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doi:10.2204/iodp.pr.334.2011

Site summaries

Site U1378

Site U1378 was drilled into the middle slope of the Costa Rica margin, 41 km offshore Osa Peninsula along BGR99 Line 7 (Fig. F2) above the unlocked portion of the plate boundary according to interplate earthquake relocation and geodetic measurements (S.L. Bilek, pers. comm., 2003; LaFemina et al., 2009). The margin here consists of an upper plate basement underlying about 750 m thick slope sediments. The seismic sections show that this site is above the seaward edge of one of the high-amplitude reflectors interpreted as displacement surfaces. Site U1378 was designed to penetrate one of these surfaces. The primary purpose of drilling Site U1378 was to determine the nature, composition, and physical properties of the upper plate basement and to understand the nature of the landward-dipping seismic reflectors. Two holes were drilled at this site, Holes U1378A and U1378B, penetrating 456.9 and 523.9 m into the seafloor, respectively. Hole U1378A was dedicated to LWD operations to measure the in situ physical properties of the material in the borehole. Hole U1378B was designed to retrieve as much of the sediment coverage and basement as possible within the specified time window. Overall, 526.39 m of sediment were retrieved with an average total recovery of 100.48%. The basement was not reached in any of the holes because drilling was terminated early as a result of unfavorable hole conditions.

Based on lithologic characteristics, the sediments recovered at Site U1378 can be divided into two main lithostratigraphic units (Fig. F7). Unit I, ~128 m thick, is composed of mainly soft, dark greenish gray, terrigenous silty clay. Intercalated in this silty clay is a series of ≤5 cm thick fining-upward sequences consisting of lithic sands and 21 tephra layers ranging in thickness from 0.5 to 7 cm. The basal boundary between the silty clay and the sand and tephra sequences are sharp, whereas the top boundaries are gradational. Unit II, ~385.73 m thick, consists predominantly of massive, well consolidated, olive-green terrigeneous clayey silt(stone) and silty clay(stone) with minor layers of sand(stone), sandy silty clay(stone), clay, clayey silt(stone), and 61 tephra layers that remain unlithified to 560 mbsf. The basal contact between the background sedimentation and the tephra layers is sharp, whereas the top contact is gradational. Within this monotonous sequence, sandy intervals become thicker and more common with depth. Throughout all of Unit II, fining- and coarsening-upward decimeter-scale sequences of sand are present. In the coarser sand layers, rip-up clasts, rounded clay lenses, and abundant shell fragments are commonly observed. Framboidal pyrite was observed in many of the smear slides throughout Unit II. Horizons of abundant calcareous concretions or lithified carbonate mud clasts are rare and concentrated within a 2 m interval of Core 334-U1378B-27X (Sections 334-U1378B-27X-4 through 27X-CC; 222.08–224.41 mbsf).

The 82 identified tephra layers seem to be quite heterogeneous in composition, 30 of which are light gray to pinkish/brownish white felsic tephras, 40 are pinkish gray/brown more mafic tephras, and 12 are pinkish/greenish black mafic tephras. Mafic tephra beds account for ~15% of the total tephra bed assemblage in Hole U1378B. The felsic tephra layers are mainly (>90 vol%) composed of fresh, clear, colorless, fine to coarse ash-sized glass shards varying from angular blocky, cuspate, flat, and Y-shaped shards with nearly no bubbles to highly vesicular, pumiceous textures with many elongated bubbles. Dark gray mafic ash layers consist predominantly of dark to light brown glass shards. Most of the glass shards have blocky shapes and are medially to poorly vesicular and show strong signs of alteration, especially in the deeper part of the hole.

Generally, the sediment cored in Hole U1378B is characterized by low-angle bedding planes (<30) as well as by healed and open faults with relatively steep dipping angles. Sediment-filled vein structures were observed from 262.2 mbsf downward. The main fault populations, characterized by zones of intense brecciation and sediment deformation, occur at 279.2–281.4, 361.9–376.5, 475.9–477.2, and 514.2 mbsf to an unknown lower boundary. The second fault zone shows the most intensive deformation and corresponds to a low-density/high-porosity interval identified by LWD.

Preliminary X-ray diffraction (XRD) analysis of the sediment suggests that there is little variation in mineralogical composition among the different lithostratigraphic units. XRD data indicate that the major mineral components are clay minerals including illite, smectite, kaolinite, and attapulgite, as well as calcite, anorthite, and quartz. Amphibole (hornblende and richterite), chlorite, pyroxene (augite and hypersthene), olivine, and pyrite peaks are present.

The physical property data obtained from cored material display expected behavior with depth and reflect the different units cored (Fig. F7). Physical property measurements were made after sediment cores reached thermal equilibrium with ambient temperature at ~20°C. Wet bulk density values increase with depth, likely a result of dewatering caused by overburden pressure and are well described by a linear trend. A small offset in wet bulk density values marks the boundary between lithostratigraphic Units I and II. Grain densities are relatively constant with depth, with an average value of 2.7 g/cm³; however, they are quite variable in Unit II, ranging between 2.5 and 2.9 g/cm³. There is no discernible offset between values characterizing lithostratigraphic Units I and II. On the whole, these values suggest a terrigenous origin. Porosity, inversely correlated with bulk density, decreases from ~70% at the seafloor to 40% at the bottom of the hole. A slight increase in porosity between Units I and II corresponds to the decrease in wet bulk density at this boundary. An increase of porosity is observed at ~440 mbsf and corresponds to a sandy interval. Porosity increases a few percent between ~480 and 529 mbsf and may correspond to a decrease in the clay content. Magnetic susceptibility in the sedimentary sequence is low, indicating an abundance of noniron-bearing clays and scarcity of magnetic minerals. However, a region of generally higher values is present between ~85 and 195 mbsf. Other notable regions of relatively high values occur at 335, 355, 440, and 460 mbsf. These excursions show high–wave number variability and may be due to lithologic variations between sands, silts, and clays. NGR values indicate a small positive trend through lithostratigraphic Unit I and are relatively constant through Unit II. Notable excursions to higher values in Unit II occur at ~200 and 480 mbsf. The lowermost NGR high is associated with scoria and a high abundance of mafic glass particles. Thermal conductivity measurements show that the thermal conductivity generally increases with depth and is inversely correlated with porosity. In the upper 100 mbsf, variability is significant and likely reflects gas concentrations in the core. Downhole temperature measurements using the APCT-3 show a linear equilibrium temperature increase; coupled with the average bottom water temperature and thermal conductivity measurements, they give a least-squares geothermal gradient of 51.4°C/km and a heat flow of 44 mW/m². This value is consistent with forearc values of heat flow.

Generally, all the observations summarized above are consistent with changing depositional conditions in a more downslope environment. The cover sequence recovered from Hole U1378B is a terrestrially sourced slope sequence that is consistent with high sediment accumulation rates throughout the depositional interval. Recognition of wood debris and thin layers (<5 cm thick) of normal graded sands with sharp erosional bases within Unit I at this site are consistent with deposition in the distal facies of a clastic turbidite sequence.

This similarity is also consistent with the results of shipboard studies of calcareous nannofossils and foraminifers. Biogenic components in the cored sediments have a bimodal distribution in Unit II. Whereas shell fragments, diatoms, and nannofossils are sparse but ubiquitous throughout the unit, foraminifers are partly enriched within the sediments and are a major constituent of the sand-sized fraction. Based on nannofossil biostratigraphy, the sediments of the basal core were most likely deposited in the lower Pleistocene, thus the sediments throughout the core are younger than 2 Ma. The sequence between 5.36 and 224.39 mbsf is assigned to the undifferentiated Zones NN21–NN20 of the late to middle Pleistocene based on the presence of Emiliania huxleyi in Sample 334-U1378B-1H-CC (5.36 mbsf) and the absence of Pseudoemiliania lacunosa. Good preservation allows the tentative identification of E. huxleyi in Sections 334-U1378B-1H-CC (5.36 mbsf) through 16H-CC (127.91 mbsf). Samples 28X-CC (237.47 mbsf) through 63X-CC are assigned to Zone NN20 based on the last occurrence (LO) of P. lacunosa in Sample 28X-CC (237.47 mbsf) and the absence of Discoaster brouweri. Sample 57X-CC (476.18 mbsf) contains the first occurrence of early Pleistocene marker Helicosphaera sellii. Dominant species include Helicosphaera carteri, Gephryocapsa oceanica, Gephryocapsa caribbeanica, Gephryocapsa small, and Calcidiscus leptoporus. Thus, averaged sedimentation rates are ~516 and 236 m/m.y. in the upper (<237 mbsf) and lower parts (237~524 mbsf) of the hole, respectively.

Although the boundary of planktonic foraminiferal zones was not established, a few occurrence horizons of index species are approximately concordant with nannofossil zonation. Planktonic foraminifers, which are common to rare in the upper part of Hole U1378B, are mainly dominated by tropical fauna such as Globigerinoides quadrilobatus (Globigerinoides sacculifer), Globigerinoides ruber, Orbulina universa, Globorotalia menardii, and Neogloboquadrina dutertrei. Two index marker species of planktonic foraminifers were identified in limited horizons. Sample 334-U1378B-5H-CC (43.77 mbsf) contained pink G. ruber (last appearance at 0.12 Ma), whereas Sample 4H-CC (34.62 mbsf) did not contain the pink specimen. Thus, marine isotope Stage 5e is located in the interval between these two samples, and the horizons are assigned to foraminiferal Zone PT1. Coiling change of Pulleniatina from sinistral to dextral was observed in Samples 42X-CC (351.71 mbsf) through 30X-CC (245.19 mbsf). Thus, the sediments from Section 42X-CC are older than 0.8 Ma.

Benthic foraminifers are abundant in the upper part of Hole U1378B; abundances in the lower parts are variable and range from common to rare. Generally, the majority of the observed benthic foraminiferal species are similar to modern assemblages, which are characteristic for the oxygen minimum zone in this region. Thus, the benthic foraminiferal community observed in the sediment of this hole are dominated by bathyal species. In general, the upper part of cored sediment at this site contains Pseudoparrella bradyana, Trifarina carinata, Valvulineria inflata, Cassidulina limbata, Uvigerina, and Gyroidina. Bolivina is characteristic for the upper to middle bathyal environment. In contrast to this, the sediments of the lower part of this site (>209.28 mbsf) contain species characteristic of a greater water depth (Uvigerina hispida, Melonis barleeanus, Gyroidina, and Oridorsalis umbonatus). Generally, the faunal changes observed at this site reflect continuous environmental changes from upper slope to middle slope.

Paleomagnetic investigations partially support Pleistocene deposition ages of the cored sediments. Remanent magnetization was measured on archive-half cores and discrete samples taken from the working half recovered from Hole U1378B. All archive-half cores were demagnetized in an alternating field (AF) to 15 mT and measured with the pass-through magnetometer, whereas discrete samples were subjected to complete stepwise AF demagnetization and measured in both the SRM and a JR6 magnetometer. Cores 334-U1378B-1H through 16H were cored with the APC system using a nonmagnetic cutting shoe and oriented with the Flexit orientation tool. Cores 17X through 63X were cored with the XCB system using a standard cutting shoe. For the APC cores, inclination data from neither archive half sections nor discrete sample measurements show any sign of reversed polarity magnetization. Declination data corrected by the Flexit tool cluster near the present day geomagnetic North. Thus, the sediments in the APC cores (<127.8 mbsf) were deposited within the Brunhes period (<0.78 Ma). This age assignment is consistent with the shipboard micropaleontological data, which suggest that the foraminifer fossil G. ruber (pink) found in Core 5H may be 0.125 Ma in age. Paleomagnetic measurements on the XCB cores are severely affected by drilling-induced remagnetization and inconclusive, but so far they have not revealed any reversed polarity. NRM intensities correlate with magnetic susceptibility data, suggesting some of the downhole susceptibility variation reflects change in magnetic minerals.

The geochemical trends displayed by the analyzed pore water (82 whole rounds) and gas samples (65 headspace samples, 99 void gas samples, and 173 gas samples) are reflecting the different materials cored and present at the basement (Fig. F7). The pore fluid profiles of sulfate, alkalinity, ammonium, methane, and Ca in the upper 20 mbsf at this site reflect typical changes associated with organic carbon cycling. Sulfate concentrations decrease almost linearly from the seafloor to the sulfate–methane transition zone (SMTZ) at ~13 mbsf, whereas alkalinity increases from a seawater value at the seafloor to a maximum at 24 mbsf. Likewise, ammonium concentrations increase in the zone of active SO₄ reduction and reach a local maximum below the SMTZ at 24 mbsf, reflecting on-going organic matter diagenesis. Calcium concentrations decrease from seawater value at the seafloor in this zone, reflecting the precipitation of authigenic carbonates. The highest methane concentrations are observed just below the SMTZ between 14.1 and 23.6 mbsf. The gas at these depths results from biogenic production, as indicated by the high ratio of methane to heavier homologues (ethane and propane), with CH₄/(C₂H₆ + C₃H₈) values ranging from 8,000 to 15,000. From 20.7 to 200 mbsf, the ratio mainly steadily decreases and is interpreted as a mixing zone between shallow biogenic and deep-sourced thermogenic gas transported upward by diffusion.

From 100 to ~440 mbsf, salinity, Cl, Mg, K, B, and Na concentrations mainly show a monotonic decrease with depth (Fig. F7). Dissolved Ca concentrations are variable to ~200 mbsf and increase with depth to a peak concentration at 440 mbsf that is coincident with minima in Cl, Mg, and K concentrations. Silica concentrations are variable with depth, and there is a concentration maximum at 300 mbsf and minima at 100 and 460 mbsf in the same interval as the maxima in Ca concentrations. Collectively, the pore fluid chemical profiles suggest there is a unique fluid between 420 and 500 mbsf characterized by relatively low salinity, Cl, Mg, K, alkalinity, Si, and Sr concentrations and elevated Ca concentrations. This depth interval also corresponds to a marked increase in thermogenic hydrocarbons (propane, n-butane, and iso-butane). The in situ temperature at this depth is too cold for local generation of thermogenic hydrocarbons, suggesting lateral migration of a fluid sourced in a region with temperatures high enough to support clay dehydration and thermogenic hydrocarbon production.

The three samples recovered below 500 mbsf show a steep depth gradient in salinity and chloride, indicating diffusional communication with another fluid below the cored section. Since the Cl and salinity profiles decrease below 500 mbsf, this fluid must be fresher than the deeper sourced fluid sampled between 420 and 500 mbsf. Ethane, propane, and iso- and n-butane all show maxima at 518.7 mbsf. Corresponding with the increasing concentrations and maxima of these higher chain hydrocarbons (C₂₊), the CH₄/(C₂H₆ + C₃H₈) ratios indicate the dominance of thermogenic gas at depth in Hole U1378B.

Results from LWD generally correspond to the lithostratigraphic observations as well as to the physical property data obtained from the cored sediments. The LWD tools deployed in the hole included the adnVISION 675 (density, neutron, and ultrasonic caliper), the TeleScope 675 (MWD; power and data transmission and drilling parameters), the arcVISION 675 (propagation resistivity, gamma radiation, and annular pressure), and the geoVISION 675 (resistivity images and gamma radiation). The measurements recorded by the LWD tools were downloaded and processed successfully, except for the geoVISION resistivity image data.

Two logging units were defined on the basis of the LWD measurements. Logging Unit 1 (0–82 mbsf) corresponds to a compacting sequence where density and resistivity increase and porosity decreases with depth, reaching ~1.6 g/cm³, 1 Ωm, and 60% porosity at the base of the unit. The top of logging Unit 2 (82–455 mbsf) is marked by a step increase in density and resistivity, which then increase slowly with depth (from 1.8 g/cm³ and just above 1 Ωm at the top to 1.9 g/cm³ and just below 2 Ωm at the bottom of the unit). Porosity shows a matching small decrease with depth, from 55% at the top of logging Unit 2 to 45% at the base.

The adnVISION tool collected oriented images of bulk density and borehole radius. Despite its limited azimuthal resolution (image data are sampled in 16 azimuthal sectors, i.e., every 22.5°), the images display well-defined vertical bands of large borehole radius in the interval 110–438 mbsf, interpreted as borehole breakouts caused by differences in the principal horizontal stresses. The average azimuth of the breakouts is roughly northeast–southwest to east-northeast–west-southwest, indicating that the maximum horizontal stress is oriented northwest–southeast to north-northwest–south-southeast.

Site U1379

Site U1379 was drilled in the upper slope of the Costa Rica margin, 28.2 km offshore Osa Peninsula and Cao Island along BGR99 Line 7 (Fig. F2). The site is located above the locked portion of the plate boundary according to interplate earthquake relocation and geodetic measurements (Bilek, 2003; LaFemina et al., 2009). The margin here consists of an upper plate basement underlying ~890 m thick slope sediments. The primary purpose of drilling Site U1379 is to determine the nature, composition, and physical properties of the upper plate basement. This site is also designed as a "pilot hole" in preparation for proposed deeper CRISP Program B drilling at this location. Three holes were drilled or cored at Site U1379 penetrating 960, 10.5, and 949 m into the seafloor. Hole U1379A was dedicated to LWD operations to measure in situ physical properties of the material in the borehole. The hole was drilled with an 8½ inch drill bit with LWD tools in the BHA. Hole U1379B, drilled with the APC system, was completely dedicated to high-resolution geochemical and microbiology sampling to precisely determine the depth of the SMTZ and the associated microbiological changes. Hole U1379C was designed to retrieve as much sediment and basement core as possible within the specified time window. It was drilled with the APC system to refusal at 91.2 mbsf, followed by the XCB system to a refusal depth of 949.0 mbsf. The APCT-3 was deployed six times and useable data were recovered five times. The Flexit orientation tool was deployed on all APC cores in Hole U1379C, but data were lost from the first 10 cores when a critical computer was turned off during the first tool's deployment, causing the tool to lose synchronization with the computer. All APC holes were cored with nonmagnetic core barrels. Core recovery for Site U1379 was 100.3% with APC and 84.4% with XCB. Overall, 804 m of sediment and 12 m of basement were retrieved, despite the difficult drilling conditions in the basement.

Based on lithologic characteristics, the sediments recovered from Hole U1379C are divided, from top to bottom, into five lithostratigraphic units (Fig. F8). Unit I, the relatively thin top unit, consists of medium- to coarse-grained sand with abundant shell fragments. It is worth noting that Unit I observed in this hole is not the same lithostratigraphic unit as Unit I in Hole U1378B. Unit II, ~650 m thick, is composed of mainly olive-green clayey silt(stone) and silty clay(stone) with minor layers of sand(stone), sandy silty clay(stone), clay, clayey silt(stone), and tephra. The sediments in this unit are massive and well consolidated; the tephra layers are unlithified. Superimposed on the main background sedimentation of Unit II are three different subunits mainly consisting of consolidated clay, clayey silt with intercalated carbonate and dolomite concretions, and fining- and coarsening-upward decimeter-scale sequences of silty sands and sandstone, respectively. Generally, the deposits of Unit II in this hole are lithostratigraphically similar to the clay-rich deposits of Unit II observed in Hole U1378B. Nonetheless, the difference between the abundance of calcareous concretions or lithified carbonate mud clasts observed at this site compared to Site U1378 is striking. Unit III is ~229 m thick and consists of fining- and coarsening-upward sequences (decimeter thick) of olive-green silty sands and sandstone. Smear slides indicate that the sandstones are dominated by lithic clasts composed of magmatic rock fragments and feldspar minerals. Chlorite is the most abundant accessory mineral and is followed, in decreasing order of abundance, by volcanic glass, opaque minerals, and amphibole. Trace abundances of calcite, pyroxene, and quartz are present. Tephra layers are sparse in this unit, accumulating mainly in one sequence within the upper part of the unit. Unit IV (~2 m thick) consists of carbonate-cemented medium- to coarse-grained sand with well-rounded, lithic pebble- sized clasts and thick-walled shell shards. Unit V is ~67 m thick and is composed of matrix-supported breccia with clasts of limestone, basalt, and mudstone in a fine sandy matrix intercalated with basalt in the upper part (881.75–906.72 mbsf) and a sequence of variably sandy and clayey silt in the lower part (916.40–947.52 mbsf). The basalt ranges from aphyric to moderately phyric, containing plagioclase, pyroxene, and olivine phenocrysts and showing minor signs of alteration along veins.

Overall, 53 tephra layers (2–45 cm thick) have been identified intercalated in the background sedimentation of the different units, the majority of them below 324 mbsf. Smear slides indicate that most tephras consist of felsic glass shards varying from angular blocky, cuspate, flat, and Y-shaped shards with nearly no bubbles to highly vesicular, pumiceous textures with commonly elongated bubbles. The transparent glass shards of the felsic tephras are mostly fresh without any signs of alteration until Core 334-U1379C-60X. Below this core, divitrification structures increase with depth within the glass shards, reflecting differing levels of alteration. Grain size ranges from very fine to coarse ash (up to millimeter size). Identified mineral assemblages consist of plagioclase, pyroxene, hornblende, and biotite. Plagioclase is the dominant phase, but some tephras are dominated by amphibole and biotite, which normally occurs in the most evolved felsic layers. The few observed dark gray mafic ash layers are predominantly composed of dark to light brown glass shards. Most of the glass shards are blocky and are medium to poorly vesicular and show strong signs of alteration, especially in the deeper part of the hole. The mineral assemblages of the mafic tephras include plagioclase, pyroxene, and spinel. In contrast to felsic tephras, mafic tephras contain more crystals. In general, tephra layers have a sharp basal contact to underlying terrigenous sediment but a gradual transition with overlying ash-bearing terrigeneous sediment, and many are normally graded in grain size and well sorted.

The structural characteristics of the cored sediments can be divided into three zones corresponding to the upper and lower part of the sedimentary sequences and to Unit V, respectively. The upper part of the sedimentary sequences and the basement represent zones of gently dipping bedding planes and poor fault populations. Several healed normal faults and layer-parallel faults, likely formed during early stage deformation, were found in the upper part of the sedimentary sequences (lithostratigraphic Unit II). A zone with steep bedding dips and increased fault populations characterizes the lower part of the sedimentary sequences, below 597 mbsf. This part contains at least four fault zones composed of brecciated zones, fracture zones, and weakly deformed zones. The first fault zone (642.1–652.8 mbsf) corresponds to the lowermost section of lithostratigraphic Unit II and to considerably low density and high-porosity intervals identified by LWD.

Even though, macroscopically, these sediments seem to be heterogeneous, XRD analytical results indicate that there is little variation in mineralogical composition across the lithostratigraphic units described. XRD spectra indicate that the major mineral components are calcite, anorthite, and quartz, with amphibole, chlorite, pyroxene, olivine, and pyrite accessory minerals. Although very few monocrystalline quartz grains were seen in the smear slides, it may be possible that other silica-rich components (sedimentary and magmatic lithic fragments) are producing this signal.

These findings are generally reflected by physical property data obtained on the cored material (Fig. F8). Bulk density values were determined from both gamma ray attenuation (GRA) measurements on whole cores and mass/volume measurements on discrete samples from the working halves of split cores. In general, values of wet bulk density determined from whole-round GRA measurements and measurements from discrete samples agree well. Wet bulk density increases with depth, increasing more rapidly in the upper section than in the lower section, likely a result of dewatering caused by overburden pressure. Bulk density values in the brecciated basement are ~2.3 g/cm³. Grain densities determined from discrete samples are relatively homogeneous, ranging between 2.6 and 2.8 g/cm³. Measured porosities within the sediment section are inversely correlated with bulk density, decreasing with increasing depth. Magnetic susceptibilities are, with two exceptions, low and uniform, indicating an abundance of noniron-bearing clays and scarcity of magnetic minerals. Basement values of magnetic susceptibility are overall quite low considering absolute values; it is unlikely that this material contributes to any magnetic anomalies. Measured natural gamma ray values are relatively uniform to about Core 334-U1379C-78X, below which they start to decrease, consistent with the observed downward increase of sandy sediments and the increase in grain density. NGR values in the basement are mainly low, consistent with the sandy lithologies described in this interval. P-wave velocities show a sharp increase from the sediment coverage into the basement, reflecting the low porosity and high consolidation of the sediments. Thermal conductivity increases with depth and is inversely correlated to porosity. Measured downhole temperatures using the APCT-3 show a relatively linear increase of equilibrium temperatures with depth. Coupling these temperatures with the average bottom water temperature as well as with the measured thermal conductivity results in a least-squares geothermal gradient of 41.6C°/km and a heat flow of 40 mW/m², which is consistent with forearc values of heat flow.

Generally, the observations summarized above are consistent with changing depositional conditions in a forearc basin that may range from a near-shore environment to shelf sediments to upper slope sediments (turbidites) interrupted by calcareous mud debris from close fluid-venting areas. The calcareous microfossil community identified in the cored sediments also supports this interpretation.

Shipboard studies of calcareous nannofossils and foraminifers are generally used to further constrain the depositional environment and ages of the cored sediments. Calcareous nannofossils provided excellent biostratigraphic control for most of the cored section. All observed microfossils are characteristic of the Pleistocene. Based on nannofossil biostratigraphy, sediments retrieved from the lowest core seem to have been deposited in the lowermost Pleistocene. Thus, the sediments throughout the core would be younger than 2.6 Ma, resulting in an average sediment accumulation rate for the upper (~566 mbsf) and the middle part (566~722 mbsf) of the section of ~1230 and 100 m/m.y., respectively. Although the boundary of planktonic foraminifers could not be established, a few occurring horizons of index species are approximately concordant with nannofossil zonation. The faunal changes observed in the benthic foraminiferal community reflect continuous environmental changes from continental shelf to upper slope (middle bathyal).

Paleomagnetic investigations partially support Pleistocene deposition ages and the high sediment accumulation rates. Stepwise AF demagnetization was performed on all archive-half sections (to 30 mT), and discrete samples were taken from the working half. For APC cores, inclination data from neither archive-half sections nor discrete sample measurements show any sign of reversed polarity magnetization. Declination data do not show near-180° shifts within each core. Thus, the sediments in the APC cores (<91.2 mbsf) were deposited within the Brunhes period (<0.78 Ma).

Paleomagnetic measurements on XCB cores are severely affected by drilling-induced remagnetization and are inconclusive. From the pass-through measurements of archive-half sections, one interval at ~701–704 mbsf (Section 334-U1379C-83X-3 through the top part of 83X-4) show dominantly reversed polarity after AF demagnetization. Biostratigraphic Zone NN19 of the early Pleistocene is placed at this interval. Using calcareous nannoplankton zonal schemes of the eastern equatorial Pacific for the lower boundary of Zone NN19 (2.3 Ma), this observed reversed polarity should correlate with Chron C1r.2r (1.185–1.778 Ma). If true, this would suggest a rapid sediment accumulation rate (>388 m/m.y.).

NRM intensities correlate with magnetic susceptibility data, suggesting some of the downhole susceptibility variation reflects change in magnetic minerals.

The geochemical trends displayed by the analyzed pore water samples (110 whole rounds), interstitial gas samples (118 sediment plug samples), and void gas samples (74 samples) are also consistent with the different materials cored (Fig. F8). The upper 50 mbsf at this site reflects typical changes associated with organic carbon cycling. Alkalinity and ammonium increase from seawater values to maxima at 12 mbsf, reflecting organic matter diagenesis. At ~14 mbsf, the sulfate gradient decreases, associated with a concomitant decrease in ammonium concentrations and decrease in the alkalinity gradient. Based on the sulfate data, the depth of the SMTZ is estimated to be ~30 mbsf. This zone is characterized by a decrease in dissolved Ca, suggesting authigenic carbonate precipitation between 20 and 30 mbsf. Below the SMTZ, methane concentrations increase with depth and reach the highest concentrations from 42.31 to 67.18 mbsf. The methane at these depths is dominated by microbial production, as indicated by the high ratio of methane to heavier homologues (ethane and propane), with C₁/(C₂ + C₃) values of ~8,000 to 10,000.

The pore fluid geochemistry below the zone of most intense biogeochemical cycling can be split into three zones. The first zone extends from ~50 to 500 mbsf and is characterized by a steady increase in Ca concentrations and a decrease in Mg and K concentrations with depth. These trends are most likely not the result of volcanic ash alteration to clays and zeolites, because salinities (hence Cl) decrease with depth rather than increase, as would be expected, and the tephras recovered are largely unaltered. Rather, the trends appear to result from clay-ion exchange, ongoing biogeochemical reactions, and authigenic carbonate precipitation/dissolution reactions. Below ~100 mbsf, ethane concentrations increase progressively, whereas propane to pentane are only detected in insignificant amounts. At ~360 mbsf, abrupt increases in C₂–C₅ gas concentrations correlate with an increase in ammonium concentrations observed in the pore fluids. Another peak in the heavier hydrocarbon gases occurs at 440 mbsf, and both C₂–C₅ maxima indicate the presence of thermogenic gases at and below 360 mbsf.

The second zone occurs between ~500 and 800 mbsf and is characterized by a broad zone of low Ca, salinity, Mg, ammonium, and alkalinity concentrations. The K concentration-depth profile, however, shows a steady decrease in this interval, which is similar to the gradient observed in the upper 500 m of the sediment section. The lowest C₁/(C₂ + C₃) values observed at this site occur in the zone between 598.49 and 656.55 mbsf, in which methane concentrations are between ~3000 and 6000 ppmv. A strong peak in C₃–C₅ concentrations occurs at ~650 mbsf, coincident with the minima in Ca and salinity concentrations observed in the pore fluids. The broad decrease in the major elements and decrease in the ratio of methane to the heavier hydrocarbons correlates with lithostratigraphic Unit III, which is dominated by coarser grained sediments as well as several fault zones identified below ~600 mbsf. Generally, the inorganic and organic geochemical data suggest lateral and upward flow of a low-salinity (hence Cl) fluid with elevated concentrations of thermogenic hydrocarbons (up to iso-pentanes) and K. The geothermal gradient at Site U1379 is 40°C/km; thus, the temperature between 600 and 800 mbsf ranges from 24°–32°C. This temperature range is too low for the in situ production of thermogenic hydrocarbons or for extensive clay dehydration, suggesting the fluid sampled in Unit III is from a deeper source and is migrating laterally and upward along the permeable sand horizons and faults.

The third zone occurs from ~800 to ~900 mbsf (lithostratigraphic Unit III into Unit IV) and is characterized by a strong linear increase in Ca concentrations and a decrease in Mg concentrations. These trends suggest that the basement fluid is dominated by fluid-rock reaction with basalt in the basement. Pore fluids are also enriched in hydrocarbon gases in this interval, with methane concentrations ranging from ~1,000 to >10,000 ppmv and CH₄/(C₂H₆ + C₃H₈) ratios increasing from ~300 to 1400 at 800–947.22 centimeters below seafloor.

Results from LWD generally correspond to the observations as well as to the physical property data obtained from the cored sediments. The LWD tools deployed in the hole included the adnVISION 675 (density, neutron, and ultrasonic caliper), the TeleScope 675 (MWD; power and data transmission and drilling parameters), the arcVISION 675 (propagation resistivity, gamma radiation, and annular pressure), and the geoVISION 675 (resistivity images and gamma radiation). The measurements recorded by the LWD tools were downloaded and processed successfully, except for the geoVISION data. The Schlumberger logging engineers and the Schlumberger LWD data processing center in Houston, Texas (USA, were unable to recover useful geoVISION measurements.

Based on the LWD measurements, four logging units have been defined. Logging Unit 1 (0–492 mbsf) corresponds to a compacting sequence where density and resistivity increase and porosity decreases with depth, reaching nearly constant values of ~1.9 g/cm³, 1 Ωm, and 45% porosity at the base of the unit. The top of logging Unit 2 (492–600 mbsf) is marked by a small step increase in density and resistivity (~2 g/cm³ and 1.3 Ωm). The distinguishing feature of logging Unit 3 (600–892 mbsf) is the presence of many borehole enlargements, which are likely to correspond to intervals containing unconsolidated sands or fractured intervals. Logging Unit 4 (892–955 mbsf) corresponds to the basement rocks of the sedimentary sequence and is clearly identified by a sharp shift in the baseline of NGR, density, and resistivity logs. Compared to the sediments above, the basement unit shows a markedly higher average density and resistivity (2.3 g/cm³ and 2.5 Ωm) and lower porosity (~20%).

The adnVISION tool collected oriented images of bulk density and borehole radius. Despite its limited azimuthal resolution (image data are sampled in 16 azimuthal sectors, i.e., every 22.5°), the images clearly display vertical bands of large borehole radius in the interval 292–885 mbsf interpreted as borehole breakouts caused by differences in the principal horizontal stresses. The average azimuth of the breakouts is roughly north–south to north-northwest–south-southeast, indicating that the maximum horizontal stress is oriented east–west to east-northeast–west-southwest.

Site U1380

Site U1380 was drilled as the alternate site for Site U1378 with the objective to core enough sediment to stratigraphically correlate Sites U1378 and U1380 and to core as much basement in the remaining time window as possible. Site U1380 is on the middle slope of the Costa Rica margin above the unlocked portion of the plate boundary according to interplate earthquake relocation and geodetic measurements (Fig. F2). The margin here consists of upper plate basement underlying ~550 m thick slope sediments. The site was drilled to 397 mbsf, followed by coring to total depth at ~480 mbsf, where Hole U1380A was deteriorating, making it necessary to terminate, plug, and abandon the hole. Overall, we retrieved 52.37 m of sediment at this site.

Generally, the sediments cored in Hole U1380A were assigned to a single lithostratigraphic unit consisting of olive-green, terrigeneous, well-consolidated clayey silt(stone) and silty clay(stone) with minor layers of sand(stone), sandy silty clay(stone), clay, and clayey silt(stone) (Fig. F9). Intercalated in the main background sedimentation are decimeter-scale fining- and coarsening-upward sequences of sands as well as several tephra layers. The tephra layers range in thickness from 0.5 to 35 cm. Unconformable and/or inclined bedding of the tephra was observed throughout the entire cored material. In general, the tephra layers have a sharp basal sediment contact and a gradational top contact, mainly normally graded and well sorted. The tephra layers are mainly composed of felsic (>90 vol%), transparent, colorless, and very fine to coarse ash-sized glass shards with angular blocky, cuspate, flat, or Y-shaped morphology. The majority of the glass is altered, as shown by the undulous extinction under cross-polarized light. In the coarser sand layers, rip-up clasts, rounded clay lenses, and abundant shell fragments are commonly observed. Framboidal pyrite was observed in many of the smear slides throughout lithostratigraphic Unit I.

Structurally, the material cored at Site U1380 is characterized by shallow- to moderate-dipping bedding planes (mean = ~40°) and two fault zones. The fault zones encountered between 407–419 and 454–477 mbsf show relatively intense fracturing and brecciation.

The physical property data obtained from the cored material display expected behavior with depth and reflect that only a single sediment unit, as well as a limited depth interval, was cored at this site (Fig. F9). Wet bulk densities determined from whole-round GRA measurements are relatively constant throughout the cored interval, with a mean density value of 1.87 ± 0.05 g/cm³. Grain densities determined by mass/volume measurements on discrete samples are also relatively constant with depth, with an average value of 2.67 ± 0.08 g/cm³; however, the variability increases with depth. These values suggest a terrigenous origin of the deposited material. Porosities obtained by mass/volume measurements on discrete samples using moisture and density (MAD) method C are also relatively constant through the cored interval, with a value of 49%. The magnetic susceptibility measured in the sedimentary sequence is also relatively homogeneous and low, indicating an abundance of noniron-bearing clays and scarcity of magnetic minerals. The thermal conductivity is relatively constant throughout the cored interval, with a mean of 1.05 ± 0.22 W/(m·K).

The biogenic components observed at this site are similar to those observed in lithostratigraphic Unit II at Site U1378. Biogenic components have a bimodal distribution in Unit I: shell fragments, diatoms, and nannofossils are sparse but ubiquitous throughout the unit, whereas foraminifers are partly enriched within the sediments and are a major constituent of the sand-sized fraction. Preservation ranges from poor to moderate. Based on nannofossil biostratigraphy (Zone NN19) the sediments retrieved from the basal core were deposited in the early Pleistocene; thus, the sediments cored at Site U1380 are younger than 2 Ma. However, the top and bottom boundaries of Zone NN19, defined by the LOs of P. lacunosa and D. brouweri, respectively, could not be constrained. The dominant species observed at this site include H. carteri, H. sellii, Helicosphaera neogranulata/hyalina, Coccolithus pelagicus, and C. leptoporus.

Planktonic foraminiferal zones could not be established in the sediments cored at this site because of their rare occurrences. Benthic foraminifers, on the other hand, reflect continuous paleoenvironmental changes. Planktonic foraminifers were observed in limited horizons of six core catcher samples and are characteristic of tropical fauna (G. quadrilobatus [G. sacculifer]), G. ruber, O. universa, G. menardii, and N. dutertrei). The benthic foraminifers observed at this site are similar to modern assemblages, which is characteristic for the oxygen-minimum zone in this region. Thus, benthic foraminiferal faunas mainly represent bathyal species. The faunas include many species similar to those of nearby Site U1378. In general, the following species have been observed: Bolivina argentea, Epistominella smithi, Uvigerina peregrina, Cibicidoides mckannai, and Gyroidina. These species are characteristic of upper to middle bathyal paleoenvironments.

Paleomagnetic investigation revealed behavior similar to that Site U1378. Stepwise AF demagnetization was performed on all archive-half sections (to 15 mT) and discrete samples taken from the working half. The sections and samples are severely affected by drilling-induced remagnetization, and NRM intensities are comparable to those of Site U1378. The number of discrete samples analyzed so far is too limited to confidently define the magnetic polarity for this site. Thermal demagnetization experiments on three discrete samples revealed low unblocking temperature of NRM (~60% loss by 100°C), suggesting that the main NRM carriers are Ti-rich titanomagnetite, maghemite, or goethite.

The geochemical trends displayed by the analyzed pore water and gas samples (10 whole rounds, 9 headspace samples, and 8 void gas samples) mainly follow the same general trend with depth as the fluids observed at Site U1378 (Fig. F9). Salinity, Cl, Na, H₄SiO₄, and B concentrations show a monotonic decrease with depth. This indicates diffusional communication with fluids at depth. The low chloride concentrations (380 mM) and low salinity (20 mM), ~60%–67% of seawater, reflect significant freshening of the fluid. Methane (CH₄) concentrations range between 3,346.67 and 11,925.59 ppmv in interstitial waters from sediments sampled between 398.1 and 475.1 mbsf in Hole U1380A. The gas at these depths is thermogenic in origin, as indicated by the low ratios of methane to heavier homologues (ethane and propane), with CH₄/(C₂H₆ + C₃H₈) values from 458.78 to 551.80. The C₂H₆ concentrations range between 3.89 and 15.69 ppmv, with the highest concentration at 475.1 mbsf. Propane is also present in the cores, with concentrations ranging from 1.39 to 7.33 ppmv at depth. Iso- and n-butane were detected from 398.1 to 475.1 mbsf, with concentrations between 0.55 and 3.45 ppmv. Iso-pentane concentrations ranged from 0.64 to 2.34 ppmv. Propane, butane, and pentane concentrations were all highest at 475.1 mbsf.

A clear offset in the Ca, Mg, Sr, K, ammonium, and alkalinity profiles occurs from ~400 to 500 mbsf between Sites U1380 and U1378. Ca and ammonium concentrations at Site U1380 are lower than at Site U1378, whereas K, Mg, Sr, and alkalinity concentrations are higher at Site U1380. The Ca, Mg, and K concentrations were determined twice by inductively coupled plasma–atomic emission spectroscopy at both Sites U1380 and U1378, and the ammonium and alkalinity concentrations were determined by different methods; thus, the offset is not an analytical artifact. Site U1380 is ~1 km northeast of Site U1378, and the lithostratigraphy at Site U1378 is only partially represented at Site U1380. The interval between ~400 and 500 mbsf at Site U1378 contains more and thicker sections of coarser grained sediments than at Site U1380, indicating that these horizons are either not present at Site U1380 or were not recovered and sampled. It is likely that these coarser grained horizons are the conduits for the laterally migrating fluids observed at Site U1378, which may explain why the geochemical profiles at Site U1380 show a steady increase (Ca and ammonium) or decrease (Mg and K) through the cored depth and do not display the marked anomalies observed at Site U1378.

Site U1381

One of the primary objectives of Expedition 334 was to determine the characteristics of the downward-moving plate entering the Costa Rica Subduction Zone. Fundamental to this objective is an understanding of the nature and hydrologic system of the igneous section entering the zone. In this context Site U1381 serves as a reference site. Site U1381 is also situated on BGR99 Line 7, offshore Osa Peninsula and Cao Island. Site U1381 is on a basement-relative high at common midpoint 5750 (8°25.7150′N, 84° 9.4690′W) at 2067 m below sea level (Fig. F2). The fundamental difference between this site and the other sites drilled during this expedition is that it is situated on the incoming Cocos Plate outboard of the Middle American Trench. The seismic reflection profile indicates that the basement sediment cover is ~120 m thick at this site and that it most likely consists of pelagic and hemipelagic sediments. This section disconformably overlies highly reflective basement interpreted as the igneous crust of the Cocos Ridge. Two holes were drilled at this site, Holes U1381A and U1381B. Hole U1381A was drilled to recover as much of the sediment cover and basement rock as possible in the specified time window. Hole U1381B was drilled to retrieve the uppermost 30 m of sediment for detailed geochemical sampling and to take five in situ temperature measurements to determine the geothermal gradient and heat flux at this site. Overall, 54.15 m of sediment and 35.69 m of basement were retrieved at this site, with average recovery rates of 42.30% and 54.82%, respectively.

The material cored in Hole U1381A can be divided into eight lithostratigraphic units (Fig. F10). Unit I, ~46.14 m thick, consists mainly of light greenish gray soft clay sediments, with minor layers of silty clay, and three tephra layers, ranging in thickness from 2 to 4 cm. In general, Unit I is massive with minor changes in the proportions of clay and silt. Biogenic components, especially nannofossils and diatoms, are abundant throughout the unit. Foraminifers, spicules, and radiolarians are present in trace abundances. Smear slide investigations show that the main accessory components observed in this unit are silt-sized grains of feldspar, chert, chlorite, pyroxene, amphibole, opaque minerals, calcite, glauconite, fragments of radiolarians, foraminifers, sponge spicules, glass, and rare quartz. Unit II is ~49.64 m thick and consists of mainly dark grayish to yellowish brown soft to hardened clay/(stone) with abundant intercalated tephra layers. The base of the sediment cover is partly silicified. Unit II is clearly distinguishable from Unit I by its abundant biogenic components and by an abrupt color change. Unit II sediments are >70% spicules, diatoms, radiolarians, and nannofossils. The dominantly felsic tephra layers range in thickness from 0.5 to 35 cm, are massive to soft, show normal gradation, and are well sorted. One notable exception is a 35 cm thick silicified mafic tephra layer in interval 334-U1381A-7R-1, 92 cm, through 7R-2, 9 cm, that shows parallel and cross lamination. With the exception of one tephra layer (interval 5R-5, 29–34 cm), all other tephras show devitrification structures within the glass shards and severe signs of alteration. Grain size ranges from very fine to coarse ash (up to millimeter size). The mineral assemblages observed in the tephra layers consist of plagioclase, pyroxene (hypersthene and augite), hornblende, and biotite. Bedding dips, identified at compositional boundaries or grain-size differences, of the entire cover sediment sequence are almost horizontal (mostly ≤5°). The igneous basement starts with a 29 m thick sequence of highly plagioclase phyric to glomerophyric pillow basalts separated by a set of chilled margins (Unit III). At the bottom of Unit III, there is a calcareous claystone (Unit IV) with a recovered thickness of 0.24 m. Unit V is composed of a 6 m thick sequence of aphyric to moderately plagioclase phyric pillow basalts. The contact to the underlying coarser grained basalts (Unit VI) is not clearly defined and is represented by a transitional zone of intercalated pillow basalts and coarser grained basalts. Unit VI consists of a 27 m thick sequence of coarser grained aphyric to moderately plagioclase phyric basalts. At the bottom of these relatively monotonous basalts is a clastic magmatic horizon 0.27 m thick mainly containing basalt and plagioclase clasts in a fine-grained to microcrystalline matrix (Unit VII). Unit VIII below this horizon consists of another plagioclase phyric pillow basalt unit with a chilled margin on top. We recovered 4 m of this unit before drilling stopped at 165 mbsf.

Structures of the sedimentary sequence cored at this site are characterized by gently dipping (mostly ≤5°) bedding planes, whereas structures of the cored basalt are characterized by fractures with varying dipping angles of 19°–86° partly filled with vein minerals. No brittle deformation such as faults and fractures was recognized in the sediment cores.

The physical property data obtained from the cored material are variable, which is consistent with the different lithologies cored at this site (Fig. F10). Wet bulk densities determined from whole-round GRA measurements are relatively constant throughout the cored sediment section at this site, with a mean density value of 1.40 ± 0.14 g/cm³. GRA-derived bulk densities of the basement are highly variable, because of variable filling of the core liner, with a maximum value of 2.3 g/cm³. Grain densities determined by mass/volume measurements on discrete samples of the cored sediment, although showing a large scatter, generally decrease with depth from ~2.7 to 2.5 g/cm³. Porosities obtained by mass/volume measurements on discrete samples using MAD method C are relatively constant through the cored sediment interval at 76%. Generally, porosity is expected to decrease with depth; the observed constant values could be an artifact caused by the RCB coring system. The magnetic susceptibility measured in the sedimentary sequence is low, with a mean value of 0.009 ± 0.016 SI. The magnetic susceptibility measured in the basement rocks is generally higher, increasing from the sediment/basement interface to a 140 mbsf followed by a slow decrease toward the bottom of the hole. The thermal conductivity is relatively constant throughout the cored interval, with a mean of 0.79 ± 0.08 and 1.45 ± 0.07 W/(m·K) in sediment and basement, respectively. These values are quite low for basalt and might be an artifact of the samples not being water saturated before measurement because of time constraints. Downhole equilibrium temperatures acquired using the SET increase linearly with depth and give a least-squares geothermal gradient, coupled with the average bottom water temperature, of 222°C/km. The heat flow calculated using the mean thermal conductivity of 0.8 W/(mK) is 178 mW/m². This value is significantly larger than the half-space prediction for 15 Ma crust (130 mW/m²) and larger than the observed global average heat flow for crust of this age (77 mW/m²) (Stein and Stein, 1992). This high heat flow value is an indicator for significant fluid flow within the underlying crust.

The nannofossil and foraminiferal community observed at this site provide significant biostratigraphic control of the cored sediment sequence above the basaltic basement of the Cocos Ridge and reflect the sedimentological differences described above. Based on microfossil biostratigraphy, the sedimentary layers are tentatively divided into an upper part of Pleistocene age and a lower part of middle Miocene age. Thus, the sediments just above the basement are tentatively estimated to be of middle Miocene age. The sediments just above basement basalt would be younger than 16 Ma. The zonation of planktonic foraminifers is approximately concordant with that of the calcareous nannofossils.

Two different environments, based on species, abundance, and preservation, are represented in the cored sediments at Site U1381. The upper interval (3.69–49.62 mbsf) represents a hemipelagic environment mixed with terrigenous material, whereas the lower interval (53.86–95.5 mbsf), a silicic to calcareous ooze, represents a pelagic environment. The nannofossil assemblage observed in the upper 31.9 mbsf is characteristic of lower Pleistocene Zones NN20–NN19 and contains G. oceanica, G. caribbeanica, H. carteri, and C. leptoporus. However, because of poor preservation and the lack of zonal markers, this interval cannot be biostratigraphically zoned. The interval to 41.5 mbsf is tentatively assigned to nannofossil Zone NN19 based on the occurrence of P. lacunosa and the absence of D. brouweri. However, the top boundary, defined by the LO of P. lacunosa, is undetermined. Sample 334-U1381A-6R-CC (49.62 mbsf) contains a diverse nannofossil assemblage of mixed ages ranging from the Pleistocene Zone NN19 into the upper to middle Miocene. This diverse assemblage is dominated by Pleistocene species, including G. oceanica, G. caribbeanica, H. carteri, and C. leptoporus. Also present, but rare to few in abundance, are Miocene species including Discoaster bellus, Discoaster exilis, Discoaster quinqueramus, Discoaster variabilis, and unidentifiable five- and six-rayed discoasters. The discoasters are poorly to moderately preserved, whereas the placoliths exhibit moderate to good preservation. The condition of the discoasters and the rarity or lack of biostratigraphic markers prevents the further delineation of the Pliocene and Miocene zones.

The lower interval between 53.86 and 95.5 mbsf is assigned to middle Miocene Zone NN5 based on the occurrence of Helicosphaera heteromorphus and the absence of Helicosphaera ampliaperta. The top and bottom of this zone cannot be constrained because of the uncertainty of the LOs of biostratigraphic markers. Typical species found in the samples include Sphenolithus heteromorphus, Sphenolithus moriformis, C. leptoporus, Cyclicargolithus floridanus, Coccolithus miopelagicus, D. exilis, D. variabilis, Discoaster deflandrei, and Reticulofenestra pseudoumbilicus.

Planktonic foraminifers were analyzed in nine core catcher samples. Foraminifers are abundant to common in the sediments of Hole U1381A. Preservation is good to moderate. Fragmentation of foraminifers caused by carbonate dissolution is observed in the samples of sediments from lower bathyal depths. Planktonic foraminifers, abundant to common in this hole, are much more abundant than benthic foraminifers. These trends are quite different from the trends observed in the cored sediments of the other sites. Similar to the observed nannofossil communities in this hole, the foraminiferal assemblages of the upper part of the sediment sequence are quite different from those of the lower parts. This is either caused by a hiatus or by very low sedimentation rates. The foraminiferal community of the upper sediment sequence (3.69–49.62 mbsf) is characterized by tropical fauna (G. quadrilobatus [G. sacculifer]), G. ruber, O. universa, G. menardii, and N. dutertrei). Sample 334-U1381A-3R-CC (13.34 mbsf) contains G. ruber (pink) and is assigned to the Pleistocene (older than 0.12 Ma). Sample 6R-CC (49.64 mbsf) contains sinistral coiling Pulletiatina and is assigned to be older than 0.8 Ma but younger than 4 Ma. From 53.86 to 95.55 mbsf, planktonic foraminiferal assemblages are composed of Dentoglobigerina altispira, G. quadrilobatus, Globigerinoides obliquus, Globoquadrina dehiscens, Globorotalia peripheronda, Globorotalia peripheroacuta, Paragloborotalia siakensis, and Orbulina suturalis. This sequence is tentatively assigned to planktonic foraminiferal Zone M7 (14 Ma). However, the occurrence of Praeorbulina circularis at ~95.55 mbsf may be a sign that the sediments just above the basement basalt are much older (either Zone M5 or M6).

Paleomagnetic experiments with stepwise AF demagnetization were performed on all archive-half sections and discrete samples taken from the working half. Archive-half sections were demagnetized to 15 mT for the sediments and 10 mT for the basalts. For the sediments, NRM intensity is too weak for the shipboard experiments to determine characteristic remanent magnetization (ChRM). For the basalts, the majority of the measured samples revealed a ChRM with shallow and positive inclination. Several intervals reveal a NRM with negative inclinations, possibly reflecting prolonged igneous activity.

The NRM intensity of the basalts ranges from 1 to 8 A/m. The samples from the top of the basalt section (Cores 334-U1381A-12R through 16R) frequently show NRM with shallow inclination, consistent with the low latitude, and strong intensity. The samples below Core 16R show NRM with steeper inclination of ~50°, indicating a stronger effect from drilling-induced remagnetization, and lower intensities. Thermal demagnetization experiments on three discrete samples revealed that blocking temperature of NRM is 550°–580°C, suggesting that the main NRM carrier is titanomagnetite with a low Ti content.

The geochemical trends displayed by the analyzed pore water samples (17 whole rounds) are generally controlled by organic carbon cycling, alteration of volcanic ash, and diffusive exchange with basement.

Salinity is lower than seawater value between 13 and 24 mbsf but increases gradually below this depth to the base of the sediment column (Fig. F10). Chloride concentrations are slightly below modern seawater value. A similar dilution of Cl concentrations of ~2.5% lower than modern seawater was observed in the upper 100 m of the sediment column cored at the reference site (ODP Site 1039) offshore the Nicoya Peninsula (Kimura, Silver, Blum, et al., 1997). Sodium concentrations are below seawater value throughout the cored section and reach a minimum at 35 mbsf. Potassium and Mg show similar decreases at this depth, suggesting local alteration of volcanic ash that is consistent with lithostratigraphic observations. Sodium concentrations are relatively constant below this depth. Sulfate concentrations decrease to a minimum at 23 mbsf and increase nearly linearly with depth. The alkalinity concentration-depth profile is a mirror image of the sulfate profile, reaching a maximum at 23 mbsf and decreasing toward the base of this hole. Organic matter diagenesis in the upper part of the sediment section is also observed in the ammonium profile, which reaches a maximum value at 23 mbsf. Ammonium concentrations remain nearly constant to 35 mbsf and decrease nearly linearly. Calcium concentrations reach a minimum value just below the sulfate minimum at 23 mbsf, suggesting precipitation of authigenic carbonates in the zone of active sulfate reduction and alkalinity production. Below this depth, calcium concentrations monotonically increase toward the base of the sediment column, which most likely reflects diffusive exchange with a basement fluid.

Potassium and B concentrations decrease gradually with depth and reach minimum values at the base of the sediment cover, whereas dissolved Mn concentrations increase with depth, reaching maximum values at the base. Magnesium concentrations exhibit a minimum at 35 mbsf, which is consistent with the abundant alteration of the volcanic tephra in this interval. Magnesium concentrations remain relatively constant between 45 and 81 mbsf and decrease again at the sediment/basement interface. Dissolved Si concentrations are elevated throughout the cored section, and the profile is primarily controlled by lithology. Si concentrations increase slightly from 13 to 46 mbsf and increase abruptly from 46 to 68 mbsf, reflecting the change from the clay-dominated terrigeneous sediments in the uppermost sediment column to hemipelagic sediments dominated by calcareous nannofossils and diatoms below. Silica concentrations remain elevated and constant throughout the hemipelagic nannofossil ooze section. Strontium concentrations increase slightly with depth, reaching a maximum at the base of the sediment section. The slight increase in Sr concentrations in the calcareous nannofossil-rich sediments suggests they are relatively unaltered and have not undergone significant diagenetic modification. This interpretation is corroborated by the pristine appearance of the nannofossils within these sediments.

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