Site 1252 is located on the flank of an anticline in the basin east of Hydrate Ridge. The objectives at this site are to sample the accretionary complex at shallow depth and to learn when the anticline formed. Site 1252 is located near a prominent BSR. The shipboard organic geochemistry program at Site 1252 included only the analyses of hydrocarbon gases. A description of the methods used for the gas analyses is summarized in "Organic Geochemistry" in the "Explanatory Notes" chapter.
The levels of methane (C1), ethane (C2), ethylene (C2=), and propane (C3) remaining in the cores were measured using the headspace technique. The results are reported in Table T4 and plotted as parts per million by volume (ppmv) of gas component vs. depth in Figure F16. Methane content increases rapidly from levels of 2-261 ppmv in the shallowest samples at 0-4 mbsf, to ~10,165 ppmv at a depth of 6.4 mbsf, and then to levels of ~20,000-50,000 ppmv at greater depths. This sharp increase in headspace methane occurs at the approximate depth of the SMI, as seen previously at Sites 1244, 1245, 1247, 1251, and elsewhere. The vial headspace ppmv C1 concentration was recalculated to express the millimolar methane concentrations remaining in the cores. These estimates of absolute gas concentration are meaningful only in shallow zones where sediments are undersaturated or slightly supersaturated at surface conditions with respect to dissolved methane. The calculated dissolved CH4 concentration is shown in Figure F17, along with the sulfate depletion profile (see "Sulfate, Methane, and the Sulfate/Methane Interface" in "Interstitial Water Geochemistry"). The general pattern of methane increase and sulfate decrease is consistent with an SMI depth of ~5 mbsf.
Ethane content of headspace gas is maintained at low levels (<10 ppmv) in core samples from Site 1252 from the seafloor to ~200 mbsf. Only at the base of the cored sequence from 225 to 254 mbsf do ethane contents increase to values ranging from 24- to 36-ppmv levels. Propane is present in a few samples at 1-3 ppmv. Ethylene and propylene (C3=) are also sporadically present in traces (0.6-3.3 ppmv) throughout the depth interval cored (Table T4; Fig. F16).
The compositions of gas samples collected from voids or expansion gaps in the core liner are listed in Table T5 and plotted on Figure F18. The void gas (vacutainer) samples are relatively pure methane, generally with minor air contamination. The ethane content of void gas shows regular increase with depth at Site 1252, despite the unexplained high ethane content in the sample from a depth of 16.28 mbsf. Propane is present in only a few samples and shows no variation in concentration with depth. Other heavier hydrocarbons (C4-C6) are absent.
Gas composition expressed as the C1/C2 ratio of headspace and void gas is plotted vs. depth in Figure F19. The C1/C2 ratios for both headspace and void gas show systematic decrease with increasing depth and temperature. There are some slight offsets in the C1/C2 ratio resulting from an increase in ethane content in sediments, especially in the depth interval from ~100 to 130 mbsf. Similar deviations from the general trend were observed at other sites (Sites 1244, 1245, and 1247), where gas hydrate recovery or other evidence for the presence of hydrate was noted. However, there is no shift to lower C1/C2 ratios at the approximate depth of the base of the methane hydrate stability zone (170 mbsf), unlike what was observed at Site 1251. Although a prominent BSR is present east of Site 1252, there is no strong evidence for the presence of gas hydrate in the headspace or void gas compositions, nor did we observe a BSR directly beneath Site 1252.
Additional analyses of sediments will be done as part of shore-based research, along with the stable isotopic characterization of the gases.