Strontium isotopic composition of Pliocene and Pleistocene molluscs from emerged marine deposits, North American Arctic

Abstract

High-precision strontium (Sr) isotopic measurements were obtained for 53 Pliocene and Pleistocene molluscan shells from emerged marine deposits around the coasts of Arctic North America to test whether such data can be used for chronostratigraphic purposes. 87Sr/86Sr ratios from Sr isotopic measurements on many marine fossils from Arctic Ocean borderland sites are broadly consistent with their expected values based on independent age control and on a comparison with the Sr isotopic evolution of seawater recorded in deep-sea cores. All 87Sr/86Sr ratios measured for shells from Middle and Late Pleistocene deposits are consistent with expected values, but only 9 of 22 ratios in shells from older deposits are consistent with independent age estimates. Aberrant 87Sr/86Sr ratios are consistently higher than expected. At Nome, Alaska, and Baffin Island, Canada, all 87Sr/86Sr ratios are higher than expected. Because these shells were formed along the Pacific Ocean and Atlantic Ocean margins, respectively, their high 87Sr/86Sr ratios cannot be attributed to possible differences in the Sr isotopic evolution of the Arctic Ocean relative to that of remainder of the world's oceans. Radiogenic Sr from proximal river-water input, or leaching of detritus within the shell matrix, may have changed the 87Sr/86Sr ratios by as much as about 5 × 10−5, but these mechanisms cannot account for the very high 87Sr/86Sr values (from 20 × 10−5 to 200 × 10−5 higher than those of modern seawater) measured for some shells. Alteration by diagenetic fluids rich in radiogenic Sr is the most plausible explanation for the aberrant results. Diagenesis is recognized petrographically in the most altered shells by micritic overprinting of the original shell microstructure; in addition, one shell enriched in 87Sr from Baffin Island exhibits a broad range (170 × 10−5) of 87Sr/86Sr ratios across the shell, and an oxygen isotopic gradient (l.6‰) that is greater than the expected primary variability. Although our data suggest that Sr isotopic data from young Arctic molluscan fossils may offer a viable dating method, criteria for screening altered shells must be devised before the technique can be considered a reliable chronostratigraphic tool.