2020 Volume 129 Issue 6 Pages 757-777
Reported are the origins and chemistry of hot spring water unique to the Ohmi–Hakuba region in the northern Japan Alps, central north Honshu, where a variety of rock types, derived from accretionary complex formed in the Ediacaran (presumably ca. 620 Ma), are regionally exposed. One of the largest bodies of serpentinized peridotite in Japan intersects the Quaternary volcanic front. Featuring an unusual geochemistry, the resulting unique hydrothermal hot springs yield a high pH (ca. 10-11) and a continuous supply of H2. Research reveals four types of hot spring in the Hakuba region: (1) serpentinite hosted hot spring water, (2) high-salinity and carbonated water, (3) Archean type low pO2 hot spring water, and (4) acidic and sulfuric hot spring water with a H2S gas input from magma. The high alkali and H2-enriched hot spring water (Type 1) differs remarkably from other hot springs in this region. In terms of geochemistry, there is a dissolved oxygen content due to the production of abundant H2, which is the reason why a Hadean-type microbial community is present. The origins and evolution of life are closely related to atmospheric oxygen level. Generally, anaerobic microbes inhabit subsurface areas where free oxygen is limited, while oxygen adaptive creatures cannot survive in an anaerobic environment. This means anaerobic microbes have not evolved, and remain as “living fossils”. Hakuba OD1 is one of the most important candidates for the oldest form of life directly connected to LUCA, because it has survived in a Hadean-like environment since emerging. The next research target is the ecosystem in a H2-enriched environment without free oxygen.