Deep sea: Earthquake as engine for carbon cycle

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The research vessel Kaimei set sail from the port of Yokusuka City on April 13,
The research vessel Kaimei set sail from the port of Yokusuka City on April 13, 2021. Sediment deposits in basins up to eight kilometers deep at the bottom of the Japan Trench were sampled using a special deep-sea drilling rig. © JAMSTEC

As part of an international deep-sea expedition, a team of researchers under the co-leadership of Innsbruck geologist Michael Strasser took the deepest samples ever obtained from the seafloor at a depth of more than 8000 meters in the Japan Trench in 2021. Large amounts of dissolved carbon and enormous methane reservoirs were discovered in the seafloor, the formation of which is favored by the strong earthquake activity there. The impact of processes in deep-sea trenches on the global carbon cycle has hardly been explored so far. The latest findings on this have now been published in Nature Communications.

Deep-sea trenches of the so-called hadal zone (from the Greek "hades" for "underworld") form the deepest parts of the seafloor from about 6000 meters below sea level and are among the least explored places on Earth. An international team of researchers, co-led by Michael Strasser from the Institute of Geology at the University of Innsbruck, systematically sampled the seafloor along the 7 to 8 kilometer deep Japan Trench two years ago as part of Expedition 386 "Japan Trench Paleoseismology" of the International Ocean Discovery Program (IODP). The first scientific results from this expedition provide insights into the relationship between earthquakes and newly discovered process flows of a dynamic carbon cycle deep beneath the seafloor.

New records in deep-sea drilling

The Japan Trench is part of the Pacific Ring of Fire, a region of particular interest for earthquake and deep-sea research. Here, parts of the oceanic crust bend downward several kilometers, forming deep-sea trenches and sliding jerkily along so-called subduction zones beneath overthrusting earth plates deep into the earth’s interior. In the process, large earthquakes occur, such as the Tohoku-oki earthquake of 2011, which made headlines primarily because of the tsunami and the Fukushima nuclear disaster. Such earthquakes also flush gigantic amounts of organic carbon from shallow water into the deep-sea channel. The sediment layers deposited there therefore provide information about the history of earthquakes and the carbon cycle in the deep sea floor. Using novel technologies optimized for the deep sea, sediment samples were obtained from the research vessel with a so-called "giant piston corer" - a rope-guided, very large piston corer - at 15 locations along the 500-kilometer-long Japan Trench from boreholes up to almost 38 meters deep, and a total of 832 core meters were analyzed. "This unprecedented area-wide scientific sampling in water depths between 7445 to 8023 meters below sea level breaks two new records in the more than 50-year history of the deep-sea scientific drilling program," Michael Strasser is pleased to report. "We drilled the deepest water site at a water depth of 8023 meters and were able to collect a seafloor sample from a record depth of 8060.74 meters."

Breakthrough in geochemical pore water analyses

The pore water of the deep-sea sediments was also systematically sampled. Using high-resolution organic geochemical analyses on dissolved carbon, Prof. Rui Bao of the Ocean University of China and his team have made a breakthrough. The results show that large amounts of labile dissolved carbon are stored in sediment pore water. The storage of dissolved carbon indicates active remineralization of organic carbon in the deep-sea trenches, which is much larger than in other previously known deep-sea environments of the open ocean. Advanced radiocarbon dating methods have allowed the aging and accumulation of dissolved organic and inorganic carbon in the seafloor to be quantified for the first time. "These dissolved carbon fractions can have major impacts on microbial-driven carbon cycling processes in the deep seafloor and affect the long-term global carbon cycle, as they can bury carbon in deep-sea channels and eventually in the subduction zone," said Mengfan Chu, a doctoral student at Ocean University of China and lead author of the study.

Enormous methane reservoirs

Results of sediment and gas-phase analysis conducted on the cores support these hypotheses. Huge methane reservoirs were found in sediments along the entire Japan Trench during the expedition, indicating intense microbial methane formation. This methane generation is enhanced by recurrent large earthquakes along the subduction zone. The team also discovered traces of hydrated carbonates crystallized in the sediment, indicating active transformation of carbon between its different forms (sedimentary, dissolved, gaseous, and mineral) and implying that the Japan Trench hosts a dynamic carbon cycle linking the Earth’s surface and deeper interior along the subduction zone. By increasing the supply of organic carbon and dynamically affecting the physical and chemical properties of sedimentary deposits, earthquakes thus act as a powerful driver of carbon cycling in deep-sea channels and for the metabolism of the so-called deep biosphere in these extreme ecosystems. "These discoveries are convincing evidence that deep-sea gullies in the hadal zone do not have quiet or constant environmental conditions, as previously thought," Rui Bao explains.

Earthquake research and global carbon cycle impacts

"The standard analyses and age dating of the sediment cores carried out during the expedition show that we can study past earthquake processes and their effects on the hadal zone in detail up to at least 24,000 years into the past," Michael Strasser emphasizes. 36 scientists with expertise in various geoscientific disciplines from Austria, Australia, China, Finland, France, Germany, India, Japan, the United Kingdom and the United States are further evaluating the expedition data. Through state-of-the-art analyses, this will characterize earthquake recurrence patterns for reliable hazard assessment and further improve knowledge of the dynamic carbon cycle beneath the seafloor of the Japan Deep Sea Trough. Implications on the global carbon cycle will be able to be more concretely defined by the international and interdisciplinary team in the future.

Chu, M., Bao, R., Strasser, M., Ikehara, K., Everest, J., Maeda, L., Hochmuth, K., Xu, L., McNichol, A., Bellanova, P., Rasbury, T., Kölling, M., Riedinger, N., Johnson, J., Luo, M., März, C., Straub, S., Jitsuno, K., Brunet, M., Cai, Z., Cattaneo, A., Hsiung, K., Ishizawa, T., Itaki, T., Kanamatsu, T., Keep, M., Kioka, A., McHugh, C., Micallef, A., Pandey, D., Proust, J. N., Satoguchi, Y., Sawyer, D., Seibert, C., Silver, M., Virtasalo, J., Wang, Y., Wu, T. W., Zellers, S. (2023). Earthquake-enhanced dissolved carbon cycles in ultra-deep ocean sediments. Nature Communications.’023 -41116-w.