An evaluation of shape indices as palaeoenvironmental indicators using quartzite and metavolcanic clasts in Upper Cretaceous to Palaeogene beach, river and submarine fan conglomerates

The feasibility of using quantitative shape measurements to discriminate between clast populations from different depositional settings was evaluated using samples from 11 fluvial, six submarine fan and four beach conglomerates from south-west California; these origins had been established previously by facies analysis. Quartzite and metavolcanic clasts were characterized by the following indices: modified Wentworth roundness (R_w), maximum projection sphericity (δ_p), oblate-prolate index (OPI) and long (L), intermediate (I) and short (S) axial ratios. These indices were compared with those documented previously for modern gravels. The results show that certain indices are useful palaeoenvironmental indicators, despite inherited differences in shape due to texture, provided that multiple sites are sampled and a statistical approach is used. Statistically, the most effective shape indices are δ_p and S/L which give good results with the Zingg classification (I/L vs. S/I); better results are also obtained using quartzite clasts. The OPI is useful for discriminating between beach and river conglomerates, which consist largely of oblate and prolate clasts, respectively. The relative abundance of blade-shaped clasts is a useful index of sediment maturity, being greatest for river clast samples and smallest for submarine fan clast samples. The latter are dominated by spherical particles. No correlation between palaeoenvironment and R_w is observed, hence the abundance of disc-shaped clasts in the beach conglomerates studied is attributed to selective transport in suspension and sediment by passing during fluvial transport prior to deposition in the surf zone. Selective transport of rollers (spheres and rods) by traction in a shallow marine setting, prior to redeposition by mass transport, may be responsible for the dominance of spherical particles in submarine fan conglomerates.     

On the rotational temperature of airglow hydroxyl emissions

It is argued that available observational information does not support the contention (recently advanced by Suzuki and Tohmatsu, 1976) that the rotational temperature derived from an airglow hydroxyl emission band depends systematically on the quantum number (ν′) of the vibrational state from which the band originates. In particular, it is shown that bands originating from ν′ = 6, 7 and 8, measured simultaneously, exhibit rotational temperatures which (within the experimental errors) are equal.     

Petrogenesis and Mineralization of Two‐Stage A‐Type Granites in Jiuyishan,South China: Constraints from Whole‐rock Geochemistry,Mineral Composition and Zircon U‐Pb‐Hf Isotopes

The Jiuyishan complex massif, located in the northern section of the Nanling region, is a combination of five plutons, namely, the Xuehuading, Jinjiling, Pangxiemu, Shaziling and Xishan plutons. Whole-rock geochemistry, mineral electron microprobe analysis, zircon U-Pb dating and Hf isotope analysis were carried out for the Jinjiling and Pangxiemu plutons. The zircon U-Pb dating yields weighted mean ages of 152.9±0.9 Ma for the Jinjiling pluton and 151.7±1.5 Ma for the Pangxiemu pluton, with a narrow gap between them. The Jinjiling and Pangxiemu plutons both have geochemical characteristics of high SiO2, Al2O3, Na2O, K2O and low TiO2, MgO, CaO, P2O5 contents, with intense depletionS in Sr, Ba, Ti, Eu and enrichments?in Ga, FeOT and HFSE, and these characteristics reflect an A-type affinity. From the Jinjiling to the Pangxiemu plutons, the mineral composition of mica changes from lepidomelane to zinnwaldite, with increases in F, Li2O and Rb2O contents. The mineral composition of zircon changes from low Zr/Hf to high Zr/Hf, with increasing HfO2, P2O5 and UO2+ThO2+Y2O3 contents. The mineral compositions of feldspar indicate that the Pangxiemu pluton contains more alkali feldspar than the Jinjiling pluton. The whole-rock geochemistry and mineral compositions reveal a higher degree of differentiation for the Pangxiemu pluton. The nearly uniform εHf(t) indicates the same source region for the two plutons: both were derived from partial melting of the lower crust, with small contributions of mantle materials. In addition, higher F, lower Nb/Ta and Zr/Hf ratios in the Pangxiemu Pluton suggest a closer relationship with the rare metal mineralization than for the Jinjiling pluton.     

Allometric relationships and peak-season community biomass stocks of native shrubs in Senegal's Peanut Basin

Allometric equations and community biomass stocks are presented for Guiera senegalensis J.F. Gmel (Gs) and Piliostigma reticulatum (DC.) Hochst (Pr) – two native shrub species in the Sahel. These shrubs are of interest because they dominate semi-arid sub-Sahalien Africa but have been largely overlooked as a key biomass component and regulator of ecosystem composition and function in this landscape. In Year 1, best predictors of aboveground biomass were height and number of stems (Gs) and crown diameter (Pr); and for belowground biomass were height and basal diameter (Gs) and basal diameter (Pr). In Year 2, height and crown diameter were the best predictors of aboveground biomass (R² = 0.90 for Gs and 0.87 for Pr), whereas basal diameter and number of stems (Gs) and basal diameter (Pr) were best predictors of belowground biomass. Peak-season biomass estimates ranged from 0.44 to 4.58 ton ha^?1 (mean = 2.38 ton ha^?1) in the Gs sites and from 0.33 to 7.38 ton ha^?1 (mean = 3.71 ton ha^?1) in the Pr communities. Both species exhibited unusually large root:shoot ratios (4.5:1 for Gs and 10.2:1 for Pr). Although models differ between years, allometric relationships provide reasonable biomass estimates for Gs and Pr.     

The orientation of helical flow in curved channels

The planform patterns of meandering submarine channels and subaerial fluvial bends show many similarities that have given rise to strong analogies concerning the fluid dynamics of these channel types. Existing models of helical motion in open‐channel bends depict flow that is characterized by surface flow towards the outer bank, and basal flow towards the inner bank. This paper investigates and compares, through an analytical model and physical experiment, flows within fluvial meanders, and submarine channel bends that contain density‐driven gravity currents. The results indicate that the sense of helical motion can be reversed in submarine bends that contain density currents when compared with fluvial bends, and that the orientation of the helical flow is dependent on the vertical distribution of downstream velocity. Specifically, the sense of helical motion is reversed in bends when the maximum downstream velocity is near the bed, resulting in near‐bed flow towards the outer bank. These findings suggest that the dynamics of sediment transport and deposition in curved channels with such velocity profiles will be fundamentally different to those currently assumed from sinuous open‐channels.     

Petrology and Geochemistry of West Philippine Basin Basalts and Early Palau-Kyushu Arc Volcanic Clasts from ODP Leg 195, Site 1201D: Implications for the Early History of the Izu-Bonin-Mariana Arc

Site 1201D of Ocean Drilling Program Leg 195 recovered basalticand volcaniclastic units from the West Philippine Basin thatdocument the earliest history of the Izu–Bonin–Marianaconvergent margin. The stratigraphic section recovered at Site1201D includes 90 m of pillow basalts, representing the WestPhilippine Basin basement, overlain by 459 m of volcaniclasticturbidites that formed from detritus shed from the Eocene–Oligoceneproto-Izu–Bonin–Mariana island arc. Basement basaltsare normal mid-ocean ridge basalt (N-MORB), based on their abundancesof immobile trace elements, although fluid-mobile elements areenriched, similar to back-arc basin basalts (BABB). Sr, Nd,Pb and Hf isotopic compositions of the basement basalts aresimilar to those of basalts from other West Philippine Basinlocations, and show an overall Indian Ocean MORB signature,marked by high ²⁰⁸Pb/²⁰⁴Pb for a given ²⁰⁶Pb/²⁰⁴Pb and high¹⁷⁶Hf/¹⁷⁷Hf for a given ¹⁴³Nd/¹⁴⁴Nd. Trace element and isotopicdifferences between the basement and overlying arc-derived volcaniclasticsare best explained by the addition of subducted sediment orsediment melt, together with hydrous fluids from subducted oceaniccrust, into the mantle source of the arc lavas. In contrastto tectonic models suggesting that a mantle hotspot was a sourceof heat for the early Izu–Bonin–Mariana arc magmatism,the geochemical data do not support an enriched, ocean islandbasalt (OIB)-like source for either the basement basalts orthe arc volcanic section. KEY WORDS: back-arc basalts; Izu–Bonin–Marianas; Philippine Sea; subduction initiation; Ocean Drilling Program Leg 195     

Contrasted abyssal basalt liquidus trends: evidence for mantle major element heterogeneity

Natural submarine basalt glasses define liquidus trends which may be summarized as curvilinear regression lines in the normative plagioclase-pyroxene-olivine ternary. Individual basalt suites separated in time or space may differ in major element composition; these differences translate into systematic differences in normative plagioclase, leading to efficient discrimination of individual trends in this ternary. Comparison of two contrasted sets of trends to available experimental and petrographic data confirms the similarity of the natural basalt trends to those predicted by 1-atm experiments, in general agreement with earlier studies based on whole-rock data. These contasted liquidus trends are believed to reflect major element heterogeneity and varying degrees of melting of the mantle source. This major element heterogeneity is not simply correlated with heterogeneity in incompatible trace elements, and like incompatible trace element heterogeneity, there is growing evidence that it may vary abruptly in space and time. The existence of these compositionally contrasted basalt suites must be considered in petrogenetic modelling; specifically, they will introduce scatter in most generalized variation diagrams, and will increase the likelihood of “misfits” in fractionation calculations utilizing parent and residual compositions drawn from different suites.     

Exploring the geophysical and socio-economic determinants of land cover changes in Eastern Mau forest reserve and Lake Nakuru drainage basin,Kenya

Understanding the linkages between the biogeophysical and socio-economic processes that operate at different spatial and temporal scales is important for land cover change mitigation. This study analysed several factors that explained the forest-shrubland conversions, grassland conversions and cropland expansions in Lake Nakuru drainage basin and Eastern Mau forest reserve in Kenya from 1985 to 2011. Logistic regression models were developed using a combination of remote sensing-based land cover data, and geographical information systems-based geophysical and socio-economic data (i.e., temperature, rainfall, elevation, slope, aspect, topographic wetness, curvature, soil pH, soil cation exchange capacity (CEC), population density and distance to road, river and town). The results were varied; for example, in the period 1985–2000, forest-shrubland conversions were linked to distance to road), distance to town, soil pH, soil CEC, rainfall, topographic wetness, curvature and aspect. The same factors, in addition to slope and distance to river, also determined the likelihood of forest-shrubland conversions in the period 2000–2011. Overall, significance of the determining factors varied depending on time and nature of land cover change. For example, topographical factors influenced grassland conversions in the period 1985–2000, while soil-related factors did not. But in the period 2000–2011, the converse was true. Therefore, policies for restoration, conservation and sustainable management of critical ecosystems (e.g., forests) should be spatially targeted and time-specific. These results broaden our knowledge of land cover dynamics in this locality, and provide a base for effective environmental policy formulation, planning and management.