Oxygen and carbon isotopic compositions of foraminifers at DSDP Hole 72-517

Oxygen and carbon isotope analyses were performed on monospecific or mixed-species samples of benthic foraminifers, as well as on the planktonic species Globigerinoides ruber from a 24-m hydraulic piston core raised on the western flank of the Rio Grande Rise, at DSDP Site 517 (30°56.81'S and 38°02.47'W, water depth 2963 m) in the southwestern Atlantic. This site is presently located in the core of North Atlantic Deep Water (NADW). This is the first long isotopic record of Quaternary benthic foraminifers; it displays at least 30 isotopic stages, 25 of them readily correlated with the standard sequence of Pacific Core V28-239. The depths of both the Bruhnes/Matuyama boundary and the Jaramillo Event based on oxygen isotope stratigraphy agree well with paleomagnetic results. Quaternary faunal data from this part of the Atlantic are dated through isotopic stratigraphy and partially contradict data previously published by Williams and Ledbetter (1979). There was a substantial increase in the size of the earth's major ice sheets culminating at Stage 22 and corresponding to a l per mil progressive increase of d18O maximal values. Further, ice volume-induced isotopic changes were not identical for different glacial cycles. Oxygen and carbon isotope analyses of benthic foraminifers show that during Pleistocene glacial episodes, NADW was cooler than today and that Mediterranean outflow might still have contributed to the NADW sources. The comparison of coiling ratio changes of Globorotalia truncatulinoides with planktonic and benthic oxygen isotope records shows that there might have been southward excursions of the Brazil Current during the Pleistocene, perhaps related to Antarctic surface water surges. The question of the location of NADW sources during glacial maxima remains open.本研究针对西南大西洋里奥格兰德海隆西侧深海钻探计划（Deep Sea Drilling Project, DSDP）517站位（30°56.81'S、38°02.47'W，水深2963 m）获取的24 m长液压活塞岩心，开展了氧、碳同位素分析；分析样品涵盖单种或混合种底栖有孔虫（benthic foraminifers），以及浮游有孔虫红色抱球虫（Globigerinoides ruber）。该站位目前处于北大西洋深层水（North Atlantic Deep Water, NADW）的核心区域。本数据集为首条第四纪底栖有孔虫长同位素记录，共识别出至少30个同位素阶段，其中25个可与太平洋V28-239岩心的标准序列进行可靠对比。基于氧同位素地层学（oxygen isotope stratigraphy）得到的布容-松山边界（Bruhnes/Matuyama boundary）与贾拉米洛事件（Jaramillo Event）的深度数据，与古地磁结果吻合度极佳。大西洋该区域的第四纪生物群数据通过同位素地层学定年，其结果与Williams与Ledbetter（1979）此前发表的部分研究数据存在矛盾。地球主要冰盖的规模在第22同位素阶段达到峰值，期间δ¹⁸O最大值呈现1‰的渐进式升高，对应冰盖体量的显著增长。此外，冰体体积变化引发的同位素偏移在不同冰期旋回中并不一致。底栖有孔虫的氧、碳同位素分析结果显示，更新世（Pleistocene）冰期阶段的北大西洋深层水（NADW）温度较现代更低，且地中海溢流（Mediterranean outflow）可能仍为北大西洋深层水的物源之一。将截形圆幅虫（Globorotalia truncatulinoides）的旋向比变化与浮游、底栖氧同位素记录进行对比后发现，更新世期间巴西海流（Brazil Current）可能存在向南偏移事件，该现象或与南极表层水涌升（Antarctic surface water surges）相关。冰期盛期北大西洋深层水的物源位置这一科学问题，目前仍无定论。