Pollen and spores recovered from the Nenana Gravel include Pinus*, Picea, Larix, Tsuga*, Abies*, Betula, Alnus, Diervilla-Weigela*, and a number of herbaceous taxa that are rare or absent from Miocene deposits from the same region (taxa marked with asterisks in this abstract no longer grow in the Alaska Range or elsewhere in interior Alaska). Some herb taxa, such as Polemonium, did not evolve until Pliocene time, and others appear to have played only a very minor role in the regional vegetation prior to the Pliocene. The presence during the Pliocene of taxa such as Tsuga and Abies that are now missing from the interior region suggests that a highly continental climate was not yet fully developed. MAT in the lowlands of the interior was probably near 3° C, and permafrost was absent except at higher elevations. As mentioned above, deposition of the Nenana Gravel may have begun by ca. 5 Ma (based on the uplift history of the Alaska Range) and may have continued until ca. 2.8 Ma (based on the age of the Jumbo Dome igneous intrusion that deformed deposits of this gravel unit in one area of the Alaska Range at 2.8 Ma [minimum date]).
Another apparent Pliocene gravel deposit was found in ancient terrace deposits of the Yukon River near Circle, Alaska. It contains a pollen flora that matches closely that in the Nenana Gravel. The pollen flora does not match closely any known Miocene or Pleistocene floras from the region. The plant macrofossils identified from the gravel deposits near Circle contain taxa that range in age from late Miocene to Pliocene. Paleomagnetic samples from one exposure of the terrace gravel deposits have reversed polarity, which provides some small constraint on the age of the deposit. At present, I believe that the age of the gravels is probably between ca. 5 and 3 Ma. The gravel deposits contain wood (Picea, Abies*, Pinus*), cones (Picea and an undescribed species of Larix), needles (Picea, Pinus), and seeds (Prunus*, Epipremnum*, Hypericum*, Aracites*, Weigela*, Aralia*, and many others). This flora represents a transitional stage between the late Miocene floras, which are relatively rich in conifer taxa but poor in herbaceous taxa, and Quaternary interglacial boreal forest floras which are relatively depauperate in conifer and broadleaf taxa but rich in herbaceous taxa.
Another informative site is at Ch'ijee's Bluff, an exposure on the Porcupine River in northern Yukon. Unit 1 at the base of the bluff contains a flora that is probably mid-Pliocene age (ca. 3.5-2.5 Ma), but this age assignment is based mostly on the seed assemblage and needs to be refined through additional research. The unit 1 deposits are rich in wood, including beaver-chewed Abies*, Pinus* (Strobus group), Larix, and Picea. Cones in the deposit are identified as Picea, Larix cf. L. minuta, and Pinus* (5-needle and 2-needle types). Seeds from the deposit include Betula, Alnus, Larix*, Comptonia*, Sambucus*, Carex, Sparganium, and Potamogeton.
An additional very important Pliocene site is Lost Chicken Mine in east- central Alaska, where the sediments contain a tephra that has been fission- track dated at 2.9 + 0.2 Ma (see Adam, this volume). This late Pliocene deposit contains a proto-boreal forest fossil assemblage of Picea, Larix (not L. laricina, the only larch species in Alaska today), Pinus*, Abies?*, Corylus*, Betula, Alnus, and Ericaceae. In contrast to the older Pliocene floras from the Nenana Gravel and the terrace gravels near Circle, the Lost Chicken Mine site has yielded only a sparse herbaceous flora. This may reflect a dense forest cover, with few openings for herbaceous taxa, and perhaps a mossy ground cover. The Lost Chicken flora lacks Tsuga, in contrast to late Miocene and older Pliocene floras from interior Alaska. The climate was warmer than the present (Holocene) interglacial in Alaska. MAT was probably ca. 2-3° C, vs. ca. -7° C today at the nearby community of Chicken.
On the North Slope of Alaska, the Fish Creek site provides some information on the early development of tundra or tundra-like vegetation during the late Pliocene, probably about 2.4 Ma. Tree and shrub elements of the older Fish Creek pollen flora include Picea*, Pinus*, Larix*, Abies*, Betula, Alnus*, Salix, and Ericaceae. Herbs are well-represented, and include Cyperaceae (most abundant), Gramineae, Artemisia, Compositae (Tubuliflorae), Caryophyllaceae, and Valeriana. Pollen data within the Fish Creek site records a progressive loss of trees and shrubs, reflecting onset of colder glacial climates in northern Alaska.
The Pliocene history of climate and ecological changes in Alaska and northern Canada are still poorly understood, although progress is being made to identify and investigate such deposits. One of the major problems inhibiting progress has been inadequate age control for some of the most important fossil localities. Because the precise biostratigraphic ranges of many of the Pliocene taxa from the region are poorly known, the fossil assemblages alone can only provide broad-range age assignments. The best hope of reconstructing a detailed Pliocene terrestrial record from the region may be from the drilling project planned for 1994 at Fort Yukon, in interior Alaska. At that location we may obtain a long Neogene record (about 15 Ma) in about 300 meters of sediment core. A water well log from Fort Yukon suggests that the subsurface sediments are mostly lacustrine.