海底沉积物孔隙水钡循环对天然气渗漏的指示

冷泉流体的渗漏活动强烈地影响着海底沉积物孔隙水钡循环。冷泉流体中的Ba2+ 向上扩散与孔隙水硫酸盐反应,在硫酸盐—碳氢化合物转化带（SHT）之上沉淀重晶石。随着沉积物的埋藏,先前沉淀的重晶石被埋藏于SHT之下的硫酸盐亏损带,将发生溶解,溶解的钡向上扩散,在SHT之上再次沉淀重晶石。当体系中向上扩散的Ba2+超过埋藏的重晶石中的钡时,在剖面上形成“钡锋”。向上渗漏的碳氢化合物（甲烷为主）通量控制了SHT的深度,二者之间存在很好的地球化学耦合关系,从而,可以用“钡锋”来评价天然气渗漏活动的特征。在总结和分析国际海底冷泉渗漏活动区沉积物孔隙水的甲烷和钡循环的研究进展基础上,综述了海底沉积物孔隙水钡循环对现在和过去天然气渗漏的指示,总结了渗漏成因重晶石的地质和地球化学特征。

Barium Cycling in Pore Water of Seafloor Sediment: Indicator of Methane Fluxes

Methane and other hydrocarbons flux upward through deep-marine sediment may strongly impact the cycling of barium near the seafloor.Dissolved barium crossing sulfate-hydrocarbon transition(SHT) reacts with SO~(2-)_4,precipitating barium fronts composed of barite immediately above the SHT.During sediment burial,barite moves downward from SO~(2-)_4-rich to SO~(2-)_4-depleted pore water where it dissolves.In turn,Ba~(2+) dissolved diffuses upward from SO~(2-)_4-depleted to SO~(2-)_4-rich pore water where it precipitates as barite.In systems where upward Ba~(2+) diffusion exceeds downward barite burial,this barium cycling can result in a "barium front",a short interval of anomalously high labile barium concentrations immediately above the depth of SO~(2-)_4-depletion.Because the depth of SHT is controlled by the fluxing of methane and other hydrocarbons,the fluxes of methane should be coupled with barium cycling.Thus "barium front" may become an indicator of present and past methane fluxing and can be used to reconstruct changes in the upward flux of methane.The methane and barium cycles in sediment pore water near the seafloor has been comprehensively reviewed.Moreover,the indicator of barium cycling to present and past gas seeping in sediment pore waters at seafloor has been discussed.Finally,the geological and geochemical characteristics of seep related barite have been summarized.