Above is a typical Bigwoods Remnant Forest in SE Minnesota |
Bigwoods
of Minnesota
A Brief Summary of Field Methods and Proxies |
Oak Savanna/Woodland |
Lake Sediment Coring Just like the rings of a tree, the sediments in a lake can provide a history for an area. For most lakes, sediment accumulates each year and this sediment contains fossils like pollen (plants) and charcoal (fire) as well as the remains of diatoms (biogenic silica), magnetic particles, and more generically the bodies of plants and animals (organic matter). Lakes sediment (mud) is collected in a variety of ways, and can be collected during the summer or winter. We have used a surface corer (big polycarbonate tube, ~ 2 meters long) for soft, young sediment as well as a Livingston Corer (smaller diameter, 1 meter long stainless steel tube) for older, stiffer sediments. Both corers are driven into the sediment with stiff, light-weight, magn.-titanium alloy rods and both corers contain a piston which is anchored into place with a steel cable. When the corer is pushed past the piston a vacuum is formed which helps to reduce compacting of the mud as the corer accumulates more sediment. Once collected, the mud is extruded (and see summer) from the corer. With the Livingston corer -- and the addition of casing -- multiple cores can be taken from the same spot and we have cored as much as 14 meters of sediment from two lakes (Sharkey Pond and Kimble Lake). In other words, since the glaciers retreated from this area 14 yards of sediment accumulated in the bottoms of these lakes. As remarkable as that may seen, what should give pause for thought is that it took only 150 years for the last 1-2 yards to accumulate. In other words, erosion rates into the lakes in this region have increase x10 fold since the onset of plow-based agriculture. Proxies Proxy means stand-in and the fossils we examine are just that, stand-ins for the actual processes we are interested in. Descriptions of the protocols used can be obtained from the University of Minnesota Limnological Research Center which has been an international leader in the study of lake sediments. The proxies we have used include:
Cores are dated using a combination of radiocarbon dates and identification of depth of major increase in ragweed (Ambrosia) pollen which is a marker for plow agriculture. Note that ragweed is a native plant and there are several different perennial and annual species present in prairie. Selected References |
Close-up of Prairie in Fall
|
|
Prairie Fire |