青藏高原南部及邻区深部碳释放规模与成因

Fluxes and genesis of deep carbon emissions from southern Tibetan Plateau and its adjacent regions

不同构造背景下的深部碳释放通量与机制研究对于深刻理解长时间尺度的气候变化具有重要意义,以往的相关研究多集中在洋中脊、大洋俯冲带和大陆裂谷等地质单元,缺少对大陆碰撞带深部碳释放规模与机理的关注,从而制约了对大陆碰撞带深部碳循环过程及其气候环境效应的进一步认识。青藏高原起源于印度和欧亚大陆的碰撞,是研究大陆碰撞带深部碳循环的理想地区。为此,在近年来青藏高原温室气体释放野外观测与研究的基础上,本文估算了高原南部及邻区火山-地热区的CO_(2)释放规模并探讨了其释放模式。气体He-C同位素地球化学与温泉水热活动特征等显示,青藏高原南部及邻区的深部碳释放主要受深部岩浆房、断裂和浅部水热系统等因素的控制。依据深部流体源区和上升运移控制因素的差异,可以将青藏高原南部及邻区的深部碳释放划分为三大类:(1)以壳内水热系统脱碳为主的藏南地区;(2)深大断裂控制的以水热系统脱碳为主的川西地区;(3)深部岩浆房和浅部水热系统共同控制的滇西南地区。青藏高原南部土壤微渗漏CO_(2)释放通量介于18.7~52.3Mt/yr之间,温泉溶解无机碳释放通量约为0.13Mt/yr;高原邻区的川西、滇东南断裂带温泉CO_(2)释放规模为0.1Mt/yr;腾冲火山区CO_(2)释放通量介于4.5~7.1Mt/yr之间。本研究认为,青藏高原南部及邻区每年向大气圈释放CO_(2)的规模为23.4~59.6Mt,这一规模与全球其他构造背景(如洋中脊、大洋俯冲带、大陆裂谷等)火山区深部碳释放规模相当,表明以青藏高原为代表的大陆碰撞带是地质源CO_(2)释放的重要场所。

Deep carbon fluxes and emission mechanisms in different tectonic settings are important for the long-term climate change.Previous studies mostly focused on the carbon cycling in the mid-ocean ridges,oceanic subduction zones and continental rift systems,and reckoned without that in continental collision zone.Along with India-Asia continental collision,the Tibetan Plateau is considered to participate significantly in the geological carbon budget of the Earth,yet,the carbon degassing fluxes are poorly constrained by the lack of detailed studies.Based on the results of surface carbon observations in Tibetan Plateau in recent years,we estimate the scale of carbon dioxide emissions from the volcanic-geothermal areas,and discuss the carbon emission patterns in different regions combined with He-C isotopes of spring gases and their emission characteristics.Our results show that,the deep carbon degassing from Tibetan Plateau and its adjacent regions are controlled by deep magma chamber,regional faults and shallow hydrothermal systems.Considering the difference of volatiles’sources and controlling factors during their ascending processes,three types of deep carbon degassing patterns are recognized:(1)southern Tibetan Plateau,which is dominated by hydrothermal degassing in the crust;(2)western Sichuan,which is dominated by hydrothermal systems controlled by deeply-derived faults;and(3)southwestern Yunnan,which is dominated by double control of deep magma chamber and shallow hydrothermal systems.The calculated results show that in southern Tibetan Plateau,the CO_(2)fluxes from soil diffusive emissions ranges from ca.18.7 Mt/yr to 52.3 Mt/yr,and the annual flux of dissolved inorganic carbon-type degassing from mineral springs is ca.0.13 Mt.The deep-sourced CO_(2)flux from regional fault systems and Tengchong volcanic field in southeastern Tibetan Plateau are ca.0.1 Mt/yr and ca.4.5~7.1 Mt/yr,respectively.Thus,the annual CO_(2)emissions from the southern Tibetan Plateau and its adjacent regions would be ca.23.4~59.6 Mt,being at the same magnitude as that of other tectonic settings(e.g.,mid-ocean ridge,oceanic subduction zone,continental rift,etc.)and indicating the continental subduction zone as one of the important tectonic regimes for the geological carbon degassing.