We estimated the amount of oil remaining in Prince William Sound, Alaska, 12 yr after the 1989 Exxon Valdez spill to assess its importance as a long-term reservoir of toxic hydrocarbons. We found oil on 78 of 91 beaches randomly selected according to their oiling history. Surface oiling was recorded for randomly placed quadrats, which were then excavated and examined for subsurface oil. The cumulative area of beach contaminated by surface or subsurface oil was estimated at 11.3 ha. Surface oil varied little with tide height, but subsurface oil was more prevalent at the middle tide heights. The mass of remaining subsurface oil is conservatively estimated at 55 600 kg. Analysis of terpanes indicated that over 90% of the surface oil and all of the subsurface oil was from the Exxon Valdez and that Monterey Formation oil deposited after the 1964 Alaska earthquake accounted for the remaining surface oil. These results indicate that oil from the Exxon Valdez remains by far the largest reservoir of biologically available polycyclic aromatic hydrocarbons on beaches impacted by the spill and that biota dependent on these beaches risk continued exposure.

Although population mixtures often include contributions from novel populations as well as from baseline populations previously sampled, unlabeled mixture individuals can be separated to their sources from genetic data. A Gibbs and split-merge Markov chain Monte Carlo sampler is described for successively partitioning a genetic mixture sample into plausible subsets of individuals from each of the baseline and extra-baseline populations present. The subsets are selected to satisfy the Hardy-Weinberg and linkage equilibrium conditions expected for large, panmictic populations. The number of populations present can be inferred from the distribution for counts of subsets per partition drawn by the sampler. To further summarize the sampler's output, co-assignment probabilities of mixture individuals to the same subsets are computed from the partitions and are used to construct a binary tree of their relatedness. The tree graphically displays the clusters of mixture individuals together with a quantitative measure of the evidence supporting their various separate and common sources. The methodology is applied to several simulated and real data sets to illustrate its use and demonstrate the sampler's superior performance.Résumé : Bien que les mélanges de populations contiennent des contributions des nouvelles populations en plus de celles des populations originales déjà échantillonnées, les individus non identifiés du mélange peuvent être séparés en fonction de leur source par leurs caractéristiques génétiques. Nous décrivons un échantillonneur de Gibbs de type Monte Carlo avec procédure de séparation-regroupement par chaînes de Markov qui sépare successivement un échantil-lon contenant un mélange génétique en sous-ensembles plausibles d'individus à la fois de la population d'origine et des populations additionnelles présentes. Les sous-ensembles sont sélectionnés de manière à satisfaire aux exigences de l'équilibre Hardy-Weinberg et de l'équilibre de liaison attendus dans de grandes populations panmictiques. Le nombre de populations présentes peut être estimé à partir de la distribution des nombres de sous-ensembles par partition retirés par l'échantillonneur. Afin de mieux résumer le produit de l'échantillonneur, les probabilités d'attribution conjointe des individus du mélange aux mêmes sous-ensembles sont calculées à partir des partitions et elles servent à construire un arbre binaire de leur degré de parenté. L'arbre représente graphiquement les groupements d'individus du mélange de même qu'une mesure quantitative des données qui appuient leurs différentes sources séparées et communes. Nous appliquons la méthodologie à plusieurs ensembles de données réelles et simulées afin d'en illustrer l'utilisation et de démontrer la performance supérieure de cet échantillonneur.[Traduit par la Rédaction] Pella and Masuda 596

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Oil stranded by the 1989 Exxon Valdez spill has persisted in subsurface sediments of exposed shores for 16 years. With annualized loss rates declining from ∼68% yr -1 prior to 1992 to ∼4% yr -1 after 2001, weathering processes are retarded in both sediments and residual emulsified oil ("oil mousse"), and retention of toxic polycyclic aromatic hydrocarbons is prolonged. The n-alkanes, typically very readily oxidized by microbes, instead remain abundant in many stranded emulsified oil samples from the Gulf of Alaska. They are less abundant in Prince William Sound samples, where stranded oil was less viscous. Our results indicate that, at some locations, remaining subsurface oil may persist for decades with little change.