Abstract:

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 SO₄^2- 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 SO₄^2- 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.

Key words: gas hydrate, ammonia ionvphosphate ion, pore water, marine sediment