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:

  • Charcoal. The burnt remains of fires. We work with charcoal trapped by 120-180 micron sieves (e.g with openings that are ~1/10 of 1 mm). This size charcoal most likely originated within 100-200 meters of the edge of the lake.
  • Pollen. This is what plants produce as part of sexual reproduction. The outer walls of the pollen grain are very resistant to decomposition. Pollen grains can be identified based on their size and shape. They have to be magnified to x400 to be viewed and are isolated from the sediment through a series of many mechanical and chemical processing steps. The proxies displayed elsewhere on this site include Ulmus (elm), Tilia (basswood), Ostrya (hop-horn beam), Ambrosia (ragweed), Quercus (oak), Poaceae (grass).
  • Magnetics. Lake sediment often contains magnetic particles that can be used to infer some combination of changes in (a) climate, (b) lake levels and local erosion, and (c) changes in the oxygenation level at the sediment surface. Common measurements are of the concentration of magnetic particles (SIRM in plots) and the size of particles (ARM/SIRM which gets larger with smaller particles).
  • Loss-On-Ignition (LOI). This is a way of describing the basic make-up of the sediment. Sediment is described in terms of its percentage organic matter (organic carbon), carbonates, and inorganic matter, based on measuring the loss in weight of the sediment after exposing it to different temperatures. Organic matter burns at 550C, carbonates (in this area assumed to be calcium carbonate) at 1000C. Change in LOI reflect changes in erosion outside of the lake, changes in the productivity of algae and other organisms in the lake as well as decomposition processes happening in the sediment.

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

(St. Olaf College Homepage)