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Background, motivation, and overview

Subseafloor observatories

Circulation obviation retrofit kits (CORKs) (Davis et al., 1992) have been used in recent decades to isolate igneous basement from overlying sediments and seawater, allowing the borehole to return to its native hydrological and biogeochemical state (Fisher et al., 2011). Initially, the principal function of CORKs was to monitor pressure and temperature in situ within single intervals within sealed boreholes (Davis et al., 1992). CORKs have evolved and become increasingly sophisticated, allowing fluid sampling for chemical analyses and biological sampling and experiments, all of which can be conducted at multiple sealed observatory intervals within basaltic basement (Wheat et al., 2011). Hence, CORKs have become powerful tools for studying in situ hydrological, geochemical, and biological process in igneous oceanic crust.

The principal driver for installation of CORK observatories during Integrated Ocean Drilling Program (IODP) Expedition 336 at North Pond was to study a section of young (8 Ma), cool (<20°C), “average” ridge flank in order to assess the role of microbiological processes in alteration of igneous oceanic crust. These observatories complement ongoing CORK observatory work being conducted in young and warm oceanic crust on the Juan de Fuca Ridge flank (Fisher et al., 2011) but target a crustal end-member that is more representative of global ridge flank conditions. Like the CORKs installed on the Juan de Fuca Ridge flank during IODP Expedition 327 (Fisher et al., 2011), the North Pond Expedition 336 CORK observatories are designed to target multiple horizons within upper oceanic crust for long-term in situ experimentation and monitoring (i.e., colonization experiments, fluid samplers; Orcutt et al., 2011; Wheat et al., 2010) and real-time formation fluid collection with seafloor sleds and submersible-supported systems (Cowen et al., 2012).

North Pond setting, history of study, and initial characterization

To study the potential for life in relatively young and cool oceanic crust, Expedition 336 visited the legacy North Pond drilling site at 22°45′N on the western flank of the Mid-Atlantic Ridge (Expedition 336 Scientists, 2012a) (Fig. F1). North Pond is an isolated northeast-trending sediment pond bounded by basement ridges as tall as 2 km and a range of sediment thicknesses up to 300 m in the southernmost part of the basin. This site was a drilling target during Deep Sea Drilling Project (DSDP) Leg 45 (Site 395) in 1974/1975 to examine crustal properties to characterize the geology of young oceanic crust (8 Ma). This relatively young and permeable crust is affected by vigorous seawater circulation that allows warming of circulating fluids to only 10°–15°C before they are discharged from the crust. Since initial drilling, the site has been revisited seven times for logging, hydrogeological studies, and other survey work (mapping, seismics, shallow coring, and heat flow): twice during DSDP and the Ocean Drilling Program (ODP) for logging and downhole experiments (DSDP Leg 78B and ODP Leg 109); a second time during ODP for logging and CORK installation (ODP Leg 174B); once with the submersible Nautile for logging by wireline reentry; and once by the R/V Atlantis and submersible Alvin for detailed heat flow, coring, and pore pressure surveys (Becker, Malone, et al., 1998). North Pond was revisited again with the R/V Maria S. Merian for additional survey work in 2009 and 2012. Borehole observatories and experiments were installed and sediment and volcanic rocks were cored during Expedition 336 in the fall of 2011, with a focus on subseafloor microbial systems.

Hydrologically, North Pond is characteristic of areas where volcanic crustal rocks are exposed across large areas, and continuous cover by sediment is the exception rather than the rule (Becker et al., 1984; Langseth et al., 1992). Drilling, coring, logging, and limited borehole experiments suggest that the upper crust in this area is highly porous and permeable (Bartetzko et al., 2001; Becker, 1990; Gable et al., 1992; Hickman et al., 1984). Samples and data collected during a heat flow and sediment coring expedition in 2009 suggest a crustal fluid having considerable dissolved oxygen and seawater-like concentrations for most major and trace ions and nutrients, indicating a short residence time for fluids within basaltic basement (Ziebis et al., 2012). Because temperatures are low, sluggish abiotic rates of reaction favor alteration by kinetically enhanced biotic reactions (Knab and Edwards, in press). These reactions could support microbial communities that alter the crust directly. North Pond was selected for new drilling and CORK installation during Expedition 336 in part because of the contrasts it provides with the Juan de Fuca Ridge flank environment—the only other dedicated site for deep crustal microbiology studies (Cowen et al., 2012; Fisher et al., 2011; Orcutt et al., 2011; Wheat et al., 2010). Namely, there is extensive, vigorous circulation of relatively cold fluids in the crust of North Pond (Fig. F2), likely more circulation per unit of basement rock than observed where fluids are hotter and more altered on the eastern flank of the Juan de Fuca Ridge. Delineating fluid flow pathways from recharge to discharge around North Pond is challenging, however, because of the large expanses of exposed rocky seafloor.

North Pond upper oceanic crust, borehole conditions, and CORK overview

The primary objective of Expedition 336 was the installation of several single- and multilevel CORKs targeting upper oceanic crust within the North Pond study site (Fig. F2). At the conclusion of Expedition 336, two observatories were installed in the southern region of North Pond (DSDP Hole 395A and IODP Hole U1382A) and two observatories were initiated in the northeastern region of the study site (IODP Holes U1383B and U1383C; Expedition 336 Scientists, 2012a) (Figs. F1, F2). Here, we describe conditions of the upper oceanic crust in these holes directly related to the deployment of borehole observatories.

The original drilling at Site 395 proceeded smoothly despite several rubbly intervals, with core recovery of ~25% and ~50 m of fill/​rubble in the base of the 664 m deep Hole 395A. Subsequent logging on several cruises documented a consistently clean and open hole to just deeper than 600 meters below seafloor (mbsf), including during the most recent Expedition 336. The original CORK installed in Hole 395A during Leg 174B consisted of a fairly simple thermistor string (603 m long internal string with 10 thermistors, a data logger, and pressure sensors) within the sealed hole. Hole 395A was an underpressured hole and drew down bottom seawater at a rate of roughly 1000 L/min for 21 y after its initial drilling (Becker, Malone, et al., 1998) prior to installation of a CORK. Subsequent logging and initial CORK results suggested that 21 y of downhole flow through Hole 395A prior to CORKing had a negligible effect on the hydrology of the North Pond area (Becker, Malone, et al., 1998). A new multilevel observatory was deployed in Hole 395A during Expedition 336; however, the observatory was not successfully installed, as described in more detail below.

Site U1382 includes a basement-penetrating hole (U1382A) located ~50 m west of Hole 395A (Figs. F1, F2; Expedition 336 Scientists, 2012a). Basement (encountered at 90 mbsf) was cored in Hole U1382A and wireline-logged (105.6 m of open hole in basement); rotary core barrel (RCB) coring recovered basement from 110 to 210 mbsf (Cores 336-U1382A-2R through 12R). Core recovery in basement was 32%, yielding a number of volcanic flow units with distinct geochemical and petrographic characteristics. A unit of sedimentary breccia containing clasts of basalt, gabbroic rocks, and mantle peridotite was found intercalated between two volcanic flow units and was interpreted as a rock slide deposit (Expedition 336 Scientists, 2012a). Downhole hydrologic (packer) tests failed because ship heave up to 3 m prevented the packer from remaining set in the casing for >10 min. A single-level CORK was installed in order to sample/​monitor upper basement between 110 and 210 mbsf.

Site U1383 is located 5.9 km north-northeast of Site U1382 (Figs. F1, F2); Holes U1383B and U1383C with observatory components are ~25 m apart at this site (Expedition 336 Scientists, 2012a). Our strategy for Hole U1383B was to install 20 inch casing through sediments (53 mbsf) followed by installation of 16 inch casing into uppermost basement. We then planned to deepen the hole with a 14¾ inch tricone bit to install 10¾ inch casing to ~140 mbsf. However, this operation was not completed because a cone on the tricone bit broke off in the hole at 89.8 mbsf. A decision was made not to fish the lost component but to start over with a new hole. Nevertheless, this hole remained open and we planned to deploy a modified CORK with downhole experiments during a remotely operated vehicle (ROV)-based cruise in April of 2012, as described in more detail below. A landing platform was installed in order to facilitate the future work. We then initiated Hole U1383C, where we jetted in 16 inch casing before drilling into basement and sealing the sediment section with 10¾ inch casing to 60.4 mbsf. This hole was then RCB cored to 331.5 mbsf, logged, and completed with a three-level CORK that spans a zone of thin basalt flows with intercalated limestone (~70–146 mbsf), a zone of glassy, thin basaltic flows and hyaloclastites (146–200 mbsf), and a lowermost zone (~200–331.5 mbsf) of more massive pillow flows with occasional hyaloclastites in the upper part (Expedition 336 Scientists, 2012a). Drill string packer experiments were attempted in Hole U1383C but again were not successful (Expedition 336 Scientists, 2012a).