Geological Time Spans Research Articles

Annealing kinetics of Kr-tracks in monazite: Implications for fission-track modelling

This paper reports data related to the potential of the mineral monazite for reconstructing the low-temperature thermal histories of geological samples from confined fission-track length measurements. The data are derived from isothermal annealing experiments on 300 MeV 86Kr tracks. Simplified versions of the favoured annealing models for apatite are adequate for describing the dataset. A non-compositional model predicts that: (1) track accumulation sets in when a sample cools under ~ 100 °C; (2) a substantial reduction (~ 2.5 μm) takes place at ambient and lower temperatures on a geological timescale; (3) an additional reduction of similar magnitude (~ 2.5 μm) takes place during exposure for a geological time span to the annealing conditions in the earth's upper crust; and (4) etching at elevated or room temperature is expected to affect the length of induced fission tracks but not that of fossil tracks.

A novel procedure to detect low molecular weight compounds released by alkaline ester cleavage from low maturity coals to assess its feedstock potential for deep microbial life

In recent years the widespread occurrence of microorganisms was demonstrated in deep marine and terrestrial sediments. With this discovery inevitably the question of the potential carbon and energy sources for this deep subsurface microbial life arises. In the current study a new method for the investigation of low molecular weight (LMW) organic acids linked to the kerogen matrix is presented. These LMW organic acids form a potential feedstock for deep microbial populations. Twelve coal samples of different maturity (vitrinite reflectance (R0) of 0.28–0.80%) from several coal mines on the North and South Island of New Zealand (NZ) were examined to assess the amount of bound LMW organic acids. Formate, acetate and oxalate were detected in significant amounts whereas the amounts of these compounds decrease with increasing maturity of the coal sample. This decrease of LMW organic acids mainly correlates to the phase of diagenetic alteration of the organic matter characterized by the release of oxygen containing compounds. Concomitantly, it coincides with temperature conditions assumed to be still compatible with microbial life in the deep subsurface. First assessments of the feedstock potential and generation rates of LMW organic acids indicate that the NZ coals investigated exhibit the potential to feed deep terrestrial microbial life with appropriate substrates over geological time spans.

Restrictions on fluorine depth profiling for exposure age dating in archaeological bones

Archaeological fragments of bone and teeth that are exposed to a humid environment take up fluorine from the surrounding soil. The fluorine ion replaces the hydroxyl group in the mineral phase of the bone, forming chemically more stable fluorapatite. In cortical parts of the long bone diaphysis a fluorine concentration profile can be observed, which decreases from the outer surface and the marrow cavity towards the inner parts of the bone matrix. Geological time spans are needed for this process to reach equilibrium and for the distribution to become uniform. As the shape of the profile, which can be described by a diffusion model, contains information on the exposure duration of the fossil object, several attempts to use fluorine profiling as a dating method have been undertaken. The distribution of fluorine in an archaeological sample however is strongly influenced by environmentally induced processes of bone diagenesis, i.e. alteration in the structure and composition of the mineral phase and degradation of organic components that may make the time information indistinct. The primary chemical composition of bones can thus be obscured by diagenesis within tens, hundreds or thousands of years. This depends more on the diagenetic environment than on the geological age. To observe the impact of environmental influence on the profile shape, samples from several burial sites featuring various soil conditions have been analyzed for their fluorine distribution and preservation state. This paper provides an overview on the restrictions that have to be considered when attempts are undertaken to relate a fluorine diffusion pattern to the archaeological age of a bone specimen.

Organic Matter in Black Slate Shows Oxidative Degradation Within Only a Few Decades

Abstract Low-grade metamorphic black slates were examined for the impact of oxidative weathering after exposure to the atmosphere for well-constrained time periods. Measurements of organic carbon were carried out on roofing slates and alum slate mining waste, and were compared against data obtained from freshly mined material. The results show that the organic-carbon content of roofing slates decreased from initially 0.73–1.7 wt.% to 0.51, 0.42, and 0.50 wt.% after 90, 94, and 100 years, respectively. The organic-carbon content of alum slate mining waste decreased from > 7.5 wt.% at the base (5 m below surface) to < 1 wt.% at the top of a dump during an exposure time of 53 years. The thermal maturity of this black slate organic matter is of anthracite stage. The degradation of organic matter is controlled exclusively by its accessibility within the exposed rock. After intense weathering, different black slate types (alum slate, roof slate) show similar amounts (less than 1 wt.%) of a poorly accessible OM fraction. The results demonstrate that oxidative weathering of organic matter in black slates is a fast process. A substantial decrease of organic matter occurs within only some decades to a century, rather than over geological time spans.

Accounting for unequal variances in evolutionary trend mechanisms

Evolutionary trends seemingly abound throughout the history of life. Perhaps the best-known example is Cope's rule, the tendency for lineages to increase in size over time. Other examples include hypothesized trends toward increasing complexity and fitness. What kinds of mechanisms cause such large-scale trends—trends occurring in higher taxa over geological time spans—and how can we effectively visualize them? In a seminal paper, Stanley (1973) proposed that apparent directional trends such as Cope's rule can result from unbiased change away from small initial values, analogous to diffusion away from a lower bound. Fisher (1986) generalized this idea, and Gould (1988, 1996) argued that such trends are properly viewed as increases in variance rather than trends in the average. These types of trend mechanisms were labeled by McShea (1994) as passive, in contrast to driven mechanisms in which change is biased in a particular direction because of (for instance) adaptive benefit or ontogenic constraint. In McShea's usage (McShea 1994, 1998b, 2000), the passive and driven categories include a broad range of evolutionary dynamics, of which unbiased diffusion and biased change are the simplest. More generally, McShea defines a driven trend mechanism as one resulting from an unstructured state space in which rates of change, speciation, and extinction are constant for all lineages. In contrast, a passive trend mechanism is one in which change among lineages follows a state space in which the rates vary in a structured way (McShea 2000). A similar distinction is the passive/active classification proposed by Wagner (1996), although Wagner's active trends constitute a broader category than McShea's driven trends. Much empirical and methodological work has been devoted to distinguishing passive and driven trends. Some recent papers in the paleontological literature include those of Carlson (1992), Boyajian and Lutz (1992), McShea (1993, …

Characterisation of early Archaean chemical sediments by trace element signatures

Rare earth element (REE) plus yttrium (Y) patterns of modern seawater have characteristic features that can be used as chemical fingerprints. Reliable proxies for marine REE+Y chemistry have been demonstrated from a large geological time span, including Archaean banded iron formation (BIF), stromatolitic limestone, Phanerozoic reef carbonate and Holocene microbialite. Here we present new REE+Y data for two distinct suites of early Archaean (ca. 3.7–3.8 Ga) metamorphosed rocks from southern West Greenland, whose interrelationships, if any, have been much debated in recent literature. The first suite comprises magnetite-quartz BIF, magnetite-carbonate BIF and banded magnetite-rich quartz rock, mostly from the Isua Greenstone Belt (IGB). The REE+Y patterns, particularly diagnostic anomalies (Ce/Ce*, Pr/Pr*), are closely related to those of published seawater proxies. The second suite includes banded quartz-pyroxene-amphibole±garnet rocks with minor magnetite from the so-called Akilia Association enclaves (in early Archaean granitoid gneisses) of the coastal region, some 150 km southwest of the IGB. Rocks of this type from one much publicised and highly debated locality (the island of Akilia) have been identified by some workers [Nature 384 (1996) 55; Geochim. Cosmochim. Acta 61 (1997) 2475] as BIF-facies, and their 13C-depleted signature in trace graphite interpreted as a proxy for earliest life on Earth. However, REE+Y patterns of the Akilia Association suite (except for one probably genuine magnetite-rich BIF from Ugpik) are inconsistent with a seawater origin. We agree with published geological and geochemical (including REE) work [Science 296 (2002) 1448] that most of the analysed Akilia rocks are not chemical sediments, and that C-isotopes in such rocks therefore cannot be used as biological proxies. Application of the REE+Y discriminant for the above two rock suites has been facilitated in this study by the use of MC-ICP technique which yields a more complete and precise REE+Y spectrum than was available in many previous studies.

Long-term persistence of bacterial DNA

The persistence of bacterial DNA over geological timespans remains a contentious issue. In direct contrast to in vitro based predictions, bacterial DNA and even culturable cells have been reported from various ancient specimens many million years (Ma) old [1–8]. As both ancient DNA studies and the revival of microorganisms are known to be susceptible to contamination [8–10], it is concerning that these results have not been independently replicated to confirm their authenticity. Furthermore, they show no obvious relationship between sample age, and either bacterial composition or DNA persistence, although bacteria are known to differ markedly in hardiness and resistance to DNA degradation [11]. We present the first study of DNA durability and degradation of a broad variety of bacteria preserved under optimal frozen conditions, using rigorous ancient DNA methods [8–10]. The results demonstrate that nonspore-forming gram-positive (GP) Actinobacteria are by far the most durable, out-surviving endosporeformers such as Bacillaceae and Clostridiaceae. The observed DNA degradation rates are close to theoretical calculations [9], indicating a limit of ca. 400 thousand years (kyr) beyond which PCR amplifications are prevented by the formation of DNA interstrand crosslinks (ICLs). The twelve permafrost samples (0-8.1 Ma) investigated were obtained from northeast Siberia and Beacon Valley, Antarctica. DNA preservation at these sites is exceptional due to constant subzero temperatures, largely neutral pH, and anaerobic conditions. Epifluorescence microscopy revealed ~107cells/gram wetweight in the bacterial size range. The cell counts are in agreement with previous results obtained on permafrost [2,3]. 16S rDNA sequences of 120 bp and 600 bp could be reproducibly amplified from samples up to 400–600 kyr, and show an inverse relationship between PCR amplification efficiency and fragment length that is typical of ancient DNA [8–10,12]. Controls for surface contamination during sampling were negative. Chimeric sequences were excluded from analysis, along with sequences that failed a bootstrap test for independent reproducibility [13]. DNA concentrations and taxonomic diversity were found to decrease with age until 400–600 kyr, at which point the percentage of templates with ICLs reached 100% (Figure 1A–C). Sequences from the older samples appear to be a subset of those from younger material, and all identified bacterial taxa are known soil inhabitants, indicating that permafrost is a nonextremophile environment. There were clear age-related patterns in taxon survival across geographically widespread samples (separated up to 1400 km). Sequences of non-sporeforming GP Actinobacteria, affiliated largely to the genus Arthrobacter (99–100% similarity), consistently persisted for the longest time, followed by GP endospore-forming Bacillaceae and Clostridiaceae and finally gram-negative (GN) bacteria, mostly Proteobacteria (Figure 1D).

Molecular phylogeny of the endemic Philippine rodent Apomys (Muridae) and the dynamics of diversification in an oceanic archipelago

We analysed the phylogenetic relationships of ten of the 13 known species of the genus Apomys using DNA sequences from cytochrome b. Apomys, endemic to oceanic portions of the Philippine archipelago, diversified during the Pliocene as these oceanic islands arose de novo. Several of the speciation events probably took place on Luzon or Mindanao, the two largest, oldest, and most topographically complex islands. Only one speciation event is associated with vicar- iance due to Pleistocene sea-level fluctuation, and a Pleistocene diversification model in which isolation is driven by sea-level changes is inconsistent with the data. Tectonic vicariance is nearly absent from the Philippines, in which tectonic coalescence plays a significant role. Most speciation events (about two-thirds) are associated with dispersal to newly developed oceanic islands. The data imply that the species have persisted for long periods, measured in mil- lions of years after their origins; further implications therefore are that faunal turnover is very slow, and persistence over geological time spans is more prominent than repeated colonization and extinction. Neither the equilibrium nor the vicariance model of biogeography adequately encompasses these results; a model incorporating colonization, extinction, and speciation is necessary and must incorporate long-term persistence to accommodate our observations. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society, 2003, 80, 699-715. ADDITIONAL KEYWORDS: biogeographical models - cytochrome b - dispersal - island biogeography - Pleistocene - speciation - vicariance.

Geomorphology of the Arun Valley, Eastern Nepal

The Arun Valley can be divided into the Upper Arun Periglacial Valley (UAPV) and Lower Arun Fluvial Valley (LAFV). The area separated by the linearly arranged ridges of Jaljale Danda, Harale Danda, and Sichok Danda. The UAPV lies in the north whereas the LAFV occupies the southern territories. The UAPV is further divided into four geomorphic provinces with fifteen distinct Holocene terrace levels, whereas the LAFV is divided into four geomorphic provinces with thirteen Pleistocene terrace levels.&#x0D; The concept of geomorphic province is introduced to denote distinct geomorphological features formed in a type locality within a given geological time span. The identified geomorphic surfaces are morpho-stratigraphically defined at respective villages and have a wide geographic distribution.&#x0D; This paper attempts to classify geomorphologically the whole Arun watershed and to compare the position of various terraces in terms of neo-tectonic activities. The main geomorphic features of the watershed and their relationship with the major geological structures are also discussed. The boundaries of the geomorphic province often coincide with the reported thrusts in the area.

Quantitative analogue flume‐model study of river–shelf systems: principles and verification exemplified by the Late Quaternary Colorado river–delta evolution

ABSTRACT Physical modelling of clastic sedimentary systems over geological time spans has to resort to analogue modelling since full scaling cannot be achieved within the spatial and temporal restrictions that are imposed by a laboratory set‐up. Such analogue models are suitable for systematic investigation of a sedimentary system's sensitivity to allocyclic changes by isolating governing parameters. Until now, analogue models of landscape evolution were mainly qualitative in nature. In this paper, we present a quantitative approach. The quantitative experimental results are verified and discussed by comparison with high‐resolution data from the Colorado river–shelf system of the Texas shelf that we used as a prototype.The model's dimensions are proportionally scaled to the prototype, except for a vertical exaggeration. Time is scaled using a Basin Response factor to maintain a similar ratio between the period of change and the system's equilibrium time for model and prototype. A Basin Fill factor was used to compare the ratio between the time‐averaged sedimentation rate and the rate of change in accommodation space of model and prototype. The flume‐model results are in the form of sediment budgets that are related to shelf cannibalism and fluvial supply, which are compared with the ancestral Colorado river–delta evolution of the last 40 kyr.Model and prototype have similarities in delta evolution in response to one cycle of sea‐level change. With sea‐level change as the isolated variable, the flume model generates a significant supply pulse caused by headward erosion of the shelf in response to the sea‐level fall. This pulse adds to the yield of the hinterland. The supply induced by sea‐level change persists during the early rise, although its rate declines. A similar trend is observed on the east Texas shelf. We argue that shelfal and fluvial degradation cycles induced by sea‐level changes can significantly influence the timing and amount of sediment supply to basins and must therefore be taken into consideration.

A test of whether rates of speciation were unusually high during the Cambrian radiation.

The Cambrian radiation represents an interval when nearly 20 animal phyla appear in the fossil record in a short geological time span; however, whether this radiation also represents a period of extremely rapid speciation remains unclear. Here, a stochastic framework is used to test the null hypothesis that diversity changes in one of the dominant Early Cambrian groups, the olenelloid trilobites, could be produced by tempos of speciation known to have operated during later time periods. Two continuous-time models, the Yule model and the birth and death process model, and one discrete-time model, the Bienaymé-Galton-Watson branching process model, were used. No statistical evidence for uniquely high rates of speciation during the radiation in these trilobites was found when the continuous-time models were used with low or moderate extinction rates, the rates typically associated with the Cambrian radiation, although the p values are fairly low or, in one case, significant when high extinction rates were used. However, rates of speciation were higher than the average Phanerozoic rates of speciation. The discrete-time model produced equivocal results: either rates were unusually high or the model is inapplicable during the Cambrian radiation. This suggests that there was nothing unique about evolutionary processes relating to the tempo of speciation during the Cambrian radiation.

Geochemistry of metabauxites in the Bergsträsser Odenwald (Mid German Crystalline Rise) and palaeoenvironmental implications

Pre- to early Variscan metamorphosed volcano-sedimentary series of the western (Bergstrasser) Odenwald have been intruded by Variscan calc-alkaline magmatites with plate margin affinities. Within the NE-SW trending metabasic and metapelitic series, intercalations of anomalously aluminous- and iron-rich compositions representing former bauxites are distinguishable. Geochemical data indicate that the Al-, Fe-rich rocks are the metamorphic equivalents of a former bauxitic-ferralitic weathering profile, now comprising spinel fels (top) with relics of pisolitic structures, corundum-chlorite fels, corundum-cordierite-plagioclase gneiss, sillimanite-cordierite-plagioclase gneiss, cordierite-plagioclase gneiss and kinzigite (base) displaying a decrease of weathering in the order as listed. The evolution of such terrestrial sediments is related to specific climatic, environmental, and physico-chemical conditions similar to those of the present-day tropical to subtropical humid regions with high seasonal rains and intensive drainage. Comparison with palaeomagnetic and palaeoclimatic data indicate that the Odenwald metabauxites originally formed during the Lower to Mid-Devonian. Our results imply that, during this geological time span, parts of the precursors of the Mid-German Crystalline Rise not only transversed low latitudes but also were exposed to terrestrial weathering.

Uncorrelated change produces the apparent dependence of evolutionary rate on interval

An intriguing phenomenon in the study of evolutionary rates of morphological change measured from fossil lineages has been the dependence of these rates on the inverse of the measurement interval. This effect has been reported across wide ranges of species as well as within single lineages, and has been interpreted as representing a smooth extension of evolutionary rate from generational timescales to paleontological timescales, suggesting that macroevolution may be simply microevolution extended over longer intervals. There has been some debate about whether this inverse dependence is a real feature of evolutionary change, or a mathematical or psychological artifact associated with the interpretation of data.Our analysis indicates that the strong inverse dependence of rate on interval is an artifact produced by the phenomenon of spurious self-correlation. Spurious self-correlation can appear in any calculation when a ratio is plotted against its denominator, as is done in plotting rate versus interval, and when these two quantities are not well correlated with one another. We demonstrate that the effect of spurious self-correlation appears in seven published data sets of evolutionary rate that range from taxonomically broad compendia to studies of single families. The effect obscures the underlying information about the dependence of evolutionary change on interval that is present in the data sets. In five of the seven data sets examined there is no significant correlation between the extent of evolutionary change and elapsed time. Where such a correlation does exist, the inverse dependence of rate on interval length is weakened. We describe the role played by taxonomic, dynamic, and character inhomogeneity in producing the lack of correlation of change with interval in each of these data sets. This lack of correlation of change with interval, and the accompanying inverse correlation of rate with interval, most likely arises from discontinuous modes of evolutionary change in which a distinct long-term dynamic dominates net change over geological time spans. It is poorly explained by the extrapolationary microevolutionary models that have been said to account for this phenomenon.

Enamel Ultrastructure of Cattle from the Quaternary Period in India

The present paper examines tooth enamel of three species of cattle, Bos acutifrons, Bos namadicus and Bos indicus, which are supposed to have been phylogenetically related and belong to the Quaternary period. It aims to show whether in a short geological time span of 2myrs, the changing environs that cattle were exposed to during the Early Holocene have caused any microstructural changes in their tooth enamel.Mammalian teeth exhibit a very complex arrangement of prisms in the enamel. The prisms are bundles of hydroxyapatite crystallites, arrangement of which is genetically determined and subject to evolutionary change. Hypsodonty or high crowned molars are such examples where concomitant masticatory stress factors near the enamel-dentin junction (EDJ) are known to have affected and altered the enamel microstructure in several large mammalian genera. Modified radial enamel in the deep enamel layer in some ungulates, including cattle, is an adaptive response to these stress factors which appeared much earlier (Tertiary) in the evolutionary history of these large mammals.The cattle enamel analysed here revealed several levels of structural complexity indicating its functional designations. The schmelzmuster in cattle are formed of three enamel types: radial enamel (outer enamel), Hunter-Schreger Bands (mid enamel), and modified radial enamel (inner enamel). There is an increase through time in the percentage of Hunter-Schreger Bands (HSB), which cover up to about 68% of the entire enamel surface among recent cattle. The modified radial enamel and radial enamel do have a receding trend in the Holocene, compared to their Pleistocene ancestors.

The evolution of the G matrix: selection or drift?

The evolution of quantitative characters can be described by the equation Deltaz=GP-1S where Deltaz is the vector of mean responses, G is the matrix of additive genetic variances and covariances, P is the matrix of phenotypic variances and covariances and S is the vector of selection differentials. This equation can be used to predict changes in trait values or to retrospectively estimate the selection gradient and is thus a central equation of evolutionary quantitative genetics. Genetic variances and covariances will be reduced by stabilizing selection, directional selection and drift, and increased by mutation. Changes in trait values resulting from directional selection that are comparable with differences observed among species are readily obtainable in short geological time spans (<5000 generations) with selection intensities so small that they would have an insignificant effect on the G matrix (of course it is possible that such changes came about by strong selection over a few generations, followed by long periods of stasis; there is insufficient evidence to presently distinguish these two possibilities). On the other hand, observed effective population sizes are sufficiently small that considerable changes in G can be expected from drift alone. The action of drift can be distinguished from selection because the former produces a proportional change in G whereas the latter, in general, will not. A survey of studies examining variation in G suggests that the null hypothesis that most of the variation can be attributed to drift rather than selection cannot be rejected. However, more research on the predicted statistical distribution of G as a result of selection and/or drift is required and further development of statistical tests to distinguish these two forces needs to be made.

サハリン（樺太）島の地質と地形 珪藻化石層序によるサハリン島南部のＭａｒｕｙａｍａ層・Ｋｕｒａｓｉ層の年代と北海道天北地域の新第三系との対比

A diatom biostratigraphic study of the Maruyama Formation in the Makarov and Dolinsk areas of the southern part of Sakhalin Island indicates that its geologic time span ranges from ca. 14 Ma to 3.5 Ma or possibly younger, which is much longer than previous estimates (e.g. 9.5-5.3 Ma by Ingle, 1992). A middle Miocene age of the Kurasi Formation in the Makarov area is also given on the basis of diatoms. These results are presented with correlation to the Neogene sections in the Tenpoku area, northern Hokkaido.The diatom zones of Akiba (1986) and Yanagisawa and Akiba (1998) recognized in the Makarov area include the Denticulopsis simonsenii Subzone of the D. hyalina Zone (NPD4Bb; middle middle Miocene, 14.6/14.5-13.1 Ma) in the Kurasi Formation and the 1st Member of the lower part of the Maruyama Formation, and the D. praedimorpha Zone (NPD5B; upper middle Miocene, 12.9-11.5 Ma) in the 2nd Member of the lower part of the Maruyama Formation. In the middle part of the Maruyama Formation in the Dolinsk area, the type area for the formation, numerous specimens of reworked diatoms were observed. The youngest zone shown by those assemblages is the Thalassiosira oestrupii Subzone of the Neodenticula kamtschatica Zone (NPD7Bb; lower Pliocene, 5.5-3.9/3.5 Ma), thus the middle part of the Maruyama Formation in the Dolinsk area should be of early Pliocene or younger ages. As similar assemblages are known from the Yuchi Formation in the Tenpoku area of northern Hokkaido, which is of Pleistocene age at least in part, the middle part of the Maruyama Formation in the Dolinsk area may be Pleistocene in age, which should be tested by further studies. In addition, our re-evaluation of Sheshukova-Poretzkaya's (1967) data from the Aniva area, south of Yuzhno-Sakhalinsk, suggests that the formation also contains an interval correlative to the Rouxia californica Zone (NPD7A; upper upper Miocene, 7.6-6.4 Ma).The present study geochronologically constrains other geological data such as paleontology, paleomagnetism, and paleoenvironments from the studied sections and their correlative strata. The considerably long geochronologic interval represented by the Maruyama Formation suggests stratigraphic gaps within the formation, thus a further study of the formation may elucidate the history of relative sea-level changes which have localor global implications.

Global change and the decline of coral reefs

Abstract. Ever since coral reefs exist, changing environmental conditions have periodically led to their decline. However, within the perspective of geological time-spans, corals have always managed to re-install themselves. Today, human activity has enhanced stress factors and added new ones that cause a rapid and (on the human time-scale) irreversible decline of many reef ecosystems. The reasons for the disturbance of these complex communities are multiple, but global warming is a key factor. As a result, coral reefs lose their vital role of protecting coastal areas from flooding and storm impact and of creating habitats for numerous marine organisms. In this short article, natural and anthropogenically induced stress factors are discussed, and measures for mitigating or stopping coral-reef decline are proposed.

The distribution and interactions of some Paleozoic fungi

The span of geologic time extending from the Early Devonian to Late Permian is examined for several sources of information about four major groups of fungi. The Lower Devonian Rhynie chert is characterized by a mycota consisting of a diverse assemblage of chytrids and zygomycetes. In Carboniferous coal-swamp communities, zygomycetes may have been the principal cellulose and lignin decomposers, in part because of their hypogeous habit. By late Permian time wood-rotting basidiomycetous fungi of several types were common and these can be traced into the Mesozoic. Although the remains of fungi are infrequently recognized in the fossil record, thus potentially biasing the stratigraphic distribution of major groups, the complexity of the fungus-host interactions during these geologic times suggests that most modern groups should extend well back into the Precambrian.

Modelling the links between vegetation and landforms

Vegetation is the most important intermediate through which climate and land use modify geomorphological processes and landforms. In this paper we explore three types of model which attempt to simulate the links to uncultivated vegetation, all sharing a common basis in hydrology. The first simulates the relationship between climate, vegetation and erosion rates for a fixed topography and soil cover. The second group of models takes into account longer term interactions with soil properties, which are thought to be important on the times scales of current concern with global change. An expanded illustration of this interaction is the relationship between wash erosion processes, vegetation cover and the evolution of microtopography. The third group of models looks at geological time spans, for which consistent differences in vegetation and landform between climatic areas create responses in the pattern of erosion rates, jointly to a combination of past and present climates.

Experimental diffusion of hydrogen into synthetic fluid inclusions in quartz

Abstract Quartz‐hosted, synthetic CO2‐H2O fluid inclusions behave as open systems with respect to diffusional transfer of hydrogen during laboratory‐simulated metamorphic re‐equilibration at 650, 750 and 825°C and 1.5 kbar total pressure with fO2 defined by the C‐CH4 buffer. Microthermometry and Raman spectroscopy show that the initial CO2‐H2O inclusions become CO2‐CH4‐H2‐H2Oinclusions after diffusive influx of hydrogen from the reducing confining medium. Measurable changes are observed in inclusion compositions after only 15 days of re‐equilibration, implying significant hydrogen mobility at still lower temperatures over geological time spans. Results of synthetic inclusion re‐equilibrium experiments have profound implications for the interpretation of natural fluid‐inclusion data; failure to account for potential hydrogen migration in inclusions from high‐temperature geological environments may lead to erroneous estimates of P‐T, and/or the compositions of metamorphic fluids.