DISTRIBUTION OF BENTHIC FORAMINIFERAL FAVOCASSIDULINA FAVUS IN WESTERN PACIFIC DURING EARLY QUATERNARY AND ITS IMPLICATIONS

Abundances of benthic foraminiferal species of Favocassidulina favus are compared among ODP Sites of 807, 1143 and 1146 for the period of early Quaternary (2 500~1 600 ka). The results show that abundance of the species reach 15% at Site 807 in the western Pacific, and below 7% at Sites 1143 and 1146 in the South China Sea. The species continuously lived in the Ontong Java Plateau of the western Pacific during early Quaternary, but survived intermittently in the South China Sea (SCS) during 2 500~2 100 ka, 1 950~1 830 ka, and 1 750~1 600 ka respectively. Two water masses, the Pacific Deep Water (PDW) and the Upper Circumpolar Deep Water (UCDW) within approximately same density (and depth) range, met and balanced at the low latitude area of the Pacific. The UCDW flows in and the PDW out. The PDW is marked by low temperature, low oxygen and low salinity, high nutrients, high silica, and old age. Comparing to the PDW, the UCDW has higher salinity and lower sediment budget. Previous studies show that the species lived in the waters deeper than 2600 m are influenced by UCDW. Based on the stable isotopic data of oxygen and carbon of benthic foraminiferal species and F. favus studies, we conclude that variation in F. favus in the South China Sea may indicate the balance of the two waters masses of UCDW and PDW. F. favus appeared in the SCS only when the front of the UCDW reached at least 20°N and the PDW retreated to the north of 20°N or further, which happened during the periods of 2 500~2 100 ka, 1 950~1 830 ka, and 1 750~1 600 ka. F. favus disappeared from the SCS when PDW expanded with its front across the equator to the south hemisphere, which happened in the periods of 2 100~1 950 ka and 1 830~1 750 ka.