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doi:10.2204/iodp.proc.329.112.2011

Results

Geologic context/geophysical survey (Abrams, Harris, Pockalny)

Underway geophysical survey (Abrams, Pockalny)

Underway geophysical data were collected across nearly the entire width of the Pacific plate in the Southern Hemisphere between 20° and 45°S (Fig. F9). The mapped ocean 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 the beginning of the survey to about 6 Ma (Chron 3An.1n) at the easternmost limit of the survey. Calculated spreading rates range from slow-intermediate (<20 km/m.y., half-rate) to ultrafast (>80 km/m.y., half-rate).

The location of seismic surveys and coring stations also covered a relatively wide range of crustal ages, spreading rates, and tectonic/volcanic environments (Table T12). The depth and crustal age of each coring site correlates well with the predicted depth versus age curve (Stein and Stein, 1992), which suggests the sites are located on representative crust (Fig. F10). 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 Site SPG-5 and, possibly, Site SPG-6 masked the original seafloor fabric. Sediment thickness ranges from <3 m to 130 m and generally increases to the 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 Ma. 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., Site SPG-5).

Thermal gradient measurements (Harris)

The time series from a successful deployment of thermistors shows the temperature-time history of coring operations and the penetration of the piston core in the sediments (Fig. F11). The frictional heating of penetration, asymptotic decay of temperatures, and frictional heating of pullout is clear. The bottom water temperature is ~1.5°C.

Thermal conductivity measurements (Harris)

Thermal conductivity results for Sites SPG-2 through the upper portion of Site SPG-5 are very consistent with values ~0.72 W(m·K) (Fig. F12). The lower portion of the penetrated sediment at Site SPG-5 shows somewhat higher thermal conductivity as the sediment goes from a red clay to carbonaceous ooze. The increase in thermal conductivity with depth at Site SPG-6 correlates to an increase in density and is consistent with compaction.

Heat flow determinations (Harris)

Heat flow values are plotted as a function of age (Fig. F13). Heat flow determinations that fall off the best-fitting model are interpreted as indicating advective fluid flow. With the exception of Sites SPG-2 and SPG-4, heat flow values are thought to reflect advective fluid flow. Existing heat flow data (Fig. F2) around Sites SPG-1 and SPG-4 show large variability, which is also indicative of advective fluid flow (Fig. F14).

Oceanographic context (Dorrance, Goldstein, Halm, Morse)

Concentration/temperature/depth recorder results (Dorrance, Halm)

The data taken by the CTD show a slight decrease in salinity from 35.57 to 35.6 psu at Site SPG-1 in the west to 35.26 psu at Site SPG-6 in the east at a depth of 80 meters below sea level (mbsl) and an increase in the surface water to 36.1 psu at Site SPG-7 (Table T13). The temperature at the water surface increased from 23° to 24°C. During the northern west–east transect, the chlorophyll maximum sank from 100 mbsl at Site SPG-1 to 200 mbsl at Site SPG-7. The chlorophyll concentration decreased from 0.39 µg/L at Site SPG-1 to 0.8 µg/L at Site SPG-7. The ammonium concentration increased from 0.11 µM at Site SPG-1 to 0.054 µM at Site SPG-7.

The sites of the southern transect are characterized by different near-surface water mass properties than the sites of the northern transect. At the southern sites, salinity is between 34.24 and 35.6 psu and the water is colder (10.5°–11.5°C at 100 mbsl to 17.5°C at 20 mbsl). Chlorophyll occurs at all depths in a range of 0.2–0.3 µg chl-a/L, with its maximum at 100 mbsl and greater depths. At Sites SPG-10 and SPG-11, a smaller second chlorophyll maximum occurs beneath the thermocline. From Site SPG-9 onward, the ammonium concentration is between 0.096 and 0.29 µM.

Marine mammal monitoring and mitigation (Goldstein, Morse)

For this expedition, the National Oceanographic and Atmospheric Administration/National Marine Fisheries Service (NOAA/NMFS) issued a Incidental Harassment Authorization (IHA) permit to Scripps Institution of Oceanography to authorize nonlethal takes of certain marine mammals incidental to a marine seismic survey in the South Pacific Ocean. Behavioral disturbance of marine mammals is considered to be “take by harassment” under the provisions of the US Marine Mammal Protection Act.

The temporary or permanent impact of seismic exploration and other anthropogenic sound sources to any marine mammals is unknown. Nonetheless, to minimize the possibility of an injurious effects (auditory or otherwise) and to document the extent and nature of any disturbance effects, NOAA/NMFS requires that seismic research conducted under an IHA include provisions to monitor for marine mammals, sea turtles, and other protected marine species and to shut down/power down the seismic sources when these animals are detected within designated safety radii. Safety radii were defined based on the estimated radius at which the received level of seismic sounds (on an rms basis) was expected to diminish to 180 dB at 1 µPa, as specified by NMFS. The IHA also required monitoring and mitigation procedures to minimize potential harassment of sea turtles using the same safety zone.

Two marine mammal observers were onboard the cruise to monitor and mitigate as directed by the regulations described in the IHA permit. Observers stood watch for a total of 43.17 h during seismic operations and 306.95 h while seismic operations were secured. There were 23 sightings during the cruise (Table T14). No sightings were made during seismic operations.

Sedimentology (Hasiuk, Stancin)

During the expedition, 11 sites were successfully cored on two transects of the South Pacific Ocean (D’Hondt et al., 2009). The sediment recovered constituted between 4% and 100% of the sediment present at each site (Fig. F15). The northern transect surveyed sites on progressively younger crust from west to east, and the southern transect surveyed sites on crust that generally increased in age from east to west. The easternmost site (SPG-7) is located at the center of the lowest productivity section of the ocean (as determined by satellite measurement of surface chlorophyll concentration).

The northern transect consisted of seven sites (SPG-1 –SPG-7) from 23.85°S, 165.64°W (5554 mbsl on 102.5 Ma crust) to 27.68°S, 117.57°W (3920 mbsl on 6.1 Ma crust). Sediment at the first four sites (SPG-1–SPG-4) was homogeneous very dark brown clay (Fig. F16). Manganese nodules were common at the first three sites and rare at the fourth. There was some mottling, and smear slides showed micronodules and were barren of fossils. The first major change in sediment character occurred at Site SPG-5, with dark yellowish brown nannofossil-bearing clay at 100 cmbsf in Section 5. Washing of sediment at Sites SPG-3 and SPG-5 yielded cosmic spherules; sediments were not washed at other sites. Sites SPG-6 and SPG-7 had increasing amounts of calcareous nannofossils as the depth shoaled across the carbonate compensation depth.

The second transect consisted of four sites (SPG-9–SPG-12) from 38.06°S, 133.09°W (5013 mbsl on 39 Ma crust) to 45.96°S, 163.18°W (5310 mbsl on 73 Ma crust). The sediment from Site SPG9-11 is lithologically similar (homogeneous clay) to Sites SPG-1–SPG-4, yet was lighter in color (brown versus dark brown). Sediments at these sites also display extensive mottling (yellowish brown tubular mottles). Site SPG-12 was the only site cored outside of the South Pacific Gyre. It is dominated by siliceous ooze (abundant diatom debris and sponge spicules) and displays major gradational color changes on short spatial scales and several dark horizons, 2–10 cm thick. Recent worm burrows were also evident at Site SPG-12.

Manganese nodules

Approximately 260 manganese nodules were recovered from seven sites (SPG-1–SPG-4 and SPG-9–SPG-11), ranging in size from a few millimeters to almost 10 cm in diameter. Cores at two sites had distinct Mn crusts: Site SPG-1 at 735–740 cmbsf and Site SPG-10 at 220–225 cmbsf. Almost all cores recovered from Site SPG-10 contained two nodules, with one at the surface and one at 30–50 cmbsf, both with an average diameter of 8 cm. Most nodules recovered are spherical to subspherical, except for those at Site SPG-9, which are aggregates of centimeter-diameter nodules. Nodules recovered from Site SPG-4 and some from Site SPG-9 have encrusted anastamosing worm tubes, 0.1 mm in diameter and centimeters long, on nodules that average 1 cm in diameter. Smear slides included silt-sized nodules (micronodules) in almost all cases. Washing clay yielded silt- to sand-sized nodules (mininodules).

Sedimentation rates

By combining estimates of crustal age (interpreted from seafloor magnetic anomalies) with sediment thickness (interpreted from 3.5 kHz seismic data), average sedimentation rates were calculated (Fig. F17; Table T15). In this calculation, no account was made for interstitial water volume or changes in interstitial water volume downcore.

For Sites SPG-1–SPG-3, sedimentation rate decreased from 0.7 to 0.08 m/m.y., consistent with decreasing sediment supply at the more centrally located sites in the South Pacific Gyre. Sedimentation rates for Sites SPG-4–SPG-6 range from 0.3 to 1.1 m/m.y., which may have resulted in part from increasing percentages of carbonate sediment caused by shallower depths. Site SPG-7 has a sedimentation rate of 0.2 m/m.y., which could reflect its location at the center of the South Pacific Gyre chlorophyll minimum or a diagenetic removal of carbonate. This rate is likely suggestive of background noncarbonate sedimentation rates at this location. Sites SPG-9–SPG-12 show a general increase in sedimentation rate from 0.5 to 1.8 m/m.y. as sites became increasingly more distal from the center of the gyre. Site SPG-12 had the highest sedimentation rate of all sites visited on the cruise because of its location on the outer portion of the South Pacific Gyre in more nutrient-rich waters.

Although the range in sedimentation rates calculated for these sites spans three orders of magnitude, these rates are still among the lowest sedimentation rates that occur on the Earth’s surface.

Physical properties (Hasiuk, Rogers, Stancin)

Core-logging results (Rogers)

GEOTEK’s software processed the raw data according to calibrations performed during the cruise. These parameters are stored in proprietary binary.pro files. The accompanying graphs of the processed data show section lengths, density, magnetic susceptibility, and P-wave velocity (Figs. F18, F19, F20, F21, F22, F23, F24, F25, F26, F27, F28). The attached graphs show the P1 core in red and the P2 core in green for the sites where both were taken. Other shorter cores are various colors. There is no processed gravity core data because of their smaller diameter.

The first necessary result is the 25 cm deionized water sample run before each core. This should be within 1% of 1 g/cm³ with near-zero magnetic susceptibility for the subsequent data to be trusted. The gamma processing was recalibrated if this drifted. There are some obvious errors near the edge of each core caused by end cap thickness. These could be compensated for with further postprocessing, as the deionized water standard has the same end caps as the cores. The magnetic susceptibility loop seemed to have low spatial resolution, so edge effects of a few centimeters were unavoidable. These are especially bad in the P1 cores or other heavily sampled cores. The P-wave velocity rollers had some data quality issues. Electrical tape, end caps, dust, and other irregularities in the core liners caused poor connection and resulted in low P-wave amplitude. The velocity data are therefore noisier than they otherwise might be but still appear to be valid.

Most cores had homogeneous properties. Sites SPG-1, SPG-2, SPG-5, and SPG-10 had some interesting features, although some of the early sites’ magnetic data may have suffered from sudden temperature changes in the MST van. Logs of Site SPG-5 cores record the clay to carbonate transition at 6 mbsf, and logs of Site SPG-10 show manganese nodules at a variety of depths.

Further processing could include normalization of each core based on the measured and known densities of the calibration section. Edge effects could also be reduced using the same calibration section and the thickness deviations found in the raw data. The effects of temperature variations on magnetic susceptibility in the first two cores could also be better understood. If the magnetic signatures shown in those graphs are real, they show an interesting pattern.

Conductivity results (Hasiuk, Stancin)

Variability in conductivity among cores generally decreases from 30–45 mS/cm at the surface to 30–40 mS/cm at ~2 mbsf, reflecting decreasing water content (Fig. F29). Below 2 mbsf, between-site variability increases, suggesting influence of diagenetic processes peculiar to each site. Cores SPG-1-P2, SPG-9-P2, and SPG-10-P3 all show change in conductivity downcore, whereas the other cores recovered show little to no variability downcore.

Biogeochemistry (Fischer, Fuldauer, Graham, Griffith, Schrum, Smith, Spivack)

O₂ distributions (Fischer)

The differences between Sites SPG-1–SPG-11 in respect to oxygen profiles were low (D’Hondt et al., 2009; Fischer et al., 2009). The most obvious feature of the profiles measured on the cores is the presence of oxygen throughout the whole length of the cores (as deep as 8 m) at all sites except Site SPG-12 (Fig. F30). At the time, this was the deepest oxygen penetration ever measured in marine sediments. All profiles show the highest drop in concentration within the uppermost 50 to 100 cm. At greater depths, oxygen remained relatively constant or declined slightly and nearly linearly (on the order of a few micrometers per meter) with depth. These profiles indicate extremely low downward fluxes of oxygen and aerobic respiration rates.

Site SPG-12 differed completely from this pattern. Here, the oxygen penetration was limited to ~100 cm.

Small-scale profiles measured with microelectrodes on multicores showed a zone of relatively high respiration, limited only to the uppermost few centimeters (Fig. F31), not to the first ~50 cm (unlike in Fig. F30). This result is in good agreement with the in situ profiles obtained with the electrode profiler at Sites SPG-7 and SPG-10 (Fig. F32). The discrepancy between the small-scale electrode measurements and the measurements on the cores is most easily explained by a coring artifact, with the top section of the long cores (particularly the piston cores) mixed by initiation of the coring process.

Interstitial water chemistry (Fuldauer, Graham, Griffith, Schrum, Spivack)

At most of the sites, alkalinity concentrations are either relatively stable or shift to slightly higher values downcore (Fig. F33A) (D’Hondt et al., 2009). However, at Sites SPG-5 and SPG-7, alkalinity decreases in the deepest sediments, indicating alkalinity consumption or transport of alkalinity from the sediment to the underlying basement. The greatest change in alkalinity occurs at Site SPG-12, where it gradually increases from 2.6 mM at the top of the core to ~3.2 mM at 2 mbsf and then remains stable for the remainder of the core (to 5 mbsf). This result suggests that the shallowest sediments of Site SPG-12 are characterized by greater oxidation of organic matter than any of the other sediments recovered on this expedition.

Dissolved nitrate concentrations tend to be between ~35 and ~45 µM at Sites SPG-1 to SPG-11 (Fig. F33B) (D’Hondt et al., 2009). At these sites, nitrate is generally stable or increases slightly with sediment depth. These increases presumably result from oxidation of buried organic nitrogen. Gradual downcore decreases occur at Sites SPG-5 and SPG-7, perhaps due to transport of dissolved nitrate through the sediment to the underlying basement. Of these sites, Site SPG-11 has the most significant change in nitrate concentrations over the shortest distance; values increase by ~11 µM over 3.05 m, indicating that organic nitrogen is oxidized at a much higher rate in the shallow sediment of this site than in the sediments of Sites SPG-1 through SPG-10. At Site SPG-12, dissolved nitrate decreases rapidly from a peak of 44 µM to zero at ~2.5 mbsf, indicating significant nitrate reduction at that depth. This occurrence of nitrate reduction is consistent with the much higher levels of organic matter oxidation at that site than at the other South Pacific Gyre sites.

Measurements of dissolved sulfate concentrations at the South Pacific Gyre sites exhibit a fair bit of scatter (Fig. F33C). Scatter aside, dissolved sulfate concentrations appear to be basically stable or, perhaps, decrease slightly downcore at the various sites.

Chloride profiles tend not to show obvious trends downcore but exhibit scatter (Fig. F33D). At some sites, such as Sites SPG-1, SPG-6, and SPG-9, chloride concentrations appear to shift to higher values with depth in the sediment column. Error analysis for chloride, bromide, sulfate, and nitrate is based on average deviations of duplicate analyses of the same dilution.

The downcore bromide profiles show considerable scatter for nearly all sites. In general, bromide varies from ~84 to 88 µM. Site SPG-6 shows a distinct increase in bromide from the top of the core to ~1 mbsf, after which point the concentration decreases and stabilizes.

Methane concentrations are below our detection limit (1 ppm) at all sites.

H₂ distributions (Smith)

Of the 167 sediment samples analyzed for H₂ concentration, only four samples contained measurable amounts. No H₂ was detected in any sample from Sites SPG-1–SPG-9. One sample at Site SPG-10 (0.7 mbsf) contained 35 nM H₂. Samples collected immediately above (0.5 mbsf) and below (0.95 mbsf) this sample contained no detectable H₂. The deepest sample analysed at Site SPG-12 contained 1315 nM H₂.

Microbial activity (Ferdelman, Soffientino)

Hydrogenase activity (Soffientino)

Because the liquid scintillation counter malfunctioned soon after departing Apia, only partial results of Sites SPG-1–SPG-7 were obtained. The radio-labeled slurries from all sites were shipped frozen to URI for postexpedition processing and analysis.

Microbiology (Durbin, Forschner, Harrison, Horn, Kallmeyer, Lever, Puschell, Smith)

Microscopic results (Kallmeyer)

Cores from Sites SPG-2–SPG-11 show a rather uniform pattern, with cell abundances of 7 × 10⁵ to 4 × 10⁶ cells/cm³ at the sediment/water interface, dropping steeply by about two orders of magnitude in the upper 0.5 m, and staying almost constant downcore (Fig. F34A, F34B) (D’Hondt et al., 2009). At these sites, surface water concentrations were ~10⁵ cells/cm³ and bottom water concentrations were 10⁴–10⁵ cells/cm³.

Cell counts at Site SPG-1 appeared slightly elevated relative to some of the other sites but exhibited large shifts between adjacent samples, indicating that the efficiency of the cell separation varied between samples. Therefore, cell separation on Site SPG-1 samples was repeated postcruise.

At Site SPG-12, cell numbers were higher and the distribution was completely different than at all other sites (D’Hondt et al., 2009). At the sediment/water interface, cell abundances were 1 × 10⁶ cells/cm³, basically in the same range as other cores but not higher. However, at this site no multicorer samples were recovered and the uppermost gravity core sample may not represent the sediment/water interface; possibly a few centimeters of the shallowest sediment were lost. Highest cell abundances (2 × 10⁶ cells/cm³) were found at ~1 mbsf. This subsurface maximum in cell density corresponds with the depth at which oxygen is depleted. Below this depth, cell numbers decline steadily to 2 × 10⁵ cells/cm³ at 5 mbsf. These subseafloor cell numbers are about two orders of magnitude higher than at the other sites. The surface water cell concentration at this site was also higher than at the other sites (slightly above 10⁶ cells/cm³).

These shipboard counts show that there is a microbial community in these sediments. Cell abundances at Sites SPG-1–SPG-11 are the lowest ever encountered in the marine environment. Compared to cell counts on samples from Ocean Drilling Program (ODP) cruises, the cell abundances found in cores from this cruise are on average two orders of magnitude lower. The exception occurs at Site SPG-12, which has cell counts at the lower end of the ODP counts (Fig. F35).

The cell counts of all the field samples were well above the counts for the blank samples. Usually 0 or 1 cell in 400 fields of view at 63× magnification was found in a blank sample, with 4 cells being the highest count in a blank. These cell counts correspond to cell densities of 7 × 10¹ to 3 × 10² cells/cm³. However, the lowest actual cell counts (~1 × 10³ cells/cm³) revealed <10 cells in 200 fields of view; therefore, these data are statistically much less well constrained as those from samples with higher cell abundances. Further counting postcruise will be necessary to validate these numbers.

Molecular biology (Durbin, Lever)

In shipboard tests on fresh sediments, archaeal DNA, extracted from the upper meter of sediment from six stations, was uniformly below the detection limit of PCR assays with general archaeal primers (40 cycles amplification + 40 cycles reamplification). Further nucleic acid extractions were attempted postcruise (Durbin and Teske, 2010).

Cultures (Forschner, Puschell)

No contaminants have been detected in the cultures of A. Puschell. There was no detectable shipboard growth in those cultures, except in the media for heterotrophs at Sites SPG-1 and SPG-2 where the lowest dilution (10^–1) appeared to have a hazy film over the sediment, which could be an indicator of growth. The enrichment cultures by G. Horn also did not show strong growth during the expedition. Two of the FeS gradient tubes exhibited haziness in the upper portion of the plug that may indicate growth. The remaining gradient tube cultures did not show signs of growth during the expedition.

Gram-positive bacterial cultivations resulted in growth barely visible to the naked eye. Out of 855 plates inoculated, only 10 showed visible growth during the expedition. All 10 were plated within the first week of sampling and took 4 weeks for growth to appear. Plates continued to be incubated postexpedition at 4°C. Further studies, including investigations into bioactive metabolites, will commence once pure cultures have been isolated.

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