A new generation of ⁸⁷Sr/⁸⁶Sr, d¹⁸O and d¹³C curves for Phanerozoic seawater, based on several thousands of stratigraphically well defined and well preserved low-Mg calcitic skeletal components, results in much better constrained secular curves. For strontium isotopes, the steep parts of the curves can be utilized for correlation purposes and for “relative” dating, often with a resolution comparable to that of the best biostratigraphy. The major practical constraint that precludes delineation of an unequivocal Sr isotope curve for Phanerozoic seawater is the uncertainty in the assignment of absolute ages to samples. As a result, the curve is in reality a band. This is a reflection of the limitations of biostratigraphy and geochronology that are inherited by any derivative correlation and dating technique including isotope stratigraphy. It is therefore unlikely that in the near future, Sr isotope stratigraphy will surpass the resolution capability of biostratigraphy as a global correlation tool. On the other hand, if complemented by lithostratigraphy, it is potentially a valuable tool for correlation of sequences on intrabasinal and regional scales. In terms of causative factors, the first order structure of the curve is well understood, as a reflection of contribution of radiogenic strontium from river inputs vs. nonradiogenic inputs from submarine hydrothermal systems. This is not the case for the higher order structure, where the model scenarios are equivocal. In many cases, however, sharp short term peaks tend to coincide with hiatuses or with slow sedimentation rate intervals.

The new generation of ¹⁸O/¹⁶O and ¹³C/¹²C measurements yields secular trends that are, in general features, comparable to those previously described for the whole rocks, with superimposed higher order oscillations. The trend for the d¹⁸O suggests about 5 ± 2 enrichment from Cambrian to today, with higher order peaks, for example, in the Ordovician-Silurian, coincident with glacial episodes. In contrast, the d¹³C rise during the Palaeozoic is followed by its decline in the Mesozoic and Cenozoic. Optical (textural) and chemical criteria suggest that the interior “secondary” layer of the brachiopod shells, the material that carries the signals, is well preserved in many samples and the extracted isotopic trends are therefore a primary feature of the geologic record. In my view, the ¹⁸O enrichment in progressively younger samples is principally, although not exclusively, a reflection of the evolving ¹⁸O/¹⁶O composition of seawater. If so, a delineation of this trend may ultimately result in the development of a valuable palaeoclimatic and palaeoceanographic tracer for the Phanerozoic.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah