Hydrothermal plumes in the Gulf of Aden, as characterized by light transmission, Mn, Fe, CH4 and δ13C-CH4 anomalies

We conducted water column surveys to search for hydrothermal plumes over the spreading axes in the Gulf of Aden between 45 degrees 35'E and 52 degrees 42'E. We measured light transmission and chemical tracers Mn, Fe, CH4 and delta C-13 of CH4 in seawater taken using a CID-Carrousel multi-sampling system at 12 locations including a control station in the Arabian Sea. We recognized three types of hydrothermal plumes at depths of similar to 650 to similar to 900 m (shallow plumes), similar to 1000 to similar to 1200 m (intermediate plumes), and > 1500 m (deep plumes). The shallow plumes were apparently originated from newly discovered twin seamounts (12 degrees 03-06'N and 45 degrees 35-41'E) at the westernmost survey area, where two-dimensional distributions of light transmission and Mn were mapped by tow-yo observations of the CTD-sampling system with an in situ auto-analyzer GAMOS. The maximum concentrations of Mn, Fe, and CH4 of 46 nM, 251 nM, and 15 nM, respectively, were observed for collected seawater within the shallow plumes. The intermediate plumes were characterized by anomalies of light transmission, Mn, Fe, and delta C-13 of CH4, but by little CH4 anomalies, suggesting that CH4 had been consumed down to the background level during the aging of the plumes. Anomalies of delta He-3 already reported by the World Ocean Circulation Experiment (WOCE) program exhibited a hydrothermal plume-like peak at similar to 2000 m depth in the Gulf of Aden, which seems to coincide with the deep plumes observed in this study. The endmember delta C-13-CH4 values for the shallow and the deep plumes were estimated to be in a range between 10 parts per thousand and 15 parts per thousand, demonstrating that the sources of CH4 are not biogenic but magmatic as similarly observed at sediment-starved mid-oceanic ridges. (C) 2015 Elsevier Ltd. All rights reserved.