Biological productivity in the Southern Ocean has a strong impact on the silica cycle in the World Ocean. Silica is an important nutrient in the marine environment and is utilized by diatoms, silicoflagellates, radiolarians, and sponges to build up their frustules and spicules. It is estimated that up to 37% of the global biogenic silica accumulation is taking place in Southern Ocean sediments, particularly within the pelagic "opal belt" south of the Polar Front (Ragueneau et al., 2000). Moreover, it is suggested that variations in atmospheric CO2 concentrations, which contributed to the distinct glacial-interglacial cyclicity of the late Quaternary climate, are linked to changes of carbon dioxide uptake in the Southern Ocean (Kumar et al., 1995; Francois et al., 1997; Elderfield and Rickaby, 2000). Therefore, the reconstruction of paleoproductivity in the Southern Ocean and its connection to global environmental changes both on long-term and short-term timescales is of major importance for our understanding of the Earth climatic system.
In the cold surface waters of the Southern Ocean, production of diatoms prevails. Consequently, biogenic components in the underlying deep-sea sediments mainly consist of the remains of this siliceous phytoplankton (e.g., Goodell, 1973). The linkage between biosiliceous production in the euphotic zone and the accumulation of opal within the seabed, however, is quite complex because various processes influence the preservation of the siliceous tests (e.g., Ragueneau et al., 2000). The most important processes are (1) dissolution in the water column and the surficial sediment and (2) sediment redistribution ("winnowing" and "focusing") (Nelson and Gordon, 1982; van Bennekom et al., 1988; Francois et al., 1997; Schlüter et al., 1998; Frank et al., 2000; Pondaven et al., 2000). Furthermore, supply of lithogenic and/or calcareous particles controls the opal concentrations in the sediment by both dilution and preservation effects (Archer et al., 1993; DeMaster et al., 1996; Ragueneau et al., 2000). Therefore, the potential influence of these processes on the opal record, which is archived in the sediments, needs to be considered when using the opal signal as a proxy for paleoproductivity. Nevertheless, opal records in upper Quaternary sedimentary sequences from the Bellingshausen Sea apparently reflect climatic-induced productivity changes in the surface waters (Pudsey and Camerlenghi, 1998; Hillenbrand, 2000; Pudsey, 2000).
In this chapter, we present contents and accumulation rates of biogenic opal in upper Miocene to Quaternary drift sediments recovered on the continental rise west of the Antarctic Peninsula. We will discuss how dissolution processes and supply of nonbiosiliceous particles have affected opal preservation in the drift deposits. It will be shown that the opal record can be used as a proxy for the reconstruction of relative productivity changes in the polar Southern Ocean on long-term timescales, although the original productivity signal is masked. We highlight how both regional paleoclimatic conditions and Southern Ocean circulation, which is strongly coupled to global paleoceanography, influenced the productivity changes observed in the high-latitude environment west of the Antarctic Peninsula.
Sites 1095, 1096, and 1101 are located in the Bellingshausen Sea, which represents a marginal sea in the Pacific sector of the Southern Ocean (Fig. F1). These sites were drilled on the upper continental rise west of the Antarctic Peninsula, which is characterized by nine large sediment mounds, elevated up to 900 m above the surrounding seafloor (Rebesco et al., 1996, 1998). The mounds, which were interpreted as asymmetric contourite drifts, are elongated toward the abyssal plain and are separated by 5-km-wide and 150-m-deep channels originating in dendritic feeder gullies at the base of the continental slope.
All three sites are located within the Antarctic Zone that is bounded by the Polar Front in the north and by the southern boundary of the clockwise flowing Antarctic Circumpolar Current (ACC) in the south (Orsi et al., 1995). Because of seasonal sea-ice coverage, open-water conditions in this zone prevail only during spring and summer, for about 8 months above Drift 4 and for about 3 months above Drift 7 (Parkinson, 1994). During these periods, light permeates the surface waters and thus promotes phytoplankton productivity. The water column in the Bellingshausen Sea includes a thin surface layer of cold Antarctic Surface Water (AASW), underlain by warm, saline Circumpolar Deep Water (CDW), which locally protrudes onto the continental shelf (Hofmann et al., 1996). CDW in the Pacific sector of the Southern Ocean represents a mixture of North Atlantic Deep Water (NADW) and recirculated waters from the Indian and Pacific Oceans (Patterson and Whitworth, 1990). The flow of relatively warm NADW to the Southern Ocean is assumed to govern the heat budget of the CDW (e.g., Denton, 2000). Warmth delivered by the CDW has a strong impact on the overlying surface waters and on the survival of sea ice (e.g., Hofmann et al., 1996; Jacobs and Comiso, 1997).
Along the flanks of Drift 7, bottom-water flow with a mean current velocity of ~6 cm/s follows the bathymetric contours in an counterclockwise manner (Camerlenghi et al., 1997). The contour current was concluded to represent the southwestward extension of Weddell Sea Deep Water (WSDW) (Camerlenghi et al., 1997), which leaves the Weddell Sea via deep topographic gaps in the South Scotia Ridge and flows along the lower continental slope around the northern tip of the Antarctic Peninsula up to at least 65°W (Nowlin and Zenk, 1988). On the other hand, gradients in dissolved oxygen, nutrients, and density within the bottom water of the Bellingshausen Sea are interpreted to indicate a confinement of WSDW to the South Shetland Trench (Sievers and Nowlin, 1984) and an eastward supply of bottom water from the ACC to the Antarctic Peninsula continental rise (Chernaykova and Stunzhas, 1998; Orsi et al., 1999). A bottom-water circulation from west to east, however, might be precluded at least for the upper continental rise because such a flow pattern would contradict the current directions measured near Drift 7.