SITE 1086

Site 1086 is located in the southernmost area of the Cape Basin in 780 m deep water (Fig. 1). The primary objective for drilling at this site was to explore the early history of the Benguela Current in the southern Cape Basin and to detect possible Agulhas Current influences. We expect to obtain information about the supply of warm water from the Indian Ocean, through the Agulhas Retroflection, and from the Subtropical Convergence Zone, which are nearby. Both warm-water and cold-water eddies can be shed from the retroflection and the front, but the position of the Subtropical Convergence Zone and the transport by the Benguela Current will be crucial in determining which is more likely to reach the location of the site. The site is also located close to the continent and should detect upwelling signals and signals from continental climates, as well as sea-level changes.

Drilling at Site 1086 recovered a relatively continuous section spanning the upper Miocene to lower Pleistocene. The upper Pleistocene record apparently is missing. Sediments form one lithostratigraphic unit and are composed of clay-rich nannofossil-foraminifer ooze, and foraminifer-nannofossil ooze, foraminifer-rich nannofossil ooze, and nannofossil ooze (Fig. 2). Most of the sediment is moderately bioturbated, and burrows range in diameter from 1 mm to over 1 cm. The detrital component is dominated by clay and trace abundances of silt-sized, subangular mono- and polycrystalline quartz grains. Authigenic minerals are rare or present in trace abundances. Pyrite is present as silt-sized aggregates of euhedral crystals or as framboids. The biogenic component in all smear slides consists of abundant to very abundant nannofossils. Foraminifers are abundant to common.

Sedimentation rates are around 35 m/m.y. between 120 and 200 mbsf (Miocene to lower Pliocene sediments) and are much lower (20 m/m.y.) in the upper Pliocene and Pleistocene sediments.

Calcareous nannofossils and planktonic foraminifer datums are in good agreement. Siliceous microfossils are rare or absent in much of the core. A significant amount of tropical to subtropical planktonic foraminifer species between 140 and 150 mbsf may indicate a warming event or increased input from the warm Aghulhas Current. There are but few radiolarian species at 25 mbsf; this suggests a warm and low-productivity ocean. Dinoflagellate cysts are common in the upper Miocene sediment between 140 and 206 mbsf (~5.9-7.5 Ma).

Except for the uppermost 30 mbsf, the cores show significant coring-induced remagnetization. The magnetostratigraphic interpretation at Site 1086 is based mainly on the inclinations after AF demagnetization at 20 mT. All chrons from the Olduvai (C2n) to C4n (~7.5 Ma) were identified.

Detailed comparisons between the magnetic susceptibility records generated on the MST and color reflectance measured with the Minolta spectrophotometer demonstrated complete recovery of the sedimentary sequence down to 232 mcd.

Sediments at Site 1086 are low in marine organic matter, with TOC concentrations fluctuating between 3.6% and nil. Concentrations of CaCO3 are generally between 85% and 75%. The interstitial water composition is dominantly controlled by the high carbonate content and low organic carbon concentrations in the sediments. The chemical gradients at Site 1086 are even more gradual than those found at the lithologically similar Site 1085. For example, sulfate is not completely consumed until 180 mbsf, which was the greatest depth observed during Leg 175, and the alkalinity maximum at 125 mbsf is only 17 mM. Carbonate and phosphate precipitation reactions are inferred from the profiles of dissolved calcium, magnesium, and phosphate.

Physical sediment properties were determined both by high-resolution MST core logging and index properties measurements. Magnetic susceptibility and GRAPE signals reveal pronounced cyclicities.

Although the Quaternary record is missing, a continuous record back into the upper Miocene was recovered. Sedimentation rates are high in the lower part of the section, into the lower Pliocene. Subsequently, rates drop until they reach zero in the lowermost Quaternary sediment. At the water depth near 800 m, the site records properties of (and activity within) the upper water layers in the Southern Cape Basin. Increased winnowing since the lower Pliocene section apparently is reflected in the drop in sedimentation rate and results in an increasing proportion of sand fraction in the younger sediments.

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