Synopsis of the international WVU-symposium “sustainable development in coastal regions — A comparison between north atlantic coast and Baltic Sea”

The WVU-Symposium, organized by the Wissenschaftsverbund Umwelt at the Rostock University, was held in Rostock, Germany, from April 15–20, 1996. 109 participants and 22 students attend the symposium. 13 invited lectures and 22 lectures were given and 16 posters presented. In two plenary discussions the general results achieved were discussed as well as gaps, problems and main trends in the next 10–20 years. The synopsis gives an overview of the main points.     

“Andesitic water”: a phantom of the isotopic evolution of water-silicate systems. Comment on “Isotopic shifts in waters from geothermal and volcanic systems along convergent plate boundaries and their origin” by W.F. Giggenbach

On account of the low porosity of the lithosphere, intracrustal fluids behave very differently from surface fluids, in that they are changing their geochemical and isotopic labels according to the geological environment. Given a heat source, meteoric waters can be supplied plentifully and their rates of throughput in geothermal systems are sufficiently high to exhaust the compositional signals of a given rock buffer. In contrast, fluids exsolved from magma, and subducted fluids, would be supplied at less than about one tenth of the meteoric rate over the life time of a system. Based on up-to-date flow models, the isotopic evolution of meteoric water interacting with crustal rock follows a curved to L-shaped track in the δD versus δ¹⁸O plot. Instantaneous (present-day) tie-lines between recharge and discharge are secants of such tracks and can have a range of slopes. At the start of an interaction, waters have δD and δ¹⁸O values approaching equilibrium with the original rock composition (water “W1”). Using known hydrogen isotope fractionation factors, W1 values generally plot in the region of “andesite” or “andesitic” waters of various authors. Since the W1 waters have δD values that are on average more positive, and also less variable than those of the meteoric recharges, most tracks and tie-lines have positive slopes, and the plotting of a large number of tie-lines will produce a focus on the field of W1 waters, regardless of the original water source.     

Recycling and “re-hydration” of degassed magma inducing transient dissolution/crystallization events at Stromboli (Italy)

Intrusive degassing and recycling of degassed and dense magma at depth have been proposed for a long time at Stromboli. The brief explosive event that occurred at the summit craters on 9 January 2005 threw out bombs and lapilli that could be good candidates to illustrate recycling of shallow degassed magma at depth. We present an extensive data set on both the textures and the mineral, bulk rock and glassy matrix chemistry of the “9 Jan” products. The latter have the common shoshonitic–basaltic bulk composition of lavas and scoriae issued from typical strombolian activity. In contrast they differ by the heterogeneous chemistry of their matrix glasses and their crystal textures that testify to crystal dissolution event(s) just prior magma crystallization upon ascent and eruption. Comparison between mineral paragenesis of the natural products and experimental phase equilibria suggest water-induced magma re-equilibration. We propose that mineral dissolution is related to water enrichment of the recycled degassed magma, via differential gas bubble transfer and to some extents its physical mixing with volatile-rich magma blobs. However, all these features illustrate transient processes. Even though evidence of mineral dissolution is ubiquitous at Stromboli, its effect on the bulk magma chemistry is minor because of the subtle interplay between mineral dissolution and crystallization in magmas having comparable bulk chemistry.     

Flux and dispersion of gases from the “Drachenschlund” hydrothermal vent at 8°18' S, 13°30' W on the Mid-Atlantic Ridge

Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH₄ to ³He in the neutrally buoyant plume is quite high, 4 × 10⁸. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol s^− 1 in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH₄ calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of ³He is estimated to be between 0.14 and 1.2 nmol s^− 1. In either case, the ³He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23 yr^− 1, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m²m² s^− 1. These high values may be due to tidally driven internal waves across the promontory on which the vent is located.