167 Preliminary Report

Site 1010 (Proposed Site CA-14A)

One of the primary objectives of Site 1010 (Fig. 1) was to provide chronological control for biostratigraphic events in the California Current region through the middle Miocene. Because subtropic and subarctic flora and fauna mix along the coast of California, the detailed biostratigraphy fits imperfectly with schema developed for either the tropics or the subarctic North Pacific. A second important goal is the Neogene paleoceanography of the southern region of the California Current. In the modern oceans, the California Current can be distinguished by its temperature and salinity characteristics to the southern tip of Baja California. Site 1010 provides a means to monitor this southern region because it is located underneath the approximate core of the current. Site 1010 will also be used to study organic matter diagenesis. In addition, high-resolution stable isotope profiles of intersitial water will be used to study ice volume at the last glacial maximum. A secondary goal was to obtain a representative sample of the basaltic basement for igneous petrology and geochemistry.

The sedimentary sequence at Site 1010 (Fig. 2) consists of an apparently continuous, 185-m-thick interval of Quaternary to middle Miocene sediments. They are divided into four lithologic units. Unit I (0-18.5 meters below seafloor [mbsf]) consists of siliciclastic sediments (silty clays and clayey silts) with abundant vitric ash layers and disseminated volcanic glass throughout. The top of Unit II (18.5-66.0 mbsf) is defined by increased biogeneous sediment components and consists of interbedded units alternating between nannofossil ooze and silty clay. Volcanic ash layers (both vitric volcanic ash and altered) are abundant. Unit III (66.0-178.0 mbsf) represents increased siliceous components interbedded throughout with clay and nannofossil sediments. Interbedding occurs on a scale of 20-80 cm. Unit IV (178.0-basement) is composed of alternating porcellanite, nannofossil chalk, and clay. The tagged basement consists of basalt.

Core-core correlation between the 5 holes successfully established a continuous composite record down to 110 mbsf. More detailed work is in progress for cores below this depth. Scatter in bulk density values determined from discrete sediment samples matches the bulk densities obtained using the gamma-ray attenuation porosity evaluator (GRAPE) well. Velocities increase slowly downhole with very slight variation. High velocities correspond to low porosity values.

The section includes an upper 60-m-thick sequence containing variable but often abundant planktonic foraminifers and few diatoms, radiolarians, and calcareous nannofossils from the Quaternary through the lowermost Pliocene and uppermost Miocene (Fig. 2). This is underlain by a 70-m-thick sequence of late Miocene to late middle Miocene age marked by an almost complete absence of planktonic foraminifers and generally uncommon calcareous nannofossils, diatoms, and radiolarians. This is underlain by a 55-m-thick sequence of rapidly deposited diatom ooze of middle middle Miocene age (base: diatom Denticulopsis hyalina Zone, 13.1-13.9 Ma; Nannofossil Zone CN4, 13.6 to 15.8 Ma). These diatom oozes contain assemblages of diatoms, radiolarians, and planktonic foraminifers indicative of extensive upwelling of cool waters associated with the California Current. The radiolarian assemblages are the best-preserved Miocene examples from an intense upwelling province, and include numerous new species never observed before. Likewise, the planktonic foraminifer assemblages provide a rare insight into the characteristics of faunas associated with intense Miocene upwelling. The sequence of changes in planktonic foraminifers suggests the need to establish a new zonation for the southern California Current system.

A complete magnetostratigraphy was determined from Holes 1010C and 1010E (Fig. 2). All chrons from the Brunhes to the top of Chron C3Bn (7 Ma) could be identified in the upper 75 mbsf. The age-depth plot based on magnetic reversals shows three linear segments, representing 3 different sedimentation rates in the upper 75 mbsf. Below 75 mbsf, the intensity of magnetization decreases. The drilling-induced remanent magnetization dominates between 75 and 160 mbsf and prevents the identification of polarity reversals.

No significant amounts of gas were measured in the sediment column. Carbonate contents vary strongly between 0 and 80 wt% in the upper 110 mbsf. Below this depth, carbonate contents are generally higher than the upper section because of higher nannofossil concentration. Organic carbon values decrease with depth from about 0.4 to less than 0.2 wt%.

Interstitial water samples for shipboard analysis were taken 1 per core for the first 10 cores and then 1 every third core to about 170 mbsf. A high-resolution suite of samples was also taken from one hole at 1/section spacing (about 1.5 m) for shorebased study. The interstitial water geochemistry is typical of an open ocean site, showing the influence of reactions in the underlying basalt via diffusion, of the relatively low organic carbon content of the sediments, and of the dissolution of biogenic silica. Chloride increases slightly with increasing depth, from 559 to 569 mM at around 80 mbsf, then decreases again. Calcium increases with increasing depth, with an average gradient of 4.2 mM/m, and magnesium decreases with increasing depth, with an average gradient of -3.3 mM/m. Calcium and magnesium are linearly correlated with each other. Alkalinity increases only slightly to values over 4 mM at depths of 20-60 mbsf. Sulfate indicates a minor amount of organic carbon oxidation via sulfate reduction, with values between 24 and 29 mM throughout the section. Phosphate has elevated values around 6 µM in the upper 10 m, with values between 2 and 3 µM at greater depth. Ammonia increases with depth to values around 130 mM. Silicate increases with increasing depth to values greater than 1000 µM by 75 mbsf.

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