Beneath Waves: Petit-spot volcanoes release CO2 & methane in depths

Scientists have made groundbreaking discoveries about the hydrothermal activity of "little dot" volcanoes, shedding light on their previously unexplored nature. By analyzing samples collected at a record depth of 5.7 kilometres underwater, researchers have revealed crucial information.

Deep-sea volcanoes impact global carbon

The study published in Nature suggests that the small volcanoes, located on the deepest seafloor known to date, could potentially release carbon dioxide (CO2) and methane, significantly affecting the global carbon cycle.

Submarine volcanism and its associated hydrothermal activity play a vital role in marine biogeochemical cycles, particularly the carbon cycle. However, until now, the hydrothermal activity of the small volcanoes has remained largely unrevealed.

Recent revelations indicate that these small volcanoes, found around the world in regions where oceanic plates flex, spew alkaline magma enriched in carbon dioxide (CO2). Furthermore, the production of a volcanic rock known as peperite, resulting from the heating of water-rich sediments, suggests the occurrence of hydrothermal fluid production and methanogenesis.

Consequently, it is hypothesized that small-spot volcanoes can emit methane-containing hydrothermal fluids. Understanding the hydrothermal activity of these volcanoes is imperative to accurately assess their contributions to the marine biogeochemical cycle.

Deepest hydrothermal activity and formation

In a recent study, a team of scientists, led by Assistant Professor Keishiro Azami of Waseda University, conducted research on the hydrothermal deposits of a small volcano located 5.7 kilometers deep in the Japan Trench, located in the western North Pacific Ocean.

"The submarine hydrothermal activity that we have described in our paper is the deepest known to date. Based on our findings, we have further estimated the hydrothermal interactions that occur at small-spot volcanoes," explains Azami. The research team also included Dr. Shiki Machida from the Chiba Institution of Technology and Associate Professor Naoto Hirano from Tohoku University.

As part of their research, the team analyzed the chemical and mineral composition of dredge samples obtained from the ocean floor near the petit-spot volcano. The results revealed that the samples consisted mainly of iron (Fe) and manganese (Mn) oxides, indicating their hydrothermal origin, precipitated directly from the hydrothermal fluid.

These findings not only establish the presence of petit-spot hydrothermal activity as the driving force behind the formation of these oxides, but also confirm that the petit-spot volcano is the deepest known site of hydrothermal activity. The chemical and mineral compositions of the samples provided evidence of low-temperature hydrothermal activity.

To gain more information, the researchers used X-ray fluorescence spectroscopy to identify the elemental distribution within cross sections of the sample. By performing an independent component analysis on the elemental distribution data, they unraveled the formation process of the Fe-Mn oxides.

Unveiling Small-Spot Volcanoes' Environmental Impact

Their analysis suggested that the formation of these oxides begins with low-temperature hydrothermal fluid generated by small-point magma. The fluid rises through the sediment column, precipitating manganese oxides at the interface with seawater. As more manganese oxide accumulates, a layer of manganese oxide containing traces of silicate grows towards the seafloor.

Eventually, this debris is altered and Fe oxides are deposited through a similar process at the interface between the low-temperature hydrothermal fluid and Mn oxides. After the cessation of hydrothermal activity, a hydrogen rim forms on the exposed surface, which interacts with seawater.

Explaining the importance of his research, Azami states: "Based on previous studies, we can estimate that the hydrothermal fluids from small-spot volcanoes are enriched in CO2 and methane compared to those from mid-ocean ridges. This implies that the hydrothermal activity of the Small spots around the world may have crucial implications for global biogeochemical cycles, particularly the carbon cycle."

These findings highlight the presence of hydrothermal activity on cold, aged oceanic plates, emphasizing the importance of further study of small-spot volcanoes. The knowledge obtained from these investigations will contribute to a better understanding of the intricate processes that occur within the Earth's oceans and their impact on the environment.

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