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doi:10.2204/iodp.proc.331.101.2011

Drilling strategy

We drilled five sites during Expedition 331: the active hydrothermal vent site and sulfide-sulfate mound at North Big Chimney (NBC) (Site C0016) (Fig. F3), three sites east of NBC at distances of ~100, 450, and 1550 m from the active vents (Sites C0013, C0014, and C0017, respectively), and one site on a hill ~600 m northwest of the active vents that represents a potential migration path for hydrothermal fluid (Site C0015) (Fig. F6).

The NBC hydrothermal mound at Site C0016 is 20 m high and 6 m in diameter (Fig. F7). We attempted to drill one hole into an active high-temperature (310°C) vent at its summit in which the pipe broke and we failed to recover any core. We drilled a second hole 20 m away, immediately at the base of the mound on its western side. Because of the high temperatures and because we expected to recover hard rock, we used conventional hard rock drilling equipment supplied by Baker-Hughes Inteq (BHI) specifically for Expedition 331. The BHI system collects 4 inch diameter core in aluminum liners in lengths of 9, 18, or 27 m, but the cores cannot be retrieved by wireline and so require a time-consuming pipe trip for each core.

At Sites C0013, C0014, and C0017, we drilled the relatively high, moderate, and low heat flow areas to the east of the Iheya North hydrothermal field to investigate subseafloor microbial habitats and communities within broad gradients of physical and chemical variation, both laterally and vertically, that could be affected by mixing between discharging hydrothermal solutions and recharging ambient bottom seawater. These sites were drilled using the hydraulic piston coring system (HPCS) to first refusal and then again in any softer intervals encountered deeper in the hole, alternating as necessary with the extended punch (EPCS) and extended shoe (ESCS) coring systems, and, for one run at Site C0013, the BHI system, to penetrate the harder layers. Both the EPCS and ESCS systems were able to penetrate the harder layers, with a slight edge to the ESCS for the hardest, but the EPCS was generally much better at core recovery.

We penetrated the margin of the local discharge-recharge zone to depths of 55, 137, and 151 mbsf at Sites C0013, C0014, and C0017, respectively, coring variably hydrothermally altered sediment and pumiceous deposits (Fig. F5). We were able to measure in situ temperature at two of these sites using the advanced piston corer temperature tool (APCT3) shoe (upper calibration limit = 55°C) as part of the HPCS, combined with commercial thermoseal (Nichiyu Giken Co., Ltd.) strips taped to the outer surface of the core liner. The strips estimate temperature in 5° or 10°C increments using chemically impregnated wafers that turn black when exposed to the designated temperature. They can each be used over one of several limited ranges between 75° and 250°C. Temperature measured at the distal flank Site C0017 reached 44°C at 112 mbsf and 90°C at 151 mbsf. Temperature at the intermediate flank Site C0014 was 22°C at 6.5 mbsf; it exceeded 55°C at 16 mbsf and exceeded 210°C at only 50 mbsf. We were not able to measure temperature at proximal flank Site C0013, but the gradient was likely higher than at the other two flank sites. This contention is based on the alteration mineral assemblages and the fact that the acrylide plastic core liners began to melt, which during onboard tests began to soften and deform at 82°C, at only 12 mbsf. Melting of the plastic core liners severely limited our ability to core to depth at Site C0013. Our one attempt to use the BHI system at this site, with its 4 inch diameter aluminum core liners, recovered only 2.1 m of core from a 9.2 m penetration and required nearly 22 h to collect, mainly because the interval included soft layers and the BHI system is optimized for hard rock coring. During the port call in Okinawa halfway through Expedition 331, we acquired eighteen 9 m sections of standard 2.44 inch diameter aluminum core liner, which were mostly used to retrieve good quality core to the depth we reached at Site C0014.

Site C0015 is located 600 m northwest of the hydrothermal mounds, where the seismically detected negative polarity sequences become thick. We targeted for drilling a heavy concentration of shells of dead mussels, which suggest that methane-rich hydrothermal fluids upwelled here in the recent past. We saw no live mussels at this site. Up to 400 m of penetration would be required to penetrate the deepest of the negative polarity reflectors, but we had time to take only two HPCS cores to 9.4 mbsf before we had to depart for Okinawa for a crew change midway through the expedition.

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