167 Preliminary Report
Site 1020 is located on the east flank of the Gorda Ridge at a water depth of 3040 mbsl (Fig. 1). It is about 170 km west of Eureka, California, and forms the high-resolution, deep-water site on the Gorda Transect. The site is located on an abyssal hill which trends northeast and rises 50-100 m above the surrounding seafloor on 5.1-Ma ocean crust. The primary objective of drilling at this site was to study paleoceanographic conditions near the core of the Northern California Current during the critical time interval when the Northern Hemisphere ice sheets began to form. The development of a correlation between biostratigraphy and the paleomagnetic chronostratigraphy was another key objective at this site. Site 1020 will provide information about organic carbon diagenesis and about minor-element geochemistry through pore-water profiles and through solid-phase analyses. Paleomagnetic studies will provide an avenue to study the deformation of the Gorda Plate. Provided that stable magnetic declinations can be obtained, the rotation of the Gorda Plate can be monitored through time. A secondary goal was to obtain a representative sample of the basalt basement for igneous petrology and geochemistry.
The sedimentary sequence recovered from the 4 holes at Site 1020 consists of a well-dated, apparently continuous, 275-m-thick interval of upper lower Pliocene to Quaternary (3.79-0.0 Ma) sediments. Siliciclastic clay is found throughout the cored interval and is the dominant component in the upper part of the sequence. The middle part is dominated by nannofossil clay with frequent interbedding of clayey nannofossil ooze. Diatoms are also present, but only as a minor component ranging from 10% to 30% of the total sediment. The lower portion is dominated by interbeds of nannofossil clay and clayey nannofossil chalk. Two thin (decimeter-scale) intervals of dolomite are found at 78 mbsf and 178 mbsf. Turbidite deposition is relatively unimportant at this site compared to previous sites except for a few thin, graded beds in the uppermost portion and a slightly thicker interval at about 68 mbsf. Lithostratigraphic Unit I is predominantly composed of clay-rich sediments mixed with minor quantities of nannofossils and diatoms. Thin intervals of nannofossil ooze occur throughout this unit, and diatoms rarely exceed 30% of the sediment. Unit II consists of clayey nannofossil chalk mixed sediments in which pyrite nodules are abundant and scattered throughout. Clay content increases to nearly 95% near the bottom. Approximately 5 cm of basalt was recovered at the base of the hole.
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 242 mcd, with the exception of coring gaps at 126, 137, and 200 mcd, which could not be covered by overlap. Bulk densities in the upper part of the section slowly increase downhole with some scatter corresponding to the interbedding of clay and nannofossil clay. Densities are constant between 120 and 220 mbsf and increase sharply at 220 mbsf. The increase is caused by higher carbonate content, corresponding to the top of the nannofossil chalk mixed with clay.
A well-constrained biostratigraphy and chronology is provided by a combination of calcareous nannofossil, planktonic foraminifer, radiolarian, and diatom datums, and paleomagnetic reversals for the upper lower Pliocene and Quaternary. Most of the sequence contains common radiolarians and mostly common to abundant diatoms. All of the microfossil groups are clearly dominated by cool, high-latitude elements throughout the late Neogene. Radiolarians are entirely represented by subarctic forms, and the assemblages exhibit noticeably lower diversity than at all other sites cored during Leg 167. Diatoms are dominated by North Pacific subarctic assemblages in addition to rare temperate elements. Relative high abundances of Gephyrocapsa carribeanica may indicate cooler episodes within the Quaternary. Planktonic foraminifer assemblages are dominated by subarctic to cool temperate forms, with subtropical elements absent. Radiolarians are represented by forms not characteristic of upwelling regions throughout the entire sequence. Likewise, diatoms are represented by open-ocean forms not characteristic of coastal upwelling regions, except during the early late Pliocene when upwelling forms are present in relatively low frequencies.
Planktonic foraminifer assemblages are made up entirely of cool temperate to subarctic taxa. Distinct changes in planktonic foraminifer assemblages in the uppermost Pliocene and Quaternary clearly reflect glacial to interglacial oscillations. In contrast, radiolarians do not. At greater depths in the section, short-term, climatically related faunal changes are much less conspicuous. Late early Pliocene planktonic foraminifers (3.8 Ma) exhibit the highest diversity in the sequence, and are considered to reflect the warmest surface-water temperatures. The first consistent occurrence of sinistrally coiled Neogloboquadrina pachyderma marks a distinct cooling step at 1.14 Ma.
Benthic foraminifers in the upper Pliocene and Quaternary are typical lower bathyal, deep-sea assemblages indicative of well-oxygenated bottom waters. The faunas exhibit little change throughout the entire upper Pliocene and Quaternary, and exhibit no clear oscillations associated with glacial/interglacial change. In the lowermost part of the section, the benthic foraminifer assemblages may represent deposition in lower middle bathyal water depths as compared with lower bathyal depths in the sequence above. The changes in benthic foraminifer assemblages represent evidence for rapid tectonic subsidence in the lower 30 m of the sequence. The changes in benthic foraminifers would indicate subsidence of 500 to 1000 m during this interval. No evidence for paleodepth change is evident from benthic foraminifers above 256 mbsf during the late early Pliocene through Quaternary.
AF demagnetization at 20 mT revealed a complete magnetostratigraphic record between 0 and 130 mbsf that allowed the identification of the Brunhes (C1n) and the Jaramillo (C1r.1n) normal polarity intervals. Reorientation of the magnetic declination using the Tensor tool supports the interpretation of magnetic polarity zonation based on the inclination data.
Headspace volatile hydrocarbon concentrations range from 5 to 8900 ppm methane. Ethane or propane gases are near the detection limit. Low calcium carbonate concentrations between 1 and 20 wt% were found in the upper 220 mbsf. Below this depth, CaCO3 concentrations increase up to 58 wt%. Organic carbon contents vary around 0.7 wt%, and show slightly decreased values where carbonate content is high. Low total organic carbon to total nitrogen ratios between 4 and 9 indicate a marine dominance of the organic material.
Chemical gradients in the interstitial waters 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 of >20 mM, whereas sulfate concentrations decrease to values below the detection limit (approximately 0.2 mM) by 107.25 mbsf. Phosphate concentrations increase to values >55 µM, and ammonium concentrations increase to maximum values >3 mM. Dissolved silicate increases to concentrations >1000 µM, and strontium increases to 230 µM. Calcium concentrations decrease to as low as 5.2 mM, then increase with increasing depth to 26.3 mM at 260.30 mbsf. Magnesium concentrations generally decrease throughout the section to 19.6 mM at 260.30 mbsf.
The 4 Adara temperature measurements yield a thermal gradient of 189°C/km. Using an average measured thermal conductivity of 0.899 W/(m-K) provides a heat-flow estimate of 170 mW/m2 at Site 1020 (Fig. 5). The vicinity of the Gorda Ridge explains this relatively high heat-flow value.
Reflectance data are consistent with the major lithological units. In lithostratigraphic Unit I reflectance is consistently low for the 450-500 nm band. Unit II shows greater and more variable reflectance than Unit I. A ratio of the 850- to 900-nm (nIR) band to the 450- to 500-nm (blue) band was used as a qualitative proxy for opal content. In Subunit IA, where clay predominates, the nIR/blue is generally low. As Subunit IA grades into Subunit IB, the proportion of diatoms increases, as does the mean value and variability of the ratio of nIR to blue.
Hole 1020B was logged with the Triple Combination, FMS/Sonic, and GHMT tool strings from 86 to 275 mbsf. Overall log quality at this site was excellent to very good below 170 mbsf, and fair in the washed out section above 170 mbsf. Comparison between the log and core MST data demonstrate that the logs can reliably reproduce first-order features of the records generated from measurements of the sediments using the MST track particularly in the interval below 170 mbsf where hole conditions are very good. Core-log comparison suggests that the cored sediment, and therefore the mcd scale, is expanded by about 10% relative to its true depth range.
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