HOLE 1079C

Position: 11°55.7969'S, 13°18.5607'E

Start hole: 1155 hr, 5 September 1977

End hole: 2230 hr, 5 September 1997

Time on hole: 10.58 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 749.2

Total depth (drill pipe measurement from rig floor, mbrf): 876

Distance between rig floor and sea level (m): 11.3

Water depth (drill pipe measurement from sea level, m): 737.9

Penetration (mbsf): 126.8

Coring totals:

    Type: APC

    Number: 14

    Cored: 126.8 m

    Recovered: 129.9 m (102.4%)

Lithology:

    Unit I: olive-gray silty clay with varying amounts of nannofossils and foraminifers

 

Principal results: Site 1079 is located outside the Bight of Angola in 1738-m deep water. Situated between the high-productivity regions off the Congo River to the north and off Namibia to the south, the site is part of a transect that will provide information on "pelagic background" sedimentation for the latest Neogene. Sediments from this region indicate lower primary productivity in overlying waters compared with the adjacent upwelling areas. Thus, the influence of the open ocean is more pronounced and will provide a tie-in of coastal ocean history to the record of the pelagic en-vironment. This will allow us to study the cross-correlations of climate-driven ocean dynamics across these two regimes. One of the intriguing aspects of this record is the low opal content associated with high organic matter accumulation. This paradox indicates a strong influence of the quality of subsurface waters, which is set elsewhere in the system, possibly in the Subtropical Convergence Zone. Other topics of importance are the control of variation by precession, reflecting the changing dominance of trade-wind and monsoonal effects.

 

Three holes were cored with the advanced hydraulic piston corer (APC) at Site 1079 to a maximum depth of 128.3 meters below seafloor (mbsf), which recovered an apparently continuous hemipelagic sedimentary section spanning the last 700 k.y. of the Pleistocene. Hole 1079A was cored with the APC to 121.0 mbsf. Twenty-two cores were taken at Hole 1079B with the APC to a depth of 128.3 mbsf, and Hole 1075C was cored to 126.8 mbsf.

 

Sediments form one lithostratigraphic unit composed predominantly of uniform olive-gray silty clay with varying amounts of nannofossils and foraminifers. There also are a few discontinuous light olive-gray silt laminae (1–2 mm thickness) present below 80 mbsf. Rare gastropods and frequent shell fragments are disseminated throughout the uppermost 60 mbsf. Whitish gray nodules, 1–2 mm in diameter, are sparse in some of the uppermost cores and become more frequent below 90 mbsf. The calcium carbonate content ranges from 7 to 19 wt%, averaging ~13 wt%.

The silt component is dominated by smectite, kaolinite and/or illite, quartz, the feldspar minerals albite and microcline, and muscovite. The biogenic component is represented by frequent foraminifer fragments and nannofossils. Secondary minerals include dolomite, glauconite, and pyrite. Feldspar, in contrast to quartz, is not supplied by the Congo River; it originates from igneous complexes in southern Africa and therefore probably represents a southern sediment source fed by the Kunene River or eolian dust. Sedimentation rates are high (~400 m/m.y.) in the uppermost 90 mbsf and lower (~50 m/m.y.) between 90 and 120 mbsf.

 

Detailed comparisons between the magnetic susceptibility record 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 132 meters composite depth (mcd).

 

Preservation of calcareous nannofossil specimens is good to very good. The overall abundance ranges from very abundant to abundant throughout the entire section. Reworked specimens (Neogene) are rare to common between 90 and 120 mbsf. Based on the oldest identified datum, the bottom age of Hole 1079A is estimated at 0.7 ± 0.05 Ma. The benthic foraminifers are well preserved and abundant at Site 1079; however, the diversity is relatively low and dominated by Bolivina species. Planktonic foraminifers are common in the upper 50 mbsf, but abundance levels fall drastically below 60 mbsf. Diatoms, silicoflagellates, and radiolarians are absent throughout the entire section.

Magnetic inclinations and declinations after alternating-field (AF) demagnetization at 20 mT indicate that only the Brunhes (C1n) normal polarity Chron is recorded. Short reversal events in the Brunhes Chron were not found, despite the high sedimentation rates.

 

Sediments average 3.0 wt% total organic carbon (TOC), which is high for ocean margin areas and reflects a history of elevated primary production in this area. Interstitial water chemistry studies document a sequence of diagenetic processes in the upper 50 mbsf that are caused largely by the degradation of organic matter and carbonate dissolution-reprecipitation reactions. Among these are moderately high levels of methane and carbon dioxide generated by in situ microbial activity. Increases in interstitial water sulfate, chloride, and salinity from 70 mbsf to the bottom of Hole 1079A at 120 mbsf may reflect the influence of evaporite dissolution and brine formation. Profiles of salinity, dissolved chloride, and methane do not indicate the presence of gas hydrate in Site 1079 sediments.

 

Physical sediment properties were determined both by high-resolution MST core logging and index properties measurements. Magnetic susceptibility and gamma-ray attenuation porosity evaluator (GRAPE) signals reveal pronounced cyclicities, which were used for high-quality stratigraphic correlation in conjunction with digital color data.

Site 1079 was drilled as a companion to Site 1078 to make an east–west transect in the Eastern Angola Basin. High regional productivity and a strong influx of terrigenous clays and silts from a rapidly rising shore (with cliffs and deep, narrow canyons near Lobito) provide an expanded record of the late Quaternary. High productivity is greatly favored by the supply of nutrients from subsurface waters, probably involving the Angola Dome. Changes in productivity should record the intensity of domal and coastal upwelling and, possibly, the movement of the Angola-Benguela Front. Variations in the supply of marine water or terrigenous matter will track climate changes in the drainage basins supplying the sediment, as well as the rates of uplift along the Angolan coast. Such uplift could be caused by salt tectonics below the shelf, stimulated by increasing amplitudes of sea-level change. If so, terrigenous influx should have greatly increased during the last 1 m.y.