Recent deep-sea benthic foraminiferal distributions in the South China and Sulu Seas

Abstract

Thirty-nine surface sediment samples collected from the South China and Sulu Seas were examined to determine the bathymetric distribution of benthic foraminifera in these two basins and the factors responsible for these distributions. The benthic foraminiferal faunas of both basins are highly diverse but have low absolute abundances; benthic number and species diversity systematically decrease with increasing water depth. Four faunal assemblages were identified for each basin by Q-mode principal component analysis. The Globocassidulina subglobosa/Uvigerina assemblage (< 1500 m) and the Uvigerina assemblage (< 1400 m) represent the shallowest assemblages in the South China Sea and Sulu Sea, respectively, and are found within the oxygen minimum zone of each basin where the sedimentary organic carbon content is highest and the pore water oxygen penetration depth (Z o) is shallowest. The Bulimina aculeata assemblage occurs at water depths between 1700 and 2000 m in the southeastern South China Sea and is also associated with relatively high sedimentary organic carbon content. The Astrononion pusillum assemblage is found in South China Sea at the depths between 1500 and 3200 m. Below the lysocline (∼ 3200 m) in the South China Sea, this assemblage is replaced by the agglutinated Rhabdammina abyssorum assemblage. This assemblage reflects the presence of a water mass that is highly undersaturated with respect to calcite. In the Sulu Sea, the Pyrgo murrhina assemblage occupies water depths from 1400 to 2200 m. Below 2200 m, two deep water assemblages are present that are both dominated by Oridorsalis umbonatus. A correlation between various environmental and benthic foraminiferal variables indicates that salinity, temperature, sediment grain size, water depth, and bottom water oxygen concentration are not key factors controlling the benthic foraminiferal distribution in either the South China Sea or the Sulu Sea. Rather, organic carbon content and oxygen penetration depth in sediments appear to play important roles in determining the distribution of benthic foraminifera in both basins.