Census of calcareous nannofossil taxa expressed as percent of total number of individuals counted, ODP Legs 113, 114, 119, 120

Quantitative analysis of upper Eocene-upper Oligocene calcareous nannofossil assemblages from five Ocean Drilling Program sites in the Atlantic and Indian Ocean sectors of the Southern Ocean reveals an abrupt increase in cool-water taxa at the top of magnetic Subchron C13R ca. 35.9 Ma, coincident with an enrichment of ~1? d18O in the planktonic foraminifers at these sites. The synchrony of the abrupt increase in cool-water taxa in the Southern Ocean renders this event a useful biostratigraphic datum at southern high latitudes. This earliest Oligocene cool-water taxa increase was the sharpest and largest during the late Eocene-late Oligocene interval and indicates a drop in surface-water temperature of more than 3°C in the Southern Ocean. This suggests that the earliest Oligocene d18O shift represents primarily a temperature signal; a small portion (~0.2?) is attributable to a global ice-volume increase.对南大洋大西洋与印度洋区段内5个大洋钻探计划（Ocean Drilling Program, ODP）站位的始新统上部-渐新统上部钙质超微化石（calcareous nannofossil）组合开展定量分析，结果显示：在磁亚带C13R顶部（约35.9 Ma），冷水型类群出现突变式富集，这与同期站位浮游有孔虫的δ¹⁸O值升高约1‰完全吻合。南大洋冷水型类群突变增多事件的同步性，使其成为南极高纬度地区极具实用价值的生物地层标志面。该渐新世最早期的冷水型类群增多事件，是始新世晚期至渐新世晚期时段内幅度最陡、规模最大的一次，指示南大洋表层水温下降幅度超过3℃。研究结果同时表明，渐新世最早期的δ¹⁸O偏移主要反映温度信号，仅约0.2‰的贡献来自全球冰量增加。