HOLE 1083D

Position: 20°53.7138'S, 11°13.0734'E.

Start hole: 2310 hr, 18 September 1997

End hole: 1900 hr, 19 September 1997

Time on hole: 19.83 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 2189.9

Total depth (drill pipe measurement from rig floor, mbrf): 2386

Distance between rig floor and sea level (m): 11.6

Water depth (drill pipe measurement from sea level, m): 2178.3

Penetration (mbsf): 196.1

Coring totals:

    Type: APC

    Number: 21

    Cored: 196.1 m

    Recovered: 201.94 m (102.98%)

Lithology:

    Unit I: clayey nannofossil ooze

 

Principal results: Site 1083 is the deep-water drill site (2180 m) of the Walvis Ridge/Walvis Basin transect, which includes Ocean Drilling Program Sites 1081 and 1082 and Deep Sea Drilling Project Sites 532 and 362. Because of its relatively low sedimentation rates, one of the main objectives for drilling at Site 1083 is to provide an extended advanced hydraulic piston core (APC) record back in time. The greater water depth should also result in well-expressed carbonate cycles via cyclic dissolution intensity. In addition, this site is farthest from shore and should have the best representation of pelagic signals.

 

Four holes were cored with the APC at Site 1083 to a maximum depth of 202.3 meters below seafloor (mbsf), which recovered a relatively continuous hemipelagic sedimentary section spanning the last 2.6 m.y. Hole 1083A was cored with the APC to 201.3 mbsf. At Hole 1083B, 22 APC cores were taken to 202.3 mbsf. One failed mudline core was taken at Hole 1083C, and Hole 1078D was cored with the APC to 196.1 mbsf.

 

The sediments form one lithostratigraphic unit composed of moderately bioturbated clayey nannofossil ooze. This unit has been subdivided into two subunits based on the changing abundance of diatoms, which become more abundant below 130 mbsf. The stratigraphic variation in diatom abundance and the lithostratigraphic definitions are comparable to those at Sites 1081 and 1082. A major characteristic of sediments at Site 1083 is the repeated occurrence of dark–light color cycles throughout the drilled sequence. Similar to Site 1082, the lighter colored cycles are more calcium carbonate–rich compared with the adjacent, more clay-rich, darker colored cycles. Clay-rich intervals are generally 60 to 150 cm thick and occur approximately every 2 to 5 m.

 

The detrital component of the sediments consists of clay with rare silt-sized, angular and subangular, mono- and polycrystalline quartz and feldspar grains. Muscovite and biotite are present in trace amounts. Grain sizes of identifiable detrital components are relatively constant. Authigenic minerals, such as framboidal pyrite and dolomite rhombs, are rare or present in trace abundances only. Carbonate minerals are generally rare to frequent in abundance and, at times, are common.

 

The biogenic fraction of the sediment revealed abundant to very abundant nannofossils, abundant to common foraminifer fragments, rare siliceous sponge spicules, and trace amounts of radiolarians and silico-flagellates. Diatom abundances vary from common to barren. The relative abundances of the biogenic components change frequently within one core. The intercalated dark olive-brown and black clay intervals have distinctly lower abundances of biogenic components and occasionally show higher abundances of silt-sized mono- and polycrystalline quartz grains.

Sedimentation rates range from 60 to 140 m/m.y., with the highest values located within the last 0.26 m.y. and between 0.8 and 0.96 Ma (from the onset of the Jaramillo Chron to the onset of the Brunhes Chron).

 

An integrated biostratigraphic framework composed of both calcareous and siliceous microfossils was established, resulting in a well-constrained age model for Site 1083. Preservation of nannofossil specimens is good to very good. The scarcity of the late Pliocene index species Discoaster is probably related to colder than average surface-water temperatures (a combination of increased upwelling intensity and advection of subantarctic surface water) over the southwest African and Namibian continental margins during the last 500 k.y. of the Neogene. The changes in planktonic foraminiferal species indicate a northward shift in position of the Benguela Current associated with a shift in Southern Ocean circulation, or a reduced seasonality in southerly penetration of the Angola Current. Alternatively, the biotic change may indicate increased advection of Southern Ocean water in the Benguela Current system, or a combination of all factors. Radiolarians are present throughout. In most of the investigated samples, radiolarians are abundant and preservation is good. Diatom preservation is moderate. As was the case for Sites 1081 and 1082, the record of diatom abundance points to high deposition rates during the late Pliocene and early Pleistocene. The diatom assemblage is similar to that at Sites 1081 and 1082 and consists mainly of a mixture of upwelling-indicator and oceanic species. Two middle- to high-latitude cold-water indicator species suggest periods of intensified Southern Ocean input into the Benguela Current system.

 

A complete magnetostratigraphy was generated at Site 1083 after alternating-field (AF) demagnetization at 20 mT. All chrons from the Brunhes (C1n) to the termination of the Gauss (C2An) at 2.58 Ma could be identified.

Dark–light color cycles, in which concentrations of calcium carbonate and organic carbon (C_org) vary between 17 and 82 wt% and between 0.7 and 7.5 wt%, respectively, reflect fluctuations in the elevated marine production associated with the Benguela Current. Most interstitial water chemical trends are intermediate between the neighboring Walvis Sites 1081 and 1082. Sulfate is completely consumed within the upper 25 mbsf; both alkalinity and ammonium display strong increases through this depth range. The concentration of interstitial water strontium reaches a maximum that is two to three times higher than that observed at Sites 1081 and 1082, reflecting the greater availability of biogenic calcite at Site 1083.

 

Physical sediment properties were determined both by high-resolution multisensor track (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.

 

Of the Walvis Ridge/Walvis Bay transect sites, 1083 stands out because of its excellent microfossil record, which comprises both calcareous (foraminifers and nannofossils) and siliceous fossils (diatoms and radiolarians). This will allow the intercalibration of the messages contained in these various assemblages regarding the history of change in current regimes and upwelling. With the results obtained from this and other sites in this transect, a very exact reconstruction of conditions should be possible for the last 2.6 m.y. Intercalibration of physical properties (MST data and sediment color) and of chemical stratigraphy (CaCO₃ and C_org) with well-constrained information from microfossils should be of special interest at this site.