Emission and plume transport of methane and hydrogen from the Mid-Atlantic Ridge
Zusammenfassung der Projektergebnisse
In cooperation with the universities of Bremen and Hamburg, gas measurements were conducted on 5 cruises to the Mid-Atlantic Ridge, 2 in the South Atlantic and 3 to the Logatchev Hydrothermal Field at 13 15°N. The gases measured included methane concentration and 13C/12C ratio, hydrogen concentration and 3He/4He ratio. The measurements were made on samples collected from the water column and vent fluids. Methane measurements provided an important indication of venting activity found subsequently at 8° 18‘S. This turned out to be a black smoker that has one of the highest ratios of CH4 to 3He found in hydrothermal vent systems. In part this is due to the production of methane by serpentinization in the sub-surface hydrothermal circulation. In addition, it appears that the 3He to heat ratio is unusually low, perhaps because of the off-axis position of this vent. The vertical distribution of methane in the region surrounding the vent indicates that vertical mixing is at least an order of magnitude stronger than on the flank of the Mid-Atlantic Ridge and in the open ocean. All methane from plume and vent fluid samples collected during this project had a relatively heavy 13C/12C ratio, consistent with what had been previously observed at other vent sites in the North Atlantic. Carbon isotope ratios at Logatchev do not follow a functional trend with decreasing methane concentrations, even when these concentrations are normalized to the conservative 3He concentration. This appears to be because two opposing processes affect its carbon isotope ratio. As consumption proceeds, the 13C/12C ratio of the methane remaining in solution increases due to kinetic isotope fractionation. However, as the plume is progressively diluted with seawater containing low methane concentrations the 13C/12C ratio decreases since the background methane is isotopically lighter than that of the vent fluid. Model simulation indicates that the relative progression of mixing and oxidation as the plume “ages” is important in determining the methane δ13C structure within the plume.
Projektbezogene Publikationen (Auswahl)
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(2006), Discovery of new hydrothermal vents on the southern Mid-Atlantic Ridge (4°-11°S) during cruise M68/1. InterRidge News, 15, 9- 14
Koschinsky, A., A. Billings, C. Devey, N. Dubilier, A. Deuster, D. Edge, D. Garbe-Schönberg, C. German, O. Giere, R. Keir, K. Lackschewitz, H. A. Mai, H. Marbler, J. Mawick, B. Melchert, C. Mertens, M. Peters, S. Sander, O. Schmale, W. Schmidt, R. Seifert, C. Seiter, U. Stöber, I. Suck, M. Walter, S. Weber, D. Yoerger, M. Zarrouk, F. Zielinski
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(2007), Young volcanism and related hydrothermal activity at 5°S on the slow-spreading southern Mid-Atlantic Ridge. Geochem. Geophys. Geosyst., 8, Q11002
Haase, K.M., S. Petersen, A. Koschinsky, R. Seifert, C. Devey, N. Dubilier, S. Fretzdorff, D. Garbe-Schönberg, C.R. German, O. Giere, R. Keir, J. Kuever, K. Lackschewitz, J. Mawick, H. Marbler, B. Melchert, C. Mertens, H. Paulick, M. Perner, M. Peters, S. Sander, O. Schmale, J. Stecher, H. Strauss, J. Süling, U. Stöber, M. Walter, S. Weber, U. Westernströer, D. Yoerger, and F. Zielinski
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(2008), Flux and dispersion of gases from the „Drachenschlund“ hydrothermal vent at 8°18’S, 13°30’W on the Mid-Atlantic Ridge. Earth Planet. Sci. Lett., 270, 338-348
Keir, R. S., O. Schmale, M. Walter, J. Sültenfuß, R. Seifert, and M. Rhein
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(2009), Isotope fractionation and mixing in methane plumes from the Logatchev hydrothermal field. Geochem. Geophys. Geosyst., 10, Q05005
Keir, R. S., O. Schmale, R. Seifert, and J. Sültenfuß
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(2010), A note on the fluxes of abiogenic methane and hydrogen from mid-ocean ridges. Geophys. Res. Lett., 37, L24609
Keir, R. S.
