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Site U13631

Expedition 327 Scientists2

Site summary

Five holes were cored during Integrated Ocean Drilling Program (IODP) Expedition 327 at Site U1363, adjacent to the edge of Grizzly Bare outcrop (see Tables T1, T2, T3 and Fig. F3B in the “Expedition 327 summary” chapter). Sediments are composed of turbidites interspersed with hemipelagic clay, consistent with core recovery during Ocean Drilling Program (ODP) Leg 168 and IODP Expedition 301, >50 km to the north (Davis, Fisher, Firth, et al., 1997; Fisher, Urabe, Klaus, and the Expedition 301 Scientists, 2005). Four lithologic units were distinguished.

Unit 1 is composed of hemipelagic mud (clayey silt to silty clay), thin-bedded turbidites (sand-silt-clay), and thick-bedded medium sand turbidites. Unit 2 is composed of beds of silt and sandy silt intercalated with hemipelagic mud deposits (silty clay to clayey silt). Unit 3 is composed of hemipelagic carbonate-rich claystone rich in foraminifers and nannofossils. Unit 4 is represented by a few small pieces of basalt recovered from the sediment/basalt interface in Holes U1363B, U1363D, and U1363F. The basalt is cryptocrystalline and plagioclase phyric, with glomeroporphyritic texture visible in hand specimens. Phenocrysts are large (up to 8 mm) and are anhedral to euhedral in shape. The basalt is sparsely vesicular with highly variable vesicle size and shape. Secondary minerals are present as background groundmass replacement and alteration halos as well as filling vesicles and lining hydrothermal veins.

All cores were run through the Whole-Round Multisensor Logger (WRMSL), yielding magnetic susceptibility values from <500 × 10–6 SI in clay sections to ~1400 × 10–6 SI in sandy turbidites. Point magnetic susceptibility data collected with the Section Half Multisensor Logger (SHMSL) are similar, with split-core values tending to be slightly lower than whole-round values, except in the case of turbidite sequences, where SHMSL values are consistently higher. Gamma ray attenuation (GRA) bulk density averages ~1.8 g/cm3, depending on lithology, with some compaction evident with depth in clay intervals. P-wave velocities measured on the WRMSL range from ~1.46 to ~1.87 km/s, excluding the erroneously low values derived from insufficient sediment filling within core liners.

Discrete measurements, including moisture and density (MAD), P-wave velocity, and thermal conductivity, were measured on most cores from Holes U1363B–U1363D and U1363F. Insufficient time was available to measure samples from Hole U1363G. Thermal conductivity at Site U1363 averages 1.3 ± 0.2 W/(m·K), whereas MAD bulk densities average 1.7 g/cm3, both showing bimodal distributions corresponding to clay and sand lithologies. MAD porosities range from 38% to 76%, with an average value of ~60%. P-wave velocities of discrete samples range from 1.49 to 1.75 km/s (mean = ~1.52 km/s), with considerable variability across lithologies. Velocities derived from discrete measurements agree with those measured on whole-round sections with the WRMSL. Velocities also show weak anisotropy between vertical and horizontal directions. P-wave velocity increases ~50 m/s within the uppermost 50 meters below seafloor (mbsf). On the other hand, grain density is remarkably consistent regardless of depth or lithology.

Pore water samples were recovered from five holes, providing systematic trends to assess the composition of the underlying basaltic formation fluid at these locations. Pore waters were extracted in a nitrogen atmosphere, and some analyses (alkalinity and ion chromatography) were conducted immediately to guide subsequent drilling operations. We collected 58 pore water samples: 15 from Hole U1363G, 14 from Hole U1363F, 14 from Hole U1363B, and 15 from (adjacent) Holes U1363C/U1363D, with basement depths of 17, 35, 57, and 231 mbsf, respectively. In the upper portion of sediment, biogenic processes release dissolved Mn and Fe near the sediment/water interface and consume sulfate. There is a corresponding increase in alkalinity, phosphate, and ammonium and an initial decrease in Ca resulting from carbonate formation. There are similar trends for sulfate, Mn, and Fe near the sediment/basalt interface. However, phosphate and ammonium are more influenced by diffusion and reaction within the upper basaltic basement. The cations Ca, Mg, and K show gradients near the sediment/basalt interface that are indicative of a formation fluid that is slightly altered relative to seawater. Minor and trace elements in seawater also show gradients in the basal sediment section, with greater alteration at greater distance from the outcrop.

Microbiologists collected whole-round core and pore water samples from sediments and basement pieces recovered at Site U1363. Eleven sediment intervals were targeted for microbiology sampling in Hole U1363B. Most samples were taken from hemipelagic clay layers, although some sandy turbidite layers were also sampled. The deepest sediment sample was taken from a carbonate-rich layer near the sediment/basalt interface. Thirteen sediment intervals and one basement basalt interval were sampled from Holes U1363C and U1363D. Again, sediment samples were mostly from clay-rich layers, although some samples contained sand. The basement sample from Core 327-U1363D-6X was a relatively unfractured basalt with spots of light green and orange alteration crusts. Nineteen sediment and basement samples were taken from Hole U1363F. Most samples contained either clay or sandy layers, with the exception of samples from Section 327-U1363F-4H-2 and deeper, which also contained manganese crust, basalt fragments, and lighter tan-colored sediment resembling the foraminifer-rich carbonate sediments from Hole U1363B. Sixteen samples were collected from Hole U1363G. All samples were clay rich, and no hard rock samples were recovered.

At each sampling location whole-round core samples were collected for shore-based DNA analysis, characterization of halogenated organic matter, and incubation experiments to examine dehalogenation reaction activities. Syringe samples were also collected for headspace gas analysis and microsphere contamination checks from the interior and exterior of the cores. Headspace samples were analyzed on board for safety purposes, and only a few samples contained quantifiable levels of methane or higher hydrocarbon gases. Microsphere samples were shipped to the shore-based laboratory for postcruise analysis because of time limitations at the end of the expedition. These samples will also be used for shore-based cell counting analysis and fluorescent in situ hybridization (FISH) analysis. A subset of samples was collected for analysis of dissolved organic carbon (DOC)/dissolved nitrogen (DN), particulate organic carbon (POC)/particulate nitrogen (PN), amino acids, low molecular weight organic acids, and lipid biomarkers.

Remanent magnetization measurements were made on two-thirds of core sections from Hole U1363B. Samples were demagnetized at 10 mT steps from 0 to 40 mT using the cryogenic magnetometer’s inline alternating-field (AF) coils. Although the majority of samples have positive inclinations, there is a large scatter of positive and negative inclinations, which is probably the result of core disturbance during coring.

Temperature measurements were collected with the third-generation advanced piston corer temperature tool (APCT-3) and the Sediment Temperature (SET) tool in Holes U1363B, U1363C, U1363F, and U1363G. Good measurements were obtained with both tools, and these data will be analyzed postexpedition to assess seafloor heat flow and thermal conditions in basement.

1Expedition 327 Scientists, 2011. Site U1363. In Fisher, A.T., Tsuji, T., Petronotis, K., and the Expedition 327 Scientists, Proc. IODP, 327: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.327.106.2011

2Expedition 327 Scientists’ addresses.

Publication: 5 September 2011
MS 327-106