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doi:10.2204/iodp.proc.344.203.2016

Introduction

The Costa Rica Seismogenesis Project, Program A Stage 2, drilled at five sites located offshore of the Osa Peninsula near the incoming Cocos Ridge to study the processes controlling earthquakes in an erosional subduction zone (see the “Expedition 344 summary” chapter [Harris et al., 2013a]). Among the five sites, Sites U1380, U1412, and U1413 were studied to investigate deep fluid transportation. Several studies on the sources and evolution of the magmas are well advanced, even though subduction zone settings can be quite complex (e.g., Stern, 2002; van Keken, 2003; Hall and Kincaid, 2001). In contrast, we know little about fluid migration in subduction zones. The characterization of gas isotope and conversion ratio in headspace gas (HS) and void gas (VG) could help to understand deep fluid systems and the origin of gases. In this study, we provide the molecular and stable isotopic compositions of HS and VG, which are indicators for the origin and migration of the gases. Furthermore, we show the organic matter to hydrocarbon gas conversion ratios at the drilled sites.

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