167 Preliminary Report

Site 1021 (Proposed Site CA-5A)

Site 1021 is the deep-water site of the Gorda Transect (Fig. 1). It is located about 100 km south of the Mendocino Fracture Zone, more than 360 km from the California Coast, and is situated on an abyssal hill at 4240 mbsl on 29.6-Ma crust. The primary drilling objective was to sample the longer term Neogene record, and to develop a correlation between Neogene biostratigraphic data from the northeastern Pacific and the paleomagnetic chronostratigraphy. 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. Site 1021 will also be used to study preservation of bulk organic matter and, if reliable paleoproductivity indices can be generated, to relate organic matter preservation to flux in low sedimentation rate environments.

The sedimentary sequence recovered from the 4 holes at Site 1021 consists of a well-dated, apparently continuous, 310-m-thick interval of uppermost middle Miocene to Quaternary (11.8-0 Ma) sediments. Sediments are dominated either by homogenous clay-rich intervals or by submeter-scale alternation of carbonate/biosiliceous and siliciclastic strata. All sediments contain biogenic assemblages, mainly calcareous nannofossils, diatoms, foraminifers, and radiolarians, in general order of decreasing abundance. Dominant lithologies are clay, clay with nannofossils/nannofossil ooze, and clayey diatomite/diatom clay mixed sediment. The upper part of the sequence is characterized by a dominance of calcareous nannofossils, whereas diatoms form the main biogenic component in the lower part. Fine-grained volcanic ash layers, and thin clay-rich laminations interpreted as altered ash, serve as potential marker horizons throughout the sequence. The sediments are slightly to moderately bioturbated, with increasing intensity downhole. The sedimentary succession consists of two distinct units (Units I and II) as determined by visual core descriptions and smear-slide estimates. Subunits IA, IB, IC, and IIA and IIB are distinguished by different levels of clay, carbonate, and biogenic silica. Sedimentation rates for the last 10 m.y. were remarkably constant at 30 m/m.y., and decreased during the late middle Miocene (10 to 11.8 Ma).

Index properties correspond well with lithological variations. High grain densities are typical in the nannofossil-rich interval in the upper 110 mbsf and, lower grain densities and higher porosities in siliceous, diatom-rich parts of the sequence (110-310 mbsf).

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 185 mcd.

A well-constrained biostratigraphy and chronology is provided by a combination of calcareous nannofossil, planktonic foraminifer, radiolarian, and diatom datums, and paleomagnetic reversals. Radiolarians indicate that the base of Hole 1021B is 11.8 Ma in age.

The microfossil groups at Site 1021 are clearly dominated by cool, high-latitude elements from the latest Miocene through the Quaternary. Planktonic foraminifer assemblages are especially cool during Quaternary glacial episodes and the latest Miocene. Radiolarian assemblages suggest relatively warmer conditions during the middle and early late Miocene. Radiolarian species characteristic of upwelling environments are scarce throughout most of the sequence, suggesting weak and episodic vertical advection of deep waters at this location. However, both radiolarians and diatoms indicate that upwelling was stronger during the middle through late Miocene and especially strong during the latest Miocene. Benthic foraminifers indicate that well-oxygenated bottom waters bathed Site 1021 throughout the entire late Neogene.

A complete magnetostratigraphy was determined at Holes 1021B and 1021C after AF demagnetization at 20 mT (Fig. 11). All chrons from the Brunhes (C1n) to the onset of C3n.4n (Thvera subchron) at 5.23 Ma could be identified in the upper 160 mbsf. This section will serve as a well-dated reference for the calibration of biostratigraphic datums.

Organic carbon contents are very low, varying between 0.05 and 0.6 wt%. Concentrations are decreasing downhole to values that are typical for pelagic sediments. The total nitrogen contents varies between 0.06 and 0.17 wt%, and the total sulfur content ranges from 0 to about 0.3 wt%. The low C/N ratios indicate a predominant marine origin of the organic material. Two intervals (86-105 mbsf and 222-275 mbsf) show high CaCO3 concentrations of up to 60 wt%. Low CaCO3 concentrations are observed in intervals (0-86 mbsf and 105-275 mbsf) of carbonate dissolution.

Chemical gradients in the interstitial waters reflect organic matter diagenesis, the dissolution of biogenic opal, and possibly the diffusive influence of reactions in underlying basalt. Alkalinity increases to peak values of >9 mM, whereas sulfate concentrations decrease to plateau values around 20 mM by 107.45 mbsf. Phosphate concentrations are greater than 15 µm from 2.96 to 78.95 mbsf, and ammonia concentrations increase to maximum values >500 µM. Dissolved silicate increases to concentrations >1000µM. Calcium concentrations decrease to 8.7 mM at 107.45 mbsf, then increase with increasing depth to 12.5 mM at 304.95 mbsf. Magnesium concentrations gradually decrease throughout the section to 47 mM at 304.95 mbsf.

Downhole temperature measurements yield a thermal gradient of 54°C/km (Fig. 5). Using an average measured thermal conductivity of 0.849 W/(m-K) provides a heat-flow estimate of 46 mW/m2.

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