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 87 Sr/ 86 Sr values of similarage 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 87 Sr/ 86 Sr values. The composite curve shows a “trough” of markedly lower 87 Sr/ 86 Sr values in the Aptian and early Albian stages, higher but constant values for the middle Albian-Cenomanian stages, followed by a decrease in 87 Sr/ 86 Sr values in the early Turonian.