Foraminiferal study of ODP Hole 189-1171

Sequence boundary ages determined in shallow-water sediments obtained from ODP (Ocean Drilling Program) Leg 189 Site 1171 (South Tasman Rise) compare well with other stratigraphic records (New Jersey, United States, and northwestern Europe) and d18O increases from deep-sea records, indicating that significant (>10 m) eustatic changes occurred during the early to middle Eocene (51-42 Ma). Sequence boundaries were identified and dated using lithology, bio- and magnetostratigraphy, water-depth changes, CaCO3 content, and physical properties (e.g., photospectrometry). They are characterized by a sharp bioturbated surface, low CaCO3 content, and an abrupt increase in glauconite above the surface. Foraminiferal biofacies and planktonic/benthic foraminiferal ratios were used to estimate water-depth changes. Ages of six sequence boundaries (50.9, 49.2, 48.5-47.8, 47.1, 44.5, and 42.6 Ma) from Site 1171 correlate well to the timings of d18O increases and sequence boundaries identified from other Eocene studies. The synchronous nature of sequence boundary development from globally distal sites and d18O increases indicates a global control and that glacioeustasy was operating in this supposedly ice-free world. This is supported by previous modeling studies and atmospheric pCO2 estimates showing that the first time pCO2 levels decreased below a threshold that would support the development of an Antarctic ice sheet occurred at ca. 51 Ma. Estimates of sea-level amplitudes range from ~20 m for the early Eocene (51-49 Ma) and ~25 m to ~45 m for the middle Eocene (48-42 Ma) using constraints established for Oligocene d18O records.

从大洋钻探计划（Ocean Drilling Program，ODP）189航次1171站位（塔斯曼海南部海域）采获的浅水沉积物中测定的层序界面年龄，与美国新泽西州、欧洲西北部等区域的地层记录以及深海记录中的氧同位素δ¹⁸O（d18O）升高特征吻合良好，表明始新世早期至中期（51~42 Ma）发生了显著（>10 m）的全球海平面变化。层序界面的识别与定年依托岩性、生物地层学与磁地层学、水深变化、碳酸钙（CaCO₃）含量及物理性质（如分光光度法）完成。此类界面以清晰的生物扰动面、低碳酸钙含量以及界面上方海绿石含量的突增为典型特征。研究通过有孔虫生物相及浮游/底栖有孔虫比值估算水深变化。1171站位的6个层序界面年龄（50.9、49.2、48.5~47.8、47.1、44.5及42.6 Ma），与其他始新世研究中识别出的δ¹⁸O升高节点及层序界面时代高度匹配。全球多个远源沉积区的层序界面发育与δ¹⁸O升高具有同步性，这表明海平面变化受全球因素调控，且在这个原本被认为无冰的地球时期，冰川型海平面变化机制已然运作。此前的模拟研究与大气二氧化碳分压（pCO₂）估算结果也佐证了这一结论：大气pCO₂首次降至可支撑南极冰盖发育的阈值以下的时间约为51 Ma。基于渐新世δ¹⁸O记录建立的约束条件，估算得到的海平面升降幅度为：早始新世（51~49 Ma）约20 m，中始新世（48~42 Ma）约25~45 m。