Improvements in the Record of Secular Change in Sea Water ⁸⁷Sr/⁸⁶Sr: A Stronger Basis for Dating Paleozoic Diagenetic and Depositional Events

Stephen C. Ruppel, Eric W. James, James E. Barrick

That the strontium isotope composition of the world's oceans has changed continuously throughout the Phanerozoic is well establish. Secular changes in sea water ⁸⁷Sr/⁸⁶Sr constitute the basis for a powerful geochronometer in marine sedimentary rocks. For more than a decade, an increasing number of researchers have used these secular trends to define the age and timing of complex depositional and diagenetic events in the rock record.

Current studies of seawater ⁸⁷Sr/⁸⁶Sr in conodonts of Silurian, Devonian, and Permian age, however, indicate that previously defined secular trends of ⁸⁷Sr/⁸⁶Sr for the Paleozoic are too imprecise for accurate age dating. The primary causes of this imprecision are diagenesis and miscorrelation of sample materials. Both are problems in whole rock samples commonly used in early studies, but they may also be problems in abiotic cements and non-luminescent brachiopods, two of the most popular sample materials.

Conodonts, which are ubiquitous in Paleozoic marine sedimentary rocks, overcome these problems. They contain high Sr concentrations, resist diagenetic alteration, and can be related temporally on the basis of conodont biozones, perhaps the best standard for global correlation in the Paleozoic. Using modern instrumentation and preparation techniques, high precision measurements of ⁸⁷Sr/⁸⁶Sr can be made from individual elements weighing less than 20 µg.

On the basis of our analyses of more than 160 conodont samples from continuous measured sections and biostratigraphically correlated Paleozoic sections on three continents, we submit that the most accurate depiction of secular changes in ⁸⁷Sr/⁸⁶Sr sea water chemistry during the Paleozoic must be based on conodonts. Reported sea water ⁸⁷Sr/⁸⁶Sr values from other materials must be viewed with suspicion.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995