LOWER JURASSIC FLORAS FROM HOPE BAY AND BOTANY BAY, ANTARCTICA
Rees, P.M. & Cleal, C.J. (Special Papers in Palaeontology No. 72, 2004)
Hope Bay and Botany Bay, Graham Land, Antarctica have yielded two of the most diverse floras known from the Jurassic. Because of its high diversity, as well as its early discovery and description (by T. G. Halle in 1913), the Hope Bay flora has served as a taxonomic standard for studies of other Mesozoic floras from Gondwana. This paper presents a major revision of the Hope Bay flora, based on extensive subsequent collections. A nearby flora from Botany Bay is described for the first time. Thirty-seven species are now recognised in the Hope Bay flora and 32 from Botany Bay. The floras are closely similar; 80 per cent of the Botany Bay species also occur at Hope Bay. They are shown here to be Early Jurassic, which contradicts the results of previous studies that suggested a Late Jurassic or earliest Cretaceous age. The revision of their age has special significance for our understanding of the Mesozoic geological history of the Antarctic Peninsula. It also highlights the need for reappraisal of a number of other Mesozoic Gondwanan floras that had been dated mainly on their close similarity to the Hope Bay flora. The taxonomic work has resulted in establishment of a new combination, Taeniopteris taeniopteroides
, and emendation of the diagnoses of Coniopteris oblonga, Sphenopteris nordenskjoeldii, Sphenopteris pecten
and Komlopteris indica
LATE JURASSIC CLIMATES, VEGETATION, AND DINOSAUR DISTRIBUTIONS
Rees, P.M., Noto, C.R., Parrish, J.M. & Parrish, J.T. (Journal of Geology, 2004)
The Jurassic and Cretaceous are considered to have been warmer than today on the basis of various climate data and model studies. Here, we use the available global record of climate-sensitive sediments, plants, and dinosaurs to infer broadscale geographic patterns for the Late Jurassic. These provide a context for our more detailed accounts of the Morrison and Tendaguru Formations in North America and East Africa. At the global scale, evaporites predominated in low latitudes and coals in mid- to high latitudes. We ascribe these variations to a transition from drier to wetter conditions between the equator and poles. Plant diversity was lowest in equatorial regions, increasing to a maximum in midlatitudes and then decreasing toward the poles. Most dinosaur remains are known from low-latitude to marginally midlatitude regions where plant fossils are generally sparse and evaporites common. Conversely, few dinosaur remains are known from mid- to high latitudes, which have higher floral diversities and abundant coals. Hence, there is an obvious geographic mismatch between known dinosaur distributions and their primary food source. This may be due to taphonomic bias, indicating that most dinosaur discoveries provide only a small window on the diversity and lifestyles of this group. On the basis of our global- and local-scale studies, we suggest that dinosaur preservation was favored in environments toward the drier end of the climate spectrum, where savannas rather than forests predominated. A holistic approach, incorporating climate and vegetation as well as geography, is required to better understand patterns of dinosaur ecology and evolution.
TRACING THE TROPICS ACROSS LAND AND SEA: PERMIAN TO PRESENT
Ziegler, A.M., Eshel, G., Rees, P.M., Rothfus, T.A., Rowley, D.B. & Sunderlin, D. (Lethaia, 2003)
The continuity through the past 300 million years of key tropical sediment types, namely coals, evaporites, reefs and carbonates, is examined. Physical controls for their geographical distributions are related to the Hadley cell circulation, and its effects on rainfall and ocean circulation. Climate modelling studies are reviewed in this context, as are biogeographical studies of key fossil groups. Low-latitude peats and coals represent everwet climates related to the Intertropical Convergence Zone near the Equator, as well as coastal diurnal rainfall systems elsewhere in the tropics and subtropics. The incidence of tropical coals and rainforests through time is variable, being least common during the interval of Pangean monsoonal climates. Evaporites represent the descending limbs of the Hadley cells and are centred at 10? to 40? north and south in latitudes that today show an excess of evaporation over precipitation. These deposits coincide with the deserts as well as seasonally rainy climates, and their latitudinal ranges seem to have been relatively constant through time. Reefs also can be related to the Hadley circulation. They thrive within the regions of clear water associated with broad areas of downwelling which are displaced toward the western portions of tropical oceans. These dynamic features are ultimately driven by the subtropical high-pressure cells which are the surface signature of the subsiding branches of the Hadley circulation. Carbonates occupy the same areas, but extend into higher latitudes in regions where terrestrial surface gradients are low and clastic runoff from the land is minimal. We argue that the palaeo-latitudinal record of all these climate-sensitive sediment types is broadly similar to their environments and latitudes of formation today, implying that dynamic effects of atmospheric and oceanic circulation control their distribution, rather than temperature gradients that would expand or contract through time.
THE MIDDLE JURASSIC FLORA FROM STONESFIELD, OXFORDSHIRE, UK
Cleal, C.J. & Rees, P.M. (Palaeontology, 2003)
The Stonesfield 'Slate' of Oxfordshire has yielded a diverse Middle Jurassic flora, containing 25 morphospecies, dominated by remains of araucariacean and cheirolepidiacean conifers, bennettitaleans, and leaves of the possible gymnosperm Pelourdea. It mainly represents coastal vegetation, which included mangrove-like stands of Ptilophyllum, and conifers probably growing in lowland coastal habitats that were subjected to periodic water-stress. The Pelourdea leaves may have drifted in from drier, more upland habitats. The flora is similar, but not identical to, the slightly older floras from the Cotswold 'Slate' of Gloucestershire, such as from Eyeford and Sevenhampton. Also comparable are the contemporaneous floras preserved in marine deposits in France and the Venetian Alps of Italy. It is quite different from the Middle Jurassic flora of Yorkshire, which is thought to represent vegetation growing in deltaic habitats. The new combinations Ptilophyllum pectiniformis (Sternberg), Komlopteris speciosa (Ettingshausen) and Pachypteris macrophylla (Brongniart) are proposed for species found at Stonesfield. Nilssoniopteris solitarium (Phillips) comb. nov. is proposed for the species previously named Nilssoniopteris vittata (Brongniart). Taeniopteris vittata Brongniart is retained as the type species of Taeniopteris, a morphogenus to be used for entire cycadophyte leaves that cannot be definitely assigned to the cycads or bennettitaleans. Conites is shown to be an earlier nomenclatural synonym of Bucklandia, to be used for casts and compressions of cycadophyte stems. This requires a number of new combinations for species that have hitherto been included in Bucklandia: Conites anomala (Stokes and Webb), C. gigas (Seward), C. indica (Seward), C. milleriana (Carruthers), C. pustulosa (Harris) and C. yatesii (Carruthers).
CARBON-CYCLE PERTURBATION IN THE MIDDLE JURASSIC AND ACCOMPANYING CHANGES IN THE TERRESTRIAL PALEOENVIRONMENT
Hesselbo, S.P., Morgans-Bell, H.S., McElwain, J.C., Rees, P.M., Robinson, S.A. & Ross, C.E. (Journal of Geology, 2003)
Carbon-isotope analyses of fossil wood from the Middle Jurassic Ravenscar Group, Yorkshire, NE England, reveal a significant excursion toward light isotopic values (delC13 change of -3 to -4 per mil) at about the Aalenian-Bajocian boundary (approximately 174 Ma). A positive carbon isotopic excursion is also shown for the middle Bajocian (approximately 170 Ma) but is less clearly defined. These isotopic patterns are very similar to the few published marine carbonate records available for this time, in particular one based on belemnites from the Hebrides basin, NW Scotland, and others from pelagic limestones in Italy. The similarity of the terrestrial and marine isotope curves is an indication that the observed isotopic signal is a global phenomenon. Through parts of the Ravenscar Group (the Scarborough Formation), supplementary data from bulk organic carbon and palynofacies analysis confirm that isotopic curves based on bulk analyses may be strongly influenced by the balance of terrestrial versus marine organic matter present in the samples. The negative isotope excursion at the Aalenian-Bajocian boundary marks a change from charcoal to coal as the dominant preservational mode of the macroscopic wood fossils, which is interpreted here as a shift to a more continuously humid climate in the Early Bajocian. Upsection, charcoal once again becomes common, reflecting a return to more fire-prone (presumably seasonally arid) environments in the middle Bajocian. Paradoxically, floral assemblages associated with the lithological unit in which the negative excursion occurs display characteristics that would normally be interpreted as adaptations to water stress brought about by relative aridity or salinity. Preliminary analyses of leaf stomatal densities show some evidence of raised pCO2 relative to background values at about the level of the negative excursion.
REGIONAL PHYTOGEOGRAPHY OF THE PERMIAN ANGARAN FLORA
Sunderlin, D. & Rees, P.M. (Presented at 20th MPC, Chicago, 2003)
The Angaran phytogeographic realm in the northern temperate zone of the Permian supercontinent of Pangea contains a distinct macroflora as compared to equatorial Euramerica and Cathaysia and southern temperate Gondwana. The Angaran province is floristically characterized by the dominance of cordaitids, the rarity of lycophytes, and the presence of endemic forms such as Rhipidopsis and Angaropteridium. Previously, Russian paleobotanists have recognized geographic variation of the floral collections within this region to be the broadly separate zones of Siberia, the Russian Platform, and Subangara. There are also patterns of presence or absence of plant remains that can be seen down at the basin scale within these divisions by the most experienced researchers. Here we present a multivariate phytogeographic analysis of Angaran macrofloral data compiled by the Paleogeographic Atlas Project at the University of Chicago. Genus occurrences were collected from the primary and secondary literature for macrofloral assemblages in Angara. The depositional environment and facies of preservation are known for most of the assemblages allowing the comparison of similarly preserved collections. These consist primarily of floodplain deposits in lowland settings and therefore preserve most assemblages in fine grain sediments. Detrended correspondence analysis and other multivariate analyses were performed on the floral assemblages with more than 5 genera and within the same overall depositional scheme. These analyses were performed for three bins within the Permian and show the Russian Platform/Urals and Verkhoyansk region to be separate and distinct in the Early Permian. Localities along the Vilyui River and other regions become well defined in the middle Permian and Late Permian collections in the Pechora Basin are distinct. We then place these findings in the context of paleoclimate and paleogeography of subtropical to temperate northern Pangea. Additionally, we present the use of these multivariate measures to examine macrofloral associations in the available Angaran collections. These results when combined with paleogeography, taphonomic processes and facies analysis, and paleoclimate may help define communities and greater characterize the terrestrial ecosystems throughout Northern Pangea in the Late Paleozoic. By examining the Angaran phytogeographic realm with these quantitative techniques, new patterns emerge and are helping to refine our understanding of the global Paleophytic/Mesophytic floral turnover on a regional scale.
LAND-PLANT DIVERSITY AND THE END-PERMIAN MASS EXTINCTION
Rees, P.M. (Geology, 2002)
The Permian and Triassic represent a time of major global climate change from icehouse to hothouse conditions accompanied by significant (~25 degrees) northward motion of landmasses amalgamated in essentially one supercontinent, Pangea. The greatest of all mass extinctions occurred around the Permian-Triassic boundary (251 Ma), although there is no consensus regarding the cause(s). Recent studies have suggested a meteor impact and worldwide die-off of vegetation, on the basis of sparse local observations. However, new analyses of global Permian and Triassic plant data in a paleogeographic context show that the scale and timing of effects varied markedly between regions. The patterns are best explained by differences in geography, climate and fossil preservation, not by catastrophic events. Caution should be exercised when extrapolating local observations to global-scale interpretations. At the other extreme, global compilations of biotic change through time can be misleading if the effects of geography, climate, and preservation bias are not considered.
SIMULATIONS OF PERMIAN CLIMATE AND COMPARISONS WITH CLIMATE-SENSITIVE SEDIMENTS
Gibbs, M.T., Rees, P.M., Kutzbach, J.E., Ziegler, A.M., Behling, P.J. & Rowley, D.B. (Journal of Geology, 2002)
We use a climate model to simulate two intervals of Permian climate: the Sakmarian (ca. 280 Ma), at the end of the major Permian glaciation, and the Wordian (ca. 265 Ma). We explore the climate sensitivity to various levels of atmospheric CO2 concentration, and to changes in geography and topography between the two periods. The model simulates large seasonality and high aridity in the continental interiors of both hemispheres for both periods. The northern summer monsoon weakens, and the southern monsoon strengthens, between the Sakmarian and the Wordian, owing to changes in geography and topography. The northern middle and high latitudes cool in winter, between the Sakmarian and Wordian, associated with northward shift of the continents. This high latitude cooling strengthens the winter westerlies and shifts the maximum storm track precipitation south. In the southern hemisphere, the winter westerlies weaken from the Sakmarian to the Wordian. Starting the simulations with no permanent ice fields (i.e., by assuming that the late Sakmarian postdates deglaciation), and imposing increased levels of atmospheric CO2, four times the present level, we find no tendency for re-initiation of major glaciation. Some permanent snow fields do develop in high southern latitudes, but these are primarily at high elevation. However, the combination of low CO2 levels (such as present-day levels) and a cold summer orbital configuration produces expanded areas of permanent snow. The results are based upon statistics derived from the final five years of twenty-year simulations. Paleoenvironmental indicators such as coal, evaporite, phosphate, and eolian sand deposits agree qualitatively with the simulated climate. The extreme cold simulated in high latitudes is inconsistent with estimates of high latitude conditions. Either the interpretation of observations is incorrect, or the model is incorrect, or both; a possible model deficiency that leads to cold conditions in high latitudes is the relatively weak ocean heat transport simulated by the heat diffusion parameterization of the upper ocean model.
PERMIAN PHYTOGEOGRAPHIC PATTERNS AND CLIMATE DATA/MODEL COMPARISONS
Rees, P.M., Ziegler, A.M., Gibbs, M.T., Kutzbach, J.E., Behling, P.J. & Rowley, D.B. (Journal of Geology, 2002)
The most recent global 'icehouse-hothouse' climate transition in Earth history began during the Permian Period. Warmer polar conditions, relative to today, then persisted through the Mesozoic and into the Cenozoic. We focus here on two Permian stages, the Sakmarian (285 - 280 Ma) and the Wordian (267 - 264 Ma; also known as the Kazanian), integrating floral with lithological data to determine their climates globally. These stages postdate the Permo-Carboniferous glaciation, but retain a moderately steep equator-to-pole gradient, judging by the level of floral and faunal differentiation. Floral data provide a particularly useful means of interpreting terrestrial paleoclimates, often revealing information about climate gradations between 'dry' and 'wet' end-member lithological indicators such as evaporites and coals. We applied multivariate statistical analyses to the Permian floral data to calibrate the nature of floral and geographical transitions as an aid to climate interpretation. We then classified Sakmarian and Wordian terrestrial environments in a series of regional biomes ('climate zones') by integrating information on leaf morphologies and phytogeography with patterns of eolian sand, evaporite, and coal distributions. The data-derived biomes are compared here with modeled biomes resulting from new Sakmarian and Wordian climate model simulations for a range of CO2 levels (1x, 4x, and 8x Present), presented in our companion paper. We provide a detailed grid cell comparison of the biome data and model results by geographic region, introducing a more rigorous approach to global paleoclimate studies. The simulations with 4xCO2 match the observations better than the simulations with 1xCO2 and, at least in some areas, the simulations with 8xCO2 match slightly better than those for 4xCO2. Overall, the 4xCO2 and 8xCO2 biome simulations match the data reasonably well in the equatorial and mid latitudes as well as the northern high latitudes. However, even these highest CO2 levels fail to produce the temperate climates in high southern latitudes indicated by the data. The lack of sufficient ocean heat transport into polar latitudes may be one of the factors responsible for this cold bias of the climate model. Another factor could be the treatment of land surface processes, and the lack of an interactive vegetation module. We discuss strengths and limitations of the data and model approaches.
THE EARLY PERMIAN FLORAS OF PRINCE EDWARD ISLAND, CANADA: DIFFERENTIATING GLOBAL FROM LOCAL EFFECTS OF CLIMATE CHANGE
Ziegler, A.M., Rees, P.M. & Naugolnykh, S.V. (Canadian Journal of Earth Sciences, 2002)
New Permian plant specimens are described from Prince Edward Island, Canada. They include attached specimens of leaf and stem genera Walchia and Tylodendron, enabling reconstruction of this Early Permian conifer. Although poorly preserved, the study of these floras extends our knowledge of diversity and climate conditions in the region. By placing these findings in a broader stratigraphic and geographic framework, we can document the phytogeographic and climate trends through the Carboniferous and Permian in the Maritimes Basin. Combined data on temporal trends in climate-sensitive sediments, as well as macrofloral and microfloral diversities, generally match the independently derived paleolatitudinal estimates. These show the region migrating from the southern subtropics across the equator and into the northern subtropics between the Early Carboniferous and Early Permian. Evaporites and pedogenic carbonates, together with low diversity floras, match its subtropical position in the Early Carboniferous. In contrast, coals are present in the Late Carboniferous, accompanied by high diversity macro- and microfloral remains, when the region was on or near the equator. However, the subsequent transition to pedogenic carbonates, eolian sands and lower diversity floras is not matched by significant poleward latitudinal motion. We ascribe these changes to a decrease in moisture availability, as transgressions of epeiric seas became less frequent and finally stopped altogether, causing an increase of continentality in Euramerica.
QUANTITATIVE PALAEOCLIMATE ESTIMATES FROM LATE CRETACEOUS AND PALEOCENE LEAF FLORAS IN THE NORTHWEST OF THE SOUTH ISLAND, NEW ZEALAND
Kennedy, E.M. Spicer, R.A. & Rees, P.M. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2002)
Three new plant macrofossil assemblages were collected from Late Cretaceous and Paleocene fluvio-lacustrine sediments of the Pakawau and Kapuni groups in the northwest of the South Island, New Zealand. Palaeoenvironmental interpretations were made from each locality and palaeoclimate was deduced from the dicotyledonous angiosperm leaf component of each flora. A latest Cretaceous (Pakawau Bush Road locality) flora yielded 58 different dicotyledonous leaf forms; the two Paleocene collections, Ian's Tip and Pillar Point Track, included 23 and 28 dicotyledonous leaf forms respectively. Quantitative palaeoclimate estimates were obtained using both Leaf Margin Analysis (LMA) and the Climate Leaf Analysis Multivariate Program (CLAMP). Temperature estimates suggest that there was a slight cooling from the latest Cretaceous into the early Paleocene in the northwest Nelson region of New Zealand, supporting similar Southern Hemisphere palaeoclimate findings from Antarctic data. Consistency in temperature estimates using different methods, including LMA, multivariate leaf morphological analysis (CLAMP), oxygen isotope data, regional versus local studies and global palaeoclimate models, suggests that the mean annual temperature for the Pakawau region in the latest Cretaceous was between 12 and 15 degrees C. LMA produced temperature estimates between 6.5 and 8 degrees C for the two Paleocene assemblages whereas CLAMP-produced estimates were slightly higher between 9 and 12.5 degrees C.
PALAEOENVIRONMENT AND ECOLOGY OF THE MIDDLE CRETACEOUS GREBENKA FLORA OF NORTHEASTERN ASIA
Spicer, R.A., Ahlberg, A., Herman, A.B., Kelley, S.P., Raikevich, M.I. & Rees, P.M. (Palaeogeography, Palaeoclimatology, Palaeoecology, 2002)
The Grebenka flora, from the main exposure of the Albian-Cenomanian Krivorechenskaya Formation in northeastern Russia, represents a range of plant communities from pioneer to mature forest that grew close to the mid-Cretaceous North Pole (>72 degrees N). The diversity of this flora is dominated by angiosperms followed by conifers, ferns and other plant groups. The age is constrained by 40Ar/39Ar analyses of associated volcaniclastics (~ 96.5 Ma), coupled with biostratigraphic correlation of the plant-bearing non-marine beds with marine units of the Krivorechenskaya Formation and the overlying Dugovskaya Formation. Limited palaeosol development and pronounced episodic floodplain aggradation indicate that the 100-m-thick plant-bearing volcaniclastic floodplain succession was deposited rapidly, resulting in excellent trapping and preservation of the plant communities, but dilution of the palynoflora. Analysis of the megaflora (>100 foliage taxa, plus woods and fructifications) provides a "snapshot" of the mid-Cretaceous climate, and offers reliable quantitative climatic signals of conditions near the mid-Cretaceous North Pole. Multivariate analysis of leaf physiognomy (Climate Leaf Analysis Multivariate Program) on the whole flora suggests that the plants experienced a mean annual temperature of 13.0 +/- 1.8 degrees C and a cold month mean temperature of 5.5 +/- 3.3 degrees C. However, analyses of individual florules yield slightly different results that help constrain the uncertainties inherent in such an approach. These and other foliar physiognomic data are compared across the Arctic.
SIMULATED WARM POLAR CURRENTS DURING THE MIDDLE PERMIAN
Winguth, A.M.E., Heinze, C., Kutzbach, J.E., Maier-Reimer, E., Mikolajewicz, U., Rowley D.B., Rees, P.M. & Ziegler, A.M.
During Permian stage 6 (Wordian, Kazanian) the Pangaean supercontinent was surrounded by a superocean: Panthalassa. An ocean general circulation model has been coupled to an atmospheric energy balance model to simulate the sensitivity of the Wordian climate (~265 million years ago) to changes in greenhouse gas concentrations, high-latitude geography, and Earth orbital configurations. The model simulates significantly different circulation features with different levels of greenhouse gas forcing, ranging from a strong meridional overturning circulation in the Southern Hemisphere at low CO2 concentration (present level) to more symmetric overturning circulation cells with deep water formation in polar latitudes of both hemispheres at high CO2 concentration (8 times present level). The simulated climate with 4 times present level CO2 concentration agrees generally well with climate-sensitive sediments and phytogeographic patterns. The model simulates strong subtropical gyres with similarities to the modern South Pacific circulation and moderate surface temperatures on the southern continent Gondwana, resulting from a strong poleward heat transport in the ocean. An even more moderate climate is generated if high-latitude land is removed from the southern continent so that ocean currents can penetrate into the polar regions of Gondwana.
A MAJOR CARBON-CYCLE PERTURBATION IN THE MIDDLE JURASSIC AND ACCOMPANYING CLIMATIC CHANGE ADDUCED FROM THE LAND-PLANT RECORD
Hesselbo, S.P., Morgans-Bell, H., McElwain, J., Rees, P.M. & Robinson, S.A. (Presented at EUG XI, Strasbourg, 2001)
The Middle Jurassic Ravenscar Group of Yorkshire, England, yields one of the best studied Jurassic land-plant records in the world. Carbon-isotope analyses of wood fossils shows that two major atmospheric isotopic excursions occurred during deposition of these rocks, a negative excursion at about the Aalenian-Bajocian boundary and a positive excursion in the late Bajocian. The sharp negative excursion is associated with an apparently abrupt change from charcoal to coal as the dominant mode of preservation in the succession. A gradual reversion to charcoal dominance follows and is maintained through the remainder of the section. Multivariate analysis of leaf morphology in relation to climate-sensitive lithology indicates a warm period approximately coincident with the negative excursion, and analysis of leaf stomatal indices show similar evidence of high pCO2 although the precise stratigraphic relations remain to be determined. Similarities can be seen with isotopic curves derived from European marine successions and, in common with other examples, there exists a relationship between isotopic excursion and sea-level change: the negative excursion occurred during relative sea-level rise and the positive excursion occurred during relative sea-level fall. Unlike many other prominent Phanerozoic carbon-isotopic excursions there is as yet no large igneous province reported from this time, nor significant mass extinction, and the origin of the isotopic fluctuations is not yet known.
PERMIAN CLIMATES: NEW DATA/MODEL RESULTS, AND INSIGHTS ON TERRESTRIAL VEGETATION CHANGES ACROSS THE PERMO-TRIASSIC BOUNDARY
Rees, P.M., Ziegler, A.M. & Kutzbach, J.E.
(Presented at Earth System Processes, GSA-GSL, Edinburgh, 2001)
The Permian is characterized by a Pangean world geography with ca 15 degrees northward motion of the major continents through this period, and a transition from icehouse to hothouse conditions that cannot be explained solely by such latitudinal motion. We analyzed fossil plant and lithological data to derive biomes (?climate zones?) for two Permian stages, the Sakmarian (286 - 280 Ma) and Wordian (267 - 264 Ma). These represent remnant icehouse and transition toward hothouse world conditions. The results from our data are compared with those from GCM simulations for a range of prescribed CO2 levels and orbital configurations. CO2 level is an important factor (4x and 8xCO2 producing a better match with the data than 1xCO2). The effects of orbital variations are minor. However, even these elevated CO2 levels fail to match the data in the high southern latitudes. We discuss possible reasons for this data and model discrepancy in terms of oceanic heat transport. Our approach to determining Permian climates also provides a framework for understanding changes in terrestrial vegetation across the Permo-Triassic boundary. These observed changes are usually assumed to have been synchronous and due to a global extinction. However, separate factors could have affected floras in different parts of the world and these could have operated within the limits of temporal resolution. Three main regions, Angara, Gondwana and China, have Late Permian plant remains. There is good evidence that the changes in Angaran vegetation occurred at about the beginning of Siberian flood basalt volcanism. Changes in the Gondwanan records could be largely attributed to disappearance of moisture sources and the consequent drying out of this region. The Chinese records can be ascribed to northward latitudinal motion of the North and South China blocks, these areas migrating out of the tropical everwet biome into progressively drier regimes.
GLOBAL DISTRIBUTION OF OCEAN WATER MASSES DURING THE JURASSIC: DELIMITING MARINE CLIMATE ZONES USING CLIMATE-SENSITIVE SEDIMENTS
H. Morgans-Bell, P.M. Rees, A.M. Ziegler & D.B. Rowley (Presented at Geoscience 2000, Manchester, UK)
The collection of climate-sensitive sediment data for the Jurassic has enabled an atlas of ocean water masses to be compiled for each of the period's eleven stages, which augment coeval terrestrial biomes defined using palaeobotanical information. We consider 'water masses' to be the marine equivalent to terrestrial climate zones, as studies indicate that in the present-day oceans temperature and salinity effects have apparently sharp boundaries. Certain sediment types that are indicative of very particular marine conditions determine the different types of water mass. Diagnostic sediments include evaporites, which form where evaporation exceeds precipitation, and where there is some isolation from the ocean. In contrast coals reflect positive precipitation; in such areas greater runoff may enhance surface-water productivity yielding organic-rich shales. Similarly, upwelling also contributes to high surface productivity, producing phosphorites and oil-source rocks. Reefs are limited to the low latitudes in areas of good light penetration and warm temperatures. Drawing on the character of present-day water masses, the Jurassic examples are described and their temporal distributions discussed.
JURASSIC PHYTOGEOGRAPHY AND CLIMATES: NEW DATA AND MODEL COMPARISONS
Rees, P.M., Ziegler, A.M. & Valdes, P.J. (In: Huber et al. (eds), Warm climates in Earth History: 297-318, 2000)
Leaves are a plant's direct means of interacting with the atmosphere, and their morphology is often attuned to and reflects prevailing environmental conditions. Although better-understood and documented for angiosperms (or 'flowering plants'), non-angiosperms also exhibit a phytogeographic pattern linked most strongly to the evaporation/precipitation ratio, a relationship often reflected in their foliar morphologies. We have used this to interpret Jurassic terrestrial climate conditions along a spectrum defined by climate-sensitive lithological end-members such as evaporites and coals. Global climate zones, or biomes, were determined by exploring the foliar morphology/climate relationship using multivariate statistical analysis. Jurassic plant productivity and maximum diversity was concentrated at mid latitudes, where forests were dominated by a mixture of ferns, cycadophytes, sphenophytes, pteridosperms and conifers. Low-latitude vegetation tended to be xeromorphic and only patchily forested, represented by small-leafed forms of conifers and cycadophytes. Polar vegetation was dominated by large-leafed conifers and ginkgophytes which were apparently deciduous. Tropical everwet vegetation was, if present at all, highly restricted. Five main biomes are recognised from the data: seasonally dry (summerwet or subtropical), desert, seasonally dry (winterwet), warm temperate and cool temperate. Their boundaries remained at near-constant palaeolatitudes while the continents moved through them (south, in the case of Asia, and north, in the case of North America). Net global climate change throughout the Jurassic appears to have been minimal. The data-derived results are compared here with a new climate simulation for the Late Jurassic. The use of more detailed palaeogeography and palaeotopography has improved the overall data/model comparisons. Major discrepancies persist at high latitudes, however, where the model predicts cold temperate conditions far beyond the tolerance limits indicated by the plants. Nevertheless, our approach at least enables direct comparison of global palaeoclimate interpretations from both a data and model perspective, and indicates future research directions.
LATE PALEOZOIC CLIMATE CHANGE IN EASTERN CANADA; DIFFERENTIATING GLOBAL FROM LOCAL EFFECTS
Ziegler, A.M., Rees, P.M. & Naugolnykh, S. (Presented at the AGU Spring Meeting, 2000)
Paleoclimatologists are interested in "global climate change" but tectonic plates, and the sedimentary record they carry, have changed latitude significantly through time, resulting in a local signal that incorporates both effects. For example, Laurussia (North America+Europe) moved across the equator during the Late Paleozoic, and a sequence of climates, arid-humid-arid, was recorded, but at the same time continentality was increasing as a result of continental collision and marine regression. We herein document the climate stages in the Carboniferous and early Permian strata of the Maritimes Basin, Eastern Canada using climate-sensitive sediments and soils as well as macrofloral and microfloral remains, and, we compare these climate shifts with the paleo-latitudinal path of the basin as indicated by paleomagnetic data. The Early Carboniferous contains evaporites, pedogenic carbonates and low floral diversities, and is interpreted as an arid environment. Paleomagnetic data confirm that the basin lay over the southern subtropical dry belt and that the basin then moved to the equatorial zone in the Late Carboniferous. By this time, coals enter the record and floral diversity increases dramatically, although pedogenic carbonates and red beds remain as interbeds in the sequence. This rapidly fluctuating climate cycle was influenced indirectly by Milankovitch effects by way of an alternating moisture source driven by glacio-eustatic cycles. That is, marine conditions advanced across the European Platform to the Maritime Provinces numerous times, extending some 5000 km from the nearest ocean, the Uralian Seaway. Humid conditions were eliminated from the Maritimes Basin in the Early Permian with the final retreat of the seaway while paleomagnetic evidence indicates that the area remained in the equatorial zone. The Early Permian record is dominated by redbeds, pedogenic carbonates, eolian sands and low diversity floras which show adaptations for xeric conditions. In conclusion, the Late Paleozoic arid-humid-arid cycle was driven by latitudinal changes in the Maritimes Basin during the Carboniferous, but the precipitation was limited by increasing continentality during the Permian. The best documentation of this local climate change is seen in the macrofloral and microfloral diversity cycle but the spores and pollen show moderate diversities even in the dry times. On the assumption that the microfossil assemblages include transported elements, we suggest that more favorable conditions for vegetation existed in the mountains surrounding the basin.
PERMIAN CLIMATES: EVALUATING MODEL PREDICTIONS USING GLOBAL PALEOBOTANICAL DATA
Rees, P.M., Gibbs, M.T., Ziegler, A.M., Kutzbach, J.E. & Behling, P.J. (Geology, 1999)
The most recent global icehouse-hothouse climate transition in Earth history occurred in the Permian. Warmer polar conditions relative to today existed from the middle Permian through the Mesozoic and into the Cenozoic. We focus here on one particularly well-correlated middle Permian stage that postdated the deglaciation, the Wordian (267-264 Ma), integrating floral and lithological data to determine Wordian climates globally. Paleobotanical data provide the best means of interpreting terrestrial paleoclimates, often revealing important information in the continuum between "dry" and "wet" end-member lithological indicators such as evaporites and coals. New statistical analyses of Wordian floras worldwide have enabled a greater understanding of original vegetation patterns and prevailing climate conditions. The derived climate interpretations are compared with new Wordian atmospheric general circulation model (AGCM) simulations. The model matches the data well in the tropics and northern high latitudes, but predicts colder conditions in southern high latitudes. We discuss possible reasons for this discrepancy.
PERMIAN PALEOCLIMATE DATA/MODEL AGREEMENTS AND DISCREPANCIES
Mark T. Gibbs, John E. Kutzbach, Alfred M. Ziegler & P. McAllister Rees (Presented at the May 1998 AGU meeting in Boston)
The Permian Period contains the most recent transition from a major glaciation to a fully ice-free state. The experiments reported here were conducted with the GENESIS (version 2) GCM for the Wordian (Late Permian; 268 Ma) paleogeography, with reduced solar luminosity, varied levels of atmospheric CO2, and different orbital parameters. A key goal of this work was to evaluate whether high CO2 kept the Late Permian ice-free.
Predictions made by the model are tested against new global compilations of lithologic indicators of paleoclimate such as coals, evaporites, aeolian sands, tillites, and phosphorites. Overall, the model performs very well when its predictions of P-E, wind directions, etc., are tested against these indicators. We also make extensive use of data from Permian fossil plant localities, which provides the highest resolution of paleoclimatic information. By considering spatial patterns of generic diversity as well as leaf morphologies we are able to delineate climate biomes. Monthly averages of temperature and precipitation are used to predict model biomes, providing a quantitative and comprehensive data/model comparison. The model performance is good in the tropics and the northern high latitudes, reproducing reasonably well the observed biome patterns.
With high CO2, there is no net annual snow accumulation in the model, consistent with the observed record of ice-free conditions, although summer model temperatures only just rise above freezing in the south polar region. This is the largest data/model discrepancy: tundra, or at best, cold temperate conditions are predicted by the model for high latitude Gondwana, whereas cool temperate conditions are indicated by thriving Glossopteris deciduous forests. Experiments with a hot summer orbit for the Southern Hemisphere continue to maintain cold temperate conditions at high latitudes. A possible solution to this discrepancy may well be a "warm polar current," which the models configuration (50 m "slab" ocean) cannot resolve. This current could have arisen under the paleogeographic regime of the Late Permian, where one large supercontinent lay just off the South Pole. We are currently planning new experiments with equilibrium asynchronous coupling between an atmosphere and an ocean GCM to investigate the potential effects of such a current.
PERMIAN AND JURASSIC CLIMATES: STRENGTHS AND LIMITATIONS OF THE FOSSIL PLANT DATA
Peter McA. Rees & Alfred M. Ziegler (Presented at the May 1998 AGU meeting in Boston)
Recent advances in the compilation and interpretation of global geological data enable direct comparison with results from palaeoclimate models. This is illustrated here for the Permian and Jurassic, for different paleogeographies and vegetation types as well as inferred global climate conditions. Climate signals from these non-angiosperm floras (as well as from lithological indicators such as coals and evaporites) are in good overall agreement with corresponding model results. Major discrepancies persist at high latitudes, however, where the models predict cold conditions beyond the tolerance limits indicated by the plants. These can be at least partially reconciled by increasing model CO2 levels, but discrepancies then arise at lower latitudes. Clearly, other factors must be considered in the models, such as vegetation feedbacks and ocean circulation. However, the quality of comparative data should also be assessed.
We adopted a global 'whole-flora' approach to the collection of fossil data, based upon as much as we know of the individual plant assemblages. Although imperfect, it provides our best proxy for original vegetation and therefore prevailing climate regimes in the geological past. We then conducted separate statistical ordinations of leaf genera from ca 600 Permian and 800 Jurassic localities to explore the foliar morphology/climate relationship and to identify phytogeographic patterns and biomes (climate zones) in a more rigorous and repeatable manner. From this, strong patterns emerged showing that certain leaf morphologies/taxa consistently co-occur, and that there is a good correlation between the kind of foliage preserved and its paleogeographic distribution. These patterns were used to help define the different biomes.
However, as with the corresponding models, biome resolution is relatively coarse. Detailed locality sampling is impractical for global-scale interpretations, so floral lists were compiled from the literature. Lists from individual formations were merged, both temporally and spatially (ca 100 km), obscuring plant community or Milankovitch variations. Our study assesses co-occurrences and distributional patterns of fossil leaves at the genus level. Although this has ensured a more standardized approach to identifications and comparisons, it means that we can only identify broad patterns. Inherent potential limitations of the fossil plant data include taphonomic and collection bias, taxonomic inconsistency, poor stratigraphic control, uncertainty concerning the relation of foliar organs to each other and to other plant organs, poor preservation of most material, and morphological variability of different species of the same genus. Also, uncertainties exist regarding climate tolerances of non-angiosperm fossils, since most are today either extinct or at best 'relictual' in their distributions.
Nevertheless our approach introduces greater consistency to the interpretation of global vegetation and climate patterns for different geological intervals, and indicates the potential for further refinement. By conducting the exercise on a global scale and by applying statistical methods to arrange the plant data, we can produce 'biome/paleobiome/climate/pattern' maps for different intervals in the geological past. These can then be compared directly with the corresponding global climate models.
A PERMIAN PALEOGEOGRAPHIC ATLAS
A.M. Ziegler, M.T. Gibbs, P.McA. Rees and D.B. Rowley (Presented at the International Symposium on Upper Permian Stratotypes of the Volga Region, Kazan, Russia, 1998)
Worldwide maps for the nine stage intervals of the Permian will be presented illustrating topography, climate, oceanography and biogeography. The paleogeographic base maps employ a new assessment of the paleomagnetic pole data, especially, the age assignments of the rocks which have yielded the poles. The topographic maps show contours at sea-level, 200 m, 1000 m, and 2000 m, and include restorations of about 65 Permian mountain chains around the world. The climates have been determined from a General Circulation Model study, and show seasonal temperature, precipitation and wind patterns with derivative statistics for various levels of atmospheric carbon dioxide. The oceanographic maps have been reconstructed from the climate-sensitive sediments, and show water masses based on temperature, salinity and upwelling patterns. The continental biogeography has been reconstructed using about 700 floral lists which have been subjected to multivariate statistics to determine distributional patterns, while the physiognomic structures of the floral elements provide information on the climate. The marine biogeography is being approached with help from Russian colleagues, T.A. Grunt and T.B. Leonova, who are providing data on brachiopod and ammonoid distributions, respectively.
Pangea moved about 15 degrees latitude to the north during the Permian Period, so that its southern portions rotated off the south pole while the northern portions approached the north pole. Thus the major Permo-Carboniferous glaciation of Gondwana disappeared while indications of local mountain glaciers appeared in the Verkhoyansk and Omolon regions of the Russian northeast toward the end of the period. While the change in latitude probably triggered the deglaciation in Gondwana, it must be remembered that portions of the supercontinent remained between 40 to 80 degrees, a latitudinal range occupied by the earlier ice sheets. Thus the latitudinal temperature gradient in the southern hemisphere decreased during the early Permian, and we are exploring mechanisms, such as the development of warm polar currents or increases in atmospheric carbon dioxide, to explain this climate shift. Changes observed in equatorial to subtropical regions during the Permian seem to relate to the northward drift of Pangea rather than to any global change.
CORRESPONDENCE OF PALEOLATITUDINAL DIVERSITY IN HERBIVOROUS DINOSAURS AND PLANTS DURING THE JURASSIC
J.M. Parrish, P.McA. Rees & A.M. Ziegler (Presented at the June 1996 NAPC meeting in Washington D.C.)
Diversity of herbivorous dinosaurs and of common leaf taxa were plotted on the latest paleogeographic maps produced by the Paleogeographic Atlas Project. Both assemblages showed very prominent bimodal distributions, with pronounced diversity peaks at mid-latitudes in the northern and southern hemispheres, and a relative lack of taxa at high and paraequatorial latitudes. The floral biomes include "seasonally dry subtropical", "warm temperate, and "cool temperate", with diversity being greatest in the warm temperate zone, about 40o to 60o in each hemisphere. Dinosaur diversity, however, seems to be centered on the "subtropical" zone, centered about 20o to 40o from the equator. The preserved vegetation in this zone was dominated by microphyllous conifers and cycadophytes, which would not have proved very nutritious for large herbivores. However, herbaceous forms such as ferns and sphenophytes also occur and would have flourished during wet periods. Thus, a savannah-like environment is envisaged, much like modern biomes that support large herbivores. In the Late Jurassic, the formations exhibiting the greatest diversity, the Morrison Formation in North America and the Tendaguru Formation in Tanzania are notable in the high redundancy of sauropod taxa (four genera, three families in the Tendaguru, nine genera, four families in the Morrison).The paucity of large herbivores and herbaceous plants at equatorial latitudes correlates with what appear to have been superheated, arid parts of the continental interior. Thus both diversity gradients appear to be a function of moisture as well as temperature.
MESOZOIC ASSEMBLY OF ASIA: CONSTRAINTS FROM FOSSIL FLORAS, TECTONICS AND PALEOMAGNETISM
A.M. Ziegler, P.McA. Rees, D.B. Rowley, A. Bekker, Li Qing & M.L. Hulver (In: Yin, A. & Harrison, M. (eds) The tectonic evolution of Asia: 371-400, 1996)
A floral gradient for the Early Mesozoic has been reconstructed from localities that ranged from the subtropics to the polar region of the northern hemisphere, and encompassed climates interpreted as ranging from the warm dry subtropical to the cool wet temperate regime. Our previous ordination studies on the floras demonstrated a gradual replacement of morphological types; from coniferophytes and cycadophytes with thick cuticles and small leaves in low latitudes, through broader-leaved forms of cycadophytes with filicopsids, to broad-leaved deciduous ginkgophytes and coniferophytes in near polar positions. Parallels with the Recent tropical and subtropical distributions of the cycads, and with the late Cenozoic temperate distribution of Ginkgo may be drawn.
Floral lists were assembled from eight exceptionally well sampled regions in Northern Eurasia ranging in age from Late Triassic through Late Jurassic, and were used to determine the correlation of the floral gradient with paleolatitude. Floral lists were also assembled from basins associated with the Chinese microcontinents of South China, North China and Tarim, which were converging with Northern Eurasia at the time. Their positions are therefore less well known, and our purpose is to show that the floral gradient is sensitive enough to be used as a check on the tectonic and paleomagnetic based reconstructions currently available. South and North China were in the warm temperate zone in the Late Triassic and Early Jurassic and collision with the southward moving Eurasia was complete by the Late Jurassic. During the Jurassic, the complex was moving equatorward into the dry subtropical zone. These conclusions accord with current tectonic interpretations, but the available paleomagnetic data seem to underestimate significantly the paleolatitude of these blocks during some time intervals.