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1.
温室气体减排对控制全球气候变暖具有重要意义。2014年我国农田非CO2温室气体(主要指CH4和N2O)排放占全国温室气体排放总量的4.3%,预计2030年我国实现碳达峰后,化石能源逐步被清洁能源替代,农田CH4和N2O排放占全国温室气体排放的比重也将随之增大,其减排的紧迫性和重要性将日渐凸显。然而,现有农田碳减排技术由于缺乏立法教育宣传和成果激励机制等,并未得到充分转化应用与推广示范,使得减排成果难以落地坐实,不利于我国农业碳减排目标的顺利实现。文章总结了我国农田CH4和N2O减排工作的研究进展,指出了当前我国农田CH4和N2O减排所面临的问题,并在今后长效监测平台运维、新方法技术突破、大众减排意识提升及成果推广示范加强4个方面提出了技术和政策上的建议。  相似文献   

2.
Coastal tidal wetlands produce and accumulate significant amounts of organic carbon (C) that help to mitigate climate change. However, previous data limitations have prevented a robust evaluation of the global rates and mechanisms driving C accumulation. Here, we go beyond recent soil C stock estimates to reveal global tidal wetland C accumulation and predict changes under relative sea level rise, temperature and precipitation. We use data from literature study sites and our new observations spanning wide latitudinal gradients and 20 countries. Globally, tidal wetlands accumulate 53.65 (95%CI: 48.52–59.01) Tg C yr−1, which is ∼30% of the organic C buried on the ocean floor. Modeling based on current climatic drivers and under projected emissions scenarios revealed a net increase in the global C accumulation by 2100. This rapid increase is driven by sea level rise in tidal marshes, and higher temperature and precipitation in mangroves. Countries with large areas of coastal wetlands, like Indonesia and Mexico, are more susceptible to tidal wetland C losses under climate change, while regions such as Australia, Brazil, the USA and China will experience a significant C accumulation increase under all projected scenarios.  相似文献   

3.
Resolving regional carbon budgets is critical for informing land-based mitigation policy. For nine regions covering nearly the whole globe, we collected inventory estimates of carbon-stock changes complemented by satellite estimates of biomass changes where inventory data are missing. The net land–atmospheric carbon exchange (NEE) was calculated by taking the sum of the carbon-stock change and lateral carbon fluxes from crop and wood trade, and riverine-carbon export to the ocean. Summing up NEE from all regions, we obtained a global ‘bottom-up’ NEE for net land anthropogenic CO2 uptake of –2.2 ± 0.6 PgC yr−1 consistent with the independent top-down NEE from the global atmospheric carbon budget during 2000–2009. This estimate is so far the most comprehensive global bottom-up carbon budget accounting, which set up an important milestone for global carbon-cycle studies. By decomposing NEE into component fluxes, we found that global soil heterotrophic respiration amounts to a source of CO2 of 39 PgC yr−1 with an interquartile of 33–46 PgC yr−1—a much smaller portion of net primary productivity than previously reported.  相似文献   

4.
In early 2020, unprecedented lockdowns and travel bans were implemented in Chinese mainland to fight COVID-19, which led to a large reduction in anthropogenic emissions. This provided a unique opportunity to isolate the effects from emission and meteorology on tropospheric nitrogen dioxide (NO2). Comparing the atmospheric NO2 in 2020 with that in 2017, we found the changes of emission have led to a 49.3 ± 23.5% reduction, which was ∼12% more than satellite-observed reduction of 37.8 ± 16.3%. The discrepancy was mainly a result of changes of meteorology, which have contributed to an 8.1 ± 14.2% increase of NO2. We also revealed that the emission-induced reduction of NO2 has significantly negative correlations to human mobility, particularly that inside the city. The intra-city migration index derived from Baidu Location-Based-Service can explain 40.4% ± 17.7% variance of the emission-induced reduction of NO2 in 29 megacities, each of which has a population of over 8 million in Chinese mainland.  相似文献   

5.
水泥生产的碳排放因子研究进展简   总被引：1，自引：0，他引：1  
水泥生产是除化石能源以外碳排放的重要来源。中国是世界上水泥产量最大的国家,水泥生产的碳排放问题不容忽视。2011年,中国水泥产量为20亿t占世界总产量的58.8%。中国水泥生产碳排放占世界水泥排放的比重增至60.6%,与此同时,水泥排放占中国碳排放总量的比重也增至11.3%。当前,国际默认的水泥生产碳排放因子已不能满足中国碳排放核算的需求。加强中国水泥生产碳排放因子计算方法研究,对科学、准确核算中国水泥生产的碳排放具有重要现实意义。本文在系统梳理政府间气候变化专门委员会（Intergovernmental Panel onClimate Change,IPCC）、世界可持续发展工商理事会（World Business Council for Sustainable Development,WBCSD）水泥可持续发展自愿性协议（Cement Sustainability Initiative,CSI）即WBCSD/CSI 和中国建筑材料研究总院（ChinaBuilding Materials Academy,CDMA）水泥生产碳排放因子核算边界、计算方法基础上,对上述计算方法进行了比较,在辨析参数选择、不确定性差异的基础上揭示了水泥生产的碳排放因子的影响因素,讨论了各种计算方法在中国的适应性,为未来中国水泥生产的碳排放因子计算方法的选择及构建奠定了基础。  相似文献   

6.
2002—2020年,全球野火每年约排放73.2亿吨二氧化碳（CO2）,为化石燃料排放CO2的18.5%;其中,林火碳排放约占野火碳排放20%左右（15亿吨CO2）。近年来,随着气候变化和人类活动加剧,林火释放的CO2呈增加趋势。例如,2023年5月以来的加拿大林火,截至8月29日已累计排放12.68亿吨CO2。我国在实现森林面积和蓄积量“双增长”的背景下,自2010年以来林火发生次数和面积显著减小,为减少林火碳排放、应对全球气候变化作出了重要贡献。鉴于林火已成为不可忽视的温室气体排放源,需要建立全面、客观、公正的碳排放监测与计量系统,兼顾人类活动（如化石燃料排放、工业排放）和自然林火碳排放;特别是通过采取减少林火发生频率、降低火灾强度等有效手段,降低林火碳排放。针对极端林火预测和防控的世界性难题,急需构建林火风险识别、预警-预测和防控技术体系,并加强林火过程碳排放研究,建立更加科学、全面、自主可控的碳核算体系。  相似文献   

7.
基于系统动力学方法和联合国政府间气候变化专门委员会(IPCC)排放因子法,对上海市 2000—2019 年的化石能源 CO2排放进行定量核算,构建上海市化石能源 CO2排放系统动力学模型,并模拟基础排放、低排放、高排放 3 种情景下的未来 CO2排放变化。研究发现：(1)根据 2000—2019 年统计数据,上海市化石能源的 CO2排放从 2003 年开始攀升,到 2010 年开始趋于平稳增长,其中增长最快的阶段为 2004—2007 年,增长了 32.7%;(2)上海市生产总值(GDP)年增长,2050 年低排放情景较基础情景下降 25%,高排放情景较基础情景上升 33%;(3)2020—2050 年低排放情景的能源消费量呈下降趋势,基础情景与高排放情景的能源消费量呈先增后降趋势,分别在 2029 年、2037 年达到峰值,但能源强度从高到低为低排放情景、基础情景、高排放情景,主要是由于 GDP 增长问题导致;(4)基础情景与低排放情景的 CO2排放在 2030 年达到峰值,...  相似文献   

8.
中国稻田温室气体的排放与减排   总被引：1，自引：0，他引：1  
中国是世界上最大的水稻生产国, 水稻种植面积占全球总种植面积的30%。水稻生产在粮食安全方面起着重要的作用, 稻田却是温室气体甲烷(CH4)和氧化亚氮(N2O)的重要排放源。文章综述了稻田CH4和N2O的产生过程、影响因素及时空变异规律, 总结了近年来我国稻田CH4和N2O排放总量的估算结果, 并提出了针对性的温室气体减排措施。  相似文献   

9.
Although much attention has been paid to investigating and controlling air pollution in China, the trends of air-pollutant concentrations on a national scale have remained unclear. Here, we quantitatively investigated the variation of air pollutants in China using long-term comprehensive data sets from 2013 to 2017, during which Chinese government made major efforts to reduce anthropogenic emission in polluted regions. Our results show a significant decreasing trend in the PM2.5 concentration in heavily polluted regions of eastern China, with an annual decrease of ∼7% compared with measurements in 2013. The measured decreased concentrations of SO2, NO2 and CO (a proxy for anthropogenic volatile organic compounds) could explain a large fraction of the decreased PM2.5 concentrations in different regions. As a consequence, the heavily polluted days decreased significantly in corresponding regions. Concentrations of organic aerosol, nitrate, sulfate, ammonium and chloride measured in urban Beijing revealed a remarkable reduction from 2013 to 2017, connecting the decreases in aerosol precursors with corresponding chemical components closely. However, surface-ozone concentrations showed increasing trends in most urban stations from 2013 to 2017, which indicates stronger photochemical pollution. The boundary-layer height in capital cities of eastern China showed no significant trends over the Beijing–Tianjin–Hebei, Yangtze River Delta and Pearl River Delta regions from 2013 to 2017, which confirmed the reduction in anthropogenic emissions. Our results demonstrated that the Chinese government was successful in the reduction of particulate matter in urban areas from 2013 to 2017, although the ozone concentration has increased significantly, suggesting a more complex mechanism of improving Chinese air quality in the future.  相似文献   

10.
研发海洋“负排放”技术 支撑国家“碳中和”需求   总被引：10，自引：1，他引：9       下载免费PDF全文
应对气候变化的关键是实现碳中和。实现碳中和的基本途径包括"减排"(减少向大气中排放CO2)和"增汇"(增加对大气中CO2的吸收)。我国作为碳排放大国和发展中国家,应在尽可能减排的同时想方设法增汇,也即研发负排放的方法与途径,这是实现碳中和的必由之路,应强调主动作为。海洋是地球上最大的活跃碳库,有着巨大的碳汇潜力和负排放研发前景。我国的海洋碳汇理论研究已走在国际前沿,并推动了科学与政策的连接。当前,应从顶层设计、及时布局,对外引领国际大科学计划,对内结合国情大力研发海洋负排放技术,打造负排放生态工程;建立海洋碳汇标准体系,并通过大科学计划将其推向世界,占领国际制高点,为实现碳中和宏伟目标提供有力的科技支撑。  相似文献   

11.
中国二氧化碳排放的分布动态与演进趋势   总被引：3，自引：2，他引：1  
刘华军  鲍振  杨骞 《资源科学》2013,35(10):1925-1932
研究二氧化碳排放的动态分布特征及其演进规律对于制定合理的碳减排政策并最终实现碳减排目标具有重要理论意义和参考价值.本文采用IPCC的计算方法,以中国大陆29个省(区、直辖市)1995-2010年期间不同年份的化石能源消费量精确测算了各省的二氧化碳排放量,并以碳强度为指标,采用动态分布分析方法(MEDD)对中国省际和区域二氧化碳排放的分布动态及其演进进行了实证分析.研究结论显示: ① GIS可视化方法表明中国二氧化碳排放存在显著的空间非均衡性;②基尼系数测算结果表明,在样本考察期内中国二氧化碳排放空间分布的总体差距呈现扩大趋势.其中,中西部地区均呈现扩大趋势,只有东部地区呈现下降趋势;③Kernel密度估计表明,中国二氧化碳排放的地区差距在样本考察期内呈下降态势,三大区域也均呈下降态势;④Markov链分析表明,不同水平的碳强度的组间流动性较低,表明中国碳排放具有一定的稳定性.整体来看,中国碳排放有下降趋势,整体向着低和中低水平的趋势发展.  相似文献   

12.
In this paper, we present an on-chip hand-powered membrane pump using a robust patient-to-chip syringe interface. This approach enables safe sample collection, sample containment, integrated sharps disposal, high sample volume capacity, and controlled downstream flow with no electrical power requirements. Sample is manually injected into the device via a syringe and needle. The membrane pump inflates upon injection and subsequently deflates, delivering fluid to downstream components in a controlled manner. The device is fabricated from poly(methyl methacrylate) (PMMA) and silicone, using CO2 laser micromachining, with a total material cost of ∼0.20 USD/device. We experimentally demonstrate pump performance for both deionized (DI) water and undiluted, anticoagulated mouse whole blood, and characterize the behavior with reference to a resistor-capacitor electrical circuit analogy. Downstream output of the membrane pump is regulated, and scaled, by connecting multiple pumps in parallel. In contrast to existing on-chip pumping mechanisms that typically have low volume capacity (∼5 μL) and sample volume throughput (∼1–10 μl/min), the membrane pump offers high volume capacity (up to 240 μl) and sample volume throughput (up to 125 μl/min).  相似文献   

13.
张利  雷军  张小雷 《资源科学》2012,34(1):42-49
本文根据IPCC碳排放计算指南缺省值计算了新疆1952年-2008年的碳排放量,系统分析了新疆碳排放总量、碳排放结构、碳排放强度的变化,并对新疆碳排放进行阶段划分,最后应用对数平均迪氏指数法（Logarithmic Mean Divisia Index）对碳排放量进行因素分解,定量分析了碳排放不同阶段各影响因素对碳排放的作用程度。结果表明：①1952年-2008年,新疆CO2排放总量和人均CO2排放量不断上升,万元GDP碳排放水平则呈先上升后降低的＂倒V＂型曲线,煤炭消费是碳排放的主要来源,第二产业能源消费产生的CO2排放量比重最大;②新疆碳排放经历了五个阶段,当前处于经济和碳排放快速增长阶段;③碳排放不同阶段各影响因素的作用程度不同。总体来看,经济增长是碳排放量增加的主要推动因素;能源消费强度的降低（升高）也是促使碳排放减少（增加）的主要因素;能源消费结构由于变化不大,对碳排放的影响很小;计划生育政策实施以来,人口增长对碳排放量增长的推动力逐渐减弱。能源消费强度又进一步受到产业结构和各产业能耗强度的影响,其中,第二产业能耗强度和第二产业产值比重是影响能源消费强度最主要的影响因素。  相似文献   

14.
Activation of high-energy triple-bonds of N2 is the most significant bottleneck of ammonia synthesis under ambient conditions. Here, by importing cobalt single clusters as strong electron-donating promoter into the catalyst, the rate-determining step of ammonia synthesis is altered to the subsequent proton addition so that the barrier of N2 dissociation can be successfully overcome. As revealed by density functional theory calculations, the N2 dissociation becomes exothermic over the cobalt single cluster upon the strong electron backdonation from metal to the N2 antibonding orbitals. The energy barrier of the positively shifted rate-determining step is also greatly reduced. At the same time, advanced sampling molecular dynamics simulations indicate a barrier-less process of the N2 approaching the active sites that greatly facilitates the mass transfer. With suitable thermodynamic and dynamic property, a high ammonia yield rate of 76.2 μg h–1 mg and superior Faradaic efficiency of 52.9% were simultaneously achieved.  相似文献   

15.
The global deposition of superheavy pyrite (pyrite isotopically heavier than coeval seawater sulfate in the Neoproterozoic Era and particularly in the Cryogenian Period) defies explanation using the canonical marine sulfur cycle system. Here we report petrographic and sulfur isotopic data (δ34Spy) of superheavy pyrite from the Cryogenian Datangpo Formation (660–650 Ma) in South China. Our data indicate a syndepositional/early diagenetic origin of the Datangpo superheavy pyrite, with 34S-enriched H2S supplied from sulfidic (H2S rich) seawater. Instructed by a novel sulfur-cycling model, we propose that the emission of 34S-depleted volatile organosulfur compounds (VOSC) that were generated via sulfide methylation may have contributed to the formation of 34S-enriched sulfidic seawater and superheavy pyrite. The global emission of VOSC may be attributed to enhanced organic matter production after the Sturtian glaciation in the context of widespread sulfidic conditions. These findings demonstrate that VOSC cycling is an important component of the sulfur cycle in Proterozoic oceans.  相似文献   

16.
Cell migration is an essential process involved in the development and maintenance of multicellular organisms. Electric fields (EFs) are one of the many physical and chemical factors known to affect cell migration, a phenomenon termed electrotaxis or galvanotaxis. In this paper, a microfluidics chip was developed to study the migration of cells under different electrical and chemical stimuli. This chip is capable of providing four different strengths of EFs in combination with two different chemicals via one simple set of agar salt bridges and Ag/AgCl electrodes. NIH 3T3 fibroblasts were seeded inside this chip to study their migration and reactive oxygen species (ROS) production in response to different EF strengths and the presence of β-lapachone. We found that both the EF and β-lapachone level increased the cell migration rate and the production of ROS in an EF-strength-dependent manner. A strong linear correlation between the cell migration rate and the amount of intracellular ROS suggests that ROS are an intermediate product by which EF and β-lapachone enhance cell migration. Moreover, an anti-oxidant, α-tocopherol, was found to quench the production of ROS, resulting in a decrease in the migration rate.  相似文献   

17.
Hydrothermal fluid is essential for transporting metals in the crust and mantle. To explore the potential of Cu isotopes as a tracer of hydrothermal-fluid activity, Cu-isotope fractionation factors between Cl-bearing aqueous fluids and silicate magmas (andesite, dacite, rhyolite dacite, rhyolite and haplogranite) were experimentally calibrated. Fluids containing 1.75–14 wt.% Cl were mixed together with rock powders in Au95Cu5 alloy capsules, which were equilibrated in cold-seal pressure vessels for 5–13 days at 800–850°C and 2 kbar. The elemental and Cu-isotopic compositions of the recovered aqueous fluid and solid phases were analyzed by (LA-) ICP–MS and multi-collector inductively coupled plasma mass spectrometry, respectively. Our experimental results show that the fluid phases are consistently enriched in heavy Cu isotope (65Cu) relative to the coexisting silicates. The Cu-isotope fractionation factor (Δ65CuFLUID-MELT) ranges from 0.08 ± 0.01‰ to 0.69 ± 0.02‰. The experimental results show that the Cu-isotopic fractionation factors between aqueous fluids and silicates strongly depend on the Cu speciation in the fluids (e.g. CuCl(H2O), CuCl2– and CuCl32−) and silicate melts (CuO1/2), suggesting that the exsolved fluids may have higher δ65Cu than the residual magmas. Our results suggest the elevated δ65Cu values in Cu-enriched rocks could be produced by addition of aqueous fluids exsolved from magmas. Together with previous studies on Cu isotopes in the brine and vapor phases of porphyry deposits, our results are helpful for better understanding Cu-mineralization processes.  相似文献   

18.
水泥生产碳排放测算的国内外方法比较及借鉴   总被引：4，自引：3，他引：1  
魏丹青  赵建安  金迁致 《资源科学》2012,34(6):1152-1159
进行国内外水泥生产碳排放测算方法比较分析,对制定中国水泥碳排放测算标准具有重要借鉴价值。本文在详细研究国内外相关标准和体系的基础上,从水泥生产碳排放测算的运营边界与范围、工艺排放、燃料排放、间接排放4个方面,对相关的方法进行了比较分析和准确性及适用性的讨论,并得到以下结论：①各标准和体系的整体框架和计算过程大致相同,而在运营边界的范围、排放过程中的具体计算方法以及CO2排放系数上存在差异;②运营边界的范围一般参考WBCSD-CS《I水泥行业二氧化碳减排议定书》,各国可根据实际情况来选择报告选项;③水泥生产工艺CO2排放计算熟料法和生料法理论上是等同的,在国家层面的计算一般采用熟料法;④燃料燃烧CO2排放的计算方法一般依据物料平衡法中的发热值来计算,我国应采用低位发热值来计算;⑤间接排放在国家层面只计算外购电力的二氧化碳排放和余热发电项目的抵减量。在上述基础上,本文提出了构建我国水泥生产碳排放系数体系的基本测算流程,并对下一步的研究方向进行了探讨。  相似文献   

19.
Water is transported to Earth''s interior in lithospheric slabs at subduction zones. Shallow dehydration fuels hydrous island arc magmatism but some water is transported deeper in cool slab mantle. Further dehydration at ∼700 km may limit deeper transport but hydrated phases in slab crust have considerable capacity for transporting water to the core-mantle boundary. Quantifying how much remains the challenge.

Water can have remarkable effects when exposed to rocks at high pressures and temperatures. It can form new minerals with unique properties and often profoundly affects the physical, transport and rheological properties of nominally anhydrous mantle minerals. It has the ability to drastically reduce the melting point of mantle rocks to produce inviscid and reactive melts, often with extreme chemical flavors, and these melts can alter surrounding mantle with potential long-term geochemical consequences. At the base of the mantle, water can react with core iron to produce a super-oxidized and hydrated phase, FeO2Hx, with the potential to profoundly alter the mantle and even the surface and atmosphere redox state, but only if enough water can reach such depths [1].Current estimates for bulk mantle water content based on the average H2O/Ce ratio of oceanic basalts from melt inclusions and the most un-degassed basalts, coupled with mass balance constraints for Ce, indicate a fraction under one ocean mass [2], a robust estimate as long as the basalts sampled at the surface tap all mantle reservoirs. The mantle likely contains some primordial water but given that the post-accretion Earth was very hot, water has low solubility and readily degasses from magma at low pressures, and its solubility in crystallizing liquidus minerals is also very low, the mantle just after accretion may have been relatively dry. Thus, it is plausible that most or even all of the water in the current mantle is ‘recycled’, added primarily by subduction of hydrated lithospheric plates. If transport of water to the core–mantle boundary is an important geological process with planet-scale implications, then surface water incorporated into subducting slabs and transported to the core–mantle boundary may be a requirement.Water is added to the basaltic oceanic crust and peridotitic mantle in lithospheric plates (hereafter, slab crust and slab mantle, respectively) at mid-ocean ridges, at transform faults, and in bending faults formed at the outer rise prior to subduction [3]. Estimates vary but about 1 × 1012 kg of water is currently subducted each year into the mantle [4], and at this rate roughly 2–3 ocean masses could have been added to the mantle since subduction began. However, much of this water is returned to the surface through hydrous magmatism at convergent margins, which itself is a response to slab dehydration in an initial, and large, release of water. Meta-basalt and meta-sediments comprising the slab crust lose their water very efficiently beneath the volcanic front because most slab crust geotherms cross mineral dehydration or melting reactions at depths of less than 150 km, and even if some water remains stored in minerals like lawsonite in cooler slabs, nearly complete dehydration is expected by ∼300 km [5].Peridotitic slab mantle may have much greater potential to deliver water deeper into the interior. As shown in Fig. 1a, an initial pulse of dehydration of slab mantle occurs at depths less than ∼200 km in warmer slabs, controlled primarily by breakdown of chlorite and antigorite when slab-therms cross a deep ‘trough’, sometimes referred to as a ‘choke point’, along the dehydration curve (Fig. 1a) [6]. But the slab mantle in cooler subduction zones can skirt beneath the dehydration reactions, and antigorite can transform directly to the hydrated alphabet silicate phases (Phases A, E, superhydrous B, D), delivering perhaps as much as 5 wt% water in locally hydrated regions (e.g. deep faults and fractures in the lithosphere) to transition zone depths [6]. Estimates based on mineral phase relations in the slab crust and the slab mantle coupled with subduction zone thermal models suggest that as much as 30% of subducted water may have been transported past the sub-volcanic dehydration front and into the deeper mantle [4], although this depends on the depth and extent of deep hydration of the slab mantle, which is poorly constrained. Coincidentally, this also amounts to about one ocean mass if water subduction rates have been roughly constant since subduction began, a figure tantalizingly close to the estimated mantle water content based on geochemical arguments [2]. But what is the likely fate of water in the slab mantle in the transition zone and beyond?Open in a separate windowFigure 1.(a) Schematic phase relations in meta-peridotite modified after [6,10,12]. Slab geotherms are after those in [4]. Cold slabs may transport as much as 5 wt% water past ‘choke point 1’ in locally hydrated regions of the slab mantle, whereas slab mantle is dehydrated in warmer slabs. Colder slab mantle that can transport water into the transition zone will undergo dehydration at ‘choke point 2’. How much water can be transported deeper into the mantle and potentially to the core depends on the dynamics of fluid/melt segregation in this region. (b) Schematic showing dehydration in the slab mantle at choke point 2. Migration of fluids within slab mantle will result in water dissolving into bridgmanite and other nominally anhydrous phases with a bulk storage capacity of ∼0.1 wt%, potentially accommodating much or all of the released water. Migration of fluids out of the slab into ambient mantle would also hydrate bridgmanite and other phases and result in net fluid loss from the slab. Conversely, migration of hydrous fluids into the crust could result in extensive hydration of meta-basalt with water accommodated first in nominally anhydrous phases like bridgmanite, Ca-perovskite and NAL phase, but especially in dense SiO2 phases (stishovite and CaCl2-type) that can host at least 3 wt% water (∼0.6 wt% in bulk crust).Lithospheric slabs are expected to slow down and deform in the transition zone due to the interplay among the many factors affecting buoyancy and plate rheology, potentially trapping slabs before they descend into the lower mantle [7]. If colder, water-bearing slabs heat up by as little as a few hundred degrees in the transition zone, hydrous phases in the slab mantle will break down to wadsleyite or ringwoodite-bearing assemblages, and a hydrous fluid (Fig. 1a). Wadselyite and ringwoodite can themselves accommodate significant amounts of water and so hydrated portions of the slab mantle would retain ∼1 wt% water. A hydrous ringwoodite inclusion in a sublithospheric diamond with ∼1.5 wt% H2O may provide direct evidence for this process [8].But no matter if slabs heat up or not in the transition zone, as they penetrate into the lower mantle phase D, superhydrous phase B or ringwoodite in the slab mantle will dehydrate at ∼700–800 km due to another deep trough, or second ‘choke point’, transforming into an assemblage of nominally anhydrous minerals dominated by bridgmanite (∼75 wt%) with, relatively, a much lower bulk water storage capacity (< ∼0.1 wt%) [9] (Fig. 1a). Water released from the slab mantle should lead to melting at the top of the lower mantle [10], and indeed, low shear-wave velocity anomalies at ∼700–800 km below North America may be capturing such dehydration melting in real time [11].The fate of the hydrous fluids/melts released from the slab in the deep transition zone and shallow lower mantle determines how much water slabs can carry deeper into the lower mantle. Presumably water is released from regions of the slab mantle where it was originally deposited, like the fractures and faults that formed in the slab near the surface [3]. If hydrous melts can migrate into surrounding water-undersaturated peridotite within the slab, then water should dissolve into bridgmanite and coexisting nominally anhydrous phases (Ca-perovskite and ferropericlase) until they are saturated (Fig. 1b). And because bridgmanite (water capacity ∼0.1 wt%) dominates the phase assemblage, the slab mantle can potentially accommodate much or all of the released water depending on details of how the hydrous fluids migrate, react and disperse. If released water is simply re-dissolved into the slab mantle in this way then it could be transported deeper into the mantle mainly in bridgmanite, possibly to the core–mantle boundary. Water solubility in bridgmanite throughout the mantle pressure-temperature range is not known, so whether water would partially exsolve as the slab moves deeper stabilizing a melt or another hydrous phase, or remains stable in bridgmanite as a dispersed, minor component, remains to be discovered.Another possibility is that the hydrous fluids/melts produced at the second choke point in the slab mantle at ∼700 km migrate out of the slab mantle, perhaps along the pre-existing fractures and faults where bridgmanite-rich mantle should already be saturated, and into either oceanic crust or ambient mantle (Fig. 1b). If the hydrous melts move into ambient mantle, water would be consumed by water-undersaturated bridgmanite, leading to net loss of water from the slab to the upper part of the lower mantle, perhaps severely diminishing the slab’s capacity to transport water to the deeper mantle and core. But what if the water released from slab mantle migrates into the subducting, previously dehydrated, slab crust?Although slab crust is expected to be largely dehydrated in the upper mantle, changes in its mineralogy at higher pressures gives it the potential to host and carry significant quantities of water to the core–mantle boundary. Studies have identified a number of hydrous phases with CaCl2-type structures, including δ-AlOOH, ϵ-FeOOH and MgSiO2(OH)2 (phase H), that can potentially stabilize in the slab crust in the transition zone or lower mantle. Indeed, these phases likely form extensive solid solutions such that an iron-bearing, alumina-rich, δ-H solid solution should stabilize at ∼50 GPa in the slab crust [12], but only after the nominally anhydrous phases in the crust, (aluminous bridgmanite, stishovite, Ca-perovskite and NAL phase) saturate in water. Once formed, the δ-H solid solution in the slab crust may remain stable all the way to the core mantle boundary if the slab temperature remains well below the mantle geotherm otherwise a hydrous melt may form instead [12] (Fig. 1a). But phase δ-H solid solution and the other potential hydrated oxide phases, intriguing as they are as potential hosts for water, may not be the likely primary host for water in slab crust. Recent studies suggest a new potential host for water—stishovite and post-stishovite dense SiO2 phases [13,14].SiO2 minerals make up about a fifth of the slab crust by weight in the transition zone and lower mantle [15] and recent experiments indicate that the dense SiO2 phases, stishovite (rutile structure—very similar to CaCl2 structure) and CaCl2-type SiO2, structures that are akin to phase H and other hydrated oxides, can host at least 3 wt% water, which is much more than previously considered. More importantly, these dense SiO2 phases apparently remain stable and hydrated even at temperatures as high as the lower mantle geotherm, unlike other hydrous phases [13,14]. And as a major mineral in the slab crust, SiO2 phases would have to saturate with water first before other hydrous phases, like δ-H solid solution, would stabilize. If the hydrous melts released from the slab mantle in the transition zone or lower mantle migrate into slab crust the water would dissolve into the undersaturated dense SiO2 phase (Fig. 1b). Thus, hydrated dense SiO2 phases are possibly the best candidate hosts for water transport in slab crust all the way to the core mantle boundary due to their high water storage capacity, high modal abundance and high-pressure-temperature stability.Once a slab makes it to the core–mantle boundary region, water held in the slab crust or the slab mantle may be released due to the high geothermal gradient. Heating of slabs at the core–mantle boundary, where temperatures may exceed 3000°C, may ultimately dehydrate SiO2 phases in the slab crust or bridgmanite (or δ-H) in the slab mantle, with released water initiating melting in the mantle and/or reaction with the core to form hydrated iron metal and super oxides, phases that may potentially explain ultra-low seismic velocities in this region [1,10]. How much water can be released in this region from subducted lithosphere remains a question that is hard to quantify and depends on dynamic processes of dehydration and rehydration in the shallower mantle, specifically at the two ‘choke points’ in the slab mantle, processes that are as yet poorly understood. What is clear is that subducting slabs have the capacity to carry surface water all the way to the core in a number of phases, and possibly in a phase that has previously seemed quite unlikely, dense SiO2.  相似文献   

20.
The supramolecular chemistry of nanoclusters is a flourishing area of nano-research; however, the controllable assembly of cluster nano-building blocks in different arrays remains challenging. In this work, we report the hierarchical structural complexity of atomically precise nanoclusters in micrometric linear chains (1D array), grid networks (2D array) and superstructures (3D array). In the crystal lattice, the Ag29(SSR)12(PPh3)4 nanoclusters can be viewed as unassembled cluster dots (Ag29–0D). In the presence of Cs+ cations, the Ag29(SSR)12 nano-building blocks are selectively assembled into distinct arrays with different oxygen-carrying solvent molecules―Cs@Ag29(SSR)12(DMF)x as 1D linear chains (Ag29–1D), Cs@Ag29(SSR)12(NMP)x as 2D grid networks (Ag29–2D), and Cs@Ag29(SSR)12(TMS)x as 3D superstructures (Ag29–3D). Such self-assemblies of these Ag29(SSR)12 units have not only been observed in their crystalline state, but also in their amorphous state. Due to the diverse surface structures and crystalline packing modes, these Ag29-based assemblies manifest distinguishable optical absorptions and emissions in both solutions and crystallized films. Furthermore, the surface areas of the nanocluster crystals are evaluated, the maximum value of which occurs when the cluster nano-building blocks are assembled into 2D arrays (i.e. Ag29–2D). Overall, this work presents an exciting example of the hierarchical assembly of atomically precise nanoclusters by simply controlling the adsorbed molecules on the cluster surface.  相似文献   

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