C4 photosynthesis

Chemosphere, Vol.27, No.6, pp 947-978, 1993 Printed in Great Britain

0045-6535/93 $6.00 + 0.00 Pergamon Press Ltd.

PALAEOECOLOGY, PAST CLIMATE SYSTEMS, AND C 3/C 4PHOTOSYNTHESIS Robert A. Spicer Earth Sciences Department, Oxford University, Parks Road, Oxford OX1 3PR, United Kingdom ABSTRACT The geologic record shows unequivocally that the present world is unusually cold; the so called 'greenhouse' condition has been normal for planet Earth for the past 500 million years. Continental positions, orbital parameters, and atmospheric composition strongly influence global climate on timescales ranging from 10 8 to 10 2 years. Atmospheric CO 2 is an important contributing factor in determining average global temperature, and is particularly important in influencing changes over shorter timescales (say < 10 5 years). Carbon sequestering on land has varied substantially over the past 500 million years and may be correlated with changing climate. Most terrestrial carbon sequestering operates on biological timescales (< 10 5 years) rather than geological timescales (> 10 5 years). Terrestrial carbon sequestering is strongly influenced by the biology of the organisms involved and it has been shown that terrestrial carbon sequestering is greater in ever-wet conditions. The distribution of the sites of greatest carbon sequestering switches from low latitudes during icehouse times to higher latitudes, >40 °, during greenhouse times, except maritime sites. Evolutionary factors, e.g. competition, and climate change have led to major ecosystem restructuring during the past 500 million years. Pre-change biodiversity is therefore critical in determining the nature and rate of restructuring particularly with respect to plants which are the only group of organisms capable of carbon sequestering. There exists a number of uncertainties as well as probabilities involved in estimating sequestering ratios and climate changes; Estimates of past carbon sequestering are likely to be too low because dispersed fossil organic matter is inadequately inventoried. Numerical climate model results are unreliable unless evaluated against fossil and sediment data. Terrestrial carbon sequestering is unlikely to dominate tectonic controls but as it operates on a shorter time scale it has a strong short term effect and could well tip the climate balance in critical situations. Most extant land plants have a C 3 photosynthetic pathway. However, under conditions where photorespiration can reduce photosynthetic 947

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efficiency, warmth and high 0 2 concentrations, many unrelated plants have independently evolved C 4 pathways. C 4 plants have different water relations and competitive characteristics to C 3 plants and clearly ecosystem structure and carbon sequestering are likely to change with global warming. By studying the different isotopic signature bequeathed by these systems the fossil record can provide critical data on the dynamics of plants with these systems under changing climatic conditions: data that again are essential for effective ecosystem management strategies. INTRODUCTION For eighty percent of the past 500 million years, Ma, the mean global temperature of the Earth has been higher than at present (Fig. 1) (Frakes, 1979). The "normal" condition for the planet is the so-called greenhouse condition and it is the present that is abnormally cold.

I1 Time

06years-

0 1.64

MEAN GLOBAL TEMPERATURE PERIOD Quaternary

PresentTemperature

Colder '~ Warmer ~,~.

Pliocene

5.2 Miocene

23.3 3S.4 56.5 65

Oligocene Eocene

Paleoecene

i I i j ! l

Cretaceous

i i

Jurassic

i

146 206 Triassic

245 290 362.5 408.5

Permian Carboniferous

Devonian Silurian

Figure 1. Mean global temperature relative to the present time modified from Frakes (1979).

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It follows that a great deal can be learned about the behaviour of biological, geological, and climate systems under greenhouse conditions by studying the legacy of these past times in the rock record. Although not complete this record provides the only absolute data on how Earth systems behave under conditions other than those of the present and provides the only long-term perspective open to us for assessing potential planetary management practices. The geologic record shows clearly that under different climatic regimes the sites and rates of carbon sequestering vary and these variations are predictable even if the detailed mechanisms of change are as yet poorly understood. These patterns of change are not the product of hypothetical model predictions but are a fact of the historical interaction of all this planet's systems under natural conditions. Global climate varies on a range of temporal scales. Some variations are influenced by slowly moving continental configurations (Barron et al., 1980; Barron et al. 1981; Barron, 1983; Barron et al., 1984; Barron & Washington, 1984; Worsley and Kidder 1991), and it is only over tens or hundreds of millions of years that significant change is discernible. Variations in orbital parameters take place over tens to hundreds of thousands of years and have clearly played an important role in determining the extent of glaciation within the past million years (e.g. Kutzbach & Otto-Bliesner, 1982; Kutzbach & Guetter, 1986) and similar orbital forcing factors may have determined other climatic phenomena in the Mesozoic (e.g. Barron et al., 1985). Variations in atmospheric composition due to volcanism and biotic processing appear to operate both on geologic timescales measured in millions of years and the much shorter biological timescales of less than a century. By taking a long temporal perspective we take a large sample of the changes that this planet has passed through as well as the conditions that led to those changes. It might be argued that our interpretation of the past is inevitably constrained by our experience of the present world. This is certainly true if we study ancient systems by mere extrapolation from the present, for example by the application of nearest living relative (NLR) techniques to climatic interpretation. NLRtechniques assume no evolutionary change and interpret ancient environments on the assumption that fossil forms had the same environmental tolerances as their nearest living relatives. This approach has some validity in Quaternary studies but for older fossil material the NLRtechnique becomes unreliable. Reliability declines with progressively older fossils. Fortunately most modern analyses of pre-Quaternary rocks adopt more robust approaches, reviewed in Spicer (1989b) through which

950 it is possible to study ancient systems that have no modern analogues. PALAEOCLIMATE MODELLING - STRENGTHS AND LIMITATIONS Global climate models, particularly the family of numerical atmospheric global circulation models (AGCM's), are important tools for understanding global climate change. Nevertheless they do not give consistent predictive results. This can easily be demonstrated by running several models under the same conditions: there will be as many different results as there are models (Mitchell, et al., 1990). When boundary conditions depart significantly from those of the present the disparities are amplified, but in spite of these shortcomings ACGM's have been used to model ancient climates 0Kutzbach and Gallimore, 1989; Barron, 1985; Barron, 1986) with what appears to be some success. However, for the Mesozoic greenhouse world lack of knowledge of necessary boundary conditions such as sea surface temperature result in low predictive accuracy. All models give, or are constrained to give, higher mean global temperatures under elevated atmospheric CO2, but the patterns of climate change vary between models. Inconsistency of performance between models are partly due to the fact that AGCM's are highly simplified, for example by using "swamp" oceans in which there is effectively no depth or circulation, and any significant departure from the present world for which the models have been "tuned" is likely to introduce spurious results. More realistic coupled ocean/atmosphere models, although an improvement over solely atmospheric models, are currently rather crude and truly comprehensive models await advances in computing technology. In spite of their obvious shortcomings ACGMs can be particularly helpful for identifying the relative importance of boundary conditions at any given time. However, any given climate pattern can be generated by several sets of conditions such as different continental configurations or CO 2 concentrations. Furthermore model results can only be evaluated by comparison with actual data. For the construction of most ACGM's that data has been provided by historical meteorological records spanning at most two hundred years. Useful as this historical data is it is limited compared with the range of conditions witnessed and recorded by palaeontologicai and sedimentary data spanning hundreds of millions of years. THE NATURE AND IMPORTANCE OF THE PLANT FOSSIL RECORD AND CARBON FLUXES IN A GEOLOGICAL CONTEXT Photosynthetic plants are the primary producers of organic matter and they do this by processing the atmosphere and sequestering carbon.

951 This is normally a temporary process as through respiration either within the living plant itself or post-mortem virtually all the assimilated carbon is returned to the atmosphere (Sundquist, 1985). Nevertheless a small but significant proportion of plant matter escapes this fate and provides a medium to long-term store for sedimentary carbon. It is the carbon accumulated during that geological timespan that is now being released, rapidly, back into the atmosphere. In the case of land plants the preserved remains not only provide a record of carbon sequestering but, because plants are directly exposed to the atmosphere, are fixed spatially postgermination, and have to process the atmosphere efficiently under the conditions to which they are exposed, their morphology and chemistry record ancient climatic conditions more sensitively than any other organisms (Spicer, 1989b). There have been significant advances in our knowledge of the processes that lead from the living organism to the fossil condition (Spicer, 1989a) but detailed understanding of these taphonomic processes at the microscopic scale is limited. For example conventional studies of fossil pollen oxidise most dispersed organic matter in the process of cleaning the palynological preparation and so provide minimal data on microscopic sedimentary carbon. Modern preparation techniques, however, employ little or no destructive oxidation and some studies have been undertaken that show that such "palynodebris" is not only abundant but displays environmentally related patterns (e.g. Lorente, 1990). It is clear that much if not most organic matter in the sedimentary record is preserved as microscopic particles dispersed within inorganic sediments rather than as massive carbon concentrations such as coal. Research into the nature, dispersal, sedimentation, and preservation of microscopic organic particles is underway and eventually will allow us to evaluate ancient short and long-term carbon sequestering more accurately in the face of changing physical, chemical, and biological conditions. It is apparent that estimates of current carbon fluxes to and from the various atmospheric, terrestrial and oceanic reservoirs leave considerable room for improvement. The situation is significantly worse for the past. Nevertheless we can estimate the long term effects of ancient fluxes given comprehensive data on geologic reservoir content and an understanding of the processes that lead to carbon burial in its various forms. Unfortunately reliable data on the actual abundance of sedimentary carbon is difficult to obtain because: 1) only economically important reservoirs are recorded, 2) only those so far discovered can be recorded, and

952

3) such reserves are a strategic resource and data are often proprietary. Furthermore because even finely dispersed microscopic organic matter in what might be termed "normal" sediments can be the precursor of hydrocarbons, data on organic content are collected by oil companies but much of this information is not in the public domain. In general only the most carbon-rich rocks are evaluated and a significant proportion of the more dilute organic material which is present in most sedimentary rocks therefore goes unrecorded. If an understanding of previous and long-term carbon sequestering is to be obtained a global inventory of the distribution and abundance of organic concentrations not only in coal and oil deposits, but also of dispersed organic matter in sediments, is needed. If at present a comprehensive inventory of past preserved carbon accumulation and fluxes is unavailable considerable insights can be obtained on organic productivity, accumulation, and climate change by studying the distribution, abundance and characteristics of individual fossil remains. The plant fossil record is particularly rich as most plant parts have a reasonably high preservation potential (Spicer, 1989a) and although the vagaries of fossilisation mean that reviews of past plant diversity and survivorship under stress are beset with problems (Boulter, et a1.,1988; Knoll & Niklas, 1987) the overall pattern indicates that plant diversity has in creased over time and, unlike animals, extinctions are usually preceded by innovations. This strongly implies that climate change p e r se has had relatively little effect on past diversity and that loss of species has been due to competitive stresses. It may be, of course, that climate change has enhanced the survivorship and radiation of novel plant forms that ultimately caused extinctions of less well adapted taxa, but the fossil record shows that the novel forms arose from parental stock whose diversity was not initially depleted by those changes. Clearly both terrestrial and marine environments are important in understanding global systems. However, for present purposes I shall concentrate on a carbon sequestering environment that is of special interest to Malaysia; namely the formation of peat and its geological counterpart - coal. PEAT FORMATION The occurrence and distribution of coals in space and time is relatively well known and even allowing for erosion of some deposits their distribution patterns provide a secure foundation for determining changing patterns of organic productivity and carbon

953 sequestering. However, because coals rarely preserve well the morphological and anatomical details of their constituent plants and reflect only a rather specialised environment, it is essential that climate data are also derived from the wider plant fossil and sediment record. Special circumstances are necessary for autochthonous i(~ situ) peat formation (Moore, 1987): 1) Exclusion of clastics, ie non-organic sediments such as sand or clay - if clastics are present carbon sequestering still takes place but the resulting deposit is not regarded as economic and tends to be lost from global inventories of sequestered carbon. 2) Exclusion of oxygen, this is related to the presence of water as the system has to be constantly water-rich to generate anoxia coals are thus limited to particular depositional and/or climate settings - simply summarised as "ever-wet". 3) Productivity must exceed degradation, this depends on a) rates of degradation as well as productivity, b) evolutionary changes in the organisms involved, c) climate changes. 4) Accumulation must equal or exceed subsidence rates (see 1 above) or peat will not form due to dilution by clastics (McCabe, 1984). -

Preserved coal is not a complete guide to original productivity because 1) peats today form at different rates under a wide variety of climates, but always under ever-wet conditions due either to high rainfall or constant run-off/groundwater accumulation. Evaporation must be low in relation to water supply or raised salinity will limit peat growth, 2) preservation of the peats depends on local tectonic and sedimentary factors decoupled at short timescales from the carbon cycle. If accumulation equals subsidence very thick deposits can result e.g. Powder River Basin coals (Merewether and Claypool, 1980) western USA and those of the La Trobe Valley, Victoria, Australia (Gaulton, 1990). However, in the absence of other data coals are a rough guide to where water was plentiful and productivity outstripped degradation. Coals therefore give clues as to where and at what time high terrestrial carbon sequestering was taking place, but a more complete picture of past climates and biological response can only be obtained by looking at coals in conjunction with dispersed organics whether as megafossils or palynological matter. WHAT CAN WE LEARN ABOUT TERRESTRIAL CARBON SEQUESTERING FROM THE THREE MAJOR PHASES OF COAL FORMATION? (Carboniferous/Permian, Paleocene)

Late Jurassic/Early Cretaceous, and Late

954

T H E CARBONIFEROUS The Carboniferous was time when most of the coal deposits of the USA, Europe and many other areas were being laid down. The Carboniferous mire forests were dominated by the now extinct

Reconstructions of Late Carboniferous (300ma) Trees

lOm

Lepidodendron sp.

Sigiltaria sp.

Figure 2. Reconstructionsof the Late Carboniferousmire-dwelling lycophyte trees Lepidodendronand Sigillaria. (After Stewart, 1983). arborescent or tree-stature lycopods of which the Lepidodendron and Sigillaria trees are typical examples (Fig. 2). Lepidodendron was the canopy dominant and even emergent, grew to 30m or so in height when mature, was cheaply constructed in that it had very little secondary wood, and had xeromorphic strap-like leaves with sunken stomata and small surface area (Stewart, 1983; Thomas & Spicer, 1987; Spicer, 1989b). The leaves clearly were of a type normally found in dry environments where it is important for plants to limit their water loss, and yet this plant grew in swamp forests. It seems likely that the xeromorphy was a function of the inefficient vascular system. Permineralised portions of the trunks of the Lepidodendron tree show that near the base of the trunk which might be nearly a metre across the water conducting tissue was restricted to a cylinder only a few centimetres in diameter (Fig. 3, Eggert, 1961). Furthermore the organisation of the vascular tissue shows that the tree had determinate growth: that it was monocarpic and only developed a branched crown when in its reproductive phase. Palaeoecological studies show that often the swamp forests consisted of a patchwork of even-aged stands of lycopods (Fig. 4,

955

Sections through Lepidodendron a

I

10cm

cd

outercortex

0 b

secondarycortex

~

0

middle cortex

innercortex secondaryxylem - primaryxylem

d i

¢

<"

pith

Figure 3. Sections through the trunk of Lepidodendron. Left hand diagram represents a transverse section of the trunk approximately halfway up a mature tree. Note the small amount of water-conducting tissue, xylem, in relation to the total cross-sectional area. The right hand diagram represents a longitudinal section through the xylem, and where present the pith, of a mature Lepidodendrontrunk, and associated transverse sections at various points. The letters show the positions at which the transverse sections were taken. (Modified from Stewart, 1983; based on Eggert, 1961).

f_ Pre-reproductive Phase

Reproductively Mature Phase

Figure 4. The structure of part of a Late Carboniferous monocarpic lycopoddominated mire forest showing both the immature and later mature phase. This reconstruction is based on a detailed palaeoecologicalstudy of an actual fossil site conducted by DiMichele and DeMaris (1987).

DiMichele & DeMaris, 1987). Both mature and immature stands had a very low leaf area index, LAI, and leaves appear to have been continually shed and replaced at the growing points. A low LAIwould have allowed a high through-fall of rain and transpiration would have been limited

956 by the xeromorphy. Given this combination of characteristics meteoric water supply to the swamp would have been high and transpirational cycling of water back to the atmosphere rather low: a combination of factors that must have helped maintain mire saturation and hence decreased degradation rates. The acrotelm was probably very shallow particularly as these trees only had a shallow rooting system. The rooting system of the arborescent lycopods was unlike that of any living tree. It consisted of four lobes each of which gave off equally branching main roots which bore numerous helically arranged hollow rootlets about lcm diameter (Fig. 5).

secondary cortex middle cortex protoxylem secondary xylem cambium root trace

Lepidodendron branch surface with leaves

Figure 5. Diagram of the rooting structure (Stigmaria) and leaves of a mature Lepidodendron tree., After Stewart (1983).

Such a system allowed the supply of oxygen to the respiring rooting structure but also to penetrate the otherwise anoxic substrate. It may also have acted as a carbon dioxide collecting system to boost photosythetic productivity (see below). Overall the cheap construction, low LAIand leaf shedding, which facilitated nutrient cycling in otherwise nutrient poor mires, are features seen in some modern swamp trees. (DiMichele & DeMaris, 1987) make comparisons with the Okefenokee Swamp forest dominated by T a x o d i u m . the general characteristics are an ombrotrophic setting, high basal area, random spacing, low crown volumes and high proportion of sclerophylls, and low levels of interspecific and intraspecific competition. Low sea-level also contributed to mire development and hence

957

carbon sequestering. The Late Carboniferous when coal formation was at its peak was a time of glaciated poles and all major North American and western European coal mire forests were in low latitudes (Fig. 6, Parrish et al. 1986).

Late Permian (2S0ma) ~cc

I Late Carboniferous (300ma)

(~ Tillltes (Glacial)

(~ Coals

~) Evaporites

Figure 6. Palaeogeographicmaps of the world for Late Carboniferous and Late Permian time showing the distribution of preserved tillites, coals, and evaporites. Tillites are produced by glacial action, coals by a high organic productivity relative to decay and evaporites by a low precipitation/evaporation ratio. Only bedded coals, those that could not be confused with individual coalified logs, and primary evaporites, those not formed during weathering, were included in the study by Parrish et al. (1986).

958 Polar ice sheet formation led to global sea-level fall and exposed the continental shelves where mire habitats developed. This provides an interesting analogy to the raised coastline arguments of Sieffermann et al. (1988) regarding the growth of some SE Asian peats. They suggest that reduction in base level reduces frequency of flooding and yet induces lateral water flow which prevents stagnation. This is necessary for fast initial growth. The Kalimantan peats are on podzolised soils - a similar situation to that seen below some Carboniferous coals. WAS T H E E C O L O G Y O F THE DEGRADING ORGANISMS DIFFERENT IN T H E CARBONIFEROUS COMPARED W I T H T H E PRESENT? Peat grows if the rate of production exceeds that of degradation. Clearly if the activity of degraders in the Carboniferous was low this would account for the apparent high accumulation rates. Fungi and bacteria have a fossil pedigree that long precedes the Carboniferous, but evidence is emerging that in the Carboniferous large particle invertebrate feeders such as arthropods were less efficient as detritivores than they are now. This evidence comes from a detailed examination of degradation patterns on leaves from environments not restricted to mires but it does indicate possible differences in the detritivore food chain (Chaloner et al., 1991). WAS T E R R E S T R I A L CARBON SEQUESTERING RESPONSIBLE FOR THE PERMO-CARBONIFEROUS GLACIATIONS? The time of maximum Carboniferous coal formation appears to slightly precede maximum glaciation but this does not necessarily mean that peat development was responsible for global cooling. Indeed a factor in mire development was probably ice-induced eustatic sea-level fall which provided vast areas of exposed shelf upon which swamp forests could develop and the resultant peats be buried by subsequent sea level rise. Given the disparity in the rate of onset of glaciation and the rate of continental plate movement it seems unlikely that relative continental positions alone could account for the observed cooling. Undoubtedly orbital variations may have played a part in initiating glaciations once a critical threshold condition had been reached but if as has been suggested (Frakes, 1979; Lovelock & Whitefield, 1982; Raven & Sprent, 1989) atmospheric CO 2 concentrations have shown an overall downward trend over geologic time, then one

959

might expect the Carboniferous to have high CO 2 levels, e.g. > > 10 times present, and be buffered from cooling. However, if this was the case then either 1) almost all the atmospheric CO 2 must have been sequestered to bring about cooling comparable to that seen during the recent glaciations, or 2) no matter what the base level of CO 2 slight changes bring about major changes in global climate. Neither of these situations appears very likely because 1) is only tenable if the Carboniferous swamp/ocean system was responsible for sequestering far greater amounts of CO 2 than we have a record of, which is of course a possibility, and 2) demands a rethink on the role of CO 2 as a greenhouse gas. To summarise then the factors that led to high Carboniferous peat development were as follows: 1) eustatic lowering of sea level due to ice sheet development provided extensive coastal plains upon which swamp forests could develop, 2) high rainfall in equatorial regions where, coupled with high temperatures, productivity outstripped degradation, 3) although fungi and bacteria had evolved long before, the invertebrate detritivore food chain in the Carboniferous appears not to have been as well established as it is now and degradation rates may have been lower than in equivalent systems at the present time, 4) plants became well adapted to the strictures of ombrogenous peats both structurally and ecologically. They adapted to low nutrient status by developing "cheap construction", they developed abscision strategies and a monocarpic reproductive system that led to constant cycling of nutrients, and they produced sclerophyils and a low LAI which in turn increased precipitation through-fall while decreasing transpirational loss of water from the peat. This led to a maintenance of mire saturation and hence peat development. WHY DID PEAT FORMATION DECLINE IN THE PERMIAN? Tectonic factors led to the construction of the super continent Pangea with equal land masses in each hemisphere symmetrical about the equator (Fig. 6). This led to the development of an extremely strong monsoon system (Parrish, 1982; Parrish et al., 1982; Parrish et al., 1986; Kutzbach &Gallimore, 1989; Dubiel et al., 1991) with pressure systems and precipitation zones swinging from hemisphere to hemisphere and the consequent loss of the ever-wet tropics. Warming may have been enhanced by oxidation of some of the carbon sequestered during the Carboniferous. However, significant amounts of carbon remained sequestered by being buried by marine sediments laid down perhaps in

960 part due to sea level rise as the polar ice melted. In this case tectonically driven influences on climate apparently overwhelmed any cooling and related climate change due to CO 2 reduction by biotic sequestering. The result was a greenhouse world that lasted 255 million years - until the end of the Eocene. TERRESTRIAL CARBON SEQUESTERING IN A WARM WORLD; - The Jurassic, Cretaceous and Palaeocene. The Jurassic and Cretaceous were again times of great terrestrial carbon sequestering in the form of coals. Preliminary data also suggests that in particular atmospheric CO 2 levels were at 4-8x Present levels for much of the Cretaceous,, with low values, 2-4x, in the early Cretaceous and peak values, 7-11x, in the Aptian and early AIbian, about 124-100Ma, (Arthur et.al.,1991). However, unlike the Carboniferous and early Permian, peat development was at high, not low, latitudes (Figs. 7 & 8). The greenhouse world is a world in which the poles rather than the equatorial regions get warmer, ie there is a lower equator to pole temperature gradient than during icehouse conditions (Barron, 1983; Crowley & North, 1991; Parrish & Spicer, 1988; Spicer, 1987). The Jurassic and Cretaceous polar lands were extensively forested and the preserved leaf physiognomy, together with the sediments, suggest wet regimes, at least during the growing season and near coasts. Tree growth rings showing very patterns in the leaf floras suggest a highly seasonal regime with a rapid transition between summer and winter (Smiley, 1967; Vachrameev, 1978; Spicer, 1986; Parrish a!.,1987; Spicer, 1987; Spicer & Parrish, 1990a; Parrish & Spicer, 1988). Today at high latitudes the transition between 24hrs light during the summer to 24hrs darkness in the winter takes place over a period of a few weeks (Anonymous, 1978) and based on the little latewood production and taphonomic and morphological fossil evidence there is no reason to believe the light regime 100 my ago was substantially any different to that of today (Spicer & Parrish, 1990a; Spicer & Parrish, 1990b).

961

Late Jurassic (150ma)

Early Jurassic (200ma)

i

(~) Tillites (Glacial)

-

-

(~) Coals

(~ Evaporites

Figure 7. Palaeogeographicmapsof the world for the Jurassic showingthe distribution of climaticallysensitivesediments.The early Jurassic map is modified from Parrish et al. (1986)and that for the Late Jurassic from Parrish et al. (1982). In the mid Cretaceous (Albian - Cenomanian) productivity was high

during the summer months: at 75°N mean annual temperatures were about 10°C, with warm month mean temperatures of at least 25°C, constant daylight, and water plentiful (Spicer & Parrish, 1990a). At the end of the summer most trees shed their leaves and herbaceous forms died back to underground root systems or overwintered as seeds. In the winter, however, the lack of sunlight for several months must have resulted in temperatures around freezing and possibly the cold month mean might have been as low as - l l ° C (Parrish, et al., 1987). Winter degradation of summer-accumulated productivity would have been strongly curtailed leading to a high net accumulation of organic matter (Spicer et al., In Press). This model of seasonal accumulation also applies to late Permian and Triassic coals of Antarctica and the Paleogene coals of the northern hemisphere.

962

LateTerliary (15ma) " .,,.3 ~

E ~E

" E

"It'' * ~le. E

E

t,.¢ %

'.~,'tg. : ~ ° ,

S~

-X

c~ c, CL.

Mid Cretaceous(95rna)

~) Tillites (Glacial)

~ Coals

(~ Evaporites

Figure 8. Palaeogeographicmaps of the world for the mid Cretaceousand Late Tertiary based on Parrish et al. (1982).

P O L A R FOREST COMPOSITION, PEAT M I R E DEVELOPMENT, AND E C O L O G Y Cretaceous coals of Albian-Cenomanian and Campanian-Maastrichtian age formed on the North Slope of Alaska and these have been studied in detail. The older deposits include numerous, thick beds of predominantly low-sulphur, low-ash coal. Some of the beds are composed of allochthonous organic material, but most are autochthonous and probably formed in raised mires. Conifers, especially the extinct broad-leaved Podozamites , dominated both the mire and regional communities and contributed much biomass to the peats. Ferns and Equisetites contributed herbaceous material (Fig. 9, Spicer & Parrish, 1990a; Spicer et al., In Press).

963

Early Late Cretaceous Vegetation of Northern Alaska (Palaeolatitude 75°N) Raised Mire

Levee

Lake

~P°d°zamites~Tax°diaceous(~Angiosperms~(Nilsso,.:.~type ) c o n i f e r s Cycads Ginkgophytes

V

Ferns

Equisetites

Figure 9. Reconstructionof Arctic Cenomanianvegetation after Spicer et al. (in press). The reconstruction is based on a detailed study in northern Alaska of plant megafossils,leaves,wood etc., and fossil pollen and spores coupled with analysis of the sedimentaryfacies that contained them. These coals, almost totally devoid of angiosperms, form very large reserves estimated to be 2.73 trillion tonnes or one third of all US coal reserves of all ages combined (Sable and Stricker, 1985). The younger coals are few and thin, and probably formed in lowlying, topogenous, mires. Many of these younger coals are composed of degraded, allochthonous organic debris, Spicer et al. (In Press). The LAIwas similar to modern temperate forest - probably closed canopy in the Cenomanian but later in the Cretaceous the forest became more similar to taiga with large open areas. The sedimentological and hydrological regime during the two coal-bearing intervals was similar, but a major change in climate occurred between the Albian-Cenomanian and the Campanian-Maastrichtian. Vegetational analysis and growth-ring analysis of fossil woods indicate a drop in mean annual temperature in the order of 5°C (Spicer & Parrish, 1990a). In addition, the summers during the Campanian-Maastrichtian were cooler, which would have affected plant productivity. Finally, although drying was not sufficient to affect the sedimentological regime, forest fires were common enough to produce abundant disseminated charcoal in the sediments (Parrish et al., 1987; Parrish & Spicer, 1988). Drying, which may have been associated with an intensification of the polar high pressure cell, would have hindered the development of raised mires. Cool summers, which would have restricted productivity,

964 combined not only to open the forest canopy but also to reduce the formation of coal on the North Slope through the Late Cretaceous. This cooling is also seen in low latitude marine oxygen isotope data and carbon isotopic analysis of benthic and planktonic forams shows evidence of a concomitant draw-down of atmospheric CO2, Spicer & Corfieid (In Press.). In the Early Tertiary at high northern latitudes plant evidence shows an increase in mean annual temperature of about 2°C (Spicer & Parrish, 1990a). Productivity appears to have increased as witnessed by tree ring data and the re-establishment of the formation of thicker coal seams. In the late Mesozoic of the southern high latitudes leaf and pollen data suggest a similar vegetation structure with conifers (podocarps and araucarians) being the main arborescent component with ferns and later angiosperms as subordinate elements. In coastal areas both the vegetation and climate appears to have been similar to that described above for the circum Arctic Ocean fluvio-marine deltaic plains (e.g. Spicer, 1990; Truswell, 1990; Askin, 1990). However, in more continental settings such as the opening rift valley between Australia and Antarctica late Early Cretaceous floras exhibit an evergreen component with leaf architecture similar to that seen in plants occupying semi-arid conditions. Sedimentological evidence suggests a wet but variable water supply and given the high palaeolatitude, and consequent light regime, evergreenness could only have been supported by winter temperatures low enough to limit significantly metabolic processes. This suggests severe winter freezing and, as one might expect from a more continental setting, a large mean annual range of temperature (Parrish et al., 1991). Mean annual temperatures of about 7°C seem likely. Late Mesozoic coals are rare in the southern high latitudes not because of an unsuitable climate or vegetational productivity, but because the appropriate basinal settings were absent. LOW LATITUDE ENVIRONMENTS UNDER GREENHOUSE CONDITIONS Localised topogenous and alluvial plain coals developed at latitudes as low as 40 degrees during the Jurassic and Cretaceous (Figs. 7 & 8). However, at lower latitudes still plant fossil and sedimentological data indicates frequent, erratic, but nevertheless probably seasonal aridity (Alvin et al. 1981; Francis, 1984). Vegetation at low latitudes was dominated by an extinct family of conifers known as the Cheirolepidiaceae which had a variety of adaptations for periodic

965 severe drought and generally dry conditions (Alvin, 1982), (Fig. 10). For example they had highly reduced leaves, thick cuticles, stomata sunken into pits protected by overarching papillae and even small hairs along the edges of leaves that acted as sites for moisture nucleation. The plant literally extracted water from the atmosphere (Spicer, 1989b). Nevertheless taphonomic studies suggest that at times of extreme drought leafy shoots were shed and the plant was facultatively deciduous. The dominance of the Cheirolepidiaceae waned in the late Cretaceous concomitant with the rise of the angiosperms as forest dominants and the cooling of the global climate. Other xeromorphic plants found in low latitudes at this time include the drought-tolerant fern Weichselia (Alvin, 1974), often found as charcoalified fragments, and small-leaved cycad-like bennetitaleans.

i'!

"~'~.,'

t

i I i

F A

•,

so,

Figure 10. a) Reconstructionof the extinct cheirolepidiaceousconifer Pseudofrenelopss and b) its foliage,c) the cheirolepidiaceousfemalecone Hirmeriella (redrawnfrom Alvin, 1982n).Note the smallleaf area/volumeratio of the foliage.The Cheirolepidiaceaeareknownon the basis of megafossilsand fossil pollen grains to have occupiedthe seasonallyaridlow latitudes, <40 °, throughoutthe Mesozoic. ARE SEASONALLY DRY TROPICS A FUNCTION OF CONTINENTAL POSITIONS OR ARE THEY INEVITABLE AT TIMES OF GLOBAL WARMTH IRRESPECTIVE OF PANGEAN-TYPE MONSOONS? According to qualitative models the breakdown of intense global monsoon system appears to have occurred by mid Jurassic times as the distribution of land about the equator became asymmetrical (Dubiel et

966

al., 1991), equatorial Tethys seaway developed between Gondwana and Laurasia, and tectonieally driven changes in sea level flooded large areas of the continents. Nevertheless seasonally dry tropics persisted to the end of the Cretaceous and coal deposition at high latitudes continued to the end of the Palaeogene (Fig. 8). Clearly continental configurations are not the complete explanation for changes in the distribution of peat forming sites or global climate patterns. Considerably more modelling needs to be done and evaluated against geologic data but perhaps a clue to the phenomenon of shifting productivity zones lies in the suggestion of Ziegler et al (1987) that at times of global warmth when the polar front is weak the Inter Tropical Convergence Zone, ITCZ, is free to travel more polewards in the northern summer and southern summer than it currently does. The ITCZ approximately coincides with the thermal equator and as the rising warm air cools moisture condenses in the form of rain. Thus the ITCZ is also wet. If the seasonal excursions of the ITCZ north and south of the geographic equator are limited then the ITCZ precipitation belts overlap and the equatorial regions will tend to be always wet. Under these conditions rainforest vegetation and coals can form. Large excursions away from the geographic equator lead to little or no constantly wet zones; merely seasonally wet areas. The zone of peak productivity therefore shifts from low to high latitudes at times of global warmth. Under greenhouse conditions low latitude vegetational productivity cannot outstrip degradation and peats fail to form except, perhaps, where topography is favourable for water accumulation. In situations where peat formation is marginal small shifts in precipitation patterns can make critical differences. This may be what is happening in Kalimantan today. One optimistic note is that even if Ziegler et al. (1987), are correct the Permian (Fig. 6) and Cretaceous (Fig. 8) records also show that low latitude land surrounded by sea remain relatively wet through convective precipitation. By late Tertiary time (Fig. 8) the continental positions are similar to those of the Present. The onset of widespread glaciation in the Northern Hemisphere had not yet occurred although the development of significant glaciation in Antarctica had begun, and average global temperature was slightly higher than now. At this time coal deposition had returned to equatorial regions but also occurred at middle and high latitudes. Clearly with the present continental configurations only a slight increase in global temperature is likely to give rise to a major increase in productivity and carbon sequestering at high latitudes.

967

ATMOSPHERIC COMPOSITION AND PHOTOSYNTHETIC PATHWAYS Changes in atmospheric composition, particularly in respect to the biologically active gases 0 2 and CO2, are likely to have had profound effects not only on plant productivity, but also inter-plant competition (Moore, 1983b). Although the interaction of potentially limiting factors are complex in vascular plants (Bolhar-Nordenkampf, 1980) atmospheric CO 2 concentration is positively correlated with productivity up to approximately four times present atmospheric concentrations. Above this value CO 2 concentrations induce stomatal closure, which reduces photosynthetic activity. However, stomatal closure also reduces water loss with varying degrees of efficiency between species (Bazzaz, 1980). Moore (1983b) points out that conflicts between CO 2 fixation and water have resulted in evolutionary modifications of the photosynthetic process and the way plants interact with each other, Bazzaz and Carlson (1984).

(rChloroplas0 i.iO2

PhosphoglycericacidI (Peroxisome~)

Iqibulosebisphosph~ (RUBP)(5C)

; Glycollate (20)

CO2 f

phosphoglycericacid

(3C)

/

Glycine (20)

CalvinCycle

Figure 11. Diagramof C3 photosynthesis,after Moore(1983b).Note the initial fixation of carbondioxideto give the three-carboncompoundphosphoglycericacid. The photosynthetic pathway adopted by most green plants reduces CO2 and the carbon atom is combined with a 5-carbon sugar, ribulose bisphosphate, RuBP, before giving rise to two 3-carbon molecules (Fig. 11). This is the so called C3 photosynthesis. The action of the enzyme RuBP carboxylase, RuBisCo, is hampered by the presence of oxygen and although the products of this interference are processed the overall effect of this "photorespiration" is to reduce the efficiency of the C 3 process. Higher atmospheric partial pressures of CO 2 increase

968

photosynthetic efficiency in C 3 plants but photorespiration particularly detrimental at high temperatures.

is

CAM plants initially fix carbon in a 4-carbon molecule which is stored until night time when the carbon is released within the plant and re-fixed by the conventional C 3 system, the Calvin Cycle, (Fig.12).

9

Malate (4C)

2 x Phosphoglyceric acid (3C) ~

Calvin Cycle

RUBP Carboxylase

"

~ ,

CO 2

Pyruvate (3C)

Ribulose bisphosphate (5C)

Figure 12. Diagram of CrassulaceanAcid Metabolism,CAM, photosynthesis,after Moore (1983b). CO2 is initially fixed, during the night, to form the four carbon compound malate and then stored until the daytime where it is converted to phosphoglycerieacid which enters the Calvin Cycle. At night the stomata are closed which not only limits water loss but increases the CO 2 concentration in the plant so that the CO 2 competes more effectively against oxygen for the RuBisCo (Lorimer & Andrews, 1973). CAM plants tend to be desert succulents such as cacti but CAM type photosynthesis is known to occur in 15 dicotyledonous and 4 monocotyledonous extant families as well as some polypodiaceous ferns, the gymnosperm Welwitschia and lycophyte Stylites (lsoetes) which is regarded as a descendant of the Carboniferous arborescent lycopods (Moore, 1983a; Moore, 1983b). Stylites able to concentrate carbon through its unique rooting structures (Keeley et al., 1984), which it has in common with its Carboniferous ancestors, and if such a system operated in the past it could account for some of the apparent high

969

productivity in the Carboniferous coal swamp forests without the need to invoke high CO 2 concentrations in the atmosphere. Although first studied in tropical grasses the C 4 photosynthetic system (Fig. 13) is currently known to occur in 16 dicotyledonous and 3 monocotyledonous families (Moore, 1983b). Like CAM plants the initial fixation of carbon, in a 4-carbon molecule, is separated from the secondary, oxygen mediated, C 3 fixation.

Figure 13. Diagram of C4 photosynthesiswhere malate is formed near the stomata and then moved to bundle sheath cells deep within the plant before being converted to phosphoglycericacid, based on Moore (1983b). However, unlike the temporal separation in CAM there is a spatial separation in C 4 plants in that a 4-carbon molecule is transferred to specialized cells deep inside the plant tissues around the vascular bundle where CO 2 is released and re-fixed by the C 3 system. The characteristic bundle sheath known as the 'Kranz Anatomy' has been found in Late Miocene grasses (Nambudiri et al., 1978, Tidwell and Nambudiri, 1989), but both CAM and C 4 photosynthesis may be much older may have arisen numerous times. Smith & Robbins (1974) suggest that C 4 systems, including CAM, have evolved at least 20 times in 10 families and it appears that relatively few gene changes are required to convert a C 3 to a C 4 system (Bjorkman, Gauhl, & Nobs, 1969). There are problems in identifying C 4 systems in the fossil record in that CAM plants do not display any characteristic anatomical modifications and CAM and C 4 plants tend not to be woody. Nevertheless the biochemical differences involved do lead to C 4 systems accumulating

970

different ratios of 13 C/12C. C 4 system plants produce less negative d 13C values than those with C 3 photosynthesis and this difference may be palaeontologically useful (Moore, 1983b). Preliminary work involving analysis of Tertiary coals suggests that this avenue may prove to be extremely valuable for characterising the nature of coalforming vegetation and therefore coal quality (Holmes et ai., 1987) and indirect evidence from carbon isotope ratios in dinosaur bones suggest that in sub-humid environments C 4 system plants may have been a major component of Late Cretaceous hadrosaur diet (Bocherens et al., 1988). C 4 plants have a lower CO 2 compensation point than C 3 plants, they can photosynthesize more efficiently at lower CO 2 levels, and because photorespiration is not a problem, they can have a higher optimum temperature for growth than C 3 plants (Moore, 1983b), (Table 1). High atmospheric 0 2 would tend to enhance photorespiration and thus be detrimental to C 3 plants, (Moore, 1989). If, as has been suggested, (Berner & Landis, 1988) Cretaceous atmospheres were enriched with oxygen compared to present then C4, especially CAM systems, would have been favoured because average global temperatures were higher than at present and the climate seasonally dry at low latitudes. However, at high latitudes, where the lower temperatures and light regime would favour C 3 plants, high oxygen levels would be detrimental, and yet we know from palaeobotanical evidence that high latitude vegetation was impressively productive (Parrish & Spicer, 1988; Spicer, 1987; Spicer, 1990; Spicer and Parrish, 1990a).

Variable

C3

C4

Temperatureoptimum

20-25°C

30-35°C

C02 optimum in stomatal chamber

220 ppm

120ppm

;oC~below

3000>0above

;oCod above

;oCo(~below

LOW,-22 to -33 (mean -27)

High. -10 tO -18 mean -14)

Water use efficiency

generally low

higher than C 3

Salinitytolerance

variable

generally high

Quantum yield (carbon per fixed unit of energy) ~'c ~,..

r (I$C:120}IlamPkD 1-] 111000

"L~=c ?=c~.... ~ _J

Table 1. Summaryofthe charcteristicsof C3 and C4 photosynthesis,basedon Moore (1983b). Although with the present atmospheric composition C 3 and C 4 system plants display clear distributions with respect to climate (Moore, 1981), such relationships may be altered significantly when

971

CO 2 and 0 2 levels depart from their present values. Currently we can do little except speculate about these changes, or ancient atmospheric compositions, until more data are gathered on carbon isotopic ratios in organic deposits arising from terrestrial vascular plants. In particular we need to analyse individual and identifiable plant remains as this would allow us to examine the prevalence, distribution, and evolution of individual lineages and communities through time and under changing climates and provide critical data on past natural ecosystem responses to climate change. This information would provide the framework for the development of realistic and minimally-damaging planetary ecosystem management practices in the face of future climate change, and possibly provide mechanisms for enhancing carbon sequestering to temper CO 2 emissiondriven environmental change. To embark on regional or global ecosystem management without such a natural framework and perspective would be extremely dangerous. CONCLUSIONS As with any issue as complex as this conclusions must be subject to certain caveats. The geologic record presents only a partial picture of ancient environments. Nevertheless in spite of an imperfect record strong patterns emerge and the geologic record provides the only facts we have at our disposal regarding the response of earth systems to changing conditions. It is clear that even from qualitative data that global average temperatures higher than those of the Present are normal for planet Earth. However, quantitative data, while not complete, can provide constraints and evaluation for numerical climate models through which, particularly when coupled ocean/atmosphere models are available, a more complete picture of the past may emerge. In the meantime existing geologic data show clearly that climate varies with continental positions, orbital parameters, and atmospheric composition. Atmospheric CO 2 concentration while not the only cause of past climatic change is of critical importance in determining short term variations. In conjunction with those variations carbon sequestering on land has varied substantially over the past 500 million years and key elements in the rate of sequestering are the biology of the organisms involved, the presence of high precipitation/evaporation ratios, and appropriate tectonic settings for long-term carbon burial. As a crude generalization under 'icehouse' conditions greatest organic productivity and carbon sequestering takes place at low latitudes while in a 'greenhouse' world greatest productivity is at

972

high latitudes and there is a tendency for low latitudes to be seasonally arid. The only exception is that low latitude lands experiencing a maritime climates may escape aridization. This probably explains the situation in the Paleogene when both high and low latitude forests florished under "greenhouse" conditions. Continental fragmentation was well advanced so moisture sources were now relatively close to many areas that were previously arid. Nevertheless the nature of Paleogene equatorial vegetation is not well understood and additional research is needed before its structure and physiognomy can be compared with those of modern tropical forests. Overall current estimates of past carbon sequestering are likely to be too low because dispersed matter is inadequately inventoried. This situation needs to be remedied but on the basis of available data terrestrial carbon sequestering is unlikely to dominate tectonic controls but as it operates on a shorter time scale it has a strong short term effect and could well tip the climate balance in critical situations. RESEARCH RECOMMENDATIONS 1) Better estimates of carbon fossilization pathways and fluxes from variety of "biomes" are urgently needed. The Present is a good time for this in that the modern world incorporates a range of local climates and vegetation types that fully encompasses those that we have evidence for in the past. 2) A global carbon inventory is required that includes dispersed carbon in the sedimentary record. 3) Better collation of global palaeoclimate data is needed to test and improve AGCMs and subsequently coupled ocean/atmosphere models as they become available. 4) Know productivity potential of past biomes is needed. This can be obtained through detailed palaeoecological research and a survey of carbon isotopes in terrestrial plants through time. REFERENCES Alvin, K. L., 1974. Leaf anatomy of Weichselia based on fusainized material. Palaeontology, 17, 587-598. Alvin, K. L.,1982. Cheirolepidiaceae: biology, structure and palaeoecolgy. Review of Palaeobotany and Palynology, 37, 71-98. Alvin, K. L.,Fraser, C. J., and Spicer, R. A., 1981. Anatomy and palaeoecology of Pseudofrenelopsis and associated conifers in the English Wealden. Paleontology, 24(4, 759-778.

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Anonymous, 1978. C. I.A. Handbook, 1978 Polar Regions Atlas. National Foreign Assessment Center, CIA, USA. Arthur, M. A., Allard, D., and Hinga, K. R., 1991. Cretaceous and Cenozoic atmospheric carbon dioxide variations and past global climate change. Geological Society of America. Abstracts with Program, 23, A178. Askin, R. A., 1990. Campanian to Paleocene spore and pollen assemblages of Seymour Island, Antarctica. Review of Palaeobotany and Palynology, 65, 105-113. Barron, E. J., 1983. A warm, equable Cretaceous: the nature of the problem. Earth-Science Reviews, 19, 305-338. Barron, E. J., 1985. Numerical climate modeling, a frontier in petroleum source rock prediction: results based on Cretaceous simulation. Am. Assoc. of Petroleum Geologists Bull., 69, 448-459. Barron, E. J., 1986. Mathematical climate models: Insights into the relationship between climate and economic sedimentary deposits. Society of Economic Paleontologists and Mineralogists, Short Course. Barron, E. J., Arthur, M. A., and Kauffman, E. G., 1985. Cretaceous rhythmic bedding sequences: a plausible link between orbital variations and climate. Earth and Planetary Science Letters, 72, 327340. Barron, E. J., SIoan, J. L., and Harrison, C. G. A., 1980. Potential significance of land-sea distribution and surface albedo variations as a climatic forcing factor; 180 million years to the present. Palaeogeography, Palaeoclimatology, Palaeoecology, 30, 17-40. Barron, E. J., Thompson, S. L.,and Hay, W. W., 1984. Continental distribution as a forcing factor for global-scale temperature. Nature, 310, 574-575. Barron, E. J., Thompson, S. L.,and Schneider, S. H., 1981. An icefree Cretaceous? Results from climate model simulations. Science, 212, 501-508. Barron, E. J., and Washington, W. M., 1984. The role of geographic variables in explaining paleoclimates: results from Cretaceous climate model sensitivity studies. Journal of Geophys. Res., 89, 1267-1279. Bazzaz, F. A., 1980. Consequence of elevated CO 2 concentrations on plant photosynthesis, growth and competition. In Abstracts, 5th International Congress on Photosynthesis., Halkidiki, Greece. Bazzaz, F. A., and Carlson, R. W., 1984. The response of plants to elevated CO 2. I Competition among an assemblage of annuals at different levels of soil moisture. Oecologia, 62, 196-198. Berner, R. A., and Landis, G. P., 1988. Gas bubbles in fossil amber as possible indicators of the major gas composition of ancient air.

974 Science, 239, 1406-1409. Bjorkman, O., Gauhl, E., and Nobs, M. A., 1969. Comparative studies of A t r i p l e x with and without carboxylation photosynthesis and their first-generation hybrid. Yearbook of the Carnegie Institution Washington, 68, 620-633. Bocherens, H., Fizet, M., Cuif, J., Jaeger, J., Michard, J., and Mariotti, A., 1988. Premieres mesures d'abondances isotopiques naturelles en 13C et 15N de la Matiere organique fossile de dinosaure. Application a i'atude du regime alimentaire du genre Anatosaurus, Ornitischia, Hadrosauridae. Cr Acad. Sci. Paris, 306, 1521-1525. Bolhar-Nordenkampf, H . R . , 1980. Changes in photosynthetic efficiency. In The Global Carbon Cycle, pp. 403-457. New York: John Wiley. Boulter, M. C., Spicer, R. A., and Thomas, B. A., 1988. Patterns of plant extinction from some palaeobotanical evidence. In G.P. Larwood, Ed. Extinction and Survival in the Fossil Record, Systematics Association Special Volume 34, 1-36. Oxford: Clarendon Press. Chaloner, W. G., Scott, A. C., and Stephenson, J., 1991. Fossil evidence for plant-animal interactions in the Palaeozoic and Mesozoic. Philosophical Transactions of the Royal Society London B, 333, 177186. Crowley, T. J., and North, G.R., 1988. Paleoclimatology, Oxford: Clarendon Press, 339 pp. DiMichele, W. A., and DeMaris, P. J., 1987. Structure and dynamics of a Pennsylvanian-age Lepidodendron forest: colonizers of a disturbed swamp habitat in the Herrin, No. 6 Coal of Illinois. Palaios, 4, 146157. Dubiel, R. F., Parrish, J. T., Parrish, J. M., and Good, S. C., 1991. The Pangean Megamonsoon - Evidence from the Upper Triassic Chinle Formation, Colorado Plateau. Palaios, 6, 347-370. Eggert, D. A., 1961. The Ontogeny of Carboniferous Arborescent Lycopside. Palaeontographica, 108, 43-92. Frakes, L.A., 1979. Climates Throughout Geologic Time. New York: Elsevier. Gaulton, R. J., 1990. LaTrobe Valley coal deposits Notes on the Geology. Geoengineering Division, Mine Technology Department, Sate Electricity Commission of Victoria. 12pp. Francis, J. E., 1984. Palaeogeography, Palaeoclimatology, Palaeoecology 48, 285-307. Holmes, C. W., Fiores, R. M., and Pocknall, D. T., 1987. Carbon isotopes in Powder River Basin Tertiary coals: A measure of the evolution of peat swamps. Geol. Soc. of Am., Abstracts with Program, 19, 706. Keeley, J. E., Osmond, C. B., and Raven, J. A., 1984. S t y l i t e s , a

975 vascular land plant without stomata absorbs CO 2 via its roots. Nature, 310, 694-695. Knoll, A. H., and Niklas, K. J., 1987. Adaptation, plant evolution, and the fossil record. Review of Palaeobotany and Palynology, 50, 127149. Kutzbach, J. E., and Otto-Bliesner, B. L., 1982. The sensitivity of African-Asian monsoonal climate to orbital parameter changes for 9000 years B.P. in a low-resolution general circulation model. Journal of the Atmospheric Sciences, 39, 1177-1188. Kutzbach, J. E., and Guetter, P. J., 1986. The influence of changing orbital parameters and surface boundary conditions on climate simulations for the past 18 000 years. Journal of the Atmospheric Sciences, 43, 1726-1759. Kutzbach, J. E., and Gallimore, R. G., 1989. Pangean climates: Megamonsoons of the megacontinent. Journ.of Geolophys.Res., 3341-3357. Lorente, M. A., 1990. Textural characteristics of organic matter in several subenvironments of the Orinoco Upper Delta. Geologie en Mijnbouw, 69, 263-278. Lorimer, G. H., and Andrews, J . T . , 1973. Plant photorespiration - an inevitable consequence of the existence of atmospheric oxygen. Nature, 243, 359-360. Lovelock, J. E., and Whitefield, M., 1982. Life span of the biosphere. Nature, 296, 561-563. McCabe, P. J., 1984. Depositional environments of coal and coalbearing strata. Special Publications of the International Association of Sedimentologists, 7, 13-42. Merewether, E. A., and Claypool, G. E., 1980. Organic composition of some Upper Cretaceous shale, Powder River Basin, Wyoming. Am. Assoc. of Petroleum Geologists Bulletin, 64, 448-500. Mitchell, J. F. B., Manabe, S., Tokioka, T., and Meleshko, V., 1990. Equilibrium Climate Change. In J. T. Houghton,G. J. Jenkins, and J. J. Ephraums, Eds., Climate Change The IPCC Sci. Assess., pp. 131-170. Cambridge: Cambridge Uni. Press. Moore, P. D., 1981. The varied ways plants tap the sun. New Scientist, 89, 394-397. Moore, P. D., 1983a. Photosynthetic pathways in aquatic plants. Nature, 304, 310. Moore, P. D., 1983b. Plants and the palaeoatmosphere. Journal of the Geological Society, 140, 13-25. Moore, P. D., 1987. Ecological and hydrological aspects of peat formation. In A. C. Scott, Ed., Coal and Coal-Bearing Strata: Recent Advances, Geol. Soc. Special Publication 32., pp. 7-15. Moore, P. D., 1989. Some ecological implications of palaeoatmospheric variations. Journ. of the Geol.Soc. of London, 146, 183-186.

976 Nambudiri, E. M. V., Tidwell, W. D., Smith, B. N., and Hebbert, N. P., 1978. A C 4 plant from the Pliocene. Nature, 276, 816-817. Parrish, J. M., Parrish, J. T., and Ziegler, A. M., 1986. PermianTriassic paleogeography and paleoclimatology and implications for therapsid distributions. In N. H. Hotton III,MacLean, P.D., Roth, J.J., Roth, E.C., Eds., The Ecology and Biology of Mammal-like Reptiles, pp. 109-132. Washington, D.C.. Parrish, J. M., Parrish, J. T., Hutchison, J. H., and Spicer, R . A . , 1987. Late Cretaceous vertebrate fossils from the North Slope of Alaska and implications for dinosaur ecology. Palaios, 2, 377-389. Parrish, J. T., Ziegler, A. M., and Scotese, C . R . , 1982a. Rainfall patterns and the distribution of coals and evaporites in the Mesozoic and Cenozoic. Palaeogeography, Palaeoclimatology, Palaeoecology, 40, 67-101. Parrish, J. T., 1982b. Upwelling and petroleum source beds, with reference to the Paleozoic. American Association of Petroleum Geologists Bulletin, 66, 750-774. Parrish, J. T., and Spicer, R. A., 1988. Late Cretaceous terrestrial vegetation: A near-polar temperature curve. Geology, 16, 22-25. Parrish, J. T., Spicer, R. A., Douglas, J. G., Rich, T. H., and Vickers-Rich, P., 1991. Continental Climate Near The Albian South Pole and Comparison with Climate near the North Pole. Geological Society of America, Abstracts with Program, 23, A302. Parrish, J. T., Spicer, R. A., and Grant, P., 1987. Near-polar climate in the latest Cretaceous. Geol.Soc.of Am., Abstracts with Program, 19, 800. Raven, J. A., and Sprent, J. I., 1989. Phototrophy, diazotrophy and palaeoatmospheres: biological catalysis and the H, C, N and O cycles. Journ.of the Geol. Soc. of London, 146, 161-170. Sable, E. G., and Stricker, G. D., 1985. Coal in the National Petroleum Reserve in Alaska, NPRA--framework geology and resources. Amer.Assoc.of Petroleum Geologists Bulletin, 69, 677. Sieffermann, G., Founier, M., Triutomo, S., Sadelman, M.T. and Semah, A. M., 1988. 8th IPC Congress, Velocity of tropical forest peat accumulation in Central Kalimantan Province, Indonesia, Borneo. Smiley, C. J., 1967. Paleoclimatic interpretations of some Mesozoic floral sequences. Amer.Assoc.of Petroleum Geologists Bulletin, 51, 849-863. Smith, B. N., and Robbins, M. J., 1974. Evolution of C4 photosynthesis. An assessment based on 13 C/12 C ratios and Krantz anatomy. In: Proc.of the 3rd Internat.Congress on Photosynthesis, 2 (pp. 1579-1587). Spicer, R. A., and Parrish, J. T., 1986. Paleobotanical evidence for

977 cool North Polar climates in middle Cretaceous, Albian-Cenomanian time. Geology, 14, 703-706. Spicer, R. A., 1987. The significance of the Cretaceous flora of northern Alaska for the reconstruction of the climate of the Cretaceous. Geologisches Jahrbuch, Reihe A, 96, 265-291. Spicer, R. A., 1989a. The formation and interpretation of plant fossil assemblages. Advances in Botanical Research, 16, 95-191. Spicer, R. A., 1989b. Physiological characteristics of land plants in relation to environment through time. Transactions of the Royal Society of Edinburg: Earth Sciences, 80, 321-329. Spicer, R. A., 1990. Reconstructing High Latitude Cretaceous Vegetation and Climate: Arctic and Antarctic Compared. In T. N. and. T . . T a y l o r E. L., Eds., Antarctic Paleobiology and its Role in the Reconstruction of Gondwana, pp. 27-36. New York: Springer-Verlag. Spicer, R. A., and Corfield, R., In Press.. A Review of Terrestrial and Marine Climates in the Cretaceous and Implications for Modelling the Greenhouse Earth. Geol. Mag., 129. Spicer, R. A., and Parrish, J. T., 1990a. Late Cretaceous-early Tertiary palaeoclimates of northern high latitudes: a quantitative view. Journ.of the Geol.Soc., London, 147, 329-341. Spicer, R. A., and Parrish, J. T., 1990b. Latest Cretaceous woods of the central North Slope, Alaska. Palaeontology, 33, 225-242. Spicer, R. A., Parrish, J. T., and Grant, P. R., In Press. Evolution of Vegetation and Coal Forming Environments in the Late Cretaceous of the north Slope of Alaska: A Model for Polar Coal Deposition at Times of Global Warmth. In P. J. McCabe and J. T. Parrish, Eds., Controls on the Deposition of Cretaceous Coal, Geol. Soc. Amer. Spec. Pub. Stewart, W. N., 1983. Paleobotany and the Evolution of Plants. Cambridge: Cambridge University Press. Sundquist, E . T . , 1985. Geological perspectives on carbon dioxide and the carbon cycle. In E. T. Sundquist and W. S. Broecker, Eds., The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present, Geophysical Monographs, 32, 5-60. Thomas, B. A., and Spicer, R. A., 1987. The Evolution and Palaeobiology of Land Plants. London: Croom Helm. Tidwell, W. D., and Nambudiri, E. M. V., 1989. T0mlinsonia th0masonii Gen. et sp. Nov., a permineralized grass from the upper Miocene Ricardo Formation, California. Review of Paleobotany and Palynology, 60, 503537. Truswell, E. M., 1990. Cretaceous and Tertiary vegetation of Antarctica: a palynological perspective. In T. N. Taylor and E. L. Taylor, Eds., Antarctic Paleobiology, pp. 71-88. New York: SpringerVerlag. Vachrameev, V. A., 1978. The climates of the northern hemisphere in

978 the Cretaceous in the light of paleobotanicai data. Paleontological Journal, 2, 143-154. Worsley, T. R. and Kidder, D. L., 1991. First-order coupling of paleogography and CO2, with global surface temperature and its latitudinal contrast. Geology, 19, 1161-1164. Ziegler, A. M., Raymond, A. L., Gierlowski, T. C., Horrell, M. A., Rowley, D. B., and Lottes, A. L., 1987. Coal, climate and terrestrial productivity: The present and Early Cretaceous compared. In A. C. Scott, Ed., Coal and Coal-Bearing Strata: Recent Advances, Geol. Soc Special Publication 32., pp. 25-49.