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3 Scientiﬁc Accomplishments: Fluids, Flow, and Life in the Subseaﬂoor The ability to drill deep into the seaﬂoor has increased Early Deep Sea Drilling Project (DSDP) studies focused on understanding of the role of ﬂuid ﬂow within ocean sediments making measurements in sediments, including temperature and basement rock, especially the connectivity of hydrogeo- and pressure (Box 3.1). However, the discovery of hydro- logic systems within the ocean crust. This knowledge has led thermal vents along mid-ocean ridges in the 1970s, and the to surprising achievements in the study of hydrothermal vent subsequent recognition of robust hydrogeologic systems systems, especially in understanding vent compositions and in nearly all subseaﬂoor geologic environments, served as subsurface extent, as well as in the research and recovery the catalyst for a multidecadal, multi-disciplinary effort to of gas hydrates. In addition, by providing the only direct understand those systems. Beginning with the Conference access to the subseaﬂoor biome, scientiﬁc ocean drilling on Scientiﬁc Ocean Drilling (COSOD) II in 1987, ﬂuid ﬂow has revolutionized understanding of subsurface microbial became a major focus of study in the Ocean Drilling Program communities living at the limits of life, thus enabling a new (ODP) and subsequent Integrated Ocean Drilling Program ﬁeld of scientiﬁc inquiry. (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 HEAT FLOW, FLUID FLOW, AND and geochemical anomalies to recognize ﬂuid migration GEOCHEMISTRY pathways (Moore et al., 1987; Kastner et al., 1991). Hydro- Hydrogeologic systems are present beneath the sea- geology efforts have been concentrated in two distinct zones: ﬂoor in all geologic environments, and they inﬂuence a seaﬂoor basement and sediment. In both ﬁelds the emphasis wide range of biological, chemical, and physical processes. has been on quantifying ﬂow rates and patterns and under- These processes include the magnitude and distribution of standing the links between chemical, biological, tectonic, ﬂuid pressures and related hazards, formation of continen- geophysical, and hydrogeologic processes. tal crust, hydration of plate boundary faults, initiation of Basement hydrogeology studies initially focused on explosive volcanism, generation of gas hydrates and other quantifying the circulation observed in hydrothermal vents. mineral resources, and distribution of subseaﬂoor microbial Early observations revealed that ocean bottom waters were communities. The complexity of interactions not only has being drawn down into the upper levels of the basement, challenged scientists attempting to understand the fundamen- implying that ocean crust was more permeable than the tals of subseaﬂoor processes, but also opened the door for overlying sediments (e.g., Hyndman et al., 1976). The ﬁrst exciting, interdisciplinary efforts in scientiﬁc ocean drilling. direct measurements of basement permeability (DSDP Leg 83, DSDP Hole 504B) documented a stratiﬁed permeability structure, in which permeabilities decreased exponentially Scientiﬁc Accomplishments and Signiﬁcance 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 From the time of Project Mohole (see Box 1.2), scien- tists recognized that scientiﬁc ocean drilling provided an than 550 m (Anderson et al., 1985). Drill-string packer tests, opportunity to investigate ﬂuid ﬂow processes in oceanic like those used on DSDP Leg 83, were subsequently used sediments and crust (e.g., Von Herzen and Maxwell, 1964). successfully at many other sites. The resulting datasets were 27
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28 SCIENTIFIC OCEAN DRILLING basement formations that could be used to estimate forma- Box 3.1 tion permeability (Figure 3.1). However, such open ﬂow also In Situ Measurement of Temperature represented a perturbation to in situ conditions and revealed and Pressure the need for tools with long-term in situ monitoring capa- bilities. The development of the CORK (Circulation Obvia- Temperature is a major parameter controlling tion Retroﬁt Kit) has led to widespread use for long-term dynamic Earth processes. Borehole temperature measurement of temperatures, pressures, and ﬂuid ﬂuxes measurements are important for understanding (Box 3.2). The ﬁrst long-term observatories were estab- heat transfer from Earth’s interior, lithospheric lished on the Juan de Fuca Ridge (ODP Leg 139). Pressure evolution, hotspot volcanism, gas hydrate stability, records from these observatories after 14 months showed and fluid flow in marine sediments. Consequently, high lateral ﬂuid ﬂuxes and short residence times in very temperature was one of the initial downhole proper- permeable upper basement (Davis and Becker, 2002; Fisher, ties measured during DSDP (Von Herzen and Max- 2005). The ﬁrst cross-hole experiment (ODP Leg 168), and well, 1964). Throughout DSDP, ODP, and IODP, new the ﬁrst three-dimensional CORK array (IDOP Leg 301), tools and analysis approaches have continuously also along the Juan de Fuca Ridge, continued to add to the been developed and improved (e.g., Uyeda and picture of large lateral ﬂuid ﬂuxes and high permeabilities, Horai, 1982; Horai, 1985; Fisher and Becker, 1993; Davis et al., 1997; Heeseman et al., 2006). A 2004 and recorded transient ﬂow events associated with seismic IODP workshop on downhole tools confirmed that activity and tides (e.g., Fisher et al., 2008). precise downhole temperature measurements were critical to fulfillment of programmatic objectives in all primary research themes (Flemings et al., 2004). Log [permeability (m2)] The most efficient tool for measuring temperature -11 -17 -16 -15 -14 -13 -12 -10 in boreholes is the advanced piston corer tem- perature tool (APCT), which measures sediment 100 Model, Anisotropy? temperatures as the core is being taken (Horai tidal, seismic and Von Herzen, 1985). The APCT allows for the analyses 200 measurement of in situ temperatures in the undis- Cross- hole turbed sediments that have not yet been reached 300 ? Packer by the drill bit. The APCT has undergone two major upgrades to improve sensor and data sampling 400 P T accuracy and stability while retaining the same ef- Hole 858G (≤0.2 Ma) ficient physical format (Heeseman et al., 2006). For Depth into basement (m) 500 deeper sediments that are too stiff to be sampled Hole 1024C (0.9 Ma) with the APCT, the Davis-Villinger temperature tool Hole 1025C (1.2 Ma) 600 (DVTP) was developed (Davis et al., 1997). The Hole 839B (2.2 Ma) DVTP also measures in situ pore pressure, although Hole 1026B (3.5 Ma) 700 obtaining reliable pressure measurements has been Normal basaltic basement challenging because of the long time constant of Hole 1301B (3.5 Ma) 800 the pressure response and fractures induced in the Hole 1027C (3.6 Ma) sediment when the probe is inserted (Villinger et al., Hole 504B (5.9 Ma) 900 2010). Hole 896A (5.9 Ma) 1000 Hole 395A (7.3 Ma) Hole 801C (157.4-166.8 Ma) 1100 compiled into a regional summary (Figure 3.1), which illus- Hole 857D (sill/sediment, ≤0.2 Ma) Other trates that shallow basement permeabilities are consistently basement 1200 Hole 735B (gabbro, 11.7 Ma) three to seven orders of magnitude higher than the overlying Hole 642E sediment column and supports the early observations of a (thickened crust, 56 Ma) stratiﬁed permeability structure controlled by depth within the basement (Fisher, 2005; Becker and Fisher, 2008). The FIGURE 3.1 Summary of borehole permeability determinations widespread nature of large-scale basement ﬂuid circulation in oceanic basement rocks, based on packer and temperature (ﬂow- 8, ⇑ up Figure Fisher et al. has profound implications for the formation and continua- meter) experiments. Vertical axis is depth into basement, accounting tion of subseaﬂoor microbial communities, the creation of for differences in sediment thickness. Most seaﬂoor measurements have been made in basaltic crust, but two sets of data (ODP Holes ore deposits and gas hydrates, and the overall chemical and 857D and 735B) are from sediment/sill and gabbroic lithologies, heat budget of the oceans. respectively. Note range of values and relatively consistent depth In some cases, borehole temperature measurements indi- trends. SOURCE: Fisher, 2005. cated down- or uphole ﬂuid exchange between the ocean and
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29 SCIENTIFIC ACCOMPLISHMENTS; FLUIDS, FLOW, AND LIFE IN THE SUBSEAFLOOR Box 3.2 CORKs: Subseaﬂoor Borehole Observatories Open drill holes allow significant exchange between Top plug bottom water and formation fluids following perturbations Pressure monitoring Fluid associated with drilling ocean crust. CORKs (Circulation sampling Casing Obviation Retrofit Kits) are designed to stop bottom wa- seal Free flow valve ter influx, thus allowing borehole conditions to return to a more natural hydrodynamic state (Davis et al., 1992; Lateral Becker and Davis, 2005). CORKs can be used for pres- pipe Seafloor sure, seismic, strain, and temperature monitoring; crustal T Packer 20" casing fluid sampling; and microbiological and controlled pertur- T bation experiments. CORKs were originally conceived to Sediment allow for estimates of in situ flow rates and permeability, 16" casing and scientists have more recently begun using CORKs Cement for a variety of chemical and biological experiments us- ing both downhole and seafloor samplers (Fisher et al., Basement T 2005). Samples for geochemistry can be collected over long time periods (up to 5 years) using downhole base- 10-3/4" casing T ment 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 T et al., 2003). Recent downhole experiments have been Cement deployed with mineral colonization surfaces (Orcutt et Spectra cable al., 2010), and seafloor samplers are currently in use 4-1/2" casing T on the Juan de Fuca Ridge flank CORKs to allow for the sampling of multiple fluid horizons within the CORKed Temperature borehole, both from a submersible or as a stand-alone logger sampler (Fisher et al., 2005). CORKs as subseafloor T borehole observatories offer unprecedented opportuni- Fluid sampling ties for integrating hydrogeological studies with microbial Packers and chemical processes in basement fluids. T Pressure monitoring T Microbiological substrate Perforated T collars Fluid sampling Sinker bar 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 transient, conﬁned-aquifer ﬂow, localized expulsion, and/or focused on identiﬁcation of ﬂow pathways and ﬂuxes in external ﬂuid sources, but there is still much to be learned passive and active margins. Drilling in active accretionary about speciﬁc ﬂow pathways and magnitude, as well as the margins resulted in development of new geochemical tracers role that ﬂuids play in seismicity, chemical alteration, and for inferring ﬂuid ﬂow (ODP Legs 112, 125, 131, 134, 146, volcanism. 156, 170, and 190; see Figure 3.2). Using pore water anoma- Hydrologic investigation of non-accretionary subduc- lies such as low chloride concentrations, negative chlorine tion zones proved to be more difﬁcult because of limited isotope ratios, and carbon isotopic ratios of dissolved meth- geologic records. However, drilling on seamounts in the ane, scientists showed that ﬂuids migrate tens of kilometers Mariana forearc and Costa Rica margins conﬁrmed massFigure 2, Fis ﬂuxes of ﬂuids originating from several kilometers belowExp 327, CO along focused pathways, with localized ﬂow rates two to six the seaﬂoor (see section on subduction zone processes inMS 327-107 orders of magnitude larger than steady-state models would suggest (e.g., Moore et al., 1987; Vrolijk et al., 1991; Ransom Chapter 2). et al., 1995). The ﬂow rates inferred from these data require In passive margins, drilling along the New Jersey Mar-
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30 SCIENTIFIC OCEAN DRILLING 60˚ ODP N Leg 112 Leg 125 Leg 131 Leg 134 Leg 146 Leg 156 Leg 170 40˚ Leg 190 20˚ 0˚ -20˚ 120˚ 150˚E 180˚ 150˚W 120˚ 90˚ 60˚ 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 interaction to quantify ﬂow rates and pathways responsible ﬂuid pressures and ﬂow, but pressures could only be inferred for heat, pressure, and solute transfer (e.g., Spinelli and from physical properties (Dugan and Flemings, 2000). Mud- Saffer, 2004; Spinelli and Wang, 2008); and the emergence stone pressures were subsequently measured in the Gulf of of three-dimensional (3D) seismic data as a routine tool for Mexico (IODP Leg 308) in a rigorous demonstration of the investigation of physical properties (e.g., Bangs et al., 2009). coupling between ﬂow and excess pressure (Flemings et al., 2006, 2008; Stigall and Dugan, 2010). Those concepts form Goals Not Yet Accomplished the basis for new understanding of the relationship between overpressures and slope failures along passive margins. 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 Fields of Inquiry Enabled shapes of pathways, and how they affect geologic hazards, The recognition of the magnitude of ﬂuid ﬂow within mineral resources, and the distribution of subsurface microbe sediments and beneath the seaﬂoor has led to exciting new communities. Several ﬁrst-order questions still need to be research to quantify the role that ﬂuids play in controlling addressed in order to resolve the signiﬁcance of these pro- mechanical processes along both passive and active margins, cesses, including the nature of hydraulic communication including the occurrence and magnitude of large earthquakes between basement and sediments; the effect of diagenetic along plate boundary faults and the distribution and timing of modiﬁcation of sediments on geochemical and microbial major slope failure events along passive margins. The body processes in the underlying basement; changes in ﬂow as of literature on these topics is growing at a fast rate; Screaton the ocean crust ages; links between marine and continental (2010) provide a broad synthesis of other related studies. hydrogeologic systems on passive margins; and determina- The one-dimensional nature of ﬂuid-ﬂow measurements tion of quantitative relationships between seismic activity, from boreholes has also led to the development and appli- shallow faulting, and hydrologic processes in subduction cation of new, multidisciplinary tools designed to extend zones. understanding to three dimensions. Notable accomplish- Several marine science initiatives show promise for ments include the use of inexpensive heat probes to help addressing these important questions, in particular the resolve complex patterns of ﬂuid ﬂow (e.g., Fisher and National Science Foundation’s Ocean Observatories Initia- Harris, 2010); modeling studies of ﬂuid ﬂow and ﬂuid-rock
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31 SCIENTIFIC ACCOMPLISHMENTS; FLUIDS, FLOW, AND LIFE IN THE SUBSEAFLOOR tive (OOI).1 The combination of scientiﬁc ocean drilling, 1979. Very few basement rocks were recovered, but analyses permanent observatory capabilities, and evolving drilling and of hydrothermally altered sediments suggested the presence sensor technologies has the opportunity to provide a power- of two distinct hydrothermal systems: one of short duration ful, new integrated approach to resolving key issues related and low temperatures, associated with shallow basaltic intru- to climate variability, changes in ocean ecosystems, plate sions into sediments; the other of longer duration and higher tectonics, and subseaﬂoor chemistry and biology. temperatures, associated with large magmatic intrusions (Gieskes et al., 1982). More than 10 years later, two more high-temperature HYDROTHERMAL VENT PROCESSES hydrothermal vent sites were drilled: Middle Valley and the The seaﬂoor expression of subseaﬂoor hydrothermal Trans-Atlantic Geotraverse (TAG) mound. TAG, located at 26° 08′ N on the eastern side of the Mid-Atlantic Ridge, is vent processes is spectacular, with gushing high-temperature black smokers, ﬁelds of glassy new lava ﬂows, and bushes an area of known high-temperature (>360 °C) basalt-hosted of tube worms and other chemosynthetic life forms. With venting that also supports diverse chemosynthetic life forms the conﬁrmation of seaﬂoor spreading and the discovery of (Humphris et al., 1995). On ODP Leg 158 in 1994, 17 holes deep-sea hydrothermal vents in 1977 (Corliss et al., 1979), drilled at ﬁve locations on the active TAG sulﬁde mound there was a focused effort to delve beneath the seaﬂoor to (200 m in diameter and 50 m high) revealed a massive understand the underlying water-rock reactions that create subseaﬂoor sulﬁde zone in the upﬂow zone (Humphris et spectacular deep-sea hydrothermal vents. Active hydrother- al., 1995). Combined with the seaﬂoor sulﬁde deposits, mal circulation is driven by heat provided by magma cham- geologists estimate almost 3 million tons of sulﬁde at this bers, where the circulating ﬂuids react with the roof of the hydrothermal mound, raising the level of interest among magma chamber, and convection in the crust is driven by the economic geologists (Rona, 2003). In addition, drilling temperature gradient between the ocean and the magma. This demonstrated that there is clear mineralogical zonation in the allows for easy exchange of crustal ﬂuids with the overly- crust, with evidence for huge amounts of seawater intrusion ing ocean. Beneath the seaﬂoor, hydrothermal ﬂuids evolve in the subseaﬂoor, as indicated by the presence of anhydrite, when seawater is heated and a variety of water-rock chemi- a highly soluble mineral that had not been seen in ancient cal reactions take place, such as cation exchange, where mineral deposits and ophiolites (Moores and Vine, 1971). elements such as magnesium are taken up into the rock and The formation and dissolution of anhydrite help to form the iron, zinc, manganese, and silica are released (Seyfried and brecciated sulﬁde framework that allows the sulﬁde mound Mottl, 1995). In addition, these chemical reactions create to grow over time (Humphris and Tivey, 2000). energy sources that support the chemosynthetic-fueled com- Three years earlier, drilling began at Middle Valley munities seen at vents, in which microorganisms use the huge (ODP Legs 139 and 169), located at the northern end of the amounts of volatiles and reduced compounds leached from Endeavour segment of the Juan de Fuca Ridge in the north- east Paciﬁc Ocean (41° N, 127° 30′ W). Like TAG, Middle rocks to grow, thus serving as the major food source and base of the vent ecosystem (e.g., Rau and Hedges, 1979). They Valley is a basalt-hosted site with a massive sulﬁde deposit also create subseaﬂoor habitats that cross temperature and that is actively producing high-temperature hydrothermal energy gradients, allowing for the growth of diverse micro- ﬂuids, but with the additional feature of being overlain by bial communities. These hydrothermally driven water-rock thick Pleistocene continental deposition (Zierenberg et al., reactions are a fundamental component of global geochemi- 1998). In 1996, drilling at Middle Valley on ODP Leg 169 cal cycles and are critical for understanding exchanges and penetrated both the sulﬁde deposit and the feeder-zone, ﬂuxes between the crust and the oceans. Scientiﬁc ocean through which high-temperature metal-rich ﬂuids reach drilling provides access to the subseaﬂoor, which strengthens the seaﬂoor. This deep metal-rich zone contained almost understanding of the processes responsible for the existence 16 percent copper ore (Zierenberg et al., 1998) and had not of seaﬂoor hydrothermal systems and the role these chemical previously been seen below seaﬂoor mineral deposits, further reactions play in inﬂuencing the composition of ocean crust raising the interest for mineral exploration both on land and and the regulation of ocean chemistry. in the ocean (Rona, 2003) (Figure 3.3). Cell counts and phos- pholipid proﬁles were also obtained from the sediment cores, spanning a range of temperatures, and it was found that even Scientiﬁc Accomplishments and Signiﬁcance at high temperatures (up to 185 °C) microbial populations Four active hydrothermal systems were drilled as part were still present, although at lower concentrations than at of DSDP and ODP, representing different geological settings the cooler surface temperatures (Cragg and Parkes, 1994; and highlighting diverse styles of water-rock reaction and Cragg et al., 2000; Summit et al., 2000). The fourth hydrothermal system, PACMANUS (3° 43′ S, crustal alteration. The ﬁrst active site drilled was Guaymas 151° 40′ E), was drilled on ODP Leg 193. PACMANUS is Basin in the Gulf of California (DSDP Leg 64) in 1978 and an active hydrothermal vent ﬁeld within a back-arc basin hosted in felsic volcanic rocks at a convergent margin in 1 See http://www.oceanobservatories.org/.
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32 SCIENTIFIC OCEAN DRILLING Goals Not Yet Accomplished a 856G 1035A 856H There is a continued interest in drilling deep-sea 0 1035G 1035D West East hydrothermal vents, as exempliﬁed by the August 2009 Clastic sulphide Clastic sulphide Massive sulphide Massive sulphide workshop, “Scientiﬁc Ocean Drilling of Mid-Ocean Ridge Upper feeder zone 100 Lower feeder zone and Ridge-Flank Settings” (Christeson et al., 2009). Most Copper replacement zone recently, IODP Expedition 331 drilled hydrothermally Basaltic sill complex Silicified Basaltic flows sediments active mounds in the Okinawa Trough to obtain more 200 data on subseaﬂoor microbial communities. A number of Deep copper zone proposals have been put forward to expand the different FIGURE 3.3 A cross-section of mineralization at the Middle Val- geological settings and diverse styles of water-rock reaction ley hydrothermal site’s Bent Hill massive sulﬁde deposit. SOURCE: and crustal formation drilled, with considerable attention Zierenberg et al., 1998. paid to establishing borehole observatories and linking in with cabled ocean observatories. Special technological issues remain with drilling at active hydrothermal systems, the western Paciﬁc near Papua New Guinea. Discovered and there is a strong need for improved core recovery in in 1991, it hosts both high- and low-temperature venting, young (less than 3 myr) crustal environments. Ongoing chemosynthetic communities, and extensive hydrothermal developments include hard rock re-entry systems, remotely deposits (Binns et al., 2002). Four holes drilled in the ﬁeld operated submersible drill rigs, advanced diamond core revealed that alteration is pervasive beneath the active sites barrels, and engineered muds and instruments capable of and not, as in previous sites, narrowly conﬁned to an upﬂow withstanding high (>200 °C) temperatures (Christeson et zone. Instead, permeability that controls hydrothermal vent- al., 2009). Development and testing of these important tools ing and deposition to the seaﬂoor is governed by fractures, will continue to be important for fulﬁlling scientiﬁc goals not subseaﬂoor high porosity (Binns et al., 2007). Data from in these regions. ﬂuid inclusions also demonstrated evidence for subsurface phase separation with deep-sourced hot hydrothermal ﬂuids SUBSEAFLOOR BIOSPHERE (Vanko et al., 2004). In addition, rocks from core interiors were collected from two of the holes to determine the distri- Morita and Zobell (1955) ﬁrst cultured bacteria from bution of microorganisms in the subseaﬂoor, and microbial shallow marine sediment cores and concluded that the lower cells and ATP (adenosine triphosphate, a marker for biologi- limit of Earth’s biosphere was 7.5 m beneath the seaﬂoor. It cal activity) were detected down to 99.4 and 44.8 m below was not until almost 30 years later that scientists used sedi- the seaﬂoor, respectively (Kimura et al., 2003). ment cores collected from DSDP Leg 96 in the Mississippi River delta to document microbial activity down to 167 m Fields of Inquiry Enabled beneath the seaﬂoor (Whelan et al., 1986). It took another 10 years for scientists to visualize and quantify these microbial Until scientiﬁc ocean drilling began, the only way cells at depths in excess of 500 m at ﬁve ODP sites around to study the chemical reactions and physical stockwork the Paciﬁc Ocean (Parkes et al., 1994). These ﬁndings, beneath hydrothermal vents was via collection of exiting coupled with the 1977 discovery of chemosynthetic-fueled vent ﬂuids and rocks at the seaﬂoor or by examination of life at deep-sea hydrothermal vents, sparked a new interest ophiolites. However, these approaches have the disadvan- in microbiology of the subseaﬂoor, with some estimates tage of only inferring subseaﬂoor hydrothermal processes. suggesting that more than one-third of Earth’s carbon may By providing access to samples beneath the seaﬂoor, sci- be locked in microbial biomass within the subsurface (Gold, entiﬁc ocean drilling has made a critical contribution to 1992; Whitman et al., 1998). Unlike much of the ocean, understanding active hydrothermal systems from a chemi- the subseaﬂoor environment does not depend on photosyn- cal, geological, and even a biological perspective. One of thesis; instead, the most abundant energy supply is from the more unexpected outcomes of drilling hydrothermal inorganic electron donors and acceptors (Bach and Edwards, vents was the discovery of subseaﬂoor massive mineral 2003). The possibility of an extensive population of bacteria deposits, and together with previous interest in seaﬂoor and archaea living in the subseaﬂoor raises many important massive sulﬁde deposits, there is now considerable interest and intriguing questions about the limits of microbial life, in mining seaﬂoor hydrothermal systems, particularly in the role of marine microbes in essential biogeochemical back-arc basins and arc volcanoes in water depths of less cycles, and the origin and evolution of life on Earth and its than 2,000 m (Hoagland et al., 2010). Because the scientiﬁc possibilities for other planets. Understanding the inﬂuence community’s understanding of the formation and evolution these microbes have on the chemistry of the ocean and any of these deposits and associated ecosystems is incomplete, consequences for the global carbon and climate cycles is there is a strong desire to link industry and scientists to avoid essential. potential environmental damage.
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33 SCIENTIFIC ACCOMPLISHMENTS; FLUIDS, FLOW, AND LIFE IN THE SUBSEAFLOOR Scientiﬁc Accomplishments and Signiﬁcance .001 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 .01 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, micro- .1 biologists began sailing on ODP legs to collect sediment cores for biological analysis. These expeditions focused on Depth (mbsf) paleoceanography, gas hydrates, and other scientiﬁc themes rather than on microbiology. Parkes et al. (2000) summa- 1 rized 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 10 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 100 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 1000 consequences of the subseaﬂoor biosphere. 0 2 4 6 8 10 12 Log10 bacterial numbers In 2002, the ﬁrst dedicated microbiology leg sailed to (cells/cm3) core sediments from the Peru Margin (ODP Leg 201; see Box 3.3). An international group of multidisciplinary scientists FIGURE 3.4 Compilation of cell count data from recovered sedi- examined the samples for microbial abundance, activity, ment cores from 1986 to1996, showing correlation of non-hydro- genetic composition, and contribution to biogeochemical thermal subseaﬂoor bacterial populations with depth. SOURCE: Parkes et al., 2000. 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 microbes until scientists examined samples from DSDP Leg (D’Hondt et al., 2004). Molecular-based assessments of the 70 and ODP Leg 148 (Furnes et al., 1996; Giovannoni et al., sediment samples showed that microbes are indeed active at 1996). These studies (and others) employed various DNA depth in the sediment column (Schippers et al., 2005), and stains to the rocks that suggested the presence of microbes in they are composed of genetically and phylogenetically dis- the alteration zones of the basalts. Fisk et al. (1998) examined tinct microorganisms (Biddle et al., 2008; Fry et al., 2008). more than 100 exposed and buried basalt samples, includ- The ﬁndings from ODP Leg 201 were shortly followed by ing many from DSDP/ODP archives that ranged from a few the discovery of active microbial cells in 111 myr sediments meters to 1,500 m below the seaﬂoor, to record the breadth of from >1,600 m below the seaﬂoor collected on ODP Leg 210 weathering textures and conditions under which the basalts from the Newfoundland Margin (Roussel et al., 2008), thus had formed. This research suggested that microbes may play extending the depth of known microbial life in the sediment- an important role in the basalt alteration process, such as hosted subseaﬂoor biosphere. controlling rates of alteration or the composition of altera- Meanwhile, in the late 1990s there was growing inter- tion products, and regulating the cycling of nutrients between est in the other overlooked but important component of the seawater and ocean crust. However, deﬁnitive evidence of subseaﬂoor, the rocks. The crustal aquifer is potentially the indigenous subseaﬂoor microbes growing from or altering largest habitat on Earth, with more than 60 percent of the rock was not found. ocean crust estimated to be hydrologically active (Stein The use of CORKs (Box 3.2) as subseaﬂoor microbial and Stein, 1992). Current estimates suggest that the volume observatories (Fisher et al., 2005) allows scientists to address of ocean crust capable of sustaining life is comparable in the issue of in situ microbial-rock interactions in the subsea- magnitude to that of the oceans (Heberling et al., 2010). ﬂoor. In 2003, the ﬁrst such microbiological study from ODP Although earlier examinations of surﬁcial marine basalts Hole 1026B on the Juan de Fuca Ridge ﬂank was published, had suggested a role of microbes in the transformation of where warm crustal ﬂuids were ﬁltered from a CORK and basalt to palagonite (Thorseth et al., 1992), no subseaﬂoor examined for evidence of a unique thermophilic subseaﬂoor rocks had yet been studied for the presence and activity of microbial community (Cowen et al., 2003). More recently,
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34 SCIENTIFIC OCEAN DRILLING Box 3.3 “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 develop- ment 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 re- covery 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 imme- diately, 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. Fields of Inquiry Enabled geochemical fluid osmosamplers and downhole micro- bial samplers ﬁlled with mineral incubation material that Until microbiologists began sailing on scientiﬁc ocean encourage colonization and growth have been deployed and drilling legs, scientists had no way to access the deep and retrieved in the Juan de Fuca CORKs, with plans for future continuous cores needed to determine microbial abundance deployments in other CORK borehole observatories (Fisher and activity in the marine subseaﬂoor. The scientiﬁc ocean et al., 2005; Orcutt et al., 2011). In concert with geochemi- drilling program, therefore, uniquely enabled a new ﬁeld of cal and pressure monitoring, these observatories allow for a inquiry into life in the marine subseaﬂoor. Rock-associated comprehensive view of subseaﬂoor microbial life over time microbes are virtually unaccounted for in any census of sub- and the interaction with the basement hydrogeology and seaﬂoor microbial life because of the inherent difﬁculties in chemistry. collecting rock samples and using them in biological analy- Finally, a recent study examined microbial communities sis (Santelli et al., 2010); therefore scientiﬁc ocean drilling in gabbroic rocks for the ﬁrst time as part of IODP Legs 304 is critical to success in understanding microbiology in the and 305 to the Atlantis Massif. An extremely low diversity subseaﬂoor. The ability to drill even deeper will continue to of bacteria was seen, dominated by putative hydrocarbon push our limits and understanding of microbial life in this degraders that may be living completely independently of unique biosphere. the surface biosphere (Mason et al., 2010).
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35 SCIENTIFIC ACCOMPLISHMENTS; FLUIDS, FLOW, AND LIFE IN THE SUBSEAFLOOR Goals Not Yet Accomplished seas, where organic matter builds up quickly enough to sup- port microbial methane production or where existing gas is By the end of the next decade, the potentially huge and transported into the gas hydrate stability zone (Claypool and unaccounted for subseaﬂoor habitat will become part of the Kaplan, 1974). In the United States and elsewhere, methane census of Earth’s microbial life, but only with the access hydrate occurs naturally in sediment beneath permafrost and facilities allowed by scientiﬁc ocean drilling. Obvi- and along continental margins, and in some areas may be ously, using scientiﬁc ocean drilling samples and holes for concentrated enough to augment conventional gas supplies microbiological experiments demands special considerations and provide greater domestic energy security (NRC, 2010). with respect to drilling strategy, particularly when assessing Geohazards associated with gas hydrate include large-scale contamination. The coring system is not designed for micro- slope destabilization (e.g., Maslin et al., 2004) and release biology, and surface seawater, which is pumped through the of methane, a potent greenhouse gas. Evidence collected drill string to remove tailings from the borehole, can contain from deep-sea sediments has been attributed to some mas- on the order of 1 million microbes per liter. Techniques to sive releases from methane hydrate deposits and linked with monitor contamination using a chemical tracer (perﬂuoro- major global warming episodes (e.g., Dickens et al., 1995; methylcyclohexane) and a physical tracer (fluorescent Kennett et al., 2003). Alternative hypotheses for the data are spheres) have been tested to assess contamination in the cores viable, and it is clear from ice core data that major global (Smith et al., 2000; Lever et al., 2006). Results suggest that warming episodes in the past 100 thousand years (kyr) were although the collection of mostly uncontaminated cores is not associated with atmospheric methane increases (e.g., possible, the type of coring, the nature of the formation, and Brook et al., 2000; Sowers, 2006). Box 4.2 contains more various other factors inﬂuence the level of contamination discussion on this topic. seen. This type of variability requires increased vigilance for both drilling operators and scientists when deciding how Scientiﬁc Accomplishments and Signiﬁcance best to drill holes and collect materials for microbiology. In addition, scientists are currently assessing best storage Scientiﬁc ocean drilling has been a major factor in practice for cores needed in microbial analysis in conjunc- improving understanding of the distribution and dynamics tion with core repositories in the United States and abroad, of gas hydrate in marine sediments. The ﬁrst gas hydrates and detailed notes on exactly how cores were retrieved and collected in the deep ocean were sediments at the Middle stored are essential in assessing potential contamination, America trench accretionary complex during DSDP Legs even if tracers were not used onboard. 66 and 67 in 1979, although hydrate-bearing sediments had Although there was only one dedicated microbiology previously been cored with no gas hydrate recovery during leg in all 60 years of scientiﬁc ocean drilling’s history, there DSDP Leg 11 in 1970. ODP Leg 164 to the Blake Ridge, a have been two recent IODP expeditions (South Paciﬁc Gyre passive margin sediment drift deposit, was the ﬁrst expedi- in 2010 [IODP Expedition 329] and Mid-Atlantic Ridge tion to focus primarily on gas hydrates. It was followed by Microbiology in 2011 [IODP Expedition 336]), and many ODP Leg 204 and IODP Expeditions 311 and 328 to the others have been proposed. The growing interest in the sub- Cascadia accretionary complex offshore Oregon and Van- seaﬂoor biosphere will continue to be a driver of scientiﬁc couver Island. ocean drilling in the next decade, and new developments in Drilling data are essential for calibrating and validating contamination assessment, storage practice, sample analysis, models of gas hydrate distribution, which are derived from and subseaﬂoor observatories will further enhance the ability remote sensing data. Because methane hydrate is stable of scientiﬁc ocean drilling to understand this essential and at atmospheric pressure only at temperatures below about underexplored aspect of Earth’s biosphere. Studies into this –80 oC, much of the hydrate in deep ocean cores is prob- ﬁeld of inquiry remain in their infancy, and scientiﬁc ocean ably lost because of decreases in pressure and increases drilling has been critical in advancing discovery and under- in temperature during recovery. The unique challenges of standing of the deep marine biosphere and will continue to sampling and preserving gas hydrates in cores and inferring play a pivotal role in future discoveries. the concentration and distribution of gas hydrate in situ have been addressed through development of new technologies to GAS HYDRATES recover and analyze core at in situ conditions and through calibration of a variety of proxies for gas hydrate abundance At high pressure and low temperature, some low and distribution of varying accuracy and resolution. The molecular weight gases (e.g., methane, carbon dioxide) most accurate measurements are derived from pressure core can combine with water to form gas hydrate, an ice-like samples (Box 3.4), which can only sample a very small substance. These seafloor conditions are found almost subsurface volume. Geophysical logs are used to obtain ubiquitously where the water depth exceeds 300-800 m high-resolution data at in situ conditions from the entire (depending on regional seawater temperatures). Gas hydrate borehole, including the parts of the core where sediment is most often found at continental margins and in enclosed is not recovered. Figure 3.5 presents data for several gas
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36 SCIENTIFIC OCEAN DRILLING Box 3.4 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 sedi - ments 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 condi- tions 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 in- clude 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 sedi- mentology. Measurements on pressure cores show that X-ray computed tomography images of natural gas hydrates in thin, grain-displacing subvertical gas hydrate structures clay-rich sediments collected by the JOIDES Resolution from and nodules form in uniform clay lithology whereas the Krishna-Godovari Basin. SOURCE: U.S. Department of distributed grains form in pore space in coarse-grained Energy National Energy Technology Laboratory, 2010. sediments. Although gas hydrate investigations have primarily been the focus for development of pressure coring and analysis techniques, other scientists such as microbiolo- gists (Parkes et al., 2010) have also found them to be of use. Fields of Inquiry Enabled hydrate proxies and illustrates the strong heterogeneity in vertical gas hydrate distribution (adapted from Tréhu et al., Until gas hydrates were drilled and sampled, geophysi- 2004). Analysis of pressure cores indicated the importance cally based estimates of the gas hydrate content of sediments of lithology and fracture permeability in controlling where were very poorly constrained, and estimates of the amount and how gas hydrate precipitates (e.g., Weinberger et al., of gas hydrate present on a global basis varied over many 2005; Torres et al., 2008). Drilling combined with regional orders of magnitude (Milkov et al., 2003). Calibration of geologic characterization obtained from pre-drilling site these estimates using drilling data resulted in a decrease in surveys has also provided many new insights into the ﬂuid the range, although uncertainty remains large because of ﬂow regimes that control gas hydrate distribution. the very heterogeneous distribution of gas hydrate in nature. Perhaps more important are the insights into the factors that
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37 SCIENTIFIC ACCOMPLISHMENTS; FLUIDS, FLOW, AND LIFE IN THE SUBSEAFLOOR C. D. B. RAB - Hole 1244D Concentration of CH4 (mM) A. 100 150 200 47 Resistivity (m) 0 HIgh Low HYACINTH Hydrate Core 1244E-7H Image orientation N S N Core 1244E-8Y 48 20 pore 66 water 30 49 40 100 67 Distance along the core (cm) BSR 50 50 Depth (mbsf ) Depth (mbsf ) Depth (mbsf ) 68 60 Dissolved gas 51 70 69 200 80 Free gas 52 70 90 71 100 53 Core 1244E-9H 1.5 1.7 1.9 Density (gm/cm ) 3 pore water 300 72 Run 1 Cl data from Hole 1244B; 54 Pressure core data from Run 12 Holes 1244B, C and E Run 16 460 500 540 55 Cl - (mM) 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 advanced by industry groups, following the initial efforts by obtained from drilling. These insights can be extrapolated ODP (see Box 3.4). to the many regions where only remote sensing data are Drilling has also provided insights into the mechanisms available. that allow large amounts of free gas to migrate through the Since 2006, much of the deep ocean drilling to charac- gas hydrate stability zone to form spectacular mounds of terize the distribution of gas hydrates has been undertaken gas hydrate near the seaﬂoor. Although seaﬂoor gas hydrate by the Department of Energy Joint Industry Program in the deposits may constitute only a fraction of hydrate in marine Gulf of Mexico and international programs supported by sediments, they are the most easily accessible and, therefore, Japan, India, Korea, and China on their continental margins, most well-studied. with the objective of evaluating fossil fuel potential or geo- hazards posed by gas hydrates for conventional oil drilling Goals Not Yet Accomplished and recovery. Procedures and protocols for handling and archiving gas hydrate-bearing cores during these expeditions Studies of gas hydrate dynamics on decadal and shorter have been modeled on procedures pioneered during ODP time scales, which require time series observations, will and IODP expeditions (e.g., storage of samples in pressure remain a main focus of the scientiﬁc ocean drilling commu- vessels or liquid nitrogen; immediate routine scanning of all nity and will require close collaboration between scientiﬁc core with infra-digital infra-red cameras). Technologies for ocean drilling and ocean observatories (Torres et al., 2007). recovering and studying core at in situ pressure have been The recent advanced CORK installation and connection to
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38 SCIENTIFIC OCEAN DRILLING the NEPTUNE Canada ﬁber optic cable (IODP Expedition fouled by gas hydrate formation initiated by the presence of 328) represents the ﬁrst of what should be a new genera- the sensor itself. Several attempts are currently under way tion of methane hydrate studies. A challenge unique to gas to develop probes that could be deployed through scientiﬁc hydrate studies is development of sensors that can record ocean drilling to operate in this challenging environment. natural changes in temperature, pressure, electrical resistiv- ity, pore ﬂuid ﬂow rate, and other parameters without getting