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Plant Macrofossil Remains from a Full-Glacial Landscape


by Victoria Goetcheus, Department of Geology and Geophysics


Understanding past environments is necessary in order to understand how the environment can change in the future. My study focuses on the environment and biota of a portion of the Bering Land Bridge at the height of the Last Glacial Maximum about 18,000 years ago. In 1968, Dr. David M. Hopkins discovered a land surface on the Seward Peninsula buried by a tephra about 18,000 years ago. The tephra originated from a maar eruption that blanketed the surrounding landscape with ash, burying the land surface over an area of approximately 2500 km2. Where the tephra was thicker than the active layer (>40 cm), it preserved the plant and animal remains present on the surface by in situ freezing. The buried surface is especially interesting due to its location near the center of the Bering Land Bridge. The surface records the vegetation near the time of the thermal minimum of the last glacial.

Present-day views of the Beringian environment derive mainly from faunal remains and pollen cores. Plant macrofossils from this period are very scarce, but where found, provide a more detailed picture of the plants actually growing in a region than does pollen. Macrofossil remains can be identified with greater precision than pollen, and often down to the species level. Usually macrofossils in a sediment can be assumed to have been produced relatively locally and transported only a short distance before deposition and preservation. This is in contrast to pollen that can be transported for long distances before deposition, especially in treeless, glacial environments with strong winds.

Macrofossils from the buried surface are exceptional in that they have been produced by the vegetation growing on the spot, and therefore provide a direct record of it. Much of the vegetative material has decayed over time, and thus the main types of macrofossils preserved are seeds and fruits. Table 1 lists the various taxa that have been identified from plant macrofossils, mainly seeds, found on the buried surface. These taxa provide important information for the reconstruction of the paleoenvironment of the Bering Land Bridge.

Most of the identified taxa share two characteristics: they grow on mineral soil and they prefer calcareous substrates. The vegetation appears to fall into two main groups, xerophytic plants and those with higher moisture requirements, suggesting some differentiation in moisture availability at different places. The vegetation for the northern Seward Peninsula during the Last Glacial Maximum is reconstructed as a dry meadow and herb-rich tundra, often dominated by Kobresia myosuroides with a mixture of grasses, sedges, and herbs and an understory of mosses. Prostrate shrubs formed a minor, though locally important, part of the vegetation. Snow-beds and shallow hollows provided damper habitats for a more moisture-demanding species assemblage. Acrocarpous mosses were an important element of the vegetation, tolerating and stabilizing the steady low loess input, and maintaining the complete plant cover in most places. There appears to be no exact analogues in the present day. Analogues for the physiognomy of the vegetation type can be found, but the modern communities do not contain the same species assemblages.



Table 1
Table 1


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