SITE 1084

One of the main objectives at Site 1084 (Fig. 1) is to document the northward migration of the Benguela Current system from the Miocene to the Quaternary periods, as well as the shoreward and seaward migrations of the upwelling center. Filaments of cold, nutrient-rich waters from the coastal upwelling area extend well offshore and mix with low-productivity oceanic water, forming a zone of intermediate productivity at Site 1084. Because of high sedimentation rates, this site is expected to provide a high-resolution record. The close vicinity to the Luederitz upwelling cell should result in well-expressed organic carbon, diatom, and coccolith cycles via cyclic productivity intensity. In addition, this site is closest to the area with elevated primary production and should have the best representation of coastal upwelling signals. A close tie-in between pelagic and terrigenous sedimentation is expected within the slope record.

Drilling at Site 1084 recovered a relatively continuous hemipelagic sedimentary section spanning the last 4.7 m.y. Sediments form four lithostratigraphic units defined by the changes in the major lithology between clay and ooze (Fig. 2). The uppermost lithology is composed of moderately bioturbated, intercalated intervals of clays that contain varying abundances of diatoms, nannofossils, foraminifers, and radiolarians. Three subunits are distinguished based on microfossil type and abundance. The section continues downward with about 100 m of clay-rich nannofossil diatom ooze, diatomaceous nannofossil ooze, and clay-rich nannofossil ooze. The following 50-m thick unit is comprised of clay with varying amounts of nannofossils and diatoms underlain by a unit consisting predominantly of clayey nannofossil ooze.

Several decimeter-thick intervals of dark, organic-rich clay layers occur between 120 and 410 mbsf. These horizons are characterized by lower carbonate contents that average just below 10 wt% and by higher organic carbon contents that range between 8 and 18 wt% when compared to the under- and overlying sediment layers. The detrital component of the sediments consists of clay with rare silt-sized, angular and subangular mono- and polycrystalline quartz grains. Subangular feldspar grains are present in trace amounts. Authigenic minerals include framboidal pyrite and dolomite rhombs in rare or trace amounts. The dark horizons generally contain slightly higher abundances of pyrite and exhibit significant compositional variations in the relative abundances of diatoms, foraminifers, and nannofossils. The biogenic component of the dark layers is commonly dominated by diatom resting spores.

Sedimentation rates range from 100 to 270 m/m.y. with highest values located within the last 1 m.y. (Fig. 7). A second episode of high sedimentation rate (170 m/m.y.) is associated with an upper Pliocene diatom-rich interval.

Detailed comparisons between the magnetic susceptibility and the GRAPE density records generated on the MST and color reflectance measured with the Minolta spectrophotometer demonstrated complete recovery of the sedimentary sequence down to 175 mcd.

An integrated biostratigraphic framework composed of both calcareous and siliceous microfossils was established, resulting in a well-constrained age model for Site 1084 (Fig. 7). Calcareous nannofossils are abundant within the top 280 mbsf and between 410 and 600 mbsf and provided 13 biohorizons that were constrained within an average depth interval of 3 m. Planktonic foraminifer zonation is difficult because of an absence of marker species and dissolution of planktonic foraminifers, particularly in the lower Pleistocene to upper Pliocene sediments. It is difficult to determine whether the absence of marker species is caused by ecological conditions or by selective dissolution. Both cool- and warm-water faunas are present in the same assemblages downcore and may indicate an increased contribution from cooler Southern Ocean waters. The benthic foraminifers are abundant and well preserved, and abundances correlate well with the different lithostratigraphic units. Radiolarians are abundant and well preserved in almost all samples examined. Diatom preservation is moderate throughout Hole 1084A. In contrast to the other sites, diatom abundances, although highly variable, remain moderately high throughout the Pleistocene sediments. In addition to the "background" diatom assemblage composed of a mixture of upwelling-indicator and oceanic species, we recorded many more cold-water markers characteristic of the Southern Ocean than at Sites 1081, 1082, and 1083. The intervals of greatest diatom abundances in the late Pliocene are recorded by a mixed/Thalassiothrix antarctica-rich assemblage and may represent mat deposits similar to the ones discovered in the eastern equatorial Pacific during Leg 138. The fact that these T. antarctica-rich sediments occur during persistent subantarctic water-mass influence at Site 1084 may relate to more vigorous surface circulation leading to the development of stronger frontal systems, facilitating the concentration of the diatom mats and, consequently, greater mat flux.

A complete magnetostratigraphy was determined at Site 1084 after AF demagnetization at 20 mT (Fig. 7). All chrons from the Brunhes (C1n) to the latter part of the Gilbert (~4.4 Ma) were identified. No short polarity-reversal event during the Brunhes Chron was detected in spite of the high sedimentation rates.

Sediments at Site 1084 are notably rich in marine organic matter. Well-developed light-dark sediment color cycles, in which concentrations of CaCO3 and organic carbon vary between 1% and 69% and 1.2% and 18%, respectively, record fluctuations in the elevated marine production associated with the Benguela Current. Higher concentrations of organic carbon from 0 to 392 mbsf indicate that productivity was higher during the last 2 m.y. than earlier in the history of this upwelling system.

Interstitial water chemistry profiles at this organic-carbon-rich site record some of the most extreme conditions of sediment diagenesis ever recovered in the history of DSDP and ODP drilling. Maximum values of alkalinity (172 mM) and ammonium (50 mM) are greater than those observed at any site, except Site 688, within the Peruvian upwelling system (Fig. 8). Sulfate is completely depleted within the uppermost 5 mbsf, which is much more rapidly than in any other Leg 175 site and speaks to the availability of oxidizable organic matter. These changes are accompanied by calcite dissolution and dolomite precipitation, as recorded in the calcium, magnesium, and strontium distributions.

Physical sediment properties were determined both by high-resolution MST core logging and index properties measurements. Magnetic susceptibility and GRAPE signals reveal pronounced cyclicities that were used for high-quality stratigraphic correlation in conjunction with digital color data.

Hole 1084A was logged with a full suite of sensors to continuously characterize the sedimentary changes through depth and to provide data for core-log integration. Hole conditions above 170 mbsf are poor with a regular increase of the hole size. Nineteen dolomitic layers and 114 organic rich dark layers were identified from logging data. Natural gamma-ray intensity can be used for detailed core-log correlations and for correlation with Site 1082 in the Walvis Basin.

Based on pore-water chemistry and Corg concentrations, Site 1084 records by far the highest productivity of the sites drilled. This was expected, since the site is close to the most active upwelling cell along southwest Africa; that is, the Luederitz Bay. The sediments recovered will allow high-resolution documentation of the variability of coastal upwelling for the last ~4.7 m.y.

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