Stable isotope record of foraminifera from DSDP Hole 81-552A

It is now widely accepted that expansion of continental glaciers is one manifestation of profound paleoenvironmental change in latest Miocene time. The oxygen isotopic record in deep-sea cores is inferred to document brief pulses in glaciation, based on covariance of delta18O in benthic and planktonic foraminifera, which probably lowered latest Miocene sea level by a minimum of 60 m. Additional evidence of glacial activity in the oxygen isotope record is obscured by the small signal amplitude. Before further details of paleoenvironmental change can be investigated it is essential to have a high-resolution stratigraphy and chronology. This has not been achieved previously because most studies of the latest Miocene have had a sampling interval greater than the duration of events they seek to resolve. A high-resolution stable isotope record from the north Atlantic (DSDP 552) is correlated with a second record from the southwest Pacific (DSDP 588). Each core has magnetostratigraphy, but there are two possible interpretations of magnetic results in the Atlantic core. By assuming that brief events of 18O enrichment and 13C depletion in each core are synchronous it is possible to assign the most likely age model to the Atlantic core. Results of this exercise indicate that two delta18O maxima occurred at 4.8 and 5.2 Ma. If these events reflect continental glaciation, and if sea level was lowered enough to contribute to isolation and desiccation of the Mediterranean, then the Miocene/Pliocene boundary must be close to 4.8 Ma in age because Messinian deposits are latest Miocene in age. This interpretation is consistent with biostratigraphy at DSDP 552; magnetostratigraphy, biostratigraphy, and isotope stratigraphy at a site commonly referred to because of its proximity to the Mediterranean (DSDP 397, northwest African continental margin); and recent work on land in the Mediterranean region.

目前学界已普遍认为，大陆冰川扩张是晚中新世时期重大古环境变革的表现形式之一。基于底栖有孔虫与浮游有孔虫的δ¹⁸O（delta18O）协变特征，深海岩芯的氧同位素记录被推断为记录了冰川作用的短暂脉冲事件，该事件至少使晚中新世海平面下降了60米。不过，氧同位素记录中反映冰川活动的其他证据，因信号振幅过小而被掩盖。
在对古环境变化开展更细致的研究之前，获取高分辨率地层学与年代学数据是必不可少的前提。此前这一目标尚未达成，因多数晚中新世相关研究的采样间隔长于其试图解析的事件持续时长。
本研究将北大西洋（深海钻探计划<Deep Sea Drilling Project, DSDP>552孔）的高分辨率稳定同位素记录，与西南太平洋（DSDP 588孔）的另一套同位素记录进行了关联对比。两处岩芯均开展了磁地层学分析，但北大西洋岩芯的磁学结果存在两种可供选择的解释方案。通过假设两处岩芯中δ¹⁸O富集与δ¹³C亏损的短暂事件为同步发生，可为北大西洋岩芯确定最合理的年代模式。
本次研究结果显示，共出现两次δ¹⁸O极值，分别对应4.8 Ma与5.2 Ma。若这些事件对应大陆冰川作用，且海平面下降幅度足以导致地中海被隔离并发生干涸，那么中新世/上新世边界的年代应接近4.8 Ma，因墨西拿沉积属于晚中新世地层。这一解释与DSDP 552孔的生物地层学结果相符，同时也符合地中海附近（西北非洲大陆边缘）的DSDP 397孔的磁地层学、生物地层学及同位素地层学研究成果，以及近期地中海地区的陆上研究结论。