Southern Kermadec arc stratovolcanoes (of predominantly basaltic and andesitic composition) provide a depth transect of the transition between effusive and explosive submarine volcanism. Observations along ∼4.6 km of towed-camera track from the crests and upper flanks of the Clark and Rumble III volcanoes reveal a consistent pattern of substrate types that are interpreted to record effusive and explosive eruption processes. Below 700 m water depth, substrates are dominated by massive/blocky flows, pillow lavas, pillow and angular block talus, localized sheet flows, and minor granule-sand volcaniclastic detritus. The latter typically forms a substrate mode of 5–20%. Above 450 m, sand lapilli (that is in part winnowed and rippled), have a substrate mode of 50–100%, with minor components of massive flows, pillow lavas, and coarse talus. This difference in abundance of sand lapilli between 450 and 700 m is interpreted to record the transition between effusive and explosive (both phreatic and phreatomagmatic) eruptions. Between 600 and 700 m, a varied distribution of sand-lapilli abundance is interpreted as recording a mix of effusive and explosive eruptions, consistent with possible differences in the physical interaction of magma–water and known compositional magma heterogeneity.

Hydrocarbon gases have been sampled from both cold-seeping and heat-venting areas in the New Ireland fore arc basin in the vicinity of Lihir Island, Highest concentrations of up to 10 mul/l CH4 with a delta(13)C(CH4) value of - 54.9 parts per thousand PDB have been measured in the deep ocean water within a long and narrow deep sea basin located between Edison Seamount and an uplifted structure named "Mussel Cliff". Surface sediments of the seep area were covered with chemoautotrophic deep sea fauna such as Calyptogena species and tube worms, Large authigenic calcite concretions occur in the sediments between 50- and 200-cm sediment depth. The carbon isotopes of the carbonates in the concretions range from - 15 parts per thousand to - 40 parts per thousand PDB indicating a mixture of two CO2 sources: normal marine-inorganic carbon fixed in biogenic shells and CO2 from anaerobe bacterial oxidation processes of methane. Accordingly, C-14-AMS dating suggests that authigenic calcite mineralisation incorporated relatively "young" carbon from methane oxidation, In contrast, the C1/C2 ratio of 234 and the delta(13)C(CH4) value of - 24.1 parts per thousand PDB in the hot hydrothermal vent of Libir Harbour indicates a mixture of a major abiogenic carbon source for methane formation related to magmatism associated with Lihir Volcano. The observed variable fluid characteristics within only 20-km distance between hot hydrothermal-venting and the methane-seeping deep sea area indicates highly variable heat flow situations and/or sediment distributions which control the gas geochemical characteristics in the New Ireland fore are basin. (C) 2002 Elsevier Science B.V. All rights reserved.

High-temperature (250°C) hydrothermal vents and massive anhydrite deposits have been found in a shallow water, sediment-filled graben near 66°36′N in the Tjornes Fracture Zone north of Iceland. The site is located about 30km offshore, near the small island of Grimsey. The main vent field occurs at a depth of 400m and consists of about 20 large-diameter (up to 10m) mounds and 1–3m chimneys and spires of anhydrite and talc. A north–south alignment of the mounds over a 1-km strike length of the valley floor suggests that their distribution is controlled by a buried fault. Widespread shimmering water and extensive white patches of anhydrite in the sediment between the mounds indicates that the entire 1-km2 area occupied by the vents is thermally active. A 2-man research submersible JAGO was used to map the area and to sample vent waters, gases, and chimneys. Actively boiling hydrothermal vents occur on most of the mounds, and extensive two-phase venting indicates that the field is underlain by a large boiling zone (200×300m). The presence of boiling fluids in shallow aquifers beneath the deposits was confirmed by sediment coring. The highest-temperature pore fluids were encountered in talc- and anhydrite-rich sedimentary layers that occur up to 7m below the mounds. Baked muds underlie the talc and anhydrite layers, and pyrite is common in stockwork-like fractures and veins in the hydrothermally altered sediments. However, massive sulfides (pyrite–marcasite crusts) were found in only one relict mound. Subseafloor boiling has likely affected the metal-carrying capacity of the hydrothermal fluids, and deposition of sulfides may be occurring at greater depth. Although the mounds and chimneys at Grimsey resemble other deposits at sedimented ridges (e.g. Middle Valley, Escanaba Trough, Guaymas Basin), the shallow water setting and extensive boiling of the hydrothermal fluids represent a distinctive new type of seafloor hydrothermal system.

A research cruise has documented changes in rift tectonics, volcanism, and hydrothermalism along the least studied and most enigmatic sector of a crustal complex in the southwest Pacific Ocean. Results from the longitudinal transect are expected to provide insight into processes involving the Kermadec arc‐Havre backarc (KAHB) system, a continuum from oceanic spreading to continental rifting at a convergent plate boundary KAHB forms the central sector of an active, 2000‐km arc‐backarc complex between Tonga and New Zealand (Figure 1).The expedition also engaged in the first comprehensive survey of submarine vents in the Taupo Volcanic Zone (TVZ) at the south end of the KAHB system. Identified in the off‐shore segment of TVZ were three major hydrothermal vent areas associated with late Quaternary fault structures. Data from the expedition and from other recent research in the same area addressed questions concerning the type of hydrothermal venting, magmatic heterogeneity along and across KAHB, the style of backarc rifting, and tectonic and magmatic consequences of anomalous terranes colliding with the subduction margin.

Hot springs in active geothermal areas such as Yellowstone National Park, the Geysers geothermal field in California, and the Taupo volcanic zone in New Zealand are notably enriched in the trace metals Au, Ag, As, Sb, and Hg. Such near-surface hot springs have formed many of the world's important deposits of gold and silver and some of the largest deposits of mercury. The majority of these are associated with continental geothermal systems in subaerial environments. Here we report the discovery of active mercury-depositing hot springs in a submarine setting, at nearly 200 m water depth, within the offshore extension of the Taupo volcanic zone of New Zealand. These vents contain the first documented occurrence of elemental mercury on the sea floor and provide an important link between offshore hydrothermal activity and mercury-depositing geothermal systems on land. The discovery has implications for mercury transport in sea-floor hydrothermal systems and underscores the importance of submarine volcanic and geothermal activity as a source of mercury in the oceans.