Fluid Seeps at Continental Margins: towards an Integrated Plumbing System

Abstract

Over the past 2 decades, fluid seepage has been detected at numerous continental margin sites, and engendered a sub- stantial research effort worldwide (e.g. Moore 1999). Direct observations have been made by remotely operated vehicles, and linked to evidence ofseepage from geophysical (e.g. seis- mic) and geochemical (gas flux) data. Together with the growing recognition ofancient seepage sites in the geological record, these observations have established fluid seepage as a normal and widespread feature of continental margins. The fluids include hydrocarbon-rich (oil, gas) systems and water-rich systems containing dissolved or accompanying gases. Major consequences of fluid seepage include: the sup- port of diverse biological communities; contribution to the composition ofocean water and the atmosphere; mineral pre- cipitation within the fluid plumbing system; and the potential to reduce the stability of continental margin sediments. The papers in this thematic issue represent studies of these differ- ent aspects of continental margin hydrogeology. The seepages discussed in this issue are generally low-tem- perature systems, in contrast to the hydrothermal systems encountered at seafloor spreading centres, which are not only hotter but exhibit higher rates offluid flow (Mills & Harrison 1998). However, the recent discoveries ofcarbonate precipi- tation at lower temperatures at a site away from the active Atlantic spreading centre (Kelley et al. 2001) and evidence for a hydrothermal plume above a ‘cold' seep (Vogt et al. 1997) show that the distinction between hot and cold sys- tems is not complete, and could be uncertain in the geolog- ical