Recent planktonic foraminifers and their relationships to surface ocean hydrography of the South China Sea

Abstract

The relative abundances of Recent planktonic foraminifers from surface sediments on the sea floor reveal relationships to hydrographic variables of the ocean surface. To investigate whether the effect of the vertical structure of oceanic upper-layer environments is important to the changes in the abundance of planktonic foraminifers, a set of 173 coretop faunal data from modern South China Sea (SCS) surface sediments was compiled for comparison with direct observations of sea-surface temperature (SST) and the depth of thermocline (DOT). The coretops used in this study are distributed within the area of 25°N and 105°–125°E, with water depths ranging from 68 to 3990 m. The relationships between the abundances of planktonic foraminifer species and hydrographic variables were examined using simple correlation analyses. In the analyses, four groups of planktonic foraminifers with different ecological preferences were identified: Group I – faunas primarily reliant on DOT, with positive correlations ( N. dutertrei), Group II – faunas reliant on both SST and DOT, showing negative correlations with SST and positive correlations with DOT ( G. glutinata, P. obliquiloculata, and G. bulloides, Group III – faunas reliant on both SST and DOT, showing positive correlations with SST and negative correlations with DOT (including G. sacculifer, G. menardii, and G. aequilateralis), and Group IV – faunas reliant on both SST and DOT, with positive correlations ( G. ruber). Though each of these species displays selectivity of ecological controls, these results indicate that in the SCS, correlations between faunal abundances of planktonic foraminifers and SST or DOT are about equally significant. This study also indicates a need for re-evaluating the relationships between planktonic foraminifer abundances and many other important hydrographic variables in the upper-layer of oceans on a more regional scale.