Fluid and chemical cycling at Bush Hill: Implications for gas‐ and hydrate‐rich environments

Abstract

The results of a deployment of aqueous flux meters at the Bush Hill hydrate mound show that persistent hydrologic instability is a primary feature of the globally abundant gas‐ and hydrate‐rich cold seep environment. Seven flux meters were deployed for 14 weeks in the area of the Bush Hill hydrate mound. Instruments were deployed on microbial mats, bivalves, adjacent to the surface hydrate mound, and a site without visible fauna. Flow rates were observed to range from downflow of 0.01 mm/day to upflow of >15 mm/day. Temporal variability and a major hydrological event were observed to occur on all instruments. There is evidence that this event was related to a gas expulsion episode. The two instruments which exhibited downflow during the event were near surface hydrates and bubbling vents. Nearby instruments recorded a rapid increase in flow rates at the time of the event with a subsequent decrease in rates to the previous background values. Two instruments showed significant output of seawater‐like fluids, while the others output typical methane seep‐type pore fluids. These results at a passive margin site, along with our previously published convergent margin results at gas‐rich northeast Pacific cold seeps (Hydrate Ridge, Eel River margin), illustrate the hydrologic complexity of these environments. We now propose that these and our earlier results are characteristic of seafloor environments which have abundant free gas and hydrates. These mechanisms have major consequences for the near‐surface geochemical and microbial environment and for the way we interpret measurements made in these areas.