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

• Published in: Canadian Journal of Earth Sciences
• Main Authors: Kaufman, Darrell, Carter, L. David, Miller, Gifford H., Farmer, G. Lang, Budd, David A.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1993
• Subjects: General Earth and Planetary Sciences; Arctic; Arctic Ocean; Baffin Island; Canada; Pacific; Baffin; Nome; Alaska
• Online Access: http://dx.doi.org/10.1139/e93-041; http://www.nrcresearchpress.com/doi/pdf/10.1139/e93-041

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. 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr ratios are consistently higher than expected. At Nome, Alaska, and Baffin Island, Canada, all 87 Sr/ 86 Sr ratios are higher than expected. Because these shells were formed along the Pacific Ocean and Atlantic Ocean margins, respectively, their high 87 Sr/ 86 Sr 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 87 Sr/ 86 Sr ratios by as much as about 5 × 10 −5 , but these mechanisms cannot account for the very high 87 Sr/ 86 Sr 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 87 Sr from Baffin Island exhibits a broad range (170 × 10 −5 ) of 87 Sr/ 86 Sr ratios across the shell, and an oxygen isotopic gradient (l.6‰) that is greater than the expected primary variability. Although our data suggest ...