Margaret E. Collinson

Episodic fresh surface waters in the Eocene Arctic Ocean

It has been suggested, on the basis of modern hydrology and fully coupled palaeoclimate simulations, that the warm greenhouse conditions that characterized the early Palaeogene period (55–45 Myr ago) probably induced an intensified hydrological cycle with precipitation exceeding evaporation at high latitudes. Little field evidence, however, has been available to constrain oceanic conditions in the Arctic during this period. Here we analyse Palaeogene sediments obtained during the Arctic Coring Expedition, showing that large quantities of the free-floating fern Azolla grew and reproduced in the Arctic Ocean by the onset of the middle Eocene epoch (∼50 Myr ago). The Azolla and accompanying abundant freshwater organic and siliceous microfossils indicate an episodic freshening of Arctic surface waters during an ∼800,000-year interval. The abundant remains of Azolla that characterize basal middle Eocene marine deposits of all Nordic seas probably represent transported assemblages resulting from freshwater spills from the Arctic Ocean that reached as far south as the North Sea. The termination of the Azolla phase in the Arctic coincides with a local sea surface temperature rise from ∼10 °C to 13 °C, pointing to simultaneous increases in salt and heat supply owing to the influx of waters from adjacent oceans. We suggest that onset and termination of the Azolla phase depended on the degree of oceanic exchange between Arctic Ocean and adjacent seas.

Cainozoic ferns and their distribution

The phytogeographic distribution of Cainozoic ferns is reported based upon a critical re-appraisal of the macrofossil and mesofossil record also taking account of evidence from a few highly diagnostic spores. Well-documented circum-Arctic Cainozoic floras show ferns (Woodwardia, Onoclea, Osmunda, Coniopteris and to a lesser extentAzolla) distributed around the pole to very high paleolatitudes. Some ferns are shared between the mid-paleolatitudes of North America and Europe as would be predicted from the distributions of other biota. Evidence for the composition of Cainozoic fern floras is minimal in some regions (e.g., Antarctica, Central and South America, Africa, India, South East Asia), so the absence of fern fossils from these areas has no biogeographical significance. Matoniaceae were abundant in the preceding Mesozoic. However, the absence of Cainozoic macrofossils, and the fact that no CainozoicMatonisporites spores areMatonia-like, indicates that Matoniaceae had attained their modern relict distribution by, or very early in, the Cainozoic. The important Mesozoic families Marattiaceae and Dipteridaceae are also not represented by Cainozoic macrofossils. They probably also showed Cainozoic restriction but spores are not sufficiently diagnostic to enable testing of this hypothesis. Other ferns, which were also important in the Mesozoic (e.g., Dicksoniaceae, Gleicheniaceae), have patchy, equivocal, or inadequately published Cainozoic records. The dispersed spore record may provide an opportunity to track Cainozoic Gleicheniaceae but this approach is not without problems. Most well-represented Cainozoic fern families, genera and subgenera show widespread Cainozoic ranges, typically with considerable range extensions over their living relatives, both onto other continents and north and south to higher paleolatitudes. These include Schizaeaceae (Lygodium, Anemia, and the extinctRuffordia), Osmundaceae (Osmunda), Pteridaceae (Acrostichum), Thelypteridaceae (Cyclosorus), Lophosoriaceae (Lophosoria), Cyatheaceae (theCnemidaria/Cyathea decurrens clade) and the heterosporous water fernAzolla (Azollaceae). A few well-represented ferns show Cainozoic distributions similar to those of the present day (e.g.,Salvinia [Salviniaceae] andCeratopteris [Pteridaceae] (the latter by the Neogene and based only on spores]) but even these had slightly broader ranges in the Cainozoic. Some Cainozoic ferns have apparently local distributions, e.g.,Blechnum dentatum (Blechnaceae) in Europe; and others are so far represented at only one or few sites, e.g.,Dennstaedtiopsis (Dennstaedtiaceae),Botrychium (Ophioglossales),Grammitis (Grammitidaceae), andMakotopteris andRumohra (Dryopteridaceae). Cainozoic fossils assigned toDryopteris (and some other dryopteroids) require revision along with those of Thelypteridaceae, the latter having high potential to provide useful paleobiogeographic evidence, at least of theCyclosorus group. Cainozoic records of Hymenophyllaceae and Polypodiaceae are here considered unconfirmed.

Assessing the potential for the stomatal characters of extant and fossil Ginkgo leaves to signal atmospheric CO2 change

The stomatal density and index of fossil Ginkgo leaves (Early Jurassic to Early Cretaceous) have been investigated to test whether these plant fossils provide evidence for CO(2)-rich atmosphere in the Mesozoic. We first assessed five sources of natural variation in the stomatal density and index of extant Gingko biloba leaves: (1) timing of leaf maturation, (2) young vs. fully developed leaves, (3) short shoots vs. long shoots, (4) position in the canopy, and (5) male vs. female trees. Our analysis indicated that some significant differences in leaf stomatal density and index were evident arising from these considerations. However, this variability was considerably less than the difference in leaf stomatal density and index between modern and fossil samples, with the stomatal index of four species of Mesozoic Ginkgo (G. coriacea, G. huttoni, G. yimaensis, and G. obrutschewii) 60-40% lower than the modern values recorded in this study for extant G. biloba. Calculated as stomatal ratios (the stomatal index of the fossil leaves relative to the modern value), the values generally tracked the CO(2) variations predicted by a long-term carbon cycle model confirming the utility of this plant group to provide a reasonable measure of ancient atmospheric CO(2) change.

Alternative origin of aliphatic polymer in kerogen

The origin of sedimentary organic matter (kerogen) has been attributed to random recombination reactions of biological components in sediments or to selective preservation of decay-resistant macromolecules. Neither hypothesis explains the aliphatic composition of the cuticle of fossil arthropods. Thermal maturation experiments on modern arthropods, involving confined pyrolysis at 250–360 °C, degrade the chitin-protein complex of the cuticle and transform free aliphatic components into a polymeric structure. The results of the application of electron microscopy and spectroscopic methods to modern, thermally matured, and fossil arthropod cuticles indicate that in situ polymerization of free and ester-bound cuticular lipids can lead to kerogen formation. Thus, fossil arthropod fragments can contribute to sedimentary organic matter.

Experiments in waterlogging and sedimentology of charcoal: results and implications

Abstract Fossil charcoal has a sporadic occurrence in sedimentary rocks since Devonian time. It is moderately common as a component of the organic material found in a wide variety of facies, but there are also some notable concentrations which occur locally. These occurrences have considerable palaeobotanical and palaeoecological value because the process of charring may result in excellent preservation of plant tissue. If assemblages of charred material are to be used to interpret palaeoenvironments, it is important to understand the behaviour of charcoal during transport from the site of the fire. Charcoal is an unusual sedimentary material because most fresh material floats, but with prolonged immersion becomes waterlogged and sinks. We carried out a series of waterlogging experiments on uncharred material and charcoal from a range of different plant types and tissues. These show considerable variations in the rates of waterlogging of different charred and uncharred plant tissues, and it is suggested that buoyancy contrasts are likely to result in separation during transport and deposition. Furthermore, experiments in a flume tank have shown that the process of deposition of charcoal in sand is controlled by the rate of migration of bedforms, which is in turn determined by depth, flow velocity and sediment supply. Allochthonous assemblages of charred and uncharred plant debris will most probably be biased and not fully representative of the contemporaneous plants or vegetation. The presence of charcoal concentrations may be used as an aid in interpreting palaeohydraulic conditions.

Testing the climatic estimates from different palaeobotanical methods: an example from the Middle Miocene Shanwang flora of China

The Miocene Shanwang biota from eastern China contains exceptionally well-preserved plant fossils with abundant leaf fossils and palynomorphs co-occurring at several levels in the sedimentary succession. This has provided an ideal opportunity to undertake detailed comparative quantitative palaeoclimate reconstruction, based on both mega- and microfloral assemblages, using various approaches, namely the coexistence approach (CoA), leaf margin analysis (LMA) and Climate Leaf Analysis Multivariate Programme (CLAMP). By applying these approaches to the same dataset and using multiple fossil assemblages from different levels we are able to compare results from the different methods of climate prediction. CLAMP and LMA give consistently lower temperature (mean annual temperature, MAT) predictions than the CoA. Taking errors into account all methods indicate no overall climate change through the sequence studied. Results from CoA show overall agreement in the palaeoclimate parameters obtained using both pollen and leaf datasets indicating a high degree of internal consistency with this method.

Eocene–Oligocene mammalian faunal turnover in the Hampshire Basin, UK: calibration to the global time scale and the major cooling event

A new high-resolution, c. 1 Ma mammalian record in continental Eocene–Oligocene strata in the Hampshire Basin is used to investigate the nature and timing of the major Europe-wide mammalian faunal turnover termed the Grande Coupure. Whether this was caused by climate change or by competition with taxa dispersing from Asia is controversial. The mammalian faunas in this record, after rarefaction analysis, show a sharp reduction in diversity only after the Grande Coupure. Improved correlation of NW European successions to global events confirms the Grande Coupure as earliest Oligocene. It shows that a c. 350 ka hiatus interrupts the Hampshire and Paris Basin sequences prior to the first record of post-Grande Coupure Asian taxa. Hiatus-bridging faunas from elsewhere in Europe record mainly post-Grande Coupure taxa, suggesting that the turnover occurred early in the hiatus, minimizing bias to the turnover pattern. A previously unrecorded, second, smaller turnover, involving European mammals only, immediately precedes the Grande Coupure in the Hampshire Basin, coinciding with vegetational change. This turnover is judged not to represent cooling. It is concluded that the Grande Coupure coincides with the earliest Oligocene Oi-1 glaciation and that climate change combined with competition to produce the turnover.

The oil-generating potential of plants from coal and coal-bearing strata through time: a review with new evidence from Carboniferous plants

Abstract A combination of microscopy and chemistry is used to characterize high molecular weight components of plant material which contributed to Carboniferous coals and coal-bearing strata. Material was selected from coals, paper coals, coal balls, oil-shales and shales and included cuticles from stems and leaves, periderm from stems and rooting systems, spore walls, algal cell walls and ‘resin’ rodlets. Chemical analyses were undertaken using Curie-point pyrolysis-gas chromatography (-mass spectrometry) (Py-GC-(MS)) and 13C solid state Nuclear Magnetic Resonance (NMR). Light microscopy and scanning electron microscopy were used to ensure a detailed understanding of the plant material which was analysed chemically. This study has emphasized those plants, plant tissues and organs which are known to have been dominant or major contributors to Carboniferous coals and coal-bearing sequences: arborescent lycophyte periderm (Diaphorodendron stems and Stigmaria rhizomorphs); pteridosperm cuticles (medullosan (e.g. Alethopteris) and Karinopteris), lycophyte stem cuticle (Eskdalia) and arborescent lycophyte spores (megaspores and microspores). Several algal cell walls (Tetraedron, Tasmanites and Gloeocapsomorpha) and the ‘resin’ rodlets derived from medullosan pteridosperm petioles have also been analysed. Results show that all of these elements (except resins) contain (or are dominated by) highly resistant, highly aliphatic macromolecules which have the potential to yield, upon catagenesis, n-alkanes which are found in crude oils. Resins potentially contribute cyclic hydrocarbons to crude oils. Combining all this evidence it is concluded that Carboniferous coals are oil prone and that explanations for the absence of oil-pools derived from such coals must be sought in geological or exploration factors and not in the nature of the coals themselves. A review of coal-forming floras through time, and of suggested terrestrially sourced oils and their parent coals, leads us to conclude that high molecular weight components of higher plant materials have potentially made a major contribution to oils sourced from coals and coal-bearing strata over a long period of geological time (Devonian onwards). Cell walls of freshwater algae have made a comparable contribution since the Ordovician. The oil-generating potential of each coal or organic-rich sediment will depend upon a combination of depositional environment and the floristic and chemical composition of the source vegetation and its component plants and plant parts.

Factors influencing the preservation of plant cuticles: a comparison of morphology and chemical composition of modern and fossil examples

Samples of Recent Ginkgo biloba, two Cretaceous Ginkgo and two Cretaceous conifer cuticles from different enclosing lithologies but with similar thermal maturity of the fossils, have been analysed by scanning and transmission electron microscopy (SEM, TEM), Fourier transform-infrared spectroscopy (FT-IR), and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Recent and fossil Ginkgo cuticles under SEM reveal sheets, similar in appearance, varying in the abundance and texture of the cuticular papillae. TEM of the Recent Ginkgo shows an outer amorphous cuticle layer, a structured middle layer and an inner laminated layer of cell wall. The Cretaceous Ginkgo cuticles retain the amorphous layer and a modified structured layer. SEM of Cretaceous Ahietites and Frenelopsis also shows preservation of cuticle sheets but each has distinctive morphology. These conifer cuticles are very thick (TEM), Frenelopsis cuticle has remarkable multilaminar ultrastructure whilst Abietites is amorphous. G. biloba cuticle consists mainly of the natural polyester, cutin, as revealed by FT-IR and pyrolysis, indicated by an abundance of saturated, unsaturated and hydroxy fatty acids. IR spectra of fossil cuticles, like modern cuticles, show aliphatic C-H, hydroxyl and carbonyl functions. However, in fossils, the carbonyl ester is transformed to carboxylic acid or ketone groups. Pyrolysates of fossils show phenolic constituents like modern cuticles but loss of cutin fatty acid monomers and an increased prominence of an homologous series of n-alkene and n-alkane fragments up to n-C 30 . Since most Recent cuticles, including those of conifers and Ginkgo biloba which we have studied, do not yield a non-saponifiable highly resistant residue it is proposed that organic preservation of fossil species investigated involves the diagenetic stabilisation of chemically-labile aliphatic cutin constituents along with incorporation of waxes. These general chemical modifications characterise all fossil Ginkgo and conifer cuticles, irrespective of their enclosing lithology, systematic affinity, external morphology or internal ultrastructural preservation. However there are also clear chemical differences between the fossil samples which may relate to their systematic affinity (ginkgos vs Abietites and Frenelopsis).

The ecology of Cainozoic ferns

Abstract The ecology of Cainozoic ferns is documented (excluding that based only on nearest living relatives). Free-floating water ferns (of the modern genera Azolla and Salvinia ) are widespread in the Cainozoic. They are represented by whole plants and dispersed or interconnected megaspores and microspore massulae in freshwater facies associated with a range of aquatic angiosperms. Acrostichum (a fern characteristic of mangroves today) was clearly associated with lakes and freshwater marshes in the Cainozoic. In southern England an Acrostichum / Typha association existed comparable to that which is rare today, e.g. in the Florida Everglades. Other Cainozoic ferns also grew at the margins of lakes and in mires, especially well-represented by the Princeton Chert flora (Dennstaedtiaceae, Dryopteridaceae, blechnoids and Osmunda ). In North America ferns such as Onoclea and Osmunda were associates of freshwater swamp forests dominated by taxodiaceous trees. These ferns, along with Woodwardia and the extinct Coniopteris , had a Cainozoic circum-Arctic distribution to very high palaeolatitudes. The Eocene fern flora of Yellowstone National Park, USA, grew in a disturbed volcanogenic terrain but the same ferns also occurred in backswamp settings. Gleicheniaceae were part of a fire-prone vegetation in the Miocene of Australia but other Cainozoic Gleicheniaceae are very poorly understood. Relatively little is known about the Cainozoic ecology of the Marattiaceae, Matoniaceae, Dipteridaceae, Dicksoniaceae and Cyatheaceae despite their Mesozoic importance. The Cainozoic record of tree ferns (proven by stem fossils) is very patchy but does include members of the Cyatheaceae, Dicksoniaceae and Osmundaceae ( Aurealcaulis , which grew in swampy floodplain forests). Although the epiphytic habit had evolved in extinct families of ferns in the Carboniferous there is no convincing evidence for fossils of epiphytic ferns in the Cainozoic. The fern Lygodium (for which a climbing habit is often inferred from morphological similarity with modern Lygodium ) was widespread in the Cainozoic in North America, Chile, Europe, Australia and probably China. However, there are no rachis fossils to confirm or refute the interpretation that Palaeogene to Miocene Lygodium was a climber.