HOLE 1082C

Position: 21°5.6690'S, 11°49.2342'E

Start hole: 0825 hr, 16 September 1997

End hole: 0200 hr, 17 September 1997

Time on hole: 17.58 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 1293.5

Total depth (drill pipe measurement from rig floor, mbrf): 1495.5

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

Water depth (drill pipe measurement from sea level, m): 1282.1

Penetration (mbsf): 202

Coring totals:

    Type: APC

    Number: 24

    Cored: 202 m

    Recovered: 217.83 m (107.84%)

Lithology:

    Subunit IA: intercalated intervals of nannofossil- and foraminifer-rich clay

    Subunit IB: intercalated intervals of nannofossil- and diatom-rich clay and nannofossil-rich diatomaceous clay

 

Principal results: Site 1082 is located in 1280-m deep water ~120 km to the southeast of Deep Sea Drilling Project (DSDP) Site 532, within the Northern Cape Basin. Together with Ocean Drilling Program (ODP) Sites 1081 and 1083 and DSDP Sites 532 and 362, it is part of a transect that is central to the reconstruction of the history of the Benguela Current. Site 1082 is closest to the coast and is expected to contain a direct record of upwelling history in the Walvis Bay area. The DSDP sites are well seaward of the upwelling center but contain an upwelling signal that was transported to this location by the Benguela Current and its filaments and eddies. Compared with DSDP Site 532, which shows evidence of sediment redeposition, Site 1082 offers a more continuous and less disturbed sequence.

 

Three holes were cored with the advanced hydraulic piston corer/extended core barrel (APC/XCB) at Site 1082 to a maximum depth of 600.6 meters below seafloor (mbsf). Hole 1082A was cored with the APC to 128.6 mbsf and was extended with the XCB to a depth of 600.6 mbsf. Hole 1082A was logged with a full suite of sensors (seismostratigraphic suite, lithoporosity suite, Formation MicroScanner [FMS] suite, and the geological high-sensitivity magnetic tool [GHMT]) from 599.3 to 65 mbsf. At Hole 1082B, 14 cores were taken with the APC to 127.0 mbsf. Hole 1082C was cored with the APC to 202 mbsf. Detailed comparisons between the magnetic susceptibility and the gamma-ray attenuation porosity evaluator (GRAPE) density records generated on the multisensor track (MST) and the color reflectance measured with the Minolta spectrophotometer demonstrated complete recovery of the sedimentary sequence down to 141 meters composite depth (mcd).

 

Drilling at Site 1082 recovered an apparently continuous hemipelagic sedimentary section spanning the upper Miocene to Holocene (0–5.8 Ma). The upper part of the succession is composed of moderately bioturbated, intercalated intervals of olive to black clays, which contain varying abundances of diatoms, nannofossils, foraminifers, and radiolarians. Three subunits are defined based on the various abundances and types of microfossils in the sediments: nannofossil- and foraminifer-rich clay (0–112 mbsf), diatom-rich clay (112–369 mbsf), and nannofossil clay (369–475 mbsf). The underlying lithostratigraphic unit (475–590 mbsf) consists of homogeneous, greenish gray nannofossil ooze. The lithostratigraphy at Site 1082 can be correlated to the one identified at Site 1081. Differences between these two sites are the higher sedimentation rates (70–200 m/m.y.) and the better temporal resolution at Site 1082. Sediments at Site 1082 also have higher abundances of nannofossils than those at Site 1081.

 

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. Dolomite rhombs are observed in the diatom-rich clay and the nannofossil ooze. The biogenic component is represented by varying abundances of foraminifers (whole and fragments), nannofossils, diatoms, radiolarians, sponge spicules, and silicoflagellates.

 

Fine biostratigraphic resolution was achieved by integrating datums from all microfossil groups. Calcareous nannofossils are abundant within the entire section. Planktonic foraminifers indicate upwelling at 70 mbsf, and downhole faunal variations indicate that a warm surface-water current (Angola Current) reached the region in the past. Tropical to warm subtropical species appear at 36 mbsf. The surface-water warming is associated with a decrease in upwelling, as indicated by a reduced abundance of Globigerina bulloides. The benthic foraminiferal fauna shows high diversity throughout the entire section. The record of diatom abundance points to a substantial increase in deposition during the upper Pliocene and lower Pleistocene intervals, reaching a maximum in upper Pliocene sediments, followed by a decrease within the Pleistocene interval at ~1 Ma. Overall abundances are low, or diatoms are absent, in upper Miocene and lower Pliocene sediments. The diatom content at Site 1082 probably reflects a varying nutrient supply that could be related to upwelling of nutrient-rich deeper waters and high biological productivity, especially in the upper Pliocene sediment. The diatom assemblage is similar to that at Site 1081 and consists mainly of a mixture of upwelling-indicator species.

 

The polarity of the remanent magnetization was determined from the magnetic declinations and inclinations of APC cores and from the magnetic inclinations of XCB cores after alternating-field (AF) demagnetization at 20 mT. All chrons from the Brunhes (C1n) to the onset of the C3An at ~6 Ma could be identified. A short reversal event (Cobb Mountain) was observed at all three holes within the Matuyama Chron. The Reunion event (2.14–2.15 Ma) appears at Hole 1082C between ~196 and 197 mbsf.

 

Well-developed cycles, in which concentrations of calcium carbonate and organic carbon vary between 1 and 85 wt% and <0.1 and 16.1 wt%, respectively, reflect fluctuations in the elevated marine production associated with the Benguela Current. Higher concentrations of organic carbon from 0 to 260 mbsf record higher productivities during the last 2 m.y. than earlier in the history of this upwelling system. The interstitial water chemical profiles record a relatively shallow (0–20 mbsf) region in which dia-genetic degradation of organic matter consumes sulfate and produces increases in alkalinity, ammonium, and phosphate that ultimately exceed those found at nearby Site 1081 on the Walvis Ridge. These changes are accompanied by calcite dissolution and dolomite precipitation, which are recorded by increases in dissolved strontium and decreases in dissolved magnesium and calcium.

 

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

 

Logging in Hole 1082A is characterized by a regular hole size (10- to 11-in diameter) with numerous small enlargements from 530 to 120 mbsf and by washout zones at the top and bottom of the logged interval. Thirteen dolomitic layers were identified in the downhole logs, characterized by very high velocity, resistivity, and density, and by low gamma-ray intensity. Dolomitic layers are present in the entire interval, but are particularly concentrated in the lower half. The core and log measurements of natural gamma-ray intensity are very similar and can be used for detailed correlations between the core and log data sets. In Hole 1082A, log depth is similar to core depth. The logging data from Sites 1081 and 1082 show a reliable correlation.

 

We expect that glacial/interglacial climatic cycles are well developed at Site 1082 in terms of productivity, carbonate dissolution, and terrigenous sedimentation cycles. In addition, the sediments from Site 1082 will document the supply of minerals and plant remains from land as a function of changing climate and sea level. Together with the results of DSDP Site 532, the record of ODP Site 1082 will allow the definition of offshore gradients and the high-resolution reconstruction of the intensity of the Benguela Current. We expect detailed information of the history of upwelling, both on the scale of glacial/interglacial cycles and sub-Milankovitch cycles. Trends will be compared with the records north and south of the Walvis Ridge to identify long-term changes in boundary conditions.