CAJ | 학술논문

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SO2–4、K+、Mg2+、Ca2+、Sr2+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SO2–4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SO2–4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/(m2·a)。此外,2个柱样沉积物孔隙水的Ca2+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。
남해서부륙파해역해저엄범발육마갱,기규모화수량재세계범위내균속한견,단관우타문목전적활동특정상불청초。통과대서사륭기서남부마갱구채집적량근침적주양공극수SO2–4、K+、Mg2+、Ca2+、Sr2+이급용해유궤탄(DIC)함량수심도적변화특정적연구,게시마갱내여류산근소모유관적생물지구화학과정,병추단마갱목전적활동상황。채집우마갱외적C9주양SO2–4농도변화정체정향하요적형태강저,감소적류산근시피유궤질류산염환원작용소모。채집우마갱내적C14주양SO2–4농도제도정현명현적삼단식변화,0.00~0.66 m내SO2–4농도변화주요수유궤질류산염환원작용공제,0.66~3.70 m수유궤질류산염환원화갑완결양양화공동공제,3.70 m이하부분주요수갑완결양양화작용적영향。근거C14주양3.7 m이하공극수류산근농도제도계산적류산근-갑완교접대(SMI)약재14.3 m처,갑완향상확산적통량약위0.0144 mol/(m2·a)。차외,2개주양침적물공극수적Ca2+농도균수심도명현강저,이Mg2+농도략미강저,주요여자생탄산염광물침정유관。C14적Mg/Ca화Sr/Ca수심도변화지시해주양침적물중자생탄산염암광물주요위고미방해석。2개주양적공극수지구화학특정현시목전연구구마갱활동불활약, C14마갱중함갑완류체발생미약삼루,가능처우마갱활동적쇠락기。
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 SO2–4 , K+, Ca2+, Mg2+, Sr2+, 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/(m2·a), respectively. The pore-water Mg/Ca and Sr/Ca weight ratios suggest that high Mg-calcite 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.