We present evidence for strong hydrothermal activity in the eastern Manus Basin (depth: 1700-2100 m), the existence of large scale triple-layered buoyant plumes at depths of ∼1100 m ("shallow plume"), ∼ 1700 m ("deep plume"), and ∼1400 m ("middle plume" with less extent than the other two plumes) that were revealed from water column anomalies of CH4, Mn, Al and pH observed in November to December 1990. Judging from the horizontal distribution of these parameters, the deep plume seems to originate from two distinct hydrothermal sites (eastern and western sites) in the research area, the eastern site being visually ascertained with deep-tow observations at the same time. The CH4/Mn ratio (mol mol-1) of the deep plume (0.02-0.05) is the lowest yet observed in hydrothermal plumes. The order of magnitude difference of CH4/Mn ratios between the shallow plume and the deep plume suggests that different kinds of fluid-rock interaction occurred to make the hydrothermal end members for the deep and shallow plumes. The shallow plume, which had an areal extent of more than 50 km, may be an episodic "megaplume", because it was not recognized in the previous CH4 profiles in 1986, and because it has a similar CH4/Mn ratio as the megaplume observed in the North Fiji Basin. We found that the eastern deep plume is characterized by enormously high aluminium concentrations (0.6- 1.5 μmol kg-1), pH anomalies (∼0.1) and high Al/Mn ratios (10-17). The endmember fluid for the eastern deep plume may have an unusually low pH value to dissolve this much aluminum during fluid-rock interaction, or this plume may originate from an eruption-influenced fluid.
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