HOLE 1080B

Position: 16°33.5963'S, 10°49.2043'E

Start hole: 1730 hr, 7 September 1997

End hole: 0600 hr, 8 September 1997

Time on hole: 12.50 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 2779.3

Total depth (drill pipe measurement from rig floor, mbrf): 2817.5

Distance between rig floor and sea level (m): 11.4

Water depth (drill pipe measurement from sea level, m): 2767.9

Penetration (mbsf): 38.2

Coring totals:

    Type: APC

    Number: 4

    Cored: 37.7 m

    Recovered: 40.1 m (106.3%)

    Type: XCB

    Number: 1

    Cored: 0.5 m

    Recovered: 0.0 m (0.0%)

Lithology:

    Unit I: dark greenish gray to olive-gray, diatom-bearing, and diatom-rich silty clays with varying abundances of nannofossils and foraminifers

 

Principal results: Site 1080, in the Southern Angola Basin, 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 phases of productivity variations, we expect to obtain information on dry–wet cycles in the drainage basin of the Kunene River. Of special interest in this context is the relationship of such cycles to the northern monsoon.

 

Two holes were cored with the advanced hydraulic piston corer (APC) at Site 1080 to a maximum depth of 52.1 meters below seafloor (mbsf), which recovered a hemipelagic sedimentary section probably spanning ~1 m.y. in the Pleistocene. Hole 1080A was cored with the APC to 50.8 mbsf and deepened with the extended core barrel (XCB) to 52.1 mbsf, with high torque and very little penetration in a very hard authigenic dolomite layer. Coring with the APC advanced at Hole 1080B to 37.7 mbsf when a hard dolomite layer resulted in no advance. An XCB core barrel advanced 0.5 m in 25 min of rotation and led to the decision to terminate the site.

 

Drilling at Site 1080 recovered sediments from 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. The clastic fraction is dominated by coarse silt-sized, angular, mono- and polycrystalline quartz grains with rare feldspar and detrital apatite clasts; smectite, kaolinite, and perhaps illite; the feldspar minerals albite and microcline; and muscovite. 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 gamma-ray attenuation porosity evaluator (GRAPE) density records generated on the multisensor track (MST) and high-resolution color reflectance measured with the Minolta spectrophotometer demonstrated complete recovery of the sedimentary sequence down to 42 meters composite depth (mcd).

 

Micropaleontological studies were carried out on core-catcher samples from Holes 1080A and 1080B. The downhole succession of nannofossil assemblages from both holes suggests 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 has been 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–few) and open-ocean species (trace–rare).

 

A magnetostratigraphy was determined after alternating-field (AF) demagnetization at 20 mT. The Matuyama/Brunhes boundary is present in both holes at ~10 mbsf, and the onset and termination of the Jaramillo Subchron (C1r.1n) was identified at Hole 1080A at 51 and 41 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 wt% total organic carbon (TOC), 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 this site. The profiles of alkalinity, ammonium, phosphate, and sulfate in the upper 10 mbsf reflect the 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 to <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, which 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, it was soon appreciated 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 and onset of the Brunhes Chrons.