Allochthonous ostracods in the South China Sea and their significance in indicating downslope sediment contamination

Abstract

The modern distribution pattern of allochthonous ostracods in the South China Sea was analyzed, and their palaeoceanographical significance in indicating downslope sediment contamination was testified using the sediment record of a core. General characteristics of allochthonous ostracods in surface sediments of the South China Sea include (1) a distribution limited to the shelf and slope off southern China and around coral reef-studded shoal areas; (2) a distinct depth range and assemblage composition of allochthonous forms between the areas on the continental slope and around coral reefs; and (3) a downslope decline of assemblage abundance, mean shell size and adult to juvenile ratio, although this tendency is reversed at depths below 2000 m, probably due to the increased carbonate dissolution and lowered productivity of indigenous ostracods in the deep sea. The distribution patterns confirm the idea that post-mortem transport of marine ostracods occurs in a downslope manner, and at the same time suggests that allochthonous ostracods are suitable indicators of sedimentary source. An analysis of ostracods from the core of Sonne-95 Cruise Site 17940, taken from the northern slope of the South China Sea, reveals markedly higher accumulation rates and abundance of allochthonous forms in the transitional phases from cold to warm periods than in the quiet periods in between, implying that downslope sediment transport from the shelf area was significantly intensified during transgression and regression.