SITE 1085

One of the main objectives for drilling at Site 1085 (Fig. 1) was to help document the path and strength of the Benguela Current system from the Miocene to the Quaternary periods, as well as the shoreward and seaward migrations of the upwelling center. The upwelling center inside the Benguela Coastal Current is fed from the thermocline by South Atlantic Central Water, and its intensity is related to the position and intensity of the Benguela Current system. Filaments of cold, nutrient-rich waters from the coastal upwelling area extend as much as 600 km offshore. Here, cold water mixes with low productivity oceanic water, forming a zone of intermediate productivity. Site 1085 is located offside from the mouth of the Orange River, which has water year round and delivers additional terrigenous sediments. This effect should be more pronounced during times when the Benguela Current and coastal upwelling activity were of lower intensity than today, so that sediments from ocean production were less dominant. A close tie-in between pelagic and terrigenous sedimentation is expected to be present within the slope record.

Drilling at Site 1085 recovered a relatively continuous hemipelagic sedimentary section spanning the last 15 m.y. Sediments form two lithostratigraphic units dominated by nannofossil ooze (Fig. 2). The uppermost unit has been subdivided into two subunits to reflect the decrease in foraminifer abundances downhole. The underlying unit is a reddish brown, microfaulted, and thinly laminated clay-rich nannofossil ooze. Graded, 2- to 17-cm-thick beds rich in foraminifer tests are present between 30 and 70 mbsf. Below 360 mbsf, pyrite is present as isolated fine sand-sized grains and below 420 mbsf as nodules up to 1 cm in diameter. The deepest core of Hole 1085A is interpreted as a slump block. It consists of a thinly laminated reddish to olive-brown nannofossil ooze. Packages of occasionally graded laminae within this unit are commonly microfaulted, convolutely layered, and folded. The detrital component in sediments at Site 1085 is dominated by clay and trace abundances of silt-sized, subangular mono- and polycrystalline quartz grains. Sedimentation rates range from 15 to 80 m/m.y., with the highest values located within the last 8 m.y.

Physical sediment properties were determined both by high-resolution MST core logging and index properties measurements. Detailed comparisons between the magnetic susceptibility generated on the MST and color reflectance measured with the Minolta spectrophotometer demonstrated complete recovery of the sedimentary sequence down to 289 mcd.

A biostratigraphic framework composed of both calcareous nannofossils and foraminifers was established resulting in a well-constrained age model for Site 1085. Calcareous nannofossils are abundant and well preserved throughout the entire section. Reworking is limited to the interval between 60 and 100 mbsf. The overall abundance of benthic foraminifers is high throughout the studied interval. The planktonic to benthic foraminifer ratio at this site is about ten times higher than at previous sites. Radiolarians are generally rare and show signs of dissolution in almost all samples. Radiolarian assemblages indicate that intermittent upwellings occurred through the last ~3 m.y. The presence of an Antarctic species indicates an occasional influence of cooler currents. Diatoms are rare or absent in most of the section. The interval between 89.17 and 127.45 mbsf (upper Pliocene, 1.8-2.6 Ma) contains a mixed/T. antarctica-rich diatom assemblage, similar to the one found in upper Pliocene sediments at Site 1084. Dinoflagellate cysts are common in the upper Miocene sediments (6-10 Ma).

A complete magnetostratigraphy was determined at Site 1085 after AF demagnetization at 20 mT. All chrons from the Brunhes (C1n) to the earliest part of the Gilbert (~5.5 Ma) were identified, although the quality of the record was not good because of a severe magnetic overprint.

Sediments at Site 1085 contain small amounts of marine organic matter, with TOC concentrations fluctuating between 2.8% and nil. Concentrations of CaCO3 are generally between 85% and 60%, but drop to 35% in sediments deposited during the Miocene carbonate crisis. Interstitial water chemistry is dominantly controlled by the low organic carbon and high carbonate concentrations in the sediment, which result in modest variations in the chemical gradients of many dissolved species. Alkalinity rises to a broad maximum of approximately 30 mM between 46 and 84 mbsf and subsequently decreases to the bottom of the hole. The deepest alkalinity value of 1.752 mM is by far the lowest (except for near-surface data) observed so far during Leg 175 and most likely reflects consumption via clay mineral formation. Sulfate is not completely consumed until 65 mbsf, which is also deep in comparison to previous Leg 175 sites. Carbonate and phosphate precipitation reactions throughout the sequence are also inferred from the profiles of dissolved calcium, magnesium, and phosphate.

Hole 1085A was logged with a full suite of sensors to continuously characterize the sedimentary changes through depth and to provide data for core-log integration. The recorded data are affected by poor hole conditions. Enlargements and washout zones were identified in the entire logged interval. Despite the adverse hole conditions, measurements such as resistivity, sonic velocity, natural gamma ray, and magnetic susceptibility provided reliable data and show well-expressed changes related to the various proportions of biogenic, clastic, and organic components.

Drilling at Site 1085 recovered a continuous high-resolution record with sedimentation rates of around 50 m/m.y. The carbonate-rich sediments will allow the reconstruction of the position and intensity of the Benguela Current for the last 15 m.y., including the influence of water masses coming from the Indian Ocean and the Subantarctic Region.

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