SEDIMENTATION HISTORY OF THE WESTERN PACIFIC

Site 1149 is the first complete section through the pelagic sediments (~400 m) of the Nadezhda Basin, a ~1000 km x 1000 km region in the northwestern Pacific Basin. More than 90% of the sedimentary section was either recovered or logged, and ~50 km of 80-in³ water gun data were seismically recorded across the site and in the vicinity. Site 1149 is an important reference site for Mesozoic equatorial sedimentation since the late Valanginian and for sediment that is being subducted along the entire Izu Trench. The original objectives for drilling at Site 1149 involved the following:

1. Estimating geochemical fluxes from sediments subducted into the Izu subduction zone. Tests of differences in sediment inputs in the Izu and Mariana trenches lead to geochemical differences in volcanic outputs at the two arcs. The paleolatitudes of formation of the sediments are important as the different sources of sediment, from pelagic, from eolian, or from volcaniclastic inputs will result in different sediment source compositions in the subduction zone;
2. Improving the Early Cretaceous paleomagnetic timescale; and
3. Providing constraints on mid-Cretaceous carbonate compensation depths and equatorial circulation.

Site 1149 is representative of the seismic and thus presumably sedimentary stratigraphy of the vast northwestern Pacific Basin (Abrams, this volume), which forms the primary input into the subduction zones at the western edge of the Pacific plate. Core-logging integration produced the following sedimentary and seismic section at Site 1149 (Abrams, this volume). An upper seismic transparent layer correlates with Unit I and II ash and pelagic clays. At 180 meters below seafloor (mbsf) and a two-way traveltime of 226 milliseconds below seafloor (msbsf) is the top of an upper seismic opaque layer, corresponding to the top of Unit III interbedded radiolarian cherts, porcellanites, and siliceous clay of early Aptian–Albian age. The strong velocity and density increase here produces a prominent seismic reflection that can be traced regionally (Ewing et al., 1968) and separates predominantly eolian deposits (above) from predominantly biogenic deposits (below). Below the uppermost cherts, velocity and density both increase more linearly, giving way to a lower transparent layer, corresponding to Unit IV cherts and carbonates of Early Cretaceous age (late Valanginian–late Hauterivian). At 410 mbsf (438 msbsf), basaltic basement is encountered, producing another impulsive increase in velocity and density and a strong seismic reflection. The cherts and carbonates deposited between the two prominent seismic reflectors mark the paleoequatorial passage of the Pacific plate regionally (Lancelot and Larson, 1975). This seismic sequence can be traced along the entire Izu-Bonin Trench and permits confident extrapolation of the detailed geochemical and sedimentological data collected at Site 1149 (Abrams, this volume; Plank et al., 2002a).

The seismic sequence outlined above results from the northward migration of Site 1149 by as much as 40° of latitude through most of its history across several oceanographic provinces (the following summary is from Plank, Ludden, Escutia, et al., 2000). Site 1149 was formed in the Valanginian (Bartolini, this volume) at low latitudes south of the equator (~5°–10°S), crossed the equator during the mid-Cretaceous (~100 Ma), and continued north to its current latitude (~31°N) in the western Pacific. The cherts and carbonates that make up the lower half of the site (Units IV and III) record biological events at equatorial paleolatitudes. The brown pelagic clays of Unit II record in a short interval (~60 m) most of the site's history (~100 m.y.) in sediments that are metal rich but largely barren of microfossils (with the exception of ichthyoliths). Unit I records ~6.5 m.y. of eolian deposition within the westerly wind belt and western boundary currents. This sedimentary section has led to definition of a global anoxic event in the Valanginian, as well as a high-resolution eolian record for the Pliocene–Pleistocene.

The uppermost Valanginian sediments in Hole 1149B contain biotic changes in nannofossils and radiolarians associated with the Valanginian ¹³C anomaly, which are coeval with and similar to those previously documented in the Tethys (Erba et al., 2004). This suggests a global perturbaton of marine ecosystems. A marked increase in abundance of Diazomatholithus, absence of nannoconids (Lozar and Tremolada, this volume), and a Pantanellium peak coincident with high concentrations of barium (Bartolini, this volume) characterize the Valanginian ¹³C excursion. Such changes were interpreted as being due to global enhanced fertility and a biocalcification crisis under conditions of excess CO₂. The presence of organic carbon–rich black shales in the Southern Alps and in the Pacific in this interval further suggested a Valanginian oceanic anoxic event (OAE), which Erba et al. (2004) named the "Weissert OAE" in recognition of Helmi Weissert's pioneering work on Lower Cretaceous sedimentary successions (Weissert, 1989). There is no paleontological or ¹⁸O evidence of warming during the Weissert OAE. On the contrary, both nannofossils and oxygen isotopes record a cooling event at the climax of the ¹³C excursion. Possibly, weathering of basalts and burial of organic carbon–rich black shales were responsible for CO₂ drawdown and establishment of reversed Greenhouse conditions.

A recent study (Escutia et al., this volume) focuses on the Miocene to Pleistocene volcanic ash record in Unit I and Subunit IIA sediments at Site 1149, along with a companion site ~10° farther north (ODP Site 1179; Kanazawa, Sager, Escutia, et al., 2001), within the context of the paleoceanographic and paleoclimatic evolution of the northwestern Pacific. Detailed ⁴⁰Ar/³⁹Ar analyses of ash samples, combined with magnetostratigraphy, yield a refined chronology that will be invaluable for future geochemical studies aimed at the evolution of arc volcanism in the northern circum-Pacific (Escutia et al., this volume). The new chronology also reveals peaks in ash frequency and cumulative ash thickness. Because both sites show correlated increases in ash accumulation rates, Sager and Escutia (2005) hypothesize increased transport of sediment may be associated with cooling climate events and stronger trade winds carrying both dust and ash from an increasingly arid Asia. This is supported by the position of dated ash layers at both Sites 1149 and 1179 for the last 2.1 m.y., which correlate with cold stages or cooling trends in the isotopic variations.

Lower Pliocene to Quaternary silicoflagellate biostratigraphy allowed Lozar and Mussa (this volume) to track the increasing warming of surface waters due to the diverted Kuroshio Current in relation to the Middle Pliocene closure of the Central American Seaway using three closely spaced events: the asperoid/fibuloid reversal, the change in the Distephanus/Dictyocha ratio, and the last occurrence (LO) of Distephanus Boliviensis. Diatom stratigraphy studies by Olschesky and Laws (this volume) show that zonal indicator species for the lower Pliocene–upper Pleistocene zones for both the equatorial Pacific and the North Pacific zonations are present in Hole 1149A. Additionally, the presence of coastal and benthic taxa in the diatom flora from Unit I strongly suggests some onshore to offshore transport of material to Site 1149 either by aqueous or eolian mechanisms.

The Pleistocene section of Hole 1149A provides an expanded record of eolian dust (supplied from the Asian continent), changing volcanic ash input, and siliceous plankton accumulation, with recurrent diagenetic intervals in a deep-sea environment. Urbat and Pletsch (this volume) developed a scheme for the late Pleistocene section using geochemical normative calculations on the basis of Al and Cr contents to discriminate between the major groups of components (terrigenous, volcanogenic, biogenic, and diagenetic) in combination with magnetic variations to quantify the eolian input of continental material to Site 1149.

Textural variations and other physical properties, such as electrical resistivity, of the sediments that are being subducted within the Pacific plate provide important information on mass transfer properties of fluids. A series of studies that will provide important constraints to future Integrated Ocean Drilling Program (IODP) experiments, such as for fluid evolution in Nankai sediments, were carried out on the Site 1149 sedimentary sequence (Hirono, this volume; Hirono and Abrams, this volume; Kawamura and Ogawa, this volume; Tanaka and Ogawa, this volume).

The primary objective of drilling at Site 1149 was to create a geochemical reference site for subduction at the Izu Trench, and so this ~400-m sedimentary section has been the target of a remarkably comprehensive set of geochemical studies. A great variety of elemental and isotopic measurements have been made, in many cases on the same sample powders. This is too often not the case, and so Site 1149 studies provide a rare opportunity to relate many geochemical systems. In addition to enabling precise mass flux estimates for the Izu margin (see "Geochemical Sinks and Recycling at Subduction Zones"), the geochemical data for Site 1149 sediments help define the sedimentation history for the Nadezhda Basin, as well as first-order systematics on some new isotopic systems.

The combination of geochemical logging data and comprehensive core analyses creates a useful geochemical section for Site 1149 sediments. Unit I is characterized by the intermediate silica and low CaO typical of pelagic clays, whereas Unit II is characterized by a dramatic increase in the concentration of hydrogenous elements like Th in brown clays (Fig. F4) (Plank and Kelley, 2001). Unit III is clearly identified by a rise to silica values >80% due to the appearance of chert and Unit IV by a more gradual rise in CaO due to the abundance of chalks (Fig. F4). Other geochemical tracers record the detailed motion of the site relative to different oceanographic provinces. Th/La maps several geographic transitions (Fig. F5), as it is sensitive to eolian dust and ash (high Th/La), hydrothermal inputs (rare earth element [REE] rich and low Th/La), and biogenic fluxes (phosphate and REE rich and low Th/La). Thus, as the site begins in the south in low-latitude, high-productivity zones, Th/La is initially low due to hydrothermal and biogenic inputs then rises dramatically when the site moves northward into the central gyre (near the Unit III/II boundary), and rises further to high values typical of eolian dust (loess) as the site moves into the influence of the westerly winds (Plank et al., 2002a) (Fig. F5). Pb isotopes also map similar variations, with low MORB-like values at the base, reflecting hydrothermal inputs, rising to high values throughout the rest of the site, reflecting eolian and seawater inputs (Hauff et al., 2003) (Fig. F5). Thus, geochemical variations both define the stratigraphic units and record the site's migration from equatorial regions to western Pacific mid-latitudes. Such systematic geochemical variations, clearly tied to lithological and latitudinal variations, make Site 1149 a useful reference site for calculating geochemical inputs into the Honshu-Izu-Bonin-Mariana Trench (see "Site 1149: A Geochemical Reference Site for Sediment Recycling at the Izu-Bonin-Mariana Margin" in "Geochemical Sinks and Recycling at Subduction Zones").

In contrast to geochemical tracers that reflect the primary sedimentation history of the site, carbon and nitrogen isotopes in Site 1149 sediments record predominantly diagenetic effects (Sadofsky and Bebout, 2004). The uppermost 100 m of the site shows systematic decreases in ¹⁵N (Fig. F5), as well as in ¹³C, and in N and organic carbon concentrations. Although some of this variation may relate to an increase in biologic productivity as the site moves into the influence of the western boundary currents, Sadofsky and Bebout (2004) interpret the changes largely as diagenetic. Significantly, this study is one of the first to document ¹⁵N variations in a section of deep-sea pelagic sediments, and the observed ~3 decrease in ¹⁵N downcore may help to relate high marine organic values to the low values typical of metasedimentary rocks (Sadofsky and Bebout, 2004) (Fig. F5). This early diagenetic shift in N isotopes may be representative of the first step in the progressive metamorphism of deep-sea sediments during subduction. These data, taken together, provide the basis for global subduction flux estimates for nitrogen and carbon (Sadofsky and Bebout, 2004).

Site 1149 sediments represent a classical sequence of oceanic pelagic sedimentation, including type examples of chalk, chert, brown clay, and eolian deposition. Thus, they make a convenient target for developing new stable isotope systems in geochemistry (Rouxel et al., 2002, 2003a, 2003b). Figure F5 includes a summary of these works, with some of the first data published on Se, Sb, and Fe isotopes in deep-sea sediments. More than revealing anything unusual about Site 1149 sediments, these measurements help to define the marine processes that lead to significant fractionation in these new stable isotope systems.

Se isotopic compositions of Site 1149 sediments show significant shifts in ⁸²Se/⁷⁶Se relative to bulk earth (Fig. F5), to both low values (at <25 mbsf) and high values (basal carbonate). In accord with the interpretations in Rouxel et al. (2002), the low ⁸²Se/⁷⁶Se ratios in the upper 25 m of the site may result from reduction of seawater Se(IV) and Se(VI) oxyanions, since this is the region of active organic matter degradation and suboxic diagenesis at the site (Sadofsky and Bebout, 2004; Cragg et al., this volume). The positive shift in the basal carbonate marks the heaviest ⁸²Se/⁷⁶Se composition in any earth (or meteoritic) material measured by Rouxel et al. (2002). Based on neighboring samples, this sediment contains a significant fraction of Fe-rich hydrothermal precipitate and may reflect the complement to low-⁸²Se/⁷⁶Se sulfide-rich hydrothermal deposits reported by Rouxel et al. (2002).

Sb isotopic compositions of Site 1149 sediments generally fall within the range defined by igneous rocks (Fig. F5), and indeed, Rouxel et al. (2003b) use Site 1149 in part to define continental crust values. Nonetheless, there is a range of almost 2 in ¹²³Sb in Site 1149 sediments, with the lowest values corresponding to a Unit I sample with abundant arc-derived ash (as revealed in its high Nd) (Hauff et al., 2003). The highest values, which are shifted toward seawater, are in Unit II samples that have the greatest hydrogenous Mn oxyhydroxide contribution (as revealed in their high Mn/P and positive Ce anomalies) (Plank and Kelley, 2001).

Rouxel et al. (2003a) measured the Fe isotopic composition of cherts and chalks from Units III and IV and found ⁵⁷Fe shifted to values up to 1 lower than the igneous value. In fact, the Site 1149 sediments are used in part to define a chert average of –0.11, which Rouxel et al. (2003a) argue is shifted toward likely seawater values of approximately –1.0. The lowest values at the site are for a basal carbonate, which may reflect the input of hydrothermal Fe (based on its high Fe/Al).

Other studies in progress on Site 1149 sediments include a focus on cosmogenic ¹⁰Be (Valentine et al., 2002), Li isotopes (Valentine et al., 2001), Hf-Nd isotope systematics (Marini and Chauvel, 2001), and high-precision Pb isotope systematics in the hydrothermally dominated basal sediments (V.M.C. Chavagnac, C.R. German, unpubl. data).