Terrain Age Research Articles

Biogeomorphic interactions in the Turtmann glacier forefield, Switzerland

Glacier forefields are dynamic environments dominated by active paraglacial processes and simultaneous vegetation succession, triggered by glacier retreat since the Little Ice Age. While these dynamics are accelerating in the last decades owing to climate change, interactions between vegetation and geomorphic processes and components and the resulting patterns are only partly understood. Using a biomorphic approach based on preexisting geomorphic and glaciological data, geomorphic activity was classified and mapped in the Turtmann glacier forefield, Switzerland. Vegetation and environmental parameters were sampled. Vegetation analysis was subsequently carried out with vegetation classification and ordination for identifying relationships to environmental parameters. A paraglacial impact on vegetation succession could be shown and differentiated according to geomorphic activity on constant terrain age. Biogeomorphic concepts were then applied to explain these patterns. Three biogeomorphic succession phases were identified and related to degrees of activity, species composition, and strength of interactions. Integrating our results into the paraglacial concept, we show how the paraglacial adjustment of a glacier forefield is significantly affected by biogeomorphic interactions.

Primary succession of lichen and bryophyte communities following glacial recession on Signy Island, South Orkney Islands, Maritime Antarctic

AbstractA directional primary succession with moderate species replacement was quantitatively characterized on Signy Island in zones of a glacial valley corresponding to their age since deglaciation. A continuous increase in diversity and abundance of lichens and bryophytes was observed between terrains deglaciated in the late 20th century, to areas where deglaciation followed the Little Ice Age, and others thought to be ice-free since soon after the Last Glacial Maximum. Classification (UPGMA) and ordination (principal co-ordinate analysis) of vegetation data identified three different stages of development: a) pioneer communities, which rapidly develop in a few decades, b) immature communities developing on three to four century old terrains, and c) a climax stage (Polytrichum strictum-Chorisodontium aciphyllum community) developing on the oldest terrains, but only where local-scale environmental features are more favourable. Multivariate analysis including environmental parameters (canonical correspondence analysis) indicated terrain age as being the dominant controlling factor, with other environmental factors also exhibiting significant conditional effects (duration of snow cover, surface stoniness). These findings not only quantitatively verify reports of the rapid colonization of Maritime Antarctic terrains following recent climate amelioration and associated decrease in glacial extent, but also show how local-scale environmental resistance may slow or even prevent vegetation succession from pioneer to more mature stages in future.

Hydrated minerals in the deposits of Aureum Chaos

Hydrated minerals have been detected in many martian chaos regions and chasmata, playing a major role in its past aqueous activity. Based on short wave infrared data from CRISM, imagery and elevation data, we identified and mapped hydrated minerals in Aureum Chaos to shed light on their stratigraphy and geological context.The Interior Layered Deposits (ILDs) display three stratigraphic units: The lowest unit shows massive and also layered, high-albedo monohydrated sulfate (MHS, best matching kieserite; 20–650m thick) with intercalated hydroxylated ferric sulfates (HFSs, best matching jarosite) and ferric oxides. The overlying polyhydrated sulfate (PHS) is commonly layered (20–40m thick), smooth to heavily fractured, of lower albedo and partially contains ferric oxides. Spectrally neutral, distinctly layered, and bumpy cap rock (40–300m thick) forms the top.We found spectral and morphological similarities to Aram Chaos (PHS, MHS, ferric oxides; texture of ILD and cap rock) and Juventae Chasma (HFS). Besides, the phyllosilicate nontronite was found attributed to chaotic terrain as light toned fractured exposure and within dark, smooth mantling. The coexistence of sulfates and phyllosilicates indicates changes in the geochemistry of the aqueous environment.Since sulfates and phyllosilicates could be alteration products, the observed mineralogy presumably is not the original; conversions between PHS and MHS, MHS or PHS into jarosite, jarosite into iron oxides are considered. Due to its occurrence along mantling edges and on flat surfaces of MHS without textural differences, it appears that PHS is an alteration product of MHS, e.g. due to surface exposure. The facies and relative timing of sulfate formation remains undefined. However, two different formation models are considered. The first implies contemporaneous ILD and PHS deposition and diagenetic sulfate conversion (into MHS, iron oxides) due to overburden later on. This model is less conclusive than groundwater evaporation -the second model- due to the lack of a sharp PHS–MHS boundary that would indicate a diagenetic formation.Alternatively, the second model suggests subsequent sulfate formation. Groundwater would have penetrated into pre-existing sulfate-free ILD. The permeability and porosity of ILD material would have defined the rate of water absorption and sulfate precipitation (low in cap rock?), resulting in cementation of probably aeolian deposited ILDs. We think this model is more consistent and could explain ILD stratigraphy with the potential anhydrous cap rock on top.The surface age of chaotic terrain (late Hesperian) and mantling deposits (mid to late Amazonian) limit the ILD age and possibly the emplacement of sulfates. Phyllosilicates in the mantling are presumably allochthonous. Limiting the timing of in situ phyllosilicates is more complicated; they could be Noachian (excavated material, following the phyllosian era), or instead syn- or post-chaotic. A close spatial and temporal association of sulfates and phyllosilicates, in which nontronite represents the deep facies, and sulfates the evaporitic facies is known from Earth and is also possible and would combine groundwater alteration with the observed mineralogy.The preservation of nontronite, HFS and MHS probably reflects a relatively dry environment with intermittent aqueous activity since their emplacement.

Environmental gradients and succession patterns of carabid beetles (Coleoptera: Carabidae) in an Alpine glacier retreat zone

Accelerated by global warming, retreating glaciers leave behind spatially ordered moraines with underlying primary succession and disturbance. Current knowledge of primary succession comes mainly from studies of vegetation dynamics. Information about above-ground macroinvertebrates is still scarce. We used carabid beetles (Coleoptera; Carabidae) as indicator taxon to assess the effects of (1) terrain age (species turnover along the proglacial chronosequence) and (2) small-scale habitat architecture (vegetation cover, surface texture) on the carabid assembly. For this purpose, 33 sampling sites with pitfall traps were installed throughout the glacier foreland Morteratsch (Engadine, Switzerland), adjacent sparse forests serving as reference sites. With a total of 33 carabid species on the foreland and another 2 on the reference sites, the study area yielded a very high carabid species diversity compared to other glacier forelands. In general, the age of deglaciation proved to be a highly significant predictor for the carabid distribution, especially for particularly discriminant species. Observed species richness and activity densities showed bimodal patterns with a steep increase within the first ca. 40 years, a decline between around 40–90 years, and a further increase towards the terminal moraine. There was no evidence of dispersal-stochasticity: distinct clusters of sites with similar species composition were found. Microhabitat suitability proved to be a secondary effect, embedded in a temporal framework of primary succession. Surface cover with litter, herbs and dwarf-shrubs turned out to be the crucial habitat factors. Habitat loss as a result of climate warming will primarily affect cold-stenotopic carabids, but may potentially be absorbed by active selection for cooler microhabitats.

Nutrient limitation during long-term ecosystem development inferred from a mat-forming moss

Extant models of long-term ecosystem nutrient status, largely formulated using data from mid- and low-latitude settings dominated by vascular vegetation, predict nitrogen (N) limitation in the early stages of primary succession and phosphorus (P) limitation in the latter stages. The aims of this study were a) to examine the use of mosses to establish ecosystem nutrient status; b) to test an existing model of nutrient limitation during long-term primary succession and c) to assess the connection between nutrient dynamics and the emergence of spatially structured vegetation in a high-latitude, moss-dominated community. Spatiotemporal changes in nutrient availability were inferred using tissue nutrient concentrations in a moss sampled from a long (850-year) chronosequence of lava flows in Iceland. This is the first use of bryophyte nutrient concentrations, even though they should be a good indicator of nutrient status in high-latitude habitats. Samples of the moss, Racomitrium lanuginosum, from different topographic positions were collected from seven lava flows of different ages (26-848 years) and analyzed to establish concentrations of N, P and major mineral nutrients. Concentrations of N were low on all sites and evidence of progressive N accumulation is lacking; increasing concentrations of phosphorus indicated increasing N limitation with terrain age. This pattern contrasts with other studies of long-term nutrient limitation, where N limitation reduces and P limitation increases with increasing terrain age. Concentrations of mineral nutrients did not vary significantly with terrain age. Nutrient concentrations were only weakly related to spatial patterning in the vegetation. It is likely that element concentrations were strongly influenced by inputs of allochthonous sediment. These findings suggest that mat-forming, pleurocarpous mosses may be used to infer long-term changes in N & P limitation during primary succession and that existing models of nutrient limitation may not be robust over century - millennial timescales in low productivity, high-latitude habitats.

Age and growth of the Archean Kongling terrain, South China, with emphasis on 3.3 ga granitoid gneisses

The North China craton and the Yangtze craton (South China) both contain Archean rocks in eastern China. Unlike the North China craton, where Archean rocks are widespread, in the Yangtze craton the exposed Archean rocks are only known in the Kongling terrain (360 km²). Zircon U-Pb ages and Lu-Hf isotopic compositions of three granodioritic-trondhjemitic gneisses and three metasedimentary rocks from the Kongling terrain were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Igneous zircons in one trondhjemitic gneiss in the north of the Kongling terrain have an age of 3302±7 (1σ) Ma. Evidence from cathodoluminescence imaging, variations in Th/U and degree of U-Pb age discordance suggest that apparently younger zircons in the same population are variably disturbed 3302 Ma grains. Thus, this trondhjemitic gneiss is the oldest known rock in South China and predates the earlier reported ∼2900 Ma granitoid magmatism by 400 Ma. Zircon cores from one granodioritic gneiss in the north of the Kongling terrain also give a concordant age group at 3200 to 3300 Ma. Regardless as inherited or not, these cores crystallized from a magma indistinguishable in age with the trondhjemite. Concordant U-Pb ages for igneous zircons in one granodioritic gneiss in the south of the Kongling terrain yielded a weighted average ²⁰⁶Pb/²⁰⁷Pb age of 2981±13 Ma (2σ, MSWD=9.7, n=21). The zircon age and initial Hf isotopic compositions are similar to those of widespread granitoid gneisses from the north of the Kongling terrain (2903-2947 Ma), and indicate that the south and north of the Kongling terrain are correlative. The results also reinforce that magmatism of the whole Kongling terrain mainly occurred at 2900 Ma. Available Hf isotopic data from the Kongling terrain show that juvenile crustal additions occurred mainly between 3150 and 3800 Ma with a significant peak at 3300 to 3500 Ma. The ∼3300 Ma zircons from the trondhjemitic gneiss have Hf crust formation ages of 3450 to 3730 Ma, some of which have nearly chondritic ε_Hf (t). The whole-rock depleted mantle Nd model age of this rock is 3400 Ma, close to its age of magmatism and consistent with the Hf model age. Its ε_Nd value at 3300 Ma is nearly chondritic (1.26). These lines of evidence suggest that the 3300 Ma trondhjemite represent juvenile crust additions to the pre-existing continental crust.

Regional Variation in Successional Trajectories and Rates of Vegetation Change on Glacier Forelands in South-Central Norway

Variability in the vascular plant species composition of four stages of primary succession was investigated on 39 glacier forelands in the Jotunheim and Jostedalsbreen regions of south-central Norway. The relative frequencies of species were recorded in the pioneer zone adjacent to the glacier snout, in vegetation on terrain ages of c. 70 years and c. 250 years, and in mature vegetation outside the foreland (approximately 10,000 years since deglaciation). Sørensen dissimilarity, non-metric multidimensional scaling, and cluster analysis were used to compare the relative variability in species composition of these four stages and to identify patterns of succession within four altitudinal belts. Indicator species analysis identified characteristic species within each stage. Variation partitioning was used to quantify the relative influence of altitude and continentality on species composition. Variability increased between pioneer and later successional stages at all but the highest altitudes, which showed no significant difference in variability between stages. The results suggest that up to an altitude of around 1600 m succession on glacier forelands follows a divergent trajectory: above this altitude little successional change occurs. Rate of successional change also varies with altitude: below about 1000 m, in the sub-alpine belt, the transition from pioneer vegetation to birch woodland occurs within 70 years; above about 1600 m in the high-alpine belt, herbaceous pioneer vegetation can persist indefinitely; at intermediate altitudes, the dwarf-shrub and snowbed vegetation types of the low- and mid-alpine belts develop within c. 250 years. The explanatory power of altitude and continentality on compositional variation and the relative importance of altitude increased with successional stage.

Detection of small lunar secondary craters in circular polarization ratio radar images

The identification of small (D < a few kilometers) secondary craters and their global distributions are of critical importance to improving our knowledge of surface ages in the solar system. We investigate a technique by which small, distal secondary craters can be discerned from the surrounding primary population of equivalent size based on asymmetries in their ejecta blankets. The asymmetric ejecta blankets are visible in radar circular polarization ratio (CPR) but not as optical albedo features. Measurements with our new technique reveal 94 secondary craters on the Newton and Newton‐A crater floors near the lunar south pole. These regions are not in an obvious optical ray, but the orientation of asymmetric secondary ejecta blankets suggests that they represent an extension of the Tycho crater ray that crosses Clavius crater. Including the secondary craters at Newton and Newton‐A skews the terrain age inferred by crater counts. It is reduced by few percentages by their removal, from 3.8 to 3.75 Gyr at Newton‐A. Because “hidden rays” like that identified here may also occur beyond the edges of other optically bright lunar crater rays, we assess the effect that similar but hypothetical populations would have on lunar terrains of various ages. The average secondary crater density measured at 1 km diameter is equivalent to the crater density at 1 km on a 3.4 Gyr surface [Neukum et al., 2001]. Younger surfaces (i.e., younger crater ejecta blankets) would be dominated by secondary craters below 1 km if superposed by a hidden ray.

Patterns of larch establishment following deglaciation of Ventina glacier, central Italian Alps

Rapid glacier retreat often results in extensive areas of deglaciated terrain where primary successional processes occur. We studied biotic and abiotic factors underlying the invasion of larch (Larix decidua Mill.) on a glacier foreland and tested the hypothesis that the pattern of its establishment was controlled primarily by terrain age.The time span between deglaciation and establishment of larch and the importance of the position and shape of the glacier on the forest structure were studied over ∼150 years of glacial retreat. We related three data sets (forest structure, topography and spatial position) using ordination methods. Simple and partial Mantel tests and kriging interpolation were used to map and identify the key factors underlying the spatial patterns of larch establishment.The elapsed time between deglaciation and the germination of the larch trees was estimated between 14 and 34 years, with lower values found closer to the glacier terminus. Sparse and young stands developed near the glacier terminus and Mantel tests indicated significant correlations between spatial distance and ecological distance. The kriging interpolation showed strong agreement between tree age and the spatial pattern of the retreating glacier.The most influential factors in determining stand density and age were proximity to the glacier terminus and seed sources, as well as litter cover and elevation. The age of trees was strongly influenced by terrain age or time since deglaciation, but as succession progressed forest structure became more complex.

Crater modification and geologic activity in Enceladus' heavily cratered plains: Evidence from the impact crater distribution

Although we can observe current activity on Saturn's satellite Enceladus with Cassini, insight into past activity is best achieved (for now) through studying the impact crater distributions. Furthermore, approximation of terrain ages can only be attained through calculations using crater densities and estimations of impact rates in the saturnian system. Here we focus on what the impact crater distribution in Enceladus' heavily cratered plains can tell us about Enceladus' geologic history. We use Cassini ISS images to count craters in the heavily cratered plains on Enceladus, along with Rhea, Dione, Tethys and Mimas as references, to develop and compare their size-frequency distributions. Comparisons of our counts show that Enceladus' cratered plains distribution is unique in that it appears to have a relative deficiency of craters for diameters ⩽ 2 km and ⩾ 6 km compared to the other satellites' heavily cratered plains. Our data also indicates that the impact crater density within the cratered plains changes with latitude. Specifically, both the north and south mid-latitude regions have approximately three times higher density than the equatorial region. We hypothesize that the “missing” small and large craters in Enceladus' cratered plains is due to a combination of viscous relaxation of the larger craters, and burial of the relaxed large craters and small craters by south polar plume and possibly E-ring material. We also conclude that the spatial density distribution is not consistent with recent polar wander.

The spatiotemporal dynamics of a primary succession

Summary Conceptual models of ecosystem development commonly predict a phase of initial colonization characterized by the nucleation, growth and coalescence of discrete patches of pioneer plants. Spatiotemporal dynamics during subsequent development may follow one of three different models: the classical model, in which initially discrete patches of competitive dominant (secondary) colonists coalesce to form a homogeneous cover; the patch dynamics model, in which renewal mechanisms such as disturbance create a shifting mosaic of patches at different stages; and the geoecological model, in which the vegetation gradually differentiates along edaphic gradients related to the underlying physical template. These models of spatiotemporal dynamics were tested using vegetation and soil data from an 850‐year chronosequence, comprised of seven lava flows on Mt Hekla, Iceland. The scale and intensity of spatial pattern were quantified on each flow using spatial analyses (mean‐variance ratios, quadrat variance techniques and indices of autocorrelation). Changes in spatial pattern with increasing terrain age were compared with predicted trajectories, in order to identify which of the models of spatiotemporal dynamics was most consistent with the observations. The early stages of ecosystem development were characterized by colonization of the pioneer species, especially Racomitrium mosses, in discrete patches (‘Pioneer colonization stage’, < 20 years), which then grew laterally and coalesced to form a continuous, homogeneous carpet (‘Pioneer expansion stage’, 20–100 years). Later in the sequence, higher plants established in discrete patches within this pioneer matrix (‘Higher plant colonization stage’, 100–600 years). Over time, heterogeneity re‐emerged at a larger spatial scale as the vegetation differentiated according to topographic variations in the underlying substrate (‘Differentiation stage’, > 600 years). Synthesis. The spatiotemporal dynamics observed in the early stages of this succession were consistent with a model of pioneer nucleation in micro‐scale safe sites, followed by growth, coalescence and eventual fragmentation of pioneer patches. The spatial patterns which emerged later in development support the geoecological model, with spatial differentiation of vegetation related to meso‐scale substrate topography. The findings provide insight on how vegetation patterns emerge at different stages of ecosystem development in response to differing scales of heterogeneity in the underlying physical environment.

Magnetic anomalies and metamorphic boundaries in the southern Nagssugtoqidian orogen, West Greenland

Within the southern Nagssugtoqidian orogen in West Greenland metamorphic terrains of both Archaean and Palaeoproterozoic ages occur with metamorphic grade varying from low amphibolite facies to granulite facies. The determination of the relative ages of the different metamorphic terrains is greatly aided by the intrusion of the 2 Ga Kangâmiut dyke swarm along a NNE trend. In Archaean areas dykes cross-cut gneiss structures, and the host gneisses are in amphibolite to granulite facies. Along Itilleq strong shearing in an E–W-oriented zone caused retrogression of surrounding gneisses to low amphibolite facies. Within this Itivdleq shear zone Kangâmiut dykes follow the E–W shear fabrics giving the impression that dykes were reoriented by the shearing. However, the dykes remain largely undeformed and unmetamorphosed, indicating that the shear zone was established prior to dyke emplacement and that the orientation of the dykes here was governed by the shear fabric. Metamorphism and deformation north of Itilleq involve both dykes and host gneisses, and the metamorphic grade is amphibolite facies increasing to granulite facies at the northern boundary of the southern Nagssugtoqidian orogen. Here a zone of strong deformation, the Ikertôq thrust zone, coincides roughly with the amphibolite–granulite facies transition. Total magnetic field intensity anomalies from aeromagnetic data coincide spectacularly with metamorphic boundaries and reflect changes in content of the magnetic minerals at facies transitions. Even the nature of facies transitions is apparent. Static metamorphic boundaries are gradual whereas dynamic boundaries along deformation zones are abrupt.

Surface pollen deposition on glacier forelands in southern Norway I: local patterns of representation and source area at Storbreen, Jotunheimen

The relationship between vegetation and surface pollen deposition is examined at Storbreen glacier foreland where a clear plant succession exists. The aim is to determine whether the distinct plant communities present produce characteristic pollen assemblages. The influence of environmental factors is also considered. Pollen assemblages from moss polsters, collected from 22 paired sampling sites across the foreland, are compared with local vegetation. Two-way indicator species analysis and detrended correspondence analysis are employed to identify clusters and sequences, initially in the vegetation data and subsequently in the pollen data sets. Vegetation and pollen data are compared simultaneously using canonical correspondence analysis. Three main plant communities are distinguished: pioneer, heath and snowbed. Broadly, each community produces characteristic pollen assemblages. Boundaries between groups are not clear-cut, reflecting the mosaic of plant communities present. Recognition of distinct plant communities is hampered by the prevalence of long-distance arboreal pollen and poor representation of entomophillously pollinated taxa. Late in the succession up to 78% of pollen could originate locally. Use of the non-arboreal pollen sum significantly improves correspondence with vegetation. The importance of indicator taxa is considered and both Salix and Empetrum are found to distinguish successfully early phases of succession from later phases. Strong correlations exist between the primary ordination axes of vegetation and pollen and with terrain age and altitude (for example, the correlation between altitude and total land pollen Axis 1 is r= -0.76). The surface data add new information to the interpretation of tree colonization in the area during the Holocene.

Epigean Arthropod Succession along a 154-year Glacier Foreland Chronosequence in the Forni Valley (Central Italian Alps)

The 154-year (1850–2004) chronosequence of the Forni Glacier foreland has been studied by sampling ant, centipede, ground beetle, and spider species assemblages. Species numbers increase with terrain age along the chronosequence from 2 to 26 on the oldest soils. Thirty-nine species were collected; species richness and diversity (Shannon’s Index) of communities are correlated to the year of soil deglaciation. Shannon Index values increase with sites deglaciated between 1 and 61 years ago; sites deglaciated between 61 and 78 years ago produce similar values, and those deglaciated 78 to 154 years ago show a further increase in diversity. Ground beetles and spiders are found at all sites, while ants and centipedes were associated with mature forest soils. On the glacier surface, pioneer species such as the wolf-spider Pardosa saturatior and the ground beetle Oreonebria castanea permanently inhabit the supraglacial detritus surviving on trophic resources. Wingless ground beetle species are associated with mature soils, especially those with high hydric stability. Open land species typical of primary succession are found in the pioneer and intermediate stages, while community assemblages found on older terrain are linked to forest vegetation structure and dynamics.

Short-term Periglacial Processes, Vegetation Succession, and Soil Development within Sorted Patterned Ground: Jotunheimen, Norway

Small (1 ≤ m diameter) sorted patterned ground features were studied on the Little Ice Age forelands of three Jotunheimen glaciers. Patterned ground appears to be most active near the ice margins, declining in intensity of activity with distance from the glaciers. Vegetation and soil development are negligible within patterned ground that is “Recent” (decadal time frame). Significant (P < 0.05) fine scale differences in vegetation and soil development occur within patterned ground on terrain ∼70 yr in age, with patterned ground borders having higher values of vegetation cover and thicker soils than that in patterned ground centers. With increasing age of terrain and patterned ground, soil development and vegetation encroach inward toward patterned ground centers, implying that a short-term, active periglacial zone exists near the ice margin, decaying with time and glacier retreat. Specifically, terrain that has been deglaciated for ∼70 years and is approximately ∼350–500 m from the ice margin shows a significant decline in frost activity, allowing for the initiation of pedogenesis and vegetation colonization.

Assessment of factors regulating axon growth between the cortex and spinal cord in organotypic co-cultures: effects of age and neurotrophic factors.

Axon growth failure in the central nervous system (CNS) of adult animals is thought to be attributable to several factors, including an inadequate intrinsic growth response, the presence of inhibitory molecules, and a lack of adequate neurotrophic support. Here we use a new in vitro assay system to quantitatively assess growth of axons in cortex/spinal cord organotypic co-cultures from neonatal rats. Co-cultures of cortex and spinal cord were prepared from neonatal rats at P3 or P7, and by pairing cortex and spinal cords from different ages. Axon growth from the cortex to the spinal cord was assessed using DiI tract tracing techniques. Axons could be traced from the cortex to the spinal cord in co-cultures in which both tissues were obtained from P3 animals, whereas few axons crossed the cortex/spinal cord boundary in co-cultures from P7 animals. A larger number of axons could be traced across the boundary in co-cultures from P3 animals that were treated with neurotrophins (NGF, BDNF, or NT3), whereas neurotrophins produced minimal growth enhancement in P7 co-cultures. In mixed age co-cultures of P7 cortex with P3 spinal cord, moderate numbers of axons extended between the cortex and spinal cord when cultures were treated with neurotrophins, but few if any crossing axons were detected in co-cultures of P3 cortex with P7 spinal cords. These results indicate that successful growth of axons from the cortex to the spinal cord depends on the developmental age of the tissue terrain (the spinal cord and/or the interface between cortex and spinal cord explants), and to a lesser extent on the developmental state of the cortical neurons, and that axon growth between cortex and spinal cord can be enhanced by exogenous neurotrophins. These co-cultures provide a potentially useful assay for factors that affect axon growth that is intermediate between assays based on dissociated neurons and the intact tissue terrain.

Geologic settings of Martian gullies: Implications for their origins

Martian gullies are found on steep slopes of all origins, on all sorts of terrains of all ages, scattered across nearly all of Mars. Gullies are observed on all manner of substrates (layered, massive, shattered, rubble), with or without nearby mantling deposits. Gullies are most common in the southern midlatitudes but also occur in the northern hemisphere, in near polar terrain, on equatorial volcanoes, and on northern plains. Most gullies in the southern hemisphere are on south‐facing slopes, but they occur on slopes of all orientations. Gullies are among the youngest features on Mars but locally are overlain by eolian deposits and cut by faults. Old or eroded gullies are rare, and those found have been partially stripped from slopes, leaving no rock debris behind. Most gully deposits contain no detectable rocks. These data are inconsistent with published hypotheses of gully formation, including seeps and breakouts of water or brine, hydrothermal activity, cryovolcanism, and breakouts from liquid carbon dioxide. The data are consistent with gullies being dry flows of eolian material (dust and silt), comparable to climax snow avalanches on Earth. Eolian sedimentation should be correlated little with underlying geology: cause of slope, age of terrain, type of terrain, or the nature of the rocks. Eolian sedimentation should be correlated with wind deceleration (which will cause suspended sediment to drop), and areas with common gullies are those with strong wind deceleration (predicted by global circulation model). In such areas, sediment will be deposited preferentially in the lee of obstacles; for the gully‐rich areas of the southern midlatitudes, winds blow from the NNW, so that sediment is deposited on SSE‐facing slopes (i.e., poleward). These predictions are in accord with observations.

Primary plant succession on recently deglaciated terrain in the Canadian High Arctic

AbstractAim Primary succession in a high arctic environment was examined to determine whether the patterns conform to models based on studies in temperate regions. The location also presented an unique opportunity to examine a well preserved pre‐Little Ice Age plant community and organically enriched palaeosol that are being released from the glacier along with the glacial and glaciofluvial sediments.Locations The main location was the recently deglaciated terrain of the Twin Glacier foreland at Alexandra Fjord, Ellesmere Island (79° N). The position of the glacier from air photos in 1959 and 1981, and periodic direct monitoring of the position of the terminus since 1980 provided good chronological control. Surveys of succession patterns were also conducted at four other sites in the Alexandra Fiord region of eastern central Ellesmere Island.Methods Vegetation cover of mosses and vascular plants (lichens were not present) was measured in 1994 and 1995 using a stratified random design. twinspan and canonical correspondence analysis were used together to examine successional vegetation patterns in relation to a set of environmental variables. Species richness, total vegetation cover, and a combined set of soil parameters (pH, organic matter content, moisture and fine substrate) were compared between the two principal soil types, palaeosol and glaciofluvial sediment.Results Terrain age accounted for most of the variation in species composition over the study area. The succession followed a directional‐replacement series with four main stages of dominance in at least 44 years: (1) mosses → (2) graminoid‐forb → (3) deciduous shrub‐moss → (4) evergreen dwarf‐shrub‐moss. There was little difference in species richness patterns over time on the palaeosol compared with the glaciofluvial sediment. However, total vegetation cover was significantly higher on palaeosol and there was a significant difference in a combined set of soil parameters between the two soil types, which indicates that palaeosol may provide conditions more favourable for establishment and growth.Main conclusions Directional‐replacement succession is possible in a high arctic oasis environment; however, this mode of succession is probably atypical of much more extensive high arctic environments such as polar deserts. Descriptive results from four other glacier forelands sampled in 1995 on east‐central Ellesmere Island showed directional succession with no species replacement at two sites and non‐directional succession without species replacement at another site. Primary succession in these high arctic sites is strongly controlled by local environmental conditions.

Long‐term retention of impact crater topography on Ganymede

Previous analyses into viscous relaxation of impact craters on Ganymede have predicted short characteristic relaxation times (of the order of 100 Myr or less for larger craters), in disagreement with the apparent great ages of Ganymede's terrains. By applying improved understanding of rheologic parameters and initial crater shapes to a viscoelastic model, we calculate upper limit (zero heat flow) relaxation times in excess of the age of the solar system, eliminating this discrepancy. These very long relaxation times are not due to elastic effects, but are simply due to the effective viscosity of water ice at the appropriate temperatures and stresses.

The Shergottite age paradox and the relative probabilities for Martian meteorites of differing ages

The apparent paradox that the majority of impacts yielding Martian meteorites appear to have taken place on only a few percent of the Martian surface can be resolved if all the shergottites were ejected in a single event rather than in multiple events. The apparent degree by which shergottites are overrepresented among Martian meteorites is thereby reduced to about fourfold relative to other types of Martian meteorites for nominal cratering rate models, and further reduced for higher cratering rate models. If the shergottite‐ejection event is assigned to one of three craters in the vicinity of Olympus Mons that were previously identified as candidate source craters for the SNC (shergottites, nakhlites, Chassigny) meteorites, and the nakhlite/chassignite event is assigned to a candidate crater in the vicinity of Ceraunius Tholus, the implied ages of the surrounding terrains agree well with crater density ages. Because of their young crystallization ages, the likely origin of the shergottites is in the Tharsis region even for cratering rates up to twice the nominal value (minimum terrain ages). For these higher cratering rates, the nakhlite source need not be in the Tharsis region. Since the source of the ancient orthopyroxenite, ALH84001, was probably in the ancient southern uplands, meteorite‐yielding impacts may have been widely dispersed on the Martian surface.