Last Glacial Maximum equilibrium-line altitude trends and precipitation patterns in the Sangre de Cristo Mountains, southern Colorado, USA

Precipitation patterns during the Last Glacial Maximum (LGM) in the Rocky Mountains varied due to the influence of the continental ice sheets and pluvial lakes. However, no constraints have been placed on potential changes of southeasterly Gulf of Mexico-derived moisture that today contributes considerable precipitation to the easternmost ranges of the southern and middle Rocky Mountains. The Sangre de Cristo Mountains of southern Colorado are ideally situated to assess the relative importance of westerly and southeasterly-derived moisture during the LGM. Based on reconstructions of 30 palaeoglaciers in the Sangre de Cristo Mountains, we find that LGM equilibrium-line altitudes (ELAs) on the east side of the range were systematically 100–200 m lower than ELAs on the west side. The observed ELA pattern is strikingly similar to modern precipitation patterns in the study area, suggesting that southeasterly-derived precipitation had a significant influence on the mass balances of LGM glaciers.     

Evaluating delta asymmetry using three‐dimensional facies architecture and ichnological analysis,Ferron ‘Notom Delta’, Capital Reef,Utah, USA

Delta asymmetry occurs where there is strong wave influence and net longshore transport. Differences in the morphology and facies architecture between updrift and downdrift sides of asymmetric deltas are potentially significant for exploration and exploitation of resources in this class of reservoirs. Although delta asymmetry has been recognized widely from modern wave‐influenced deltaic shorelines, there are few documented examples in the ancient record. Based on an integrated sedimentological and ichnological study, the along‐strike variability and delta asymmetry within a single parasequence (Ps 6) is documented in continuously exposed outcrops of the Cretaceous Ferron Sandstone Member of the Mancos Shale Formation near Hanksville in southern Utah. Two intra‐parasequence discontinuity surfaces are recognized which allow subdivision of the parasequence into three bedsets, marked as Ps 6‐1 to Ps 6‐3. Four facies successions are recognized: (i) wave/storm‐dominated shoreface; (ii) river‐dominated delta front; (iii) wave/storm‐reworked delta front; and (iv) distributary channel and mouth bar. Dips of cross‐strata within distributary‐mouth bars and shorefaces show a strong downdrift (southward) component. Ps 6‐3 predominantly consists of river‐dominated delta‐front deposits, whereas Ps 6‐1 and Ps 6‐2 show an along‐strike facies change with shoreface deposits in the north, passing into heterolithic, river‐dominated delta‐front successions south to south‐eastward, and wave/storm‐reworked delta‐front deposits further to the south‐east. Trace fossil suites correspondingly show distinct along‐strike changes from robust and diverse expressions of the archetypal Cruziana Ichnofacies and Skolithos Ichnofacies, into suites characterized by horizontal, morphologically simple, facies‐crossing ichnogenera, reflecting a more stressed, river‐dominated environment. Further south‐eastward, trace fossil abundance and diversity increase, reflecting a return to archetypal ichnofacies. The overall facies integrated with palaeocurrent data indicate delta asymmetry. The asymmetric delta consists of sandier shoreface deposits on the updrift side and mixed riverine and wave/storm‐reworked deposits on the downdrift side, similar to that observed in the modern examples. However, in contrast to the recent delta asymmetry models, significant paralic, lagoonal and bay‐fill facies are not documented in the downdrift regions of the asymmetric delta. This observation is attributed to a negative palaeoshoreline trajectory during delta progradation and subsequent transgressive erosion. The asymmetric delta was induced by net longshore transport from north to south. The forced regressive nature of the delta precludes significant preservation of topset mud.     

Calcite–dolomite geothermometry in the system CaCO3–MgCO3–FeCO3: an experimental study

Abstract An experimental study of the system CaCO₃–MgCO₃–FeCO₃ was undertaken in order to calibrate the iron correction to the calcite–dolomite geothermometer, which is based on the solubility of magnesium in calcite in the assemblage calcite + dolomite. The experiments, at 450°C and lower temperatures, resulted in products with a very small grain size and incomplete equilibration. However, application of a carefully-devised automatic data processing algorithm to analyses of the phases in experimental charges, combined with a thermodynamic analysis, results in geothermometer diagrams which should be preferred to previous theoretical predictions.     

Noble gas measurements in the L/LL5 chondrite Knyahinya

Abstract— The L/LL5 chondrite Knyahinya had an approximately spherical shape, and as it experienced a single stage exposure history, it represents a very interesting object to study depth profiles of cosmic-ray-produced nuclide concentrations. Such data are required to improve and to validate model calculations of production rates. We report Ne, Ar, Kr and Xe isotopic abundances in five bulk samples. The adopted procedure of noble gas extraction included two pyrolysis steps at 450 °C and 650 °C, respectively, followed by a combustion step in pure O₂ at 650 °C before melting the sample. This procedure allows for the separation of a significant fraction of the trapped Kr and Xe, leading to an enrichment of the cosmic-ray produced component, which is released in the melting step. The isotopic composition of the trapped Xe component measured in the combustion step is found to be identical with the OC-Xe composition (Lavielle and Marti, 1992) and supports the suggestion that ordinary chondrites formed in a homogeneous trapped noble gas reservoir. Cosmic-ray produced Kr and Xe components and depth profiles were measured, including for the first time a ⁸¹Kr profile. The calculated exposure age of 39.5 ± 1.0 Ma, based on the ⁸¹Kr-Kr method, is found to be in excellent agreement with previous determinations. The concentrations of trapped and fissiogenic noble gas components are clearly lower than those generally observed in type 5 ordinary chondrites and may suggest diffusion losses before a meter-sized object was exposed to the cosmic radiation.     

Selection of calibration objective fonctions in the context of rainfall-ronoff modelling in a Sudanese savannah area

Abstract

In the context of rainfall-runoff modelling carried out on the Sudanese savannah area in the northwest of the Ivory Coast, attempts are being made to reconstitute the flow at the outlets of catchments in 10 day time steps. By using algorithms with automatic fitting procedures for the parameters, it appeared necessary to make a choice concerning the calibration objective functions to be used. The paper presents the algorithms, data and objective functions that have been used. The results obtained from the calibrations made have been analysed. That analysis was done principally with the help of a comparative evaluation modulus which takes into account elements other than the value of the objective function alone and which enables the quality of the results to be picked out from a hydrological point of view. At the conclusion of the analysis, the objective function defined by Nash seems to stand out quite clearly in relation to the other formulae examined.     

The relation between glacial ages and terrestrial magnetism

An attempt is made to show that the change of angular momentum of the earth, caused by glaciations, may compensate the slowing down effect from tidal friction forces and even reverse the polarity in the terrestrial magnetic field.     

Properties of debris flow deposits and source materials compared: implications for debris flow characterization

In the Alps, debris flow deposits generally contain <5% clay‐size particles, and the role of the surface‐charged <2 μm particles is often neglected, although these particles may have a significant impact on the rheological properties of the interstitial fluid. The objective of this study was to compare debris flow deposits and parent materials from two neighbouring catchments of the Swiss Alps, with special emphasis on the colloidal constituents. The catchments are small in area (4 km²), 2·5 km long, similar in morphology, but different in geology. The average slopes are 35–40%. The catchments were monitored for debris flow events and mapped for surface aspect and erosion activity. Debris flow deposits and parent materials were sampled, the clay and silt fractions extracted and the bulk density, <2 mm fraction bulk density, particle size distribution, chemical composition, cation exchange capacity (CEC) and mineralogy analysed. The results show that the deposits are similar to the parent screes in terms of chemical composition, but differ in terms of: (i) particle size distribution; and (ii) mineralogy, reactivity and density of the <2 mm fraction. In this fraction, compared with the parent materials the deposits show dense materials enriched in coarse monocrystalline particles, of which the smallest and more reactive particles were leached. The results suggest that deposit samples should not be considered as representative of source or flow materials, particularly with respect to their physical properties.     

Determinism within human–environment research and the rediscovery of environmental causation

The popularity of the work Guns, germs and steel (GGS) has served to bring the question of human–environment connections once again to the forefront of popular thought. We assert that the recent success of GGS represents both a persistence of environmental determinist logic and a contemporary trend that privileges the environment as the primary influence on human–environment relationships. The historical development of the human–environment field is reviewed from the cultural and political ecology (CAPE) perspective, with particular attention to illustrating the varying emphasis between humans and their environment. GGS is situated within this developing field through a critical analysis of the arguments and methods forwarded by Jared Diamond. The book is found to mirror earlier environmental determinism by failing to take into account many of the advances in human–environmental thought since the early twentieth century. Its popular success suggests the pitfalls of failures to acknowledge the complex, intertwined and indivisible relationship that exists among humans and their environment. Furthermore, there is evidence that the environmental determinism espoused in GGS has caught the attention of international development policymakers potentially influencing future outlays of aid and assistance to the developing world. These conclusions raise cautionary flags against repeating past theoretical mistakes by accepting simplistic, causal explanations based largely on a deterministic conception of the natural environment.     

Freshwater organisms that build stromatolites: a synopsis of biocrystallization by prokaryotic and eukaryotic algae

In freshwater environments such as river and stream bottoms, rocks and submerged vegetation are covered with a biological felt (also called a periphyton, microbial mat, biofilm, etc.) that is susceptible to calcification. Compilation of an extensive bibliography and our own observations have allowed the identification of 44 species of Coccogonophyceae, 122 Hormogonophyceae, 2 Chrysophyceae, 35 Chlorophyceae, 3 Xanthophyceae, 2 diatoms, and 3 Rhodophyceae that grow on calcareous tufa and coat vegetation. Diverse genera include species that are also calcified but impossible to determine because they lack reproductive organs. Crystals have been described from 74 species in the literature and we have observed 53 others. They can be classified into 10 groups: (1) platelets on cell walls (Volvocales, analogues of coccolithophorids) (2) crystals in mucilage (Synechococcus, diatoms, Hydrurus) and calcified stalks (Oocardium) (3) sheaths containing crystals in the form of simple or three-branched needles, dendritic crystals, and crystals with box-work fabric (Geitleria, Scytonema) (4) sheaths containing calcite spherulites (5) stalks intersecting a large crystal (Cymbella) (6) micrite tubes (Phormidium, Schizothrix) (7) isolated rhombohedra (Zygnema, Scytonema), rhombohedra in clusters or chains (Nostoc parmelioides) (8) sparite platelets (Vaucheria) or isodiametric crystals (Scytonema, Chaetophora) (9) large crystals crosscut by many parallel filaments (Rivularia, Batrachospermum), and (10) fan-like crystals (Phormidium). These crystals can be arranged in clusters or form regular laminations. They can transform into isodiametric sparite crystals to form fan-like or radial palisadic structures. Knowledge of primary crystals and their diagenetic transformations is necessary to correctly interpret freshwater stromatolites. The latter always result from intense calcification and are a diagenetic transformation of a biological felt made of many prokaryotic and eukaryotic algal species, small invertebrates, and organic and mineral debris.