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The nature of life in the sediment beneath mid-ocean gyres is very poorly known. Almost all sites where subseafloor sedimentary life has been studied are on ocean margins (Ocean Drilling Program [ODP] Legs 112, 180, 201, and 204 and Integrated Ocean Drilling Program [IODP] Expeditions 301, 307, and 323) or in the equatorial ocean (ODP Legs 138 and 201). Despite those studies, the extent and character of subseafloor life throughout most of the ocean remains unknown (Ocean Studies Board, 2003). This absence of knowledge is largely due to ignorance of subseafloor life in the major ocean gyres, which collectively cover most of the area of the open ocean.

The South Pacific Gyre is the ideal region for exploring the nature of subseafloor sedimentary communities and habitats in the low-activity heart of an open-ocean gyre. It is the largest of the ocean gyres and its center is farther from continents than the center of any other gyre. Surface chlorophyll concentrations and primary photosynthetic productivity in the seawater are lower in this gyre than in other regions of the world ocean (Behrenfeld and Falkowski, 1997) (Fig. F1). Its surface water is the clearest in the world (Morel et al., 2007). The sediment of this region has some of the lowest organic burial rates in the ocean (Jahnke, 1996). Our recent survey cruise demonstrated that shallow sediment of this region contains the lowest cell concentrations and lowest rates of microbial activity ever encountered in shallow marine sediment (D’Hondt et al., 2009).

The South Pacific Gyre is also ideal for testing hypotheses of the factors that limit hydrothermal circulation and chemical habitability in aging oceanic crust (sedimentary overburden, basement permeability, and decreasing basal heat flux). It contains a continuous sweep of oceanic crust with thin (1–100 m) sedimentary cover spanning thousands of kilometers and >100 m.y. of seafloor age.

The South Pacific Gyre contains the largest portion of the seafloor that has never been explored with scientific ocean drilling. Consequently, IODP Expedition 329 will advance scientific understanding across a broad front, will help to constrain the nature of crustal inputs to the subduction factory, and will constrain the origin of the Cretaceous Normal Superchron (CNS) and tectonic history of a region as large as Australia. Recovery of sedimentary interstitial waters at several of the proposed sites will provide novel constraints on glacial–interglacial pCO2 models.

Expedition history

Expedition 329 is based on IODP drilling Proposal 662-Full3, “Life beneath the seafloor of the South Pacific Gyre” (available at​scienceops/​expeditions/​south_pacific_gyre_microbio.html). Following ranking by the IODP Scientific Advisory Structure, the expedition was scheduled for the R/V JOIDES Resolution, operating under contract with the US Implementing Organization (USIO). The expedition started in Papeete, Tahiti, on 8 October 2010 and ended in Auckland, New Zealand, on 13 December. Further details about USIO and the facilities aboard the JOIDES Resolution can be found at Supporting site survey data for Expedition 329 are archived at the IODP Management International, Inc., Site Survey Data Bank ( The KNOX-02RR site survey expedition report is available in this volume (see D’Hondt et al., 2011).

Geological setting

Expedition 329 sites span nearly the entire width of the Pacific plate in the Southern Hemisphere between 20°S and 45°S (Fig. F2). This oceanic crust was accreted along at least four different plate boundaries (e.g., Pacific/Phoenix, Pacific/Antarctic, Pacific/Farallon, and Pacific/Nazca). Crustal ages range from ~100 Ma (Chron 34n) at Site U1365 to ~6 Ma (Chron 3An.1n) at alternate proposed Site SPG-7A. Calculated spreading rates range from slow–intermediate (<20 km/m.y., half-rate) to ultrafast (>80 km/m.y., half-rate).

The site locations cover a relatively wide range of crustal ages, spreading rates, and tectonic/volcanic environments. The depth and crustal age of each site correlates well with the predicted depth versus age curve (Stein and Stein, 1994), which suggests the sites are located on representative crust. Calculated spreading rates at each site are somewhat biased toward fast and ultrafast spreading rates (28–95 km/m.y., half-rate). Surprisingly, the 95 km/m.y. value is one of the fastest spreading half-rates measured globally. The abyssal hill fabric is relatively well defined for most coring sites. However, off-axis volcanism at alternate proposed Site SPG-5A and Site U1368 masked the original seafloor fabric. Sediment thickness ranges from <3 to 122–130 m and generally increases west and south of our survey area. This sediment thickness trend is consistent with greater sediment cover on older crust and on crust located farther away from the center of the gyre. The notable exception to this trend is along the northern transect on crust accreted along the Pacific-Farallon spreading system and older than ~30 m.y. Sediment at each of the sites generally appears as pelagic drape, with some localized mass wasting deposits. Seismic images also reveal areas of bottom current activity occasionally resulting in localized scouring of all sediment above volcanic basement (e.g., alternate proposed Site SPG-5A).

Microbiological setting

The sedimentary communities and activities of shallow (0–8 meters below seafloor [mbsf]) South Pacific Gyre sediment are unlike those in any sediment of equal depth previously explored by scientific ocean drilling (D’Hondt et al., 2009). The survey expedition, KNOX-02RR, demonstrated that cell concentrations and organic-fueled respiration in the shallow sediment of Sites U1365–U1370 are orders of magnitude lower than concentrations in previously examined sediment of equivalent depth (Fig. F3) (D’Hondt et al., 2009). Dissolved oxygen (O2) penetrates extremely deeply (Fig. F4A) (D’Hondt et al., 2009; Fischer et al., 2009).

These pilot results demonstrated that, at least in the shallow sediment, (1) net metabolic activities are low and oxygen is the principal net terminal electron acceptor and (2) biomass is substantially different than in any previously examined deep-sea sediment. In contrast, on the southern edge of the gyre, where sea-surface chlorophyll content is much higher, cell concentrations and dissolved chemical concentrations in the shallow (0–4 mbsf) sediment (D’Hondt et al., 2009) resemble those of ODP Site 1231 (on the northeastern edge of the gyre) (Figs. F3, F4), where most of the subseafloor interstitial water is anoxic and the community may be principally supported by oxidation of organic matter coupled to reduction of Mn(IV), Fe(II), and NO3 migrating up from the underlying basaltic aquifer (Shipboard Scientific Party, 2003; D’Hondt et al., 2004). These results suggested that biomass and microbial activity in subseafloor sediment may vary predictably with sea-surface chlorophyll content.

Seismic studies/Site survey data

From 18 December 2006 to 27 January 2007, Cruise KNOX-02RR, aboard the R/V Revelle, surveyed all 11 drilling sites (Fig. F1). Sediment was geophysically imaged and cored at all 11 sites. Cores from the survey are curated at the US National Science Fund–supported Rock and Core Facility at the University of Rhode Island (USA). Sites U1365–U1370 are in the central portion of the South Pacific Gyre. Site U1371 is below higher productivity water at the gyre’s southern edge.

All of the sites are at a crossing point of two track lines or on a single track line immediately adjacent to a crossing point. The multibeam and seismic results are provided in figure form for each site (see D’Hondt et al., 2010).

Geophysical data collected at each site include SIMRAD EM120 swath map bathymetry and Knudsen digitally recorded 3.5 kHz and multichannel seismic reflection. Data were collected at 4.5–6 kt with continuous GPS navigation, including at least one set of intersecting track lines. Following each geophysical survey, shallow sediment (0–8 mbsf) was recovered using gravity, piston, and multicores. At sites with water depths deeper than 4 km, this shallow sediment is principally abyssal clay capped by manganese nodules. At sites in shallower water (proposed Sites SPG-6A and SPG-7A), the shallow sediment is clayey nannofossil ooze.

All Expedition 329 drill sites were supported by seismic, navigation, and bathymetric data and classified as “1Aa” by the Site Survey Panel. The 1Aa classification indicates that all required data are in the site survey data bank and have been reviewed by the Site Survey Panel and that they image the target adequately and there are no scientific concerns of drill site location and penetration.

Scientific objectives

Our study has the following fundamental objectives:

  • To document the habitats, metabolic activities, genetic composition, and biomass of microbial communities in subseafloor sediment with very low total activity;

  • To test how oceanographic factors (such as surface ocean productivity) control variation in sedimentary habitats, activities, and communities from gyre center to gyre margin;

  • To quantify the extent to which subseafloor microbial communities of this region may be supplied with electron donors by water radiolysis, a process independent of the surface photosynthetic world; and

  • To determine how basaltic basement habitats, potential activities and, if measurable, microbial communities vary with crust age and hydrologic regime (from ridge crest to abyssal plain).

We proposed to meet these objectives by

  • Coring the entire sediment column at several sites along two transects in the region of the South Pacific Gyre (Figs. F1, F2);

  • Coring and logging the upper 100 m of the basaltic basement at three key sites; and

  • Undertaking extensive microbiological, biogeochemical, geological, and geophysical analyses of the cores and drill holes.

The project results address several significant questions. Are the communities in mid-gyre subseafloor sediment uniquely structured? Do they contain previously unknown organisms? What are their principal sources of metabolic energy? Do their principal metabolic activities and composition vary with properties of the surface world, such as sea-surface chlorophyll concentrations or organic flux to the seafloor? Is microbial activity is sustainable in subseafloor basalt by mineral oxidation (e.g., oxidation of iron and sulfur species in the basaltic minerals) for tens of millions of years after basalt formation? Are microbial communities recognizably present in subseafloor basalt older than 13 Ma?

These questions can be framed as hypotheses to be tested. For example, we hypothesized the following:

  • A living community persists in the most organic poor sediment of the world ocean.

  • Organic-fueled metabolic activity is extremely low and oxygen is the principal net terminal electron acceptor in this sediment. Consequently, the degree of anaerobic activities is far lower here than in previously examined subseafloor sediments.

  • The biomass, metabolism, and composition of this subseafloor sedimentary community is distinctly different from the communities observed in organic-rich anaerobic subseafloor ecosystems on the continental margins.

  • H2 from water radiolysis is a significant electron donor for microbial respiration in the most organic-poor subseafloor sediment.

  • Open flow continues in the very old basalt of the western gyre.

  • Basalt oxidation may support chemolithotrophic microbial activity for 100 m.y. here.

  • Biomass and activity decrease with basement age as electron donor accessibility decreases.

Even if all of the above hypotheses are falsified, the results of Expedition 329 will significantly advance understanding of the subsurface world. Postcruise research by Expedition 329 scientists will test the extent to which distinct oceanographic and geologic provinces contain distinct subseafloor habitats and distinct subseafloor communities. They will document the extent to which life in the low-activity gyre sediment depends on the surface photosynthetic world—and the extent, if any, to which it is metabolically independent of the surface world. They will place firm constraints on the potential for microbial redox activity in ancient subseafloor basalt and how that potential varies with crustal age over 100 m.y. or more. They will place firm constraints on estimates of total subseafloor biomass and habitable space on our planet.

The results of Expedition 329 and subsequent shore-based studies will also test

  • The factors that control evolution of geothermal circulation and chemical alteration in oceanic crust,

  • Models of regional tectonic history,

  • Geodynamo models, and

  • Models of glacial–interglacial ocean-climate change.

Explanation of primary objectives

Our first primary objective is to document the habitats, metabolic activities, genetic composition, and biomass of microbial communities in subseafloor sediment with very low total activity.

Key questions are: What are the principal microbial activities in mid-gyre subseafloor sediment? What are the rates of those activities? How dense are the populations in this sediment? What are the communities active here? How do these communities and activities compare to those in subseafloor sediment with much higher levels of activity? How unique are their organisms?

Expedition 329 provides a crucial opportunity to document microbial habitats, activities, and community composition in a subseafloor sedimentary ecosystem that has never been explored by scientific ocean drilling.

The penetration of high concentrations of O2 and NO3 meters into the sediment at KNOX-02RR survey sites suggested that the net rate of electron-accepting activity is even lower in the South Pacific Gyre than at Site 1231 in the Peru Basin (D’Hondt et al., 2009). Along with chemical data from Deep Sea Drilling Project (DSDP) Leg 92, these O2 and NO3 profiles also suggested that the principal electron-accepting activity in the subseafloor sediment of this region may be different than at any of the sites where subseafloor sedimentary communities have been previously explored. A sequence of sites was drilled during Leg 92 at ~20°S in the northern portion of the gyre (Leinen, Rea, et al., 1986). At all Leg 92 sites where interstitial water chemistry was analyzed, dissolved NO3 is present throughout the entire sediment column, which is as thick as 50 m (Gieskes and Boulègue, 1986). Because NO3 concentrations do not significantly change with depth in the KNOX-02RR survey cores (except at Site U1371) or the Leg 92 cores, O2 was predicted to be the principal net electron acceptor in subseafloor sediment throughout the South Pacific Gyre (D’Hondt et al., 2009).

Quantification of subseafloor biomass in the South Pacific Gyre will place strong constraints on the size of Earth’s subseafloor biomass and total biomass.

On the basis of acridine orange direct counts in subseafloor sediment of relatively high activity sites, subseafloor sedimentary biomass has been estimated to comprise one-tenth to one-third of total carbon of living biomass on Earth (Parkes et al., 2000; Whitman et al., 1998). A subsequent study of intact polar lipids estimated living archaeal biomass in subseafloor sediment to be 90 Pg (Lipp et al., 2008; Lipp and Hinrichs, 2009), equal to ~15% of Earth’s living biomass. These estimates make a key assumption that needs to be tested by Expedition 329; they assume that the sites where cells were counted are representative of biomass content in the subseafloor sediment of all regions. However, most sites used for these biomass estimates are in relatively organic-rich, high-biomass sedimentary environments and therefore may not accurately represent much of the world ocean. As described earlier, cell concentrations in the KNOX-02RR cores are orders of magnitude lower than concentrations at the same depths in all previous IODP/ODP sediment (D’Hondt et al., 2009) (Fig. F3). Therefore, living subseafloor biomass may be significantly lower than 10%–30% of Earth’s total biomass. Expedition 329 provides fundamental data to address this issue.

Genetic analyses of bulk sediment samples and cultured bacterial isolates from deep beneath the seafloor have demonstrated that similar subseafloor sedimentary environments separated by thousands of kilometers contain similar phylogenetic types of microbial communities. For example, hydrate-bearing sediment of the Peru margin and the northwestern US margin (Hydrate Ridge) resulted in statistically similar compositions of 16S rRNA gene clone libraries (Inagaki et al., 2006). In contrast, nearby sites with different sedimentary environments contain very different populations, suggesting that environmental factors on energetic constraints and availability may significantly affect the geographic distribution of microbial communities and their metabolic processes (Inagaki et al., 2006).

Other studies demonstrated the presence of previously undiscovered and therefore uncharacterized phylogenetic lineages in subseafloor sediment, even within a few meters of the seafloor (Sørensen et al., 2004). For example, archaeal 16S rRNA gene clone libraries from shallow sediment (≤2.1 mbsf) collected during the KNOX-02RR cruise show that the diversity of predominant archaeal components shift from Nitrosopmilus-related ammonia oxidizers (α-subgroup) to physiologically unknown members of η- and υ-subgroups within the Marine Crenarchaeota Group I (Durbin and Teske, 2010).

In combination with the metabolic points described above, these genetic discoveries underscore three important points about potential community composition in subseafloor sediment of the South Pacific Gyre. First, the gyre’s subseafloor microbial communities may greatly differ from those of any subseafloor environments explored to date. Second, the very low activity and potentially aerobic communities of the deepest sediment in the South Pacific Gyre are likely to contain unique and previously undiscovered microorganisms. Third, exploration of the gyre sediment may provide deep insight into community composition and structure throughout much of the open ocean, because the genetic communities of subseafloor sediment in the other major ocean gyres may resemble those of the South Pacific Gyre.

The second primary objective is to test how oceanographic factors (such as surface ocean productivity) control variation in sedimentary habitats, activities and community compositions from gyre center to gyre margin.

Key question is: How are subseafloor sedimentary activities and community compositions affected by oceanographic properties that vary predictably from gyre center to gyre margin?

Expedition 329 provides an unprecedented opportunity to document how the nature of subseafloor sedimentary life varies with oceanographic properties in the least biologically active region of the world ocean.

Interstitial water surveys of regions with higher organic fluxes to the seafloor, such as the equatorial Pacific and continental margins, suggest that the principal electron-accepting activity and the net rates of electron-accepting activities vary predictably with sea-surface chlorophyll content and organic flux to the seafloor (D’Hondt et al., 2002, 2004). These relationships appear to be principally due to reliance of subseafloor sedimentary communities on burial of photosynthesized organic matter for electron donors. Other sedimentary properties that vary with measures of oceanic productivity include sediment composition, which depends on the rate of microfossil production in the overlying water column as well as on the position of the carbonate compensation depth (CCD), and sediment thickness, which largely depends on the rate of calcium carbonate and biogenic silica production in the overlying water column.

The KNOX-02RR results (D’Hondt et al., 2009) suggest that the principal electron-accepting activity, net rates of activities, and cell concentrations in the shallow subseafloor sediment of the South Pacific Gyre may vary with sea-surface chlorophyll content from within the gyre to outside the gyre and perhaps from site to site within it. Expedition 329 will test these predictions.

The third primary objective is to quantify the extent to which these sedimentary microbial communities may be supplied with electron donors by water radiolysis.

Key question is: To what extent does the ecosystem of organic-poor sediment depend on in situ radiolysis of interstitial water?

Expedition 329 provides an unprecedented opportunity to determine if subseafloor life in very low activity sediment is nourished to a significant extent by H2 from in situ radiolysis of water.

Buried organic matter appears to be the principal source of electron donors in subseafloor sediment of ocean margins and the equatorial Pacific (D’Hondt et al., 2004). However, the burial flux of organic carbon is so low in the South Pacific Gyre that in situ radiolysis of water may be the principal source of electron donors there (D’Hondt et al., 2009) (Fig. F5).

Water radiolysis has been described as a possible source of energy for ecosystems in hard rock far beneath continental surfaces (Pedersen, 1996; Lin et al., 2005a). The longest-lived products of water radiolysis are the electron donor H2 and the electron acceptor O2 (Debierne, 1909). H2 can be supplied by in situ radiolysis or, in theory, by transport of radiolytic H2 from a much deeper biologically dead environment (e.g., the mantle and oceanic basement deep beneath sediment).

The potential importance of water radiolysis to subseafloor sedimentary communities can be assessed by comparing radiolytic H2 production to organic-fueled respiration. For this comparison, in situ H2 production by water radiolysis must be quantified from

  • Logging estimates of uranium, thorium, and potassium concentrations (D’Hondt, Jørgensen, Miller, et al., 2003) or inductively coupled plasma–mass spectrometry data (Blair et al., 2007);

  • Shipboard physical properties measurements (porosity and density) (D’Hondt, Jørgensen, Miller, et al., 2003); and

  • A numerical model of water radiolysis (Blair et al., 2007).

Water radiolysis may provide a higher flux of electron donors than buried organic matter in gyre sediment. Organic-fueled respiration rates are much lower in this sediment than in any previously drilled regions of the world ocean. Clay-rich sediment contains much higher concentrations of radioactive elements than other types of deep-sea sediment. Furthermore, in fine-grained porous sediment, most alpha- and beta-energy production occurs within striking range of interstitial water. Consequently, clay-rich sediment will yield much higher rates of water radiolysis than other geological environments.

Quantification of rates of microbial uptake of radiolytic H2 requires measurement of dissolved H2 concentrations in the cored sediment (Fig. F6). The expected rate of radiolytic H2 production is so high (D’Hondt et al., 2009) that in situ H2 concentrations are measurable onboard if the H2 is not biologically utilized. If H2 is biologically utilized, its in situ concentrations will be below detection; because of its high activation enthalpy, the recombination of O2 and H2 does not occur at measurable rates at temperatures below 400°C. Bacterial catalysis allows this reaction on a timescale of minutes.

A number of postexpedition studies will help to further constrain the role of radiolysis in the subseafloor sediment and basalt of the South Pacific Gyre. Measurement and transport modeling of He–4 concentration profiles will constrain estimates of radiolysis rates independently of estimates based on abundances of radioactive elements. Postcruise experiments with sterilized samples of cored sediment and basalt plus tritium-labeled water may be used to verify rates of H2 production by water radiolysis. Similar (unlabeled) experiments with artificial radiation sources were done by Lin et al. (2005b). Radiolysis rates may be compared to measured hydrogen turnover in sediment incubations with known numbers of microbial cells using an array of relevant electron acceptors (O2, NO3, and oxidized metals). In this way, possible rates of microbial hydrogen oxidation in the subsurface ecosystem can be constrained.

The fourth primary objective is to determine how basement habitats, potential activities and, if measurable, communities vary with crust age and hydrologic regime (from ridge crest to abyssal plain).

Key questions are: How does the habitability of subseafloor basalt change with crust age? How does this change depend on basement hydrologic evolution and mineral alteration? Do fractures remain open for flow in basalt as old as 100 Ma? What is the role of sediment cover in controlling hydrologic flow, alteration, and habitability in subseafloor basalt? Are dissolved oxidants (oxygen and nitrate) available in thinly sedimented subseafloor basalt as old as 100 Ma? Are reduced elements (e.g., iron and sulfur) available for oxidation in basalt as old as 100 Ma? Does mineral oxidation support measurable microbial activity in this basalt?

Expedition 329 provides a unique opportunity to determine how basement habitability and microbial communities vary with crust age, sediment cover, and hydrologic conditions over 100 m.y. or more of basement history and in the most thinly sedimented region of the world ocean.

Subseafloor basalt has been proposed as the largest potential microbial habitat on Earth (Fisk et al., 1998). Glass alteration textures have been interpreted as evidence of microbial colonization in subseafloor basalt as old as 145 Ma (Fisk et al., 1998; Furnes and Staudigel, 1999). Laboratory experiments indicate that microbial communities may play an important role in basalt weathering (Staudigel et al., 1998). Other experiments with cultured isolates and biomineralogical studies using seafloor basalts suggest that microbes can receive their energy from rock weathering (e.g., oxidation of reduced iron and sulfur in minerals) (Edwards et al., 2003). A study of fluid samples from a cased borehole in 3.5 Ma basalt on the eastern flank of the Juan de Fuca ridge showed that thermophilic microbes exist in the borehole (Cowen et al., 2003; Nakagawa et al., 2006; Orcutt et al., 2011).

Despite this evidence, the nature and extent of subseafloor basaltic communities remain largely unknown and the factors that control the “metabolic habitability” of the basalt (the ability of the basalt to fuel microbial reactions) are largely unexplored. The biological significance of the textural features and the factors that control alteration and its timing in crust are not well constrained.

Metabolic habitability of subseafloor basalt ultimately depends on the supply of electron donors (including reduced Fe and S) in the basalt and hydrogen from in situ radiolysis and the supply of electron acceptors (e.g., O2 and NO3) from seawater flowing into and through the basalt. Processes that change these supplies over the basalt’s lifetime include mineral alteration, which changes the supply of electron donors, and the evolution of hydrologic flow through the basalt, which changes the supply of electron acceptors.

Evolution of crust alteration and metabolic habitability

Compilation of DSDP/ODP geochemical data, mostly from the North Atlantic, suggests that oxidation of Fe and S in the upper few hundred meters of subseafloor basalt occurs principally during the first 10 m.y. after basalt formation (Bach and Edwards, 2003). During this interval, the Fe(III)/ΣFe ratio of the bulk basalt in the database increases from ~0.15 to 0.35 and most of the sulfur is oxidized (Bach and Edwards, 2003). Whether this alteration is microbially mediated or not, it changes the redox habitability of the basalt. Alteration patterns are heterogeneous within and between cores, with greatest alteration in permeable zones, such as brecciated pillows (Bach and Edwards, 2003).

Recent studies of altered basalt at ODP Site 801 (~170 Ma) showed that alteration characteristics of this ancient oceanic basement are generally similar to those observed in much younger crust (e.g., ODP Hole 504B at ~6 Ma), suggesting that most alteration takes place when oceanic crust is young (Alt et al., 1992). If oxidative alteration ceases 10–15 m.y. after basalt formation, then the oceanic crust inhabitable by mineral-oxidizing microbes is limited to basalt younger than 10–15 Ma.

Other evidence suggests that oxidative alteration need not be limited to basalt younger than 10–15 Ma. Older basalt has potential for continued redox alteration, and it probably occurs in some geochemical/hydrological regimes. Geophysical measures of matrix density suggest that about half of all intergrain-scale crustal alteration in the uppermost basalt occurs in basement older than 10–15 Ma (Jarrard et al., 2001). In the equatorial Pacific, dissolved O2 and NO3 are present in basement as old as 35 Ma (D’Hondt et al., 2004). As much as 65% of the Fe remains as Fe(II) in shallow oceanic basalt older than 10 Ma (Bach and Edwards, 2003). Whether this Fe(II) continues to be oxidized at very slow rates where exposed to dissolved O2 and NO3 or is physically inaccessible to oxidation remains to be determined.

Sediment thickness provides one explanation for the average change in redox alteration at 10–15 Ma. Once sedimentary cover is thick, laterally extensive, and anoxic enough to seal basement from contact with oxidized seawater, redox disequilibria (habitability) may disappear and oxidative alteration cease. Drilling basement of different ages beneath the unusually thin sediment cover in this region of the Pacific presents a unique test of this hypothesis. If a critical sediment thickness is necessary to curtail oxidative alteration, then the young (13.5 Ma), moderate (33.5 Ma), and old (84–125 Ma) basement sites drilled during Expedition 329 will exhibit more intense or differing styles of alteration relative to other fast-spreading sites of comparable age and greater sediment cover (e.g., Site 801, ~400 m of sediment at ~170 Ma; ODP Site 1256, ~200 m of sediment at 15 Ma; and Hole 504B, ~250 m of sediment at ~6 Ma). If this is the case, the basaltic environment of the South Pacific Gyre is redox habitable for many tens of millions of years after basalt formation.

Evolution of crustal hydrology and chemical habitability

The timing and distribution of shallow basement alteration is intriguingly linked to evidence of the hydrologic evolution of subseafloor basalt. Seismic velocity and macroporosity data suggest that porosity and bulk permeability of subseafloor basalt decrease rapidly during the first 10–15 m.y. after basalt formation (Jarrard et al., 2001). These decreases are thought to derive from secondary mineralization in the basalt. This mineralization reduces surface area within the basalt and may limit electron donor and nutrient availability in the basaltic aquifer.

Global compilations of heat flow data indicate that advective heat loss is high in young seafloor and decreases with increasing age until ~65 Ma, where, on average, advective heat loss ceases (Stein and Stein, 1994) (Fig. F7). Fisher and Becker (2000) explain these observations by invoking closure of small-scale permeability in basement within 10–15 m.y. but ongoing flow for 65 m.y. in large fractures and faults that fill a relatively small volume of the rock. Three mechanisms are thought responsible for limiting hydrothermal circulation in old crust:

  • Buildup of laterally extensive low-permeability sediment cover that isolates the oceanic crust from overlying seawater;

  • Ongoing mineralization, which decreases basement permeability with age; and

  • Decreasing basal heat flux with age, which reduces the driving force for buoyant fluid flow.

Basement permeability is commonly thought to ultimately control the cessation of hydrothermal circulation (Stein and Stein, 1994; Jarrard et al., 2001). However, a growing body of evidence suggests that oceanic basalt remains permeable enough to sustain advective heat loss through its life. This evidence includes the significant variance in age-dependent heat flow averages (Stein and Stein, 1994) (Fig. F7), large variations in heat flow survey data of Cretaceous-aged basement (Von Herzen, 2004; Fisher and Von Herzen, 2005) velocity logs, macroporosity data, matrix data (Jarrard et al., 2001), present-day fluid flow within ~132 Ma basement at ODP Site 1149 (Shipboard Scientific Party, 2000), and celadonite precipitation ages that indicate low-temperature fluid circulation at the Trodos ophiolite (Gallahan and Duncan, 1994). Given these lines of evidence, the termination of the average heat flow deficit at ~65 Ma probably signifies that much of the open circulation between basaltic basement and ocean has largely stopped by then, rather than that hydrothermal flow has ended (Anderson and Skilbeck, 1981; Jacobson, 1992; Stein and Stein, 1994).

We hypothesize that old seafloor can host advective fluid and that the thin sediment cover characterizing the South Pacific Gyre facilitates fluid flow in old oceanic crust. If this hypothesis is true, the apparent waning of hydrothermal circulation at ~65 Ma is controlled by sediment thickness or declining heat flow, rather than by basement permeability. The South Pacific Gyre offers a unique opportunity to test this hypothesis because its sediment is thin and its basement relief is relatively variable. Further, this hypothesis implies large-scale permeability will be high regardless of basement age and heat flow data at the drill sites may deviate significantly from conductive values. Surface heat flow data from our recent survey (Fig. F7) and from scattered older measurements within 200 km of Sites U1365 and U1367 suggest that an active flow system may be present in the basement throughout the region of proposed drilling.

An additional implication of this hypothesis is that the supply of the dissolved electron acceptors O2 and NO3 to the upper basement of the South Pacific Gyre may remain high long past the first 10–15 m.y. after basalt formation. If drilling of the lowermost sediment and the basalt shows the occurrence of these electron acceptors in the upper basement at the sites, either (1) the metabolic habitability of South Pacific Gyre basalt remains sufficient to support life for as long as 100 m.y. or (2) the metabolic habitability of this basalt is ultimately controlled by the inaccessibility of electron donors in the basalt rather than by access to electron acceptors. If the first case applies, increased oxidative alteration may be evident in crust of increasingly greater age (e.g., alteration at Site U1365 will be greater than alteration at Site U1367, which will be greater than alteration at Site U1368). If the second case applies, oxidative alteration will be similar at all three sites.

Assessment of the extent and relative importance of secondary alteration to the basaltic basement will require an integrated program of petrographic, geochemical, and borehole analyses. At hand-sample and thin section scales, we will carefully describe general alteration textures and characteristics (e.g., veins, halos, vesicle filling, mineral/matrix replacement, and glass palagonitization), principal secondary mineralogy (e.g., saponite, celadonite, calcite, Fe oxyhydroxide, etc.), and the size, distribution, and orientation of veins and other structural features in the crust. Discrete samples of the core, representing “fresh” (e.g., pristine glass), average, and end-member altered domains, will be powdered and analyzed for bulk major, trace, and volatile element chemistry, as a means of characterizing the bulk crustal composition and geochemical effects of alteration. Borehole logging and core-log integration are invaluable for reconstructing recovery gaps and estimating bulk geochemical and structural characteristics of deep basement drill sites (Barr et al., 2002; Révillon et al., 2002; Kelley et al., 2003; Pockalny and Larson, 2003). Postexpedition radiogenic isotope measurements will place important constraints on the timing of alteration at each site. For example, calcite formed during crust alteration often contains high concentrations of uranium but little to no lead, making the lead isotopic system a potentially useful calcite precipitation geochronometer, especially in old oceanic crust (Hauff et al., 2003).

Basement community composition

Drilling of the basalt sites during Expedition 329 provides a direct opportunity to test the existence and composition of microbial communities in oceanic basement of three very different ages (13.5, 33.5, and 80–125 Ma). Molecular ecological and biomineralogical studies of uncontaminated basalt from the site with youngest basement will test whether or not microbes take full advantage of the redox habitability of relatively young basalt in open exchange with the overlying ocean while analyses of samples from the sites with older basement will test whether or not ongoing oxidation of the subseafloor basalt (by occurrence of reduced Fe and supply of dissolved O2 and NO3 by ongoing advection and diffusion through the sediment) sustains similar (or different) communities, albeit at slower rates of redox activity. Collectively, the study of these sites will track the compositional and functional evolution of basement microbial communities >100 m.y. in a thinly sedimented region of fast-spreading crust, where we hypothesize supply of dissolved oxidants to continue for as long as 100 m.y. (or more).

Explanation of secondary objectives

Oceanic crust inputs to the subduction factory

The altered oceanic crust at our oldest basement sites (U1365 and U1366) is outboard of the Tonga-Kermadec subduction zone. Characterizing the style and extent of alteration in this subducting plate will provide an important reference for assessing the role of altered oceanic crust in the subduction process. Of global arcs, the Tonga arc has perhaps the smallest sediment flux (Plank and Langmuir, 1998) and the altered oceanic crust in this system is proportionally more influential in the subduction process than at most other arcs. If the low sedimentation rate at this site has resulted in unusual alteration characteristics in the subducting oceanic crust, these differences may translate into geochemically distinct signatures in the magmas produced at the Tonga arc.

Regional tectonics

Sites U1365 and U1366 are centrally located in ocean crust accreted during the CNS. The tectonic history of this Australia-sized area is poorly constrained because correlatable magnetic seafloor anomalies are not present. Although limited bathymetry data suggest a general north–south spreading direction, the actual direction(s) and the presence of a failed rift within the region are poorly constrained. Drilling and logging ocean crust at these sites would address these questions. Radiometric dating of plagioclase within the recovered basalt would provide important age constraints (Koppers et al., 2003) and structural analysis of dipping flow units with core logging data would provide a spreading direction (Pockalny and Larson, 2003). Resolving the tectonic history of this region is critical to understanding the effect of large igneous provinces on tectonic processes and whether the Ontong Java, Manihiki, and Hikurangi plateaus were created by one or multiple mantle plumes (Taylor, 2006).


The causal mechanisms for the Cocos-Nazca spreading center are still debated. Some authors have proposed the presence of a strong magnetic field during superchrons (Larson and Olson, 1991; Tarduno et al., 2001), which would argue for an efficient geodynamo (i.e., large intensities during low reversal frequency). Others argue for a weak field (Loper and McCartney, 1986; Pick and Tauxe, 1993), suggesting that increased convective vigor in the core would increase the reversal rate by generating frequent instabilities. If the latter model applies, the Cocos-Nazca spreading center record is low in paleointensity but may contain frequent reversals. Further adding to this controversy, the limited number of paleointensity measurements from the Cocos-Nazca spreading center yielded very different results for different methods (Pick and Tauxe, 1993; Tarduno et al., 2001). Drilling basement at Site U1365 will provide important data and samples to test these models and methods. The paleointensity methods could be compared and the measurements would provide important data (in conjunction with the radiometric age) for the origin of the Cocos-Nazca spreading center and its relationship to the geodynamo.


Interstitial waters of the South Pacific Gyre represent a unique archive of glacial-aged water from which relict NO3 can be used to test hypotheses of glacial–interglacial ocean-climate change with significantly lower uncertainty than through proxy measurements.

It has been hypothesized that changes in strength of the biological carbon pump caused the dominant variation in Earth’s climate and atmospheric CO2 over the last 1 m.y. (Broecker, 1982). We will test the two principal models of this variation by reconstructing the preformed NO3 content and deep ocean δ15NO3 of the last glacial maximum (LGM) through measurement of O2, NO3, and δ15NO3 in the interstitial fluid of gyre sediment. Our tests utilize the fact that pore fluids from depths greater than ~30–50 mbsf (e.g., at Sites U1365 and U1370) are samples of paleo–bottom water that have effectively been out of diffusive contact with the ocean since the LGM.

Relationship to previous drilling

Site U1365 is located in the western portion of the gyre, near DSDP Sites 595 and 596 (Menard, Natland, Jordan, Orcutt, et al., 1987). There are no DSDP/ODP/IODP sites near any of the other Expedition 329 sites. The closest sites were cored during Leg 92, which recovered Oligocene and younger sediment from a series of sites at 20°S (Leinen, Rea, et al., 1986). The Leg 92 sites are located beneath higher productivity waters than the central gyre (Fig. F1). The entire sediment column was cored at the Leg 92 sites; basement was encountered between 1 and 50 mbsf (with sediment depth increasing westward).

Coring-drilling strategy

Our general strategy during Expedition 329 was to core the entire sediment column multiple times at seven sites and to core the upper basement at three sites. The sites collectively underlie the full range of surface-ocean productivity conditions present in the South Pacific Gyre, ranging from the extremely low productivity conditions of the gyre center (Site U1368) to the moderately high (for open ocean) productivity at the southern edge of the gyre (Site U1371, at the northern edge of the Antarctic Convergence) (Figs. F1, F2). This series of sites is composed of two transects (Fig. F1), with the first transect centered at ~26°S, beneath the heart of the South Pacific Gyre, and the second transect centered at ~42°S in the southern portion of the gyre.

The sites in the northern sequence have been continuously far from shore and beneath the low-productivity gyre waters for many tens of millions of years (Figs. F1, F2). They provide an ideal opportunity to meet our first objective (to document the nature of life in subseafloor sediment with very low biomass and very low rates of activity). In combination with the southern transect, the northern transect is also necessary to meet our second objective (to determine how subseafloor sedimentary microbial activities and communities vary from gyre center to gyre margin).

Sites U1365 to U1371 are necessary for our third objective (to quantify the extent to which subseafloor communities in organic-poor sediment are sustained by H2 from radiolysis of water). Sites U1365, U1370, and U1371 are particularly crucial for this objective because their sediment columns are thick enough that their dissolved He–4 (alpha particle) concentrations and fluxes will be measurable.

The sites in the second transect have been in the southern portion of the present gyre (Sites U1369 and U1370) or south of the gyre (Site U1371) for tens of millions of years. Particularly at Site U1371, chlorophyll-a concentrations and primary productivity are much higher than at all of the sites in the northern transect (Figs. F1, F2). This transect is necessary to meet our second objective (to document how subseafloor sedimentary microbial activities and communities vary from gyre center to gyre margin). Because Site U1371 provides an anoxic standard of comparison for the other sites, it is also crucial for documenting the potential uniqueness (or ubiquity) of the microbial communities and activities that persist in the low-activity, low-biomass sediment beneath the gyre center.

The northern sequence of sites (U1365–U1368) is placed on basaltic basement of steadily increasing age from east to west (Fig. F2). Basaltic basement ranges in age from 7 to as much as 125 Ma (Site U1365). Basement age of the southern sites ranges from 39 to 73 Ma. Their water depths generally follow the classic curve (Parsons and Sclater, 1977) of increasing water depth with increasing basement age (Fig. F2). These sites are necessary to meet our fourth objective (to document the evolution of basalt hydrology and its implications for metabolic habitability and microbial communities in ocean crust under very thin sediment cover).