The chemical composition of shallow-water hydrothermal fluids in Tutum Bay, Ambitle Island, Papua New Guinea and their effect on ambient seawater

Submarine, hydrothermal venting occurs at Tutum Bay in shallow (5–10 m) water along the inner shelf that contains a patchy distribution of coral–algal reefs. Two types of venting are observed. (1) Focused discharge of a clear, two-phase fluid from discrete orifices, 10–15 cm in diameter. Discharge temperatures are between 89 and 98°C and estimated flow rates are as high as 300 to 400 l/min. (2) Dispersed or diffuse discharge that consists of streams of gas bubbles ubiquitous in the area. The composition of the gas is mainly CO2 (92.6–97.9%) with minor amounts of N2 (2.2–4.7%), O2 (0.43–0.73%), CH4 (0.6–2%) and He (∼0.01–0.02%). Based on their geographic position and chemical composition, the vents have been divided into two groups, A and B. Area B vents have higher K, Rb, Sb, Cs, Tl, and As and lower Ca, Li, Mn, Fe, and Sr concentrations. Their chemical difference is likely caused by subsurface mixing of a CO2-rich water with a deep reservoir neutral chloride fluid in varying proportions. A two- or possibly three-step process controls fluid evolution and final chemical composition: (1) phase separation in the deep reservoir beneath Ambitle Island produces a high temperature vapor that rises upward and subsequently reacts with cooler ground water to form a low pH, CO2-rich water of approximately 150–160°C. (2) The steep topography causes lateral movement of this CO2-rich fluid towards the margin of the hydrothermal system where it mixes with the marginal upflow from the deep reservoir. This produces a dilute chloride water of approximately 165°C. A third step may be the entrainment of minor amounts of ground or seawater during its final ascent. Based on a B–Rb/Cs mixing model, it has been estimated that approximately 10% of the deep reservoir fluid reaches the surface. Compared to seawater, the hydrothermal fluids are depleted in Cl, Br, SO4, Na, K, Ca, Mg, and Sr and enriched in HCO3, B, Si, Li, Mn, Fe, Rb, Cs, Sb, Tl and As. Although some elements are significantly enriched, they do not have a clear impact on ambient seawater composition because their concentration is buffered by mixing and uptake into secondary minerals. Only the surface water in Tutum Bay carries a clear imprint of the hydrothermal fluids.