Investigation into extremely acidic hydrothermal fluids off Kueishan Tao, Taiwan, China

摘要

Kueishan Tao (24°51′N, 121°55′E) is located at a tectonic junction of the fault system extension of Taiwan and the southern rifting end of the Okinawa Trough. A cluster of over 30 vents, at a water depth of about 10-20 m off the eastern tip of the tao emits hydrothermal fluids and volcanic gases such as H_2S. A sulfur chimney or mound, formed by condensation of the sulfur contained in the hydrothermal fluid, can usually be seen around the vents. The tallest chimney reaches 6 m. Vents discharging a yellowish fluid have temperatures between 92 and 116℃ and flow rates as high as 158 t/h; vents discharging a whitish fluid have lower temperatures of between 48 and 62℃ and lower flow rates of about 7.0 t/h. These world-record, breaking low pH (as low as 1.52) fluids are totally different from those found in the black and white-chimneys of the mid-ocean ridges. Magnesium and SiO_2 data indicate that these hydrothermal fluids probably originate from a depth of 915～1 350 m below the surface. While the ratios of major ions relative to the sodium of these hydrothermal fluids are quite similar to open ocean water, the ratios of SO_4 and chloride to sodium seem to be higher for some of the vents. It is suggested that the volcanic gases contribute SO_4 and chlorine to the fluids, hence increasing their ratios relative to sodium. Some hydrothermal fluids, however, are found to be depleted of the major elements which can have been caused by phase separation. The concentrations of iron and manganese in the fluids are much lower than those found in the mid-ocean ridges, while the aluminium content is higher. Four species of benthos (Xenograpsus testudi-natus, a snail, a sea anemone, and a Sipuncala), 1 species of algae (Corallinaceae), and 1 species of fish (Siganus fusescens) were recorded near the hydrothermal vents. A mitochondria DNA sequence comparison of Xenograpsus testudinatus with 6 other decapod species shows the greatest number of nitrogen base differences in the DNA fragments from 225 to 270. The gene diversity in this DNA region possibly results from the unusual habitat. A sediment core was collected at a pond near the eastern side of the tao. No volcanic ash could be detected in the core sample spanning 3 620 a.