note t h a t the above analysis has proceeded under the unrealistic assumption t h a t 02 variation would have no effect other than to change pyricity. 02 is basically poisonous to fife, and eukaryotic organisms accept various physiological costs to protect themselves from its influence. The direct effects of 02 on fife will compound the effects manifested through fire. For example, ribulose diphosphate carboxylase, the main enzyme in C3 photosynthesis and probably the most common enzyme on Earth, is very sensitive to the mixing ratio of O2:CO2, and is inhibited by excess 02 (Edwards and Walker, 1983; Farquhar, 1986; Von Reis, 1988). The photosynthetic constraint may produce a sharper 02 threshold than do pyric considerations, and a flora t h a t was stressed, simultaneously by fire and declining photosynthetic efficiency is likely to have responded differently than one confronted with fire alone.

Acknowledgements I thank my dissertation committee (U.C. Santa Barbara, Geog.) for leaving me latitude, the National Center for Atmospheric Ressearch and NSF Grant 86-09211 for support during the

236

gestation period and the E a r t h System Sci. Center of Penn. S t a t e U. for time in an intellectually stimulating environment. I would like specifically to acknowledge: B.H. T i f f n e y (UCSB, Geol.), for numerous suggestions, and for mention of animals; H. Holland (Harvard, E a r t h Plan. Phys.), whose queeries about fire p r o m p t e d the first draft; A.H. Knoll (Harvard, Bot.) for criticism on a previous draft; A. Davis and D. Glick (PSU Energy and Fuels Res. Lab) for access to coal data, W. Kinneman (PSU Combustion Lab) for insights on fire behavior; L. K u m p (PSU ESSC) and R. Berner (Princeton, Geol. Geophys.) for clarification of geophysical processes; J. von Reis at (Calif. Polytech. Inst., Botany) for bringing R u B P to my attention; and the NCAR library staff for locating Watson (1978). Those acknowledged may not agree with parts of the text, and I accept sole responsibility for any errors t h a t it m a y contain.

Appendix A - - N o t e s on high O2 as affecting global fire at the K / T boundary T h e iridium spike t h a t marks t h a t K / T boundary is associated, in at least three sites, with a spike of fine particles of soot and elemental C (Wollbach et al., 1988). Extrapolated globally, the mass C represented in this spike amounts to about 10% of the C mass of the current biosphere. T h e presumed C source is combustion of living a n d / o r fossil biomass through fires caused by the impact event. T h e a m o u n t of material burned is likely to have been an order of magnitude higher t h a n the showing up in the spike. If the atmosphere of the late Cretaceous was, indeed, high in 02 , it becomes difficult to account for a C flux of the size estimated based on combustion of living biomass alone. Although high O 2 would have increased flammability, the Cretaceous biosphere would have already been thinned by fire at the time of the boundary event. Hence the mean Cretaceous biomass loading of 1.5 g C cm -e used by Wollbach et al., 1988 and which the authors themselves state is an upper limit, is probably too excessive, even

for denser forests, and much too high for the global mean. F u r t h e r m o r e the biomass t h a t was present was probably less available t h a n would be assumed based on modern evidence. This weighs in favor the supposition t h a t a significant fraction of the combusted material was at least peat, if not fossil fuel, and suggests t h a t useful lines of inquiry would include (a) investigation to see if regression, in the late Cretaceous, m a y have exposed peat and carbonaceous shales and made large amounts of C available for burning; and (b) looking for a large increase in the fusain content of coals and shale beds at the K / T boundary.

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