RECONSTRUCTING A JURASSIC ISLAND FOREST, TALKEETNA VOLCANIC FORMATION, TALKEETNA MOUNTAINS, ALASKA

The Early Jurassic (~Pleinsbachian) Talkeetna Volcanic Formation of Alaska’s Talkeetna Mountains preserves a marine and terrestrial sedimentary succession that evolved on an island arc complex offshore of western North America. Land plant fossils from collections made at the Sheep Mountain, Hicks Creek, and East Boulder Creek localities include foliage compressions/impressions of cycadophytes (e.g. Otozamites, Taeniopteris, Pterophyllum), seed ferns (e.g. Sagenopteris), and conifers (e.g. Pagiophyllum, Brachyphyllum). Reproductive elements of conifers are also preserved in compression and conifer wood is coalified and, occasionally, permineralized by silica.

In this study we placed the recovered foliage taxa in Early Jurassic global distribution paleogeographic context among all reported occurrences of each taxon. We cross-sectioned specimens of permineralized wood and made earlywood and latewood cells counts, measured earlywood and latewood ring width, and noted the occurrence of false rings over a >65 year growth period. From these data we calculated earlywood/latewood ratio, annual sensitivity, and performed mean sensitivity analysis.

Tectonic paleogeographic reconstructions have placed the island arc in the northern tropical to subtropical latitudes at the time of Early Jurassic sedimentation in the Talkeetna Volcanic Formation (TVF). This is partially consistent with our foliage data as most collected foliar taxa in the TVF also occur within the range of tropical and/or subtropical paleolatitudinal distribution on the nearest (west) continental margin. However, many of the same foliar taxa are also found in more eastern continental localities that have been reconstructed to be warm temperate. Distinct wood growth rings suggest that the island setting was subject to seasonal variation in growing conditions, perhaps driven by variance in precipitation in a monsoonal system or temperate belt seasonality. The foliar data, together with the near absence of false rings and a mean sensitivity analysis value near the traditional border of “complacent” and “sensitive” growth at ~0.31, suggest a warm and wet set of environmental conditions during the growing season, without extreme inter-annual variation.