POSTCRUISE SCIENTIFIC RESULTS

Biostratigraphy and Paleoceanography

Two studies investigated the fossil record in Site 1179 sediments. Winter et al. (this volume) developed a biostratigraphy from diatoms (Fig. F4). This study complements the shipboard biostratigraphy, which was based primarily on radiolarians and dinoflagellates. The authors report that preservation of diatom frustules is good, with abundances ranging from rare to abundant and no barren samples encountered. Because the sediments contain both tropical and temperate diatom species, it was possible to correlate the fossil assemblages with those of equatorial and North Pacific biostratigraphies. The succession of diatoms appears nearly complete, with only one zonal boundary missing. Furthermore, it agrees well with the previously defined shipboard biostratigraphy. Diatom abundance was found to be lower in the Miocene and greater in the Pliocene and Pleistocene, consistent with shipboard observations of an increase in siliceous sedimentation with time as Site 1179 came under the influence of the Kuroshio Current.

McCarthy and coworkers (McCarthy et al., 2004a, 2004b) examine the implications of several calcareous layers that occur in the otherwise siliceous sediment section of Site 1179 at depths of ~37, 61, and 92 mbsf, corresponding to ages of 0.7–0.9, 1.6, and 2.4–2.5 Ma, respectively. These layers were deposited >1 km below the carbonate compensation depth (CCD) and contain well-preserved foraminifer tests, including some dissolution-prone species. These layers also contain synchronous peaks of pollen that indicate aridification and cooling, leading the authors to conclude that the layers were deposited during glacial periods. These layers correspond to similar high carbonate content layers found in other North Pacific sediment cores recovered during ODP Leg 145 and with sections of Chinese loess in mainland Asia. The authors explain the layers by fertilization of surface waters by dust from Asia (containing growth-limiting nutrients) during glacial periods with strong winds. The result was higher productivity in surface waters and concomitant high sedimentation at the seafloor, allowing the carbonate sediments to be deposited and buried before dissolution could remove them. McCarthy and coworkers (2004a) speculate that these brief periods of high sedimentation were widespread and that the sequestration of carbon dioxide in carbonate sediments may have augmented global cooling during these times by reduction of the greenhouse effect. The authors also note that the palynological record is key to recognizing the provenance and paleoclimate implications of terrigenous particles blowing to the ocean from Asia.

Sediment Properties

Using X-ray absorption techniques, Fukukawa et al. (this volume) measured major element and rare earth element abundances for sediment samples from the upper 35 m of Site 1179. In particular, they examined the oxidation state of Mn and Ce and the effect of element abundances on several geochemical ratios used as provenance indicators. Mn is reduced to Mn(II) below 0.6 mbsf, with low abundance in sediments and high abundance in pore waters. The authors concluded that the Mn/TiO₂ ratio, often used to deduce depositional environment, must be affected by diagenesis. In contrast, the ratio La/Ce changes little with depth, suggesting it is not significantly affected by diagenesis. On a plot of La/Ce vs. Al₂O₃/(Al₂O₃ + Fe₂O₃), used to infer sediment source region, Site 1179 samples plot between continental and pelagic zones, leading the authors to conclude that there is significant terrigenous input to the sediments, despite the distance of Site 1179 from land.

Arguing that knowledge of sediment permeability and porosity is critical to understanding fluid flow, Kwon et al. (this volume) made transient pulse method measurements of permeability and porosity for six sediment samples from Site 1179. Both quantities are high for Units I and II (porosities >80% and permeabilities of 342–1779 µD), consistent with physical property measurements made on these diatomaceous sediments during Leg 191. The single sample measured from Unit III, the pelagic brown clay, had much lower porosity (~66%) and permeability (1.9 µD).

The data report by Horner-Johnson and Sager (this volume) presents anisotropy data for anhysteretic remanent magnetization and magnetic susceptibility. According to previous studies, anisotropy is low in sediments that have low compaction but increases in sediments that have compacted as magnetic grains rotate to the same plane with vertical shortening. The authors find that in Units I and II, the high porosity, low-density diatomaceous section, anisotropies are low and near isotropic. In contrast, sediments in the pelagic brown clay, Unit III, show oblate spheroid (pancake shaped) distributions of anisotropy, indicating significant effects of compaction and implying that magnetic grains have been rotated toward the horizontal plane. This finding suggests that Unit III sediments are probably ill-suited for determining paleomagnetic inclination, owing to possible rotation of the magnetic vector.

Ash Chronology and Stratigraphy

Escutia et al. (unpubl. data) recognized 14 volcanic events within the upper 200 m of the sedimentary section (Unit I) at Site 1179 (Figs. F2, F5). They conducted radiometric dating on some of the ash layers using ³⁹Ar/⁴⁰Ar radiometric dating techniques for ash shards. Other ash-layer dates were derived by comparing the ⁴⁰Ar/³⁹Ar chronology framework to magnetostratigraphy for Site 1179 (and also northwest Pacific Site 1149) (see Escutia et al., unpubl. data). The refined chronology will be invaluable for future geochemical and volcanologic studies to determine the evolution of arc volcanism in the northern circum-Pacific. It allowed the authors to study the stratigraphic evolution of the northwestern Pacific since Messinian time (Escutia et al., in press).

Results from the ash study are relevant to "subduction factory" studies because of their bearing on the composition and total cumulative thickness of ash being subducted in the Kurile Trench. Tephras from Site 1179 are chemically bimodal (i.e., basaltic and rhyolitic), with rhyolitic volcanic events dominating during Messinian and early Pliocene times (i.e., 4.3–5.3 Ma) and through most of the Quaternary. Cumulative thickness of ash at Site 1179 is ~1 m, ~0.5% of the total sediment deposited during the last 7 m.y. Taking into account the mass fraction of dispersed ash in the background sediment, as indicated by nonquantitative shipboard smear slide estimates, the total cumulative thickness of ash in the sediment column that will be subducted in the Kurile Trench could vary between 3 and 13 m, or 1.5% and 6.5%.

Relevant to paleoceanographic and paleoclimatic studies are the inferred sedimentation rates at Site 1179, which exhibit a nearly linear increase from the Messinian (24 m/m.y.) to the late Pleistocene (31.6 m/m.y.). Such high sedimentation rates, at an oceanic site distant from land sources, are attributed to high productivity in nutrient-rich waters that characterizes the area of convergence between the Kuroshio Front and the Oyashio Current, where Site 1179 is located. An increase in sedimentation rates (from 27.4 to 38.6 m/m.y.) during the late Pliocene at ~2.5–3.0 Ma coincides with maximum frequency of volcanic events at this site (2.1 and 2.5 Ma) and with one of the maximum peaks in cumulative ash thickness. Interestingly, this time (3.0–2.5 Ma) coincides with the cooling trend associated with the initiation of Northern Hemisphere glaciation. The coincidence of high sedimentation rates and high input of volcanic ash observed at ~2.5 Ma at Site 1179 is interpreted by Escutia et al. (in press) to be the result of climatic cooling related to increased volcanic activity. Cooling may have resulted in stronger winds, bringing more ash and dust from an increasingly arid Asia. In addition, these conditions proved more favorable for the growth of diatoms in the northwest Pacific, leading to greater deposition of siliceous tests.

Isotope Geochemistry

Sano and Hayasaka (this volume) measured Sr and Nd isotope compositions of 19 basalt samples from Site 1179 basement. Initial ratios range ⁸⁴Sr/⁸⁶Sr = 0.7029–0.7040 and ¹⁴³Nd/¹⁴⁴Nd = 0.5131–0.5132 (_Nd(t) = 9.3–10.8), but age corrections change the values for Sr to 0.7026–0.7033 and _Nd(t) to 9.2–10.6. Plotting these data relative to one another (Fig. F6) shows that the Sr ratio is higher than the normal Sr ratio of mid-ocean-ridge basalt (MORB), implying that crust at Site 1179 may have been affected by plume volcanism from nearby Shatsky Rise, which was active at the same time the Site 1179 crust formed. This result is similar to previously published findings from crustal basalt cored at nearby Deep Sea Drilling Project (DSDP) Site 304 (Janney and Castillo, 1997).

Radiometric Dating

Initial description of the igneous section indicated that alteration in the basement rocks is low, raising hopes that a high-precision radiometric date would be determined (Shipboard Scientific Party, 2001b). Because Site 1179 is near the middle of a well-defined magnetic isochron (M8), such a date would be important as a tie-point for the geomagnetic polarity reversal timescale. Unfortunately, this effort did not work as hoped.

The radiometric dating analysis was done by Anthony Koppers using techniques for ³⁹Ar/⁴⁰Ar dating of altered sample groundmass that have given accurate results with other seawater-altered basalts (Koppers et al., 2000). He analyzed three samples, and two gave age plateaus, but with values of 103–105 Ma, which do not appear reasonable for rocks of M8 age (~129 Ma by the Gradstein et al., 1995, timescale). He gives several reasons why the calculated ages appear inaccurate (A. Koppers, pers. comm., 2004). K/Ca ratios of the rocks are extremely low and indicate that there is virtually no primary K residing in the rock grains. The existing K was probably introduced by seawater alteration. Age plateaus are short and show evidence for extreme ³⁹Ar recoil, which is common for samples with pervasive fine-grained alteration. Furthermore, total fusion ages are extremely discordant (~140–160 Ma), implying that the Ar system has not been closed, violating a fundamental condition needed for reliable age determination

Paleomagnetism

Noting that paleomagnetic results from Site 1179 could be important because few Pacific boreholes have penetrated as deep as 100 m into oceanic basement, Sager and Horner-Johnson (this volume) studied 122 basalt samples from 41 of 48 igneous units. Although the number of units appears large, statistical comparison implied that the data comprise only 13 independent magnetic units (i.e., units between which significant time has passed for secular variation of the geomagnetic field to occur) (Fig. F7). This implies that many flows are serially correlated and probably erupted within a short time of one another. Indeed, all of the units in the lower 33 m of the section were statistically indistinguishable, indicating that units in flow group 3 (olivine-rich basalts) were emplaced rapidly. From the 13 independent measurements, a mean paleolatitude of 1.9°N ± 6.8° was determined, assuming the negative inclinations of most samples represent a reversed magnetic polarity formed north of the equator. This paleolatitude implies that the crust at Site 1179 formed near the equator and has drifted ~39° northward since then. The authors caution that uncertainty in the paleolatitude remains because the 13 independent units may not have completely averaged secular variation; nonetheless, the paleolatitude results are important because they can be used in compilations of Pacific paleomagnetic data (e.g., Sager, in press).

In a separate data report, Sager (this volume) reports measurements made on basalt samples from eight other sites drilled during prior DSDP and ODP cruises. Most of these cores were previously overlooked because they sampled only a few igneous units and therefore were unlikely to produce unequivocal paleolatitude results. However, as noted above, such data are nonetheless valuable when averaged with other coeval data (Sager, in press). Sager (this volume) reports results from three holes from DSDP Leg 17, one hole common to DSDP Legs 86 and 88, and one hole each from DSDP Leg 92 and ODP Legs 129, 130, and 143.

Goldberg et al. (this volume) analyzed data from the Multi-sensor Spectral Gamma Ray Tool (MGT), a new, high-resolution gamma ray logging device deployed for the first time during Leg 191. These data were compared with lower-resolution data from the standard gamma ray logging tool (Hostile Environment Gamma Ray Sonde; HNGS). Logs from the two different sensors agreed in overall shape, but the vertical resolution of the MGT was several times better than the HNGS. Differences between concentrations of radioactive elements K, U, and Th estimated from the two sensors caused some concern and were blamed on differences in the borehole between logging runs, low radioactive element concentrations in the sediments, and differences in processing algorithms. The authors conclude that the new tool will be useful for high-resolution studies, especially the correlation of thin clay beds between core and logging data.

The information presented in this section is paraphrased from a report detailing progress on two western Pacific borehole seismic observatories set up by ODP, including the WP-2 observatory at Site 1179 (Shinohara et al., in press). After installation during July and August of 2000, the WP-2 observatory was visited by a remotely operated vehicle (ROV), which activated the equipment on 29 October 2000. Subsequent ROV visits in January 2001 and June 2002 retrieved data recordings totaling 417 days. Because the initial data set indicated that the vertical channel on the seismometer was somewhat noisy and had an unexpected spectral response, it was switched off and a backup vertical unit was activated. It is thought that the original vertical sensor may have been damaged during installation, but this supposition cannot be tested because the sensors are cemented in the Site 1179 borehole.

Analysis of WP-2 data indicates that the borehole noise spectrum is similar to other submarine borehole observatories and is comparable with quiet stations on land. In particular, the long-period noise shows little time variation, which allows more reliable comparisons of different event records. Noise from ocean waves appears attenuated over seafloor installations, a result of installing the sensors within a basement borehole. Noise with a period of several seconds is observed by the observatory and is thought to be related to wind stress over the ocean.

Many seismic events have been recorded by the WP-2 observatory, including both local and teleseismic events. Comparison with published earthquake records indicates that the WP-2 observatory can distinguish distant events with magnitudes (M) > 5. For earthquakes in the local region, the observatory shows even smaller events, with magnitudes as low as M = ~4.