The main objectives for Site 1255 with respect to the organic geochemistry were similar to those at Site 1254. They are (1) characterizing the hydrologic system along the décollement observed during Leg 170 at Site 1043; (2) determining the chemistry and composition of pore fluids and gases from the décollement to compare with profiles measured during Leg 170 and to evaluate possible spatial heterogeneity and any possible changes through time for diagenetic and hydrologic modeling; (3) constraining pathways of fluid return to the surface and to evaluate the effects of this flow system on element fluxes; and (4) determining the target horizon for long-term fluid flow monitoring. Because of time constraints, only limited coring was attempted (four cores from 123 to 157 mbsf, with recovery between 132.7 and 152.6 mbsf) for determining the horizon to target long-term fluid flow monitoring. However, limited core recovery permitted only a few samples to be measured, and interpretations should be regarded as preliminary.
At the prism Site 1255, volatile hydrocarbon gases (primarily methane, ethane, and propane) were sampled by headspace and vacutainer techniques and analyzed by gas chromatography (see "Organic Geochemistry" in the "Explanatory Notes" chapter). Headspace and vacutainer samples were taken and analyzed as soon as possible to allow for "real-time" monitoring. Headspace results are summarized in Table T9 and shown in Figure F22, and vacutainer data are compiled in Table T10.
In contrast to the cores at Site 1254, no gas voids were apparent at Site 1255 and, hence, the gas samples taken with vacutainers show generally low methane concentrations but large contamination with air (Table T10), a result of the very low sediment recovery (7 m out of 34 m of core) as well as core disturbance from RCB drilling; both conditions facilitate loss of methane and aeration of the voids in the core. Therefore, at this site, the results of the headspace samples are preferred over the vacutainer data.
Headspace methane concentrations were high (>3000 ppmv) in Core 205-1255A-2R (132-135 mbsf) and then dropped drastically to ~5 ppmv at ~145 mbsf (Section 205-1255A-3R-2). Ethane concentrations are ~1000 times lower and disappear totally at ~144 mbsf. Based on the data from Sites 1254, 1040, and 1043 (Kimura, Silver, Blum, et al., 1997), propane was chosen as an indicator for flow from deeper sediment layers through the flow conduit system along the décollement (see also "Organic Geochemistry" in the "Site 1254" chapter). Propane was present but concentrations were low (~1 ppmv) at 132-135 mbsf and completely absent at greater depths. Higher hydrocarbons could not be detected in the gas samples. Compared to Site 1043, headspace hydrocarbon concentrations were constantly lower but showed a similar trend in the respective depth interval. Unfortunately, at both Sites 1254 and 1255, recovery was very low and, hence, the assigned depths at this site are approximate values and only comparable to Site 1254 data within a few meters (e.g., Core 205-1255A-2R could shift 7.12 m, Core 3R could shift 5.21 m, and Core 4R could shift 9 m). From the K, Ca, Mg, and Na data (see "Inorganic Geochemistry") it can be inferred that Core 205-1255A-2R lies above the décollement as determined at Site 1043 (Kimura, Silver, Blum, et al., 1997), whereas the decrease in hydrocarbon concentrations takes place across the lithologic boundary between prism and underthrust sediments at 144.08 mbsf (see "Structural Geology").
The CH4/C2H6 ratio for two samples only (Table T9) shows values of ~1000 above the lithologic boundary, indicating migrated thermogenic hydrocarbons (e.g., propane).
The results of inorganic carbon (reported as calcium carbonate), organic carbon, nitrogen, and sulfur measurements in the solid phase from the cores of Hole 1255A are reported in Table T11 (for details see "Organic Geochemistry" in the "Explanatory Notes" chapter). In agreement with Site 1043 data, calcium carbonate concentrations are generally low (<5 wt%) in the cored intervals (i.e., in the prism as well as the uppermost underthrust). For the prism sediments, the total organic carbon (TOC) content shows values (~1 wt%) similar to those at Site 1043 (~1.4 wt%). In the uppermost underthrust sediments, the TOC content increases to the high surface concentrations (~1.7 wt% in Section 205-1255A-4R-1) observed at Site 1039 and also observed at Site 1043 below the décollement (Kimura, Silver, Blum, et al., 1997). Total sulfur (TS) concentrations are ~0.4 wt% in the prism sediments and, thus, lower than those at Site 1043 (however, the accuracy of the TS data was very poor); they increase to 1.0 wt% in the underthrust sediments.
To characterize the type of organic matter in the sediments, total organic carbon/total nitrogen (TOC/TN) values and hydrogen index (HI) values from Rock-Eval pyrolysis have been used. Unfortunately, the S3 channel of the Rock-Eval did not record any data and, hence, no oxygen index or S2/S3 ratio could be calculated (Table T12).
The TOC/TN ratio ranges from 6 to 8 (Table T11) in the prism as well as the underthrust sediments, indicating a predominantly marine origin of the organic material. The Redfield ratio of 6.6 gives the typical average for marine organic matter. As at Site 1254, the low HI values (100-140 mg HC/g TOC) (Table T12) suggest a significant input of terrigenous material (Emeis and Kvenvolden, 1986). This was not observed at this site. The discrepancy is most probably due to analytical problems with the Rock-Eval instrument. However, lithologic variations cannot be excluded. The low content (~0.2) of pyrolizable carbon as well as the combination of temperatures of maximum hydrocarbon generation (Tmax) <440°C and low production indices (production indices < 0.2) reflect organic matter that is immature for oil production.