Using Geomorphological Indicators to Predict Earthquake Magnitude (MOb‒Max): A Case Study from Cao Bang Province and Adjasent Areas (Vietnam)

Geotectonics - This paper presents analysis of geomorphological indices for predicting the maximum observed earthquake (MOb?Max) in research region, conducted in three steps: (i)...     

Sources and Exchange of Particulate Organic Matter in an Estuarine Mangrove Ecosystem of Xuan Thuy National Park, Vietnam

The spatio-temporal variations in stable isotope signatures (??¹³C and ??¹⁵N) and C/N ratios of particulate organic matter (POM), and physicochemical parameters in a creek water column were examined in an estuarine mangrove ecosystem of Xuan Thuy National Park, Vietnam. The objective was to examine the factors influencing creek water properties, and the sources and exchange of POM in this important mangrove ecosystem. The diel and seasonal variations in water temperature, flow velocity, pH, dissolved oxygen, and salinity demonstrated that tidal level, season, and biological factors affected the creek water properties. Mangroves had relatively low ??¹⁵N and very low ??¹³C values, with respective average values of 1.5?±?0.9?? and ?28.1?±?1.4??. The low mangrove leaf ??¹⁵N indicated minor anthropogenic nitrogen loading to the mangrove forests. A significant positive correlation between POM?C??¹³C and salinity along the axis of Ba Lat Estuary, Red River, indicated that marine phytoplankton (??¹³C value, ?21.4?±?0.5??) was the predominant source of POM at the estuary mouth. Based on the co-variation of ??¹³C and C/N ratios, marine phytoplankton and mangrove detritus were predominant in POM of major creeks and small creeks, respectively. During the diurnal tidal cycle, the dynamics of POM were affected by sources of organic matter, tidal energy, and seasonal factors. The contribution of mangrove detritus to POM reached a maximum at the low tide and was enhanced during the rainy season, whereas marine phytoplankton contribution was highest at high tide.     

The application of δ13C and C/N ratios as indicators of organic carbon sources and paleoenvironmental change of the mangrove ecosystem from Ba Lat Estuary, Red River, Vietnam

In this study, two sediment cores (~70 cm) were collected from separate mangrove forests straddling the Ba Lat Estuary, Red River of northern Vietnam, to examine the origins of sedimentary organic carbon (SOC) and reconstruct the paleoenvironment. In addition, mangrove leaves and particulate organic matter were collected and measured for δ¹³C to trace the origins of SOC. The cores were analyzed by high-resolution sections for δ¹³C, TOC, C/N ratios, sediment grain size, water content, and porosity, with values of δ¹³C, TOC, and C/N ratios ranging from −28.19 to −22.5‰, 2.14–30.94 mg/g, and 10.29–18.32, respectively. The δ¹³C and TOC relationship indicated that there were some small residual effects of diagenetic processes on TOC and δ¹³C values in mangrove sediments. However, the shifts of δ¹³C and C/N ratios from the bottom to the surface sediment of the cores explained the change in organic matter sources, with values of C/N > 12 and δ¹³C < −25‰, and C/N < 12 and δ¹³C > −25‰ indicated terrestrial (e.g., mangrove litter) and marine phytoplankton sources, respectively. The covarying δ¹³C, C/N ratios, and sediment grain sizes during the past 100 years in sediment cores showed that the paleoenvironment may be reconstructed into three environments (subtidal, tidal flat, and intertidal mangrove). General trends in δ¹³C and C/N followed a gradual increase in the C/N ratio and a concomitant decrease in δ¹³C from the subtidal, through to tidal flat, and to the intertidal mangrove. δ¹³C and C/N ratios are therefore effective in measuring the continuum of environmental change in mangrove ecosystem.     

The Sin Quyen Cu–Fe–Au–REE deposit (northern Vietnam): composition and formation conditions

The Sin Quyen Cu–Fe–Au–REE deposit is localized in the Proterozoic deposits of the Phan Xi Pang zone, northern Vietnam. The mineralization is formed by lenticular and sheet-like bodies occurring concordantly with the host rocks. Seventeen orebodies have been recognized in the deposit, which form an ore horizon up to 140 m in total thickness, about 2 km in strike, and up to 350 m in dip. The ores are of simple mineral composition: Au-rich copper and iron sulfides (chalcopyrite, pyrite, pyrrhotite) and iron oxides (magnetite, hematite). Gold and silver are distributed unevenly in the ores: Their contents vary from hundredths and tenths of ppm to 1.8 ppm. Copper sulfide ores are the main concentrator of gold and silver. All ores are characterized by high REE contents, tens and hundreds of times exceeding the element clarkes. The highest contents have been revealed for Ce and La. Orthite is the main carrier of REE. No correlation between REE and ore elements of sulfide-oxide ores has been revealed, which points to the independent formation of the mineralization. Orebodies together with the host rocks underwent metamorphism at 500–600 to 630–685 °C and 3–7 kbar. The spatial association of the mineralization with amphibolites (metamorphosed basites) and the mineral composition of ores suggest that the Sin Quyen deposit is of Cyprian volcanogenic type.     

Tracing carbon transfer and assimilation by invertebrates and fish across a tropical mangrove ecosystem using stable isotopes

Mangroves are highly productive ecosystems that exhibit a diverse range of habitats, including tidal creeks and flats, forest gaps and interior forest with varying understory light intensity, tidal dynamics, geomorphological settings, and overall biological production. Within mangrove ecosystems, invertebrates and fish feed on heterogeneous food sources, the occurrence of which is unevenly distributed across the system. This provides a basis for testing models of carbon transfer across mangrove ecosystems. We hypothesized that the carbon transfer and assimilation by fish and invertebrates will vary across the different mangrove habitats and that such variations can be predicted by their stable isotope compositions. We analysed δ¹³C and δ¹⁵N signatures of consumers and their potential organic carbon sources across a tropical mangrove ecosystem in Vietnam. The δ¹³C values of crabs and snails significantly decreased from the tidal flat to interior forest, indicating that variations in carbon transfer and assimilation occurred at small scales <30 m. Reduced variation in δ¹³C of suspension‐feeding bivalves suggested that tidal water was a vector for large‐scale transport of carbon across the mangrove ecosystem. An analysis of co‐variance using habitat as a fixed factor and feeding habit and movement capacity of consumers as co‐variates indicated that habitat and feeding types were major features that affected the δ¹³C values of invertebrates and fish. The findings demonstrate that carbon transfer and assimilation across mangrove ecosystems occur as a diverse combination of small (<30 m) and large (>30 m) scale processes.     

A variable source area for groundwater evapotranspiration: impacts on modeling stream flow

Evapotranspiration (ET) plays a crucial role in catchment water budgets, typically accounting for more than 50% of annual precipitation falling within temperate deciduous forests. Groundwater ET is a portion of total ET that occurs where plant roots extend to the capillary fringe above the phreatic surface or induce upward movement of water from the water table by hydraulic redistribution. Groundwater ET is spatially restricted to riparian zones or other areas where the groundwater is accessible to plants. Due to the difficulty in measuring groundwater ET, it is rarely incorporated explicitly into hydrological models. In this study, we calibrated Topographic Model (TOPMODEL) using a 14‐year hydrograph record and added a groundwater ET pathway to derive a new model, Groundwater Evapotranspiration TOPMODEL (GETTOP). We inspected groundwater elevations and stream flow hydrographs for evidence of groundwater ET, examined the relationship between groundwater ET and topography, and delineated the area where groundwater ET is likely to take place. The total groundwater ET flux was estimated using a hydrological model. Groundwater ET was larger where the topography was flat and the groundwater table was shallow, occurring within about 10% of the area in a headwater catchment and accounting for 6 to 18% of total annual ET. The addition of groundwater ET to GETTOP improved the simulation of stream discharge and more closely balanced the watershed water budget. Copyright © 2013 John Wiley & Sons, Ltd.     

Repartition non orthogonale des sens du premier mouvement des ondes P

Usually, one can explain the orthogonal distribution of first movement by the dislocation model of seismic sources, namely a uniform slip. However, other representations may be obtained if the normal component of the dislocation is present. When this component is less than 0.6 times the tangential one, we obtain conical distributions. Some models which simulate a volume phenomenon provide analogous results. We show that they could agree with some observations.     

Siderite layers in the fresh-water Neogene sediments of Vietnam

Siderites forming beds and lenses in the Neogene lacustrine—swampy sediments of the Rinh Chua Formation (northern Vietnam) are considered in detail. Results of the mineralogical and chemical study of siderites and host terrigenous–clayey siltstones are reported. New analytical data characterize the composition and structure of microbiomorphic (bacterial) bodies in the siderites and terrigenous sediments. Microstructures (porosity) and compositional peculiarities (up to 18% P₂O₅) of individual horizons of the sediments testify to their lacustrine-swampy genesis. It is established that the siderites in association with the layer silicates were formed during the microbiochemogenic decomposition of terrigenous components, including quartz.     

兰坪金顶蜂子山铅锌矿床的构造控矿:来自三维空间数据分析的证据

滇西金顶超大型铅锌矿床的热液成矿和构造控矿特点突出,对其主要容矿岩石和断层耦合控矿机制仍不清楚.利用三维空间数据分析方法,对代表性的蜂子山铅锌矿床开展了矿化结构及其空间展布特征的综合研究.结果 表明:Pb和Zn矿化中心呈现出东西向分带及差异性分段富集的特点,空间上并与黄铁矿、天青石(—重晶石)和石膏矿体共置;成矿流体的运聚受近SN向逆冲断层和近EW向走滑断层的共同控制,为其提供通道和沉淀空间;而金顶群(N1j)的湖相氧化性含矿卤水在两组方向断层的联合驱动下,在三合洞组上段(T3S3)地层中与其赋存的还原性流体幕式混合成矿,是该矿床发育的重要机制.尤其是,近SN向逆冲断层和近EW向走滑断层联合控矿特征的发现,为深入理解金顶超大型铅锌矿床成矿作用及指导找矿勘查提供了新的依据.