The origin of the African savanna has been traced back to 10–6 million years (Ma) ago, but the mechanisms driving its evolution are hotly debated, and include global atmospheric CO2, regional fire ...activity, herbivore competition, and hydrological climate change. Here, we present the first microcharcoal-based fire activity records covering the last ~7 Ma at four International Ocean Discovery Program (IODP) sites near southern Africa. The records show that fire activities in both savanna and non-savanna regions were stable during this interval. Grass vegetation in burnt biomass continued to expand from 6 Ma into the present savanna region, whereas no grass expansion was observed in non-savanna regions. A compilation of regional data suggests asynchronous C4 grass expansion since 6 Ma on the African Continent. We consider that CO2 concentrations and wildfires might have caused the first appearance of C4 plants at ~10 Ma and ~ 7–6 Ma, respectively. Since 6 Ma, the regional climate promoted expansion of C4 plants into the present-day savanna habitat.
•Microcharcoals in four IODP boreholes surrounding the southern Africa are studied.•Microcharcoal influxes show the stable wildfire trends since 6 Ma ago.•Microcharcoal morphologies show the C4 vegetation expanded along specific routes.•The C4 expansion is attributed to the regional climate rather than CO2 or wildfire.
Understanding the evolution of river systems in southern Africa is fundamental to constrain the evolution of landscape and sediment dispersal patterns. It is widely considered that the upper Zambezi ...River was connected with the Limpopo River during the Cretaceous, forming what was then the largest river in Africa. Crustal flexure during the Paleogene severed the upper Zambezi drainage from the Limpopo, setting the framework of the modern Zambezi and Limpopo River systems. We present first evidence—based on heavy‐mineral assemblages from cores drilled offshore of the Limpopo River mouth and samples collected in different reaches of the modern Limpopo River, integrated with magnetic susceptibility, detrital‐zircon geochronology, and geomorphological analysis—suggesting that the current Limpopo River formed recently in the Plio‐Quaternary. Plio‐Quaternary climate change is envisaged to have controlled the recent dynamics of river drainage and consequent distribution of sediment loads, as observed in many other transcontinental rivers worldwide.
Plain Language Summary
Landscape evolution, sediment dispersal patterns, and the accumulation of hydrocarbon and mineral resources in southern Africa, were largely determined by the evolution of the vast Zambezi‐Limpopo River system. It is widely believed that this river system attained its current configuration during the late Paleogene as a consequence of mantle‐driven dynamic uplift. Based on heavy‐mineral assemblages from river sediments and an International Ocean Discovery Program ocean core, we argue here instead for a Plio‐Pleistocene age for the formation of the modern characteristics of the Limpopo River (i.e., tens of millions of years younger than previously thought). This challenges the current paradigm on both formation modality and driving mechanisms of this vast river system. Plio‐Pleistocene climate change is envisaged to have controlled the dynamics of river drainage and consequent distribution of sediment loads, as observed in several other transcontinental rivers worldwide, underscoring the importance of these climate changes in driving river evolution world‐wide.
Key Points
The modern Limpopo River of South Africa formed in the Plio‐Pleistocene
Plio‐Pleistocene drainage reorganization was driven by climate change
Formation age and driving mechanisms of South African rivers are revisited
In the military field, determining how to increase the hole-expanding ability of shaped charge warheads is a key and difficult issue with respect to warhead development. Amorphous alloys have grains ...or grain boundaries, with unique mechanical properties. Zr41.2Ti13.8Cu12.5Ni10Be22.5 can be used as the liner material of shaped charges, resulting in high-speed particle flows that differ from those of traditionally shaped charges. In this paper, based on the analysis of the mechanical response characteristics of Zr41.2Ti13.8Cu12.5Ni10Be22.5 and its fracture morphology under impact, combined with the formation theory of shaped charge jets, a semi-empirical formula is derived to calculate the velocity of non-cohesive high-speed particle flow considering the elastic strain energy loss. Additionally, the reliability of the proposed theoretical model is verified through experiments. The penetration process of Zr-based amorphous alloy high-speed particle flow into a concrete target is theoretically analyzed, and the penetration stages of the high-speed particle flow into the target are clearly distinguished. Combined with the penetration theory of shaped charge particle jets, a high-speed particle flow penetration model is proposed, and a pore expansion model is established through an energy method. The experimentally obtained data on depth of penetration are in agreement with the theoretical calculation results.
Furfurylation with a low concentration of furfuryl alcohol (FA) promotes the improvement of the properties and the effectiveness of FA on cell–wall action without darkening the furfurylated wood to ...the point that it affects its applications. In this paper, the effects of furfurylation on the hygroscopicity and water uptake dimensional stability of poplar (Populus sp.) and Chinese fir (Cunninghamia lanceolata) were analyzed. Meanwhile, the distribution of FA resin, the relationship between wood and water, the change in pore size distribution, and the weight percentage gain and cell wall bulking coefficient of wood were also investigated. The results were as follows: (1) A low concentration of FA could better enter the cell walls of the Chinese fir than the poplar, as FA resin was almost cured in the secondary walls, cell corners, and compound middle lamellae when a 10% concentration of FA was applied to the Chinese fir and poplar. When the FA concentration was increased to 30%, there were no significant increases in the amount of FA entering the cell walls and the amounts of FA cured in the cell lumen of the poplar were greater than those of the Chinese fir. Meanwhile, the modification of cell walls was more suitable in poplar than in Chinese fir. (2) The pointed ends of the pit chambers and the pit apertures (800–1000 nm) in the poplar and the small pores of the pit membranes and the pit apertures (1–6 μm) in the Chinese fir were partially deposited by the FA resin, which formed new pores in the size ranges of 80–600 nm and 15–100 nm, respectively. The porosity of the poplar was greater than that of the Chinese fir, and the bulk density of the poplar was less than that of the Chinese fir before and after modification. (3) Furfurylation with a low concentration of FA was able to better reduce the equilibrium moisture content, improve the anti-swelling efficiency, and enhance the dimensional stability of the poplar wood compared to the Chinese fir. Furfurylation effectively reduced water uptake due to the hydrophobic property of the FA resin. The water uptake of the Chinese fir increased by 17%–19% in second cyclic water soaking when treated with FA with various concentrations, which indicated the loss and leaching of FA resin during the test. Low-field NMR was used to demonstrate that the furfurylation not only reduced the amount of water but also affected the combination state of bound and free water with wood. Thus, furfurylation at a low concentration is a feasible method by which to extend applications of furfurylated wood.
Furfurylation can effectively improve the quality of fast-growing wood, but its leachability is unclear. In this study, fast-growing poplar (Populus sp.) and Chinese fir (Cunninghamia lanceolata ...(Lamb.) Hook.) were impregnated with low concentrations of 5%–20% furfuryl alcohol (FA), and the chemical and microscopic changes during leaching tests were analyzed by UV spectra and confocal laser scanning microscopy (CLSM). The results show that FA impregnation can regulate the weight percentage gain, but its effectiveness in regulating the cell wall bulking coefficient decreased as the impregnation concentration was increased. Impregnation with 15% and 20% FA showed no significant difference in the effect on volume swelling efficiency. The inverse relationship between the concentration of FA and the leaching rate was demonstrated by leaching tests, UV spectra, and CLSM. Notably, the leaching rate of poplar and Chinese fir wood was more than 30% when impregnated with 5% FA. Although the entirety of the furfuryl alcohol was deposited in the cell wall when impregnated with low concentrations of FA, the binding was not stable. The weight percentage gain of furfurylated Chinese fir was greater than that of poplar, but its leaching rate was lower, indicating that the cured furfuryl alcohol resin in poplar was not as stable as that in Chinese fir. Therefore, differences in tree species should be considered in low-concentration FA impregnation, as the improvement effect of concentrations below 10% on the properties of fast-growing wood is weak and the leaching rate of FA is significant.
Rubber seed shell (RSS) was modified by grafting treatment using glycidyl methacrylate (GMA) at various concentrations. The RSS was then used to reinforce high-density polyethylene (HDPE). The ...effects of modification on the mechanical, water absorption, and thermal properties of the RSS/HDPE composites were studied using a mechanical testing instrument, weighing method, Vicat softening temperature (VST) testing, thermogravimetry, and dynamic mechanical analysis. The results showed that the GMA grafting produced an improvement in the flexural and tensile properties of the composites. The water absorption rate of the composites also had an obvious decrease. While a slight increase in VST was found, the various concentrations of GMA showed no improvement in VST. GMA modification also could elevate the thermal stability of the composites at the initial decomposition stage. The optimum grafting concentration of GMA (2.5%) led to the lowest thermal weight loss (37.07% and 26.56%) during the first and second decomposition stages. The E’ values of the composites had a significant increase with the addition of GMA. There were two peaks of tan δ for the untreated samples, but the modified samples exhibited a shift in the transition peak at higher temperatures; moreover, the second peak disappeared.
Torque rheological properties of wood flour/chitosan/PVC (WF/CS/PVC) compounds were measured by a torque rheometer using roller-style rotating blades at various setting temperatures (175 and 185 °C) ...and rotation speeds (30, 45, 60, and 75 rpm). The torque rheological parameters were calculated based on the Marquez model and Arrhenius equation. The torque rheological curves of WF/CS/PVC composites were similar to WF/PVC composites without chitosan. The classical Marquez model was verified to be suitable for both WF/PVC and WF/CS/PVC composites. Specifically, the activation energy (ΔE), n value, and range of C(n)m for the former and latter were 27.698 kJ•mol-1 and 29.237 kJ•mol-1, 0.382 and 0.381, and 4.415 to 5.749 N•m•sn and 4.652 to 6.079 N•m•sn, respectively. The rheological properties of WF/CS/PVC composites did not show a great qualitative enhancement compared to WF/PVC composites.
The thermal degradation behavior and pyrolysis kinetic models of wood flour (WF)/PVC composites before and after adding chitosan were studied using thermogravimetry (TGA) and nine common kinetic ...model functions (f(α)). The results indicated that the thermal degradation temperature of WF/PVC composites was delayed to a higher value after adding chitosan. The first-order reaction order (L1) model and second-order reaction order (L2) model were found to be the best reaction order functions for the description of mass loss kinetics of WF/PVC without chitosan during the first and second stages. Two L2 models were suitable for both degradation stages of WF/CS/PVC. Activation energy (E) and frequency factor (A) for WF/PVC and WF/CS/PVC corresponded to 26.05 kJ·mol-1, 4.08×103 s-1, and 40.89 kJ·mol-1, 2.11×1010 s-1 at the first degradation stage, respectively, and 97.83 kJ·mol-1, 1.11×107 s-1 and 92.88 kJ·mol-1, 1.56×1011 s-1 at the second degradation stage.
This study mainly aimed at investigating the effects of volatile chemical components of wood species on mould and termite resistance of wood plastic composites (WPC) using artificial accelerated ...tests. The morphology characterization of surface and fracture of WPC was estimated by SEM and digital instrument. Volatile chemical components of wood species extractives were analyzed by GC–MS. The results indicated that the sequences of mould resistance of WPC were ranked as: Cunninghamia lanceolata and Melaleuca leucadendra (level 0) > Eucalyptus grandis × Eucalyptus urophylla (level 1) > Pinus massoniana (level 2) > Liquidambar formosana and Ricinus communis (level 4). The sequences of termite resistance of WPC were ranked as: C. lanceolata (level 1, ML = 4.52%) > M. leucadendra (level 1, ML = 5.73%) > E. grandis × E. urophylla (level 1, ML = 6.48%) > L. formosana (level 2, ML = 6.72%) > P. massoniana (level 2, ML = 7.08%) > R. communis (level 3, ML = 10.40%). It was also suggested that 8-propoxy-cedrane, cedrol, α-cedrene and β-cedrene in C. lanceolata, 2,3-dihydro-2,2-dimethyl-3,7-benzofurandiol, 3-demethyl-colchicine and squalene in M. leucadendra; 2,3-dihydro-2,2-dimethyl-3,7-benzofurandiol and stigmast-4-en-3-one in E. grandis × E. urophylla were potentially crucial to provided positive effects on biodegradation resistance. Longifolene, caryophyllene and α-pinene in P. massoniana; 4-hydroxy-3,5-dimethoxy-benzaldehyde, 3,5-dimethoxy-4-hydroxycinnamaldehyde and cinnamyl cinnamate in L. formosana; 5-hydroxymethylfurfural and 1,6-anhydro-β-d-glucopyranose in R. communis led to the opposite results.
•Wood species was a potential factor to biodegradation resistance of wood plastic composites.•The channels between wood flour and PVC existed for movement of spores and hyphae.•WPC made from Cunninghamia lanceolata and Melaleuca leucadendra had excellent abilities to biodegradation resistance.•WPC made from Liquidambar formosana, Pinus massoniana and Ricinus communis had weak abilities on biodegradation resistance.
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