(Table 1) Abundances of Discoaster species in ODP Hole 130-806C

Members of the calcareous nannofossil genus Discoaster have been used extensively to subdivide Tertiary deep-sea sediments into biostratigraphic zones or subzones (e.g., Martini, 1971; Bukry, 1973). Haq and Lohmann (1976) mapped biogeographic migrations of this group through time and over latitude. They suggested that expansions and contractions of Discoaster-dominated assemblages across latitudes reflect sea-surface temperature changes. Subsequently, late Pliocene Discoaster species were counted at closely spaced sample intervals from various Atlantic sites (Backman et al., 1986; Backman and Pestiaux, 1987; Chepstow-Lusty et al., 1989, 1991), and Indian Ocean as well as Pacific Ocean sites (Chepstow-Lusty, 1990). In addition to the biostratigraphic information revealing positions and the precision by which the different late Pliocene Discoaster species can be determined, these studies also demonstrated that discoasters strongly fluctuate in abundance as a function of time. These abundance variations occur in equatorial as well as temperate temperature regimes, and show periodicities that reflect orbital frequencies. Chepstow-Lusty et al. (1989, 1991) also suggested that the oscillating abundances partly represent productivity pressure, because discoasters tend to show low abundances under high productivity conditions and vice versa.

钙质超微化石（calcareous nannofossil）圆盘石属（Discoaster）的化石类群已被广泛用于将第三纪（Tertiary）深海沉积物划分为生物地层带或亚带（如Martini, 1971; Bukry, 1973）。Haq与Lohmann（1976）绘制了该类群随时间与纬度变化的生物地理迁移图谱，并提出以圆盘石属为主的生物组合在不同纬度间的扩张与收缩，可反映表层海水温度的变化。此后，学界在大西洋多个站位（Backman et al., 1986; Backman and Pestiaux, 1987; Chepstow-Lusty et al., 1989, 1991）以及印度洋与太平洋站位（Chepstow-Lusty, 1990），以高分辨率采样间隔对上新世晚期（late Pliocene）的圆盘石属物种开展了计数统计。除了可明确不同上新世晚期圆盘石属物种的产出层位与鉴定精度的生物地层学信息外，上述研究还证实，圆盘石属的丰度随时间呈现显著波动。此类丰度变化在赤道与温带温度环境中均有出现，其周期特征反映了轨道频率。Chepstow-Lusty et al. (1989, 1991)还提出，该丰度振荡部分源于生产力压力的影响：圆盘石属在高生产力条件下丰度往往偏低，反之亦然。