Aminostratigraphy of Pliocene-Pleistocene high-sea-level deposits, Nome coastal plain and adjacent nearshore area, Alaska

Abstract

Amino acid epimerization ratios (aIle/Ile) measured in 295 fossil molluscan shells of 3 genera provide a basis to evaluate the number and relative timing of high-sea-level events represented by deposits blanketing the Nome coastal plain and adjacent nearshore area, northwestern Alaska. Frequency-distribution analysis, along with a new approach that relies on the ranked-data distribution, was used to identify clusters (aminozones) in the aIle/Ile data, each of which presumably represents a discrete period of marine inundation of the study area. Six periods of high relative sea level were distinguished; some are recognized only by redeposited shells rather than in situ bio- or lithostratigraphic units. The oldest period is represented by shells recovered from the deepest sections of two offshore boreholes; aIle/Ile ratios in these shells (1.0) are much higher than in any found onshore and probably date to the late Miocene/early Pliocene. The oldest aminozone lies below nonfossiliferous sediments, which, in turn, are overlain by glacigenic and marine deposits containing largely fragmented shells with aIle/Ile ratios ranging from 0.35 to 0.65. This range coincides with aIle/Ile ratios from various onshore deposits previously ascribed to the middle Pliocene (∼3 m.y.) Beringian transgression. Ratios in this range can be subdivided into three evenly spaced aminozones, each of which may represent multiple, short-duration (10 4 yr) high-sea stands separated by a longer-period (>10 5 yr) sea-level oscillation. On the basis of a new empirical model of parabolic epimerization kinetics and an assumed age of 3.2 m.y. for the oldest Beringian aminozone (the approximate age of the initial submergence of Bering Strait), the estimated length of time represented by the three Beringian aminozones is between 0.5 and 1.7 m.y. The youngest Beringian highstand was followed in the late Pliocene or early Pleistocene by a lengthy period of nonmarine deposition during which local valley glaciers advanced onto the coastal plain at least once. Relative sea level apparently did not again surpass that of today until the middle Pleistocene Anvilian transgression, the oldest higher-than-present stand of the Pleistocene, yet considerably younger than previously thought. Glaciers overran the Nome coastal plain at least once following the Anvilian transgression, incorporating abundant shells into their drift. The drift is notched by a shoreline formed during the Pelukian transgression of the last interglaciation. Although well represented onshore, no Pelukian deposits have been found offshore.