9. Site 1250¹

Shipboard Scientific Party²

INTRODUCTION

Site 1250 (proposed Site HR4a) was drilled in ~792 m of water ~100 m west of the southern summit of Hydrate Ridge and ~100 m east of the carbonate chemoherm known as the Pinnacle (see Fig. F1 in the "Leg 204 Summary" chapter). The Pinnacle is the only carbonate mound on southern Hydrate Ridge, whereas at northern Hydrate Ridge several major chemoherms are known (Torres et al., 2002; Teichert et al., in press). It has near-vertical flanks, rising ~40 m above the seafloor. The carbon source for formation of the Pinnacle is known to be biogenic methane from the very low ¹³C values (Teichert et al., in press). ²³⁰Th/²³⁴U data indicate that it formed during the last 11.4 k.y. (Teichert et al., in press). It is located within a high-reflectivity patch mapped by a deep-towed side-scan sonar survey (Johnson et al., in press), which is interpreted to indicate scattered carbonates close to the seafloor and/or shallow gas hydrates (see Fig. F7 in the "Leg 204 Summary" chapter). The precruise three-dimensional (3-D) seismic-reflection survey data show that seismic Horizon A (~150 meters below seafloor [mbsf] at Site 1250) meets the bottom-simulating reflector (BSR) (~114 mbsf at Site 1250) just below the Pinnacle (see Fig. F7 in the "Leg 204 Summary" chapter).

The primary objective at Site 1250 was to sample the sediments, aqueous fluids, gases, and gas hydrate under the high-backscatter seafloor flanking the Pinnacle. The sediments at Site 1250 were expected to be strongly affected by the upward fluid migration that has resulted in the formation of the Pinnacle chemoherm. In this context, understanding the role of Horizon A as a conduit for fluid flow and its interaction with the BSR was of special interest.

Six holes were drilled at Site 1250 (Fig. F1). Recording of the logging-while-drilling (LWD) resistivity-at-the-bit (RAB) tool failed during the first run in Hole 1250A as a result of depleted batteries; therefore, the LWD operation was repeated in Hole 1250B. Holes 1250C and 1250D were cored using the advanced piston corer (APC) and extended core barrel (XCB) to 145 mbsf. Holes 1250C and 1250D are each composed of 19 cores, with average core recoveries of 82% of the total penetration (see Table T1 in the "Leg 204 Summary" chapter). Hole 1250E, which was dedicated to biogeochemical sampling, comprises two cores, with 92% core recovery. Because of relatively high levels of higher hydrocarbons encountered near Horizon A at Site 1248, we decided not to penetrate Horizon A until a better understanding of possible hazards associated with this horizon had been obtained from drilling through it further downdip. After coring through Horizon A at Sites 1245 and 1247, we returned to Site 1250 to core Hole 1250F from 100 to 180 mbsf.

¹Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
²Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

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