167 Preliminary Report

Site 1014 (Proposed Site CA-15A)

Site 1014 is located in Tanner Basin, within the outer band of California Borderland basins (Fig. 1). The primary objective was to sample a high-resolution section from the late Miocene to Quaternary to study the evolution of the California Current system and to study oceanographic processes in intermediate waters as Northern Hemisphere glaciations expanded. The site will also provide important information about organic carbon diagenesis and about minor-element geochemistry through pore-water profiles and through solid-phase analyses. Pore-water sampling especially within the upper 100 m 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 4 holes at Site 1014 consists of a well-dated, apparently continuous, 325-m-thick interval of upper Pliocene to Quaternary sediments, underlain by a relatively poorly dated, 124-m-thick sequence of early Pliocene to possible latest Miocene age.

The sediments are homogeneous throughout the entire sequence, and consist dominantly of calcareous nannofossils and foraminifers and siliciclastic clays. The sequence is divided into two lithologic subunits. Subunit IA (0-140 mbsf) contains interbedded clay with foraminifers and nannofossil ooze with foraminifers and clay. Subunit IB (140-449 mbsf) contains an increased amount of calcareous nannofossils, and is composed of nannofossil ooze and nannofossil chalk alternating with clay-rich intervals. Discrete ash layers and thin dolostone beds occur in the lower part of the sequence.

Detailed comparisons between the magnetic susceptibility 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 160 mbsf.

Biostratigraphic age control was provided by a combination of calcareous nannofossil, planktonic foraminifer, and radiolarian datums for the upper Pliocene and Quaternary. The base of the sequence is not well dated, but calcareous nannofossils suggest a late Miocene age of between 5 and 7 Ma.

Diatom and radiolarian assemblages suggest weak to strong upwelling cycles during the late Pliocene leading to high-productivity episodes on the continental margin. Middle to upper Miocene diatom and radiolarian species suggest a persistent input of reworked material throughout this sequence. Planktonic foraminifer and radiolarian assemblages indicate relative warmth from the early Pliocene through the late Pliocene until 2.5 Ma. Cooling at thermocline depths is suggested after 3.0 Ma by cooler radiolarian assemblages. This was followed at 2.5 Ma by a major surface-water cooling. Low oxygen concentrations in basinal bottom waters during the earliest Quaternary through latest Pliocene coincided with strong upwelling conditions. During the Quaternary, benthic foraminifer assemblages change in association with glacial-interglacial oscillations. This suggests changes in upper intermediate water circulation during late Quaternary climatic cycles.

AF demagnetization at 20 mT revealed a good magnetostratigraphic record between 0 and 100 mbsf. The Brunhes (C1n) and the Jaramillo (C1r.1n) normal polarity intervals were identified. An age-depth plot based on the reversal boundaries gives a constant sedimentation rate of 79 m/m.y. for the past 2.6 m.y.

Methane to ethane ratios determined from vacutainer and headspace samples are high throughout the sediment column. Average values of calcium carbonate contents steadily increase from 30 wt% at the top of the core to about 55 wt% at 250 mbsf and decrease again at the bottom. The pattern show a high-amplitude fluctuation ranging from 20 to 35 wt%. Organic carbon values are very high (2 to 9 wt%). According to low C to N ratios, the organic material is mainly of marine origin.

The interstitial water geochemistry (Fig. 4) reflects the influence of organic carbon diagenesis by sulfate reduction, of biogenic opal dissolution, and of possible authigenic mineralization reactions. Dissolved sulfate reaches concentrations <1 mM by 17.05 mbsf. Alkalinity increases to values >100 mM, dissolved phosphate to >200 µM, and ammonium to 40 mM. Opal dissolution is indicated by the increase of dissolved silicate to values >1000 µM by 136.6 mbsf. Nonconservative profiles of calcium and magnesium indicate the potential importance of authigenic mineralization.

Index properties show a rapid increase in density and associated decreases in void ratio, porosity, and water content to about 50 mbsf, where coring was switched from the APC to the extended core barrel (XCB) system. Below this depth, the downhole physical property changes are slow, with few fluctuations, most likely corresponding to changing amounts of clay and carbonate. However, at approximately 140 mbsf, densities shift to higher values, whereas void ratio, porosity, and water content values drop. At this depth an increase in carbonate content occurs and is marked as the change from lithologic Unit IA to IB.

Thermal conductivity is low, 0.842 W/(m-K) on average, and provides a heat-flow estimate of 49 mW/m2 (Fig. 5).

Color reflectance data were used to predict high-resolution carbonate measurements aboard ship. Two separate regression equations were used, one based on Site 1012 reflectance and carbonate data and the other based on a combined data set from Sites 1012 and 1013. The combined equation was an effort to compensate for the effect of high organic carbon content at Site 1014. Carbonate content was simulated well by both equations, generally matching the laboratory measurements in both amplitude and phase.

Logging was conducted at Hole 1014A from the base of pipe set at 80 mbsf to a sub-bottom depth of 445 mbsf. Hole conditions were excellent, with the exception of a few washouts. The log physical property data closely matched the measured core density, porosity, and susceptibilty (Fig. 6) over the core-log data overlap.

The log gamma-ray values exhibit very high values between 100 and 160 mbsf. The gamma-ray activity throughout the hole is predominantly caused by variations in the uranium content, which is strongly correlated to measured variations in sediment organic carbon content. The uranium-organic carbon linkage appears to reflect authigenic uranium fixation in these strongly reduced sediments. High sedimentation rates at Site 1014 provide an opportunity to examine the core and log resolution of orbital and millennial-scale bedding cycles (Fig. 6). Comparison of the FMS record (averaged to 2 mm resolution) with the digital video brightness (L*) channel data (decimated to 4 mm resolution) suggests that periodic variability in carbonate composition at the 20-30 cm scale (equivalent to 2-3 k.y.) can be faithfully resolved in the log data.

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