167 Preliminary Report

Site 1013 (Proposed Site BA-2B)

Site 1013 is located in San Nicolas Basin, within the middle band of basins of the California Borderlands in water depth of 1575 m (Fig. 1). The primary objective of drilling at this site was to sample a high-resolution section from the early Pliocene to Quaternary to study the evolution of the California Current system and to study oceanographic processes in intermediate waters as Northern Hemisphere glaciations expanded. Site 1013 will also provide information about organic carbon diagenesis and about minor-element geochemistry through pore-water profiles and through solid-phase analyses. Because of its location, away from the turbidites that fill the inner Borderland basins, we expect most of the organic matter in the basin to be marine in origin.

The sedimentary sequence recovered from the three holes at Site 1013 consists of an apparently continuous, 146-m-thick interval of upper upper Pliocene (2.7 Ma) through Quaternary sediments. Sediments gradually change from mixtures of siliciclastic and biogenic components to mixtures of biogenic and minor siliciclastic components. Interbedding of the sediments is on a scale of less than a meter to several meters. Siliciclastic clay and silt are found throughout the cored interval, but strongly decrease downhole. Calcareous nannofossils and to a lesser extent foraminifers strongly increase downhole, and dominate the calcareous fraction of the sediments. The biosiliceous component is negligible. Thin terrigenous siliciclastic sand layers occur in the upper part of the sequence and distinct ash layers throughout the lower part of the sequence.

Detailed comparisons between the magnetic susceptibility 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 97 mbsf.

Calcareous nannofossils are abundant throughout and preservation is moderate to good. Planktonic foraminifers are abundant and well preserved in the Quaternary, and abundant to rare and moderately well preserved in the uppermost Pliocene. Radiolarians and diatoms are absent in the sequence except for conspicuous reworking of middle Miocene species. Reworked calcareous nannofossils of middle Miocene and Eocene age occur in the Quaternary. A well-constrained biostratigraphy and chronology is provided by calcareous nannofossil and planktonic foraminifer datums for the upper part of the Pliocene and Quaternary.

Latest Pliocene through Quaternary planktonic foraminifer assemblages indicate large-scale oscillations in sea-surface temperatures associated with glacial/interglacial episodes. Benthic foraminifer assemblages suggest relatively low oxygen concentrations in the basin during the late Neogene. Near suboxic to suboxic basinal conditions occurred during the latest Pliocene to earliest Quaternary.

AF demagnetization at 20 and 25 mT revealed a complete magnetostratigraphic record between 0 and 95 mbsf. The Brunhes (C1n), the Jaramillo (C1r.1n), possibly the Cobb Mountain, and the top of the Olduvai (C2n) normal polarity intervals were identified. An age-depth plot based on the reversal boundaries gave a sedimentation rate of 65 m/m.y around the Jaramillo and a lower sedimentation rate below the Cobb Mountain.

Headspace volatile hydrocarbons rapidly increase at about 20 mbsf and stay consistently high throughout the sedimentary column. Methane to ethane ratios, however, are in the normal range of biogenic methanogenesis. The calcium carbonate record from 0 to 50 mbsf shows slightly increasing values between 5 and 30 wt% with a low fluctuation. Below 50 mbsf, the fluctuation increases to about 40 wt%, and highest values occur in the deepest part of the hole. Organic carbon ranges from 1 to 6 wt% (to a maximum of 9 wt%). The ratios of organic carbon to total nitrogen range from 8 to 12, indicating a marine provenance of the organic matter at this site.

The interstitial water geochemistry (Fig. 4) reflects the influence of organic carbon diagenesis by sulfate reduction, of biogenic opal dissolution, and of possible authigenic mineralization reactions. Dissolved sulfate reaches concentrations <1 mM by 19 mbsf. Alkalinity increases to as high as 60 mM, dissolved phosphate to 150 µM, and ammonium to 13 mM. Opal dissolution is indicated by the increase of dissolved silicate to values >1000 µM by 76 mbsf. Nonconservative profiles of calcium and magnesium suggest the importance of authigenic mineralization.

Physical properties show very little variation downhole corresponding to the sedimentological findings. The few variations most likely correspond to fluctuating amounts of clay and carbonate. Three downhole temperature measurements were taken using the Adara tool, and gave a geothermal gradient of 72°C/km (Fig. 5). The heat-flow estimate at Site 1013 is 65 mW/m2.

The ODP Digital Color Video images correlate very well with those obtained using the Oregon State University Color Reflectance tool. Additionally, there appears to be a relationship between color and discrete index properties, in particular, density. This is probably a result of the color variations associated with carbonate concentration, and the strong correlation between density and carbonate content. Reflectance data were used to predict high-resolution carbonate concentrations in real time.

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