Isotope paleohydrology and environment of Devonian bivalve fossils from the Weatherall and Hecla Bay formations, Melville Islands, Canadian Arctic

Abstract

Sr, O and C isotopic measurements of Devonian bivalves were performed to elucidate the isotope paleohydrology of an unusual bivalve-dominated marginal marine sequence from the siliciclastic Weatherall and Hecla Bay formations of Melville Island. To confirm that measured isotopic compositions had not been completely overprinted by diagenesis, considerable evidence was amassed indicating that the bivalves had been subjected to relatively closed-system diagenesis. XRD analysis indicated that the fossils contain up to 18 % relic aragonite, finely disseminated on a millimetre scale within the recrystallized shell. The 87Sr/86Sr - Ca/Sr diagram was used to separate potential diagenetic trends from environmental trends. Trend lines defined by individual species from single handsamples were extrapolated toward lower Ca/Sr and lower 87Sr/86Sr, resulting in convergence of the trend lines within a relatively narrow range of 87Sr/86Sr ratios, which is interpreted as the mean 87Sr/86Sr range for the depositional waters, averaged over all facies. Using the 87Sr/86Sr ratios and estimates of the 87Sr/86Sr and Sr concentration of Devonian marine and freshwaters, Sr material balance calculations yielded paleosalinities between 0 ‰ and 14 ‰ for the bivalves' depositional waters and a 1000 Sr/Ca ratio of 12.5 for Devonian seawater. Oxygen isotope balance calculations yielded paleosalinities between 0 ‰ and 19 ‰ While the paleosalinity ranges determined using these two methods are similar, salinity values calculated for individual species are consistent only in certain specimens of Limoptera sp., suggesting evaporative enrichment of 180 in the mixing zone or diagenesis. The calculated 87Sr/86Sr salinity values are consistent with sedimentological studies of the area, which suggest a deltaic environment of deposition.