Multiple geochemical analyses (total carbon [TC], total nitrogen [TN], total sulfur, isotopic compositions of δ 13 C org and δ 15 N org , and Rock-Eval pyrolysis) were performed on bulk sediment samples from Integrated Ocean Drilling Program Expedition 344 to assess geochemical characteristics and variations with depth at each drilled site. Downcore profiles of geochemical analyses vary with site and composition and do not correlate with lithostratigraphic units. In addition, the downcore profiles of TC, total organic carbon (TOC), total inorganic carbon (TIC), and TN analyzed postcruise displayed similar trends to those determined from onboard analyses at all sites. Based on Rock-Eval pyrolysis, most of the organic matter is at an immature stage of Type III (vascular land plants) evolution, which is inconsistent with the results from TOC/TN, δ 13 C org , and δ 15 N org that indicated that most of the organic matter originates from an admixture of marine and freshwater algae (Sites U1380 and U1413) or from marine algae (Site U1412). vided into three units (Units I, II, and III), and Site U1414 is divided into five units (Subunits IA, IB, IIA, and IIB and Unit III) (see the "Expedition 344 summary" chapter [Harris et al.,

Gas stable isotopes (δ 13 C CH4 , δD CH4 , and δ 13 C CO2 ) and the conversion ratios of headspace gas and void gas have been analyzed at Integrated Ocean Drilling Program Expedition 344 Sites U1380, U1412, and U1413 to reveal the origin and migration of gas. The results of headspace gas and void gas analyses illustrate that methane (C 1 ) is the dominant component and ethane (C 2 ) is a minor component. Variations in C 1 /C 2+ ratios with depth display differing trends across sites. The δ 13 C CH4 and δD CH4 values are less than -57.0‰ and -181‰, respectively, indicating that hydrocarbon gas might be originated through CO 2 reduction. In addition, the organic matter to hydrocarbon gas conversion ratios of biogenic void gas at Sites U1412 and U1413 range from 22% to 30%. IntroductionThe 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. Methods and materials Gas compositionThe HS samples were analyzed onboard by an Agilent 6890 Series II gas chromatograph (GC3) (see the "Methods" chapter [Harris Harris, R.N., Sakaguchi, A., Petronotis, K., and the Expedition 344 Scientists