Miocene stable isotope stratigraphy of ODP Hole 120-747A

We correlated Miocene d18O increases at Ocean Drilling Program Site 747 with d18O increases previously identified at North Atlantic Deep Sea Drilling Project Sites 563 and 608. The d18O increases have been directly tied to the Geomagnetic Polarity Time Scale (GPTS) at Site 563 and 608, and thus our correlations at Site 747 provide a second-order correlation to the GPTS. Comparison of the oxygen isotope record at Site 747 with records at Sites 563 and 608 indicates that three as-yet-undescribed global Miocene d18O increases may be recognized and used to define stable isotope zones. The d18O maxima associated with the bases of Zones Mila, Milb, and Mi7 have magnetochronologic age estimates of 21.8, 18.3, and 8.5 Ma, respectively.
The correlation of a d18O maximum at 70 mbsf at Site 747 to the base of Miocene isotope Zone Mi3 (13.6 Ma) provides a revised interpretation of four middle Miocene normal polarity intervals observed between 77 and 63 mbsf at Hole 747A. Oxygen isotope stratigraphy indicates that the reversed polarity interval at 70 mbsf, initially interpreted as Chronozone C5AAr, should be C5ABr. Instead of a concatenated Chronozone C5AD-C5AC with distinct Chronozones C5AB, C5AA, and C5A (as in the preliminary interpretation), d18O stratigraphy suggests that these normal polarity intervals are Chronozones C5AD, C5AC, and C5AB, whereas Chronozones C5AA-C5A are concatenated. This interpretation is supported by the d13C correlations.
The upper Miocene magnetostratigraphic record at Hole 747A is ambiguous. Two upper Miocene d18O events at Site 747 can be correlated to the oxygen isotope records at Site 563 and 608 using the magnetostratigraphy derived at Hole 747B. Our chronostratigraphic revisions highlight the importance of stable isotope stratigraphy in attaining an integrated stratigraphic framework for the Miocene.

我们将大洋钻探计划（Ocean Drilling Program, ODP）747站位的中新世δ¹⁸O升高事件，与此前在北大西洋深海钻探计划（Deep Sea Drilling Project, DSDP）563及608站位识别出的δ¹⁸O升高事件进行了关联对比。563与608站位的δ¹⁸O升高事件已直接绑定至地磁极性年表（Geomagnetic Polarity Time Scale, GPTS），因此本研究对747站位的关联分析，可为地磁极性年表提供二级关联校准。对比747站位与563、608站位的氧同位素记录可知，尚有3个尚未被描述的全球性中新世δ¹⁸O升高事件可被识别，并可用于界定稳定同位素带。与Mila、Milb及Mi7带底界对应的δ¹⁸O极值，其磁地层年龄估算值分别为21.8 Ma、18.3 Ma及8.5 Ma。
将747站位海底以下70米（meters below seafloor, mbsf）处的δ¹⁸O极值与中新世同位素Mi3带底界（13.6 Ma）进行关联，可对747A孔77至63米海底以下深度段识别出的4个中中新世正极性间隔给出修正解释。稳定同位素地层学研究表明，此前被归为磁极性带C5AAr的70米海底以下深度处的反极性间隔，实际应为C5ABr。相较于初步解释中将C5AB、C5AA及C5A作为独立磁极性带的C5AD-C5AC复合带，δ¹⁸O地层学结果显示，上述正极性间隔分别对应磁极性带C5AD、C5AC及C5AB，而C5AA与C5A则为复合磁极性带。碳同位素关联分析也支持这一解释方案。
747A孔的上中新世磁地层记录存在歧义。借助747B孔获得的磁地层数据，可将747站位的2个上中新世δ¹⁸O事件与563及608站位的氧同位素记录进行关联。本次年代地层学修正凸显了稳定同位素地层学在构建中新世综合地层框架中的重要价值。