Vailulu'u Seamount, Samoa : life and death on an active submarine volcano

Abstract

Submersible exploration of the Samoan hotspot revealed a new, 300-m-tall, volcanic cone, named Nafanua, in the summit crater of Vailulu'u seamount. Nafanua grew from the 1,000-m-deep crater floor in <4 years and could reach the sea surface within decades. Vents fill Vailulu'u crater with a thick suspension of particulates and apparently toxic fluids that mix with seawater entering from the crater breaches. Low-temperature vents form Fe oxide chimneys in many locations and up to 1-m-thick layers of hydrothermal Fe floc on Nafanua. High-temperature (81°C) hydrothermal vents in the northern moat (945-m water depth) produce acidic fluids (pH 2.7) with rising droplets of (probably) liquid CO2. The Nafanua summit vent area is inhabited by a thriving population of eels (Dysommina rugosa) that feed on midwater shrimp probably concentrated by anticyclonic currents at the volcano summit and rim. The moat and crater floor around the new volcano are littered with dead metazoans that apparently died from exposure to hydrothermal emissions. Acid-tolerant polychaetes (Polynoidae) live in this environment, apparently feeding on bacteria from decaying fish carcasses. Vailulu'u is an unpredictable and very active underwater volcano presenting a potential long-term volcanic hazard. Although eels thrive in hydrothermal vents at the summit of Nafanua, venting elsewhere in the crater causes mass mortality. Paradoxically, the same anticyclonic currents that deliver food to the eels may also concentrate a wide variety of nektonic animals in a death trap of toxic hydrothermal fluids.