167 Preliminary Report

Site 1016 (Proposed Site CA-11E)

Site 1016 is located about 150 km west of Point Conception, and forms the deep-water site on the Conception Transect (35°N). The site is located on a northeast trending abyssal hill and rises 50-100 m above the surrounding seafloor (Fig. 1) in water 3846 m deep. Basement is 22.5-Ma oceanic basalt. The site was chosen to provide material to investigate the longer term Neogene record as well as to assess paleoceanographic conditions near the core of the California Current. Site 1016 occupies an important transitional zone for modern flora and fauna, and provides a good opportunity to link magnetochronology and biostratigraphy. Geochemical indices of paleoproductivity and microfossil assemblages obtained from this site will provide important data on nutrients carried by the California Current. Organic carbon deposition should be relatively high compared to typical pelagic sedimentary sections, yet the sedimentation rate is low. This environment provides one of the end members needed to study preservation of bulk organic matter and of specific organic molecules.

The sedimentary sequence recovered from the 4 holes at Site 1016 consists of a well-dated, apparently continuous, 308-m-thick interval ranging from Quaternary to late Miocene age. Sediments are dominated by decimeter to meter scale alternations of carbonate and siliciclastic layers. Several fine-grained sand layers and volcanic ash bands, each up to several centimeters thick, occur in the upper two thirds of the sequence. The base consist of porcellanite and chert horizons of unknown thickness. The sediments are divided into three lithologic units. Unit I (0-71 mbsf) is characterized by the relative abundance of clay and the prevalence of diatom ooze with clay, diatomite, and diatoms with clay. Unit II (71-163 mbsf) contains an increased amount of calcareous nannofossils and is composed of nannofossil ooze with diatoms. This unit is subdivided in two subunits on the basis of carbonate content. Unit III (163-316 mbsf) is dominated by diatomite and diatom ooze, and contains several volcanic ash layers and blebs of solid bitumen. The base of this unit consists of porcellanite and black chert. Sedimentation rates are high, averaging 50 m/m.y. from the Quaternary to the upper Pliocene, are drastically lower during the early to middle Pliocene (10-15 m/m.y.), and average 30 m/m.y. in the late Miocene.

Detailed comparisons between the magnetic susceptibility and GRAPE density record generated using the MST, and high-resolution color reflectance measured using the Oregon State University system, demonstrated complete recovery of the sedimentary sequence down to 245 meters composite depth (mcd), with the exception of a core gap at 172.6 mcd, which could not be covered by overlap (Fig. 7).

A well-constrained biostratigraphy and chronology is provided by a combination of calcareous nannofossil, planktonic foraminifer, diatom, and radiolarian datums for the upper Pliocene and Quaternary. The upper Miocene to lower Pliocene (below 154 mbsf) is dated by calcareous nannofossils, diatoms, and radiolarians. The base of the sequence is late Miocene (less than 7 Ma) in age. Diatom and radiolarian assemblages suggest two major episodes of strong upwelling during the upper Miocene and the upper Pliocene through lower Quaternary. These two episodes are separated by an interval marked by decreased vertical advection of deep waters during the lower to middle Pliocene, resulting in relatively low sedimentation rate. Cooling at thermocline depths is suggested after 3.0 Ma by Arctic radiolarian assemblages. This was followed at 2.5 Ma by major surface-water cooling. Lower bathyal benthic foraminifer assemblages appear to change little throughout the upper Pliocene and Quaternary, including between glacial and interglacial episodes.

A magnetic polarity stratigraphy could not be obtained because magnetic intensities were below the noise level of the magnetometer.

Calcium carbonate values vary from 1 to 62 wt%. Between about 2 and 6 Ma (75 to 165 mbsf), the CaCO3 values are distinctly higher. Organic carbon concentrations are high compared to normal open ocean environments (average 0.93 wt%). According to organic carbon to total nitrogen ratios, the organic material is mainly of marine origin. Headspace methane values are very low throughout the sediment column, indicating that no significant methanogenesis occurred.

Chemical gradients in the interstitial waters reflect organic matter diagenesis, the dissolution of biogenic opal and calcium carbonate, the diffusive influence of reactions in the underlying basalt, and the influence of authigenic mineral precipitation. Alkalinity increases to peak values >17 mM, phosphate to nearly 60 µM, and ammonium to >2 mM. Calcium decreases with depth to as low as 5.2 mM, then increases to 13.2 mM. Magnesium decreases throughout the section to 26.1 mM, with the decrease in the lower part of the section linearly correlated to the increase in calcium.

The porosity profiles can be divided into three units corresponding to the three lithological units. The upper 100 mbsf with high porosities around 70%-75% in sediments composed of clays and diatomite, an interval of low porosities oscillating around 60%-70% from 100 to 200 mbsf, in carbonate-rich sediments, followed by an increase of porosity downhole with values between 70% and 80%, in the diatomite-rich unit. Highs in PWL velocity and GRAPE density correspond well with reflections on the 3.5-kHz seismic site-survey record (Fig. 8). The impedance contrasts that generate the reflectors correspond to the sandy turbidite layers in the upper 70 mbsf of the section.

Three good-quality temperature measurements were obtained: 4.9°C at 36.1 mbsf, 7.0°C at 55.1 mbsf, and 9.0°C at 74.1 mbsf. Using an average measured thermal conductivity of 0.838 W/(m-K) provides a heat-flow estimate of 88 mW/m2.

Major lithologic units were identified using color reflectance data. In Unit I reflectance for the 450-500 nm (blue) band is generally low. As Unit I grades into Unit II, the proportion of nannofossils increases, as does blue band reflectance. Subunit IIB, which is predominantly nannofossil ooze interbedded with diatom ooze, has the highest reflectance of any stratigraphic unit at Site 1016. The signal of Unit IIB is variable: high reflectance values generally match nannofossil-enriched layers, and low values match more diatomaceous zones. In Unit III, diatoms replace nannofossils as the dominant microfossil component, and reflectance is low. The near infrared (nIR; 850-900 nm) to blue (450-500 nm) ratio is generally greatest when the diatom content is greatest and lowest, where clays and nannofossils predominate. These results imply that color reflectance is sensitive to the spectral character of diatom-rich sediments, or a sedimentary component that covaries with the diatom content.

Logging at Hole 1016B consisted of two full passes with the Triple Combination

tool string, one full pass with the FMS-sonic tool string, and two full passes with the magnetic susceptibility-total moment tool string. Hole conditions were fair from 220 to 300 mbsf, and excellent above 220 mbsf and up to the base of pipe at about 60 mbsf. The log physical property data closely matched the measured core density, porosity, and magnetic susceptibility over the core-log data overlap. The log variations clearly delineate the major and minor lithologic boundaries, particularly the transition to diatomites. Initial log-core comparisons suggest that decimeter-scale variations in lithology are reliably recorded by the logging tools. This provides the opportunity to assess the degree of rebound and deformation of the core material, and will be especially useful for putting together continuous records even where material is missing at core gaps.

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