Deciphering factors that constrain the extent of the deep biosphere in a subduction zone1
Published May 2016
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Determining factors that limit the biomass, diversity, and activity of subseafloor microbial communities is one of the major scientific goals to be addressed by scientific ocean drilling. In the International Ocean Discovery Program (IODP) T-Limit Project, we will drill and core at new boreholes in the immediate vicinity of Ocean Drilling Program (ODP) Sites 1173, 1174, and 808 off Cape Muroto, Japan, in the central Nankai Trough, where anomalously high heat flow regimes result in temperatures of 110° to 140°C at the sediment/basement interface. Because of their location in the trench (Site 1173) and landward protothrust zone of the Nankai Trough accretionary prism (Sites 808 and 1174), the sites have different geotectonic and thermal histories that have resulted in contrasting (bio)geochemical modes of hydrocarbon gas production and consumption. Although the upper temperature limit appears well constrained at relatively energy-rich hydrothermal vent systems at just above 120°C, it is unknown in energy-starved sedimentary subseafloor settings but is generally presumed to be lower and thus expected to be covered by our target sites. During the IODP T-Limit Project, we aim to
- Comprehensively study the factors that control biomass, activity, and diversity of microbial communities in a subseafloor environment where temperatures increase from ~30° to ~130°C and thus likely encompasses the biotic–abiotic transition zone; and
- Determine geochemical, geophysical, and hydrogeological characteristics in sediment and the underlying basaltic basement and elucidate if the supply of fluids containing thermogenic and/or geogenic nutrient and energy substrates may support subseafloor microbial communities in the Nankai accretionary complex.
Because of the D/V Chikyu’s schedule, these scientific objectives cannot be achieved within a single expedition. During the first T-Limit expedition (370), we will drill and retrieve core samples from sedimentary sections (200–1210 m below seafloor) and basement basalt (1210–1260 m below seafloor) at the protothrust site near ODP Site 1174 and measure temperatures in situ.
1Hinrichs, K.-U., Inagaki, F., Heuer, V.B., Kinoshita, M., Morono, Y., and Kubo, Y., 2016. Expedition 370 Scientific Prospectus: T-Limit of the Deep Biosphere off Muroto (T-Limit). International Ocean Discovery Program. http://dx.doi.org/10.14379/iodp.sp.370.2016
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