DIFFUSELY VENTING HYDROTHERMAL EDIFICES: SIGNIFICANT HABITATS FOR MICROBIAL AND MACROFAUNAL COMMUNITIES

Diffusely venting, intermediate temperature hydrothermal edifices within submarine environments are gaining significant attention because they are important ecosystems for both microbial and macrofaunal communities. Two such structures (~2 m in height, ~1840 kg) were recovered from the Mothra Hydrothermal Field, Endeavour Segment, Juan de Fuca Ridge as part of the Edifice Rex Project in 1998. In contrast to black smokers, which are characterized by through-going high flow-through open conduits, these pinnacles lack large open centralized channels. Instead these pinnacles are characterized by an almost sponge-like porous permeability network, and a few large cavities that facilitate diffuse flow.

Detailed mineralogical and chemical analysis of one of these structures (Roane) revealed four major zones that roughly form a radial zonation pattern, despite the fact that this structure lacks a central conduit. Progressing from the exterior to the interior these zones include the Fossil Tube Worm Zone, Silica Zone, Outer Sulfide Zone, and Inner Sulfide Zone. The porous flow structure within the edifice randomizes the distribution of sulfide minerals such that the zones are differentiated by the abundance of non-sulfide minerals (amorphous silica ± clay or barite). Chemical gradients across the zones follow broad scale trends with major metal concentrations decreasing towards the exterior and most of the trace metals peaking near the exterior. Within the zones there is significant variability on a fine scale. Porous flow and episodic fracturing of the edifice allow for conductively cooled fluids, high temperature fluids, and mixed fluids to reach nearly all areas of the pinnacle, setting up moderate physical-chemical gradients within the structure. Moderate conditions within diffuse structures are more hospitable than those in vigorous black smoker chimneys and may provide optimal habitats to the numerous and varied microbes that have been found throughout these structures. In addition the warm fluids emanating from all sides of the structure provide the necessary heat and nutrients to support dense macrofaunal communities.