CONSTRAINING MATERIAL PROPERTIES AND AGE OF MIMA MOUNDS

Earthen mounds are widespread but unusual and enigmatic features that have drawn the curiosity of professionals and lay people since they were first discovered due to their size and extensive repetitious pattern. The Mima Mounds near Olympia, Washington are the archetype for similar appearing mounds around the world. Numerous mechanisms have been proposed for their formation including gophers, seismic shaking, erosion, as well as the indigenous people. While there is little doubt that mounds in different regions of Earth have formed due to diverse mechanisms, the Mima Mounds continue to serve as the archetype for mound-like features worldwide. The Mounds formed in recessional outwash deposits dating to about 14 ka following the maximum extent of the Vashon Stade of the Fraser glaciation. Here, we further control the age of the mounds by using cosmogenic Be-10. The exposure age of clasts in the center of a mound (shielded by 2+ m of A-horizon material) is compared to that of clasts from the inter-mound area at depth of a few decimeters. The exposure age will also help determine whether the mounds are degrading due to soil creep and erosion or stable, perhaps being rebuilt while they erode. A characteristic of the Mima Mound soils that has not received much research attention, is that the A horizons are extremely dark believed to be from black carbon formed in the Mima Prairie as it was burned to facilitate the agricultural practices of the Native Americans for growing Camus species; this, however, has not been documented. We compare the black carbon content of A horizons in adjacent prairie versus non-prairie area by chemical oxidation of the soil followed by nuclear magnetic resonance; a high aromatic content indicates a higher black carbon content and suggests that this area burned more often. To provide additional characterization, the organic matter in the particular mounds that are dated by Be-10 is also carbon dated.