C. E. J. de Ronde

Elemental mercury at submarine hydrothermal vents in the Bay of Plenty, Taupo volcanic zone, New Zealand

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 worlds 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.

Towed-camera investigations of shallow–intermediate water-depth submarine stratovolcanoes of the southern Kermadec arc, New Zealand

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.

Siliceous sublacustrine spring deposits around hydrothermal vents in Lake Taupo, New Zealand

Chimneys around hydrothermal vents at a depth of c. 140 m in the Horomatangi Reefs area in Lake Taupo, New Zealand, are composed of amorphous silica (opal-A) with lesser amounts of Mn-, Fe-, and Hg-rich precipitates. Detrital quartz and feldspar grains are found in the surrounding sediments. The associated biota includes fish, bacteria, sponges, crayfish, amphipods, copepods, ostracodes, annelids, and other unidentified organisms. Much of this biota, however, is not preserved in the opal-A deposits. Instead, the silicified biota includes filamentous microbes and diatoms, scattered coccoid microbes, fungal hyphae and spores, rare sponge spicules, and rare worm(?) tubes. The diatoms, coccoid microbes, and fungi were brought into the area with the detrital sediment. In ancient successions, distinction between sublacustrine, terrestrial, and some marine hot spring deposits may be difficult because the precipitates share many similarities. Distinction based on the biota is viable only if palaeoecological information can be inferred from accurately identified organisms. Unfortunately, silicification commonly disguises microbes and precludes their accurate identification. Diagenetic transformation of opal-A to its more stable polymorphs also destroys many original depositional fabrics and silicified microbes. Distinction between the different types of spring deposit may therefore depend on the interpretation of their overall sedimentological and stratigraphic setting.

Molten Sulfur Lakes of Intraoceanic Arc Volcanoes

Intraoceanic arcs of the world are dominated by submarine volcanoes, many of which host active hydrothermal systems. A considerable number of the morphological features common to subaerial volcanoes are also present on the submarine edifices, including summit craters. Surprisingly, some of the craters, such as at Daikoku and Nikko volcanoes of the Mariana Arc, and Macauley Cone of the Kermadec Arc, are host to lakes of molten sulfur, both ancient and modern. These lakes, up to ~200 m in diameter, act as condensers of gases that derive from the underlying magmas. Volcanic vents beneath these lakes provide a steady outflow of hot gases that continuously generate molten sulfur. At Daikoku, an extraordinary lake of liquid sulfur is in constant convective and gas escape-driven motion. Smaller pools of molten sulfur occur on Nikko, and there is evidence of older, larger lakes on both this volcano and Macauley, based on the accumulation of large quantities of sulfur in the subsurface. The elemental S at these sites is produced largely by the reaction 2H2S + SO2 = 3S + 2H2O and the disproportionation of magmatic SO2. Anomalous concentrations of Au and Cu in the lakes are most likely transported by vapor.