Depth, age, strontium isotopic ratios and trace elemental ratios of samples from thirteen DSDP and ODP holes

Large variations exist between published mid-Cretaceous (late Barremian to early Turonian stages) seawater Sr-isotope stratigraphies; this has resulted in disparate interpretations of crustal production rates. We report on a detailed investigation of seawater Sr-isotope stratigraphy based on foraminifers and, where available, on inoceramid bivalves from 12 mid-Cretaceous Deep Sea Drilling Project and Ocean Drilling Program sections. The effects of diagenesis are assessed using scanning electron microscope observations and trace-elemental analyses, but are best distinguished by comparing the 87Sr/86Sr values of similar-age samples from different sites. Strontium-isotope analyses compiled from 9 of 12 sites that have detailed age control define one band of common values. This band is used as a composite curve, which presumably represents seawater 87Sr/86Sr values. The composite curve shows a "trough" of markedly lower 87Sr/86Sr values in the Aptian and early Albian stages, higher but constant values for the middle Albian-Cenomanian stages, followed by a decrease in 87Sr/86Sr values in the early Turonian.
Variations between published mid-Cretaceous Sr-isotope records result from diagenetic alteration, analytical problems, and the diverse biostratigraphic approaches and assumptions used to estimate sample ages. When preexisting age data are made consistent, the composite record shows close similarities with data sets derived from measurements of macrofossils in land sections of Europe and North America.
The interval of decreased 87Sr/86Sr values in the Aptian-Albian stages overlaps with the pulse of mid-plate volcanic activity that produced the Ontong Java, Manihiki, and Kerguelen Plateaus. The exact age and the shape of the trough, however, are consistent with increased spreading rates at oceanic ridges, given the existing data on the timing of mid-plate volcanic activity.

已发表的白垩纪中期（巴雷姆阶晚期至土仑阶早期）海水锶同位素地层学（seawater Sr-isotope stratigraphy）记录之间存在显著差异，这进而导致学界对地壳生成速率的解读存在分歧。本研究基于有孔虫（foraminifers）以及可获取的叠瓦蛤双壳类（inoceramid bivalves），对12个白垩纪中期深海钻探计划（Deep Sea Drilling Project, DSDP）与大洋钻探计划（Ocean Drilling Program, ODP）钻孔剖面的海水锶同位素地层学开展了详细研究。成岩作用（diagenesis）的影响通过扫描电子显微镜（scanning electron microscope, SEM）观测与微量元素分析进行评估，但通过对比不同站位同龄样品的87Sr/86Sr比值可最佳区分其影响。从12个具备精确年代约束的站位中选取9个的锶同位素分析数据，可得到一组具有共性的数值区间。将该数值区间作为复合曲线，其大概率代表了海水的87Sr/86Sr比值。该复合曲线显示，在阿普第阶与阿尔布阶早期存在一个87Sr/86Sr比值显著偏低的「低谷」，阿尔布阶中期至森诺曼阶时期的比值较高且保持稳定，随后在土仑阶早期再次出现87Sr/86Sr比值的下降。

已发表的白垩纪中期锶同位素记录之间的差异，源于成岩蚀变、分析误差，以及用于估算样品年代的各类生物地层学方法与假设前提。当对已有年代数据进行统一校准后，该复合记录与欧洲、北美陆相剖面中大型化石（macrofossils）测量得到的数据集高度相似。

阿普第阶至阿尔布阶期间87Sr/86Sr比值降低的时段，与形成翁通爪哇海台（Ontong Java Plateau）、马尼希海台（Manihiki Plateau）及凯尔盖朗海台（Kerguelen Plateau）的板内火山活动脉冲相重合。但结合现有板内火山活动的年代数据来看，该低谷的确切年龄与形态，与洋中脊（oceanic ridges）扩张速率加快的结论相符。