Stable and strontium isotope ratios and age determination of ODP Site 130-803

Stratigraphic information from strontium, oxygen, and carbon isotopic ratios has been integrated with diatom and planktonic foraminifer datums to refine the Oligocene to early Miocene chemostratigraphy of Site 803. The Sr isotope results are based on analyses of mixed species of planktonic foraminifer and bulk carbonate samples. 87Sr/86Sr ratios of bulk carbonate samples are, in most cases, less radiogenic than contemporaneous seawater. Estimated sediment ages based on planktonic foraminifer 87Sr/86Sr ratios, using the Sr-isotope-age relation determined by Hess and others in 1989, are in moderately good agreement with the biostratigraphic ages. Chronological resolution is significantly enhanced with the correlation of oxygen and carbon isotope records to those of the standard Oligocene section tied to the Geomagnetic Polarity Time Scale at Site 522. Ages revised by this method and other published ages of planktonic foraminifer datums are used to revise the Oligocene stratigraphy of Site 77 to correlate the stable isotope records of Sites 77 and 803. Comparison of the Cibicidoides stable isotope records of Sites 77 and 574 with paleodepths below 2500 m in the central equatorial Pacific, and Site 803 at about 2000-m paleodepth in the Ontong Java Plateau reveals inversions in the vertical d18O gradient at several times during the Oligocene and in the early Miocene. The shallower water site had significantly-higher d18O values than the deeper water sites after the earliest Oligocene 18O enrichment and before 34.5 Ma, in the late Oligocene from 27.5 to at least 25 Ma, and in the early Miocene from 22.5 to 20.5 Ma. It is not possible to ascertain if the d18O inversion persisted during the Oligocene/Miocene transition because the deeper sites have hiatuses spanning this interval. We interpret this pattern to reflect that waters at about 2000 m depth were cold and may have formed from mixing with colder waters originating in northern or southern high-latitude regions. The deeper water appear to have been warmer and may have been a mixture with warm saline waters from mid- or low-latitude regions. No apparent vertical d13C gradient is present during the Oligocene, suggesting that the age difference of these water masses was small.本研究将锶、氧、碳同位素比值的地层数据与硅藻及浮游有孔虫（planktonic foraminifer）基准面数据整合，以优化803站位渐新世至早中新世的化学地层学（chemostratigraphy）序列。锶同位素分析结果基于混合种浮游有孔虫与块状碳酸盐样品的测试分析。多数情况下，块状碳酸盐样品的87Sr/86Sr比值的放射成因程度低于同期海水。基于浮游有孔虫87Sr/86Sr比值、结合Hess等人1989年确立的锶同位素-年龄关联关系估算的沉积年龄，与生物地层年龄吻合度中等。通过将氧、碳同位素记录与绑定于522站位地磁极性年表（Geomagnetic Polarity Time Scale）的标准渐新世剖面的同位素记录对比，地层年代分辨率得到显著提升。通过该方法修正的年龄数据，以及已发表的浮游有孔虫基准面年龄数据，被用于修正77站位的渐新世地层序列，以实现77与803站位的稳定同位素记录对比。 将77、574站位的拟抱球虫属（Cibicidoides）稳定同位素记录，与赤道太平洋中部2500米以深的古水深数据，以及翁通爪哇高原约2000米古水深的803站位数据进行对比后发现，在渐新世及早中新世的多个时期，垂直方向的δ¹⁸O梯度出现反转。在渐新世早期¹⁸O富集事件之后、34.5 Ma之前，以及晚渐新世27.5 Ma至至少25 Ma、早中新世22.5 Ma至20.5 Ma期间，浅水区站位的δ¹⁸O值显著高于深水区站位。由于深水区站位存在跨越该时段的沉积间断，无法确定δ¹⁸O反转是否在渐新世/中新世过渡期持续存在。我们认为该模式反映出约2000米水深的水体温度较低，其可能由南北高纬度区域来源的冷水混合形成。而深水区水体温度相对较高，可能是中低纬度区域来源的温暖盐水混合形成的。渐新世期间未出现明显的垂直δ¹³C梯度，表明这些水团的年龄差异较小。