The equivalent dose of different grain size quartz fractions from lakeshore sediments in the arid region of north China

Equivalent dose (D_e) values were measured by using medium aliquots of different grain size quartz fractions of five lakeshore sediments from the arid region of north China. There are two different relationships between D_e values and grain sizes of these five samples. The first relationship is that the D_e values obtained from various grain sizes are in agreement within 1 delta errors. The second relationship is that D_e values are similar to each other for fractions between 125 and 300 μm, while the D_e value of the 63–90 μm fraction is 40～55% smaller than others. For example, the D_e values obtained for sample #3 are 20.15 ± 1.19 Gy, 19.80 ± 0.83 Gy and 20.93 ± 1.06 Gy for fractions of 90–125, 125–150 and 250–300 μm respectively, but are 10.79 ± 0.84 Gy for the 63–90 μm fraction. The second relationship can't be interpreted by previous studies of both dosimetry and heterogeneous bleaching. It is deduced for sample #2, #3 and #6 that fine particles (<90 μm) intruded after the dominant sedimentation. Comparison of OSL ages from different grain size fractions of sample #2 with a radiocarbon age from the same lithologic layer supports that fractions coarser than 125 μm yield more reliable burial ages, while the fraction finer than 90 μm yields underestimated ages for some lakeshore sediments from this arid region.     

Moisture origin and stable isotope characteristics of precipitation in southeast Siberia

The paper presents oxygen and hydrogen isotopes of 284 precipitation event samples systematically collected in Irkutsk, in the Baikal region (southeast Siberia), between June 2011 and April 2017. This is the first high-resolution dataset of stable isotopes of precipitation from this poorly studied region of continental Asia, which has a high potential for isotope-based palaeoclimate research. The dataset revealed distinct seasonal variations: relatively high δ¹⁸O (up to −4‰) and δD (up to −40‰) values characterize summer air masses, and lighter isotope composition (−41‰ for δ¹⁸O and −322‰ for δD) is characteristic of winter precipitation. Our results show that air temperature mainly affects the isotope composition of precipitation, and no significant correlations were obtained for precipitation amount and relative humidity. A new temperature dependence was established for weighted mean monthly precipitation: +0.50‰/°C (r² = 0.83; p <.01; n = 55) for δ¹⁸O and +3.8‰/°C (r² = 0.83, p < 0.01; n = 55) for δD. Secondary fractionation processes (e.g., contribution of recycled moisture) were identified mainly in summer from low d excess. Backward trajectories assessed with the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model indicate that precipitation with the lowest mean δ¹⁸O and δD values reaches Irkutsk in winter related to moisture transport from the Arctic. Precipitation originating from the west/southwest with the heaviest mean isotope composition reaches Irkutsk in summer, thus representing moisture transport across Eurasia. Generally, moisture transport from the west, that is, the Atlantic Ocean predominates throughout the year. A comparison of our new isotope dataset with simulation results using the European Centre/Hamburg version 5 (ECHAM5)-wiso climate model reveals a good agreement of variations in δ¹⁸O (r² = 0.87; p <.01; n = 55) and air temperature (r² = 0.99; p <.01; n = 71). However, the ECHAM5-wiso model fails to capture observed variations in d excess (r² = 0.14; p < 0.01; n = 55). This disagreement can be partly explained by a model deficit of capturing regional hydrological processes associated with secondary moisture supply in summer.     

On the relation of moderate-short term anomaly of earth resistivity to earthquake

IntroductionMany anomalies due to earthquake have been recorded in observation of earth-resistivity for30 years and over, which showed that there objectively existed the anomalies of each-resistivity.The crustal strUcture and medium conditions are quite complex, so the complexity of the temporal,spatial and intensive development of the anomalies is inevitable. Both of time and amplitUde ofanomalies among some stations near an epicenter are different (even among different observational directi…     

The settling of resuspended lake sediment related to physicochemical properties of particles of different sizes: Implication for environmental remediation

Sediment resuspension is an important way for shallow lake internal pollution to interact with the overlying water column,and the pollution risks are reasonably related to the retention of resuspended sediment particles in overlying water.In the current study,the settling of resuspended sediment particles was comprehensively investigated under different disturbances using five urban lake sediments.The results show that the particle size distributions of resuspended sediment from different lakes exhibited similar variations during settling with disturbance,although varied settling times were observed under static conditions.During settling with and without disturbance,sediment particle sizes were mainly within 8-63μm at the initial stage,and were<8μm in the later stages of settling.Based on these settling characteristics,the sediment particle size was divided into sand(>63μm),silt(8-63μm),and very fine silt and clay(<8μm)fractions.Kinetic analysis suggested that sediment settling for different particle sizes could be well described by the first-and second-order kinetic equations,especially when settling was disturbed(r²=0.727-0.999).The retention of resuspended sediment could be enhanced as particle sizes decreased and disturbance intensities increased.Furthermore,a water elutriation method was successfully optimized,with separation efficiencies of 56.1%-83%,to separate sediment particles into the defined three particle size fractions.The chemical compositions of sediment were found to change with different particle sizes.Typically,calcium tended to form large-size sediment,while the total contents of aluminum,iron,magnesium,and manganese showed significantly negative correlations with sediment particle sizes(p<0.01)and tended to distribute in small-size particles(e.g.,<8μm).Overall,the sediment particle size related settling dynamics and physicochemical properties suggested the necessity on determining the pollution of resuspended sediment at different particle sizes for restoration of shallow lakes.     

Revised ΔR values for the Barents Sea and its archipelagos as a pre-requisite for accurate and robust marine-based 14C chronologies

The calibration of marine ¹⁴C dates requires the incorporation of regionally specific marine reservoir offsets known as ΔR, essential for accurate and meaningful inter-archive comparisons. Revised, regional ΔR (‘ΔR_R’) values for the Barents Sea are presented for molluscs and cetaceans for the two latest iterations of the marine calibration curve, based on previously published pre-bomb live-collected and radiocarbon-dated samples (‘ΔR_L’; molluscs: n = 16; cetaceans: n = 18). Molluscan ΔR_R, determined for four broad regional oceanographic settings, are: western Svalbard (including Bjørnøya), −61 ± 37 ¹⁴C yrs (Marine20), 94 ± 38 ¹⁴C yrs (Marine13); Franz Josef Land, −277 ± 57 ¹⁴C yrs (Marine20), −122 ± 38 ¹⁴C yrs (Marine13); Novaya Zemlya, −156 ± 73 ¹⁴C yrs (Marine20), 0 ± 76 ¹⁴C yrs (Marine13); northern Norway, −86 ± 39 ¹⁴C yrs (Marine20), 74 ± 24 ¹⁴C yrs (Marine13). Molluscan ΔR_R values are considered applicable to other marine carbonate materials (e.g., foraminifera, ostracods). Cetacean ΔR_R are determined for toothed (n = 10) and baleen (n = 8) whales, and a combined toothed-baleen group (n = 18): toothed, −161 ± 41 ¹⁴C yrs (Marine20), 1 ± 41 ¹⁴C yrs (Marine13); baleen, −158 ± 43 ¹⁴C yrs (Marine20), 8 ± 41 ¹⁴C yrs (Marine13); combined baleen-toothed whales, −160 ± 41 ¹⁴C yrs (Marine20), 4 ± 49 ¹⁴C yrs (Marine13). Where identification and separation of baleen and toothed whales is impossible the combined ΔR_R term may be used. However, we explicitly discourage the application of existing cetacean ΔR_R terms to other marine mammals. Our new ΔR_R values are applicable for as long as those broad oceanographic conditions (circulation and ventilation) have persisted, i.e., through the Holocene. We recommend using the latest iteration of the marine calibration curve, Marine20, which seems to better capture the time-variant nature of R compared to Marine13. More ΔR_L datapoints for both molluscs and cetaceans would improve the accuracy and precision of ΔR_R. In the meantime, our new ΔR terms facilitate the calibration of marine ¹⁴C dates across the region, paving the way for meaningful and accurate late Quaternary histories and inter-regional comparisons.     

Seismic <Emphasis Type="Italic">b</Emphasis>-Value and the Assessment of Ambient Stress in Northeast India

Seismicity data of northeast India, recorded between 1986 and 1999 by a local network, are analysed for estimation of b-values. Based on the obtained values, viz. low (b ≤ 0.5), moderate (0.5 < b ≤ 0.7) and high (b > 0.7), the study area is classified into different seismic-domains. An assessment of stress level is also carried out in identifying seismic-domains. Seismic activities, though mostly confined in some sectors, are presumably triggered by mutual interaction of the Shillong Plateau, Mikir Hills, Indo-Burman Ranges and the easternmost part of the Himalayas, and the contributions from deep-seated fractures cannot be ignored. The results resemble the seismic character of a foreland setting adjacent to a convergent margin. The b-values estimated for 240 square grids of dimension 0.6° × 0.6° over five seismic domains indicate wide variation. An analysis of cumulative seismic moment release (M _O) in different layers also indicates an anomaly in reference to the total seismic-energy budget of the five zones. The lower b-value and higher M _O recorded at relatively lower depth (~30 km) towards the southwest of the study area might be associated with upward bulging of a strong lithosphere. The bulging is perhaps regionally compensated by the downward flexing of the descending Indian lithosphere beneath the Upper Assam area; features unequivocally observed in any foreland setup. Towards the north and east of the study area, random variations of in both b-value and M _O along the converging zone suggest a varied tectonic environment with active interaction between the tectonic elements in these areas.     

Body-wave Attenuation in the Region of Garda, Italy

We analyzed the spectral amplitude decay with hypocentral distance of P and S waves generated by 76 small magnitude earthquakes (M_L 0.9–3.8) located in the Garda region, Central-Eastern Alps, Italy. These events were recorded by 18 stations with velocity sensors, in a distance range between 8 and 120 km. We calculated nonparametric attenuation functions (NAF) and estimated the quality factor Q of both body waves at 17 different frequencies between 2 and 25 Hz. Assuming a homogeneous model we found that the Q frequency dependence of P and S can be approximated with the functions Q _P = 65 f ^0.9 and Q _S = 160 f ^0.6 , respectively. At 2 Hz the Q _S /Q _P ratio reaches the highest value of 2.8. At higher frequencies Q _S /Q _P varies between 0.7 and 1.7, suggesting that for this frequency band scattering may be an important attenuation mechanism in the region of Garda. To explore the variation of Q in depth, we estimated Q at short (r ≤ 30 km) and intermediate (35–90 km) distance paths. We found that in the shallow crust P waves attenuate more than S (1.3 < Q _S /Q _P < 2.5). Moreover, P waves traveling along paths in the lower crust (depths approximately greater than 30 km) attenuate more than S waves. To quantify the observed variability of Q in depth we considered a three-layer model and inverted the NAF to estimate Q in each layer. We found that in the crust Q increases with depth. However, in the upper mantle (~40–50 km depth) Q decreases and in particular the high frequency Q _S (f > 9 Hz) has values similar to those estimated for the shallow layer of the crust.     

Changes in evapotranspiration components following replacement of Eucalyptus regnans with Acacia species

Forest species composition may change following a disturbance. This change can affect long term water yield from forested catchments when the replaced and replacement species have different evapotranspiration rates. Following strip‐thinning experiments that removed 50% of the overstorey basal area in several Eucalyptus regnans water supply catchments in south eastern Australia, Acacia spp. (Acacia dealbata and Acacia melanoxylon) became the dominant overstorey species in most of the cut strips. More recently, low regeneration of E. regnans following wildfires in 2009 may result in mixed Acacia and E. regnans stands in some catchments. We compared transpiration of E. regnans and Acacia stands in the uncut and cut strips of a catchment that was strip‐thinned in early 1980s (Crotty Creek). We also compared transpiration and throughfall in a mixed E. regnans–A. dealbata regrowth stand 20 years after clear‐fell logging (Road 8). Sap flow was measured for 13 and 6 months at Crotty Creek and Road 8, respectively. In both studies, mean daily sap flow density of Acacia spp. was lower than of E. regnans. Estimated Leaf Area Index of E. regnans stands was slightly greater than that of Acacia spp. Stomatal conductance (g_c), estimated by inverting the Penman–Monteith equation, differed between the species suggesting species‐level physiological differences with Acacia being more sensitive to vapour pressure deficit than E. regnans. Throughfall measurements at Road 8 indicated interception was slightly higher in A. dealbata but only enough to offset about 13% of the difference in transpiration. Replacement of E. regnans by Acacia dominated stands may, therefore, decrease catchment evapotranspiration and increase streamflow.     

Zoobenthos of Lake Uluabat,a Ramsar Site in Turkey,and Their Relationship with Environmental Variables

The numerical and proportional distributions of zoobenthos in lake Uluabat, which is located in the Northwestern part of Turkey and having international importance according to the Ramsar Convention, were determined from August 2004 to July 2005 at monthly intervals (except for December 2004, January and February 2005) at 12 different stations. Thirty‐three taxonomic groups were recorded. It was found that the zoobenthos consisted of 35.6 % Oligochaeta, 27.7 % Nematoda, 12.3 % Chironomidae larvae, 10.7 % Gastropoda, 3.6 % Ostracoda, and 10.1 % Varia by numbers (Bivalvia, Ceratopogonidae, Hirudinea, Odonata, Ephemeroptera, Asilidae, Hydraacarina, Hemiptera, Argulidae, and Gammaridae). The average number of invertebrates was 160 individuals for 33 taxa at the 12 different stations. Also some environmental parameters of the lakewater were analyzed (temperature, pH, dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, NO₂–N, NO₃–N, NH₃–N, PO₄^–3, fecal coliform, and total coliform). The relationships between the dynamics of the Oligochaeta and the physicochemical variables were supported by the Pearson correlation index and the canonical correspondence analysis (CANOCO). It was found that the relation between the average number of P. hammoniensis and P. albicola (p < 0.05, r = 0.590 and 0.593, respectively) and L. hoffmeisteri (p < 0.01, r = 0.777) was directly proportional while the relation between the average number of some taxa Trichodrillus sp., Rhyacodrilus coccineus, Nais communis, N. variabilis, and N. barbata and NO₃–N (p < 0.05, r = –0.685) was inversely proportional.     

Dynamic evolution of nutrient discharge under stormflow and baseflow conditions in a coastal agricultural watershed in Ishigaki Island,Okinawa, Japan

Excessive terrestrial nutrient loadings adversely impact coral reefs by primarily enhancing growth of macroalgae, potentially leading to a phase‐shift phenomenon. Hydrological processes and other spatial and temporal factors affecting nutrient discharge must be examined to be able to formulate effective measures for reducing nutrient export to adjacent reefs. During storm events and baseflow periods, water samples were obtained from the tropical Todoroki River, which drains an intensively agricultural watershed into Shiraho coral reef. In situ nutrient analyzers were deployed for 6 months to hourly measure dissolved nutrient (NO₃⁻‐N and PO₄³⁻‐P) concentrations. Total phosphorus (TP) and suspended solid concentration (TSS) were increased by higher rainfall intensity (r = 0·94, p < 0·01) and river discharge Q (r = 0·88, p < 0·01). In contrast, NO₃⁻‐N concentration tends to decrease drastically (e.g. from 3 to 1 mg l⁻¹) during flood events. When base flow starts to dominate afterwards, NO₃⁻‐N manifested an increasing trend, but decreases when baseflow discharge becomes low. This counter‐clockwise hysteresis for NO₃⁻‐N highlights the significant influence of groundwater discharge. N delivery can therefore be considered a persistent process compared to sediment and P discharge, which are highly episodic in nature. Based on GIS analysis, nutrient concentration along the Todoroki River was largely affected by the percentage of sugarcane/bare areas and bedrock type. The spatial distribution of N concentration in the river reflects the considerable influence of subsurface geology—higher N levels in limestone‐dominated areas. P concentrations were directly related to the total length of artificial drainage, which enhances sediment transport. The use of high‐resolution monitoring data coupled with GIS‐based spatial analysis therefore enabled the clarification of control factors and the difference in the spatio‐temporal discharge characteristics between N and P. Thus, although erosion‐reduction schemes would reduce P discharge, other approaches (e.g. minimize fertilizer) are needed to reduce N discharge. Copyright © 2010 John Wiley & Sons, Ltd.     

Erosional unloading,hillslope geometry,and the height of the Cascade Range,Washington State,USA

Peaks in the Cascade Range in northern Washington State are on average ～800 m higher than in southern Washington. The influences of differential valley excavation and variations in hillslope length and average slope on these altitudinal trends were tested using a 3‐dimensional model for isostatic rock uplift and calculations of hillslope length and slope respectively. The magnitude of isostatic peak uplift calculated by the model is highly dependent on the flexural rigidity (D) and the related effective elastic thickness (T_e) of the crust of this region. Crustal rigidity was constrained using published estimates and by estimating the depth of the seismogenic zone in the area (D > 1 × 10²³ Nm and T_e > 24 km). With these constraints, isostatic compensation due to differential erosion added < 700 m and 300 m, or < 25% overall, of height to peaks in the northern and southern Cascades, respectively. Deeper valley incision in the northern Cascades accounts for < 300 m of the 800 m difference in peak altitudes between north and south. Similarly, variation in valley spacing and slope account for < 350 m of the difference in mean altitude between northern and southern regions. Hence, at least several hundred m difference in altitude between the northern and southern regions of the Cascades in Washington must be due to tectonic, geologic, or geophysical factors rather than surficial and geomorphic effects like isostatic response to valley incision and hillslope geometry. Copyright © 2009 John Wiley & Sons, Ltd.     

Run-off affected by climate and anthropogenic changes in a large semi-arid river basin

A rapid reduction in run-off has been observed in the middle reaches of the Yellow River basin in recent decades. Understanding the contributions of climate change and human activities, such as vegetation restoration and water consumption, to surface water resource reduction has become urgent and very important for future regional planning. Here, we use attribution approaches to explore the effects of climate change and human activities on run-off over the past six decades. The results showed that the observed annual run-off at Tongguan station, which is located within the mainstream of the Yellow River, exhibited a significant decreasing trend of −0.69 mm year⁻¹ (p < .01) and varied from −0.28 to −1.46 mm year⁻¹ (p < .01) in the eight selected tributaries from 1960 to 2015. Two relatively abrupt changes in the double mass curves occurred around 1979 and 1999; compared with Period 1 (P1; 1960–1979), the average catchment run-off decreased 32% during Period 2 (P2; 1980–1999) and up to 49% during Period 3 (P3; 2000–2015). We calculated that approximately 29% of the reduction in the run-off during P2 and 18% during P3 were attributed to climate change. Increased surface water consumption resulted in effective run-off reduction, with relative contributions of approximately 27% and 28% during P2 and P3, respectively. With the implementation of the “Grain-for-Green” project, the vegetation coverage rapidly increased from 36% in P1 to 52% in P3 and reduced run-off by 35% during P3. These findings explain the run-off reduction and benefit water resource management in the middle reaches of the Yellow River basin.     

Attenuation of High Frequency P and S Waves in the Gujarat Region,India

The local earthquake waveforms recorded on broadband seismograph network of Institute of Seismological Research in Gujarat, India have been analyzed to understand the attenuation of high frequency (2–25 Hz) P and S waves in the region. The frequency dependent relationships for quality factors for P (Q _P) and S (Q _S) waves have been obtained using the spectral ratio method for three regions namely, Kachchh, Saurashtra and Mainland Gujarat. The earthquakes recorded at nine stations of Kachchh, five stations of Saurashtra and one station in mainland Gujarat have been used for this analysis. The estimated relations for average Q _P and Q _S are: Q _P = (105 ± 2) f ^{0.82 ± 0.01}, Q _S = (74 ± 2) f ^{1.06 ± 0.01} for Kachchh region; Q _P = (148 ± 2) f ^{0.92 ± 0.01}, Q _S = (149 ± 14) f ^{1.43 ± 0.05} for Saurashtra region and Q _P = (163 ± 7) f ^{0.77 ± 0.03}, Q _S = (118 ± 34) f ^{0.65 ± 0.14} for mainland Gujarat region. The low Q (<200) and high exponent of f (>0.5) as obtained from present analysis indicate the predominant seismic activities in the region. The lowest Q values obtained for the Kachchh region implies that the area is relatively more attenuative and heterogeneous than other two regions. A comparison between Q _S estimated in this study and coda Q (Qc) previously reported by others for Kachchh region shows that Q _C > Q _S for the frequency range of interest showing the enrichment of coda waves and the importance of scattering attenuation to the attenuation of S waves in the Kachchh region infested with faults and fractures. The Q _S/Q _P ratio is found to be less than 1 for Kachchh and Mainland Gujarat regions and close to unity for Saurashtra region. This reflects the difference in the geological composition of rocks in the regions. The frequency dependent relations developed in this study could be used for the estimation of earthquake source parameters as well as for simulating the strong earthquake ground motions in the region.     

Biogeochemistry of dissolved carbon,major, and trace elements during spring flood periods on the Ob River

Detailed knowledge of the flood period of Arctic rivers remains one of the few factors impeding rigorous prediction of the effect of climate change on carbon and related element fluxes from the land to the Arctic Ocean. In order to test the temporal and spatial variability of element concentration in the Ob River (western Siberia) water during flood period and to quantify the contribution of spring flood period to the annual element export, we sampled the main channel year round in 2014–2017 for dissolved C, major, and trace element concentrations. We revealed high stability (approximately ≤10% relative variation) of dissolved C, major, and trace element concentrations in the Ob River during spring flood period over a 1‐km section of the river channel and over 3 days continuous monitoring (3‐hr frequency). We identified two groups of elements with contrasting relationship to discharge: (a) DIC and soluble elements (Cl, SO₄, Li, B, Na, Mg, Ca, P, V, Cr, Mn, As, Rb, Sr, Mo, Ba, W, and U) negatively correlated (p < 0.05) with discharge and exhibited minimal concentrations during spring flood and autumn high flow and (b) DOC and particle‐reactive elements (Al, Fe, Ti, Y, Zr, Nb, Cs, REEs, Hf, Tl, Pb, and Th), some nutrients (K), and metalloids (Ge, Sb, and Te), positively correlated (p < 0.05) with discharge and showed the highest concentrations during spring flood. We attribute the decreased concentration of soluble elements with discharge to dilution by groundwater feeding and increased concentration of DOC and particle‐reactive metals with discharge to leaching from surface soil, plant litter, and suspended particles. Overall, the present study provides first‐order assessment of fluxes of major and trace elements in the middle course of the Ob River, reveals their high temporal and spatial stability, and characterizes the mechanism of river water chemical composition acquisition.     

UAV RGB,thermal infrared and multispectral imagery used to investigate the control of terrain on the spatial distribution of dryland biocrust

Biocrusts (topsoil communities formed by mosses, lichens, bacteria, fungi, algae, and cyanobacteria) are a key biotic component of dryland ecosystems. Whilst climate patterns control the distribution of biocrusts in drylands worldwide, terrain and soil attributes can influence biocrust distribution at landscape scale. Multi-source unmanned aerial vehicle (UAV) imagery was used to map and study biocrust ecology in a typical dryland ecosystem in central Spain. Red, green and blue (RGB) imagery was processed using structure-from-motion techniques to map terrain attributes related to microclimate and terrain stability. Multispectral imagery was used to produce accurate maps (accuracy > 80%) of dryland ecosystem components (vegetation, bare soil and biocrust composition). Finally, thermal infrared (TIR) and multispectral imagery was used to calculate the apparent thermal inertia (ATI) of soil and to evaluate how ATI was related to soil moisture (r² = 0.83). The relationship between soil properties and UAV-derived variables was first evaluated at the field plot level. Then, the maps obtained were used to explore the relationship between biocrusts and terrain attributes at ecosystem level through a redundancy analysis. The most significant variables that explain biocrust distribution are: ATI (34.4% of variance, F = 130.75; p < 0.001), Elevation (25.8%, F = 97.6; p < 0.001), and potential solar incoming radiation (PSIR) (52.9%, F = 200.1; p < 0.001). Differences were found between areas dominated by lichens and mosses. Lichen-dominated biocrusts were associated with areas with high slopes and low values of ATI, with soil characterized by a higher amount of soluble salts, and lower amount of organic carbon, total phosphorus (P_tot) and total nitrogen (N_tot). Biocrust-forming mosses dominated lower and moister areas, characterized by gentler slopes and higher values of ATI with soils with higher contents of organic carbon, P_tot and N_tot. This study shows the potential to use UAVs to improve our understanding of drylands and to evaluate the control that the terrain has on biocrust distribution.     

Nature and significance of small volume fall deposits at composite volcanoes: Insights from the October 14, 1974 Fuego eruption,Guatemala

The first of four successive pulses of the 1974 explosive eruption of Fuego volcano, Guatemala, produced a small volume (∼0.02 km³ DRE) basaltic sub-plinian tephra fall and flow deposit. Samples collected within 48 h after deposition over much of the dispersal area (7–80 km from the volcano) have been size analyzed down to 8 φ (4 μm). Tephra along the dispersal axis were all well-sorted (σ _φ = 0.25–1.00), and sorting increased whereas thickness and median grain size decreased systematically downwind. Skewness varied from slightly positive near the vent to slightly negative in distal regions and is consistent with decoupling between coarse ejecta falling off the rising eruption column and fine ash falling off the windblown volcanic cloud advecting at the final level of rise. Less dense, vesicular coarse particles form a log normal sub-population when separated from the smaller (Md_φ < 3φ or < 0.125 mm), denser shard and crystal sub-population. A unimodal, relatively coarse (Md_φ = 0.58φ or 0.7 mm σ _φ = 1.2) initial grain size population is estimated for the whole (fall and flow) deposit. Only a small part of the fine-grained, thin 1974 Fuego tephra deposit has survived erosion to the present day. The initial October 14 pulse, with an estimated column height of 15 km above sea level, was a primary cause of a detectable perturbation in the northern hemisphere stratospheric aerosol layer in late 1974 to early 1975. Such small, sulfur-rich, explosive eruptions may substantially contribute to the overall stratospheric sulfur budget, yet leave only transient deposits, which have little chance of survival even in the recent geologic record. The fraction of finest particles (Md_φ = 4–8φ or 4–63 μm) in the Fuego tephra makes up a separate but minor size mode in the size distribution of samples around the margin of the deposit. A previously undocumented bimodal–unimodal–bimodal change in grain size distribution across the dispersal axis at 20 km downwind from the vent is best accounted for as the result of fallout dispersal of ash from a higher subplinian column and a lower “co-pf” cloud resulting from pyroclastic flows. In addition, there is a degree of asymmetry in the documented grain-size fallout pattern which is attributed to vertically veering wind direction and changing windspeeds, especially across the tropopause. The distribution of fine particles (<8 μm diameter) in the tephra deposit is asymmetrical, mainly along the N edge, with a small enrichment along the S edge. This pattern has hazard significance.     

Characteristics of foreshock and its identi-fication

Introduction The characteristics of generalized foreshock and direct foreshock and their identification,as well as their application to medium and short-term prediction of strong earthquake is a major study objective in seismometry both in China and abroad.China has made many short-term and imminent earthquake predictions.Among the ones with clear hazard-mitigating effect and social manifestation,direct foreshock has made an obvious contribution,for example,the MS=7.2Menglian earthquake occu…     

Methane Oxidation in the Water Column of Xiangxi Bay,Three Gorges Reservoir

Four field campaigns are carried out to quantify the methane (CH₄) oxidation rate in Xiangxi Bay (XXB) of the Three Gorges Reservoir (TGR), China. The water depth of the sampling site varied from 13 to 30 m resulting from the water level fluctuation of the TGR. The CH₄ oxidation rates are measured in situ as the decline of dissolved CH₄ concentration versus time in incubated, and those rates. The CH₄ oxidation rates range from 1.18 × 10^?3 to 3.69 × 10^?3 µmol L^?1 h^?1, with higher values and stronger variation during summer. A static floating chamber method is used to measure CH₄ emitted to the atmosphere resulting in an annual mean flux of 4.79 µmol m^?2 h^?1. The CH₄ emission rate is significantly negatively correlated with the water level. The results show that a large fraction of CH₄ is consumed in the water column with a range of 28.97–55.90 µmol m^?2 h^?1, accounting for ≈69–98% of the total CH₄ input into the water column, and more than 90% is consumed outside the summer, when the water level is lowest. Water depth, which is dominated by water level of the TGR, is a potentially important driver for CH₄ oxidation and atmospheric emission in the tributary bay.     

Characterization of shape and terminal velocity of tephra particles erupted during the 2002 eruption of Etna volcano,Italy

In this paper, we present a complete morphological characterization of the ash particles erupted on 18 December 2002 from Etna volcano, Italy. The work is based on the acquisition and processing of bidimensional digital images carried out by scanning electron microscopy (SEM) to obtain shape parameters by image analysis. We measure aspect ratio (AR), form factor (FF), compactness (CC), and rectangularity (RT) of 2065 ash particles with size between 0.026 and 1.122 mm. We evaluate the variation of these parameters as a function of the grain-size. Ash particles with a diameter of <0.125 mm vary from mostly equant to very equant, ash particles between 0.125 and 0.250 mm have an intermediate shape, and particles with diameters >0.250 mm are subelongate. We find that, on average, particles with a diameter of <0.250 mm are subrounded, particles between 0.250 and 0.50 mm are subangular, and particles >0.50 mm are angular. Using this morphological analysis and an empirical relation between the drag coefficient (C _D) and Reynolds number (R _e) of Wilson and Huang (Earth Planet Sci Lett 44:311–324, 1979), we calculate the terminal settling velocities (V _WH). The comparisons between these velocities and those calculated with the formula of Kunii and Levenspiel (Fluidization engineering. Wiley, New York, pp 97, 1969) (V _KL), which considers ash particles as spheres, show that V _KL are in average 1.28 greater than V _WH. Hence, we quantify the systematic error on the spatial distribution of the mass computed around the volcano carried out by tephra dispersal models when the particles are assumed to be spherical.     

Large 14C age offsets between the fine fraction and coexisting planktonic foraminifera in shallow Caribbean sediments

Absolute chronologies in paleoceanographic records are often constructed using the ¹⁴C dating of coarse fraction foraminifera (>150 μm). However, due to processes such as changes in sediment sources or abundances, sedimentation rates, bioturbation, reworking, the adsorption of modern carbon, etc., several studies conducted in different environmental settings have shown time-lags between records obtained from various granulometric fractions. In this study, we examined temporal phasing between the coarse foraminifera and fine fractions by studying changes in the abundances of δ¹⁸O, the ¹⁴C ages of the planktonic foraminifera Globigerinoides ruber (G. ruber, 250–350 μm), and the sediment fine fraction (<63 μm) over the last 45 ka in a core obtained from the northern Caribbean Sea. All of the records were found to be in phase during part of the Holocene (at least for the last ≈6 ka). As determined from δ¹⁸O records and ¹⁴C ages, the fine fraction was younger than G. ruber during the Last Deglaciation (of 1.89 ka). The coupling between bioturbation and changes in the fine fraction, and G. ruber abundances, as tested using a numerical model of the bioturbation record within a mixed-layer depth of 8 cm, was sufficient to explain the results. ¹⁴C age discrepancies increased from 5.64 to 8.5 ka during Marine Isotopic Stages (MIS) 2 and 3, respectively. These chronological discrepancies could not be explained by only one process and seemed to result from the interplay between mechanisms: size-differentiated bioturbation (for 1.5 to 2.5 ka), the adsorption of modern atmospheric CO₂ (for 3.04 to 5.92 ka), and variations in sedimentological processes that influenced the fine carbonate fraction. However, even if variations in the mineralogical composition of the fine carbonate fraction were identified using scanning-electron microscopy observations, X-ray diffraction measurements, and geochemical analyses (the mol % MgCO₃ of magnesian calcite and the Sr/Ca ratio of the bulk fine fraction), they can not account for the observed age differences. The results presented for core MD03-2628 extend beyond this case study because they illustrate the need for a detailed characterization of the various size fractions prior to paleoclimate signal interpretations, especially for chronological studies.