Warm climate in the late Miocene of the south coast of Alaska and the occurrence of Podocarpaceae pollen

Abstract

A study of the Homerian type section in the upper Beluga Formation (Upper Miocene) of the Kenai Group of southern Alaska has yielded two surprising discoveries: (1) warmth‐loving taxa and (2) the presence in Alaska of a ‘new’ gymnosperm family, Podocarpaceae. A well‐preserved pollen and spore flora is present in Upper Miocene coal beds of the Kenai lowland, near Homer, Alaska. Stump horizons, abundant wood fragments, wood grain, and amber within the coal attest to a forested swamp. Pollen assemblages from the Homerian type section include elements of both Mixed Northern Hardwood and warm‐temperate Mesophytic forests and are far richer than the flora previously defining the Homerian type section, which had suggested a less diverse, cooler assemblage. Within the Homerian type section, the flora exhibits no definite taxonomic chronology; in general, Alnus dominates, with up to 45% of the total counts, followed by Pinaceae and Taxodiaceae pollen types (30 to 35%) and thermophiles (c. 14%). At least 36 genera are represented, including Carya type, Corylus, Ilex, Juglans, Myrica, Ostrya/Carpinus, Pterocarya, Quercus/Quercus‐type, and Ulmus/Zelkova, eight dicot genera have not previously been reported from the type Homerian. Presence of these hardwoods in moderate to minor amounts suggests that the climate during the Homerian (Late Miocene) was only slightly cooler than that of the Seldovian (Early to Middle Miocene). Unexpectedly, Dacrydium and Podocarpus are present as minor elements in most of the samples. They apparently coexisted with the other Miocene taxa, because the pre‐Paleogene Kenai‐Chugach terrane to the southeast, which supplied sediments to the Kenai Group, is mostly of oceanic plate provenance and is unlikely to have been the source of the pollen. A uniform orange fluorescence of all the pollen, including the podocarps and any potentially reworked pollen, also suggests a contemporaneous origin for all the taxa. The flora from the Homerian type section may precede or coincide with uplift of the Alaska Range to the north. Thus, further comparison with Homerian taxa at localities north and south of the Alaska Range will be important as it may reveal a possible rain shadow effect.