I. PROJECT OVERVIEW
The completion of Integrated Ocean Drilling Program (IODP) Exp 327 in Summer 2010 launched a series of multi-year, single-hole and cross-hole experiments in young oceanic crust (eastern flank of the Juan de Fuca Ridge (JFR)), to assess hydrogeologic, solute transport, and microbiological processes in multiple directions, depths and at numerous spatial and temporal scales (meters to kilometers, minutes to years) [1] The infrastructure to complete this work includes six boreholes (Holes 1026B, 1027C, 1301A, 1301B, 1362A, and 1362B) that are cased though the sediment section and open to the basaltic crust below. These boreholes are instrumented with sensors, loggers, and samplers located at depth in the crust and attached to wellheads at the seafloor. These systems are being used to monitor ambient subseafloor conditions in response to perturbations and quantify complex system responses within the crust [2, 2a]. Collectively, these experiments are elucidating fundamental questions related to linked properties and processes within the crust: (1) What is the nature (magnitude, distribution) of permeability in upper oceanic crust? (2) What are the magnitudes and directions of driving forces, fluid fluxes, and associated solute and heat transport? (3) What is the nature of fluid storage properties within the crust, including their pressure-, temporal- and spatial-dependence? (4) What are relationships between fluid flow, vertical and horizontal compartmentalization, distribution of microorganisms, alteration, structure, and primary crustal lithology? (5) How large are crustal fluid reservoirs, what are fluid and colloid flow velocities, and how do these respond to perturbations (pumping, tides, seismic events)? (6) What is spatial and temporal variability in phylogenetic and functional diversity of resident microbial communities, and what are the nature and magnitude of dominant metabolic processes?
We are collecting samples and data using crustal borehole observatories ("CORKs") to address these questions, taking two primary approaches. First, we have placed sensors, samplers, and experiments at the seafloor, using wellhead instrumentation to monitor conditions and collect fluids from depth. Earlier studies have focused on installing and servicing these seafloor systems to recover samples and data, and to collecting discrete samples of crustal fluids and microbial materials using tubing that extends from the seafloor to depth. Second, we have placed autonomous and self-contained instruments deep below the seafloor, within the volcanic ocean crust, in five of the borehole observatories (Holes 1026B, 1301A/B, 1362A/B). These instruments include temperature loggers, fluid samplers, and microbial incubation habitats and enrichment studies, which will have been operating for 4 to 5 years in 2014, collecting samples and data that record the response to drilling operations, observatory installation, subsequent perturbation and response to tracer injection. As described later in this proposal, a small number of seafloor samples collected in Summer 2011 show that a multi-tracer transport experiment, initiated during IODP Expedition 327 (with injection of metal salts, inert gas tracer, and fluorescent particles, as part of a 24-hour pumping test), has resulted in direct evidence for tracer transport hundreds of meters between basement boreholes. The seafloor samples are limited in temporal and spatial resolution, in ocntrast contrast to the downhole systems deployed prior to tracer introduction to the crust, which will that provided greater spatial coverage and a continuous (multi-year) record. This proposal seeks support to recover samples from deep within instrumented boreholes, completing the first cross-hole hydrogeologic and tracer experiments attempted in the ocean crust.
We request support for a 12-day on site ROV/submersible expedition to recover downhole instruments installed in subseafloor borehole observatories in 2009 and 2010. In addition, we propose to recover an autonomous flowmeter that is recording long-term discharge from one of the basement boreholes (as part of the cross-hole perturbation and sampling experiment), complete a final round of fluid and microbial sampling using seafloor valves and fittings to gather large-volume samples from depth, retrieve remaining wellhead samplers, and close valves to seal the CORKs so that they are available for future studies. After collection of samples and data, we will conduct an extensive analytical program to quantify concentrations and transport behavior of co-injected tracers (SF6, salts of Cs, Er, and Ho, two sizes of fluorescent microspheres and DAPI-stained cells, and chlorinity (for freshwater inputs); [1,2]), characterize microbial materials, and interpret the cross-hole and multi-depth pressure, thermal and chemical response to long-term discharge of fluids from two boreholes (1362A/B).
This is a multi-investigator and multi-university proposal, with six co-PIs: Wheat (UAF), Fisher (UCSC), Cowen (UH), Clark (UCSB), Edwards (USC), and Becker (Miami). Four of the co-PIs (Wheat, Fisher, Cowen, and Edwards) also are co-PIs of the Center for Dark Energy Biosphere Investigations (C-DEBI), a Science Technology Center funded separately by NSF. These four co-PIs will draw upon existing C-DEBI support for partial support for students, postdocs, technicians, supplies, travel, and other expenses, significantly reducing budgets associated with the current proposal. In addition, although we are requesting UNOLS support for ship and ROV time as part of this proposal, we have submitted a complementary proposal to the Schmidt Ocean Institute (SOI), which would provide ship (R/V Falkor) and ROV (ROPOS) support should that proposal be approved. The SOI does NOT provide funds to conduct research, only ship and ROV time. This SOI proposal is currently in review (outcome anticipated in late August or early September), and if it is funded, we will not need NSF-funded ship or ROV support through UNOLS. The technical and scientific program we propose will be identical whether it is completed with SOI or UNOLS assets. The main difference would be the number of berths available, but we are planning for a lean shipboard party in any case, and a focused scientific plan comprising essential shipboard and land-based activities so that this suite of experiments can be completed.
II. SCIENTIFIC MOTIVATION
Most of the ocean floor is hydrogeologically active [e.g., 3, 4, 5] and the volcanic oceanic crust comprises a global-scale aquifer, containing as much fluid as that stored in ice caps and glaciers [6]. Hydrothermal fluid flows through the oceanic crust are equal to or greater than those from rivers to the ocean, effectively recycling the ocean every 105 to 106 yr [7-9]. Global hydrothermal flows influence: alteration of the lithosphere and the composition of flowing fluids and the ocean; subseafloor microbial ecosystems; and diagenetic, seismic, and magmatic activity along plate-boundary faults [e.g., 10-14]. Scientific ocean drilling has long sought to quantify the dynamics and impacts of fluid flow through the oceanic crust [15]. Although there have been notable successes in drilling seafloor spreading centers, particularly ore deposits and/or in sedimented environments, much work has focused on "ridge flanks" areas far from the magmatic influence of seafloor spreading, where overlying sediments stabilize the drillstring, and cooler basement conditions make sampling and experiments more tractable.
Hydrogeologic testing of the ocean crust [16], has traditionally involved very short (20-30 min), single-hole experiments [17-19]. The recent development of pressure-tight, subseafloor observatories (CORKs) comprises a major advance in marine hydrogeologic studies [20, 21]. These systems allow thermal, pressure, chemical and microbial disturbances associated with drilling to dissipate, allowing monitoring, sampling and in situ experimentation. Most CORK systems have been used mainly for passive monitoring and sampling in single holes. Initial tracer tests with these systems have been limited to single holes and limited in scope [Gieskes et al., unpublished work in 504B, 22, 23]. Our project uses a three dimensional network of CORKed boreholes to run the first controlled, cross-hole tests, the first in -situ microbial enrichment experiments, and the most sophisticated microbial colonization experiments attempted in volcanic crust. These tests and experiments are possible only now, with the recent development of novel seafloor sampling systems (GeoMICROBE Sled; [24]), an autonomous flow meter [25], downhole osmotic systems [26], in-situ colonization and enrichment experimental systems [63], and high-resolution pressure recorders [2]. With such instrumentation and novel wellhead innovations, we are able to conduct the first hydrogeologic studies of the crust to assess vertical compartmentalization, and to quantify azimuthal (directional) fluid flow properties in oceanic crust. These experiments also are designed to provide the most pristine and most continuous multi-year record of ridge-flank crustal (borehole) fluids anywhere on the planet.
These experiments are globally unique because of the investment from scientific ocean drilling. The eastern flank of the JFR has become a de-facto "type setting" for ridge flank hydrothermal processes, in part because there is a long history of multidisciplinary research in this area, including scientific drilling. There are other locations where complementary studies are underway, most notably the North Pond field site, on young crust (8 Ma) west of the Mid-Atlantic Ridge, where conditions are cooler [28] but crustal fluid flow paths are less well defined. The eastern flank of the JFR has the most extensive network of observatories and instrumentation, arranged with a geometry that allows both single and cross-hole experimentation, including evaluation of lateral and vertical variations in scalar and vector (flow, transport) properties. There is nowhere else on the planet where these kinds of experiments can be completed at present, not without considerably greater investments in drilling time, infrastructure, and scientific instrumentation. Considerable work and resources have been focused on the JFR eastern flank in the last decade, and success has not come easily. But having made the long-term commitment and taken significant technical risks, we are poised to reap considerable scientific rewards with this final phase of operations.
Proposed work will provide samples and data that are unique; are essential for understanding ocean crustal hydrogeology, biogeochemistry and microbiology; and cannot be obtained in any other way. Earlier short-term hydrologic tests in ocean crustal boreholes [18. 19, 29], and tests from open holes resulting from natural or induced differential pressures [30,31], have provided no information on formation compressibility, which is essential for understanding crustal response to transient pressure events. The use of natural formation overpressure to run long-term free flow experiments, as we are doing, quantifies formation transmissive and storage properties across a continuum of crustal scales (meters to tens of kilometers). The bulk formation properties indicated by earlier cross-hole responses in this area suggest permeability that is lower than determined with single hole tests [29], and several orders of magnitude less than estimated by numerical modeling and analyses of formation responses to tidal and tectonic perturbations. This discrepancy (which is contrary to expected scaling of permeability with the lateral scale of testing) may be reconciled if the upper crust in this area is azimuthally anisotropic with respect to basement permeability, with higher permeability in the "along-strike" direction (trending N20E). Testing this hypothesis is a fundamental goal of the complete experimental program, along with evaluating whether distinct physical and chemical zones in the crust are hydrogeologically connected or share common microbial ecosystems. Applying a suite of techniques using a single network of boreholes is the only way to accurately determine the nature of crustal permeability and storage properties, resolve scaling influences, and test the validity of idealized crustal representations commonly used in models [e.g., 32-37]. The JFR is an ideal place to address these issues, because individual tests can be run for years, delineating the scale-dependence of hydrologic properties using one experimental method.
Similarly, there is no other way to assess actual fluid, solute, and particle flow directions and rates except through a tracer experiment. This kind of experiment has never before been attempted in the ocean crust. The extent of water mixing and water-rock interaction within an aquifer depends on effective porosity (fraction of open space involved in fluid flow) and hydrodynamic dispersivity (spreading of solutes by mechanical dispersion and diffusion) [38-43]. Understanding these properties is critical to successful reactive-transport modeling and interpreting the age distributions of subseafloor fluids, but these properties have never been quantified in the oceanic crust. Simply observing tracer arrival across meters to kilometers is a significant accomplishment, but the most valuable records of tracer transport during these experiments remain samplers deep below the seafloor. remain within the boreholes that provide more robust and additional quantification of fluid flow properties. Samplers and loggers currently deployed at depth in existing CORKs contain long-term records of temperature, geochemical conditions, and microbial populations, from before, during, and after the initiation of cross-hole experiments. Recovery of this instrumentation will provides critical information contributing to the success of the multidisciplinary long-term experiments..
III. FIELD SITE AND EARLIER RESULTS
Eastern Flank of the Juan de Fuca Ridge (JFR). Numerous studies summarize geology, geophysics, basement-fluid composition, hydrogeology, and microbiology within young seafloor on the eastern flank of the Endeavour segment of the JFR [e.g., 11, 23, 24-32]. Topographic relief associated with the JFR axis and abyssal hill bathymetry on the ridge flank has helped to trap turbidites flowing from the continental margin to the east (Fig. 1) [77]. This has resulted in burial of young oceanic basement rocks under thick (200 -700 m) sediments. Sediment cover is regionally thicker and more continuous to the east, but there are seamounts and smaller basement outcrops located up to 100 km east of the spreading center on 3.5-3.6 Ma crust. Regional basement relief is dominated by linear ridges and troughs oriented parallel to the spreading center and produced mainly by faulting, variations in ridge magmatism, and off-axis volcanism [78, 79]. Low-permeability sediment limits advective heat loss regionally, and leads to strong thermal and chemical differences between bottom seawater and basaltic formation fluids [e.g., 22].
ODP Exp. 168 (1996) and IODP Exp. 301 (2004). Ocean Drilling Program (ODP) Leg 168 drilled a transect of eight sites on 0.9 to 3.6 Ma crust east of the JFR [50]. These operations included installation of four CORKs that extended into uppermost basaltic crust, two of which are located on 3.5–3.6 Ma seafloor near the eastern end of the drilling transect (Holes 1026B and 1027C, Fig. 1) [51, 52]. Prior to ODP Leg 168, there was a largely two-dimensional view of the dominant fluid circulation pathways across the eastern flank of the JFR, with recharge occurring across large areas of basement exposure close to the ridge (near the western end of the Leg 168 transect), then flowing towards the east. However, results from ODP Leg 168 were inconsistent with this view and consistent with flow dominantly parallel to the spreading system to the west with seamounts to the south and north providing conduits for seawater inflow and egress [53, 54]. Numerical models of outcrop-to-outcrop hydrothermal circulation between recharging and discharging seamounts show that rapid flow is sustained (as a "hydrothermal siphon") and temperatures matched to observations if basement permeability along the flow path is about 10–11 m2 [55].