Hydrothermal Fluids at the Mid-Atlantic Ridge (15°N and 4-11°S) as Media for the Transport of Energy and Mass from the Crust into the Hydro- and Biosphere
Final Report Abstract
During this project, a purpose-designed inert fluid sampling system (KIPS) has been significantly improved and allowed to sample and study the fluid geochemistry of three hydrothermal fields, located in different geological environments at the MAR. The Logatchev-1 field at 14°45ʼN and the Nibelungen field at 8°18ʼS are both located several kilometres off-axis in tectonic-controlled settings with heterogeneous mafic-ultramafic lithologies, whereas the vent field area at 5°S is hosted in the neovolcanic zone on the ridge axis and includes the hottest hydrothermal field found so far. The possibility provided within the SPP1144 framework to revisit the target sites several times during numerous cruises allowed to investigate the temporal evolution of hydrothermal systems depending on their geological setting and physical conditions. In the Nibelungen vent field, hydrothermal fluids vent from a single smoking crater at temperatures of about 375°C. The composition of hydrothermal fluids is controlled by the hybrid alteration of mafic and ultramafic rocks and is similar to other hightemperature, ultramafic-hosted hydrothermal systems with respect to key parameters, which collectively define a unique geochemical fingerprint attributed to mantle rock-fluid interaction in heterogeneous lithosphere at high fluid temperatures. This includes strong enrichments of hydrogen and methane, low concentrations of dissolved H2S, B, Si, and Li relative to basaltic-hosted hydrothermal systems, and an enriched ∆2HH2O signature. The high mobility of transition metals like Fe and Cu at Nibelungen is related to the relatively high fluid temperatures at the emanation site. The stability of vent fluid composition at Logatchev-1 (for 13 years) and Nibelungen (for 3 years) is in agreement with 2D 3phase modeling results and can be related to the high-pressure setting of these systems controlled by tectonic faults, and to the limited magmatic activity. In contrast, hydrothermal activity at 5°S (Turtle Pits, Comfortless Cove, Red Lion) is controlled by mafic rock-seawater interaction, with the first two sites venting at extremely high temperatures up to 425°C related to a recent lava eruption, while Red Lion has more moderate fluid temperatures of about 350°C. The fluids are affected by “supercritical” phase separation leading to the emanation of (condensed) vapor-phase fluids with about half the seawater salinity, high metal concentrations and a unique rare earth element and yttrium (REY) distribution that appears to be affected by anhydrite precipitation/dissolution. The style and temporal evolution of hydrothermal activity associated with magmatic eruptions differ strongly between fast- and slow-spreading ridges. Repeated fluid sampling between 2005-2009 disclosed stable major element fluid composition and temperature. This temporal stability observed over a period of 6 years in an actively phase-separating hydrothermal system is likely a function of the physical conditions during hydrothermal convection, in contrast to changes in heat and chemical fluxes within month or years at fast-spreading ridges in shallower water depths. Interaction between biological and hydrothermal processes in deep-sea hydrothermal environments was studied with focus on the role of metal-organic complexation. The biological availability of metals dissolved in fluids strongly depends on the presence of organic molecules which may bind chemically to the metals. The presence of elevated amino acid concentrations in hydrothermal fluids measured in both focused and diffuse flow fluid samples implies that they are available as a nutrient source to vent organisms and as potential complexing agents for metals, possibly influencing their bioavailability and solubility. Cultivation experiments of hydrothermal microbial cultures under variable Cu concentrations indicated that, independent of amino acid concentrations, added microbes produce organic Cu-binding ligands, possibly to mediate Cu toxicity stress. These results point to a microbial influence on dissolved metal concentrations, speciation and mineral precipitation in the hydrothermal vent environment. Geochemical modeling of the influence of organic ligands on metal export from hydrothermal vents demonstrated that organic metal complexation may contribute up to >10% of the total budgets of dissolved Fe and Cu in open ocean seawater.
Publications
- (2007) Geochemistry of hydrothermal fluids from the ultramafic-hosted Logatchev hydrothermal field, 15°N on the Mid-Atlantic Ridge: Temporal and spatial investigation. Chemical Geology 242 (1), 1-21
Schmidt, K., Koschinsky, A., Garbe-Schönberg, D., de Carvalho, L.M. and R. Seifert
- (2008) Hydrothermal venting at P-T conditions above the critical point of seawater, 5°S on the Mid-Atlantic Ridge. Geology 36 (8), 615-618
Koschinsky, A., Garbe-Schönberg, D., Sander, S., Schmidt, K., Gennerich, H. H. and Strauss, H.
- (2010) Fe-Si-oxyhydroxide deposits at a slow-spreading centre with thickened oceanic crust: the Lilliput hydrothermal field (9°33´S, Mid-Atlantic Ridge). Chemical Geology 278 186-200
Dekov VM, Petersen S, Garbe-Schönberg C.-D., Kamenov GD, Perner M, Kuzmann E, Schmidt M
- (2010): Geochemical and thermal structure of the hydrothermal plume at the ultramafic hosted Logatchev hydrothermal field at 14°45’ on the Mid Atlantic Ridge. Marine Geology 271 187-197
Marbler, H., Koschinsky, A., Pape, T., Seifert, R., Weber, S., Baker, E.T., de Carvalho, L.M., and Schmidt, K.
- (2010): Rare earth element distribution in >400ºC hot hydrothermal fluids from 5ºS, MAR: Controls on anomalous and highly variable distribution patterns. Geochimica et Cosmochimica Acta 74, 4058-4077
Schmidt, K., Garbe-Schönberg, D., Bau, M., and Koschinsky, A.
- (2010): Rare earth elements in mussel shells as tracers of hidden high-temperature hydrothermal systems. Earth and Planetary Science Letters 299 310–316
Bau, M., Balan S., Schmidt, K. and Koschinsky, A.
- (2011) Concentrations and distributions of dissolved amino acids in fluids from Mid-Atlantic Ridge hydrothermal vents. Geochemical Journal 44, 387-397
Klevenz, V., Sumoondur, A., Ostertag-Henning, C. and Koschinsky, A.
- (2011) Fluid elemental and stable isotope composition of the Nibelungen hydrothermal field (8°18'S, Mid-Atlantic Ridge): Constraints on fluid-rock interaction in heterogeneous lithosphere. Chemical Geology 280, 1-8
Schmidt K., Garbe-Schönberg, D., Koschinsky, A., Strauss, H., Jost C.L., Klevenz V. and Königer, P.
- (2011): Metal flux from hydrothermal vents increased by organic complexation. Nature Geoscience, 4, 145-150
Sander, S. and Koschinsky, A.
