Kiminori Shitashima

Chemical characteristics of hadal waters in the Izu-Ogasawara Trench of the western Pacific Ocean

Vertical profiles of potential temperature, salinity, and some chemical components were obtained at a trench station (29°05′N, 142°51′E; depth = 9768 m) in the Izu-Ogasawara (Bonin) Trench in 1984 and 1994 to characterize the hadal waters below ∼6000 m depth. We compared portions of both the 1984 and 1994 profiles with nearby data obtained between 1976 and 2013. Results demonstrated that the hadal waters had slightly higher potential temperature and nitrate and lower dissolved oxygen than waters at sill depths (∼6000 m) outside the trench, probably due to the effective accumulation of geothermal heat and active biological processes inside the trench. The silicate, iron, and manganese profiles in 1984 showed slight but significant increases below ∼6000 m depth, suggesting that these components may have been intermittently supplied from the trench bottom. Significant amounts of 222Rn in excess over 226Ra were detected in the hadal waters up to 2675 m from the bottom, reflecting laterally supplied 222Rn from the trench walls.

Monitoring techniques using Autonomous Underwater Vehicles for potential seepage of CO2 from sub-seafloor storage sites

Although a number of Carbon Capture and Storage sub-seafloor storage sites are now either in operation or planned for CO2, little is known about the effect of potential seepage on marine ecosystems. Here we describe a comprehensive field campaign to the North Sea (RRS James Cook Cruise 77) that used Autosub 6000 to test methods for detection of seepage, including formation fluids, natural gas and CO2, as it passes through the sedimentary overburden and into the water column, and develop monitoring strategies suitable for all offshore carbon capture and storage projects. In this paper we describe the Hugin Fracture, a 2 km long discontinuity imaged on the seabed, and associated fluid flow activity, revealed by geophysical observations including high reflectivity acoustic anomalies within the overburden. Further results in favour of active fluid flow along this fault will be presented, using a combination of multidisciplinary datasets comprising video photography, Eh sensor and sediment samples.