南海西部海底巨型麻坑活动性示踪研究  

Tracing study on the activity of mega-pockmarks in southwestern Xisha Uplift,South China Sea

作　　者：关永贤[1] 罗敏[2] 陈琳莹[3] 王淑红[3] 颜文[3] 王宏斌[1] 陈多福[2] 

机构地区：[1]国土资源部广州海洋地质调查局,广东广州510760 [2]中国科学院广州地球化学研究所边缘海地质重点实验室,广东广州510640 [3]中国科学院南海海洋研究所边缘海地质重点实验室,广东广州510301

出　　处：《地球化学》2014年第6期628-639,共12页Geochimica

基　　金：国家自然科学基金(91228206);中国科学院广州地球化学研究所“135”前沿项目(Y234021001)

摘　　要：南海西部陆坡海域海底广泛发育麻坑,其规模和数量在世界范围内均属罕见,但关于它们目前的活动特征尚不清楚。通过对西沙隆起西南部麻坑区采集的两根沉积柱样孔隙水SO^2–4、K^＋、Mg^2＋、Ca^2＋、Sr^2＋以及溶解有机碳（DIC）含量随深度的变化特征的研究,揭示麻坑内与硫酸根消耗有关的生物地球化学过程,并推断麻坑目前的活动状况。采集于麻坑外的C9柱样SO^2–4浓度变化整体呈向下凹的形态降低,减少的硫酸根是被有机质硫酸盐还原作用消耗。采集于麻坑内的C14柱样SO^2–4浓度梯度呈现明显的三段式变化,0.00-0.66 m内SO2–4浓度变化主要受有机质硫酸盐还原作用控制,0.66-3.70 m受有机质硫酸盐还原和甲烷缺氧氧化共同控制,3.70 m以下部分主要受甲烷缺氧氧化作用的影响。根据C14柱样3.7 m以下孔隙水硫酸根浓度梯度计算的硫酸根-甲烷交接带（SMI）约在14.3 m处,甲烷向上扩散的通量约为0.0144 mol/（m^2·a）。此外,2个柱样沉积物孔隙水的Ca^2＋浓度均随深度明显降低,而Mg2＋浓度略微降低,主要与自生碳酸盐矿物沉淀有关。C14的Mg/Ca和Sr/Ca随深度变化指示该柱样沉积物中自生碳酸盐岩矿物主要为高镁方解石。2个柱样的孔隙水地球化学特征显示目前研究区麻坑活动不活跃, C14麻坑中含甲烷流体发生微弱渗漏,可能处于麻坑活动的衰落期。Pockmarks are widespread on the seabed offshore southwestern Xisha Uplift, South China Sea. Some of them are so enormous that they are rare worldwide, but their activities were previously poorly known. We collected two gravity-piston cores from this pockmark field, one （C9） from outside a giant pockmark and the other （C14） from inside a giant pockmarks. The geochemistry of the pore waters, including SO^2–4 , K^＋, Ca^2＋, Mg^2＋, Sr^2＋, and dissolved inorganic carbon （DIC） was analyzed to elucidate the biogeochemical processes associated with sulfate consumption and to evaluate the current pockmark activity. The sulfate concentration-depth profile of C9 is predominantly in response to organoclastic sulfate reduction （OSR）, whereas the sulfate concentrations of C14 exhibit three zones of different concentration gradients resulting from varying proportions of contributions from OSR and anaerobic oxidation of methane （AOM）. Based on the sulfate concentration gradient of C14 below 3.7 m, the depth of the sulfate-methane interface （SMI） and the methane diffusive flux in C14 are calculated to be-14.3 m and-0.0144 mol/（m^2·a）, respectively. The pore-water Mg/Ca and Sr/Ca weight ratios suggest that high Mg-calcite＆amp;nbsp;is in equilibrium with respect to pore water or has recently precipitated from pore water in C14. The integrated analysis of pore water geochemistry of the two cores implies that this pockmark field is currently not so active and the pockmark from which C14 was obtained may currently be sluggish in activity with methane-bearing fluid weakly seeping from subsurface sediments.

关 键 词：孔隙水 地球化学 海底麻坑 西沙隆起 南海北部 

分 类 号：P593[天文地球—地球化学] P67[天文地球—地质学]