Major and rare earth element composition of ODP Site 191-1179 sediments, northwest Pacific

Depth profiles of major element and rare earth element (REE) abundances in sediment samples (mainly siliceous ooze and clay) recovered from Holes 1179B and 1179C at Site 1179, Ocean Drilling Program Leg 191 (41.4°N, 159.6°E) were determined. The oxidation states of Mn and Ce were determined by X-ray absorption near-edge structure. Some geochemical indicators were tested, including the MnO/TiO2 ratios, a bivariate diagram of La/Ce vs. Al2O3/(Al2O3+Fe2O3), and other discrimination diagrams. The oxidation state of Mn is reduced Mn(II) in the depth profile below 0.60 meters below seafloor (mbsf), which is consistent with relatively low and high abundances of Mn in the sediments and pore waters, respectively. It is possible that the diagenetic effect on the oxidation state and abundance of Mn makes it difficult for the MnO/TiO2 ratio to reflect the depositional environment. The normalized ratio of La and Ce does not change very much with depth, suggesting that the diagenetic effect does not affect the REE signature in the sediments. On the diagram of La/Ce vs. Al2O3/(Al2O3+Fe2O3), the sediments studied here plot at the boundary of the pelagic and continental margin fields. This suggests that continental material has contributed to the sediment to some degree, even though Site 1179 is in a pelagic region of the northwestern Pacific Ocean, >1600 km from Japan.

Argon composition and new combined chronologies of ODP Sites 185-1149 and 191-1179

Despite the different scientific objectives of Legs 185 and 191, the sedimentary sections recovered from Sites 1149 and 1179 are the two most complete sections recovered from the northwestern Pacific Basin by either the Deep Sea Drilling Project (DSDP) (i.e., Legs 6, 20, 32, and 86) or ODP (i.e., Legs 185 and 191). During Leg 185, a complete sedimentary section (410 m) and an additional 133 m of highly altered volcanic basement were recovered. The Miocene to Pleistocene section (i.e., upper ~150 m) recovered from Site 1149 includes lithostratigraphic Unit I (0-118.2 meters below sea floor [mbsf]) and Subunit IIA (118.2-149.5 mbsf) of Plank, Ludden, Escutia, et al. (2000, doi:10.2973/odp.proc.ir.185.2000) and consists of ash- and biogenic silica- bearing clay, radiolarian-bearing clay, silt-bearing clay, ash-bearing siliceous ooze, and diatomaceous clay, with numerous discrete volcanic ash layers (Plank, Ludden, Escutia, et al., 2000, doi:10.2973/odp.proc.ir.185.2000). During Leg 191, a near-continuous 375-m-thick sedimentary section was recovered in addition to 100 m of basaltic basement. The upper 221.5 m of the sedimentary section at Site 1179 (i.e., within lithostratigraphic Unit I of Kanazawa, Sager, Escutia et al. [2001, doi:10.2973/odp.proc.ir.191.2001]) consists of upper Miocene to Pleistocene clay- and radiolarian-bearing diatom ooze containing numerous discrete ash layers. The presence of discrete ash layers within the Miocene to Pleistocene sedimentary section at both Site 1149 and 1179 provides a unique opportunity to conduct 40Ar/39Ar ash chronology to refine the excellent magnetostratigraphic records (based on the scale of Berggren et al., 1995) obtained shipboard from both sites (Plank, Ludden, Escutia, et al., 2000, doi:10.2973/odp.proc.ir.185.2000; Kanazawa, Sager, Escutia, et al., 2001, doi:10.2973/odp.proc.ir.191.2001).In this data report we present the analytical results from the 40Ar/39Ar incrementally heated analyses and provide a new combined late Miocene to Pleistocene 40Ar/39Ar and magnetostratigraphic chronology for the northwestern Pacific.

Magnetic properties of ODP Site 191-1179 sediments

As reported by Shipboard Scientific Party (2001b, doi:10.2973/odp.proc.ir.191.104.2001) in the Site 1179 chapter of the Initial Reports volume, Leg 191 Site 1179 is located on abyssal seafloor northwest of Shatsky Rise, ~1650 km east of Japan. This part of the Pacific plate was formed during the Early Cretaceous, as shown by northeast-trending M-series magnetic lineations that become younger toward the northwest (Larson and Chase, 1972, doi:10.1130/0016-7606(1972)83[3627:LMEOTW]2.0.CO;2; Sager et al., 1988, doi:10.1029/JB093iB10p11753; Nakanishi et al., 1989, doi:10.1029/1999JB900002). The site is situated on magnetic Anomaly M8 (Nakanishi et al., 1999, doi:10.1029/1999JB900002), corresponding to an age of ~129 Ma and the Hauterivian stage of the Early Cretaceous (Gradstein et al., 1994, doi:10.1029/94JB01889; 1995). The sediments recovered at Site 1179 are split into four lithostratigraphic units based on composition and color (Shipboard Scientific Party, 2001b, doi:10.2973/odp.proc.ir.191.104.2001). Unit I (0-221.52 meters below seafloor [mbsf]) is a dominantly olive-gray clay- and radiolarian-bearing diatom ooze. Unit II (221.52-246.0 mbsf) is a yellowish brown to light brown clay-rich and diatom-bearing radiolarian ooze. Unit III (246.0-283.53 mbsf) is composed of brown pelagic clay. Unit IV (283.53-377.15 mbsf) is composed of chert and some porcellanite; any softer sediments present were washed out of the core barrel by the fluid circulating during the coring process.

Total phospholipid fatty acid concentrations structrures of ODP Hole 191-1179 samples (Table 1)

Sediment samples ranging from 0.05 to 278 m below sea floor (mbsf) at a Northwest Pacific deep-water (5564 mbsl) site (ODP Leg 191, Site 1179) were analyzed for phospholipid fatty acids (PLFAs). Total PLFA concentrations decreased by a factor of three over the first meter of sediment and then decreased at a slower rate to approximately 30 mbsf. The sharp decrease over the first meter corresponds to the depth of nitrate and Mn(IV) reduction as indicated by pore water chemistry. PLFA-based cell numbers at site 1179 had a similar depth profile as that for Acridine orange direct cell counts previously made on ODP site 1149 sediments which have a similar water depth and lithology. The mole percentage of straight chain saturated PLFAs increases with depth, with a large shift between the 0.95 and 3.95 mbsf samples. PLFA stable carbon isotope ratios were determined for sediments from 0.05 to 4.53 mbsf and showed a general trend toward more depleted d13C values with depth. Both of these observations may indicate a shift in the bacterial community with depth across the different redox zones inferred from pore water chemistry data. The PLFA 10me16:0, which has been attributed to the bacterial genera Desulfobacter in many marine sediments, showed the greatest isotopic depletion, decreasing from -20 to -35 per mil over the first meter of sediment. Pore water chemistry suggested that sulfate reduction was absent or minimal over this same sediment interval. However, 10me16:0 has been shown to be produced by recently discovered anaerobic ammonium oxidizing (anammox) bacteria which are known chemoautotrophs. The increasing depletion in d13C of 10me16:0 with the unusually lower concentration of ammonium and linear decrease of nitrate concentration is consistent with a scenario of anammox bacteria mediating the oxidation of ammonium via nitrite, an intermediate of nitrate reduction.

(Table T1) Paleomagnetic properties of ODP Hole 191-1179D basalts

During Ocean Drilling Program Leg 191, ~100 m of mid-Cretaceous igneous crust was cored at Site 1179 (41.08°N, 159.96°E), located within magnetic Anomaly M8 on the abyssal plain of the northwest Pacific Ocean near Shatsky Rise. Paleomagnetic data from this section are significant because they can constrain the mid-Cretaceous Pacific plate paleolatitude and paleomagnetic pole, both of which can be used to infer tectonic drift and other geodynamic processes. In this study, we analyzed the paleomagnetism of 122 samples from 40 flows in the Site 1179 basalt section. Comparison of inclination data among flows implies 13 independent measurements of the paleomagnetic field. Assuming a reversed magnetic polarity because of the site location within Anomaly M8, the data give a mean paleocolatitude of 88.1° ± 6.8° (corresponding to a paleolatitude of 1.9°N). The paleocolatitude is consistent with other mid-Cretaceous Pacific paleomagnetic data that indicate ~39° northward drift of the western Pacific plate since mid-Cretaceous time. Comparison of observed between-flow colatitude variance with that expected from secular variation data suggests that secular variation may not have been completely averaged with the 13 independent groups sampled at Site 1179. Colatitude scatter in the section is markedly less in the deepest 33 m of the hole, indicating a shift from rapidly erupted flows in the bottom ~33 m of the section to more slowly emplaced flows above.

(Table T1) Permeabilities and bulk properties of ODP Hole 191-1179C

One of the objectives of drilling at Site 1179 was to search for microbes or biochemical evidence of microbial activity as part of the ongoing exploration of the depth and extent of the deep biosphere. The existence of living microbes has not been confirmed, but the chemistry of pore waters from the site, such as sulfate and ammonium profiles, is consistent with sulfate reduction and nitrification by anaerobic bacteria. However, chemical profiles are affected by the movement of molecules and ions through porous sediments by diffusion and advection. Permeability is thus an important consideration in the interpretation of pore water chemistry profiles. Moreover, diatomaceous sediments have some unique and, as yet, poorly understood physical properties. The purpose of this research is to measure hydraulic conductivity (permeability) in a suite of sediment samples from Ocean Drilling Program Site 1179 by the transient-pulse method. The sample set consists of four diatom ooze samples from Unit I, one radiolarian ooze sample from Unit II, and one pelagic clay sample from Unit III. The permeability of the clay is 1.92 µd, whereas the permeabilities of the overlying radiolarian and diatom oozes range from 289 to 1604 µd. Among these samples, permeability increases with porosity and grain size, in keeping with the results of previous studies.