Several geochemical analyses were performed on samples to assess their geochemical characteristics and variations according to vertical depth and site location. Total carbon, total organic carbon (TOC), and total nitrogen (TN) content decreased from northeast to southwest along the transect of Integrated Ocean Drilling Program Expedition 311 Sites U1329, U1327, U1325, and U1326. Variations with depth showed differing trends with sites and compositions. Most organic matter was at an immature stage and Type III evolution based on Rock-Eval pyrolysis. However, results from TOC/TN, δ 13 C org , and δ 15 N org analyses showed that organic matter has an admixture of origin.
IntroductionOperations during Integrated Ocean Drilling Program (IODP) Expedition 311 drilled five sites across the northern Cascadia margin (see the "Expedition 311 summary" chapter). Among the five sites, the northeast-southwest transect of Sites U1329, U1327, U1325, and U1326 was cored to investigate gas hydrate occurrence and formation. Cold vent Site U1328 was also drilled. Based on recovered core sections, Sites U1329, U1327, U1326, and U1328 are divided into three lithostratigraphic units (I, II, and III) and Site U1325 is divided into five lithostratigraphic subunits (IA, IB, II, III, and IV) (see the "Expedition 311 summary" chapter). Characterization of elements (total carbon [TC], total nitrogen [TN], and total sulfur [TS]) and organic matter in bulk sediments according to transect and lithostratigraphic units could be helpful in understanding the depositional environment, diagenesis, and origin of organic matter and give additional information about gas hydrate occurrence. Hence, this study reports several geochemical results on bulk sediments from the northern Cascadia margin.
Methods
Elemental analysisA total of 311 squeeze cake samples from Sites U1325-U1329 were used for geochemical analyses. A part of each squeeze cake was dried for 24 h using a freeze dryer at the Korea Institute of Geoscience and Mineral Resources. After drying the bulk sediment sam- ples were ground and homogenized in an agate mortar. TC and TN contents were measured by combustion method using a LECO CHN-900 apparatus with detection limits of 0.001% for TC and 0.01% for TN. TS contents were determined using a LECO SC-132 apparatus with a detection limit of 0.001%.
Rock-Eval pyrolysisUsing Rock-Eval pyrolysis, free and adsorbed hydrocarbons released during programmed heating of a sample are recorded as the first peak in a pyrogram (S 1 ) under low temperature. The second peak (S 2 ) in the pyrogram represents hydrocarbons released by kerogen cracking when the sample is heated to 550°C. The temperature at the maximum of the S 2 peak is defined as T max . CO 2 , shown as the third peak (S 3 ) in the program, is also generated by kerogen degradation. When these components are normalized to the TOC content, the S 2 peak becomes the hydrogen index (HI = S 2 × 100/TOC) and S 3 becomes the oxygen index (OI = S 3 × 100/TOC) (Tissot and Welte, 1984; Peters, 1986)...
