The paleoenvironment of the Olympia beds based on fossil beetles from Discovery Park, Seattle, Washington, U.S.A.

摘要

Two fossil insect assemblages with ages of 17,959 and 19,786 cal BP are reported from cross-laminated silts and sands within the Olympia beds at Magnolia Bluff, Discovery Park, Seattle, Washington (N 47° 39', W 122° 25'). The silts accumulated during MIS 2 at a time when the Puget Lobe of the Cordilleran Ice Sheet was advancing southward. Eventually the Olympia beds were buried sequentially by lacustrine beds, outwash sands and gravels and finally till. The depositional environment based on paleoecology and sedimentary facies was a floodplain of a large braided river. The fossils are disarticulated skeletal parts representing a diverse fauna of 132 beetle species, all extant. The habitats represented by fossils include sparsely vegetated sand and gravel bars (Opisthius richardsoni), open muddy areas (Elaphrus californicus), and vegetation-shaded banks (Elaphrus clairvillei and Elaphrus purpurans). Outside of the floodplain, the habitat was grassland (Carabus taedatus) with occasional stands of coniferous trees indicated by scolytids (bark beetles). Large mammals probably used the river as a watering hole, based on the abundance of dung-feeding Aphodiinae (scarabs) and staphylinid beetles (Tachinus). Most of the beetle species have modern geographic ranges which include northern Washington State. Notably, however, species characteristic of the temperate rainforest of the Pacific Northwest are absent. For the older of the two fossil assemblages, mean summer temperature is estimated to be 17 ℃, or similar to Seattle today. The climate was more continental and winters would have been colder than at Seattle today. The occurrence of the northern species Pelophila borealis and Olophrum boreale in the younger of the two assemblages indicates a colder climate estimated to represent a lowering of mean summer temperature by 1-2° C. The climatic cooling is possibly associated with the advance of the Puget Lobe. Both pollen and fossil beetle evidence are in agreement that there was less available moisture, possibly one half of today's precipitation. Palynologists have referred to the MIS 2 vegetation as tundra, cold steppe, or subarctic parkland. None of the Discovery Park fossils are obligate tundra species, and because the majority of taxa have ranges which overlap in northern Washington State today, we prefer to use the term prairie for the temperate to cold-temperate MIS 2 grassland.