167 Preliminary Report

Site 1012 (Proposed Site BA-1)

Site 1012 is located in East Cortez Basin, within the middle band California Borderland basins in a water depth of 1783 m (Fig. 1). The primary objective was to sample a high-resolution section from the early Pliocene to Quaternary to study the evolution of the California Current system and oceanographic processes in intermediate waters as Northern Hemisphere glaciations expanded. Paleoceanographic proxies for surface-water properties will be sampled at a high resolution, including those for sea-surface temperature, paleoproductivity, and water mass. The site will also be important for high-resolution paleomagnetic studies and will provide important new information about organic carbon diagenesis and about minor-element geochemistry through pore-water profiles.

The sedimentary sequence recovered at Site 1012 consist of an apparently continuous, 264-m-thick interval of upper lower Pliocene to Quaternary sediments. It consists of a single lithologic unit with two subunits. Lithologic Unit I consists of interbedded silty clay, nannofossil mixed sediment, and nannofossil ooze, and their lithified equivalents. The sediments are organic-rich throughout, and cyclic variation in organic matter content is superimposed upon the carbonate/siliciclastic cycles. Subunit IA is composed of silty clay, nannofossil mixed sediment, and nannofossil ooze. Mean carbonate composition increases downcore from 15% to 50%. Subunit IB is composed of silty clay, and lithified clayey nannofossil mixed sediment and nannofossil chalk, along with isolated beds of dolostone. Mean carbonate values remain approximately at 50%, although bed-to-bed variation is great. Dolostone beds and the presence of glauconite are associated with an upper Miocene to upper lower Pliocene hiatus or condensed interval at the base 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 94 mbsf. The correlation between carbonate content and bulk density is excellent. The scatter reflects the alternating carbonate-rich and carbonate-poor layers. Downhole temperature measurements yield a thermal gradient of 82°C/km. Using an average thermal conductivity of 0.905 W/(m-K) provides a heat-flow estimate of 74 mW/m2.

Uppermost Miocene planktonic foraminifers (5.6 to 6.2 Ma) at the bottom of Hole 1012A suggest that much of the lower Pliocene is missing at Site 1012 or that there is a highly condensed lower Pliocene sequence. Calcareous nannofossils are abundant and well preserved in the Quaternary and upper Pliocene, and poorly preserved and fragmented in the upper lower Pliocene and uppermost Miocene. Planktonic foraminifers are highly abundant and very well preserved in the Quaternary, abundant to common and generally well preserved throughout the Pliocene, and are few but well preserved in the uppermost Miocene. Radiolarians and diatoms are absent in the sequence except for conspicuous reworked middle Miocene species.

A well-constrained biostratigraphy and chronology is provided by calcareous nannofossils and planktonic foraminifer datums for the upper Pliocene and Quaternary. Extensive reworking of calcareous nannofossils in the upper lower Pliocene and uppermost Miocene made biostratigaphic determinations more difficult based on calcareous nannofossils. Lower upper Pliocene microfossil assemblages indicate relatively warm-temperate conditions, which change in the latest Pliocene to Quaternary to cooler conditions with major sea-surface temperature changes related to glacial/interglacial oscillations.

Paleomagnetic alternating field (AF) demagnetization at 20 mT revealed an excellent magnetostratigraphic record between 0 and 130 mbsf. The Brunhes (C1n), the Jaramillo (C1r.1n), Cobb Mountain, and the Olduvai (C2n) normal polarity intervals were identified. An age-depth plot based on the reversal boundaries gives a constant sedimentation rate of 65 m/m.y.

The calcium carbonate record is characterized by a very high fluctuation of values, ranging from 5 to about 70 wt%. The carbonate concentration increases steadily downcore to 120 mbsf and reaches constant values around 50 wt%. Organic carbon contents are high throughout the sediment column and dominated by marine organic matter according to low C to N ratios. Episodic input of terrigenous organic matter leads to increased organic carbon values. Volatile hydrocarbons are consistently very high, but are of no safety or pollution concern because of high C1 to C2 ratios.

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 18 mbsf. Alkalinity increases to as high as 60 mM, dissolved phosphate to 120 µM, and ammonium to 14 mM. Opal dissolution is indicated by the increase of dissolved silicate to values >1000 µM by 130 mbsf. Nonconservative profiles of calcium and magnesium suggest the importance of authigenic mineralization.

A multiple linear regression was used to construct an empirical relationship between calcium carbonate and reflectance measurements gathered at Site 1011. Using this regression equation, carbonate content was predicted at Site 1012. The predicted values matched laboratory data well. Fluctuations are highly cyclic-preliminary spectral analysis of reflectance data in a crude age model framework show power in the Milankovitch frequencies. The high-resolution reflectance data imply calcite variations not resolved by the lower resolution carbonate samples.

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