3. Radiolarians from Northern Cape Basin, Site 1082¹

A.L. Weinheimer²

INTRODUCTION

A primary objective of Leg 175 was to investigate the upwelling history of the Benguela Current. Site 1082 (21° 5.6373'S, 11° 49.2361'E) lies at the landward end of a transect extending from the coast across the Benguela Current. Located adjacent to the Walvis Bay in 1280 m water depth, Site 1082 recovered sediments are expected to reveal variability in upwelling dating as far back as the late Pliocene. Off the coast of Walvis Bay, the Benguela Current transports subpolar transition waters northward to the Walvis Ridge. A front forms there between the Benguela Current and tropical waters in the southward-flowing Angola Current (Salat et al., 1992). This front moves in response to seasonal-interannual changes in the wind field (Shannon, 1985; Shannon et al., 1986, 1987; Meeuwis and Lutjeharms, 1990). Upwelling along the coast is found over the shelf in several well-established cells, as well as along the shelf-slope break, and extends over the 1000-m isobath. Streaming filaments along the coast also carry upwelled water off shore (Shannon, 1985). The upwelled nutrient-rich waters are sourced from the South Atlantic central water mass, which is a mixture of subtropical and subantarctic water masses. Below the central water mass lies Antarctic intermediate water (Shannon and Hunter, 1988; Stramma and Peterson, 1989).

The upwelling system supports a robust marine community (Shannon and Pillar, 1986) where radiolarians are abundant (Bishop et al., 1978). The endemic nature of radiolarians makes them useful in reconstructing the paleocirculation patterns. The biogeographic distribution of many species is limited by water-mass distribution. In a given geographic region, species may also have discrete depth habitats. However, their depth of occurrence can change worldwide because the depths of water masses vary with latitude (Boltovskoy, 1999). Consequently, species found at shallow depths at high latitudes (cold-water fauna) are observed deeper in the water column at lower latitudes. The low-latitude submergence of cold-water species broadens their distribution, resulting in species distributions that can cover multiple geographic regions (Kling, 1976; Casey, 1971; Boltovskoy, 1988). Since radiolarian distribution is closely related to water-mass distribution and controlled by climatic conditions rather than geographic regions, similar assemblages characterize the equatorial, subtropical, transition, subpolar, and polar regions of ocean basins (Petrushevskaya, 1971a; Casey, 1989; Boltovskoy, 1999).

Numerous radiolarian species found in water masses in the Angola and Benguela Current systems have also been observed in plankton net samples, sediment traps, and surface-sediment studies in the Atlantic sector of the Southern Ocean, where they exhibited particular water-mass affinities (Abelmann, 1992a, 1992b; Abelmann and Gowing, 1997). This report presents data on the radiolarian fauna recovered from Site 1082 sediments in the form of a survey of species reflecting the latitudinal migration of the Angola-Benguela Front and upwelling. The data constitute a time series of relative radiolarian abundances at very high resolution (every 20 cm) of the upper 12 m of Hole 1082A.