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.
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.
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.
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.