Fumio Inagaki, PhD
Director, Principal Senior Scientist
Mantle Drilling Promotion Office (MDP),
Institute for Marine-Earth Exploration and Engineering (MarE3),
Japan Agency for Marine-Earth Science and Technology (JAMSTEC)
Professor, Graduate School of Science and Faculty of Science,
Department of Earth Sciences, Tohoku University.
Guest Professor, Research Organization for Nano & Life Innovation, Waseda University.
Guest Professor, Graduate School of Integrated Arts and Sciences, Kochi University.
Work Address: JAMSTEC Yokohama Institute for Earth Sciences (YES)
Showa-machi 3173-25, Kanazawa-ku, Yokohama 236-0001, JAPAN
MY RESEARCH INTEREST
The Deep Subseafloor Biosphere
Over the past decades, scientific ocean drilling has explored subseafloor environments at various oceanographic and geological settings, which resulted in numerous discoveries on Earth’s planetary sub-systems —the atmosphere, hydrosphere, geosphere, and biosphere. The dynamism fostering the co-evolution of life and the Earth system is principally constrained by extra- and intra-terrestrial energy sources. The lithosphere consisting of sediments, crusts and upper mantle plays a significant role as an interface between the Earth’s asthenosphere and the overlying hydrosphere and atmosphere. Drilling the Earth’s lithosphere down to the upper mantle has significantly expanded our understanding of Earth’s sub-systems and will continue to do so in the future. To date, only little is known about how Earth’s various spheres interact, despite the awareness that such spheres connect and interact with each other. Building this knowledge will provide useful insights at various levels into the past, present and future of our Earth and human society.
With respect to the deep biosphere, accumulating evidence from ocean margin sites indicates that remarkable numbers of anaerobic microbial cells are present at least down to at least ~2.5 km below the ocean floor (Inagaki et al., Science, 2015). In open ocean sites, the occurrence of microbial communities and oxygen was observed in the entire sediment column of the ultra-oligotrophic South Pacific Gyre, qualifying up to ~37% of the global oceanic sediment as aerobic biosphere (D'Hondt et al., Nature Geoscience, 2015). These recent findings through scientific ocean drilling have characterized the deep biosphere as one of the important Earth’s sub-systems, where microbial life inhabiting the vast oceanic lithosphere influences whether several important elements are sequestered for millions of years or returned to the ocean as active agents with an impact on life and climate (Hinrichs & Inagaki, Science, 2012).
The Limits of Deep Life and Geosphere-Biosphere Interactions
The International Ocean Discovery Program (IODP) Expedition 370
“Temperature-Limit of the Deep Biosphere off Muroto”
During the IODP Expedition 370 in 2016, using the deep-Earth drilling vessel Chikyu, we established a ~1.2 km-deep borehole (~4.8 km water depth) at Site C0023, the Nankai Trough protothrust zone off Cape Muroto, Kochi Prefecture, Japan. A total of 112 cores and over 13,000 sub-samples were successfully obtained to answer the key questions: What are the limits to life in the plate subduction zone? What environmental factors do constrain the Earth's planetary habitability of life deep beneath the ocean floor? What is the evolutionary nature of deep subseafloor microbial communities?
(see Heuer et al., Science, 2020; Beulig et al., Nature Communications, 2022)
Biogeochemical Carbon Cycling and Evolution of the Deeply Buried Microbial Ecosystem beneath the Ocean
The Integrated Ocean Drilling Program (IODP) Expedition 337
“Deep coalbed biosphere off Shimokita”
The Integrated Ocean Drilling Program (IODP) Expedition 337 was the first expedition dedicated to subseafloor microbiology that used riser-drilling technology with the deep-Earth drilling vessel Chikyu. The drilling Site C0020 is located in a forearc basin formed by the subduction of the Pacific Plate off the Shimokita Peninsula, Japan, at a water depth of 1180 m. Primary scientific objectives during Expedition 337 were to study the relationship between the deep microbial biosphere and a series of ∼2 km deep subseafloor coalbeds and to explore the limits of life in the deepest horizons ever probed by scientific ocean drilling.
In September 2012, we penetrated a 2466 m deep sedimentary sequence with a series of lignite layers buried during the late Oligocene and Miocene, ranging from warm-temperate coastal backswamps to a cool water continental shelf. The occurrence of small microbial populations and their methanogenic activity were confirmed down to the bottom of the hole by microbiological and biogeochemical analyses.
Expedition 337 provided a test ground for the use of riser-drilling technology to address geobiological and biogeochemical objectives and was therefore a crucial step toward the next phase of scientific ocean drilling
(see Inagaki et al., Science, 2015).
Exploring Planetary Habitability and Sustainability
The Earth has experienced several catastrophic perturbations during its history but remained habitable. How will planet Earth and life co-evolve in the future? How will life adapt and transform itself across the environmental changes ahead of us? How will life continue to shape the Earth? How do microbial life and ecosystems naturally adapt to and co-evolve on the human to geologic timescales? How can we enhance the Earth’s habitability and expand sustainability into the deep future?
Only a better understanding of the Earth’s multi-spheres interactions through scientific ocean drilling will enable informed conclusions regarding the origins and evolution of life, oceans and Earth—the characterization and monitoring of multi-spheres boundaries, including the limits to the deep biosphere, will highlight the organization and interactions of Earth’s sub-systems and provide critical information enabling the discovery and utilization of new functions of Earth’s multi-spheres deep beneath the ocean (Inagaki & Taira, Oceanography, 2019).
Missions to the Mantle: The most challenging endeavor in geoscience
To date, the impact of anthropogenic climate and environmental changes on the Earth’s ecosystem and human society is becoming increasingly apparent. Understanding complex feedback-mechanisms that may affect the trajectory of our planetary systems, such as the balance of carbon and energy as the future human population grows, is an urgent need for the sustainability of the ocean and Earth’s health as well as human well-being. From a mid-to-long-term perspective, scientific ocean drilling will provide unique opportunities for the international and multidisciplinary scientific community to gain new insights into geosphere-biosphere interactions from the past and present to the future. For example, ecosystem responses to environmental changes could be deciphered from ancient DNA, biomarkers, and/or indigenous microbial communities that persist in sediment over geologic time. The planetary habitability involving interactions between the Earth’s surface and subsurface biospheres remains largely elusive in energy, space, and time. Also, it is almost completely unknown what types of pre-biotic/abiotic chemical reactions occur beneath the limit of the deep biosphere and what roles of the chemical evolution are in the emergence and evolution of life.
There are many places to drill and study mantle-ocean-life-atmosphere interactions. For example, at hydrothermally active volcanic hotspots, spreading ridges, outer-rise faulting systems, and even deep subseafloor abyssal plains, relatively low-temperature oxic seawater may deep penetrate the oceanic crust and react with mantle peridotites to supply nutrient and energy substrates for in-situ or shallower life. Because the mantle occupies 83% of the Earth’s volume and 67% of its weight, and given the fact that its energy convection intimately controls the Earth’s dynamism, drilling, characterizing, and observing the oceanic lithosphere down to the upper mantle penetrating through the Mohorovičić discontinuity will be the most comprehensive, challenging, and innovative endeavor on Earth, which will be able to accomplish using state-of-the-art technologies.