Discovery of active hydrothermal venting in Lake Taupo, New Zealand

The Horomatangi geothermal system of Lake Taupo, New Zealand, is a sub-lacustrine equivalent of subaerial geothermal activity nearby in the Taupo Volcanic Zone (TVZ). The setting of this system is rare within the TVZ as it is directly associated with an individual volcanic feature, that of the 1.8 ka Taupo eruption vent. Two distinct hydrothermal vent areas, named Te Hoata and Te Pupu, have been discovered during dives with the submersible Jago. Venting of gases was seen at both sites and hot water (up to 45‡C) discharges at the Te Pupu site. Dilute water samples have concentrations of SO4, Cl, Na and SiO2 above ambient lake water values. Gas samples have compositions similar to other TVZ geothermal systems. Gas geothermometers indicate the existence of a hightemperature hydrothermal environment beneath the lake with reservoir temperatures in excess of 300‡C. Chimney structures were found at the Te Pupu site. They are up to 30 cm tall and mineralized by an ‘epithermal’ suite of elements, including S, Hg, As, Sb and Tl. The walls of the chimneys are largely composed of diatoms and strands of silicified filamentous bacteria embedded in an amorphous silica groundmass. Bacterial mats are commonly associated with the gas vents and also occur at two hot springs. Close to the vents, commonly perched on top of dead chimneys and/or exposed outcrops, are dense assemblages of what are probably a new species of sponge of the genus Heterorotula. The sponges host a notably diversified, associated invertebrate fauna and represent a previously unseen biomass on the lake floor. The sponges appear to have bored into the mineralized chimneys.