A sedimentary section of 408 m total thickness was recovered in the four holes that were drilled at Site 1149. The sediments predominantly consist of carbonate-free clays with variable admixtures of volcanic ash and siliceous microfossils, cherts, porcelanites, and calcareous nannofossil chalks or marls. On the basis of the distribution of these lithologies, the sedimentary column between 0 and 408.2 mbsf has been divided from top to bottom into five lithologic units (Fig. F9).
Unit I, of late Miocene to late Pleistocene age, reaches 118 m thickness and consists of carbonate-free clay with common ash particles and siliceous microfossils. Unit II consists of 62 m of undated dark brown pelagic clays with several discrete ash layers in the upper 30 m (Subunit IIA). Unit II clays are barren of any siliceous or calcareous microfossils but contain ichthyoliths. At this time, however, their age is undetermined. Unit III is a 104-m-thick alternation of radiolarian chert with porcelanite and siliceous clay, the age of which is again to be determined by shore-based research. Unit IV comprises 125 m of upper Valanginian to upper Hauterivian intercalated radiolarian chert, porcelanite, and siliceous chalks or marls. Unit V, which is upper Valanginian recrystallized calcareous marlstone, was found only in fractures in the upper 2 m of basement in Hole 1149B.
The evolution of the depositional environments at Site 1149 can be divided into four broad episodes, starting in the late Valanginian (~133 Ma) with the onset of calcareous pelagic sedimentation on oceanic crust, and subsidence to below the CCD after 7 m.y.; thereafter, 100 m of radiolarian cherts, porcelanites, and clays without detectable carbonate accumulated, probably still in the Early Cretaceous. The change from siliceous deposition to the very slowly accumulated brown pelagic clays of Unit II presumably occurred during the Late Cretaceous to Paleogene. This abrupt decrease in siliceous deposition may correlate with the changing direction and the more rapid northward movement of the Pacific plate, which probably brought the site to below the oligotrophic waters of the large gyre that covered much of the mid-latitude Pacific during the Cenozoic. At some unknown point in time, but before the late Miocene, Site 1149 approached the Izu-Bonin and Japan volcanic arcs as documented by numerous discrete ash layers and significant amounts of dispersed ash in the sediments of Unit I. Subsequent to the onset of volcanic input, biosiliceous deposition resumed with the accumulation of both planktonic and benthic siliceous microfossils. Very high sediment accumulation rates and the mineral composition of the youngest sediments suggest that Site 1149 was in the reach of the Asian dust plumes after the early Pleistocene.
Site 1149 is located ~100 km east of the Bonin Trench axis in a zone of relatively thick and undeformed sediments, as discussed in "Tectonic Setting and Magnetic Anomalies". Sediments were cored in all four holes drilled at Site 1149 with a maximum total thickness of 408.2 m in Holes 1149A and 1149B, below which volcanic basement was encountered. The basement was found to be 7 m shallower in Hole 1149C and 107 m shallower in Hole 1149D, the latter being located only 5 km to the southeast (see Fig. F5). On the basis of the distribution of the major lithologies recovered at Site 1149, as defined on the basis of shipboard smear-slide analyses (see "Site 1149 Smear Slides"), the sedimentary column between the seafloor and basement has been divided from top to bottom into five lithologic units (Table T3).
Lithologic Unit I consists primarily of clayey lithologies with varying amounts of siliceous microfossils and volcanic grains. Colors are dominantly dark greenish gray, dark gray, or dark brown. There is a downward trend toward slightly lighter and more saturated colors (higher chroma values). Siliceous microfossils are common to abundant throughout Unit I and consist mostly of diatoms and radiolarians with minor siliceous sponge spicules and rare silicoflagellates. In the upper part of the unit there are occasional thin beds (typically 3 cm thick) that are slightly darker than the surrounding sediments that contain abundant diatom frustules. Below Core 185-1149A-11H, the abundance of siliceous microfossils decreases rapidly from common (10%-30%) to traces at the top of Core 185-1149A-13H over an interval of <15 m. Calcareous microfossils, as well as any other carbonate components, are generally absent throughout Unit I, except for trace amounts of redeposited calcareous nannofossils in Sample 185-1149A-3H-1, 83 cm (see "Calcareous Nannofossils").
The volcanic component includes glass, centimeter-sized pumice and smaller rock fragments, and mineral grains. Apart from the volcanic particles in the background sediment, there are at least 100 discrete ash layers in Unit I. The color of the ash varies between dark gray (10Y 3/1) and light gray (10YR 6/1). Thicknesses of the discrete ash layers typically vary from a few millimeters to 5 cm. Thicker layers, however, are not uncommon. For example, ash layers in Sections 185-1149A-8H-2 and 8H-3, 10H-5, 16H-2, and 16H-3 are 20, 45, 30, and 25 cm thick, respectively. Core 185-1149A-16H not only contains two of the thicker ash layers of 30 and 25 cm, but it is also the core with maximum frequency of ash layers. Ash layers typically have sharp basal contacts and diffuse tops and are normally graded (Fig. F10). The bases of the ash layers, although sharp, are not erosional. Grain size at the base, as determined visually and by examination of shipboard smear slides, is medium or fine sand to silt and grades toward the top to clay-size particles (Fig. F10A). The diffuse tops of the ash layers are in some cases clearly the result of bioturbation (Fig. F10B). In other cases, however, the diffuse tops may be the result of slower deposition and mixing of the ashes with the clays because of winnowing of the main ash fallout (Fig. F10A). Nonquantitative analysis of the sand and silt components of the ash layers in the shipboard smear slides shows that ash layers consist of 60%-98% glass shards and volcaniclastic particles. Smear-slide estimates of ash content in the background clay lithology of Unit I range from 10% to 30%.
Pumice fragments, up to 3 cm across (Fig. F11), are scattered throughout the unit. One larger piece of pumice from Sample 185-1149A-11H-6, 72-74 cm, contains planktonic foraminifers with preserved calcareous tests in a matrix of dark clay that fills fractures and vugs in the pumice (Fig. F12).
X-ray diffraction (XRD) analysis of the background sediment of Unit I has revealed reflections characteristic of quartz, feldspar, and clay minerals, including a 14 Å mineral (probably chlorite), mixed-layer minerals in the 10-15 Å range, and discrete illite (Fig. F13). Judging from the intensities of the XRD peaks of quartz and feldspar, these minerals are probably more abundant than suggested by the smear-slide data, where both minerals together usually accounted for no more than ~5%-10% of sediment. This discrepancy may be because some of these minerals probably occur in the clay and very fine silt size fraction and are difficult to identify in smear slides. High intensities around 3.19 Å and relatively low intensities at ~3.25 Å suggest that most feldspar is plagioclase. The diffractogram from the uppermost core, Core 185-1149A-1H, shows peaks at 14.3, 7.05, and 4.71 Å that are attributed to a chloritic mineral, which is subdued or nonexistent in the underlying sediment (Fig. F13).
Sedimentary structures and bioturbation are rarely discernible other than near ash layers and rare bedding planes, where sediment composition or texture change (Fig. F14). However, there are common green clay lamina, singly or in bundles, which are often associated with slightly more indurated intervals (Fig. F15). XRD of such a clayey layer in Sample 185-1149A-2H-1, 22-23 cm, revealed a composition similar to the background sediment (Fig. F13) except for the presence of a well-defined peak at 3.18 Å, that is related to plagioclase.
Lithologic Unit II dominantly consists of dark brown pelagic clay with rare silt- and sand-sized components. Siliceous microfossils are absent or are present in trace amounts in the upper part of the unit. The upper boundary of Unit II was placed at the top of Core 185-1149A-14H, where siliceous microfossils disappear downhole except for occasional occurrences of dissolution-resistant siliceous sponge spicules in trace amounts. The lower boundary of Unit II corresponds to the reappearance downhole of siliceous facies, including indurated lithologies such as porcelanite and chert. Unit II is divided into two subunits.
Subunit IIA is characterized by dark brown pelagic clay with rare volcanic glass grains as a background sediment and intercalated ash layers. The upper 7 m of Subunit IIA displays a wide variety of colors, ranging from olive gray (5Y 4/2) and light reddish brown (5YR 6/4) to dark reddish gray (5YR 4/2). As in Unit I, visible bioturbation is usually restricted to interfaces of differently colored sediments (Fig. F16A, F16B, F16C, F16D).
As mentioned above, the upper boundary of Subunit IIA was placed below the last downhole occurrence of dissolution-susceptible radiolarian and diatom tests. This upper boundary of Subunit IIA corresponds to a sharp decrease in porosity from close to 80% to values generally below 70% (see "Index Properties"). The transition to Subunit IIB was placed below the last downhole occurrence of a discrete layer with rare volcanic fragments in Sample 185-1149A-17H-2, 132 cm.
Apart from the disappearance of silica, the mineralogical composition of Subunit IIA sediments is remarkably similar to that of Unit I, especially in the upper part (Fig. F17). XRD shows that the dark brown clays have somewhat lower relative intensities of the 3.2-Å feldspar peak and a higher intensity of the nonbasal reflection of the clay minerals around 4.5 Å (Fig. F17).
The dominant lithology of Subunit IIB is the same dark brown pelagic clay recovered in Subunit IIA, but it contains no discrete ash layers and <5% or trace amounts of volcanic particles.
The clay from the top of the subunit is mineralogically similar to the background sediment from the overlying subunit, but there is a decrease in relative quartz abundance downhole (Fig. F17). Subunit IIB corresponds to an interval of downhole increasing gamma-ray intensity and X-ray fluorescence (XRF) K2O/Al2O3 that was recorded in the cores (see "Natural Gamma Radiation" and "Authigenic Clay Formation: Unit II") and in the potassium spectral band of the logging data (see "Natural Radioactivity"). The potassium maximum correlates with the appearance of zeolites of the heulandite-clinoptilolite group at the bottom of the subunit (Fig. F17). Although shore-based analyses are required to determine the exact mineral species, both heulandite and clinoptilolite are known to be potassium rich (Kastner, 1979; 1981). Besides the zeolites, X-ray diffractograms from the base of the subunit show weak, but significant peaks at 10.5 and 6.4 Å that are interpreted to correspond to palygorskite. Palygorskite, a magnesium-rich fibrous clay mineral is thought to form authigenically within marine sediments, most probably from a smectitic precursor (Couture, 1977; Pletsch, 1998). Additional evidence for authigenesis comes from the anomalous chemical composition of the sediments and interstitial waters (see "Authigenic Clay Formation: Unit II" and "Ash Alteration and Formation of Authigenic Clays"). Notably, a slight decrease in dissolved Cl- is reported that may be related to the transformation of smectite to palygorskite, which is accompanied by the release of interlayer water. Again, shore-based analyses are required to validate the amount and the authigenic formation of palygorskite.
The otherwise very monotonous clays are cut by at least two steep faults, one of which shows a normal sense of displacement (Fig. F18A, F18B). Although it is unknown to which degree the core-cutting process is responsible for these faults, it seems clear that the rheological character of Subunit IIB is remarkably different from the other lithologic units because deformational features with notable displacement were not recorded in either the overlying clays or in the underlying indurated sediments. The susceptibility of Subunit IIB sediment to deformation is supported by the marked decrease in shear strength (see "Shear Strength"), a phenomenon that might be responsible for the logging and operational difficulties encountered in Holes 1149B and 1149C (see "Hole 1149B" and "Hole 1149C").
Lithologic Unit III is characterized by the presence of alternating siliceous lithologies ranging from soft, opal-CT-rich, and zeolitic clay to hard but porous radiolarian porcelanite to vitreous and dense radiolarian chert. As with the many previous DSDP and ODP efforts to core intervals with chert layers, recovery dropped dramatically in this unit, typically to values below 5%. Given this low recovery and the tendency for softer lithologies to be washed away as an effect of the higher pumping rates (see "Hole 1149A"), even the very small amounts of soft material recovered in the core material must be considered a major lithology of Unit III.
The top of Unit III is defined by the first occurrence of indurated siliceous lithologies such as chert and porcelanite at ~180 mbsf. Laminated zeolitic clays and radiolarian porcelanites with more massive, irregular intercalations of radiolarian chert were recovered both in the uppermost and the lowermost part of the unit (Figs. F19A, F19B, F20A, F20B, F21). These sediments often have a peculiar mineralogy that is typical for siliceous diagenetic environments (Fig. F22). Thus, opal-CT appears at the top of the unit and persists into the upper 20 m of the underlying Unit IV. Palygorskite, detected at the bottom of Unit II, is also found in Unit III (Fig. F22), and zeolite, common throughout the unit, locally forms zeolite silts. The first downhole occurrence of opal-CT is in Core 185-1149A-21X at ~180 mbsf. This level corresponds to a significant decrease in dissolved silica in the interstitial waters, which is likely related to the crystallization of opal-CT (see "Biogenic Sedimentation" and "Diagenesis of Biogenic Silica and Carbonate").
A P-wave velocity increase was noted in the lower part of Unit III (see "Seismic Velocity") that persists downsection in the more uniformly indurated lithology of Unit IV, but no concomitant lithologic change was observed in the core.
Unit IV is characterized by the presence of calcareous lithologies that are either interbedded with radiolarian cherts and porcelanites similar to those of Unit III or present in vugs within, and halos around, the siliceous sediments (Fig. F23). There is a downward increase in carbonate content of the recovered calcareous lithologies (Table T4), which is supported by the logging data (see "Geochemical Log"). The marly and chalky lithologies often show wavy, discontinuous laminations, part of which may be a result of the flattening of burrows. A significant amount of flattening is indicated where there is differential compaction around rapidly indurated parts of the sediment, such as large, sediment-filled burrows (Fig. F24A). High initial water contents are also indicated by a water escape structure that was observed in about the same interval (Fig. F24B).
One major mineralogic characteristic of Unit IV is the downhole disappearance of opal-CT below the top 20 m of the unit (Sample 185-1149-18R-1, 9-11 cm; 301.7 mbsf). Thin sections from this core show that radiolarians and diatoms make up a considerable amount of the calcareous lithologies at the top of Unit IV (Fig. F25A, F25C, F25D). The opaline skeletons are largely preserved, though probably with secondary opal-CT, as indicated by XRD data (Fig. F26), whereas the lumens are filled with chalcedony (Fig. F25B, F25C). The calcareous component mostly consists of calcareous nannofossils, which show a broad range of preservation from very good to strongly recrystallized or broken (Fig. F25D). Common veins filled with calcite and chalcedony testify to the downward increasing recrystallization of both calcareous and siliceous components (Fig. F25E). Below ~300 mbsf, opal-CT disappears and quartz takes its place (Fig. F26). The siliceous microfossils, however, become replaced by calcite (Fig. F25F). Much of the previously dissolved silica may thus have precipitated elsewhere.
Besides the lighter colored chalks and marls, there are rare and scattered darker layers (Fig. F27). One such layer was found to contain elevated quantities of euhedral barite, along with dolomite rhombs and phosphatic fish remains. The observed increase in barium minerals is closely matched by chemical analyses that show the highest values in this interval (see "Biogenic Sedimentation"). The mineral assemblage is characteristic of deposits that are, or used to be, enriched in organic matter. Strikingly, a marked gamma-ray maximum in the uranium spectral band is recorded in the logging data in the same interval (see "Natural Radioactivity"). Common sedimentary components with elevated uranium content are phosphate and organic matter. The peculiar mineral assemblage and the logging data may thus indicate that there was a prolonged period of enhanced carbonaceous accumulation in the late Hauterivian. Unfortunately, no organic geochemical data are available to substantiate this interpretation. Testing this hypothesis is particularly interesting because a correlative organic-rich interval has been found in the Cretaceous western Tethys (Cecca et al., 1994; Baudin et al., 1999).
Below 300 mbsf, cherts display a wide variety of structures and colors, which include greenish and bluish tinges (Fig. F28). There are often several generations of differently colored chert in one piece, and some of these inhomogeneities resemble lamination, bedding, and bioturbation. These patterns, in particular the heavily bioturbated type ("swiss cheese chert"), were not observed in the brown cherts of Unit III. Toward the base of the unit, color changes are also observed in the calcareous lithologies (Fig. F29). The lowermost 10 m of the sedimentary section above the basalt is characterized by unusual, highly saturated colors (i.e., with high chroma values) like purplish blue and red. Most of this color change can be attributed to a visible downward increase in the proportion of manganese micronodules and iron oxides. This increase in a presumed hydrothermal component is also seen in an increase in Fe/Al in the bulk sediment geochemistry (see "Metalliferous Sources" and "Degradation of Organic Matter and Associated Redox Environments").
The more indurated calcareous lithologies of Unit IV often show subvertical and oblique dissolution seams. Although their origin is not well understood, these features seemed to have formed as a result of pervasive rupture of the semi-indurated sediment. The restriction of these deformational features to the hard limestones may provide some evidence for the relative timing of lithification in calcareous vs. siliceous lithologies (Fig. F30A, F30B, F30C).
Lithologic Unit V consists of calcareous marlstones that fill fractures in the brecciated basalt. The marlstones are more indurated than the sediments at the base of Unit IV. Colors are typically shades of pinkish gray. Macroscopic veins of carbonate rimmed with smectite and iron oxide are common (Fig. F11) (see "Lithologic Units"). These altered sediments appear to be derived from a precursor that was very similar to the nannofossil marls of lithologic Unit IV, but more pervasive dissolution and overgrowth of calcareous nannofossils (see "Calcareous Nannofossils") has made the remnants virtually unrecognizable. In thin section, there is a noticeable increase in secondary, equant calcite crystals. Carbonate analyses (Table T4) yield ~65 wt% carbonate for the altered sediments of this interval, which is significantly lower than the average chalks of Unit IV (84-88 wt% in Samples 185-1149B-28R-2, 114-117 cm, and 29R-1, 3-4 cm) but almost exactly the same as the lowermost marls immediately overlying the basement. The lower carbonate abundance relative to the overlying lithologies is due to a downward increase in clay content.
X-ray diffractograms provide a qualitative impression of the differences between Units IV and V (Fig. F31). They show that feldspar, which is not detected in the chalks of Unit IV above the basalts, is present in Sample 185-1149B-29R-1, 134-135 cm, from the basalt fracture fill (Unit V). Smectite also shows a strong peak at 15 Å in Unit V and decreases upward. Smectite is a typical alteration product of basalt and was observed in thin section near basalt-sediment and basalt-calcite vein interfaces. Also observed in thin section were stacked aggregates of secondary clays in the sediment groundmass; these may also be smectite. The presence of feldspar may be due to thermal dehydration of primary clay minerals at the contact to the basalt. In thin section, feldspar was observed as finely disseminated, >5 µm, euhedral to subeuhedral crystals of low relief and low-order interference colors under cross-polarized light.
The overall depositional history at Site 1149 appears to be intimately related to the large-scale vertical and horizontal movements of the Pacific plate. The newly formed oceanic crust subsided to below the CCD within ~7 m.y. of its formation (see "Post-Hauterivian Carbonate-Free Siliceous Sedimentation" and "Sedimentation Rates"). The plate tectonic movement of Site 1149 from low southern latitudes during the Early Cretaceous toward its current position at ~31°N resulted in crossing a number of paleoceanographic and paleoclimatic boundaries, including the high-productivity belt along the Late Cretaceous paleoequator (Lancelot and Larson, 1975; Sager and Pringle, 1988; Larson and Sager, 1992). The evolution of the depositional environments at Site 1149 is divided into four broad episodes.
Upper Valanginian marls and chalks were the first sediments to be deposited in fractures within, and on top of, the freshly formed oceanic crust at Site 1149. The vivid colors and elevated iron and manganese contents in the lowermost 20 m of the sedimentary section give way to more drab colors and normal Fe/Al and Mn/Al values upsection, which probably records the increasing distance of the site from the volcanic ridge axis. Decreasing carbonate contents and increasing proportions of siliceous microfossils in the overlying calcareous lithologies probably reflect the deepening of the site in the ensuing Hauterivian.
In the late Hauterivian or afterward, carbonate supply from the euphotic zone became insufficient to balance dissolution at the increasing depth of Site 1149. From this time on, sedimentation was carbonate free, except for occasional intervals where traces of calcareous material became preserved as a result of rapid sedimentation or isolation from ambient deep water. Calcareous lithologies were replaced by poorly indurated siliceous clays and radiolarian porcelanite. Most of the silica varieties, as well as several silicate minerals, probably owe their existence to the diagenesis of siliceous microfossils. The dominance of siliceous microfossils is commonly used as proxy evidence for elevated paleo-productivity at the sea surface, which in turn is usually related to upwelling of nutrient-rich waters. As such, the siliceous sediments of Unit III would indicate a period of high plankton productivity, likely related to an oceanic divergence zone. Paleomagnetic data show that Site 1149 was on or within 5° north or south of the equator during the deposition of Unit III (see "Paleolatitude"). It therefore seems likely that the high biological productivity required for the accumulation of the thick sequence of siliceous deposits was related to the position of the site below the paleoequatorial divergence.
The transition from the siliceous deposits to the dark brown pelagic clays is abrupt. If the relationship between lithofacies and paleoproductivity is correct, then this dramatic change in sedimentation suggests that productivity also declined abruptly. Although the movement of the Pacific plate has changed both direction and speed in the past, this change was probably not fast enough to bring siliceous sedimentation to an abrupt end. The transition from Unit III to Unit II may be a result of a paleoceanographic crisis such as the restriction of the broad Eocene equatorial zone of biological productivity at about the Eocene/Oligocene boundary. This event was accompanied by the sharpening of latitudinal dissolution gradients in carbonates (Worsley and Davies, 1979), but the same probably applies to silica deposition. After the site moved away from the high-productivity zone, accumulation rates dropped to extremely low values, as suggested by the sizeable amounts of fish teeth and other ichthyoliths in the clays of Unit II.
Toward the end of the pelagic sedimentation that characterizes Unit II, discrete ash layers and volcanic particles appear in the pelagic clays and become more abundant upsection in the rapidly deposited Unit I (see "Sedimentation Rates"). Since bulk sediment accumulation rates probably were more than an order of magnitude lower in Unit II, the frequency of ash layers in that interval suggests that long time intervals passed between the deposition of individual ash beds. The ash layers are interpreted to be the result of ash fallout from explosive eruptions likely west of Site 1149. An Oligocene to middle Miocene major pulse of volcanism is recorded from the Bonin-Mariana-Yap arc and from Japanese arc volcanoes (Karig, 1975) (i.e., preceding the deposition of the ash- and siliceous microfossil-bearing clays of Unit I). At this time, Site 1149 was probably relatively far away from the volcanic eruption centers, which resulted in the low accumulation rates between the volcanic events that characterize Unit II. The more frequent presence of ash layers in Unit I, of late Miocene to Pleistocene age, either points to increased subduction-related volcanic activity in the northwest or to a closer position to the volcanic centers, or both. Since paleolatitudes for Unit II were similar to present latitude (see "Paleolatitude"), the vigor and frequency of the eruptions may have played an important role, but the proximity to the Izu-Bonin arc is clearly an additional factor. Four thick layers (>20 cm) at 70.0, 86.4, 139.4, and 140.7 mbsf may result from some of the major eruptions. As the pulses of volcanism waned, volcanic ash-fall deposition greatly decreased and intervals of ash and diatom/radiolarian-bearing clay were deposited.
Although the frequency of ash layers does not significantly increase in the middle Pleistocene to Holocene part of the section and actually decreases in the uppermost core of Site 1149, the accumulation rates in this interval are on the order of 30 m/m.y. Increasing proportions of detrital quartz and terrestrial, nonvolcanogenic minerals such as chlorite provide evidence that the elevated accumulation rates in this interval may be related to an increased eolian input from mainland Asia.