STRONTIUM ISOTOPIC COMPOSITION OF PLIOCENE AND PLEISTOCENE MOLLUSKS FROM EMERGED MARINE DEPOSITS, NORTH-AMERICAN ARCTIC

High-precision strontium (Sr) isotopic measurements were obtained for 53 Pliocene and Pleistocene molluscan shells from emerged marine depos its around the coasts of Arctic North America to test whether such dat a can be used for chronostratigraphic purposes. Sr-87/Sr-86 ratios fro m Sr isotopic measurements on many marine fossils from Arctic Ocean bo rderland sites are broadly consistent with their expected values based on independent age control and on a comparison with the Sr isotopic e volution of seawater recorded in deep-sea cores. All Sr-87/Sr-86 ratio s measured for shells from Middle and Late Pleistocene deposits are co nsistent with expected values, but only 9 of 22 ratios in shells from older deposits are consistent with independent age estimates. Aberrant Sr-87/Sr-86 ratios are consistently higher than expected. At Nome, Al aska, and Baffin Island, Canada, all Sr-87/Sr-86 ratios are higher tha n expected. Because these shells were formed along the Pacific Ocean a nd Atlantic Ocean margins, respectively, their high Sr-87/Sr-86 ratios cannot be attributed to possible differences in the Sr isotopic evolu tion 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 Sr-87/Sr-86 rat ios by as much as about 5 X 10(-5), but these mechanisms cannot accoun t for the very high Sr-87/Sr-86 values (from 20 x 10(-5) to 200 x 10(- 5) higher than those of modem seawater) measured for some shells. Alte ration by diagenetic fluids rich in radiogenic Sr is the most plausibl e explanation for the aberrant results. Diagenesis is recognized petro graphically in the most altered shells by micritic overprinting of the original shell microstructure; in addition, one shell enriched in Sr- 87 from Baffin Island exhibits a broad range (170 X 10(-5)) of Sr-87/S r-86 ratios across the shell, and an oxygen isotopic gradient (1.6 par ts per thousand) that is greater than the expected primary variability . Although our data suggest that Sr isotopic data from young Arctic mo lluscan fossils may offer a viable dating method, criteria for screeni ng altered shells must be devised before the technique can be consider ed a reliable chronostratigraphic tool.