Petit-spot volcanoes involve the deepest known submarine hydrothermal activity, possibly release CO2 and methane

Newswise — Subaquatic volcanic activity within the Earth's crust significantly contributes various elements to the marine ecosystem. Thus, it holds vital significance in the ocean's biogeochemical and chemosynthetic processes. Despite extensive research on hot springs in the mid-ocean ridge, a chain of submerged volcanoes outlining diverse oceanic plates, limited data exists regarding cooler springs in alternative volcanic formations like "petit-spot" volcanoes.

Compact volcanoes, known as petit-spot volcanoes, are scattered worldwide, typically forming in regions where oceanic plates undergo flexing. Recent investigations conducted in the eastern area of the Japan Trench have revealed that petit-spot volcanoes emit alkaline magma with heightened levels of carbon dioxide (CO2). Moreover, these volcanoes generate a distinctive volcanic rock known as peperite, which arises from the heating of sediment abundant in water. This occurrence suggests the production of hydrothermal fluids and the potential for methanogenesis. Consequently, it is hypothesized that petit-spot volcanoes might release hydrothermal fluids containing methane. These discoveries underscore the necessity for a comprehensive comprehension of the hydrothermal behavior exhibited by petit-spot volcanoes to accurately assess their impact on the marine biogeochemical cycle.

In a recent analysis, a group of researchers, including Assistant Professor Keishiro Azami from Waseda University, undertook an examination of hydrothermal sediments derived from a petit-spot volcano situated at a remarkable depth of 5.7 km within the Japan Trench, located in the western region of the North Pacific Ocean. Assistant Professor Azami elucidates, "The submarine hydrothermal activity documented in our study stands as the deepest known occurrence thus far. Our observations have allowed us to estimate the hydrothermal interactions prevalent in petit-spot volcanoes." Collaborating with Assistant Professor Azami were Dr. Shiki Machida from Chiba Institution of Technology and Associate Professor Naoto Hirano from Tohoku University. The research paper detailing their findings has been published in Communications Earth & Environment.

As part of their investigation, the scientific team conducted an analysis of dredge samples collected from the seafloor in close proximity to the petit-spot volcano. The focus was on examining the chemical and mineralogical composition of these samples. The team made a significant observation: the samples predominantly consisted of iron (Fe) and manganese (Mn) oxides. Furthermore, the characteristics of these oxides strongly suggested a hydrothermal origin, indicating that they precipitated directly from hydrothermal fluids. These findings not only support the notion that petit-spot volcanoes exhibit hydrothermal activity but also establish the petit-spot volcano studied as the deepest-known hydrothermal site to date.

Additionally, the researchers discovered that the chemical and mineral compositions of the samples indicated the occurrence of low-temperature hydrothermal activity. This further adds to our understanding of the nature and dynamics of petit-spot volcanoes and their associated hydrothermal processes.

Following that, the team conducted x-ray fluorescence spectroscopy to determine the elemental distribution of the sample cross-sections. They subsequently applied independent component analysis to the elemental distribution data to unravel the formation process of the Fe-Mn oxides. The outcomes indicated that the genesis of these oxides commences with the emission of low-temperature hydrothermal fluid by the petit-spot magma. This fluid then ascends through the sediment column, causing Mn oxides to precipitate at the interface with seawater. As more Mn oxide accumulates, a downward growth of the Mn oxide layer, containing silicate debris, occurs towards the seabed. Subsequently, this debris undergoes alteration. The next step involves the deposition of Fe oxides at the interface between the low-temperature hydrothermal fluid and the Mn oxides, utilizing similar mechanisms. Following the cessation of hydrothermal activity, a hydrogenetic rim forms on these deposits when exposed to seawater.

"Drawing upon earlier studies, we can infer that hydrothermal fluid originating from petit-spot volcanoes is likely to exhibit higher concentrations of CO2 and methane in comparison to that emanating from the mid-ocean ridge," Azami elucidates. "Consequently, this implies that the elemental contributions resulting from hydrothermal activity in petit-spot volcanoes across the globe might hold significant ramifications for the global biogeochemical cycles, specifically the carbon cycle."

These findings underscore the presence of hydrothermal activity in cold and old oceanic plates and highlight the need for further studies on petit-spot volcanoes.