Sr Isotope Stratigraphy Research Articles

Strontium isotope stratigraphy

The 87Sr/ 86Sr ratio of sea water has changed significantly with time in response to the input of varying proportions of Sr derived from continental crust and upper mantle sources, as moderated and buffered by carbonate recycling. Because of these fluctuations, the marine Sr isotope record can be used for stratigraphic correlation and absolute dating purposes. Recent work has been reviewed and results for 220 samples collated in δ 87Sr units. They show four main periods of change over the past 100 million years within which variations during the Holocene to Late Miocene, Middle Miocene to Middle Eocene and Late Cretaceous (Maastrichtian and Campanian) are the most promising for Sr isotope stratigraphy with age uncertainties of better than ±1 myr. Equations describing the Sr isotope-age relationships are presented. Application of the method using carefully selected carbonate samples is now feasible; its extension to unfossiliferous sediments appears promising but it remains necessary to establish that specific authigenic phases retain the Sr isotopic compositions of contemporaneous sea waters.

Geochronology and paleothermometry of Neogene sediments from the Vøring Plateau using Sr, C and O isotopes

The Neogene sediments from DSDP site 341 on the Vøring Plateau, Norwegian Sea, contain a thin glauconitic pellet-bearing subunit, which separates underlying pelagic clays from overlying glacial-marine sediments. Oxygen isotope measurements of benthic foraminifera show aδ 18O shift of + 1%. during deposition of this subunit, probably a combined effect of a drop in bottom water temperature and a rise in seawaterδ 18O. The chronology of this sedimentological and O isotope transition is, however, poorly constrained by fossil evidence. Rb Sr dating of glauconitic pellets indicates that the lower part of the glauconitic subunit was deposited 11.6 ± 0.2 Ma ago. Further geochronological evidence, derived from the Sr and C isotopic compositions of foraminifera compared with known seawater-time variations, indicates that the lower pelagic clays are early to middle Miocene, deposited at a mean rate of ∼ 15 m/Ma. The glauconitic subunit contains part of the middle Miocene and probably all of the late Miocene in a condensed sequence with a very low mean depositional rate (∼ 0.2 m/Ma). The overlying glacial marine sediments are probably Pliocene, with a high mean rate of deposition, ∼ 45 m/Ma. This is the first application of C, O and Sr isotopic stratigraphy combined with Rb Sr dating of glauconitic minerals, and it illustrates the applications of this integrated approach in geochronology.

Strontium-Isotope Stratigraphy of a Red Sea Core

The concentrations and isotopic compositions of strontium of the noncarbonate fractions of sediment from a piston core taken in the median valley of the Red Sea have been determined. The results support the hypothesis that the silicate fraction of the sediment in this core is a two-component mixture. The observed 87 Sr/ 86 Sr ratios and Sr concentrations fit a hyperbolic curve that results from the mixing of volcanic-basaltic and terrigenous-sialic components. The Sr concentrations of these components were then derived by assuming reasonable values for the 87 Sr/ 86 Sr ratios. The concentration of the basaltic component in the sediment was calculated for each sample, using its observed Sr concentration. The variation of the abundance of volcanogenic material with depth in the core was used to define stratigraphic layers having either a normal, high, or low content of the volcanic component. The resulting Sr-isotope stratigraphy complements and refines the lithologic stratigraphy obtained by conventional means.