Shallow water submarine hydrothermal activity - A case study in the assessment of ocean acidification and fertilization
Abstract
Most natural Shallow Water submarine Hydrothermal activates (SWH) along coastlines are related to hydrothermal eruptions involving heating of groundwater with the volcanic gas. These SWHs supply nutrients such as phosphorus and micro nutrients like iron to the euphotic zone, contributing to the overall natural fertility and primary productivity of coastal waters. However, SWHs also have a negative effect, dispersing toxic materials such as mercury and arsenic, and affecting the acidification of the surrounding waters. In this study, we evaluate the impact of "iron supply" and "ocean acidification" on the primary production in a coastal marine environment, at a SWH area discovered off Gueshandao Island, northeast Taiwan. In the past three years, expeditions were conducted and observations made around this SWH site. Divers, small boats and a research vessel (R/V OR1, Ocean University National Taiwan) were used to survey successively larger areas around the site. Some of the results obtained are as follows. Hydrothermal vents are located in a hilly terrain rich with hot spring water with gas erupting intermittently. There are two types of vents, roughly divided by color, yellow hot spring water with higher temperature >110 degC ejected from sulfur chimneys of various sizes, and colorless water with lower temperature ~80 degC ejected directly from the crevices of the andesitic bedrock. Natural sulfur solidifying in the mouth of a small chimney was captured by a video camera, and explosions were also observed at intervals of a few minutes. Sediment, sand and particles of sulfur were deposited on the sides to a radius of about 50 m condensing around the chimney. The bottom type changes from sand/particles to outcrop/rock away from the vents. Moreover, gas samples were collected from the vents; the ratios of gas concentrations (N2/Ar) and isotopic composition of noble gas (3He/4He) suggest that these volcanic gases are mantle-derived. Hydrothermal fluid with high pH values between 2.1 ~ 3.5 erupts from the vents, and diffuses toward the ocean surface affecting water up to 1 km way. The high pCO2 in the surface seawater is widely distributed, and the low pH value is also observed widely in mid-depth water (5 ~ 15 m). Water samples collected around the SHW site show higher concentrations of nutrients Si and P, and typically higher micro nutrients Mn, and even Fe as compared to normal seawater. Chlorophyll a in particular shows high values of several tens to hundreds times greater than normal subtropical seawater. We find for the first time in the nature, that SHW enhances the growth of phytoplankton, and contributes to primary productivity of the surrounding waters.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMOS42A..07Z
- Keywords:
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- 4808 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Chemical tracers;
- 4825 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Geochemistry;
- 4832 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Hydrothermal systems;
- 4845 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Nutrients and nutrient cycling