Abstract
The composition of hydrothermal plumes reflects the physical and chemical characteristics of seafloor hydrothermal fluids, which in turn reflects the host rock and subseafloor reaction conditions as well as the water column processes that act to alter the plumes as they disperse and age. Here, we show that the turbidity, current, pH value, dissolved Fe (dFe), and dissolved Mn (dMn) compositions of hydrothermal plumes can be used to understand the spatial distribution and source of hydrothermal systems in the submarine geological environment. Data were obtained from 18 hydrocast stations, among which the water column samples were collected at 8 stations during the MANUS cruise of R/V KEXUE in 2015. The results showed that the Satanic Mills plume and Fenway plume rose approximately 140 m and 220 m above the seafloor, respectively. In the Satanic Mills plume, dFe remained longer than dMn during lateral plume dispersal. There was a clear intersection of the Satanic Mills plume and Fenway plume between 1625 m and 1550 m in the PACMANUS hydrothermal field, and the varied dispersion trends of the mixed plumes were affected by current velocities at different depths. The physical and chemical properties of the seawater columns in the Manus Basin were affected by the input of high-Mn, high-Fe, and low-Mg vent fluids. The turbidity and dFe, dMn, and dissolved Mg concentrations in the sections of the plumes proximal to the Satanic Mills, Fenway, and Desmos vent sites were generally higher (turbidity, Mn, and Fe) and lower (Mg) than those in the sections of the plumes that were more distal from the vent sites. This implied that the plumes proximal to their vent fluid sources, which were interpreted to have relatively young ages, dispersed chemically over time, and their concentrations became more similar to those of the plumes that were more distal from their vent fluid sources.
Highlights
Hydrothermal plumes represent a significant dispersal mechanism for chemicals released from seafloor vents to the oceans
The highest dissolved Ca (10.2 mmol/kg) concentrations were observed in the background seawater samples from the Desmos field, and dissolved Na (432.0 mmol/kg) concentrations were observed in the seawater column samples in the Fenway vent site from the PACMANUS field (Tables 1 and 2)
These differences were consistent with the differences in the dissolved Mg, Cl, dissolved Fe (dFe), dissolved Mn (dMn), and SO42- concentrations of the vent fluids between the PACMANUS and Desmos fields [27, 43], in which the maximum SO42- concentration (147 mmol/kg; [31]) of the vent fluids was observed in the Desmos field and the maximum dFe (13400 μmol/kg) and dMn (4540 μmol/kg) concentrations of the vent fluids were observed in the Fenway vent site of the PACMANUS field ([27, 29, 43]; Tables 1 and 2). This showed that variations in the major components of anomalous layers in the seawater columns in the Manus Basin are controlled by chemical variations in the intensity and position of the sources of the vent fluids [27, 43]. All these findings suggest that the fluid sources of hydrothermal plumes in the PACMANUS field have the characteristics of higher dissolved Cl, dFe, and dMn and lower SO42- abundance compared with those of the Desmos field and imply that the difference in the plume chemistry may reflect the differences in the vent fluid chemistry
Summary
Hydrothermal plumes represent a significant dispersal mechanism for chemicals released from seafloor vents to the oceans. They are of interest to geochemists because they can be exploited to detect and locate new hydrothermal fields [1]. Hydrothermal circulation in young oceanic crust involves cold seawater that penetrates the seafloor and reacts with the underlying rocks, where it undergoes chemical and thermal modification and is converted into hydrothermal fluid enriched in dissolved Fe (dFe) and dissolved Mn (dMn) that is often one million times more concentrated than the background deep ocean concentrations [1,2,3,4].
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