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doi:10.2204/iodp.proc.331.201.2016

Introduction

Hydrothermal activity at seafloor spreading centers is known to convert sedimentary organic matter into petroleum (Simoneit et al., 1984). Key variables that govern the type of hydrocarbons present within hydrothermally altered sediments include the type of organic matter found within host sediments, its quantity, and the temperature of pyrolysis (Simoneit, 1993). Iheya North hydrothermal field is located in the middle Okinawa Trough at 27°47.415′N, 126°53.8605′E. It is located within a subsea collapsed-caldera complex that is filled with a mixture of mass-wasted volcanoclastic sediments and hemipelagic sediments, many of which have been hydrothermally altered into hydrothermal lithologies (see the “Expedition 331 summary” chapter [Expedition 331 Scientists, 2011]). Some sediments encountered by drilling were not hydrothermally altered, and in these locations volcanic pumice and hemipelagic mud are the most frequently encountered sediment types. The main type of sedimentary organic matter expected in sediments before hydrothermal alteration would be allochthonous terrestrial (derived from the Yangtze and Yellow Rivers ~1000 km east and north) and autochthonous (pelagic) marine sedimentary organic matter. Present-day maximum temperatures encountered during drilling were 300°C for Site C0013, ~180°C for Site C0014, and <80°C for Site C0017. Excepting the highest temperatures recorded, temperatures are actually relatively low with respect to the rapid pyrolysis described by Simoneit (1993) for hydrothermal activity at seafloor hydrothermal ridges, but on geological timescales they would be considered sufficient for pyrolysis to occur (Killops and Killops, 2005). The solvent extractable organic matter present at these sites, if it is indigenous, would be expected to possess a biomarker fingerprint characteristic of both higher plant and marine plankton. Bitumen would be expected to be mobile within the more permeable pumice units but less mobile in the mud units. The purpose of this report is to record the chemical composition of bitumen from hydrothermally altered sediments and determine whether it could have been generated by high-temperature pyrolysis of local sediments.

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