Clamgulchian (Miocene–Pliocene) pollen assemblages of the Kenai Lowland, Alaska, and the persistence of the family Podocarpaceae

Pollen diagrams from three areas of the Kenai Lowland of Alaska demonstrate the pollen floras of the Clamgulchian Stage (∼8–2.5 Ma) of Late Miocene/Early Pliocene age. These sections, originally described based on their plant megafossils and pollen, are revisited for a more precise determination of their pollen stratigraphy. The floras from Clam Gulch on the west margin of the Kenai Lowland are compared with Clamgulchian sections of the same age on the east margin, in canyons at the head of Kachemak Bay. The results substantiate that in the Clamgulchian sections thermophile pollen types (exotic warmth-loving woody genera) represent about 2–10% of the count (and include 15 thermophile taxa), while in the earlier Homerian Stage (∼14.5–8 Ma) thermophiles are more important and more diverse (28 taxa), representing 5–15% of the count. A second difference between the Homerian and Clamgulchian is the increased diversity of herbaceous groups (such as Ranunculaceae, Urticaceae, Onagraceae, Primulaceae and others), which are few in the Homerian (∼4–5 taxa), but many in the Clamgulchian (≤15–18 taxa). These herb groups show increasing diversity near the top of the Neogene in the Alaskan flora. With a decrease in thermophiles, the Clamgulchian taxa show a transition towards a cooler climate. Pollen of the exotic gymnosperm family Podocarpaceae chiefly of the Southern Hemisphere are diverse minor elements during the entire Neogene of Alaska. These consistent elements may represent resident associates of the hardwood forests in southern Alaska as the likelihood of their pollen occurrences being the result of long-distance transport seems unlikely.