Hydrothermal activity in a coral reef ecosystem, Tutum Bay, Ambitle Island, Papua New Guinea.
Creators: name: Pichler, Thomas.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1998
Subjects: subject: Geology.

description: Not much is known about hydrothermal venting and its petrologic definition within shallow-water carbonate settings associated with volcanic islands. The fringing reef around Ambitle Island, Papua New Guinea is possibly the only present day coral reef exposed to the extensive discharge of a hot, mineralized hydrothermal fluid. Two types of venting are observed: (1) focused discharge of a clear, two-phase fluid from discrete orifices; (2) dispersed or diffuse discharge that consists of streams of gas bubbles ubiquitous throughout area. The composition of the gas is mainly CO2. Fluid evolution and final chemical composition is controlled by a two- or possibly three-step process: (1) Phase separation in the deep reservoir beneath Ambitle Island produces a high temperature vapor that rises and 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 and 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 by ground or seawater during its final ascent. Approximately 10% of the deep reservoir fluid reaches the surface. Aragonite, ferroan calcite and Fe(III) oxyhydroxide are the prominent hydrothermal precipitates in Tutum Bay. Aragonite encrusts dead coral fragments, volcaniclastic boulders and pebbles, and fills secondary fracture and remaining primary intergranular porosity within volcaniclastic arenite. Microcrystalline ferroan, low-Mg calcite occurs as crusts commonly interlaminated with Fe(III) oxyhydroxides and also locally replacing aragonite adjacent to Fe(III) oxyhydroxides. Hydrothermal deposits are unique in their chemical composition, displaying anomalous Sr isotope ratios that are closer to island-arc basalt than to seawater. Fe(III) oxyhydroxide shows very low Mn values and elements usually enriched in Fe(III) oxyhydroxide, such as Co and V, are below crustal abundances and well below their concentrations in island-arc volcanics. Seven individual vents in four different areas within Tutum Bay discharge an estimated 1500 g of arsenic per day into an area of approximately 50 by 100 m. Despite the amount of arsenic released, corals, clams and fish do not show a direct response to the elevated values. Two mechanisms efficiently control and buffer the arsenic concentration: (1) dilution by seawater and (2) incorporation in and adsorption on Fe(III) oxyhydroxides that precipitate when the hydrothermal fluids mix with ambient seawater. The Fe(III) oxyhydroxides that precipitate in Tutum Bay can contain up to 75,000 ppm As. The skeletons of scleractinian corals and the shells of Tridacna gigas clams do not show elevated concentrations of arsenic. However, they possess a distinct delta13C, delta18O, 87Sr/86Sr and Sr signature. Of the various direct and indirect hydrothermal effects recorded by the corals, thermal stress is likely the single most important. (Abstract shortened by UMI.)