167 Preliminary Report
Site 1017 is located about 50 km west of Point Arguello on the continental slope just south of Santa Lucia Bank in water 966 m deep (Fig. 1). It is the shallow-water site in the Conception Transect (35°N). The major objective for drilling was to sample a high-resolution, upper Pliocene to Holocene sediment section to compare with Santa Barbara Basin (Site 893). Site 1017 is near an important upwelling center off Point Conception and will be used to define the history and cyclicity of upwelling near 35°N. The oxidation-reduction history at this site will be affected by changes in the oxygen minimum depth through time. Waters drawn into Santa Barbara Basin also come from this vicinity, and data from Site 1017 should confirm whether Santa Barbara Basin oxygenation is being driven by changes in intermediate-water source. The site will also provide important new information about organic carbon diagenesis and about minor-element geochemistry through pore-water profiles and through solid-phase analyses. Pore-water sampling will be used to define the rates of organic matter oxidation and the removal of oxidants from the pore waters and sediments, whereas organic geochemical analysis will provide data on organic matter preservation in a low-oxygen environment.
The sedimentary sequence recovered from the five holes at Site 1017 consists of a well-dated, apparently continuous, 204-m-thick interval of Quaternary (1.2-1.4 to 0.0 Ma) age. The sediments consist almost entirely of silty clay to clayey silt with minor, variable quantities of intermixed foraminifers, nannofossils, and siliciclastic sand. Thin, discrete layers of quartzofeldspathic or foraminiferal sand turbidites are a frequent, but volumetrically minor, component of the upper two thirds of the sequence. Cemented limestone and dolostone occur at only a few horizons. Bedding is indistinct, gradational, and very thick (about 1 cycle/10 m) until becoming more distinct and thinner (several meter scale) below approximately 125 mbsf. The stratigraphic sequence is grouped into a single lithologic unit, with two subunits. Subunit IA is composed of silty clay to clayey silt. Subunit IB is composed of silty clay to clayey silt, and nannofossil clay mixed sediment, all containing minor amounts of foraminifers and diatoms. Sedimentation rates are around 100-120 m/m.y. on average.
Detailed comparisons between the magnetic susceptibility and GRAPE density records generated using the MST, and high-resolution color reflectance measured using the Oregon State University system, showed that a continuous sedimentary sequence may exist down to about 130 mcd. However, exact tie points between adjacent holes could not be established in most cases and precluded the construction of a spliced section.
Calcareous nannofossils are of highly variable abundance and quality of preservation throughout the sequence. The section above 175 mbsf is marked by mostly abundant to common and well-preserved planktonic and benthic foraminifers. Below 175 mbsf to the base of the hole, the section is essentially barren of planktonic foraminifers and benthic foraminifers are either absent or occur in low abundances. Diatoms are almost exclusively limited to reworked forms and radiolarians are absent to rare in the sequence. Changes in planktonic foraminifers indicate strong glacial to interglacial oscillations throughout. Overall, both interglacial and glacial planktonic foraminifer assemblages reflect relatively cooler conditions than in all earlier drilled sequences of Leg 167. This is almost certainly because of the sites location in the heart of the coastal upwelling zone off Point Conception.
Benthic foraminifer assemblages exhibit large differences between glacial and interglacial episodes. Assemblages associated with glacial episodes reflect relatively higher oxygen concentrations of bottom waters, whereas those associated with interglacial episodes reflect distinctly lower oxygen concentrations in bottom waters, as in Santa Barbara and Tanner Basins.
Other materials observed in the sand-sized fraction of many of the core-catcher samples include prominent fish debris, including fish scales, charcoal fragments, sponge spicules, and echinoid spines. Also observed were large numbers of tar (asphalt) globules that have glued together a variety of biogenic materials, especially sponges spicules. The tar was almost certainly derived from natural oil seeps in the region during the entire Quaternary.
After AF demagnetization, the magnetization of most cores was just around the sensitivity limit of the magnetometer. With the exception of one single section, there was no interval of reverse polarity in Hole 1017B. The positive inclinations of the top 110 mbsf represent most likely the Brunhes Chron C1n. Below the normal polarity interval, an interpretation of the inclination record was not possible because of the low magnetic intensity and core disturbance by XCB coring.
Sediments at Site 1017 are characterized by carbonate values ranging from 2 to 12 wt%. At 130 mbsf, an increase in the concentration and fluctuation can be observed. The organic carbon concentration shows a similar increase with depth from average values of about 1.5 to 2.7 wt% at 170 mbsf. The C to N ratio record displays a remarkably low fluctuation around the average value of 10. A good correlation between Uk¢37, C37 alkenone abundance, and total organic carbon content (Fig. 9) exists. Both C37 alkenone abundance and total organic carbon content show higher concentrations in warmer (interglacial) than in cooler (glacial) periods.
Volatile hydrocarbons are high throughout the sediment column, but no significant amounts of ethane could be detected, indicating that no thermogenic hydrocarbons occurs.
Chemical gradients in the interstitial waters (Fig. 4) reflect organic matter diagenesis, the dissolution of biogenic opal and calcium carbonate, and the influence of authigenic mineral precipitation reactions. Alkalinity increases to peak values >40 mM, whereas sulfate concentrations decrease to values below the detection limit (approximately 1.4 mM) by 19.25 mbsf. Phosphate concentrations increase to values >100 µM and ammonium concentrations increase to an average of 8.5 mM. Dissolved silicate increases to concentrations near 1000 µM, and strontium increases to >150 µM. Calcium concentrations decrease to around 2.4 mM, then increase with increasing depth to 5.3 mM. Magnesium concentrations generally decrease throughout the section.
Velocities shallower than 10 mbsf were high, ranging from 1586 to 1607 m/s. Below this depth, gas expansion attenuated the signal and precluded the determination of further velocity measurements. Downhole temperature measurements gave a thermal gradient of 74°C/km (Fig. 5). Using an average thermal conductivity of 0.937 W/(m-K) yields a heat-flow estimate of 70 mW/m2 at Site 1017.
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