The Cenozoic seawater ⁸⁷Sr/⁸⁶Sr curve: Data review and implications for correlation of marine strata

Abstract

The strontium isotopic ratio ( 87 Sr/ 86 Sr) in seawater changes slowly over geologic time. This variation is caused by changes in the relative contribution of Sr from various isotopically distinct sources within the crust. The most important of these are high‐ratio sialic rocks from continents and low‐ratio mafic volcanic and mafic intrusive rocks from continental margins and ocean basins. A plot of Sr isotope ratio versus age for Phanerozoic marine samples produces a curve exhibiting many episodes of increasing and decreasing values. This variation can be used as a basis for temporal correlation of marine carbonate, sulfate, and phosphate sediments. Temporal correlations can be made between high‐latitude and low‐latitude sequences, deepwater and shallow‐water sequences, and normal‐marine and restricted‐marine (hypersaline/hyposaline) sequences. Satisfactory biostratigraphic correlations between such sequences are often hampered by either the absence of age‐diagnostic fossils or by the provinciality of faunal and floral assemblages. Rapid change that took place in the 87 Sr/ 86 Sr of seawater during most of the Cenozoic makes this era particularly well suited for precise temporal correlation. The seawater curve for the Cenozoic is subdivided into three segments: Quaternary to mid‐Miocene, mid‐Miocene to late Eocene, and late Eocene to early Paleocene. The mid‐Miocene to late Eocene curve segment exhibits a particularly steep slope, making this a promising interval for high‐resolution stratigraphic correlation. Although current data generally support the present configuration of the seawater curve, some revision of the curve is probably required in the vicinity of the Oligocene‐Eocene boundary. Establishment of the general configuration of the seawater curve for the Cenozoic has promoted efforts to refine the curve on the basis of construction of detailed Sr isotope profiles within individual stratigraphic sequences. A Sr isotope profile at Deep Sea Drilling Project (DSDP) site 590B suggests a complex Neogene seawater curve characterized by minor slope changes in the Pliocene and Miocene. These slope changes are not specifically identified in the seawater curve constructed from multilocation data. On the basis of this more complex curve, and in the absence of diagenetic complications, the ultimate Neogene stratigraphic resolution is estimated to range from 0.1 to 2 million years. Both the verification and the general stratigraphic applicability of this more complex Neogene curve are largely dependent on the degree of preservation of the original seawater ratio in marine samples.