Evaluation of oxygen isotopes in carbonate as an indicator of lake evolution in arid areas: The modern Qinghai Lake, Qinghai–Tibet Plateau

The oxygen isotopic composition of carbonate in lakes has been used as a useful indicator in Palaeolimnological research, and has made some important contributions to our understanding of lacustrine systems. For modern lakes in arid or cold areas, however, there are few data available to test the effect of lake salinity and temperature on the oxygen isotopic composition of various carbonate sources such as ostracod, bulk carbonate, and fine-grained carbonate (< 60 μm). Here we examined the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonates, as well as that of coexisting water from Lake Qinghai and the smaller surrounding lakes and ponds on the Qinghai–Tibet Plateau. Our investigation highlights three key effects. First, the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonate in the lakes and ponds shows a clear response to lake water δ¹⁸O values, and these vary with water salinity. The relationship between lake water δ¹⁸O and salinity is not only dominated by the evaporation/freshwater input ratios, but is also controlled by the distance to the mouth of the major rivers supplying to the lake. Second, the ostracod, bulk carbonate, and fine-grained carbonate show similar isotopic change trends in the study area, and oxygen isotopic differences between ostracods and authigenic carbonate may be explained by the different water temperatures and very small ‘vital offsets’ of ostracods. Finally, the effect of water depth on temperature leads to increasing δ¹⁸O values in carbonates as water depth increases, both in benthic ostracods living on the lake bottom, as well as in bulk carbonate precipitated at the water surface.For arid, high-altitude Lake Qinghai, our results suggest that variations in the δ¹⁸O values of carbonate in Lake Qinghai are mainly controlled by the oxygen-isotope ratio of the lake water changing with water salinity. As a secondary effect, increasing water depth leads to cooler bottom and surface water, which may result in more positive δ¹⁸O values of ostracod and bulk carbonate.     

金星的地幔对流、岩石圈演化和表面更新

金星在大小、轨道半径等方面与地球非常接近,但现今的观测显示金星在大气、表面温度、重力与地形特征等方面与地球非常不同.对固体金星而言,最显著特征是有火山活动但没有活动的板块构造运动、表面平均大约500±200 Ma的年龄显示金星发生过全球性表面快速更新、重力与地形强相关.本文在简要介绍已有工作的基础上,主要介绍我们基于这些观测特征开展的金星动力学研究,包括现今地幔的对流模式、相变对表面更新的影响、地壳厚度、岩石圈演化与表面更新等研究工作和取得的初步认识.     

Effects of riverbed incision on the hydrology of the Vietnamese Mekong Delta

The hydrogeomorphology of the Vietnamese Mekong Delta (VMD) has been significantly altered by natural and anthropogenic drivers. In this study, the spatiotemporal changes of the flow regime were examined by analysing the long-term daily, monthly, annual and extreme discharges and water levels from 1980 to 2018, supported by further investigation of the long-term annual sediment load (from the 1960s to 2015), river bathymetric data (in 1998, 2014 and 2017) and daily salinity concentration (from the 1990s to 2015) using various statistical methods and a coupled numerical model. Then, the effects of riverbed incision on the hydrology were investigated. The results show that the dry season discharge (i.e., in March–June) of the Tien River increased by up to 23% from the predam period (1980–1992) to the postdam period (1993–2018) but that the dry season water level at My Thuan decreased by up to −46%. The annual mean and monthly water levels in June at Tan Chau and in January and June–October at My Thuan in the Tien River decreased statistically, even though the respective discharges increased significantly. These decreased water levels instead of the increased discharges were attributed to the accelerated riverbed incision upstream from My Thuan, which increased by more than three times, from a mean rate of −0.16 m/year (−16.7 Mm³/year) in 1998–2014 to −0.5 m/year (−52.5 Mm³/year) in 2014–2017. This accelerated riverbed incision was likely caused by the reduction in the sediment load of the VMD (from 166.7 Mt/year in the predam period to 57.6 Mt/year in the postdam period) and increase in sand mining (from 3.9 Mm³ in 2012 to 13.43 Mm³ in 2018). Collectively, the decreased dry season water level in the Tien River is likely one of the main causes of the enhanced salinity intrusion.     

策克台井下宽频带地震计GL?S120B正弦波标定衰减因子的讨论

为解决EDAS?24GN6数据采集器对策克台井下宽频带地震计GL?S120B进行正弦标定的过程中产生的响应波形限幅问题,分析了数据采集器?地震计这一系统的标定过程及原理,结合GL?S120B的幅频响应特征,推导出正弦标定频点参数中的频率f、衰减因子λ与标定响应波形限幅之间的关系。分析表明,在GL?S120B平坦响应频带内,为避免出现标定波形响应限幅,频率f与衰减因子λ的乘积应至少大于限幅下限50.93;而在其衰减响应频带内,由于地震计电压灵敏度的衰减,频率f与衰减因子λ的乘积最小取值也相应减小。据此,对衰减因子λ进行校正,并再次对GL?S120B进行正弦标定。标定及计算结果显示:标定波形未限幅,GL?S120B三分向幅频响应特征较为一致;灵敏度对出厂值的变化率保持在5%范围内。本研究结论可为采用EDAS?24GN6和GL?S120B地震观测系统的台站进行正弦波标定提供频点参数,并为其他台站解决正弦标定波形限幅问题提供分析依据。     

Disaster risk evaluation using factor analysis: a case study of Chinese regions

Natural Hazards - Regional risk to natural disasters is a critical multi-criteria decision-making (MCDM) problem in the literature due to the complicated and usually conflicting evaluation index...     

中国铁路风沙防治的研究进展

基于中国沙区铁路的发展历程,系统分析了铁路沙害类型、特点以及风沙防治方案、措施和技术体系。针对我国沙区地域特征和铁路沙害类型,作为个例分类剖析了途径流沙地表、高寒环境与戈壁大风区铁路风沙危害与防治技术取得的成效和面临的问题。结合目前我国沙区铁路的发展趋势,从理论研究和工程防沙实践出发,提出复杂地形、特殊环境铁路沙害是今后风沙工程学领域的重点和理论研究趋势,也是沙区铁路沙害防治的关键和难点。     

单井与群井结合抽水试验在地面沉降实测中的应用研究

以天津市某工程项目抽水试验数据为实测基本资料,目的是确定地基土的渗透性和富水性,计算含水层的水文地质参数,并实时监测试验期间地面沉降监测结果。结果表明,单井和群井抽水试验在抽水过程中,地面高程呈现出下降的趋势,累计达到最大沉降量,达到23. 57 mm,随着抽水时间较短和抽水时间较长等原因,在地面具有附加荷载时,沉降变大。因此,试验实时监测可以有效预防存在问题,为工程的设计与施工提供安全的风险控制。     

广利河口拦门沙发育动态和河口航道的选择

根据现场调查和历史资料,获取了广利河口拦门沙水动力特征、海底地形和底质特征,对拦门沙的动态发育和波浪作用下拦门沙运动状态进行了分析,探讨了广利河口的航道选择方案。分析结果认为,广利河槽外航道宜从东偏南向入海。     

Effects of Concentrated Freshwater Discharge on Hydrography and Mesozooplankton Community in a Dammed Estuary

Ocean Science Journal - This study examines the different effects of high and low freshwater discharge on the water physical structure of the water column and plankton community in a tide-dominated...