Late Eocene-earliest Miocene Sr isotopic reference section of DSDP Site 73-522

We present a revised calibration of Sr isotopes to the geomagnetic polarity timescale (GPTS) using closely spaced (~0.15 m.y. resolution) samples from the classic uppermost Eocene through lowermost Miocene section at Site 522, eastern South Atlantic. The Sr isotopic data are fit with two linear segments with a sharp change in slope at circa 27.5 Ma from 0.000038/m.y. (27.5 to 34.4 Ma) to 0.000051/m.y. (23.8 to 27.5 Ma). Regression analysis indicates that stratigraphic resolution ranges from ±1 m.y. (for one analysis) to ±0.6 m.y. (for three analyses) for the younger interval and ±1.2 m.y. (for one analysis) to ±0.7 m.y. (for three analyses) for the older interval, representing an increase in resolution from previous studies of ±1-2 m.y. The paleoceanographic significance of this change in slope is unclear. It occurs during an interval of intermittent Antarctic glaciation, between the Oi2a and Oi2b glaciations. The subsequent interval from circa 27 to 24 Ma appears to be an interval of minimal glaciation. Thus this observation does not support previous suggestions that increases in rates of Sr isotopic change are directly associated with the frequency of Antarctic glaciations. Rather, the increase in slope may be related to increased weathering associated with the "mid-Oligocene" glaciation.本研究针对南大西洋东部522站位的经典始新统顶部至中新统底部地层序列，采用间距紧密（分辨率约0.15百万年）的样品，完成了锶同位素对地磁极性年表（GPTS）的校正修订。锶同位素数据可通过两段线性模型拟合，其斜率在约27.5百万年处发生突变：27.5~34.4百万年时段的斜率为0.000038/百万年，23.8~27.5百万年时段的斜率为0.000051/百万年。回归分析结果显示，年轻地层段的地层分辨率范围为±1百万年（单次分析）至±0.6百万年（三次分析），老地层段的分辨率范围为±1.2百万年（单次分析）至±0.7百万年（三次分析），相较于此前研究±1~2百万年的分辨率实现了提升。该斜率突变的古海洋学意义尚不明确，其发生于Oi2a与Oi2b冰期之间的间歇性南极冰盖发育时段。随后约27~24百万年的时段则表现为冰盖发育程度极低的阶段。因此本观测结果不支持此前提出的“锶同位素变化速率提升与南极冰盖发育频率直接相关”的假说。相较而言，斜率的升高或许与“渐新世中期”冰期伴随的风化作用增强有关。