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doi:10.2204/iodp.sp.331.2010

Drilling strategy

Proposed drill sites

Drilling at proposed Site INH-1D is designed to penetrate the major hydrothermal discharge path along the normal fault and precisely penetrate the huge North Big Chimney (NBC) (a center of hydrothermal discharge directly from a deeply sourced fluid). We will penetrate the associated hydrothermal mound to a maximum depth of 50 mbsf (Fig. F1B). This hydrothermal mound is ~30 m in height and ~6 m in diameter, so drilling will sample proximal hydrothermal deposits and fluids (Fig. F8). These samples will be used for age estimation of the deposits, for detailed mineral and fluid chemistry, and for microbiological investigation of subvent microbial communities along the main discharge path. This hydrothermal mound is expected to consist of massive metal sulfides and sulfates. It will therefore be drilled with conventional drilling equipment supplied by Baker-Hughes Inteq (BHI), which has a better chance of obtaining good core recovery from hard materials.

Proposed Sites INH-4D and INH-5D are designed to drill and core the relatively high and low heat flow areas near the discharge Site INH-1D, penetrating the margin of the local discharge-recharge zone to depths of 100 and 200 mbsf, respectively. Based on seismic reflection data, they are expected to penetrate sediments, pumiceous deposits, and the interface with igneous basement (Fig. F5). These cores will provide information on subseafloor microbial habitats and communities within more vertically and horizontally broad gradients of physical and chemical variation, which could be affected by mixing with ambient seawater recharge, formation of liquid and/or solid CO₂ with methane, and subseafloor microbial activity along hydrothermal flow paths. These sites will be drilled using the hydraulic piston coring system (HPCS) to refusal, followed by coring with the BHI system.

Drill site priority

Completion of Site INH-4D (HPCS coring to refusal, industrial drilling and coring as deep as possible, and casing and capping) is the first priority, as this site has the highest potential to access functionally active subseafloor microbial communities and their geochemical environments occurring in the mixing zones between hydrothermal discharges and recharges. HPCS coring at Site INH-5D (to refusal) is proposed as the second priority to obtain additional microbiological and geochemical samples potentially hosting microbial communities and more moderate geochemical mixing environments. Because industrial drilling and coring would take considerable time to complete, we would move to the third prioritized operation of industrial drilling and coring at Site INH-1D (as deep as 18 mbsf). When industrial drilling and coring at Site INH-1D is completed, we would return to Site INH-5D and complete industrial drilling and coring as deep as possible, along with casing and capping. These operations would provide subseafloor samples such as relatively clean samples for microbiological and geochemical (fluid chemistry) investigations at two different subseafloor hydrothermal mixing zones and hydrothermal deposits at sites influenced by the highest, moderate, and lowest input of the end-member hydrothermal fluid. If these operations are completed before the end of the expedition, we will be able to complete operations at Sites INH-1C and INH-3D.

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