Three recurring mollusk association types are defined in the northern Gulf of Alaska: (1) the shallow-water Yoldia-Siliqua-Lyonsia sand association; (2) the shallow to intermediate depth Cyclocardia-boreal turrid mud association (with a typical phase developing in offshore muds and a Clinocardium-Nitidella nearshore mud phase developing in Yakutat and Icy Bays); and (3) the deep-water Cadulus-thin shelled protobranch mud association. Associations are defined from 148 bottom samples containing 113 species.

Substrate exerts a strong influence on shallow-water species composition. An unidentified depth-related factor, independent of substrate type, influences both species composition and taxonomic structure. Dramatic changes in taxonomic structure that occur with depth on fine-grained glacial marine sediments in the Gulf of Alaska provide a model for paleobathymetric interpretation of high latitude late Cenozoic fossil mollusk faunas. The major structural shifts include: decrease in the proportion of heterodont, suspension-feeding bivalves; increase in the proportion of thin-shelled, deposit-feeding protobranch bivalves; and increase in the proportion of carnivorous neogastropods, particularly small-shelled, toxoglossate turrids.

The most abrupt and most readily recognized faunal break occurs at 200 m, separating the typical Cyclocardia-boread turrid mud association from the Cadulus-thin shelled protobranch mud association. Although the two associations have species in common, many species drop out as the 200-m isobath is approached and others appear at or not far below it. More precise definition of this faunal break should be explored because of its potential application in paleoecologic analysis.

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