167 Preliminary Report
Site 1018 is located about 75 km west of Santa Cruz, California, on a sediment drift just south of Guide Seamount at a water depth of 2477 mbsl (Fig. 1). The primary objectives at this site were to sample a high-resolution upper Miocene to Holocene sediment section from the central California Margin to study evolution of the California Current, as well as the history of upwelling and productivity. It provides continuity between the Gorda Transect at 40° N and the southern California Transects. The site also provides a dipstick to sample midwaters during the time in which the Northern Hemisphere ice sheets were formed. It will also be used to collect new data on organic carbon diagenesis and minor-element geochemistry through pore-water profiles and solid-phase analyses. Organic carbon contents and terrestrial organic matter input should be moderately high. 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 underneath a well-oxygenated water column.
The sedimentary sequence recovered from the 4 holes at Site 1018 consists of a well-dated, apparently continuous, 426-m-thick interval of uppermost lower Pliocene to Quaternary (3.5-0.0 Ma) sediments. Sediments vary from siliciclastic to interbedded mixtures of biogenic and siliciclastic components. Siliciclastic clays are found throughout the cored interval, but are predominant in the upper part. The middle part is dominated by diatom clay and diatom clay mixed sediment with frequent interbedding of clayey nannofossil ooze, whereas the lower part is dominated by interbeds of nannofossil clay and clayey nannofossil chalk. Diatomaceous layers tend to correspond to darker and less bioturbated intervals compared to nannofossil-rich layers. Fine-grained feldspar quartz sand occurs as thin normally graded layers (possibly turbidites), especially in the upper part of the sequence, whereas glauconite occurs both as discrete layers and disseminated in the clay matrix in the lower part of the sequence. Vitric volcanic ash layers are rare except in the lowermost part of the sequence.
The sediments are divided into two lithologic units. Unit I is a siliciclastic unit composed mainly of clays with varying amounts of diatoms and sporadical occurrence of nannofossils. Unit II is characterized by continuous and slightly increasing content of nannofossils and slightly decreasing content of diatoms.
Detailed comparisons between the magnetic susceptibility and the 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 193 mcd. Sedimentation rates range from 100 to 200 m/m.y. and average 130 m/m.y.
A well-constrained biostratigraphy and chronology is provided by a combination of calcareous nannofossil, planktonic foraminifer, radiolarian, and diatom datums for the upper Pliocene and Quaternary. Planktonic foraminifers suggest that the base of the section is about 3.4 Ma in age. All microfossil groups are clearly dominated by cool, high-latitude elements throughout the late Neogene. Site 1018 is sufficiently far north in the California Current to exclude most to all subtropical elements even during interglacial episodes. Radiolarians are entirely represented by subarctic forms. Diatoms are dominated by subarctic forms with the addition of much less abundant temperate elements. Planktonic foraminifer assemblages are dominated by subarctic to cool temperate forms, and subtropical elements are absent, except during warmest interglacial episodes. Planktonic foraminifers exhibit glacial to interglacial oscillations throughout. Radiolarians, however, do not exhibit such changes, almost certainly because they largely live at greater water depths.
Changes in microfossil assemblages provide evidence of progressive cooling during the late Neogene. The first consistent occurrence of mostly common to abundant populations of sinistrally coiled Neogloboquadrina pachyderma marks a distinct cooling step at about 1.3 Ma. Likewise, changes in radiolarian assemblages indicate a further step towards cooler conditions at about 1.0 Ma. Diatoms are dominated by oceanic forms, but during the Quaternary include a small but distinct littoral assemblage that typically lives on sea grass. These forms were reworked from shallow waters, and appear to be most abundant during times of higher input of terrigenous sediments.
Positive paleomagnetic inclinations of the top 88 mbsf most likely represent 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.
Calcium carbonate contents are very low, ranging from 0 to 5 wt% in the upper 350 mbsf. Spikes of high values up to 17 wt% frequently occur. In the lower part of the column, CaCO3 concentrations distinctly increase to values between 5 and 25 wt%. Total organic carbon variation is very stable around an average value of 1.2 wt%. The organic material is mainly of marine origin, as indicated by low C to N ratios of about 6 to 8. This is also supported by very high methane concentrations throughout the sediment column. No significant amounts of ethane or propane were observed in the headspace samples.
Chemical gradients in the interstitial waters (Fig. 4) reflect organic matter diagenesis, the dissolution of biogenic opal and calcium carbonate, the influence of authigenic mineral precipitation reactions, and the diffusive influence of reactions in underlying basalt. Alkalinity increases to peak values >50 mM, whereas sulfate concentrations decrease to values below the detection limit (approximately 1.3 mM) by 9.35 mbsf. Phosphate concentrations increase to values >160 µM, and ammonium concentrations increase to maximum values >7 mM. Dissolved silicate increases to concentrations >1000 µM, and strontium increases to >250 µM. Calcium concentrations decrease to as low as 3.6 mM, then increase with increasing depth to 11.5 mM. Magnesium concentrations generally decrease throughout the section to values as low as 22 mM.
The porosity decreases downhole to 70 mbsf, scatters around a constant value until about 345 mbsf, and then continues to decrease downhole. This trend corresponds well with the lithological boundaries. The diatom-rich unit between 200 and 345 mbsf represents an interval of low grain densities. The increase of bulk and grain densities and decrease of porosity at 345 mbsf correspond to the increase in carbonate concentration in the nannofossil-rich unit. High-resolution opal content was predicted using a multiple linear regression equation generated from site-survey color reflectance and opal data (Fig. 10). Results are consistent with the major lithologic units, and indicate an average opal content of 1.96 wt%.
In situ temperature measurements at Site 1018 gave a thermal gradient of 32°C/km (Fig. 5). Using an average measured thermal conductivity of 0.847 W/(m-K) yields a heat-flow estimate of 27 mW/m2.
Logging conditions at Hole 1018A were poor. A ledge at approximately 220 mbsf was encountered, and one pass with the Triple Combination tool string was made from 220 mbsf to the mudline. Pipe was then set at 240 mbsf and a second pass with the Triple Combination tool string was made from 350 mbsf, where another ledge was found, up to the base of pipe at 240 mbsf. Because of poor hole conditions with many washouts, and difficulties with lowering the tool string past ledges in the borehole, logging with the FMS and Geological High-Sensitivity Magnetic Tool (GHMT) string was not attempted.
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