Table T1. Calculated depth of the base of gas hydrate occurrence (in meters below seafloor) from various geophysical and geochemical techniques. (See table notes.)
| Technique | Site U1325 | Site U1326 | Site U1327 | Site U1328 | Site U1329 | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| U1325A | U1325C | U1326A | U1326D | U1327A | U1327D | U1327E | U1328A | U1328C | U1329A | U1329C | U1329D | |
| Precruise seismic BSR* | 230 | 234 | 223 | 219 | 125 | |||||||
| Depth below seafloor (s) | 0.2817 | 0.2883 | 0.272 | 0.2673 | 0.154 | |||||||
| LWD resistivity | 240 | x | 260 | x | 230 | x | x | 219 | x | ND | x | x |
| CWL computed velocity | x | DNRB | x | 260 | x | x | 228 | x | 220 | x | x | 125 |
| CWL resistivity | x | DNRB | x | 260 | x | 235 | 235 | x | 220 | x | x | 125 |
| Deepest Cl– and IR anomaly | x | 238.9 | x | 264 | x | 222 | x | x | 220 | x | 128 | x |
| C1/C2 | x | 243 | x | x | x | 225 | x | x | 214 | x | 119 | x |
| i-C4/n-C4 or C3 | x | x | x | x | x | 225 | x | x | 219 | x | x | x |
| VSP | x | x | x | x | x | 245 | x | x | ND | x | x | x |
| Downhole thermal measurements | 250–300 | 250–270 | 225–250 | 220–245 | 127–129 | |||||||
| Best estimate of BGHSZ: | 240.5 | 264 | 230 | 219 | 124 | |||||||
Notes: * = velocity used for BSR depth conversion from two-way traveltime (1636 m/s = maximum average velocity from seafloor to BSR; 1619 m/s = minimum average from seafloor to BSR); reported depth is the average of minimum and maximum estimates (note that velocity was determined from VSP at ODP Site 889). BSR = bottom-simulating reflector, LWD = logging-while-drilling, CWL = coring-while-logging, IR = infrared, VSP = vertical seismic profile, BGHSZ = base of gas hydrate stability zone, ND = not detected, DNRB = wireline logging tools did not reach BSR depth. Deepest chloride and IR anomaly: Hole U1326D: salinity of sample at 263.63 mbsf is 32 (baseline = ~33.5); the next shallowest anomaly (salinity = 23) is at 253.61 mbsf (no indication of s-II hydrate); Hole U1327D: salinity of two samples at 221.96 mbsf is 3.7 and 18, respectively (background = 22); Hole U1328C: salinity of sample at 216.5 mbsf is 28 (background = ~31); Hole U1329C = freshening spike in chlorinity (not salinity) between 119 and 128 mbsf (502–508 mM over a background of ~520 mM). Break in slope of C1/C2 ratios: Hole U1325C: below 243.85 mbsf (C1/C2 > 60,000 above; C1/C2 < 20,000 below); Hole U1327D: in void gas at ~225 mbsf; Hole U1328C: between 209.3 (C1/C2 > 10,000) and 217.7 mbsf (C1/C2 < 1,500); Hole U1329C: at 119.3 mbsf. Break in slope of i-C4/n-C4 at 224.62 mbsf in Hole U1327D. i-C4 enriched samples at 207.1 and 209.3 mbsf and C3 enriched samples at 217.1 and 218.8 mbsf in Hole U1328C; values are much lower below. Best estimate of BGHSZ: Sites U1325 and U1327–U1329: simple average value, rounded to nearest 0.5 m. Site U1326: deepest chlorinity anomaly is deeper than all other proxies, but there is no indication of s-II gas hydrate.