孔隙水中NH_4^+和HPO_4^(2-)浓度异常:一种潜在的天然气水合物地球化学勘查新指标

Anomaly of Ammonia and Phosphate Concentration in Pore Waters: A Potential Geochemical Indicator for Prospecting Marine Gas Hydrate

在海洋天然气水合物的勘查与研究中地球化学方法发挥了重大作用。利用海洋沉积物孔隙水中各种离子含量和同位素异常可以有效示踪天然气水合物的存在, 其中Cl-浓度异常、SO42-浓度梯度和氧同位素组成异常已成为指示天然气水合物存在的灵敏示踪剂。通过对比有和没有天然气水合物存在的海域浅表层沉积物中孔隙水的NH4+和HPO42-含量, 发现有天然气水合物区的孔隙水中NH4+和HPO42-浓度明显偏高, 且存在与SO42-类似的浓度梯度曲线, 认为孔隙水中NH4+和HPO42-浓度异常有可能成为一种新的地球化学示踪剂, 指示天然气水合物的存在; 探讨了引起孔隙水中NH +和HPO 2-浓度异常的可能机理及精确测定这些离子含量的方法。

Geochemical methods have been widely used in prospecting and studying marine gas hydrate. Concentration anomalies of cations, anions, and isotopes in pore waters are useful tools to indicate the existence of gas hydrate. In particular, anomalies of Cl^- concentration and oxygen isotopes, and SO_4^(2-) gradients in pore water have been proved to become sensitive tracers for gas hydrate. In this study, we compared the variations of ammonia and phosphate concentrations in pore water of shallow marine sediments with depths, and found that in the gas hydrate occurrence area the pore water show significantly higher ammonia and phosphate concentrations. We also found an increasing trend of phosphate concentrations in pore waters with depths, which match the SMI curve of SO_4^(2-) gradients. Therefore, we suggest that the anomalies of ammonia and phosphate concentrations in pore water may be used as a new geochemical tracer to prospect marine gas hydrate. We further discussed the possible mechanisms that cause the anomaly of ammonia and phosphate concentrations in pore water, and developed a simple and useful method to analyze concentrations of these ions in pore water and seawater.