矽卡岩化作用排碳到大气中的途径SCIEI北大核心CSCD

鉴于大陆岩浆弧上盘广泛分布有碳酸盐岩,岩体上升侵位过程使其碳通量明显的高于洋内岛弧,从而有可能影响着地质历史长时间尺度(百万年)的气候变化。陆弧碳的排放可分为两部分,一部分是与喷出岩相关的火山作用,另一部分是与侵入岩相关的脱碳过程。侵入岩的体量一般是喷出岩体量的10倍以上,由此与侵入岩有关的脱碳作用释放的碳通量不可忽视,可能类似与喷出岩有关的火山作用释放的碳通量甚至更大。火山作用能够将气体直接排入大气中,因此在以往的工作中研究较为充分。但与侵入岩相关的脱碳过程如:矽卡岩化,发生在地下,关于地下脱碳过程如何将碳释放到大气中还缺少详细的工作。本文,通过对比研究火山地区和非火山地区与侵入岩相关的深部脱碳过程,发现深部碳可以通过区域断层系统、地下水系统和热泉、火山通道、隐爆角砾岩筒、高频率的岩浆热液事件等途径瞬时地释放进入大气。通过初步估算,发现单个矽卡岩矿化事件所产生的二氧化碳通量(0.02~0.2Mt/yr)能够与目前地球绝大部分的普通单个火山的碳通量(<0.5Mt/yr)类比。因此岩浆深部脱碳过程对大气的影响至少与火山相当,其对气候的影响不可忽视。     

从碳源到碳汇：大陆弧演化过程中岩浆与剥蚀作用对长期碳循环的影响

在百万年时间尺度上,大气、海洋中的二氧化碳浓度(P_CO2,二氧化碳分压)和长期变化主要受岩浆-变质脱碳作用和硅酸盐风化作用(消耗二氧化碳)控制。因此,地球表层主要构造活动带的构造-岩浆活动对长期碳循环具有重要的驱动作用。本文在总结已发表文献的基础上,系统评估了大陆弧,尤其是晚白垩世大陆弧的岩浆作用和剥蚀作用的碳通量,并以此为依据探讨了大陆弧演化对于全球长期碳循环的影响。大陆弧岩浆作用以周期性(几十万年至一百百万年)岩浆爆发(magmatic flare-ups)为特征。在一个周期内,大规模岩浆喷发会导致CO₂排放量大幅度增加,促进温室效应。但同时,大规模的岩浆作用又会导致地壳增厚和和地表抬升,从而促进剥蚀作用、提高化学风化通量,进而增加CO₂消耗量。对于单个的大陆弧来说,在其演化的不同阶段对于碳循环扮演着不同的角色：演化早期由于岩浆作用起主导作用,表现为净碳源;而在岩浆作用减弱或停止后,由于剥蚀作用的持续进行,表现为净碳汇。因此,从长周期和全球尺度上讲,大陆弧岩浆活动表现的“碳源属性”受到化学风化作...     

俯冲带脱碳和固碳作用过程

俯冲带作为联系地表和地球深部系统的纽带,不仅是将地表碳带入地球深部的主要通道,也是地表物质和地球深部物质发生交换的重要场所。俯冲作用可以将地表碳以有机碳或无机碳酸盐矿物等形式带入地球深部,再通过火山作用或去气作用返回到地表系统。俯冲带深部碳循环控制着地表碳通量变化,对于研究全球气候变化和地球宜居环境具有重要意义。本文结合前人的相关研究成果,综合探讨了俯冲带的脱碳机制及固碳作用过程。俯冲带脱碳机制主要有变质反应脱碳、流体溶解脱碳和熔融作用脱碳。从俯冲板块释放的含碳流体不一定都会迁移返回地表,有一部分含碳流体在迁移演化过程中会和围岩发生反应,形成不易迁移的其他含碳相(碳酸盐、石墨或金刚石)而重新固存在俯冲板块以及上覆地幔楔中(固碳作用),进而影响碳在不同储库中的含量变化,在计算俯冲带释放碳通量时需要考虑这一过程的影响。     

俯冲板片的脱碳机制及通量估算:问题与进展SCIEI北大核心CSCD

地球98%以上的碳赋存在地球深部地幔和地核中。地球深部储库(地幔和地核)中的碳以各类岩浆作用释放到地表,而地球表层系统(大气圈、水圈、生物圈)中的碳又可以伴随板块俯冲作用进入地球深部地幔。然而俯冲过程中不同的脱碳机制会将俯冲板片中部分乃至全部碳带出板片,而后经由岛弧岩浆作用、流体扩散作用等途径返回地表。因此,板片俯冲过程中的脱碳机制及其通量深刻地影响了地质时间尺度中地表系统的二氧化碳浓度,进而改变地球的宜居性。本文总结了目前主流观点认可的五种俯冲板片脱碳机制:变质反应脱碳、流体溶解脱碳、熔融脱碳、底辟脱碳和氧化还原脱碳。另一方面,目前对于俯冲板片各种脱碳机制对应的脱碳效率还有很大的争议,因此本文进一步梳理了板片俯冲过程中不同脱碳机制相关的通量估算的研究进展与存在的问题,建议将来综合多种方法对比研究俯冲带碳循环问题,以期在俯冲带深部碳循环过程和通量方面取得突破性进展。     

地球深部碳循环与碳酸岩成因研究进展

地球深部碳循环是指地球表层的碳在俯冲带进入地幔深部,然后通过岩浆或者脱气作用再把地幔中的碳释放到地球表层系统中的过程。人类对地球深部碳赋存形式和储量、不同储库的交换方式和交换量尚缺乏清晰认识,近年来随着分析技术的发展和研究的深入,深部碳循环的研究日益丰富。本文总结了地幔中碳赋存状态、地球深部碳储量、碳进出地幔方式及通量、俯冲带碳的行为和碳酸岩成因及成矿方面的研究。地幔中碳赋存形式多样且主要受地球深部压力及氧逸度控制。相平衡实验和热动力学计算发现碳酸盐化榴辉岩在300～600 km发生部分熔融,交代地幔橄榄岩形成碳酸盐化地幔橄榄岩。碳酸盐化地幔橄榄岩的熔融又会形成碳酸岩熔体,这说明俯冲再循环物质可能对碳酸岩的成因起重要作用。碳酸岩是研究深部碳循环的良好载体,其源区特征、岩浆演化过程对示踪碳在地幔和地壳过程中的迁移至关重要。虽然深部碳循环在碳赋存形式、碳储量及通量、俯冲带碳的流变行为和碳酸岩成因对深部碳循环的启示方面已经取得了较大的研究进展,但仍有大量的科学问题亟待解决,如:沉积碳酸盐岩再循环进入地球深部后的行为、俯冲带板片流体地球化学行为、俯冲带流体氧逸度特征等,将来有必要重点开展深入研...     

碳的固定、运输、转移和排放过程：对地球深部碳循环的启示

俯冲带背景下,碳在促进岩石熔融、岩浆起源和演化、地球深部的岩石学及动力学等过程中扮演的角色尤为重要。碳的存在形式是由温度、压力、氧逸度以及溶流体的性质等条件控制的,这些以不同形式存在的碳随板块俯冲到达地球深部,而后又通过火山作用等脱气过程被返回地表,便形成了地球深部碳循环过程。固碳和脱碳反应是影响碳在固体地球、海洋和大气圈转换的主要反应。碳的固定包括硅酸盐风化作用、玄武质洋壳的热液交代、海沟外隆的蛇纹石化、有机碳的埋藏和逆风化作用等过程;碳的运输包括沉积成因和交代成因沉积物的俯冲过程;当俯冲碳被输送到地球内部时,它可能被保留在板块内,或者转移到地幔楔中,又或再被循环到地球深部,这将取决于特定构造环境的温压条件和氧化还原状态等。碳的排放包括火山作用、弧前扩散脱气、溶解脱碳、变质反应脱碳和熔融脱碳等过程,这些过程将俯冲下去的碳再一次返回大气,能够平衡俯冲带的碳输入。本文系统地总结了地表及地球深部碳的固定、运输、转移和排放过程中碳的存在形式、碳的迁移和变化以及相关碳通量计算值,分析了目前碳通量差异的原因并阐述了今后需要深入研究的一些关键科学问题。另外,工业革命后,人为成因的CO_2<...     

接触变质条件下变碳酸岩CO2释放与定量计算——以郯庐断裂南段肥东县双山为例

为了探讨接触变质带内变碳酸盐岩变质过程CO₂释放的数量和排放CO₂的物理、化学及地质条件,根据递进变质反应和时间积分流体通量模型,定量分析和定量计算了双山地区变碳酸盐岩在接触变质作用中释放CO₂的通量。计算结果得到CO₂的通量值为0.729×104~2.446×104 mol/cm2,CO₂的来源以接触变质反应释放为主;CO₂的生成释放与变质程度呈正相关关系。自白云石带至方解石带变质流体中X_CO2不断升高,但钙铝榴石带由于岩浆水影响,流体通量最高而X_CO2急剧下降。      

高压下深海碳循环的过程及其对生命活动的影响

目前约25%化石燃料来源的CO₂被海洋吸收,缓解了人类活动对气候变化的影响。海洋通过多个概念的碳泵将大气中的CO₂输送到深海。深海高压和低温的特点有利于CO₂溶解,目前已经储存了相当于大气含量50倍的无机碳,另外,深海沉积物中还储存有大量甲烷水合物。认识深海中的碳循环过程,对于保护海洋固碳能力、开发固碳潜力有重要意义。总结了国内外在海洋碳库、碳输送研究方面的进展,重点讨论了深海C元素转化循环的过程以及高压对生命活动的影响。微生物驱动了深海碳循环,大部分浮游植物所包含的有机碳在沉降过程中被微生物矿化成CO₂以及转化为难降解的有机碳,使深海成为巨大的、长周转时间的有机碳库; 高压能提高古菌甲烷厌氧氧化的活性,提升屏蔽海底甲烷释放的能力,同时,高压下氧化甲烷的过程中不仅产生碳酸氢盐,还产生可支持异养生物的乙酸,因此,全球甲烷厌氧氧化的通量可能被低估; 高压下细胞代谢额外产生的氨,可作为氨氧化古菌固定无机碳的潜在能量来源。总之,研究现在以及未来的人类活动对深海碳循环过程的影响以及环境效应,评估应用深海作为地球工程技术平台封存CO₂的可能性,都迫切需要加深对碳循环在内的深海元素循环的认识。     

藏南碳酸岩脉成因及其气候效应

始新世末期以来,全球大气CO₂浓度持续下降,但长期以来不清楚为何这一时期全球大气CO₂浓度下降,巨量的大气CO₂赋存于何处。深入研究该问题有助于准确理解未来大气CO₂浓度变化的趋势,特别是有助于进一步评估人类自身碳排放的后果。这一时期,小印度陆块持续与大亚洲陆块汇聚,导致了以喜马拉雅为代表的山脉群和青藏高原的形成。很早就有学者从地球表层碳循环的角度提出了"青藏高原的隆升导致了全球变冷"的观点,但这一观点既没有解释清楚"巨量大气CO₂到何处去"的问题,也没有讨论青藏高原本身向大气圈排放CO₂等问题,因此该观点最近受到了强烈的质疑。这些激烈的争论充分反映了传统的地球表层碳循环研究已不能充分满足当前社会的需求。本文从深部碳循环这个视角重新探讨青藏高原在全球碳循环中的作用。在印度与亚洲陆块持续汇聚期间,以喜马拉雅为代表的巨型山脉快速崛起,然后持续遭受化学风化作用,大量消耗大气CO₂。化学风化的产物堆积在喜马拉雅山前的前陆盆地内,形成了巨量含新生碳酸盐矿物和有机碳的西瓦里克沉积杂岩,随后新生的西瓦里克杂岩又随持续平板俯冲的印度陆壳被带入青藏高原内部,与平板俯冲的印度陆壳共同经历高温变质作用。俯冲板片内的(黑)云母等含水矿物发生脱水,形成花岗岩浆。花岗岩浆再与俯冲的西瓦里克杂岩内的碳酸盐岩发生交代反应,释放出含钙、镁离子、以CO₂和水为主的高温流体,本文称其为壳源火成碳酸岩浆。碳酸岩浆沿张性裂隙上侵、冷凝之后形成藏南的碳酸岩脉。虽然青藏高原内部的火山、温泉等均向大气圈排放CO₂,但所排放的碳均为再循环来自大气圈的碳,并且排放量略小于吸收量,否则消耗大气CO₂所新生的碳酸岩脉就不会在青藏高原内部保存下来。藏南大量晚新生代碳酸岩脉的发现充分说明了喜马拉雅山脉和藏南高原是一个巨大的碳储库,在其形成过程中将巨量大气CO₂转化为流体(岩浆)的形式封存于青藏高原内部,从而大幅降低了大气CO₂浓度,最终导致了全球变冷。上述过程充分说明,大气CO₂浓度的变化实质上是受控于地球内部的构造运动。进一步可推论出,"全球变化"只是一个自然现象,虽然它有独特的运行轨迹,但与人类的碳排放量无因果关系。     

松辽盆地幔源成因CO2气藏分布规律及控制因素

通过深部15s大剖面解译和典型CO₂气藏解剖,深入剖析了松辽盆地幔源成因CO₂气藏分布规律及控制因素。纵向上CO₂气主要富集在营城组火山岩和泉头组三、四段砂岩储层中。与深部热流体（热流底辟体和侵入体）直接相连的、控陷的、控制火山通道的铲式基底断裂控制着CO₂气藏呈串珠状分布的特征,断裂走向拐点、端点和侧列叠覆区控制了CO₂气出气点。深部热流体是CO₂气藏主要的气源。与之相衔接的基底断裂活动导致幔源岩浆充注盆地,同时携带的CO₂气释放进入早期喷发的火山岩中形成营城组火山岩CO₂气藏。基于控藏断裂多为断陷期和泉头组晚期-青山口组早期活动的基底断裂,推测营城组CO₂气藏主要成藏时期为泉头组晚期-青山口组早期。松辽盆地强烈的构造反转发生在明水组末期,挤压应力自南南东向北北西方向衰减的过程中,基底断裂复活形成正反转断层,CO₂气从热流底辟体中脱出上运而聚集在反转构造带上,断层式反转构造南部比北部发育,西部比东部发育,因此松辽盆地南部比北部、西部比东部CO₂气更富集。     

中国大陆新生代典型火山区温室气体释放的规模及其成因

火山活动能够将地球深部的碳输送到大气圈,是地质碳排放和深部碳循环的重要形式.火山作用不仅在喷发期能够释放大量温室气体,而且在休眠期也能释放巨量的温室气体.在全球变暖的背景下,定量化地研究火山活动对大气圈温室气体含量增加的贡献具有至关重要的意义.本文利用密闭气室法等该领域国际先进的测试技术,测量并计算了长白山、腾冲、五大连池及青藏高原南部的羊八井等典型火山区的温室气体释放规模.结果显示,我国大陆新生代典型火山区向大气圈输送的温室气体总通量约为8.13×10⁶t·a^-1,接近10⁷t·a^-1级别,相当于全球火山活动导致的温室气体(主要为CO₂)释放总量的6%左右.太平洋构造域火山区的温室气体在释放通量与总量方面均低于特提斯构造域,并且太平洋构造域火山气体的地壳混染程度较低,显示出大洋俯冲带与大陆俯冲带火山区温室气体释放的成因差异.     

非火山地热区玛旁雍热田土壤CO_2脱气研究

地热活动是地球脱气的重要形式之一,其过程常伴随大量温室气体排放。选取非火山地热区西藏玛旁雍热田作为研究对象,基于菲克扩散定律对地热田区土壤CO_2脱气量进行评估。结果表明:该区一般土壤CO_2脱气通量为0.167~0.771 kg/(m2·a),含喷气孔区域土壤CO_2脱气通量为2.054~7.877 kg/(m2·a),含喷气孔地区的土壤CO_2脱气通量是一般土壤脱气量的18.9倍;与全球火山区土壤脱气量(0.001~2.25 Mt/(m2·a))相比,其值显著偏低;但比青藏高原高寒草原生态系统土壤的CO_2排放量(187.46 g/(m2·a))大。结合区域地质背景推测地热系统中的CO_2含量主要来源于岩浆脱气和热液同长石等围岩矿物的蚀变反应。区内土壤CO_2的低脱气通量受透水性较差的碎屑岩沉积盖层约束。     

Tectonics and Topography of the Tibetan Plateau in Early Miocene

Early Miocene stratigraphy, major structural systems, magmatic emplacement, volcanic eruption, vegetation change and paleo-elevation were analyzed for the Tibetan Plateau after regional geological mapping at a scale of 1:250,000 and related researches, revealing much more information for tectonic evolution and topographic change of the high plateau caused by Indian-Asian continental collision. Lacustrine deposits of dolostone, dolomite limestone, limestone, marl, sandstone and conglomerate of weak deformation formed extensively in the central Tibetan Plateau, indicating that vast lake complexes as large as 100,000–120,000 km2 existed in the central plateau during Early Miocene. Sporopollen assemblages contained in the lacustrine strata indicate the disappearance of most tropical-subtropical broad-leaved trees since Early Miocene and the flourishing of dark needleleaved trees during Early Miocene. Such vegetation changes adjusted for latitude and global climate variations demonstrate that the central Tibetan Plateau rose to ca. 4,000–4,500 m and the northeastern plateau uplifted to ca. 3,500–4,000 m before the Early Miocene. Intensive thrust and crustal thickening occurred in the areas surrounding central Tibetan Plateau in Early Miocene, formed Gangdise Thrust System(GTS) in the southern Lhasa block, Zedong-Renbu Thrust(ZRT) in the northern Himalaya block, Main Central Thrust(MCT) and Main Boundary Thrust(MBT) in the southern Himalaya block, and regional thrust systems in the Qaidam, Qilian, West Kunlun and Songpan-Ganzi blocks. Foreland basins formed in Early Miocene along major thrust systems, e.g. the Siwalik basin along MCT, Yalung-Zangbu Basin along GTS and ZRT, southwestern Tarim depression along West Kunlun Thrust, and large foreland basins along major thrust systems in the northeastern margin of the plateau. Intensive volcanic eruptions formed in the Qiangtang, Hoh-Xil and Kunlun blocks, porphyry granites and volcanic eruptions formed in the Nainqentanglha and Gangdise Mts., and leucogranites and granites formed in the Himalaya and Longmenshan Mts. in Early Miocene. The K2O weight percentages of Early Miocene magmatic rocks in the Gangdise and Himlayan Mts. are found to increase with distance from the MBT, indicating the genetic relationship between regional magmatism and subduction of Indian continental plate in Early Miocene.     

Observation of CO2 degassing in Tianshuihai Lake Basin of the Qinghai-Tibetan Plateau

A large volume of underground gas in the permafrost region of the Qinghai-Tibetan Plateau has been identified. Although many studies were performed to investigate the soil organic carbon dynamics and Earth degassing in volcanic areas, this is the first report of a large amount of non-volcanic CO₂ contained in permafrost. The gas was mostly CO₂ (81.76 vol. %) and nitrogen (14.59 vol. %). The gas composition and the evidence from carbon stable isotope values (?23.9 ‰, PDB) suggested that the gases possibly had a deep origin. The gas emissions may be triggered by permafrost degradation, which means mitigation of the barrier effect of permafrost for the gas. In addition, plate tectonic processes may also lead to gas emissions, as the tectonic activity is strong in the area. Therefore, particular attention should be paid to the underground gases in the study of global change and permafrost degradation.     

蛇纹岩化对洋中脊超基性岩热液硫化物成矿的影响:来自青藏高原德尔尼铜矿床的启示

洋中脊超基性岩热液成矿系统通常与洋底核杂岩构造有关,多发育大型矿床,具有巨大的资源前景。然而,受大洋调查取样手段的限制,超基性岩蛇纹岩化对成矿的影响仍需进一步研究。德尔尼铜矿床是地质历史上该类矿床的典型案例,对于理解其成矿模式,以及大洋硫化物勘探具有指导意义。本文选取德尔尼铜矿床块状硫化物样品进行黄铁矿的S同位素分析,结果表明其δ³⁴S值主要分布在-0.4‰～+6.3‰。结合前人研究发现,形成于深部网脉状、条带状矿石中的δ³⁴S值为负值,而经历表层喷流和破碎作用的块状和角砾状矿石中的δ³⁴S值为正值,二者呈对称分布,这主要是由于还原条件下岩浆排气产生的SO_2和H_2S动态平衡并逐渐沉淀S^2-,表明蛇纹岩化提供的还原环境对热液系统演化产生了重要影响。然而,磁黄铁矿和矿床Ni的分布指示成矿物质中超基性岩的贡献较小,主要物质来源是洋中脊深部的基性岩浆,通过热液循环将物质运移至海底并喷流成矿。对比现今超基性岩赋矿的高温热液硫化物矿床,德尔尼铜矿床形成温度更低,代表了超基性岩赋矿热液硫化物中的中温端元,表明在距离拆离面一定距离(约2～4km)的位置也可能形成大型的热液硫化物矿床,这对于现今洋中脊热液硫化物勘探具有一定的指导意义。     

中国东北地区显生宙岩浆作用和洋-陆格局及其与气候演变的关系SCIEI北大核心CSCD

显生宙期间,地球经历了温室-冰室气候的周期性交替变化。在数百万年的时间尺度,这种古气候的转变被认为是碳源和碳汇过程耦合的结果,但一直以来关于两者贡献程度的认识尚不明确。通过全球统计分析,不同学者提出大陆弧火山脱气模型和热带弧-陆碰撞模型用于解释整个显生宙古气候的演变,分别强调了碳源和碳汇的一级控制作用。为了检验上述模型,更好地理解古气候的转变机制和演化细节,本文系统总结了中国东北地区显生宙岩浆作用-矽卡岩型矿床的时空展布和构造背景,以及弧-陆碰撞的时代、规模和古地理位置,通过数据统计和作图对比,发现东北地区岩浆作用-矽卡岩成矿峰期、弧-陆碰撞缝合带的时空迁移与大气圈CO_(2)浓度和大陆冰川沉积有很好的对应关系,暗示东北显生宙构造-岩浆过程和古气候演变的内在联系。综合东北地区及全球的研究进展,本文提出如下倾向性认识:1)洋-陆俯冲过程中火山-变质脱气的强度决定了CO_(2)排放量,而热带区域弧-陆碰撞缝合带的规模决定了全球硅酸盐风化速率和CO_(2)吸收量,在地质演化过程中两者紧密联动,共同控制了显生宙古气候的演变;2)大陆弧岩浆作用的全球爆发不一定能造成温室气候的出现,如果缺乏充分矽卡岩变质脱碳反应,大陆弧CO_(2)排放通量与岛弧、大洋中脊和板内并无显著区别;3)SO_(2)属于短期效应气体,理论和实例研究均暗示爆发式火山作用难以诱发大冰期的形成,火山作用之于长期气候应该仍是促使地球升温而非变冷。     

Major Miocene geological events in southern Tibet and eastern Asia induced by the subduction of the Ninetyeast Ridge

Cenozoic adakitic rocks in the Gangdese changed from barren continental melts to ore-forming slab melts at ~ 23 Ma. The distribution and chemical characteristics of the ore-forming adakites point to an association with the Ninetyeast Ridge. The subduction of the thick, rigid Ninetyeast Ridge changed the geometry and rheology of the eastern Tibetan Plateau lithosphere and asthenosphere, restrained the eastward escape of asthenospheric mantle as well as continental fragments, and promoted the uplift and building of the Tibetan Plateau, which consequently changed the tectonic and climatic regimes in eastern Asia.     

Preliminary analysis of the development of the Carbon Tracker system in Latin America and the Caribbean

The Carbon Tracker system will play a major role in understanding CO₂ sinks and sources, gas exchange between the atmosphere and oceans, and gas emissions from forest fres and fossil fuels in Latin America and the Caribbean. This paper discusses the trends in carbon fluxes in the biosphere and ocean, as well as emissions from forest fres and fossil fuel use in the above-mentioned region, using the Carbon Tracker (CT) system. From 2000 to 2009, the mean carbon fluxes for the biosphere, fossil fuel use, wildfires and the ocean in Latin America and the Caribbean were −0.03, 0.41, 0.296, −0.061 Pg C/yr, respectively, and −0.02, 0.117, 0.013, −0.003 Pg C/yr, respectively, in Mexico. The mean net carbon flux for Latin America and the Caribbean was 0.645 Pg C/yr, and 0.126 Pg C/yr for Mexico. The terrestrial sinks in Latin America and the Caribbean are dominated by the forest, agricultural, grass and shrub regions, as well as the Andes mountain range and the net surface-atmosphere fluxes including fossil fuel are dominant in regions around large cities in Mexico, Brazil, Chile, and areas undergoing deforestation along the Amazon River. The results confirm that forest fres are an important source of CO₂ in Latin America and the Caribbean. In addition, we can confirm that policies encouraging the use of ethanol in light vehicles in Brazil have helped to decrease carbon emissions from fossil fuel, and assume the effects of the Proárbol program on carbon sinks from the biosphere and from fire emissions sources in Mexico. Based on this analysis, we are confident that the CT system will play a major role in Latin America and the Caribbean as a scientific tool to understand the uptake and release of CO₂ from terrestrial ecosystems, fossil fuel use and the oceans, and for long-term monitoring of atmospheric CO₂ concentrations.     

Subsidence due to crack closure and depressurization of hydrothermal systems: a case study from Mt Epomeo (Ischia Island,Italy)

Levelling surveys carried out between 1990 and 2003 on the Mt Epomeo resurgent block (Ischia Island) record negative dislocations on its northern and southern flanks with a maximum subsidence rate of 1.27 cm yr⁻¹. This deformation is not associated with the cooling, crystallization or lateral drainage of magma and cannot be explained by a pressure point or prolate ellipsoid source. Results from dislocation models and the available structural and geochemical information indicate that the subsidence is due to crack closure processes along two main ENE–WSW and E–W preexisting faults, which represent the preferred pathway of CO₂ degassing from the hydrothermal system located beneath Mt Epomeo. The monitoring of the dislocations and CO₂ flux along these faults could give useful information on the dynamics of the hydrothermal system.     

Natural evidence for rapid abiogenic hydrothermal generation of CH4

Although recent hydrothermal experiments imply that abiogenic methane (CH₄) generation from hydrothermal reduction of CO₂ can occur, evidence from natural systems was still lacking. Based on the chemical and isotopic equilibrium signatures of low-temperature fumarolic gas discharges, we are able to provide hard evidence for its natural occurrence, namely in three subduction-related bi-phase hydrothermal systems of the Mediterranean, whose temperatures range from 260 to 470 °C. The attainment of equilibrium and the time spans of recent volcanic dormancy allowed us to calculate minimum rates for chemical and isotopic equilibration. These are significantly higher than those previously reported and might be due to the presence of a saturated water vapor phase in the investigated systems. The fact that nature provides conditions enabling relatively fast production of hydrocarbons from CO₂ strongly supports the concerns that were recently raised from laboratory experiments. These address the use of the carbon isotope composition of reduced carbon in Archean sediments as a tracer of early life and the occurrence of CH₄ on extraterrestrial planets as a bioindicator. In view of the potential role of abiogenic CH₄ as a precursor of life, we also present an estimate of abiogenic hydrothermal CH₄ fluxes throughout the Archean. It is not expected that these fluxes exceeded 80 Mt/yr during the past 4.0 Ga. This, however, would have been enough to facilitate HCN production on the prebiotic Earth.