Our journey on this cruise takes us to the middle of the Lau Basin, just west of Tongatapu, the main island of the Kingdom of Tonga. We are particularly interested in the 240km-long Eastern Lau Spreading Center (ELSC) and Valu Fa Ridge (VFR). Here, the crust splits apart and magma wells up from the mantle, spilling lava onto the seafloor. This is also an area of unusually high heat flow, where concentrated geothermal energy powers deep-sea hydrothermal vents.
Seafloor hydrothermal systems form when geothermal heat interacts with seawater that has percolated into the seafloor. Water and rocks chemically react at high temperatures, forming hydrothermal fluids. I like to think of it like making coffee. Hot water poured into coffee grounds dissolves certain compounds and forms a new substance. The type and flavor of coffee you get depends on what type of grounds you use and the ratios of water, heat, and coffee beans. Similarly, the type and chemistry of hydrothermal fluid depends on the types of rocks in the seafloor and the ratio of water and heat added in the process.
It is a peculiar characteristic of the ELSC that its direction forms a 17-degree angle with that of the nearby Tonga Subduction Zone. As a consequence, the distance between the ELSC and the volcanic arc that forms above the subduction zone decreases from ~100 km in the north to ~50 km in the south. At about ~70 km, there is a rapid change in the rock composition. Far to the south, at Mariner vent field, magmatic fluids such as water, carbon dioxide (CO2 ) and sulfur dioxide (SO2) mix directly with the hydrothermal fluids, a sort of super-charge mixture of high heat and low pH (acid) that extracts additional material from the rocks.
All of these differences in rock type, temperature, and fluid chemistry provide an excellent natural laboratory for exploring, in depth, the factors that influence the diversity and relationships of microbial communities at actively forming deep-sea hydrothermal deposits.
Contributed Guy Evans.