What we can see through our artificial eye at the bottom of the sea
Through World War II, there were significant improvement and development of technology and science in many fields. One of them is sound navigation and ranging (SONAR). It had long been believed that deep sea floor has no particular structure and just flat due to sedimentation. However, the Navy developed a new technology to measure the depth of the seafloor using sound waves. The investigation of the Atlantic Ocean and the Pacific Ocean using this new technology, SONAR, caused a substantial shock on geoscience academia at the time, because the depth data indicates that various topographic features exist on the seafloor. This finding initiates the idea of the plate tectonics in the 1970s as a result.
Submarine technology plays an important role in the study of the deep-sea environment. In the late 1970s, a manned submersible named Alvin succeeded in reaching the Galapagos Rift and the East Pacific Rise and discovered hot springs releasing 380℃, metal saturated acidic fluids into the seawater. Also, in these extreme environments, life form maintained by heat, gas and chemicals extracted from the volcanoes was found. Since then, geologists try to observe eruption and their environment underwater by using unmanned vehicles. At the East Pacific Rise, autonomous underwater vehicles (AUV) have enabled detailed imaging of seafloor volcanic system; the geometry of eruptive fissures, lava flows and hydrothermal vents. This observation was coupled with age dating and geochemical analyses to reconstruct the detailed history of the volcanic systems. The eruption frequencies are 5 to 15-year intervals and following eruptions, there are rising of hydrothermal vent temperature, releasing of fresh vent fluids, and formation of chimneys with elders destroying.
In contrast to mid-ocean ridge systems, arc environments typically host more silicic magmas enriched in volatiles from the subducting oceanic plates that are hydrothermally altered and covered with thick sediments. Investigation of the Mariana Arc (MA) with remotely operated vehicle (ROV) has yielded some startling discoveries. At an erupting submarine volcano of the MA, yellow sulfur-rich plumes are released from a crater and mantled the volcano’s summit. Those molten sulfur globules form by degassing and disproportionation of sulfur dioxide from magma. This magmatic degassing process even produces underwater lakes of molten sulfur which two volcanoes in the MA host. The concentration of CO2 issuing from the seafloor can reach an astounding value in arc environments also. At Northwest Eifuku, a small volcanic cone in the MA, liquid CO2 droplets rise from the seafloor and CO2 concentrations in hydrothermal fluid reach centuple of those of average mid-ocean ridges. The CO2 originates from melting of subducted carbonates and degassing from the magma chamber.
As seen briefly, these observations imply that submarine volcano might play an important role in the sulfur and carbon cycle on the Earth. Still, a large number of the arc and mid-ocean ridge submarine volcanoes have not been explored or poorly understood. We need to investigate the seafloor volcanic systems widely and thoroughly with the new technologies. Not only AUV and ROV, but also real-time monitoring that recently began will be our artificial eye at the bottom of the sea.
Figure 1. Photographs of the Champagne hydrothermal site taken with the ROPOS ROV. (a, b, and c) Small chimneys venting 103℃ vent fluid. Liquid CO2 droplets are also visible. (d) Close-up of liquid CO2 droplets rising in a stream from the seafloor. (modified from Lupton, J., et al. (2006))
