Site C0023: Expedition 370

Abstract

IODP Expedition 370 aimed to rigorously study the influence of temperature on the size, activity, and taxonomic composition of deep subseafloor microbial communities by revisiting an already well-characterized geological setting with high heat flow: the Muroto Transect in the central Nankai Trough off Japan. The expedition established Site C0023 in the vicinity of Ocean Drilling Program (ODP) Sites 1173, 1174, and 808 about 125 km offshore Kochi Prefecture, Japan. In this area, heat flow is exceptionally high and was expected to result in temperatures of ~110°–130°C at the sediment/basement interface at ~1200 mbsf. This particular geological setting not only provides suitable conditions for examining the putative temperature-dependent biotic–abiotic transition zone at relatively shallow depth but also allows the investigation of temperature effects at high resolution, because the increase of temperature with depth is still gradual enough for the establishment of distinct depth horizons with suitable conditions for psychrophilic (optimal growth temperature range <20°C), mesophilic (20°–43°C), thermophilic (43°–80°C), and hyperthermophilic (>80°C) microorganisms. Expedition 370 was designed to (1) comprehensively study the factors that control biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~2° to ~130°C and thus likely encompasses the biotic–abiotic transition zone and (2) determine geochemical, geophysical, and hydrogeological characteristics in sediment and the underlying basaltic basement in order to determine if the supply of fluids containing thermogenic and/or geogenic nutrient and energy substrates potentially supports subseafloor microbial communities in the Nankai accretionary complex.