Biodiversity and biogeography of recent benthic foraminiferal assemblages in the south-western South China Sea (Sunda Shelf)
Species composition and spatial distribution patterns of modern shallow-water and bathyal benthic foraminifera were studied on the Vietnam Shelf and Sunda Shelf of the south-western South China Sea. The investigation is based on the analyses of Rose Bengal stained benthic foraminifera from 75 sites (R/V SONNE 115 cruise, Stattegger et al., 1997). The material revealed more than 800 taxa, 90 of which occur through the entire studied water depth range (50-2000 m). The remaining species exhibit a depth related distribution. Diversity indices (H(S) and Fisher’s Alpha) are high in both areas studied, while the species dominance is low. The standing stock values are between 20 and 330 indiv./10 cm2. Shallow water (< 200 m) assemblages from the Vietnam and Sunda Shelves exhibit significantly different species composition and distinct distribution patterns correlated to current activity, the depth of light penetration, substrate properties and the levels of organic matter reaching the benthos. On the Vietnam Shelf, winnowing processes are prevalent, and there may be reduced deposition of organic matter to the sea-floor due to stronger bottom currents. The dominance of large, symbiont-bearing Rotaliida on the Vietnam Shelf reflects a nutrient-deficient and high energy environment. The bathyal assemblages are strongly influenced by the decreasing rates of C org-flux with increasing water depth. The species, depending on high C org-flux, are found in the shallower waters and are succeeded by a more ‘oligotrophic’ species at greater depths. Six statistically independent faunal associations were recognised on the Sunda Shelf area (Heterolepa aff. dutemplei-Asterorotalia gaimardii; Bulimina marginata-Neouvigerina proboscidea; Siphotextularia foliosa-Bulimina mexicana; Uvigerina ex gr. auberiana-Ehrenbergina undulata; Nuttallides rugosus-Uvigerina peregrina; Astrononion novozealandicum-Eggerella bradyi) and three on the Vietnam Shelf area (Amphistegina papillosa-Nummulites venosus; Heterolepa aff. dutemplei-Cibicidoides pachyderma; Parrelloides bradyi-Oridorsalis umbonatus). Correlation between the total foraminiferal assemblages from the Sunda continental slope and C org-flux (r2=0,962) may be used to estimate C org-flux rates from the down-core fossil record.