The ability to drill deep into the seafloor has increased understanding of the role of fluid flow within ocean sediments and basement rock, especially the connectivity of hydrogeologic systems within the ocean crust. This knowledge has led to surprising achievements in the study of hydrothermal vent systems, especially in understanding vent compositions and subsurface extent, as well as in the research and recovery of gas hydrates. In addition, by providing the only direct access to the subseafloor biome, scientific ocean drilling has revolutionized understanding of subsurface microbial communities living at the limits of life, thus enabling a new field of scientific inquiry.
Hydrogeologic systems are present beneath the sea-floor in all geologic environments, and they influence a wide range of biological, chemical, and physical processes. These processes include the magnitude and distribution of fluid pressures and related hazards, formation of continental crust, hydration of plate boundary faults, initiation of explosive volcanism, generation of gas hydrates and other mineral resources, and distribution of subseafloor microbial communities. The complexity of interactions not only has challenged scientists attempting to understand the fundamentals of subseafloor processes, but also opened the door for exciting, interdisciplinary efforts in scientific ocean drilling.
Scientific Accomplishments and Significance
From the time of Project Mohole (see Box 1.2), scientists recognized that scientific ocean drilling provided an opportunity to investigate fluid flow processes in oceanic sediments and crust (e.g., Von Herzen and Maxwell, 1964). Early Deep Sea Drilling Project (DSDP) studies focused on making measurements in sediments, including temperature and pressure (Box 3.1). However, the discovery of hydrothermal vents along mid-ocean ridges in the 1970s, and the subsequent recognition of robust hydrogeologic systems in nearly all subseafloor geologic environments, served as the catalyst for a multidecadal, multi-disciplinary effort to understand those systems. Beginning with the Conference on Scientific Ocean Drilling (COSOD) II in 1987, fluid flow became a major focus of study in the Ocean Drilling Program (ODP) and subsequent Integrated Ocean Drilling Program (IODP). The study of fluid flow included not only drilling, but also development of new technologies for sampling and long-term measurement, as well as tools for using thermal and geochemical anomalies to recognize fluid migration pathways (Moore et al., 1987; Kastner et al., 1991). Hydrogeology efforts have been concentrated in two distinct zones: seafloor basement and sediment. In both fields the emphasis has been on quantifying flow rates and patterns and understanding the links between chemical, biological, tectonic, geophysical, and hydrogeologic processes.
Basement hydrogeology studies initially focused on quantifying the circulation observed in hydrothermal vents. Early observations revealed that ocean bottom waters were being drawn down into the upper levels of the basement, implying that ocean crust was more permeable than the overlying sediments (e.g., Hyndman et al., 1976). The first direct measurements of basement permeability (DSDP Leg 83, DSDP Hole 504B) documented a stratified permeability structure, in which permeabilities decreased exponentially from very high values of 10—13 to 10—14 m2 in the upper 150 m of oceanic basement to 10—17 m2 and lower in sections deeper than 550 m (Anderson et al., 1985). Drill-string packer tests, like those used on DSDP Leg 83, were subsequently used successfully at many other sites. The resulting datasets were
In Situ Measurement of Temperature
Temperature is a major parameter controlling dynamic Earth processes. Borehole temperature measurements are important for understanding heat transfer from Earth’s interior, lithospheric evolution, hotspot volcanism, gas hydrate stability, and fluid flow in marine sediments. Consequently, temperature was one of the initial downhole properties measured during DSDP (Von Herzen and Maxwell, 1964).Throughout DSDR ODP, and IODR new tools and analysis approaches have continuously been developed and improved (e.g., Uyeda and Horai, 1982; Horai, 1985; Fisher and Becker, 1993; Davis et al., 1997; Heeseman et al., 2006). A 2004 IODP workshop on downhole tools confirmed that precise downhole temperature measurements were critical to fulfillment of programmatic objectives in all primary research themes (Flemings et al., 2004).
The most efficient tool for measuring temperature in boreholes is the advanced piston corer temperature tool (APCT), which measures sediment temperatures as the core is being taken (Horai and Von Herzen, 1985). The APCT allows for the measurement of in situ temperatures in the undisturbed sediments that have not yet been reached by the drill bit. The APCT has undergone two major upgrades to improve sensor and data sampling accuracy and stability while retaining the same efficient physical format (Heeseman et al., 2006). For deeper sediments that are too stiff to be sampled with the APCT, the Davis-Villinger temperature tool (DVTP) was developed (Davis et al., 1997). The DVTP also measures in situ pore pressure, although obtaining reliable pressure measurements has been challenging because of the long time constant of the pressure response and fractures induced in the sediment when the probe is inserted (Villinger et al., 2010).
compiled into a regional summary (Figure 3.1), which illustrates that shallow basement permeabilities are consistently three to seven orders of magnitude higher than the overlying sediment column and supports the early observations of a stratified permeability structure controlled by depth within the basement (Fisher, 2005; Becker and Fisher, 2008). The widespread nature of large-scale basement fluid circulation has profound implications for the formation and continuation of subseafloor microbial communities, the creation of ore deposits and gas hydrates, and the overall chemical and heat budget of the oceans.
In some cases, borehole temperature measurements indicated down- or uphole fluid exchange between the ocean and basement formations that could be used to estimate formation permeability (Figure 3.1). However, such open flow also represented a perturbation to in situ conditions and revealed the need for tools with long-term in situ monitoring capabilities. The development of the CORK (Circulation Obviation Retrofit Kit) has led to widespread use for long-term measurement of temperatures, pressures, and fluid fluxes (Box 3.2). The first long-term observatories were established on the Juan de Fuca Ridge (ODP Leg 139). Pressure records from these observatories after 14 months showed high lateral fluid fluxes and short residence times in very permeable upper basement (Davis and Becker, 2002; Fisher, 2005). The first cross-hole experiment (ODP Leg 168), and the first three-dimensional CORK array (IDOP Leg 301), also along the Juan de Fuca Ridge, continued to add to the picture of large lateral fluid fluxes and high permeabilities, and recorded transient flow events associated with seismic activity and tides (e.g., Fisher et al., 2008).
FIGURE 3.1 Summary of borehole permeability determinations in oceanic basement rocks, based on packer and temperature (flow-meter) experiments. Vertical axis is depth into basement, accounting for differences in sediment thickness. Most seafloor measurements have been made in basaltic crust, but two sets of data (ODP Holes 857D and 735B) are from sediment/sill and gabbroic lithologies, respectively. Note range of values and relatively consistent depth trends. SOURCE: Fisher, 2005.
CORKs: Subseafloor Borehole Observatories
Open drill holes allow significant exchange between bottom water and formation fluids following perturbations associated with drilling ocean crust. CORKs (Circulation Obviation Retrofit Kits) are designed to stop bottom water influx, thus allowing borehole conditions to return to a more natural hydrodynamic state (Davis et al., 1992; Becker and Davis, 2005). CORKs can be used for pressure, seismic, strain, and temperature monitoring; crustal fluid sampling; and microbiological and controlled perturbation experiments. CORKs were originally conceived to allow for estimates of in situ flow rates and permeability, and scientists have more recently begun using CORKs for a variety of chemical and biological experiments using both downhole and seafloor samplers (Fisher et al., 2005). Samples for geochemistry can be collected over long time periods (up to 5 years) using downhole basement fluid osmosamplers (Jannasch et al., 2004; Fisher et al., 2005, Wheat et al., 2010), as well as using seafloor samplers that can be accessed at the well-head (Cowen et al., 2003). Recent downhole experiments have been deployed with mineral colonization surfaces (Orcutt et al., 2010), and seafloor samplers are currently in use on the Juan de Fuca Ridge flank CORKs to allow for the sampling of multiple fluid horizons within the CORKed borehole, both from a submersible or as a stand-alone sampler (Fisher et al., 2005). CORKs as subseafloor borehole observatories offer unprecedented opportunities for integrating hydrogeological studies with microbial and chemical processes in basement fluids.
Schematic of casing and CORK systems deployed on the Juan de Fuca Ridge in 2004, idealized and not to scale. SOURCE: Modified from Fisher et al., 2011.
In the sedimentary realm, scientific ocean drilling focused on identification of flow pathways and fluxes in passive and active margins. Drilling in active accretionary margins resulted in development of new geochemical tracers for inferring fluid flow (ODP Legs 112, 125, 131, 134, 146, 156, 170, and 190; see Figure 3.2). Using pore water anomalies such as low chloride concentrations, negative chlorine isotope ratios, and carbon isotopic ratios of dissolved methane, scientists showed that fluids migrate tens of kilometers along focused pathways, with localized flow rates two to six orders of magnitude larger than steady-state models would suggest (e.g., Moore et al., 1987; Vrolijk et al., 1991; Ransom et al., 1995). The flow rates inferred from these data require transient, confined-aquifer flow, localized expulsion, and/or external fluid sources, but there is still much to be learned about specific flow pathways and magnitude, as well as the role that fluids play in seismicity, chemical alteration, and volcanism.
Hydrologic investigation of non-accretionary subduction zones proved to be more difficult because of limited geologic records. However, drilling on seamounts in the Mariana forearc and Costa Rica margins confirmed mass fluxes of fluids originating from several kilometers below the seafloor (see section on subduction zone processes in Chapter 2).
In passive margins, drilling along the New Jersey Mar-
FIGURE 3.2 Location map of ODP legs related to heat and fluid flow in subduction environments. This is a Mercator projection with a color range of —9,000 to 9,000 m, with white marking the 0 m depth. SOURCE: IODP-USIO.
gin (ODP Leg 174A) demonstrated coupling between excess fluid pressures and flow, but pressures could only be inferred from physical properties (Dugan and Flemings, 2000). Mud-stone pressures were subsequently measured in the Gulf of Mexico (IODP Leg 308) in a rigorous demonstration of the coupling between flow and excess pressure (Flemings et al., 2006, 2008; Stigall and Dugan, 2010). Those concepts form the basis for new understanding of the relationship between overpressures and slope failures along passive margins.
Fields of Inquiry Enabled
The recognition of the magnitude of fluid flow within sediments and beneath the seafloor has led to exciting new research to quantify the role that fluids play in controlling mechanical processes along both passive and active margins, including the occurrence and magnitude of large earthquakes along plate boundary faults and the distribution and timing of major slope failure events along passive margins. The body of literature on these topics is growing at a fast rate; Screaton (2010) provide a broad synthesis of other related studies.
The one-dimensional nature of fluid-flow measurements from boreholes has also led to the development and application of new, multidisciplinary tools designed to extend understanding to three dimensions. Notable accomplishments include the use of inexpensive heat probes to help resolve complex patterns of fluid flow (e.g., Fisher and Harris, 2010); modeling studies of fluid flow and fluid-rock interaction to quantify flow rates and pathways responsible for heat, pressure, and solute transfer (e.g., Spinelli and Saffer, 2004; Spinelli and Wang, 2008); and the emergence of three-dimensional (3D) seismic data as a routine tool for investigation of physical properties (e.g., Bangs et al., 2009).
Goals Not Yet Accomplished
Tremendous progress has been made in the past 10 years in the understanding of subsurface flow systems. However, much is still unknown about the rates of transport and the shapes of pathways, and how they affect geologic hazards, mineral resources, and the distribution of subsurface microbe communities. Several first-order questions still need to be addressed in order to resolve the significance of these processes, including the nature of hydraulic communication between basement and sediments; the effect of diagenetic modification of sediments on geochemical and microbial processes in the underlying basement; changes in flow as the ocean crust ages; links between marine and continental hydrogeologic systems on passive margins; and determination of quantitative relationships between seismic activity, shallow faulting, and hydrologic processes in subduction zones.
Several marine science initiatives show promise for addressing these important questions, in particular the National Science Foundation’s Ocean Observatories Initia-
tive (OOI).1 The combination of scientific ocean drilling, permanent observatory capabilities, and evolving drilling and sensor technologies has the opportunity to provide a powerful, new integrated approach to resolving key issues related to climate variability, changes in ocean ecosystems, plate tectonics, and subseafloor chemistry and biology.
The seafloor expression of subseafloor hydrothermal vent processes is spectacular, with gushing high-temperature black smokers, fields of glassy new lava flows, and bushes of tube worms and other chemosynthetic life forms. With the confirmation of seafloor spreading and the discovery of deep-sea hydrothermal vents in 1977 (Corliss et al., 1979), there was a focused effort to delve beneath the seafloor to understand the underlying water-rock reactions that create spectacular deep-sea hydrothermal vents. Active hydrothermal circulation is driven by heat provided by magma chambers, where the circulating fluids react with the roof of the magma chamber, and convection in the crust is driven by the temperature gradient between the ocean and the magma. This allows for easy exchange of crustal fluids with the overlying ocean. Beneath the seafloor, hydrothermal fluids evolve when seawater is heated and a variety of water-rock chemical reactions take place, such as cation exchange, where elements such as magnesium are taken up into the rock and iron, zinc, manganese, and silica are released (Seyfried and Mottl, 1995). In addition, these chemical reactions create energy sources that support the chemosynthetic-fueled communities seen at vents, in which microorganisms use the huge amounts of volatiles and reduced compounds leached from rocks to grow, thus serving as the major food source and base of the vent ecosystem (e.g., Rau and Hedges, 1979). They also create subseafloor habitats that cross temperature and energy gradients, allowing for the growth of diverse microbial communities. These hydrothermally driven water-rock reactions are a fundamental component of global geochemical cycles and are critical for understanding exchanges and fluxes between the crust and the oceans. Scientific ocean drilling provides access to the subseafloor, which strengthens understanding of the processes responsible for the existence of seafloor hydrothermal systems and the role these chemical reactions play in influencing the composition of ocean crust and the regulation of ocean chemistry.
Scientific Accomplishments and Significance
Four active hydrothermal systems were drilled as part of DSDP and ODP, representing different geological settings and highlighting diverse styles of water-rock reaction and crustal alteration. The first active site drilled was Guaymas Basin in the Gulf of California (DSDP Leg 64) in 1978 and 1979. Very few basement rocks were recovered, but analyses of hydrothermally altered sediments suggested the presence of two distinct hydrothermal systems: one of short duration and low temperatures, associated with shallow basaltic intrusions into sediments; the other of longer duration and higher temperatures, associated with large magmatic intrusions (Gieskes et al., 1982).
More than 10 years later, two more high-temperature hydrothermal vent sites were drilled: Middle Valley and the Trans-Atlantic Geotraverse (TAG) mound. TAG, located at 26° 08’ N on the eastern side of the Mid-Atlantic Ridge, is an area of known high-temperature (>360 °C) basalt-hosted venting that also supports diverse chemosynthetic life forms (Humphris et al., 1995). On ODP Leg 158 in 1994, 17 holes drilled at five locations on the active TAG sulfide mound (200 m in diameter and 50 m high) revealed a massive subseafloor sulfide zone in the upflow zone (Humphris et al., 1995). Combined with the seafloor sulfide deposits, geologists estimate almost 3 million tons of sulfide at this hydrothermal mound, raising the level of interest among economic geologists (Rona, 2003). In addition, drilling demonstrated that there is clear mineralogical zonation in the crust, with evidence for huge amounts of seawater intrusion in the subseafloor, as indicated by the presence of anhydrite, a highly soluble mineral that had not been seen in ancient mineral deposits and ophiolites (Moores and Vine, 1971). The formation and dissolution of anhydrite help to form the brecciated sulfide framework that allows the sulfide mound to grow over time (Humphris and Tivey, 2000).
Three years earlier, drilling began at Middle Valley (ODP Legs 139 and 169), located at the northern end of the Endeavour segment of the Juan de Fuca Ridge in the northeast Pacific Ocean (41° N, 127° 30’ W). Like TAG, Middle Valley is a basalt-hosted site with a massive sulfide deposit that is actively producing high-temperature hydrothermal fluids, but with the additional feature of being overlain by thick Pleistocene continental deposition (Zierenberg et al., 1998). In 1996, drilling at Middle Valley on ODP Leg 169 penetrated both the sulfide deposit and the feeder-zone, through which high-temperature metal-rich fluids reach the seafloor. This deep metal-rich zone contained almost 16 percent copper ore (Zierenberg et al., 1998) and had not previously been seen below seafloor mineral deposits, further raising the interest for mineral exploration both on land and in the ocean (Rona, 2003) (Figure 3.3). Cell counts and phospholipid profiles were also obtained from the sediment cores, spanning a range of temperatures, and it was found that even at high temperatures (up to 185 °C) microbial populations were still present, although at lower concentrations than at the cooler surface temperatures (Cragg and Parkes, 1994; Cragg et al., 2000; Summit et al., 2000).
The fourth hydrothermal system, PACMANUS (3° 43’ S, 151° 40’ E), was drilled on ODP Leg 193. PACMANUS is an active hydrothermal vent field within a back-arc basin hosted in felsic volcanic rocks at a convergent margin in
FIGURE 3.3 A cross-section of mineralization at the Middle Valley hydrothermal site’s Bent Hill massive sulfide deposit. SOURCE: Zierenberg et al., 1998.
the western Pacific near Papua New Guinea. Discovered in 1991, it hosts both high- and low-temperature venting, chemosynthetic communities, and extensive hydrothermal deposits (Binns et al., 2002). Four holes drilled in the field revealed that alteration is pervasive beneath the active sites and not, as in previous sites, narrowly confined to an upflow zone. Instead, permeability that controls hydrothermal venting and deposition to the seafloor is governed by fractures, not subseafloor high porosity (Binns et al., 2007). Data from fluid inclusions also demonstrated evidence for subsurface phase separation with deep-sourced hot hydrothermal fluids (Vanko et al., 2004). In addition, rocks from core interiors were collected from two of the holes to determine the distribution of microorganisms in the subseafloor, and microbial cells and ATP (adenosine triphosphate, a marker for biological activity) were detected down to 99.4 and 44.8 m below the seafloor, respectively (Kimura et al., 2003).
Fields of Inquiry Enabled
Until scientific ocean drilling began, the only way to study the chemical reactions and physical stockwork beneath hydrothermal vents was via collection of exiting vent fluids and rocks at the seafloor or by examination of ophiolites. However, these approaches have the disadvantage of only inferring subseafloor hydrothermal processes. By providing access to samples beneath the seafloor, scientific ocean drilling has made a critical contribution to understanding active hydrothermal systems from a chemical, geological, and even a biological perspective. One of the more unexpected outcomes of drilling hydrothermal vents was the discovery of subseafloor massive mineral deposits, and together with previous interest in seafloor massive sulfide deposits, there is now considerable interest in mining seafloor hydrothermal systems, particularly in back-arc basins and arc volcanoes in water depths of less than 2,000 m (Hoagland et al., 2010). Because the scientific community’s understanding of the formation and evolution of these deposits and associated ecosystems is incomplete, there is a strong desire to link industry and scientists to avoid potential environmental damage.
Goals Not Yet Accomplished
There is a continued interest in drilling deep-sea hydrothermal vents, as exemplified by the August 2009 workshop, “Scientific Ocean Drilling of Mid-Ocean Ridge and Ridge-Flank Settings” (Christeson et al., 2009). Most recently, IODP Expedition 331 drilled hydrothermally active mounds in the Okinawa Trough to obtain more data on subseafloor microbial communities. A number of proposals have been put forward to expand the different geological settings and diverse styles of water-rock reaction and crustal formation drilled, with considerable attention paid to establishing borehole observatories and linking in with cabled ocean observatories. Special technological issues remain with drilling at active hydrothermal systems, and there is a strong need for improved core recovery in young (less than 3 myr) crustal environments. Ongoing developments include hard rock re-entry systems, remotely operated submersible drill rigs, advanced diamond core barrels, and engineered muds and instruments capable of withstanding high (>200 °C) temperatures (Christeson et al., 2009). Development and testing of these important tools will continue to be important for fulfilling scientific goals in these regions.
Morita and Zobell (1955) first cultured bacteria from shallow marine sediment cores and concluded that the lower limit of Earth’s biosphere was 7.5 m beneath the seafloor. It was not until almost 30 years later that scientists used sediment cores collected from DSDP Leg 96 in the Mississippi River delta to document microbial activity down to 167 m beneath the seafloor (Whelan et al., 1986). It took another 10 years for scientists to visualize and quantify these microbial cells at depths in excess of 500 m at five ODP sites around the Pacific Ocean (Parkes et al., 1994). These findings, coupled with the 1977 discovery of chemosynthetic-fueled life at deep-sea hydrothermal vents, sparked a new interest in microbiology of the subseafloor, with some estimates suggesting that more than one-third of Earth’s carbon may be locked in microbial biomass within the subsurface (Gold, 1992; Whitman et al., 1998). Unlike much of the ocean, the subseafloor environment does not depend on photosynthesis; instead, the most abundant energy supply is from inorganic electron donors and acceptors (Bach and Edwards, 2003). The possibility of an extensive population of bacteria and archaea living in the subseafloor raises many important and intriguing questions about the limits of microbial life, the role of marine microbes in essential biogeochemical cycles, and the origin and evolution of life on Earth and its possibilities for other planets. Understanding the influence these microbes have on the chemistry of the ocean and any consequences for the global carbon and climate cycles is essential.
Scientific Accomplishments and Significance
The marine subseafloor can be divided into two distinct biomes: sediments (derived from both terrigenous and vol-canoclastic materials) and igneous rocks and their alteration products (Schrenk et al., 2010). The bulk of microbial data to date are derived from the sediment biome, primarily from coastal and shelf environments. In the mid-1980s, microbiologists began sailing on ODP legs to collect sediment cores for biological analysis. These expeditions focused on paleoceanography, gas hydrates, and other scientific themes rather than on microbiology. Parkes et al. (2000) summarized the findings of these first 15 years of study from 14 marine sediment sites and concluded that while microbial abundances generally decrease with increasing depth, cells are still present at depths in excess of 700 m; they can be stimulated by deep activity, such as subsurface seawater flow or gas hydrates; and they have a strategy of high biomass and low growth rate to guarantee survival (Figure 3.4). Parkes et al. (2000) estimated that microbial populations in the top 500 m of the sediment are equivalent to about 10 percent of the total surface biosphere, highlighting the potential planetary consequences of the subseafloor biosphere.
In 2002, the first dedicated microbiology leg sailed to core sediments from the Peru Margin (ODP Leg 201; see Box 3.3). An international group of multidisciplinary scientists examined the samples for microbial abundance, activity, genetic composition, and contribution to biogeochemical activity. Results indicated an active and abundant population of microbes in the subseafloor, with rates of activities and cell concentrations that varied from one environment to the other depending on electron donor and acceptor availability (D’Hondt et al., 2004). Molecular-based assessments of the sediment samples showed that microbes are indeed active at depth in the sediment column (Schippers et al., 2005), and they are composed of genetically and phylogenetically distinct microorganisms (Biddle et al., 2008; Fry et al., 2008). The findings from ODP Leg 201 were shortly followed by the discovery of active microbial cells in 111 myr sediments from >1,600 m below the seafloor collected on ODP Leg 210 from the Newfoundland Margin (Roussel et al., 2008), thus extending the depth of known microbial life in the sediment-hosted subseafloor biosphere.
Meanwhile, in the late 1990s there was growing interest in the other overlooked but important component of the subseafloor, the rocks. The crustal aquifer is potentially the largest habitat on Earth, with more than 60 percent of the ocean crust estimated to be hydrologically active (Stein and Stein, 1992). Current estimates suggest that the volume of ocean crust capable of sustaining life is comparable in magnitude to that of the oceans (Heberling et al., 2010). Although earlier examinations of surficial marine basalts had suggested a role of microbes in the transformation of basalt to palagonite (Thorseth et al., 1992), no subseafloor rocks had yet been studied for the presence and activity of microbes until scientists examined samples from DSDP Leg 70 and ODP Leg 148 (Furnes et al., 1996; Giovannoni et al., 1996). These studies (and others) employed various DNA stains to the rocks that suggested the presence of microbes in the alteration zones of the basalts. Fisk et al. (1998) examined more than 100 exposed and buried basalt samples, including many from DSDP/ODP archives that ranged from a few meters to 1,500 m below the seafloor, to record the breadth of weathering textures and conditions under which the basalts had formed. This research suggested that microbes may play an important role in the basalt alteration process, such as controlling rates of alteration or the composition of alteration products, and regulating the cycling of nutrients between seawater and ocean crust. However, definitive evidence of indigenous subseafloor microbes growing from or altering rock was not found.
FIGURE 3.4 Compilation of cell count data from recovered sediment cores from 1986 to1996, showing correlation of non-hydrothermal subseafloor bacterial populations with depth. SOURCE: Parkes et al., 2000.
The use of CORKs (Box 3.2) as subseafloor microbial observatories (Fisher et al., 2005) allows scientists to address the issue of in situ microbial-rock interactions in the subseafloor. In 2003, the first such microbiological study from ODP Hole 1026B on the Juan de Fuca Ridge flank was published, where warm crustal fluids were filtered from a CORK and examined for evidence of a unique thermophilic subseafloor microbial community (Cowen et al., 2003). More recently,
“Core on Deck!”—Core Handling, Laboratory, and Rig-Floor Procedures
Critical to the success of ODP and IODP are major innovations in the fields of ship operations, downhole logging and instrumentation, and drilling technology and engineering development. However, these innovations do not stop at the drill bit. Increasingly, JOIDES Resolution core handling, laboratory, and rig-floor procedures have been modified as new science requirements become evident. Shipboard laboratories are constantly changing, as new techniques and expedition objectives demand new core-handling priorities. Highlighted below are a few areas where the need to support science objectives has driven innovation in the internal handling procedures of the JOIDES Resolution.
The use of non-magnetic core barrels, which reduce drilling-induced magnetic overprints, and non-magnetic bottom hole assemblies, which allow for core orientation, has resulted in much improved magnetostratigraphic dating of cored sequences. The need to obtain complete stratigraphic sections generally leads to drilling or coring multiple holes at one site, so that coring breaks overlap with one another. The need to quickly correlate these cores led to the development of a fast-track multi-sensor tool (Carter and Raymo, 1999; Moran, 2000) that can quickly scan core sections and document correlation between holes, without sacrificing the detailed measurements made on a regular multi-sensor track.
The need to preserve gas hydrate samples for further study drove several significant changes to rig-floor and core-handling procedures. On ODP Leg 164, cold spots indicative of gas hydrate dissociation were used to quickly identify samples. By ODP Leg 204, any cores collected in the gas hydrate stability zone were scanned immediately after recovery with a digital infrared camera, and samples were taken rapidly and preserved by a variety of means, including storage at in situ pressure or in liquid nitrogen. Procedures for scanning cores and processing the infrared scans were further developed during IODP Expedition 311. Gas hydrate studies also drove ODP to develop technology to retrieve cores at in situ pressure (see Box 3.4), priming further industry development (Schultheiss et al., 2006).
The emergence of subseafloor microbiology led to additional dramatic changes both in rig-floor and core-handling procedures onboard the JOIDES Resolution. Although interest in microbiological samples started as early as DSDP Leg 96, it was not until ODP Leg 185 (Izu-Mariana Margin) that contaminant testing demonstrated that uncontaminated material suitable for microbiological study could be obtained from drill cores. ODP Leg 201, a return to the Peru Margin, marked the first expedition dedicated primarily to microbiological objectives. During this expedition, IODP deployed a radioactive isotope van and modified the rig-floor protocol to ensure cores were available for sampling almost immediately, before they could warm significantly. Core-handling techniques were changed on the catwalk as well, allowing extensive whole-round sampling for microbiological studies (which in some cases completely consumed the cored interval). ODP Leg 201 also marked the first use of a thermal imaging tool on the catwalk.
During the 2006-2009 JOIDES Resolution refit, a dedicated space for the radioactive isotope van was added. This fully integrated the microbiology and chemistry laboratories. As part of the retrofit, a dedicated cold laboratory for sample processing was added to the microbiology laboratory.
geochemical fluid osmosamplers and downhole microbial samplers filled with mineral incubation material that encourage colonization and growth have been deployed and retrieved in the Juan de Fuca CORKs, with plans for future deployments in other CORK borehole observatories (Fisher et al., 2005; Orcutt et al., 2011). In concert with geochemical and pressure monitoring, these observatories allow for a comprehensive view of subseafloor microbial life over time and the interaction with the basement hydrogeology and chemistry.
Finally, a recent study examined microbial communities in gabbroic rocks for the first time as part of IODP Legs 304 and 305 to the Atlantis Massif. An extremely low diversity of bacteria was seen, dominated by putative hydrocarbon degraders that may be living completely independently of the surface biosphere (Mason et al., 2010).
Fields of Inquiry Enabled
Until microbiologists began sailing on scientific ocean drilling legs, scientists had no way to access the deep and continuous cores needed to determine microbial abundance and activity in the marine subseafloor. The scientific ocean drilling program, therefore, uniquely enabled a new field of inquiry into life in the marine subseafloor. Rock-associated microbes are virtually unaccounted for in any census of subseafloor microbial life because of the inherent difficulties in collecting rock samples and using them in biological analysis (Santelli et al., 2010); therefore scientific ocean drilling is critical to success in understanding microbiology in the subseafloor. The ability to drill even deeper will continue to push our limits and understanding of microbial life in this unique biosphere.
Goals Not Yet Accomplished
By the end of the next decade, the potentially huge and unaccounted for subseafloor habitat will become part of the census of Earth’s microbial life, but only with the access and facilities allowed by scientific ocean drilling. Obviously, using scientific ocean drilling samples and holes for microbiological experiments demands special considerations with respect to drilling strategy, particularly when assessing contamination. The coring system is not designed for microbiology, and surface seawater, which is pumped through the drill string to remove tailings from the borehole, can contain on the order of 1 million microbes per liter. Techniques to monitor contamination using a chemical tracer (perfluoro-methylcyclohexane) and a physical tracer (fluorescent spheres) have been tested to assess contamination in the cores (Smith et al., 2000; Lever et al., 2006). Results suggest that although the collection of mostly uncontaminated cores is possible, the type of coring, the nature of the formation, and various other factors influence the level of contamination seen. This type of variability requires increased vigilance for both drilling operators and scientists when deciding how best to drill holes and collect materials for microbiology. In addition, scientists are currently assessing best storage practice for cores needed in microbial analysis in conjunction with core repositories in the United States and abroad, and detailed notes on exactly how cores were retrieved and stored are essential in assessing potential contamination, even if tracers were not used onboard.
Although there was only one dedicated microbiology leg in all 60 years of scientific ocean drilling’s history, there have been two recent IODP expeditions (South Pacific Gyre in 2010 [IODP Expedition 329] and Mid-Atlantic Ridge Microbiology in 2011 [IODP Expedition 336]), and many others have been proposed. The growing interest in the subseafloor biosphere will continue to be a driver of scientific ocean drilling in the next decade, and new developments in contamination assessment, storage practice, sample analysis, and subseafloor observatories will further enhance the ability of scientific ocean drilling to understand this essential and underexplored aspect of Earth’s biosphere. Studies into this field of inquiry remain in their infancy, and scientific ocean drilling has been critical in advancing discovery and understanding of the deep marine biosphere and will continue to play a pivotal role in future discoveries.
At high pressure and low temperature, some low molecular weight gases (e.g., methane, carbon dioxide) can combine with water to form gas hydrate, an ice-like substance. These seafloor conditions are found almost ubiquitously where the water depth exceeds 300-800 m (depending on regional seawater temperatures). Gas hydrate is most often found at continental margins and in enclosed seas, where organic matter builds up quickly enough to support microbial methane production or where existing gas is transported into the gas hydrate stability zone (Claypool and Kaplan, 1974). In the United States and elsewhere, methane hydrate occurs naturally in sediment beneath permafrost and along continental margins, and in some areas may be concentrated enough to augment conventional gas supplies and provide greater domestic energy security (NRC, 2010). Geohazards associated with gas hydrate include large-scale slope destabilization (e.g., Maslin et al., 2004) and release of methane, a potent greenhouse gas. Evidence collected from deep-sea sediments has been attributed to some massive releases from methane hydrate deposits and linked with major global warming episodes (e.g., Dickens et al., 1995; Kennett et al., 2003). Alternative hypotheses for the data are viable, and it is clear from ice core data that major global warming episodes in the past 100 thousand years (kyr) were not associated with atmospheric methane increases (e.g., Brook et al., 2000; Sowers, 2006). Box 4.2 contains more discussion on this topic.
Scientific Accomplishments and Significance
Scientific ocean drilling has been a major factor in improving understanding of the distribution and dynamics of gas hydrate in marine sediments. The first gas hydrates collected in the deep ocean were sediments at the Middle America trench accretionary complex during DSDP Legs 66 and 67 in 1979, although hydrate-bearing sediments had previously been cored with no gas hydrate recovery during DSDP Leg 11 in 1970. ODP Leg 164 to the Blake Ridge, a passive margin sediment drift deposit, was the first expedition to focus primarily on gas hydrates. It was followed by ODP Leg 204 and IODP Expeditions 311 and 328 to the Cascadia accretionary complex offshore Oregon and Vancouver Island.
Drilling data are essential for calibrating and validating models of gas hydrate distribution, which are derived from remote sensing data. Because methane hydrate is stable at atmospheric pressure only at temperatures below about –80 ºC, much of the hydrate in deep ocean cores is probably lost because of decreases in pressure and increases in temperature during recovery. The unique challenges of sampling and preserving gas hydrates in cores and inferring the concentration and distribution of gas hydrate in situ have been addressed through development of new technologies to recover and analyze core at in situ conditions and through calibration of a variety of proxies for gas hydrate abundance and distribution of varying accuracy and resolution. The most accurate measurements are derived from pressure core samples (Box 3.4), which can only sample a very small subsurface volume. Geophysical logs are used to obtain high-resolution data at in situ conditions from the entire borehole, including the parts of the core where sediment is not recovered. Figure 3.5 presents data for several gas
Retrieving Samples at In Situ Pressures
Pressure core samplers are key to gas hydrate investigations because they provide the only means of measuring the total amount of methane in sediments and of directly observing gas hydrate-sediment structures. During normal core recovery, the pressure decrease results in dissociation of gas hydrate and loss of methane. Development of pressure cores was initiated by ODP (Pettigrew, 1992) and provided critical data to “ground-truth” geochemical and geophysical proxies for gas hydrate during ODP Leg 204 and IODP Expedition 311. ODP pressure core samplers have primarily been used for depressurization experiments (Dickens et al., 1997a), in which gas is captured as pressure is released. Methane concentration can be calculated from collected gas volumes, given in situ pressure-temperature conditions and sediment porosity, allowing for calculation of equilibrium gas hydrate or free gas concentration.
Initial developments of wireline pressure coring technology were advanced by European Union funding of the HYACE (HYdrate Autoclave Coring Equipment) and HYACINTH (Deployment of HYACE tools In New Tests on Hydrates) programs (Schultheiss et al., 2006; Mount Albert Science Team, 2007) and by the U.S. Department of Energy (e.g., Yun et al., 2007), and include mechanisms to transfer core at full pressure into specialized pressure chambers for X-ray imaging (see Figure) and high-resolution geophysical measurements. Nondestructive geophysical measurements on pressure cores enable the nature, distribution, and morphology of gas hydrate structures to be related to the host sedimentology Measurements on pressure cores show that thin, grain-displacing subvertical gas hydrate structures and nodules form in uniform clay lithology whereas distributed grains form in pore space in coarse-grained sediments.
Although gas hydrate investigations have primarily been the focus for development of pressure coring and analysis techniques, other scientists such as microbiologists (Parkes et al., 2010) have also found them to be of use.
X-ray computed tomography images of natural gas hydrates in clay-rich sediments collected by the JOIDES Resolution from the Krishna-Godovari Basin. SOURCE: U.S. Department of Energy National Energy Technology Laboratory, 2010.
hydrate proxies and illustrates the strong heterogeneity in vertical gas hydrate distribution (adapted from Tréhu et al., 2004). Analysis of pressure cores indicated the importance of lithology and fracture permeability in controlling where and how gas hydrate precipitates (e.g., Weinberger et al., 2005; Torres et al., 2008). Drilling combined with regional geologic characterization obtained from pre-drilling site surveys has also provided many new insights into the fluid flow regimes that control gas hydrate distribution.
Fields of Inquiry Enabled
Until gas hydrates were drilled and sampled, geophysically based estimates of the gas hydrate content of sediments were very poorly constrained, and estimates of the amount of gas hydrate present on a global basis varied over many orders of magnitude (Milkov et al., 2003). Calibration of these estimates using drilling data resulted in a decrease in the range, although uncertainty remains large because of the very heterogeneous distribution of gas hydrate in nature. Perhaps more important are the insights into the factors that
FIGURE 3.5 (a) Gamma density profiles of a pressure core for ODP Leg 204 as pressure was released. Layers of very low density develop with time as gas hydrate lenses decompose in response to decreasing pressure. (b) Infrared image of the core on either side of the HYACINTH pressure core. Dark horizontal lines represent cold anomalies (6-8 ºC) resulting from gas hydrate decomposition; yellow lines represent warm anomalies (12-14 ºC) resulting from voids due to gas expansion. (c) Chlorine concentration measured in ODP Hole 1244C. Low chlorine anomalies imply that the pore space contained up to 9 percent gas hydrate in the anomalous samples. Methane concentration from pressure core data (red squares) are overlain on the chlorine data. (d) Resistivity-at-bit data (RAB) from logging-while-drilling operations in ODP Hole 1244D. A detail from 62 to 73 m below the seafloor is presented. Bright regions (high resistivity) are indicative of gas hydrate when they also correspond to low density zones. SOURCE: Tréhu et al., 2006.
control gas hydrate distribution and dynamics that have been obtained from drilling. These insights can be extrapolated to the many regions where only remote sensing data are available.
Since 2006, much of the deep ocean drilling to characterize the distribution of gas hydrates has been undertaken by the Department of Energy Joint Industry Program in the Gulf of Mexico and international programs supported by Japan, India, Korea, and China on their continental margins, with the objective of evaluating fossil fuel potential or geohazards posed by gas hydrates for conventional oil drilling and recovery. Procedures and protocols for handling and archiving gas hydrate-bearing cores during these expeditions have been modeled on procedures pioneered during ODP and IODP expeditions (e.g., storage of samples in pressure vessels or liquid nitrogen; immediate routine scanning of all core with infra-digital infrared cameras). Technologies for recovering and studying core at in situ pressure have been advanced by industry groups, following the initial efforts by ODP (see Box 3.4).
Drilling has also provided insights into the mechanisms that allow large amounts of free gas to migrate through the gas hydrate stability zone to form spectacular mounds of gas hydrate near the seafloor. Although seafloor gas hydrate deposits may constitute only a fraction of hydrate in marine sediments, they are the most easily accessible and, therefore, most well-studied.
Goals Not Yet Accomplished
Studies of gas hydrate dynamics on decadal and shorter time scales, which require time series observations, will remain a main focus of the scientific ocean drilling community and will require close collaboration between scientific ocean drilling and ocean observatories (Torres et al., 2007). The recent advanced CORK installation and connection to
the NEPTUNE Canada fiber optic cable (IODP Expedition 328) represents the first of what should be a new generation of methane hydrate studies. A challenge unique to gas hydrate studies is development of sensors that can record natural changes in temperature, pressure, electrical resistivity, pore fluid flow rate, and other parameters without getting fouled by gas hydrate formation initiated by the presence of the sensor itself. Several attempts are currently under way to develop probes that could be deployed through scientific ocean drilling to operate in this challenging environment.