SITE 1080

Site 1080 in the Southern Angola Basin (Fig. 1) was selected to sample the northern end of the Angola/Namibia upwelling region. The site should complement results obtained from Site 1081 at the Walvis Ridge. It is not only important for reconstruction of the history of the Benguela Current and coastal upwelling migration, but also for its contribution to the climatic history of southern Africa. In addition to frequencies and phase of productivity variations, we expect to obtain information on dry/wet cycles in the drainage basin of the Cunene River. Of special interest in this context is the relationship of such cycles to the northern monsoon.

Drilling at Site 1080 recovered a hemipelagic sedimentary section probably spanning about 1 m.y. in the Pleistocene. Sediments form one lithostratigraphic unit composed of moderately bioturbated, dark greenish gray to olive-gray, diatom-bearing, and diatom-rich silty clays with varying abundances of nannofossils and foraminifers (Fig. 2). The clastic fraction is dominated by coarse silt-sized, angular, mono- and polycrystalline quartz grains with rare feldspar (albite and microcline) and detrital apatite clasts, muscovite, smectite, kaolinite, and perhaps illite. The biogenic component is represented by frequent diatom fragments, foraminifer fragments, and nannofossils. Radiolarians, plant remains, and particulate organic matter are present in trace amounts. Authigenic minerals include rounded glauconitic peloids and framboidal pyrite.

Detailed comparisons between the magnetic susceptibility and GRAPE density records generated on the MST and high-resolution color reflectance demonstrated complete recovery of the sedimentary sequence down to 42 mcd.

Micropaleontological studies were carried out on core-catcher samples from Holes 1080A and 1080B. The downhole succession of nannofossil assemblages from both holes indicates that the sedimentary sequence is incomplete and disturbed, possibly by turbiditic deposition. A hiatus of at least 400 k.y. duration (Zone NN20 and upper part of Zone NN19) was identified within a disturbed interval from 12 to 25 mbsf. Planktonic foraminifers are dominated by Globigerina bulloides, a species characteristic for upwelling and a high-productivity indicator. Dissolution effects increase downhole, reducing the abundance of the planktonic foraminifers. The absence of Neogloboquadrina pachyderma may represent a change to warmer surface-water conditions, but it may also be an artifact of dissolution. Radiolarians are present throughout the section with an abundance that ranges from few to abundant. The preservation is good in all investigated samples, and no apparent reworking was identified. Diatom abundance ranges from few to abundant. In general, preservation is poor and diatom valves are fragmented. Silicoflagellates, opaline phytoliths, and sponge spicules are also present. Reworking is not evident. The presence of nonplanktonic diatoms in all core-catcher samples points to material derived from the shelf. Upwelling species dominate. Chaetoceros resting spores are abundant in all samples and are accompanied by neritic (rare to few) and open-ocean species (trace to rare).

A magnetostratigraphy was determined after AF demagnetization at 20 mT. The Matuyama/Brunhes boundary occurs at both holes ~10 mbsf, and the onset and termination of the Jaramillo Subchron (C1r.1n) was identified in Hole 1080A at 41 and 51 mbsf, respectively. A short reversal event in the Matuyama Chron was identified at both holes. The relative shortness of the Brunhes Chron compared with the distance to termination of the Jaramillo Chron suggests that the upper Quaternary record is missing at this site.

Sediments average 2.4% total organic carbon, which is rather high for ocean margin areas and reflects a history of elevated primary production in this area. Interstitial waters were gathered at a frequency of one sample per section, allowing high-resolution study of diagenetic processes through the entire cored sequence. The presence of a dolomite layer at 50 mbsf, which terminated drilling, indicates rock-forming dolomitization within the last 1 m.y. at the site. The profiles of alkalinity, ammonium, phosphate, and sulfate in the upper 10 mbsf reflect degradation of organic matter, and the distributions of dissolved calcium, magnesium, and strontium indicate active calcite dissolution and dolomite formation. Throughout this upper interval, the concentration of sedimentary calcite decreases from 26 wt% to less than 5 wt%, and dissolved strontium concentrations increase. From 25 mbsf to the dolomite layer, calcium and magnesium concentrations remain stable. The absence of decreases in these cations indicates that the dolomite layer is no longer growing.

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

The original goal to obtain a high-resolution record of the northern end of the Angola/Namibia upwelling system was not achieved. Hard dolomite layers at a depth of 35 to 50 mbsf denied recovery by APC and proved difficult to penetrate by XCB, as well. Also, we quickly discovered that the sedimentary sequence is incomplete at this site and that calcareous fossils have been largely dissolved below the upper portion of the sequence. We anticipate, nevertheless, interesting insights into the dynamics of the mid-Pleistocene climate revolution from this site, with an expanded section between the termination of the Jaramillo Chron and the onset of the Brunhes Chron.

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