HOLE 1084C

Position: 25°30.8037'S, 13°1.6670'E

Start hole: 1750 hr, 24 September 1997

End hole: 1230 hr, 25 September 1997

Time on hole: 18.67 hr

Seafloor (drill pipe measurement from rig floor, mbrf): 2003.4

Total depth (drill pipe measurement from rig floor, mbrf): 2211

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

Water depth (drill pipe measurement from sea level, m): 1991.8

Penetration (mbsf): 207.6

Coring totals:

    Type: APC

    Number: 22

    Cored: 207.60 m

    Recovered: 217.82 m (104.92%)

Lithology:

    Subunit IA: foraminifer- and diatom-bearing nannofossil clay, and foraminifer-bearing diatom-rich clayey nannofossil ooze

    Subunit IB: nannofossil-rich diatomaceous clay, clayey nannofossil ooze, and diatom-bearing clay

 

Principal results: One of the main objectives of drilling at Site 1084 was to document the northward migration of the Benguela Current system from the Miocene to the Quaternary, as well as the shoreward and seaward migrations of the upwelling center. Filaments of cold, nutrient-rich waters from the coastal upwelling area extend well offshore and mix with low-productivity oceanic water, forming a zone of intermediate productivity at Site 1084. This site is expected to provide a high-resolution record because of high sedimentation rates. The close vicinity to the Lüderitz upwelling cell should result in well-expressed organic carbon (C_org), diatom, and coccolith cycles via cyclic productivity intensity. In addition, this site is closest to the area with elevated primary production and should have the best representation of coastal upwelling signals. A close tie-in between pelagic and terrigenous sedimentation is expected within the slope record.

 

Three holes were cored with the advanced hydraulic piston corer/extended core barrel (APC/XCB) at Site 1084 to a maximum depth of 605.0 meters below seafloor (mbsf), which recovered an apparently continuous hemipelagic sedimentary section spanning the Holocene to lower Pliocene (0–4.7 Ma). Hole 1084A was cored with the APC to 149.5 mbsf and was extended with the XCB to a depth of 605.0 mbsf. Hole 1084A 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 601 to 90 mbsf. At Hole 1084B, 20 cores were taken with the APC to 182.8 mbsf. Hole 1081C was cored with the APC to 202.0 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 175 meters composite depth (mcd).

 

Sediments form four lithostratigraphic units defined by the changes in the major lithology between clay and ooze. The uppermost lithology is composed of moderately bioturbated, intercalated intervals of clays that contain varying abundances of diatoms, nannofossils, foraminifers, and radiolarians. Three subunits are distinguished based on microfossil type and abundance. The section continues downward with ~100 m of clay-rich nannofossil diatom ooze, diatomaceous nannofossil ooze, and clay-rich nannofossil ooze. The following 50-m-thick unit is composed of clay with varying amounts of nannofossils and diatoms, underlain by a unit consisting predominantly of clayey nannofossil ooze.

Several decimeter-thick intervals of dark, organic-rich clay layers are present between 120 and 410 mbsf. Compared with the under- and overlying sediment layers, these horizons are characterized by lower carbonate contents averaging just <10 wt% and higher organic carbon ranging between 8 and 18 wt%. The detrital component of the sediments consists of clay with rare silt-sized, angular and subangular mono- and polycrystalline quartz grains. Subangular feldspar grains are present in trace amounts. Authigenic minerals include framboidal pyrite and dolomite rhombs in rare or trace amounts. The dark horizons generally contain slightly higher abundances of pyrite and exhibit significant compositional variations in the relative abundances of diatoms, foraminifers, and nannofossils. The biogenic component of the dark layers is commonly dominated by diatom resting spores.

 

Sedimentation rates range from 100 to 270 m/m.y., with the highest values located within the last 1 m.y. A second episode of high sedimentation rate (170 m/m.y.) is associated with an upper Pliocene diatom-rich interval.

An integrated biostratigraphic framework composed of both calcareous and siliceous microfossils was established, resulting in a well-constrained age model for Site 1084. Calcareous nannofossils are abundant within the top 280 mbsf and between 410 and 600 mbsf and provided 13 biohorizons, which were constrained within an average depth interval of 3 m. Planktonic foraminiferal zonation is difficult because of an absence of marker species and the dissolution of planktonic foraminifers, particularly in the lower Pleistocene to upper Pliocene sediments. It is difficult to determine whether the absence of marker species is caused by ecological conditions or by selective dissolution. Both cool- and warm-water faunas are present in the same assemblages downcore and may indicate an increased contribution from cooler Southern Ocean waters. The benthic foraminifers are abundant and well preserved, and abundances correlate well with the different lithostratigraphic units. Radiolarians are abundant and well preserved in almost all samples examined. Diatom preservation is moderate throughout Hole 1084A. In contrast to the other sites, diatom abundances, although highly variable, remain moderately high throughout the Pleistocene. In addition to the "background" diatom assemblage composed of a mixture of upwelling-indicator and oceanic species, we re-corded many more cold-water markers characteristic of the Southern Oceans than were observed at Sites 1081, 1082, and 1083.

 

A complete magnetostratigraphy was determined at Site 1084 after alternating-field (AF) demagnetization at 20 mT. All chrons from the Brunhes (C1n) to the latter part of the Gilbert (~4.4 Ma) were identified. No short polarity-reversal event during the Brunhes Chron was detected, despite the high sedimentation rates.

 

Sediments from 1084 are notably rich in marine organic matter. Well-developed light–dark sediment color cycles, in which concentrations of calcium carbonate and organic carbon vary between 1 and 69 wt% and between 1.2 to 18 wt%, respectively, record fluctuations in the elevated marine production associated with the Benguela Current. Higher con-centrations of organic carbon from 0 to 392 mbsf indicate that productivity was higher during the last 2 m.y. than earlier in the history of this upwelling system.

 

Interstitial water chemistry profiles at this organic carbon–rich site record some of the most extreme conditions of sediment diagenesis ever recovered in the history of Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) drilling. Maximum values of alkalinity (172 mM) and ammonium (50 mM) are greater than those observed at any site, except ODP Site 688, within the Peruvian upwelling system. Sulfate is completely depleted within the uppermost 5 mbsf, which is much more rapidly than at any other Leg 175 site and speaks to the availability of oxidizable organic matter. These changes are accompanied by calcite dissolution and dolomite precipitation, as recorded in the calcium, magnesium, and strontium distributions.

 

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.

 

Hole 1084A was logged with a full suite of sensors to continuously characterize the sedimentary changes through depth and to provide data for core-log integration. Hole conditions above 170 mbsf are poor with a regular increase in the hole size. Nineteen dolomitic layers and 114 anomalous, probably organic-rich, dark layers were identified from logging data. Natural gamma-ray intensity can be used for detailed core-log correlations and for correlation with Site 1082 in the Walvis Basin.

 

Based on pore-water chemistry and C_org concentrations, Site 1084 records by far the highest productivity of the Leg 175 sites. This was to be expected because the site is close to the most active upwelling cell off southwest Africa, the Lüderitz Bay. The sediments recovered will allow high-resolution documentation of the variability of coastal upwelling of the last ~4.7 m.y.