10:45 AM - 11:00 AM
[MIS24-01] Geochemistry of cold seeps, mud volcanoes, and hydrothermal fluids
Keywords:cold seep, mud volcano, hydrothermal fluid, geochemistry
Hydrothermal fluids, cold seeps, and mud volcanoes are all forms of fluid discharge at the bottom of the deep sea, but hydrothermal fluids and other phenomena can be largely divided according to whether magma is involved. The only difference between cold seeps and mud volcanoes is the difference in momentum, with those involving large amounts of gas and venting explosively being called mud volcanoes, and those seeping slowly being called cold seeps. All these phenomena are caused by the high concentration of hydrogen sulfide and methane supplied by the hydrothermal fluid itself and the sulfate reduction reaction in the surface layer, which tends to crowd the chemosynthetic community on the seafloor. Since these three phenomena are listed in the session title, I would like to review the geochemical studies of these three phenomena.
The geochemistry of hydrothermal fluids begins with correlating the concentrations of other elements and species to the concentration of magnesium (Mg) and taking the end components (Mg plots). Without this, it is not possible to compare the pure hydrothermal components because the concentrations vary only with the mixing ratio of the seawater, because pure hydrothermal water contains very little Mg. It also helps that the Mg concentration in seawater is also constant in almost all areas of the ocean. When the Mg plot shows a correlation that is drawn linearly toward some component with low Mg, which is different from seawater, the composition of pure hydrothermal fluid is determined by taking the y-intercept. This also indicates that the elements or chemical species have not undergone any chemical reactions other than mixing during the mixing process. By correcting for seawater contamination in this way, the composition of pure hydrothermal water can be determined and compared with that of hydrothermal fluid around the world for the first time. The temperature of the hydrothermal source can be estimated using a silica thermometer, the hydrothermal composition can be compared with that of the rest of the world, and other variations within the same region can be discussed. In the geochemical study of hydrothermal fluids, each time a new hydrothermal fluid is found, its end-members are described and published in a database, which is a routine work, while many new processes and mechanisms are found, such as disproportionation reactions caused by volcanic gas input and alkaline hydrothermal fluids.
On the other hand, for mud volcanoes and cold seeps, if there is no abnormality in the chloride concentration, such an end-member approach cannot be taken. However, although it is not inevitable, it is often possible to take the end-member of mud volcanoes because the fluid of mud volcanoes is often mixed with fresh water from dehydration of clay minerals. However, it is often possible to take the end-member of mud volcanoes because fresh water of dehydrated origin of clay minerals is often mixed in the fluid of mud volcanoes. Rather, since this fact is observed with such a high probability, it might be better to consider it as inevitable. Rather, we can assume that the production of methane from thermal breakdown of organic matter is necessary for the driving force of mud volcanoes in the temperature environment where the dehydration of clay minerals occurs, so it is more inevitable that thermogenic methane is often detected in mud volcanoes. However, once the thermal origin is detected, the methane is not detected. However, microbial methanogenesis can occur in the sediments just below the seafloor, where thermogenic methane is once supplied and becomes reductive, and if the supply of thermogenic methane is weakened even a little, microbial methane becomes more conspicuous. In the case of mud volcanoes, the methane from chlorides is not always distributed in the surface layer. In the case of mud volcanoes, the end-members are determined using the concentration of chloride, and the geological thermometer using cations assuming equilibrium with clay minerals, the origin of methane is discussed using the carbon isotope ratio of methane, the origin of water is discussed using the isotope ratio of water, and the origin of boron and lithium is discussed using the isotope ratio of boron and lithium. Isotope ratios of boron and lithium are used to discuss the depth of origin. In the future, we expect the method of methane clumped-isotope to be able to estimate more detailed methane formation temperature.
The geochemistry of hydrothermal fluids begins with correlating the concentrations of other elements and species to the concentration of magnesium (Mg) and taking the end components (Mg plots). Without this, it is not possible to compare the pure hydrothermal components because the concentrations vary only with the mixing ratio of the seawater, because pure hydrothermal water contains very little Mg. It also helps that the Mg concentration in seawater is also constant in almost all areas of the ocean. When the Mg plot shows a correlation that is drawn linearly toward some component with low Mg, which is different from seawater, the composition of pure hydrothermal fluid is determined by taking the y-intercept. This also indicates that the elements or chemical species have not undergone any chemical reactions other than mixing during the mixing process. By correcting for seawater contamination in this way, the composition of pure hydrothermal water can be determined and compared with that of hydrothermal fluid around the world for the first time. The temperature of the hydrothermal source can be estimated using a silica thermometer, the hydrothermal composition can be compared with that of the rest of the world, and other variations within the same region can be discussed. In the geochemical study of hydrothermal fluids, each time a new hydrothermal fluid is found, its end-members are described and published in a database, which is a routine work, while many new processes and mechanisms are found, such as disproportionation reactions caused by volcanic gas input and alkaline hydrothermal fluids.
On the other hand, for mud volcanoes and cold seeps, if there is no abnormality in the chloride concentration, such an end-member approach cannot be taken. However, although it is not inevitable, it is often possible to take the end-member of mud volcanoes because the fluid of mud volcanoes is often mixed with fresh water from dehydration of clay minerals. However, it is often possible to take the end-member of mud volcanoes because fresh water of dehydrated origin of clay minerals is often mixed in the fluid of mud volcanoes. Rather, since this fact is observed with such a high probability, it might be better to consider it as inevitable. Rather, we can assume that the production of methane from thermal breakdown of organic matter is necessary for the driving force of mud volcanoes in the temperature environment where the dehydration of clay minerals occurs, so it is more inevitable that thermogenic methane is often detected in mud volcanoes. However, once the thermal origin is detected, the methane is not detected. However, microbial methanogenesis can occur in the sediments just below the seafloor, where thermogenic methane is once supplied and becomes reductive, and if the supply of thermogenic methane is weakened even a little, microbial methane becomes more conspicuous. In the case of mud volcanoes, the methane from chlorides is not always distributed in the surface layer. In the case of mud volcanoes, the end-members are determined using the concentration of chloride, and the geological thermometer using cations assuming equilibrium with clay minerals, the origin of methane is discussed using the carbon isotope ratio of methane, the origin of water is discussed using the isotope ratio of water, and the origin of boron and lithium is discussed using the isotope ratio of boron and lithium. Isotope ratios of boron and lithium are used to discuss the depth of origin. In the future, we expect the method of methane clumped-isotope to be able to estimate more detailed methane formation temperature.