Hydrocarbon gases in the cores from Hole 1253A were analyzed by the headspace technique (see "Organic Geochemistry" in the "Explanatory Notes" chapter). This was done as part of the shipboard safety and pollution prevention monitoring program. The results are summarized in Table T10. Hydrocarbon concentrations do not exceed the background values of ~3 parts per million by volume (ppmv) throughout all of the sedimentary sequences. This is in agreement with the measurements of the cores from Site 1039 during Leg 170 (Kimura, Silver, Blum, et al., 1997). As an example, the methane concentration depth profiles from Sites 1039 (Leg 170) and 1253 (Leg 205) are shown in Figure F74. Correspondingly, sulfate concentrations are high and close to the seawater value in the respective sediment strata (Table T8; Fig. F71). Hence, the strong reducing conditions needed for bacterial methane production are not achieved in the sediments of the incoming oceanic plate at Sites 1039 and 1253.
The depth distributions of inorganic carbon (represented as calcium carbonate), organic carbon, nitrogen, and sulfur in the solid phase from Hole 1253A are reported in Table T11. The combined data from Sites 1039 and 1253 are plotted for CaCO3, total organic carbon (TOC), and total sulfur (TS) vs. depth in Figure F75.
The determination of the TOC content is largely biased because of the different amounts of total carbon (TC) and accuracies of the carbon-nitrogen-sulfur (CNS) technique and the carbonate titration. At high CaCO3 contents (>10 wt%) and low TOC concentrations (close to the detection limit of the CNS element analyzer) (see "Organic Geochemistry" in the "Explanatory Notes" chapter), the TOC values only reflect the noise in the TC measurements of the CNS technique. Hence, small differences in TC with respect to CaCO3 values easily lead to even "negative" TOC concentrations. Thus, the TOC values measured at this site and its derived information (TOC/total nitrogen ratio) are considered unreliable. From these findings it can only be estimated that the TOC content of the sediments is very low—about or below the detection limit of the method (i.e., 0.3 wt%, a typical value for pelagic, carbonate-rich sediments). This is consistent with results in Unit U3 of Leg 170, Site 1039 (Kimura, Silver, Blum, et al., 1997). As expected, no significant organic matter could be observed in the igneous rock (Fig. F75; Table T11).
The carbonate content follows the general trend of Site 1039 and further decreases to ~45 wt% at 385 mbsf. Immediately above the sill (400 mbsf), CaCO3 is almost absent (<2 wt%), corresponding to a more lithified, clay-dominated layer where Si increases and K and Si(OH)4 show changes in the pore water (see "Inorganic Geochemistry"). The sediment samples analyzed between the two igneous units (431-442 mbsf) are again enriched in CaCO3 (40-60 wt%) (Fig. F75). The two igneous rock sections show no observable calcareous alteration with contact of the overlying and enclosed sediment layers (Table T11). Measurements of the alteration material in veins and voids yields carbonate contents up to 10 wt%. These values (and the other concentrations reported in Table T11) represent minimum concentrations because they are diluted by the surrounding igneous material.
TS concentrations below 370 mbsf at Site 1253 are up to 0.2 wt% and decrease to zero directly above the sill. The sediment layer between the sill and the lower igneous section shows values up to 0.1 wt%, whereas the igneous rock does not show significant amounts of sulfur, except for one vein filling with 0.2 wt% (Section 205-1253A-32R-3 [Piece 3A]) (Table T11). The sulfur minerals are most likely pyrite and chalcopyrite, which were macroscopically observed in Section 205-1253A-15R-3 (Piece 7B) (not analyzed aboard ship).