Hydrothermal activity, resulting of oceanic crust/ocean interaction, represents one of the key sources of material entering the oceans. As well as representing an important source of heat and mass from fresh-formed underlying oceanic crust, hydrothermal activity also plays host to ecosystems of perhaps unequalled originality and evolutionary significance. One of the major objectives of the MARFLUX/ATJ MAST 2 project was to locate hydrothermal sites in the area of the Azores Triple Junction.
Two hydrothermal sites have actually been discovered within E.U.'s EEZ. In addition, hydrothermal anomalies in the water column have been detected at several locations along the Mid-Atlantic Ridge southwest of the Azores Triple Junction. The gradually southward changing properties of the ocean crust from the ATJ are direct or indirect consequences of the Azores Hot Spot-(i) depth (from 800m to 2800m), (ii) chemical properties of rocks, (iii) nature of volcanism (explosive at depths shallower than 900m), and (iv) segmentation of the ridge axis.
We propose to study (i) the physical, chemical and biological fluxes from 4 contrasted hydrothermal fields as heat and matter are dispersed through a hydrothermal plume into the adjoining mid-Atlantic Ridge rift-valley (ii) highan low-temperature fluxes (physical and chemical) and structure of the hydrothermal system at the hydrothermal vent field scale (iii) the influence of depth through chemical, physical and biological processes on hydrothermal vent ecosystems.
Four cruises constitute the core of the project: A surface cruise, (1) 'FLAME' (1997) aboard U.K. R/V "Darwin" will be dedicated to the quantification of the heat and mass fluxes from a hydrothermal vent-field, up to the scale of a complete segment of the MAR. Its main objectives are to characterize the first-order physical, chemical and biological variations between plumes overlying known vent-fields. Three diving cruises using the French submersible 'Nautile' and its support vessel 'Nadir' (2)'FLORES' (1997) to document highand low-temperature fluxes (physical and chemical) and the structure of the hydrothermal systems. An important point will be to compare the hydrothermal fluxes issuing from different types of hydrothermal processes and document the relative importance of each of them on the fluxes in the ocean; (3)'MARVEL' (1997), to the study the influence of depth through chemical, physical and biological processes on hydrothermal vent ecosystems: composition (microbial and faunal) including the relative contributions from chemosynthetically and photosynthetically derived inputs, population structure and dynamics, species adaptations to toxicity of the vent fluid, the life cycles and strategies in a discontinuous (space and time), unpredictable and extreme environment;(4) 'PICO', a short diving cruise (1998)devoted to the recovery of long term experiments and measurements.
In addition, work will be done under the 'related studies' section to coordinate our effort with studies funded outside MAST. It will further the objectives of the core project in the area of Azores triple junction and constrain the regional context.