SITE 1083

Site 1083 is the deep-water drill site (2180 m) of the Walvis Ridge/Walvis Basin transect (Fig. 1), which includes Sites 1081 and 1082 and DSDP Sites 532 and 362. Because of low sedimentation rates, one of the main objectives at Site 1083 is to provide an extended 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.

Site 1083 recovered a relatively continuous hemipelagic sedimentary section spanning the last 2.6 m.y. Sedimentation rates range from 6 to 14 cm/k.y. with the highest values located within the last 0.26 m.y. as well as between 0.8 and 0.96 Ma (from the onset of the Jaramillo Chron to the onset of the Bruhnes Chron). The sediments form one lithostratigraphic unit composed of moderately bioturbated clayey nannofossil ooze (Fig. 2). 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 to 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 silicoflagellates. 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.

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. Calcareous nannofossils are abundant to very abundant throughout the entire section. The scarcity of a 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 South African and Namibian continental margins during the last 500 k.y. of the Neogene. The changes in planktonic foraminifer 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-indicators and oceanic species. Two middle- to high-latitude cold-water indicator species indicate periods of intensified Southern Ocean input into the Benguela Current system.

A complete magnetostratigraphy was generated at Site 1083 after AF demagnetization at 20 mT. All chrons from the Brunhes (C1n) to the termination of C2An.1n (Gauss) at 2.58 Ma could be identified.

Dark-light color cycles, in which concentrations of CaCO3 and organic carbon vary between 17% and 82% and 0.7% and 7.5%, 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 MST core logging and index properties measurements. Magnetic susceptibility and GRAPE signals reveal pronounced cyclicities that were used for high-quality stratigraphic correlation in conjunction with digital color data. Detailed comparisons between the magnetic susceptibility and the GRAPE density records generated on the MST demonstrated complete recovery of the sedimentary sequence down to 218.1 mcd, with a gap between 46 and 47.5 mcd (Fig. 6).

Of the sites on the Walvis Ridge/Walvis Bay transect, Site 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 intercalibration of the messages contained in these various assemblages regarding the history of change in current upwelling and regimes. Together with the other sites of 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 chemical stratigraphy (CaCO3 and Corg) with well-constrained information from microfossils should be of special interest at this site.

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