Self-organized thermoelectric conversion systems on the deep seafloor

Abstract Deep-sea hydrothermal vents are sites where thermal energy from Earth’s interior is emitted into the ocean, and sulfide- or sulfate-rich chimneys form by mixing of hydrothermal fluids and ambient cold seawater. Sulfide minerals are semiconductors and can convert thermal to electrical energy. However, regardless of the high-temperature difference (up to 400 °C) between vent fluids and seawater, the significance of thermoelectric power generation by chimneys is poorly understood. We measured the electrical conductivity and thermoelectromotive force (TEMF) of typical sulfide minerals and each part of zoned chimney walls from seafloor hydrothermal vents to evaluate their thermoelectric conversion. Young and porous chimneys consist mainly of fine-grained sulfate and Zn sulfide, with extremely low electrical conductivity, resulting in no thermoelectric conversion. In contrast, mature chimneys with inner walls rich in Cu, Fe, and Pb sulfides are n-type semiconductors with a high thermoelectric conversion performance. Given a temperature difference of 200–300 °C between the vent fluids and ambient seawater, the TEMF of the mature chimneys approaches 10–210 mV, which is comparable to the redox potential between vent fluids and seawater (~500 mV). This TEMF could play an essential role as a power source to overcome the overpotential required for the redox reactions at both sides of the chimney walls. Our findings suggest that seafloor chimneys are a unique self-organized thermoelectric conversion system that supplies substantial electrical energy to deep-sea ecosystems.