早第四纪西太平洋沉积物中底栖有孔虫Favocassidulina favus的分布及其意义

通过对早第四纪(2 500~1 600 ka) ODP 807、1143和1146站位底栖有孔虫Favocassidulina favus含量变化的对比发现,该种在西太平洋ODP 806站位中持续出现,且最高含量可以达到15%;南海ODP1143和1146站位,早第四纪的沉积物中该种的含量较少,在7%以下,并有逐渐增加的趋势,且表现出"间断性",即在2 500~1 600 ka期间,南海沉积物中该种主要在冰期时"出现",间冰期时缺乏;在约2 100~1 950 ka和1 830~1 750 ka期间,未在南海沉积物中发现该种。根据对已有研究资料的分析和西太平洋ODP 807、1146和1208站位底栖有孔虫氧碳同位素的对比发现,该种在南海的分布可能受到西太平洋北部来源的深层水(PDW)和南部来源的深层水(UCDW)控制范围变化的影响,即在该种进入南海的早第四纪冰期期间南部来的UCDW发展、扩大,其前锋最少可以影响到北纬20°,生活在UCDW中的F.favus可以随着UCDW的扩展进入南海,而在间冰期及2 100~1 950 ka和1 830~1 750 ka期间,UCDW前锋影响的范围可能在20°N以南,而此时PDW的前锋可以像现在一样影响到赤道以南的海域,其海水性质不适应F.favus生存,而使该种在南海消失。

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.