Frontiers of Earth Science

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Responses of a 234U/238U activity ratio in groundwater to earthquakes in the South Baikal Basin, Siberia
Sergei RASSKAZOV, Aigul ILYASOVA, Sergei BORNYAKOV, Irina CHUVASHOVA, Eugene CHEBYKIN
Front. Earth Sci.    https://doi.org/10.1007/s11707-020-0821-5
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In the western part of the South Baikal Basin, spatial-temporal distribution of earthquake epicenters shows quasi-periodic seismic reactivation. The largest earthquakes that occurred in 1999 (MW = 6.0) and 2008 (MW = 6.3) fall within seismic intervals of 1994–2003 and 2003–2012, respectively. In the seismic interval that began in 2013, the 234U/238U activity ratio (AR) in groundwater was monitored assuming its dependence on crack opening/closing that facilitated/prevented water circulation in an active boundary fault of the basin. Transitions from disordered, high-amplitude fluctuations of AR values to consistent, low-amplitude fluctuations in different monitoring sites were found to be sensitive indicators of both small seismic events occurring directly on the observation area, and of a large remote earthquake. The hydroisotopic responses to seismic events were consistent with monitoring data on deformation and temperature variations of rocks. The hydroisotopic effects can be applied for detecting a seismically dangerous state of an active fault and prediction of a large future earthquake.

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Trace elements of pyrite and S, H, O isotopes from the Laowan gold deposit in Tongbai, Henan Province, China: implications for ore genesis
Yuehua ZHAO, Shouyu CHEN, Jianli CHEN, Shuaiji WU
Front. Earth Sci.    https://doi.org/10.1007/s11707-019-0807-3
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The Laowan deposit is a large gold deposit in the Qinling-Dabie orogenic belt where pyrite is the main Au-bearing mineral phase. We present results from the occurrences of gold, trace elements and sulfur isotopes of pyrite, and hydrogen and oxygen isotopes of quartz and calcite to elucidate the sources of ore-forming fluid; the genesis of pyrite and the ore-forming process.

From field observations, five generations of pyrite are identified; one formed in a metamorphic-diagenetic epoch (PyI), and the others during four mineralization stages: 1) the coarse-grained pyrite-quartz stage (PyII), 2) the quartz and medium- to fine-grained pyrite stage (PyIII), 3) the polymetallic sulfide stage (PyIV), and 4) the carbonate-quartz stage (PyV). Gold mainly occurs in PyIII and PyIV. We find that Au, Ag, Pb, and Cu are incorporated into pyrite as micro-/nano-inclusions and that Co, Ni, As, and Se enter the pyrite lattice via isomorphous replacement.

The Co/Ni values and Se concentrations indicate that PyI formed from metamorphic hydrothermal fluids and that pyrites (PyII, PyIII, and PyIV) from the ore-forming stages typically reflect a hydrothermal genesis involving magmatic fluid.

The d34S values of PyI (1.45‰–2.09‰) are similar to that of plagioclase amphibole schist, indicating that S was primarily derived from wall rock, while those of PyII, PyIII, and PyIV (3.10‰–5.55‰) suggest that S was derived from the Guishanyan Formation and the Laowan granite. The four mineralization stages show a systematic decrease in dD (from −77.1‰ to −82.8‰, −84.7‰, and −102.7‰), while the δ18 O H 2O values showed a gradual decrease from 5.7 to 2.7‰, 1.0‰, and −1.3‰. These data show that the ore-forming fluid was similar to a mixture of magmatic and meteoric waters. Thus, we conclude that the Laowan gold deposit is related to magmatic-hydrothermal fluid.

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Characterizing the effects of climate change on short-term post-disturbance forest recovery in southern China from Landsat time-series observations (1988–2016)
Fangyan ZHU, Heng WANG, Mingshi LI, Jiaojiao DIAO, Wenjuan SHEN, Yali ZHANG, Hongji WU
Front. Earth Sci.    https://doi.org/10.1007/s11707-020-0820-6
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Climate change, a recognized critical environmental issue, plays an important role in regulating the structure and function of forest ecosystems by altering forest disturbance and recovery regimes. This research focused on exploring the statistical relationships between meteorological and topographic variables and the recovery characteristics following disturbance of plantation forests in southern China. We used long-term Landsat images and the vegetation change tracker algorithm to map forest disturbance and recovery events in the study area from 1988 to 2016. Stepwise multiple linear regression (MLR), random forest (RF) regression, and support vector machine (SVM) regression were used in conjunction with climate variables and topographic factors to model short-term forest recovery using the normalized difference vegetation index (NDVI). The results demonstrated that the regenerating forests were sensitive to the variation in temperature. The fitted results suggested that the relationship between the NDVI values of the forest areas and the post-disturbance climatic and topographic factors differed in regression algorithms. The RF regression yielded the best performance with an R2 value of 0.7348 for the validation accuracy. This indicated that slope and temperature, especially high temperatures, had substantial effects on post-disturbance vegetation recovery in southern China. For other mid-subtropical monsoon regions with intense light and heat and abundant rainfall, the information will also contribute to appropriate decisions for forest managers on forest recovery measures. Additionally, it is essential to explore the relationships between forest recovery and climate change of different vegetation types or species for more accurate and targeted forest recovery strategies.

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Quantifying the impact of mountain precipitation on runoff in Hotan River, northwestern China
Baofu LI, Jili ZHENG, Xun SHI, Yaning CHEN
Front. Earth Sci.    https://doi.org/10.1007/s11707-019-0811-7
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It is of great significance to quantitatively assess the impact of mountain precipitation on inland river runoff in data scarce regions. Based on the corrected TRMM precipitation and runoff data, a variety of statistical methods were used to identify which areas of precipitation have an important impact on runoff in the Hotan River Basin, and to evaluate the effects that precipitation changes have on runoff at low, mid, high, and extremely high altitudes of mountainous areas. The results showed that: 1) From 1998 to 2015, the annual runoff showed a fluctuating upward trend with a rate of 11.21 × 108 m3/10a (P<0.05). Runoff in every season also had an increasing trend, with summer runoff the most significant at a rate of 6.09 × 108 m3/10a. 2) The annual runoff and precipitation changes had certain synchronization, with a correlation coefficient of 0.45 (P<0.05). Among them, the correlations between precipitation and runoff changes were highest at low and mid- altitudes, with coefficients of 0.62 and 0.55, respectively (P<0.05). 3) 65.95% of the regional precipitation at low altitudes and 48.34% at high altitudes were significantly correlated with runoff (P<0.05), while only 38.84% and 26.58% of regional precipitation levels at mid- and extremely high altitudes were significantly correlated with runoff. 4) The annual precipitation change in the basin was 1%, which would cause the annual runoff to change by 0.24%. In 1998–2015, the change of annual runoff caused by precipitation change at high altitudes was largest at a rate of -6.01%; the change rates of annual runoff caused by precipitation change in the low, mid-, and extremely high altitudes were -3.66%, -3.62%, and -3.67%, respectively. The results have significant scientific guidance for water resource management in arid basins.

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Typhoon parameter sensitivity of storm surge in the semi-enclosed Tokyo Bay
Md. Rezuanul ISLAM, Hiroshi TAKAGI
Front. Earth Sci.    https://doi.org/10.1007/s11707-020-0817-1
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In this study, a storm surge model of the semi-enclosed Tokyo Bay was constructed to investigate its hydrodynamic response to major typhoon parameters, such as the point of landfall, approach angle, forward speed, size, and intensity. The typhoon simulation was validated for Typhoon Lan in 2017, and 31 hypothetical storm surge scenarios were generated to establish the sensitivity of peak surge height to the variation in typhoon parameters. The maximum storm surge height in the upper bay adjacent to the Tokyo Metropolitan Area was found to be highly sensitive to the forward speed and size of the passing typhoon. However, the importance of these parameters in disaster risk reduction has been largely overlooked by researchers and disaster managers. It was also determined that of the many hypothetical typhoon tracks evaluated, the slow passage of a large and intense typhoon transiting parallel to the longitudinal axis of Tokyo Bay, making landfall 25 km southwest, is most likely to cause a hazardous storm surge scenario in the upper-bay area. The results of this study are expected to be useful to disaster managers for advanced preparation against destructive storm surges.

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Erosion-deposition patterns and depo-center movements in branching channels at the near-estuary reach of the Yangtze River
Boyuan ZHU, Jinyun DENG, Jinwu TANG, Wenjun YU, Alistair G.L. BORTHWICK, Yuanfang CHAI, Zhaohua SUN, Yitian LI
Front. Earth Sci.    https://doi.org/10.1007/s11707-019-0808-2
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Channel evolution and depo-center migrations in braided reaches are significantly influenced by variations in runoff. This study examines the effect of runoff variations on the erosion-deposition patterns and depo-center movements within branching channels of the near-estuary reach of the Yangtze River. We assume that variations in annual mean duration days of runoff discharges, ebb partition ratios in branching channels, and the erosional/depositional rates of entire channels and sub-reaches are representative of variations in runoff intensity, flow dynamics in branching channels, and morphological features in the channels. Our results show that the north region of Fujiangsha Waterway, the Liuhaisha branch of Rugaosha Waterway, the west branch of Tongzhousha Waterway, and the west branch of Langshansha Waterway experience deposition or reduced erosion under low runoff intensity, and erosion or reduced deposition under high runoff intensity, with the depo-centers moving upstream and downstream, respectively. Other waterway branches undergo opposite trends in erosion-deposition patterns and depo-center movements as the runoff changes. These morphological changes may be associated with trends in ebb partition ratio as the runoff discharge rises and falls. By flattening the intra-annual distribution of runoff discharge, dam construction in the Yangtze Basin has altered the ebb partition ratios in waterway branches, affecting their erosion-deposition patterns and depo-center movements. Present trends are likely to continue into the future due to the succession of large cascade dams under construction along the upper Yangtze and ongoing climate change.

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Chemical geothermometry: application to mud volcanic waters of the Caucasus region
Olga E. KIKVADZE, Vasilii Yu. LAVRUSHIN, Boris G. POLYAK
Front. Earth Sci.    https://doi.org/10.1007/s11707-019-0810-8
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The generation temperatures of gas-water fluids released from mud volcanoes in different provinces of the Caucasian region have been constrained using Mg/Li (ТMg/Li) chemical geothermometry. Mud volcanic fluids in the Taman Peninsula (Kerch-Taman mud volcanic province) were generated at temperatures (ТMg/Li) from 41 to 137°С. The depths of the respective mud reservoirs estimated from ТMg/Li values and local geothermal gradient are in a range of 1.0 to 3.4 km which spans the Maykop Formation of marine shale. For the South Caspian province, the ТMg/Li values of waters vary from 18 to 137°C and the respective root depths НMg/Li of mud volcanoes range from ~ 0.85 to 6.5 km. The obtained TMg/Li values for the analyzed mud volcanic waters from Caucasian provinces are in positive correlation with НСО3 contents and water oxygen isotope compositions (δ18ОН2О and Dd18ОН2О) and in high negative correlation with Cl. The increase of ТMg/Li toward the Greater Caucasus Range, as well as the lateral TMg/Li patterns in the Taman and South Caspian mud volcanic provinces, support the idea that mud volcanic fluids generate at temperatures increasing progressively toward the Alpine orogenic belt.

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Climate change impact and adaptation on wheat yield, water use and water use efficiency at North Nile Delta
Marwa Gamal Mohamed ALI, Mahmoud Mohamed IBRAHIM, Ahmed El BAROUDY, Michael FULLEN, El-Said Hamad OMAR, Zheli DING, Ahmed Mohammed Saad KHEIR
Front. Earth Sci.    https://doi.org/10.1007/s11707-019-0806-4
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Investigation of climate change impacts on food security has become a global hot spot. Even so, efforts to mitigate these issues in arid regions have been insufficient. Thus, in this paper, further research is discussed based on data obtained from various crop and climate models. Two DSSAT crop models (CMs) (CERES-Wheat and N-Wheat) were calibrated with two wheat cultivars (Gemiza9 and Misr1). A baseline simulation (1981-2010) was compared with different scenarios of simulations using three Global Climate Models (GCMs) for the 2030s, 2050s and 2080s. Probable impacts of climate change were assessed using the GCMs and CMs under the high emission Representative Concentration Pathway (RCP8.5). Results predicted decreased wheat grain yields by a mean of 8.7%, 11.4% and 13.2% in the 2030s, 2050s and 2080s, respectively, relative to the baseline yield. Negative impacts of climatic change are probable, despite some uncertainties within the GCMs (i.e., 2.1%, 5.0% and 8.0%) and CMs (i.e., 2.2%, 6.0% and 9.2%). Changing the planting date with a scenario of plus or minus 5 or 10 days from the common practice was assessed as a potentially effective adaptation option, which may partially offset the negative impacts of climate change. Delaying the sowing date by 10 days (from 20 November to 30 November) proved the optimum scenario and decreased further reduction in wheat yields resulting from climate change to 5.2%, 6.8% and 8.5% in the 2030s, 2050s and 2080s, respectively, compared with the 20 November scenario. The planting 5-days earlier scenario showed a decreased impact on climate change adaptation. However, the 10-days early planting scenario increased yield reduction under projected climate change. The cultivar Misr1 was more resistant to rising temperature than Gemiza9. Despite the negative impacts of projected climate change on wheat production, water use efficiency would slightly increase. The ensemble of multi-model estimated impacts and adaptation uncertainties of climate change can assist decision-makers in planning climate adaptation strategies.

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Determination of the crustal structure and seismicity of the Linfen rift with S-wave velocity mapping
Zigen WEI, Risheng CHU, Meiqin SONG, Xiaolin YANG, Shanshan WU, Feng BAO
Front. Earth Sci.    https://doi.org/10.1007/s11707-019-0804-6
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The Linfen rift is a Cenozoic extensional rift with significant seismicity and seismic hazards. Studies of this rift shed light on deep dynamic processes and seismogenic mechanisms relevant to crustal structure and seismic activity. We first conducted a joint inversion of receiver functions and surface wave dispersion on waveform data collected from 27 broadband seismic stations to image the crustal S-wave velocity in the Linfen rift and its surroundings. We then relocated the source parameters for 10 earthquake events with depths>20 km and studied the relationship between crustal S-wave velocity and seismicity. The results show that low-velocity zones of different scales exist in the middle-lower crust, and that the depth of the seismogenic layer gradually increases from ~25 km in the south to ~34 km in the north, roughly corresponding to the bottom of the low-velocity zone. We found that most of the relocated earthquakes occurred in the low-velocity zone at depths of 18 km to 34 km, with the deepest at 32 km. Two of the greatest historic earthquakes, Linfen (Ms 7.75) in 1695 and Hongtong (Ms 8.0) in 1303, occurred at the bottom of the high-velocity zone at depths of 12 km to 18 km. Our results, combined with previous studies, suggest that the upwelling mantle material below the rift did not remarkably affect the velocity structure from the bottom of the seismogenic layer down to the uppermost mantle nor heat the crust. It is likely that neither crustal-scale faults nor mantle earthquakes exist in the Linfen rift.

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Rapid flood inundation mapping by differencing water indices from pre- and post-flood Landsat images
Ramesh SIVANPILLAI, Kevin M. JACOBS, Chloe M. MATTILIO, Ela V. PISKORSKI
Front. Earth Sci.    https://doi.org/10.1007/s11707-020-0818-0
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Following flooding disasters, satellite images provide valuable information required for generating flood inundation maps. Multispectral or optical imagery can be used for generating flood maps when the inundated areas are not covered by clouds. We propose a rapid mapping method for identifying inundated areas based on the increase in the water index value between the pre- and post-flood satellite images. Values of the Normalized Difference Water Index (NDWI) and Modified NDWI (MNDWI) will be higher in the post-flood image for flooded areas compared to the pre-flood image. Based on a threshold value, pixels corresponding to the flooded areas can be separated from non-flooded areas. Inundation maps derived from differencing MNDWI values accurately captured the flooded areas. However the output image will be influenced by the choice of the pre-flood image, hence analysts have to avoid selecting pre-flood images acquired in drought or earlier flood years. Also the inundation maps generated using this method have to be overlaid on the post-flood satellite image in order to orient personnel to landscape features. Advantages of the proposed technique are that flood impacted areas can be identified rapidly, and that the pre-existing water bodies can be excluded from the inundation maps. Using pairs of other satellite data, several maps can be generated within a single flood which would enable emergency response agencies to focus on newly flooded areas.

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