Mid-Cretaceous strontium-isotope stratigraphy of deep-sea sections

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 traceelemental analyses, but are best distinguished by comparing the ⁸⁷Sr/⁸⁶Sr values of similar-age samples from different sites. Strontiumisotope 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 mid-dle 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.