ABSTRACT: Strontium Isotope Dating of Upper Cenozoic Marine Deposits, Northwestern Alaska

D. S. Kaufman, G. L. Farmer, G. H. Miller, L. D. Carter, J. Brigham-Grette

High-precision Sr isotope measurements were obtained for 24 late Cenozoic molluskan shells from northwestern Alaska to test whether such data can be used to date arctic marine deposits. At present, the ages of geologic and climatologic events recorded by circum-arctic deposits older than the range of radiocarbon dating (>30 k.y.) are poorly known. Improving the chronological control would elucidate the interrelations between late Cenozoic global climate evolution and environmental changes in the Arctic, such as the inception of northern hemispheric ice sheets and perennial Arctic Ocean sea ice. Samples for this study were chosen from emerged and offshore marine deposits at Skull Cliff, Nome, and the Colville River area, where depositional ages are relatively well const ained on the basis of paleontologic, paleomagnetic, and amino acid criteria. At Skull Cliff, ⁸⁷Sr/⁸⁶Sr ratios were measured in molluskan shells from five upper Pliocene to upper Pleistocene stratigraphic units composed of nearshore and inner shelf sediments. The ^DgrSr values [^DgrSr= (⁸⁷Sr/⁸⁶Sr_(sample) - ⁸⁷Sr/⁸⁶Sr_(standard)) × 10⁵] range from 0.0 to -7.2, and, except for the oldest unit, decrease with increasing age. Four shells of two genera collected from a single stratigraphic horizon over a distance of 100 m have indistinguishable ^DgrSr values, within the range of external reproducibility (±1^DgrSr), confirming an absence of vital effects. Sample ages for the upper three uni s at Skull Cliff, based on the comparison of the measured ^DgrSr with the Sr isotope evolution of seawater recorded in independently dated deep-sea cores, agree broadly with the expected ages. The agreement suggests that the Arctic Ocean was in Sr isotope equilibrium with the world ocean during at least the past 1 m.y. However, Sr isotope ages for the lower two units at Skull Cliff are younger than expected. In contrast, ^DgrSr values in shells from deposits correlative to these two units, in the Colville River area, are consistent with other age estimates. This indicates that the high ^DgrSr values at Skull Cliff probably are the result of local effects and do not reflect the isolation and independent isotopic evolution of Sr in the Arctic Ocean. The ^DgrSr values in five shells from ma ine deposits in the Nome area, ranging in age from late Pliocene to middle Pleistocene, are higher than modern seawater (+2.3 to +70.8). The input of river water into the shallow Norton Sound may have affected the Sr isotope composition of coastal water at Nome. However, the ⁸⁷Sr/⁸⁶Sr of river water draining the area must have been much higher than the present value of 0.7120 to account for the very high ^DgrSr measured in the Nome fossils. Although Sr isotope data can provide useful age information for arctic sites, a better understanding of the processes controlling the Sr isotopic composition of coastal water will be necessary before Sr isotopes can be considered a reliable chronostratigraphic tool for the continental margins.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990